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Power electronics Viva voce or interview questions part-11

2025-04-24T05:21:00.001+05:30

Q-251 What is variable DC linked inverter?

Ans An inverter is called variable DC linked inverter if the input voltage is

controllable.

Q-252 What is inverter gain?

Ans The inverter gain may be defined as the ratio of the AC output to DC input

voltage.

Q-253 What are the advantages and disadvantages of variable DC linked inverter?

Ans Advantages:

1. Harmonic content does not change with output voltage.

2. Control circuit of an inverter is simple.

Disadvantages:

1. Additional chopper or control rectifier is required.

2. Efficiency of a circuit is reduced due to double conversion.

3. Transistors have to handle variable input voltages.

Q-254 Compare between Voltage source and Current source inverters

Ans Voltage source inverters

1. Input is constant voltage.

2. Short circuit can damage the circuit.

3. Peak current of power-device depends on load.

4. Current wave forms depend on load.

5. Free wheeling diodes are required in case of inducted load.

Current source inverters

1. Input is constant current.

2. Short circuit cannot damage the circuit.

3. Peak current of power-device is limited.

4. Voltage wave forms depend on load.

5. Free wheeling diodes are not required.

Q-255 Explain the principle of variable DC linked inverter?

Ans Harmonic content of the signal also changes if pulse width is varied. This

problem is taken care by DC link inverter. Instead of varying the pulses of

inverter, an input DC voltage is varied. Therefore rms value of output voltage is

varied.

Q-256 What is the commutation technique used in the parallel inverter?

Ans Complementary commutation technique.

Q-257 What is the role of the diodes D1 and D2?

Ans Diodes D1 and D2 act as free wheeling diodes, they conduct when both SCR’s

turn off. They also provide a path for conduction.

Q-258 Why is the inductor used?

Ans The inductor does not allow drastic changes in current and hence provide di/dt

protection.

Q-259 From where does the inverter derive its dc power input?

Ans It derives the dc power input from the inverter specific external VRPS.

Q-260 Why the output voltage of an inverter is to be controlled?

Ans The output voltage of the inverter is to be varied as per the load requirement.

Whenever the input DC varies the output voltage can change. Hence, these

variations need to be compensated. The output voltage and frequency of an

inverter is adjusted to keep voltage and frequency constant. Thus, the output

voltage of an inverter is to be controlled.

Q-261 What are series inverters?

Ans Inverters in which the commutating elements are permanently connected in

series with the load resistance.

Q-262 What is the condition for selecting commutating element?

Ans They are selected in such a way that the current flow through series connected

elements R, L, C is under damped

Q-263 What are the drawbacks of a basic series inverter?

Ans *If the inverter frequency exceeds the circuit ringing frequency the dc source

will be short-circuited.

*For output frequencies much smaller than the circuit ringing frequency, the

load voltage is di started.

*The source current flows only during the period when the Thyristor T1 is

conducting. This results in large ripple in the source current and peak current

rating of the source inverters.

Q-264 What are the applications of series inverters?

Ans *Induction heating

*Fluorescent lighting

*Variable speed ac motor drives

*Aircraft power supplies

*UPS

*High voltage dc transmission lines

Q-265 Why are the inductors L1, L2 and why are two capacitors needed?

Ans *The resonant frequency, which is, if it is nearby inverter output frequency,

commutation failure will take place. Hence it should be ensured that the

capacitor and inductor are so chosen that it be not near to resonant frequency.

*Equal values of L1, L1’ or C1, C1’ to be chosen so that the uniform inverter

output is maintained.

Q-266 What are the waveforms (output) obtained in inverter?

Ans The output voltage waveforms of ideal inverters are sinusoidal. But for practical

inverters they are non-sinusoidal and contain harmonics due to which the

waveforms may be square wave or quassi square wave.

Q-267 Why can’t we see current waveforms on CRO?

Ans The resistance of CRO is very high. Therefore the current measurement is

incorrect. An attempt to reduce the resistance of CRO reduces the input

impedance, which draws heavy current from the source.

Power electronics Viva voce or interview questions part-10

2025-04-24T05:20:00.002+05:30

Q-226 What are the two modes of operation of a TRIAC?

Ans *Junction gate operation

*Remote gate operation

Q-227 What are merits of TRIAC?

Ans 1. TRIAC is a bi-directional device that is it conducts in both directions.

2. TRIAC turns-off when voltage is reversed.

3. Single gate controls conduction in both directions.

4. TRIAC’s with high voltages and current ratings are available.

Q-228 What are de-merits of TRIAC?

Ans *TRIAC’s are latching devices like SCR; hence they are not suitable for DC

power applications.

*Gate has no control over the conduction once Triac is turned on.

*TRIAC’s have very small switching frequencies.

Q-229 When is the sensitivity of TRIAC greatest?

Ans The sensitivity of TRIAC is greatest in I quadrant (mode 1) when MT2 and gate

are positive with respect to MT 1 and it triggers for a low value of latching

current.

Q-230 When is the sensitivity of TRIAC low?

Ans The sensitivity is less in mode III (3rd quadrant) when MT 2 is negative and

positive gate current and it requires larger value of latching current to trigger.

Q-231 Why should we use TRIAC for bi-directional flow of current, instead we

can use an ordinary conductor (wire)?

Ans The ordinary conductor does not provide the facility to switch on or off that is,

we cannot control the flow of current. We have to manually switch on or off the

supply.

Q-232 How is the TRIAC different from RCT?

Ans The TRIAC is equivalent to two SCR’s connected in antiparallel where as the

RCT is a SCR and a diode in antiparallel direction.

Q-233 What happens when gate voltage is increased?

Ans The forward break over voltage starts decreasing.

Q-234 What is stepper motor?

Ans A stepper motor is one in which the motor rotates step by step in steps of 1.8

degree.

Q-235 Why stepper motor is also called digital motor?

Ans A stepper motor is driven by square pulses and hence does not require pulse

width modulation technique. It can be driven by simple digital circuits and hence

is called digital motor.

Q-236 How many steps are covered in one revolution?

Ans 200 steps with step angle of 1.8 degree.

Q-237 What are merits of stepper motor?

Ans It is easy to interface with digital controller, low cost solution for position

control and also zero feedback error in servo system.

Q-238 What are its applications?

Ans Teleprinters,typewriters,plotters,X ray machinery, valve control, servo

mechanism, photo printing and developing, film projectors ,cameras ,carburettor

adjusting, electronic gear box, blood analysers etc

Q-239 What is the advantage of stepper motor?

Ans The major advantage is that its speed can be varied by the digital input. We can

use a microprocessor to control the speed .It can also be rotated in steps. They

can be used for precision control of speed without using closed loop feedback.

Q-240 What is step angle?

Ans The angle through which the motor shaft rotates for each command pulse is

called the step angle. It can be 1.8, 2.5, 7.5 degrees.

Q-241 What is stepping frequency?

Ans F is the stepping frequency in pulses per second.

Q-242 What are the moving parts in stepper motor?

Ans The only moving part in stepper motor is rotor which has no winding,

commutatore, brushes.

Q-243 What are types of stepper motor?

Ans *permanent magnet motor.

*variable reluctance motor-when stator is excited ,the rotor is pulled into the

nearest

Minimum reluctance position.

Q-244 What does stepping motor consist of?

Ans *The indexer is a microprocessor capable of generating step pulses and

direction signals for the driver.

*The driver (amplifier) converts the indexer command signals into the power

necessary to energise the motor windings.

Q-245 What are its advantages?

Ans The step motor is an electromechanical device that converts digital pulses into

mechanical shaft rotation. Advantages are low cost, high reliability, high torque

at low speeds and a simple, rugged construction that operates almost in any

environment.

Q-246 What are its disadvantages?

Ans The main disadvantage in using stepper motor is the resonance effect often

exhibited at low speeds and decreasing torque with increasing speed.

Q-247 What are inverters and what are its applications?

Ans DC to AC converters is known as inverters. The function of an inverter is to

change a DC input voltage into AC output voltage of desired magnitude and

frequency. Inverters are widely used in industrial applications like variable speed

AC motor drives, induction heating, stand-by power supplies and uninterrupted

power supplies.

Q-248 Why the circuit is called parallel inverter?

Ans The circuit is called parallel inverter because the commutating capacitor is in

parallel with the primary winding of the output transformer whose secondary is

fed to the load.

Q-249 What is the main classification of inverters?

Ans Inverters can be broadly classified into two types namely, Single-phase inverters

and three phase inverters. Each type can use controlled turn-on and controlled

turn-off devices (eg. BJT’s and MOSFET’s etc) or forced commutation thyristers

depending on application.

Q-250 What is VFI and CFI?

Ans An inverter is called a Voltage Fed Inverter (VFI) if the input voltage remains

constant, a Current Fed Inverter (CFI) if the input current is maintained constant.

Power electronics Viva voce or interview questions part-9

2025-04-24T05:19:00.002+05:30

Q-201 On what basis choppers are classified in quadrant configurations?

Ans The choppers are classified depending upon the directions of current and voltage

flows. These choppers operate in different quadrants of V-I plane. There are

broadly following types of choppers: class a chopper (first quadrant); class B

(second quadrant) Class C and class D (two quadrant choppers), class C in II

quadrant and I whereas class D in IV quadrants, and I class E is four quadrant

operator.

Q-202 What are different control strategies found in choppers?

Ans The different control strategies are pulse width modulation, frequency

modulation and current limit control, variable pulse width and frequency.

Q-203 Explain the principle of operation of a chopper?

Ans A chopper acts as a switch, which connects and disconnects the load, hence

producing variable voltage.

Q-204 What are the advantages of DC choppers?

Ans * High ripple frequency, so small filters are required.

*Power factor is better.

*Efficiency is better.

*Small and compact.

*The dynamic response of choppers is fast due to switching nature of the

device.

Q-205 Define duty cycle.

Ans The duty cycle of chopper controls its output voltage. The value of duty cycle

lies between 0 and 1 and is given by Ton/(Ton+Toff).

Q-206 How can ripple current be controlled?

Ans Ripple current is inversely proportional to the frequency and hence can be

controlled by having higher frequency.

Q-207 What is step up chopper?

Ans If the output average voltage is greater than the supply voltage, then the chopper

is called step up chopper.

Q-208 On what does the commutating capacitor value depend on?

Ans It depends on the load current.

Q-209 What are the disadvantages of choppers?

Ans *They can operate only at low frequencies.

*The commutation time depends on the load current.

*The output voltage is limited to a minimum and maximum value beyond which

we cannot get the output voltage.

Q-210 How do they have high efficiency?

Ans DC choppers uses switching principle, hence they have high efficiency.

Q-211 What are the applications of dc choppers?

Ans Battery operated vehicles, switched mode power supplies, traction devices,

lighting and lamp controls, trolley cars, marine hoists, and forklift trucks. Mine

haulers etc.

Q-212 What is principle of dc motor?

Ans An electric motor is a machine, which converts electrical energy into mechanical

energy. Its action is based on the principle that when a current carrying

conductor is placed in a magnetic field it experiences a mechanical force whose

direction is given by flemings left hand rule and whose magnitude is given by

F=BIL When the field magnets of a multipolar dc motor are excited and its

armature conductors are supplied with current from supply mains they

experience a force tending to rotate the armature .By Fleming’s left hand rule it

is noted that each conductor experiences a force which tends to rotates the

armature in anticlockwise direction. These forces collectively produce a driving

torque (or twisting moment), which sets the armature rotating.

Q-213 How can the speed of the series motor controlled?

Ans *flux control method

-field divertors

-Armature divertor

*variable resistance in series with the motor.

Q-214 What are the advantages of field method?

Ans *economical,more efficient

*It gives speeds more /above the normal speed.

Q-215 What arethe disadvantages of field method?

Ans Commutation becomes unsatisfactory.

Q-216 What are the factors controlling speed?

Ans Speed can be controlled by controlling flux, resistance, voltage.

Q-217 What is the significance of back emf?

Ans When the motor armature rotates the conductors also rotates and hence cut flux.

Therefore emf is induced and direction is in opposition with the applied voltage

(Fleming’s right hand rule). Because of its opposing direction it is referred to as

back emf Eb. V has to drive Ia against the opposition of Eb.The power required

to overcome this opposition is EbIa.

Q-218 What is torque?

Ans Torque is twisting or turning moment of a force about an axis. The torque

developed by the armature of a motor is armature torque. The torque available

for useful work is known as shaft torque (available at the shaft).

Q-219 How can dc motors be classified?

Ans separately excited

*self-excited.

Q-220 What are the main losses in motors?

Ans *stator losses

*rotor losses

*mechanical losses

Q-221 Why are starter used in dc motor?

Ans Initially Eb =0 and R is usuallly very small,therefore the armature current is very

high which could damage the motor.Hence starters which is basically a

resistance connected in series with the motor.

Q-222 What is the parameter that is being varied by varying the firing angle?

Ans The armature voltage is varied which inturn varies the speed of the motor by

varying the firing angle.

Q-223 What are the operating modes of dc motor?

Ans Motoring, regenerative braking, dynamic braking, plugging.

Q-224 Why is it called TRIAC and what are its applications?

Ans The word TRIAC is abbreviated from TRI from Triode and AC. The TRIAC is

the best device for AC phase control. The input and load is AC. The power is to

be controlled in positive as well as negative half cycles. The TRIAC is triggered

in every half cycle. Thus, the necessity of anti-parallel SCR’s is eliminated by

TRIAC. Thus, it is used mainly in AC power controllers, heater, fan, triggering

device for SCR’s.

Q-225 How many terminals does a TRIAC have?

Ans A TRIAC has three terminals, main terminal (MT 1), and main terminal (MT 2),

gate (G).

Power electronics Viva voce or interview questions part-8

2025-04-24T05:18:00.001+05:30

Q-176 What is interring base resistance?

Ans Inter base resistance is the resistance between 2 bases

Q-177 What is intrinsic standoff ratio?

Ans Intrinsic standoff ratio=Rb1/(Rb1+Rb2). Its value ranges between .52 to .81.

Q-178 What is the width of the triggering pulse?

Ans TG=Rb1.C

Q-179 Why are the capacitors CIF and C used?

Ans Capacitor CIF is used to minimize the ripples and C is used for charging and

discharging so that the trigger is eventually formed.

Q-180 What is ac voltage controller?

Ans If a Thyristor switch is connected between ac supply and load, the power flow

can be controlled by varying the rms value of ac voltage applied to the load and

this type of power circuit is known as an ac voltage regulator

Q-181 What are the applications of ac voltage controllers?

Ans The most common applications of ac voltage controllers are: industrial heating,

on-load transformer tap changing, light controls, speed control of polyphase

induction motors and ac magnet controls.

Q-182 What do you mean by sequence control?

Ans The use of two or more stages voltages controllers in parallel for the regulation

of output voltage.

Q-183 Give the classification of ac voltage regulators.

Ans They are classified as:

1.single phase controllers

2.three phase controllers

Each type can be subdivided into unidirectional and bi-directional control.

Q-184 What are the two types of control?

Ans *on off control: Here Thyristor switches connect the load to the ac source for a

few cycles of input voltage and then disconnect it for another few cycles.

*phase angle control: Here Thyristor switches connect the load to the ac source

for a portion of each cycle of input voltage.

Q-185 Why are extra commutation components not required?

Ans The ac voltage controllers have main supply as input. The SCR’s in these

controllers are turned off by natural commutation. Hence extra commutation

components are not required. Therefore ac voltage controllers are simple and

easy to implement if SCR’s are used.

Q-186 What is the difference between cycloconverters and ac voltage controllers?

Ans In cycloconverters (ac to variable ac) frequency of output can be varied. In ac

voltage controller’s frequency of output is kept constant, just the output average

value is controlled (on and off times varied).

Q-187 What is diac firing circuit?

Ans A diac firing circuit consists of a diac that is used to generate trigger pulses for

the Thyristor diac can conduct in both directions and it does not have any control

terminal in the form of a gate.

Q-188 What are the merits and demerits of voltage controllers?

Ans The merits are that they are simple without commutation circuits, high efficiency

and less maintenance.

The demerits are that the load current is asymmetric (phase control) and hence

harmonics are present and intermittent supply of power in on-off control.

Q-189 Why is the trigger source for the two Thyristor isolated from each other in a

single-phase voltage controller?

Ans When one Thyristor is on, the other should be off. Both the Thyristor should not

conduct at a time.

Q-190 What is a full controlled rectifier?

Ans It is a two-quadrant ac to dc converter. It has 4 thyristors and hence all of them

can be controlled for rectification purpose. In a full converter the polarity of the

output voltage can be either positive or negative but the output current has only

one polarity.

Q-191 What is a semi converter?

Ans A semi converter is a one-quadrant converter and it has only one polarity of

output voltage and current.

Q-192 What is a dual converter?

Ans A dual converter can operate in all 4 quadrants and both output voltage and

current can be either positive or negative.

Q-193 How can we control the output voltage of a single-phase full converter?

Ans By varying the trigger angle.

Q-194 What is MCB?

Ans MCB-Miniature circuit breaker. This is used as switch, which opens or switches

off when the voltage or current is above the rated value of that of MCB.

Q-195 How many lines are there in single-phase system?

Ans Two lines- 1line

1neutral

Q-196 What is rectification mode and inversion mode?

Ans During the period alpha to 180° the input voltage Vs and input current Is are

positive and the power flows from supply to the load. The converter is said to be

operating in rectification mode. During the period 180 to 180°+alpha the input

voltage Vs and the input current Is positive and there will be reverse power flow

from load to supply. The converter is said to be operating in inversion mode.

Q-197 Where is full bridge converter used?

Ans It is mainly used for speed control of dc motors.

Q-198 What is the effect of adding free wheeling diode?

Ans Free wheeling action does not takes place in single-phase full converter

inherently as there are 4 thyristors and no diodes. From 180 to 180+alpha( ) free

wheeling diode starts conducting. It is more forward biased compared to T1 and

T2.Hence freewheeling diode conducts. The freewheeling diode is connected

across the output Vo. Hence Vo=0 during freewheeling. The energy stored in the

load inductance is circulated back to the load itself.

Q-199 What are choppers?

Ans A dc chopper converts directly from dc to dc and is also known as dc-dc

converter.

Q-200 What does a chopper consist of?

Ans It can be a power transistor, SCR, GTO, power MOSFET, IGBT or a switching

device.

Power electronics Viva voce or interview questions part-7

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Q-151 What is the maximum firing angle of R-triggering circuit and why?

Ans The maximum firing angle is 90°. This is because the source voltage reaches maximum value of 90° point and the gate current has to reach Ig(min) some where between 0-90°. This limitation means that load voltage waveform can only be varied from α = 0° to α = 90°.

Q-152 What are the disadvantages of R triggering?

Ans Trigger angle a is greatly dependent on the SCR’s Ig(min) and this value varies between SCR’s and it is also temperature dependent. · Maximum triggering angle achievable is 90°.

Q-153 In R-triggering circuit why is Rmin is connected in series with variable resistor?

Ans The limiting resistor Rmin is placed between anode and gate so that the peak gate current of the thyristor Igm is not exceeded.

Q-154 What is the maximum firing angle of RC-triggering and why?

Ans Maximum firing angle is 180°. This is because capacitor voltage and AC line voltage differ in phase. By adjusting the value of R it is possible to vary the delay in turning on the SCR from 0 to 10 msec and hence vary the firing angle from 0° to 180°.

Q-155 What is an UJT and draw its equivalent circuit?

Ans UJT-uni junction transistor. It has only one type of charge carriers. It has three terminals emitter, base 1 and base 2. (‘Duo base’ as it has 2 bases)

Q-156 Why is an UJT used in SCR firing circuit?

Ans The voltage at base 1 of UJT is smaller than the voltage needed to trigger the Scrim the voltage is high, then it will trigger the SCR as soon as the ac supply is on.

Q-157 Why is the isolation needed between Thyristor and firing circuit?

Ans The trigger circuit operates at low power levels (5-20 volts) whereas thyristors operate at high voltage levels (250 volts). Hence if the Thyristor acts as a short the entire 250volts get applied across the firing circuit causing damage. Hence isolation is needed.

Q-158 How is a pulse transformer different from other transformer?

Ans A pulse transformer is one in which the input at the primary is current which is

transformed into a pulse at the secondary. Thus it does not step-up and step-

down as other transformers.

Q-159 What are the features of pulse transformer?

Ans The primary magnetizing inductance is high, coupling efficiency is high, and

interwinding capacitance is low and has greater insulation.

Q-160 What are the advantages of using pulse transformer?

Ans *Multiple secondary windings allow simultaneous gating signals to series and

parallelconnected thyristors.

*Control circuit and power circuit can be isolated.

Q-161 Why is UJT used in SCR firing circuit?

Ans As the UJT works in a mode called as a relaxation oscillator i.e. UJT turns on or

off depending on the charging and discharging of the capacitor. Time constant

can be varied with Chance delay angle can be varied .The UJT thus gives a firing

angle range of 0- 180.Vz is supply to UJT, the discharging current when passed

through the pulse transformer triggers SCR with pulses.

Q-162 Why is the sneer diode used?

Ans The sneer diode provides a constant supply voltage for UJT. It enables

synchronization with zero crossings. Sneer diode acts as a regulator. The sneer

clamps the rectified voltage to vs. to prevent erratic firing. This sneer voltage

acts as a supply for UJT relaxation oscillator.

Q-163 What is meant by ramp control, open loop control or manual control with

respect to UJT firing circuit?

Ans Ramp control-The graph of time period in milliseconds with the firing angle in

degrees is a ramp. The ramp slope can be controlled by the potentiometer.

Manual control-The potentiometer in the kit can be used to get various firing

angles. This is manual control.

Q-164 What is a firing circuit?

Ans It is a circuit, which is used to trigger a device at various instants of time.

Q-165 Why a bridge rectifier is used?

Ans The bridge rectifier gives a full wave rectified output, which is high in efficiency

and least ripple factor.

Q-166 What is the load used?

Ans Load is high power dissipation resistor.

Q-167 What is time constant of a circuit?

Ans Time constant of a circuit=RC where R=resistance C=capacitance It gives the

time of charging and discharging of a capacitor.

Q-168 What are the merits of UJT firing circuit over RC triggering circuit?

Ans * Firing angle remains stable.

*Advantages of pulse transformer.

Q-169 What are the advantages of UJT pulse trigger circuit?

Ans The resistors, capacitors depend heavily on the trigger characteristics of the

Thyristor used. The power dissipation is high due to prolonged pulse. But the

pulse triggering can accommodate wide tolerances in triggering characteristics

by instantaneously overdriving the gate. The power level in such circuits is lower

as the triggering energy can be stored slowly and discharged rapidly when the

triggering is required.

Q-170 Why is UJT used as relaxation oscillator?

Ans The UJT is used as a relaxation oscillator to obtain sharp, repetitive pulses with

good rise time. Also it has good frequency stability against variation in the

supply voltage and temperature.

Q-171 What are the applications of UJT trigger circuits?

Ans *Used to trigger SCR’s in single-phase converters, single-phase ac regulators.

*Used in oscillators

*Used in timing circuits

Q-172 What is valley voltage?

Ans It is the voltage at which the UJT turns off and the capacitor starts charging

again.

Q-173 What is the discharging path if the capacitor?

Ans The capacitor discharges through emitter, base and primary of the pulse

transformer.

Q-174 What is relaxation oscillator?

Ans When the capacitor discharges to a valley voltage, the UJT turns off and

capacitor starts charging again. This mode of working of UJT is called relaxation

oscillator.

Q-175 What is negative resistance?

Ans After the capacitor charges to Vp it starts discharging. During this period the

voltage V decreases with increase in current, hence this portion of V-I

characteristics is called negative resistance.

Power electronics Viva voce or interview questions part-6

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Q-126 What is pinch off voltage?

Ans The voltage across gate to source at which the drain to source current becomes zero is called pinch off voltage.

Q-127 In which region does the MOSFET used as a switch?

Ans In the linear region.

Q-128 Which parameter defines the transfer characteristics?

Ans The Tran conductance Gm=Id/Vgs

Q-129 Why are MOSFET’s mainly used for low power applications?

Ans MOSFET’s have high on state resistance Rds. Hence for higher currents; losses in the MOSFET’s are substantially increased. Hence MOSFET’s are substantially increased. Hence, MOSFET’s are mainly used for low power applications.

Q-130 How is MOSFET turned off?

Ans To turn off the MOSFET quickly, the negative gate current should be sufficiently high to discharge gate source input capacitance.

Q-131 What are the advantages of vertical structure of MOSFET?

Ans *On state resistance of MOSFET is reduced.

*Width of the gate is maximized. Hence, gain of the device is increased.

Q-132 What are the merits of MOSFET?

Ans * MOSFET’s are majority carrier devices.

* MOSFET’s have positive temperature coefficient, hence their paralleling is easy.

*MOSFET’s have very simple drive circuits.

*MOSFET’s have short turn on and turn off times; hence they operate at high frequencies.

*MOSFET’s do not require commutation techniques.

*Gate has full control over the operation of MOSFET.

Q-133 What are demerits of MOSFET?

Ans *On state losses in MOSFET are high.

*MOSFET’s are used only for low power applications.

*MOSFET’s suffer from static charge.

Q-134 What are the applications of MOSFET?

Ans *High frequency and low power inverters.

*High frequency SMPS.

*High frequency inverters and choppers.

*Low power AC and DC drives.

Q-135 What is IGBT?

Ans Insulated gate bipolar transistor is the latest device in power Electronics .It is obtained by combining the properties of BJT And MOSFET.

Q-136 In what way IGBT is more advantageous than BJT and MOSFET?

Ans *It has high input impedance of the MOSFET and has low on-state voltage drop.

*The turn off time of an IGBT is greater than that of MOSFET.

*It has low onstage conduction losses and there is no problem of second Breakdown as in case of BJT.

*It is inherently faster than a BJT.

Q-137 What are on state conduction losses? How is it low in IGBT?

Ans A high current is required to break the junctions in BJT. This results in On state conduction losses. The conduction losses in IGBT are proportional To duty cycle of the applied voltage. By reducing the duty cycle conduction losses can be reduced.

Q-138 What is second breakdown phenomenon?

Ans As the collector voltage drops in BJT there is an increase in collector Current and this substantially increase the power dissipation. This Dissipation is not uniformly spread over the entire volume of the device but is concentrated in highly localized regions where the local temperature may grow and forms the black spots. This causes the destruction of BJT. This is second breakdown.

Q-139 What is switching speed?

Ans The time taken to turn on or turn off a power device is called switching Speed.

Q-140 Can we observe the transfer and collector characteristics of IGBT on CRO?

Ans No. Because the waveform which is to be observed on the CRO should Vary with respect to time otherwise we can see only a straight line on the CRO.

Q-141 What is punch through IGBT?

Ans The IGBT’s which have n+ buffer layer present are called punch through IGBT.They have asymmetric voltage blocking capabilities and have faster turn off times. Hence they are used in choppers and inverters.

Q-142 What is non-punch through IGBT?

Ans The IGBT’S without n+ buffer layer are called non-punch through IGBT’s. They have symmetric voltage blocking capabilities and are used for rectifier applications.

Q-143 What are merits of IGBT?

Ans *The drive is simple.

*Onstage losses are reduced.

*No commutation circuits are required.

*Gate has full control.

*Switching frequencies are higher.

*It has flat temperature coefficient.

Q-144 What are demerits of IGBT?

Ans *They have static charge problems.

*They are very costly.

Q-145 What are the applications of IGBT’s?

Ans *Ac motor drives. (Inverters)

*Dc to Dc power supplies. (Choppers)

*UPS systems.

*Harmonic compensators.

Q-146 Why is silicon used in all power semiconductor devices and why not Germanium?

Ans The leakage current in silicon is very small compared to germanium. The germanium is also more sensitive compared to silicon.

Q-147 What is pinch off voltage?

Ans When Vge is made negative, electrons in the n-channel get repelled Creating a depletion region resulting in a narrower effective channel. If Vge is made negative enough so as to completely eliminate the channel (High resistance, low current state), that value is called the pinch off Voltage.

Q-148 What is threshold voltage?

Ans Threshold voltage is the voltage Vge at which IGBT begins to conduct.

Q-149 How is IGBT turned off?

Ans An IGBT can turn off by discharging the gate by means of short circuiting it to the emitter terminal.

Q-150 What is the rating of IGBT?

Ans The current rating can be up to 400A, 1200V with switching frequency of 20KHz.

Power electronics Viva voce or interview questions part-5

2025-04-24T05:14:00.003+05:30

Q-101 Why does high power dissipation occur in reverse blocking mode?

Ans High power dissipation occurs because as voltage increases beyond Vbr current increases rapidly.

Q-102 Why shouldn’t positive gate signal be applied during reverse blocking Mode?

Ans If we apply positive gate signal J3 becomes forward biased. Reverse leakage current increases and Thyristor gets damaged due to large power dissipation.

Q-103 Explain reverse current Ire?

Ans When cathode voltage is positive, J2 is forward biased; J1 and J3 are reverse biased. The thyristors will be in reverse blocking state and reverse leakage current Ire flows.

Q-104 What happens when gate drive is applied?

Ans When gate drive is applied avalanche breakdown occurs at J2 causing excessive flow of charges and hence current surge. This turns the SCR into conduction state faster i.e. the Thyristor turns on at lower and lower anode to cathode voltages, which are less than Vbo.

Q-105 Differentiate between holding and latching currents

Ans Holding current is the minimum amount of current below, which SCR does not conduct. It is associated with the presence of gate terminal and concerns turn off condition. Latching current is the minimum amount of current required for the SCR to conduct. It is associated with absence of gate terminal and concerns turn on process. It is greater than holding current.

Q-106 Why is dv/dt technique not used?

Ans As this causes false triggering even when gate or voltage Vak is not applied, dv/dt technique is not used. Snubbed circuit, which is combination of a C, avoids this and R .The capacitor is placed in parallel with SCR.

Q-107 What sided?

Ans At the time of turn on, anode current increases rapidly. This rapid variation is not spread across the junction area of the thyristors. This creates local hotspots in the junction and increases the junction temperature and hence device may be damaged. This is avoided by connecting an inductor in series with an SCR.

Q-108 Why should the gate signal be removed after turn on?

Ans This prevents power loss in the gate junction.

Q-109 Is a gate signal required when reverse biased?

Ans No, otherwise SCR may fail due to high leakage current.

Q-110 What are different types of firing circuits to trigger SCR?

Ans *R firing circuit.

*RC firing circuit.

*UJT firing circuit.

*Digital firing circuit.

Q-111 What type of triggering is used in SCR?

Ans Pulse triggering.

Q-112 What is offset current?

Ans When anode voltage is made positive, J1 and J3 are forward biased, J2 is reverse biased. The Thyristor is in forward blocking or off state condition and the leakage current is known as offset current Io.

Q-113 What are the advantages of SCR?

Ans *Very small amount of gate drive is required since SCR is regenerative device.

*SCR’s with high voltage and current ratings are available.

*On state losses are reduced.

Q-114 What are the disadvantages of SCR’s?

Ans *Gate has no control once the SCR is turned on.

*External circuits are required to turn off the SCR.

*Operating frequencies are very low.

*Snubber (RC circuits) is required for dv/dt protection.

Q-115 What are applications of SCR?

Ans *SCR’s are best suitable for controlled rectifiers.

*AC regulators, lighting and heating applications.

*DC motor drives, large power supplies and electronic circuit breakers.

Q-116 What is the difference between an IGBT and SCR?

Ans IGBT comprises of a BJT and a MOSFET where as an SCR comprises of two BJT’s.

Q.117 Can we replace a SCR by a microprocessor by writing a program to exhibit characteristics of SCR?

ans. No, we can verify or test the working of SCR using microprocessor but we cannot replace it practically.

Q-117 What are MOSFET’s?

Ans Metal oxide silicon di-oxide field effect transistor is a voltage-controlled device. The parts of MOSFET are gate, drain and source.

Q-118 What is the difference between MOSFET and BJT?

Ans The MOSFET is a voltage controlled device where as BJT is a current controlled device.

Q-119 What is the difference between JFET and MOSFET?

Ans There is no direct contact between the gate terminal and the n-type channel of MOSFET.

Q-120 What are the two types of MOSFET?

Ans *Depletion MOSFET

- N channel in p substrate.

-P channel in n substrate.

*Enhancement mosfet

-virtual n channel in p substrate

-Virtual p channel in n substrate

Q-121 What is the difference between depletion and enhancement MOSFET?

Ans The channel in the centre is absent for enhancement type MOSFET but the channel is present in depletion type MOSFET.The gate voltage can either be positive or negative in depletion type MOSFET’s but enhancement MOSFET responds only for positive gate voltage.

Q-122 How does n-drift region affect MOSFET?

Ans The n- drift region increases the onstage drop of MOSFET and also the thickness of this region determines the breakdown voltage of MOSFET.

Q-123 How are MOSFET’s suitable for low power high frequency applications?

Ans MOSFET’s have high on state resistances due to which losses increase with the increase in the power levels. Their switching time is low and hence suitable for low power high frequency applications.

Q-124 What are the requirements of gate drive in MOSFET?

Ans *The gate to source input capacitance should be charged quickly.

*MOSFET turns on when gate source input capacitance is charged to sufficient level.

*The negative current should be high to turn off MOSFET.

Q-125 What is rise time and fall time?

Ans The capacitor Cgs charges from threshold voltage to full gate voltage Vgsp. The time required for this charging is called rise time. During this period, drain current rises to full value. The capacitor Cgs keeps on discharging and its voltage becomes equal to threshold voltage Vt.The time required for this discharge Cgs from Vgsp to Vt is called fall time.

Power electronics Viva voce or interview questions part-4

2025-04-24T05:12:00.006+05:30

Q-76 What is the advantage of ON-OFF control?

Ans Due to zero-voltage and zero current switching of thyristors, the harmonics generated by the switching action are reduced.

Q-77 What is the disadvantage of ON-OFF control?

Ans This type of control is applicable in systems that have high mechanical inertia and high thermal time constant.

Q-78 What is the duty cycle in ON-OFF control method?

Ans Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.

Q-79 What is meant by unidirectional or half-wave ac voltage controller?

Ans Here the power flow is controlled only during the positive half-cycle of the input voltage.

Q-80 What are the disadvantages of unidirectional or half-wave ac voltage controller?

Ans a. Due to the presence of diode on the circuit, the control range is limited and the effective RMS output voltage can be varied between 70.7percent and 100percent.

b. The input current and output voltage are asymmetrical and contain a dc component. If there is an input transformer, sdaturation problem will occur

c. It is only used for low power resistive load.

Q-81 What is meant by bidirectional or half-wave ac voltage controller?

Ans Here the power flow is controlled during both cycles of the input voltage.

Q-82 What is the control range of firing angle inac voltage controller with RL

load?

Ans The control range is(α) = Φ to 180° , where Φ = load power factor angle.

Q-83 What type of gating signal is used in single phase ac voltage controller with RL load?

Ans High frequency carrier gating signal is used for single phase ac voltage controller with RL load.

Q-84 What are the disadvantages of continuous gating signal?

Ans a. More heating of the SCR gate.

b. Increases the size of pulse transformer.

Q-85 What is meant by high frequency carrier gating?

Ans Thyristor is turned on by using a train of pulses from α to π. This type of signal is called as high frequency carrier gating.

Q-86 What is meant by sequence control of ac voltage regulators?

Ans It means that the stages of voltage controllers in parallel triggered in a proper sequence one after the other so as to obtain a variable output with low harmonic content.

Q-87 What are the advantages of sequence control of ac voltage regulators?

Ans a. System power factor is improved.

b. Harmonics are reduced in the source current and the load voltage.

Q-88 What is meant by cyclo-converter?

Ans It converts input power at one frequency to output power at another frequency with one-stage conversion. Cycloconverter is also known as frequency changer.

Q-89 What are the two types of cyclo-converters?

Ans a. Step-up cyclo-converters

b. Step-down cyclo-converters

Q-90 What is meant by step-up cyclo-converters?

Ans In these converters, the output frequency is less than the supply frequency.

Q-91 What is meant by step-down cyclo-converters?

Ans In these converters, the output frequency is more than the supply frequency.

Q-92 What are the applications of cyclo-converter?

Ans a. Induction heating

b. Speed control of high power ac drives

c. Static VAR generation

d. Power supply in aircraft or ship boards

Q-93 What is meant by positive converter group in a cycloconverter?

Ans The part of the cycloconverter circuit that permits the flow of current during positive half cycle of output current is called positive converter group.

Q-94 What is meant by negative converter group in a cycloconverter?

Ans The part of the cycloconverter circuit that permits the flow of current during negative half cycle of output current is called negative converter group.

Q-95 What is a Thyristor?

Ans Thyristor is derived from the properties of a Thyratron tube and a Transistor. It is used as another name for SCR’S. They are power Semiconductor devices used for power control applications.

Q-96 What are SCR’s?

Ans SCR’s is Silicon controlled Rectifiers. They are basically used as Rectifier

Q-97 What is Forward break over voltage?

Ans The voltage Vak at which the SCR starts conducting is called as Forward Break over voltage Vbo. This happens when the junction J2 undergoes Avalanche breakdown due to high reverse bias on junction J2.

Q-98 What is Reverse break over voltage?

Ans If the reverse voltage is increased more than a critical value, avalanche Breakdown will occur at J1 and J3 increasing the current sharply. This is Reverse break over voltage VBO.

Q-99 Why is Vbo greater than Vbr?

Ans Region at junction J2 is higher during forward bias than that of J1 and J3 under reverse bias.

Q-100 What are modes of working of an SCR?

Ans Reverse blocking mode, Forward blocking mode and Forward conduction mode are the modes of working of an SCR.

Power electronics Viva voce or interview questions part-3

2025-04-24T05:11:00.004+05:30

Q-51 What are the main classification of inverter?

Ans a. Voltage Source Inverter

b. Current Source Inverter

Q-52 Why thyristors are not preferred for inverters?

Ans Thyristors require extra commutation circuits for turn off which results inuncreased complexity of the circuit. For these reasons thyristors are not preferred for inverters.

Q-53 How output frequency is varied in case of a thyristor?

Ans The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit, i.e. the delay angle of the thyristors is varied.

Q-54 Give two advantages of CSI.

Ans a. CSI does not require any feedback diodes.

b. Commutation circuit is simple as it involves only thyristors.

Q-55 What is the main drawback of a single phase half bridge inverter?

Ans It require a 3-wire dc supply.

Q-56 Why diodes should be connected in antiparallel with the thyristors in inverter circuits?

Ans For RL loads, load current will not be in phase with load voltage and the diodes connected in antiparallel will allow the current to flow when the main thyristors are turned off. These diodes are called feedback diodes.

Q-57 What types of inverters require feedback diodes?

Ans VSI with RL load.

Q-58 What is meant a series inverter?

Ans An inverter in which the commutating elements are connected in series with the load is called a series inverter.

Q-59 What is meant a parallel inverter?

Ans An inverter in which the commutating elements are connected in parallel with the load is called a parallel inverter.

Q-60 What are the applications of a series inverter?

Ans The thyristorised series inverter produces an approximately sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

a. Ultrasonic generator.

b. Induction heating.

c. Sonar Transmitter

d. Fluorescent lighting.

Q-61 How is the inverter circuit classified based on commutation circuitry?

Ans a. Line commutated inverters.

b. Load commutated inverters.

c. Self-commutated inverters.

d. Forced commutated inverters.

Q-62 What is meant by McMurray inverter?

Ans It is an impulse commutated inverter which relies on LC circuit and an auxiliary thyristor for commutation in the load circuit.

Q-63 What are the applications of a CSI?

Ans a. Induction heating

b. Lagging VAR compensation

c. Speed control of ac motors

d. Synchronous motor starting.

Q-64 What is meant by PWM control?

Ans In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as PWM control.

Q-65 What are the advantages of PWM control?

Ans a. The output voltage can be obtained without any additional components.

b. Lower order harmonics can be eliminated or minimized along with its output voltage control. As the higher order harmonics can be filtered easily, the filtering requirements are minimized.

Q-66 What are the disadvantages of the harmonics present in the inverter system?

Ans a. Harmonic currents will lead to excessive heating in the induction motors. This will reduce the load carrying capacity of the motor.

b. If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.

c. Harmonic currents cause losses in the ac system and can even some time produce resonance in the system. Under resonant conditions, the instrumentation and metering can be affected.

d. On critical loads, torque pulsation produced by the harmonic current can be useful.

Q-67 What are the methods of reduction of harmonic content?

Ans a. Transformer connections

b. Sinusoidal PWM

c. Multiple commutation in each cycle

d. Stepped wave inverters

Q-68 Compare CSI and VSI.

Ans VSI

1.Input voltage is maintained constant

2. The output voltage does not depend on the load

3. The magnitude of the output current and its waveform depends on the nature

of the load impedance

4. It requires feedback diodes

5. Commutation circuit is complicated i.e. it contains capacitors and inductors.

CSI

1. Input current is constant but adjustable

2. The output current does not depend on the load

3. The magnitude of the output voltage and its waveform depends on the nature of the load impedance

4. It does not requires feedback diodes

5. Commutation circuit is simple i.e. it contains only capacitors.

Q-69 What are the disadvantages of PWM control?

Ans SCRs are expensive as they must possess low turn-on and turn-off times.

Q-70 What does ac voltage controller mean?

Ans It is device which converts fixed alternating voltage into a variable voltage without change in frequency.

Q-71 What are the applications of ac voltage controllers?

Ans a. Domestic and industrial heating

b. Lighting control

c. Speed control of single phase and three phase ac motors

d. Transformer tap changing

Q-72 What are the advantages of ac voltage controllers?

Ans a. High efficiency

b. Flexibility in control

c. Less maintenance

Q-73 What are the disadvantages of ac voltage controllers?

Ans The main drawback is the introduction of harmonics in the supply current and the load voltage waveforms particularly at low output voltages.

Q-74 What are the two methods of control in ac voltage controllers?

Ans a. ON-OFF control

b. Phase control

Q-75 What is the difference between ON-OFF control and phase control?

Ans ON-OFF control: In this method, the thyristors are employed as switches to connect the load circuit to the source for a few cycles of the load voltage and disconnect it for another few cycles. Phase control: In this method, thyristor switches connect the load to the ac source for a portion of each half cycle of input voltage.

Power electronics Viva voce or interview questions part-2

2025-04-24T05:09:00.006+05:30

Q-26 What is meant by input power factor in controlled rectifier?

Ans The input power factor is defined as the ratio of the total mean input power to the total RMS input volt-amperes.

Q-27 What are the advantages of six pulse converter?

Ans a. Commutation is made simple.

b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.

c. Inductance reduced in series is considerably reduced.

Q-28 What is meant by commutation?

Ans It is the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.

Q-29 What are the types of commutation?

Ans a. Natural commutation

b. Forced commutation

Q-30 What is meant by natural commutation?

Ans Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.

Q-31 What is meant by forced commutation?

Ans In this commutation, the current flowing through the thyristor is forced to become zero by external circuitry.

Q-32 What is meant by dc chopper?

Ans A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage.

Q-33 What are the applications of dc chopper?

Ans a. Battery operated vehicles

b. Traction motor control in electric traction

c. Trolly cars

d. Marine hoists

e. Mine haulers

f. Electric braking.

Q-34 What are the advantage of dc chopper?

Ans Chopper provides

a. High efficiency

b. Smooth acceleration

c. Fast dynamic response

d. Regeneration

Q-35 What is meant by step-up and step-down chopper?

Ans In a step- down chopper or Buck converter, the average output voltage is less

than the input voltage. In a step- up chopper or Boost converter, the average

output voltage is more than the input voltage.

Q-36 What is meant by duty-cycle?

Ans Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper.

Q-37 What are the two types of control strategies?

Ans a. Time Ratio Control (TRC)

b. Current Limit Control method (CLC)

Q-38 What is meant by TRC?

Ans In TRC, the value of (Ton/T) is varied in order to change the average output voltage.

Q-39 What are the two types of TRC?

Ans a. Constant frequency control

b. Variable frequency control

Q-40 What is meant by FM control in a dc chopper?

Ans In frequency modulation control, the chopping frequency f (or the chopping period T) is varied. Here two controls are possible.

a. On-time Ton is kept constant

b. Off period Toff is kept constant.

Q-41 What is meant by PWM control in dc chopper?

Ans In this control method, the on time Ton is varied but chopping frequency is kept constant. The width of the pulse is varied and hence this type of control is known as Pulse Width Modulation (PWM).

Q-42 What are the different types of chopper with respect to commutation process?

Ans a. Voltage commutated chopper.

b. Current commutated chopper.

c. Load commutated chopper.

Q-43 What is meant by voltage commutation?

Ans In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off.

Q-44 What is meant by current commutation?

Ans In this process, a current pulse is made to flow in the reverse direction through the conducting thyristor and when the net thyristor current becomes zero, it is turned off.

Q-45 What is meant by load commutation?

Ans In this process, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting thyristor.

Q-46 What are the advantages of current commutated chopper?

Ans a. The capacitor always remains charged with the correct polarity.

b. Commutation is reliable as load current is less than the peak commutation current ICP.

c. The auxiliary thyristor TA is naturally commutated as its current passes through zero value.

Q-47 What are the advantages of load commutated chopper?

Ans a. Commutating inductor is not required.

b. It is capable of commutating any amount of load current.

c. It can work at high frequencies in the order of kHz.

d. Filtering requirements are minimal.

Q-48 What are the disadvantages of load commutated chopper?

Ans a. For high power applications, efficiency becomes very low because of high switching losses at high operating frequencies.

b. Freewheeling diode is subjected to twice the supply voltage.

c. Peak load voltage is equal to twice the supply voltage.

d. The commutating capacitor has to carry full load current at a frequency of half chopping frequency.

e. One thyristor pair should be turned-on only when the other pair is commutated. This can be realized by sensing the capacitor current that is alternating.

Q-49 What is meant by inverter?

Ans A device that converts dc power into ac power at desired output voltage and frequency is called an inverter.

Q-50 What are the applications of an inverter?

Ans a. Adjustable speed drives

b. Induction heating

c. Stand-by aircraft power supplies

d. UPS

e. HVDC transmission

Power electronics Viva Voce or Interview questions part-1

2025-04-24T05:05:00.005+05:30

Q-1 Why IGBT is very popular nowadays?

Ans a. Lower heat requirements

b. Lower switching losses

c. Smaller snubber circuit requirements

Q-2 What are the different methods to turn on the thyristor?

Ans a. Forward voltage triggering

b. Gate triggering

c. dv/dt triggering

d. Temperature triggering

e. Light triggering

Q-3 What is the difference between power diode and signal diode?

Ans

Power diode

1. Constructed with n-layer, called drift region between p+ layer and n+ layer.

2. The voltage, current and power ratings are higher.

3. Power diodes operate at high speeds.

Signal diode

1. Drift region is not present.

2. Lower

3. Operates at higher switching speed.

Q-4 IGBT is a voltage controlled device. Why?

Ans Because the controlling parameter is gate-emitter voltage.

Q-5 Power MOSFET is a voltage controlled device. Why?

Ans Because the output (drain) current can be controlled by gate-source voltage.

Q-6 Power BJT is a current controlled device. Why?

Ans Because the output (collector) current can be controlled by base current.

Q-7 What are the different types of power MOSFET?

Ans a. N-channel MOSFET

b. P-channel MOSFET

Q-8 How can a thyristor turned off?

Ans A thyristor can be turned off by making the current flowing through it to zero.

Q-9 Define latching current.

Ans The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

Q-10 Define holding current.

Ans The holding current is defined as the minimum value of anode current below

which it must fall to for turning off the thyristor.

Q-11 What is a snubber circuit?

Ans It consists of a series combination of a resistor and a capacitor in parallel with

the thyristors. It is mainly used for dv / dt protection

Q-12 What losses occur in a thyristor during working conditions?

Ans a. Forward conduction losses

b. Loss due to leakage current during forward and reverse blocking.

c. Switching losses at turn-on and turn-off.

d. Gate triggering loss.

Q-13 Define hard-driving or over-driving.

Ans When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt capability.

Q-14 Define circuit turn off time.

Ans It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

Q-15 Why circuit turn off time should be greater than the thyristor turn-offtime?

Ans Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

Q-16 What is the turn-off time for converter grade SCRs and inverter grade SCRs?

Ans Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.

Q-17 What are the advantages of GTO over SCR?

Ans a. Elimination of commutation of commutating components in forced commutation, resulting in reduction in cost, weight and volume.

b. Reduction in acoustic noise and electromagnetic noise due to elimination of commutation chokes.

c. Faster turn-off, permitting high switching frequencies.

d. Improved efficiency of the converters.

Q-18 What is meant by phase controlled rectifier?

Ans It converts fixed ac voltage into variable dc voltage.

Q-19 Mention some of the applications of controlled rectifier.

Ans a. Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.

b. DC traction

c. Electro chemical and electro-metallurgical process

d. Portable hand tool drives

e. Magnet power supplies

f. HVDC transmission system

Q-20 What is the function of freewheeling diodes in controlled rectifier?

Ans It serves two process.

a. It prevents the output voltage from becoming negative.

b. The load current is transferred from the main thyristors to the freewheeling diode, thereby allowing all of its thyristors to regain their blocking states.

Q-21 What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?

Ans a. Input power factor is improved.

b. Load current waveform is improved and thus the load performance is better.

Q-22 What is meant by delay angle?

Ans The delay angle is defined as the angle between the zero crossing of the input

voltage and the instant the thyristor is fired.

Q-23 What are the advantages of single phase bridge converter over single phase mid-point converter?

Ans a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hence for same voltage and current ratings of SCrs, power handled by mid-point configuration is about

b. In mid-point converter, each secondary winding should be able to supply the load power. As such, the transformer rating in mid-point converter is double the load rating.

Q-24 What is commutation angle or overlap angle?

Ans The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.

Q-25 What are the different methods of firing circuits for line commutated converter?

Ans a. UJT firing circuit.

b. The cosine wave crossing pulse timing control.

c. Digital firing schemes.

English Speaking Basics - Section I

2025-02-14T02:00:00.001+05:30

English Speaking Basics - Section I

English Speaking Basics is for English speaking beginners who need help to understand the basics of speaking English. We will use very simple phrases and expressions to help you with your English speaking.

This first section contains the first 30 lessons. Go through them 1 by 1 until you are familiar with each sentence.

English Speaking Basics

English Speaking Basics I

1. Basic usage of 'I'm'
2. Variations of 'I'm in/at/on'
3. I'm good at
4. I'm + (verb)
5. I'm getting
6. I'm trying + (verb)
7. I'm gonna + (verb)
8. I have + (noun)
9. I have + (past participle)
10. I used to + (verb)
11. I have to + (verb)
12. I wanna + (verb)
13. I gotta + (verb)
14. I would like to + (verb)
15. I plan to + (verb)
16. I've decided to + (verb)
17. I was about to + (verb)
18. I didn't mean to + (verb)
19. I don't have time to + (verb)
20. I promise not to + (verb)
21. I'd rather + (verb)
22. I feel like + (verb-ing)
23. I can't help + (verb-ing)
24. I was busy + (verb-ing)
25. I'm not used to + (verb-ing)
26. I want you to + (verb)
27. I'm here to + (verb)
28. I have something + (verb)
29. I'm looking forward to

All terms related to illumination

2022-05-21T19:15:00.002+05:30

Important Terms:

Light: It is defined as the radiation energy from a hot body which produces the visual sensation upon the human eye. It is usually denoted by Q, expressed in lumen-hours and is analogous to watt-hour.

Luminous flux: it is defined as the total quantity of light energy emitted per second form a luminous body. It is represented by symbol F and is measured in lumens. The concept of luminous flux helps us to specify the output and efficiency of a given light source.

Luminous intensity: luminous intensity in any given direction is the luminous flux emitted by the source per unit solid angle, measured in the direction in which the intensity is required. It is denoted by symbol I and is measured in candela(cd) or lumens/steradian.

If F is the luminous flux radiated out by source within a solid angle of ω steradian in any particular direction then I =F/ ω lumens/steradian or candela (cd).

Lumen: The lumen is the unit of luminous flux and is defined as the amount of luminous flux given out in a space represented by one unit of solid angle by a source having an intensity of one candle power in all directions.

Lumens = candle power X solid angle = cp X ω

Total lumens given out by source of one candela are 4π lumens.

Candle power: Candle power is the light radiating capacity of a source in a given direction and is defined as the number of lumens given out by the source in a unit solid angle in a given direction. It is denoted by a symbol C.P.

C.P. = lumens/ω

Illumination: When the light falls upon any surface, the phenomenon is called the illumination. It is defined as the number of lumens, falling on the surface, per unit area. It is denoted by symbol E and is measured in lumens per square meter or meter-candle or lux.

If a flux of F lumens falls on a surface of area A, then the illumination of that surface is E =F/A lumens/m2 or lux

Lux or meter candle: It is the unit of illumination and is defined as the luminous flux falling per square meter on the surface which is everywhere perpendicular to the rays of light from a source of one candle power and one meter away from it.

Foot candle: It is also the unit of illumination and is defined as the luminous flux falling per square foot on the surface which is everywhere perpendicular to the rays of light from a source of one candle power and one foot away from it.

1 foot-candle = 1 lumen/ft2 =10.76 meter candle or lux

Candle: It is the unit of luminous intensity. It is defined as 1/60th of the luminous intensity per cm2 of a black body radiator at the temperature of solidification of platinum (2,0430K).

Mean horizontal candle power: (M.H.C.P) It is defined as the mean of candle powers in all directions in the horizontal plane containing the source of light.

Mean spherical candle power: ( M.S.C.P) It is defined as the mean of the candle powers in all directions and in all planes from the source of light. 1

Mean hemi-spherical candle power: (M.H.S.C.P) It is defined as the mean of candle powers in all directions above or below the horizontal plane passing through the source of light.

Reduction factor: Reduction factor of a source of light is the ratio of its mean spherical candle power to its mean horizontal candle power.

reduction factor = M.S.C.P./M.H.C.P.

Lamp efficiency: It is defined as the ratio of the luminous flux to the power input. It is expressed in lumens per watt. Specific consumption: It is defined as the ratio of the power input to the average candle power. It is expressed in watt per candela.

Brightness : When the eye receives a great deal of light from an object we say it is bright, and brightness is an important quantity in illumination. It is all the same whether the light is produced by the object or reflected from it. Brightness is defined as the luminous intensity per unit projected area of either a surface source of light or a reflecting surface and is denoted by L. If a surface area A has an effective luminous intensity of I candelas in a direction θ to the normal, than the brightness (luminance) of that surface is L = I/a cosθ The unit of brightness is candela/m2 (nits), candela/cm2 (stilb) or candela/ft2

Glare:- The size of the opening of the pupil in the human eye is controlled by its iris. If the eye is exposed to a very bright source of light the iris automatically contacts in order to produce the amount of light admitted and prevent damaged to retina this reduces the sensitivity, so that other objects within the field of vision can be only imperfectly seen. In other words glare maybe defined as brightness with in the field of vision of such a character as the cause annoyance discomfort interference with vision.

Space height ratio:- it is defined as the ratio of distance between adjacent lamps and height of their mountains.

Utilization factor or co-efficient of utilization:- It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp.

Maintenance factor: Due to accumulation of dust, dirt and smoke on the lamps, they emit less light than that they emit when they are new ones and similarly the walls and ceilings e.t.c. after being covered with dust, dirt and smoke do not reflect the same output of light, which is reflected when they are new. Lumens The ratio of illumination under normal working conditions to the illumination when the things are perfectly clean is known as maintenance factor. 𝑀𝑖𝑛𝑡𝑒𝑛𝑎𝑛𝑐𝑒 𝐹𝑎𝑐𝑡𝑜𝑟 = 𝑖𝑙𝑙𝑢𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛 𝑢𝑛𝑑𝑒𝑟 𝑛𝑜𝑟𝑚𝑎𝑙 𝑤𝑜𝑟𝑘𝑖𝑛𝑔 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠 𝑖𝑙𝑙𝑢𝑚𝑖𝑛𝑎𝑡𝑖𝑜𝑛 𝑤𝑕𝑒𝑛 𝑒𝑣𝑒𝑟𝑦 𝑡𝑕𝑖𝑛𝑔 𝑖𝑠 𝑐𝑙𝑒𝑎𝑛

Depreciation factor: this is merely reverse of the maintenance factor and is defined as the ratio of the initial meter-candles to the ultimate maintained metre-candles on the working plane. Its value is more than unity.

Waste light factor: Whenever a surface is illuminated by a number of sources of light, there is always a certain amount of waste of light on account of over-lapping and falling of light outside at the edges of the surface. The effect is taken into account by multiplying the theoretical value of lumens required by 1.2 for rectangular areas and 1.5 for irregular areas and objects such as statues, monuments etc.

Absorption factor: In the places where atmosphere is full of smoke fumes, such as in foundries, there is a possibility of absorption of light. The ratio of total lumens available after absorption to the total lumens emitted by the source of light is called the absorption factor. Its value varies from unity for clean atmosphere to 0.5 for foundries.

Beam factor: the ratio of lumens in the beam of a projector to the lumens given out by lamps is called the beam factor. This factor takes into the account the absorption of light by reflector and front glass of the projector lamp. Its value varies from 0.3 to 0.6.

Reflection factor: When a ray of light impinges on a surface it is reflected from the surface at an angle of incidence, as shown in the fallowing figure. A certain portion of incident light is absorbed by the surface. The ratio of reflected light to the incident light is called the reflection factor. It’s value always less than unity.

Plane angle: A plane angle is the angle subtended at a point in a plane by two converging lines. It is denoted by the Greek letter ‘θ’ (theta) and is usually measured in degrees or radians.

SOME IMPORTANT QUESTION OF ILLUMINATION

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Question 1. What are the characteristics of good illumination?

Answer :

The light should not strike directly the eyes.
The type and size of the lamp should be correct.
Proper location should be made.
Reflecting equipment should be suitable for purpose.
Hard and long shadows should be avoided.

Question 2. What are the factors which affect the correct illumination?

Answer :

Nature of the work
Architectural design
Surroundings
Nature of light and
Maintenance.

Question 3. What are the factors to be considered in the design of a lighting scheme?

Answer : The factors are as follows:

Illumination level
Glare
Shadow
Space height ratio
Mounting height of the lamp
Area to be illuminated
Colour of surrounding walls
Movement of the object
Utilization factor and
Depreciation factor.

Question 4. What do you mean by direct lighting?

Answer : In this lighting scheme the light does not reach the surface directly from the source, maximum light is thrown upwards to the ceiling from which it is distributed all over the room by diffuse reflectance. The glare being reduced the resulting illumination becomes softer. It is used for decoration purposes in cinemas, theaters, and hotels etc. and in workshops where large machines and other obstructions would cause troublesome shadows if direct lighting is employed.

Question 5. What do you mean by semi-direct lighting?

Answer :In this lighting scheme the total light flux is made to fall downwards directly with the help of semi-direct reflector on the working surface and also to illuminate the ceilings and walls. It is best suited to rooms with high ceilings where a high level of uniformly distributed illumination is desirable.

Question 6. What do you mean semi-indirect lighting?

Answer : In this lighting scheme the light comes partly from the ceiling by diffused reflection and party direct from the source on the working surface. As it is glare free with soft shadows it is mainly used for indoor light decoration purposes.

Question 7. What is Light?

Answer : Light is a form of energy, which is radiated by heated bodies. The light is the part of the radiant energy which produces a sensation (of light) on the human eye.

Question 8. What is Luminous Flux?

Answer : Luminous flux is the light energy radiated per second from a luminous body in the form of light waves. It is measured in lumen.

Question 9. What is Lumen?

Answer : Lumen is the unit of luminous flux which is equal to the flux emitted per unit solid angle from a uniform source of one candle power. 1 lumen = 0.0016 watt (approx).

Question 10. What is Luminous Intensity?

Answer : Luminous flux emitted by the sources per unit solid angle in any particular direction is known as the luminous intensity.

Question 11. What are Foot-candle and Meter-candle?

Answer : Foot candle or Lumen per square foot is defined as the illumination produced on the inner surface of a hollow sphere of a hollow sphere of radius one foot by a point surface at the center of intensity of one candela. Metre-candle or Lux (Lumen/m^2) is defined as the illumination produced on the inner surface of a hollow sphere of radius one meter by a point surface at the center of uniform intensity of one candela.

Question 12. What do you mean by M.H.C.P?

Answer : Mean Horizontal Candle Power (M.H.C.P) is the mean of the candle powers in all directions in the horizontal plane passing through the source of light.

Question 13. What is Depreciation Factor?

Answer : It is the ratio of illumination under normal condition of old installation to the illumination under ideal condition of new installation.

Question 14. What are the Laws of Illumination?

Answer : Illumination is directly proportional to the luminous intensity of the source.

Inverse square law – The illumination of a surface receiving its flux from a point source is inversely proportional to the square of the distance between the surface and the source.
Lambert’s cosine law – The illumination of a surface at any point is proportional to the cosine of the angle between the normal at the point and the direction of the luminous flux.

Question 15. What do you mean by brightness and its unit?

Answer : The flux emitted per unit area of the source in a direction at right angles to the surface is known as brightness. Its unit is candles / m2 or candles / cm2 or candles / ft 2.

Question 16. What is Glare?

Answer : Glare means the brightness within the field of vision of such a character as to cause annoyance, discomfort, interference with vision or eye fatigue.

Question 17. Which material is mostly used for the filament of incandescent lamps and why?

Answer : Tungsten is widely used for the filament of the incandescent lamp due to its high-temperature co-efficient, high melting point, low vapor pressure, ductility and good mechanical strength.

Energy Conversion In Power generation system

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Energy Conversion

Energy Conversion Using Steam

The combustion of coal, gas or oil in boilers produces steam, at high temperatures and pressures, which is passed through steam turbines. Nuclear fission can also provide energy to produce steam for turbines. Axial-flow turbines are generally used with several cylinders, containing steam of reducing pressure, on the same shaft.

A steam power-station operates on the Rankine cycle, modified to include superheating, feed-water heating, and steam reheating. High efficiency is achieved by the use of steam at the maximum possible pressure and temperature. Also, for turbines to be constructed economically, the larger the size the less the capital cost per unit of power output. As a result, turbo-generator sets of 500 MW and more have been used. With steam turbines above 100 MW, the efficiency is increased by reheating the steam, using an external heater, after it has been partially expanded. The reheated steam is then returned to the turbine where it is expanded through the final stages of blading.

In coal-fired stations, coal is conveyed to a mill and crushed into fine powder, that is pulverized. The pulverized fuel is blown into the boiler where it mixes with a supply of air for combustion. The exhaust steam from the low pressure (L.P.) turbine is cooled to form condensate by the passage through the condenser of large quantities of sea- or river-water. Cooling towers are used where the station is located inland or if there is concern over the environmental effects of raising the temperature of the sea- or river-water.

Despite continual advances in the design of boilers and in the development of improved materials, the nature of the steam cycle is such that vast quantities of heat are lost in the condensate cooling system and to the atmosphere. Advances in design and materials in the last few years have increased the thermal efficiencies of new coal stations to approaching 40%. If a use can be found for the remaining 60% of energy rejected as heat, fairly close to the power station, forming a Combined Heat and Power (or Co-generation) system then this is clearly desirable.

Energy Conversion Using Water

Perhaps the oldest form of energy conversion is by the use of water power. In a hydroelectric station the energy is obtained free of cost. This attractive feature has always been somewhat offset by the very high capital cost of construction, especially of the civil engineering works. Unfortunately, the geographical conditions necessary for hydro-generation are not commonly found, especially in Britain. In most developed countries, all the suitable hydroelectric sites are already fully utilized. There still exists great hydroelectric potential in many developing countries but large hydro schemes, particularly those with large reservoirs, have a significant impact on the environment and the local population.

The difference in height between the upper reservoir and the level of the turbines or outflow is known as the head. The water falling through this head gains energy which it then imparts to the turbine blades. Impulse turbines use a jet of water at atmospheric pressure while in reaction turbines the pressure drops across the runner imparts significant energy.

Particular types of turbine are associated with the various heights or heads of water level above the turbines. These are:

Pelton: This is used for heads of 150–1500 m and consists of a bucket wheel rotor with water jets from adjustable flow nozzles.
Francis: This is used for heads of 50–500 m with the water flow within the turbine following a spiral path.
Kaplan: This is used for run-of-river stations with heads of up to 60 m. This type has an axial-flow rotor with variable-pitch blades.

Typical efficiency curves for each type of turbine are shown in Figure 1.5. Hydroelectric plant has the ability to start up quickly and the advantage that no energy losses are incurred when at a standstill. It has great advantages, therefore, for power generation because of this ability to meet peak loads at minimum operating cost, working in conjunction with thermal stations – see Figure. By using remote control of the hydro sets, the time from the instruction to start up to the actual connection to the power network can be as short as 3 minutes. The power available from a hydro scheme is given by

P = rgQH (W)

where

Q = flow rate (m3/s) through the turbine;

r = density of water (1000 kg/m3);

g = acceleration due to gravity (9.81 m/s2);

H = head, that is height of upper water level above the lower (m).

Substituting,

P= 9.81QH (kW)

Gas Turbines

With the increasing availability of natural gas (methane) and its low emissions and competitive price, prime movers based on the gas turbine cycle are being used increasingly. This thermodynamic cycle involves burning the fuel in the compressed working fluid (air) and is used in aircraft with kerosene as the fuel and for electricity generation with natural gas (methane). Because of the high temperatures obtained, the efficiency of a gas turbine is comparable to that of a steam turbine, with the additional advantage that there is still sufficient heat in the gas-turbine exhaust to raise steam in a conventional boiler to drive a steam turbine coupled to another electricity generator. This is known as a combined-cycle gas-turbine (CCGT) plant, a schematic layout of which is shown in Figure 1.6. Combined efficiencies of new CCGT generators now approach 60%.

The advantages of CCGT plant are the high efficiency possible with large units and, for smaller units, the fast start up and shut down (2–3 min for the gas turbine, 20 min for the steam turbine), the flexibility possible for load following, the comparative speed of installation because of its modular nature and factory-supplied units, and its ability to run on light oil (from local storage tanks) if the gas supply is interrupted. Modern installations are fully automated and require only a few operators to maintain 24 hour running or to supply peak load, if needed.

Nuclear Power

Energy is obtained from the fission reaction which involves the splitting of the nuclei of uranium atoms. Compared with chemical reactions, very large amounts of energy are released per atomic event. Uranium metal extracted from the base ore consists mainly of two isotopes, 238U (99.3% by weight) and 235U (0.7%). Only 235U is fissile, that is when struck by slow-moving neutrons its nucleus splits into two substantial fragments plus several neutrons and 3 10 11 J of kinetic energy. The fast moving fragments hit surrounding atoms producing heat before coming to rest. The neutrons travel further, hitting atoms and producing further fissions. Hence the number of neutrons increases, causing, under the correct conditions, a chain reaction. In conventional reactors the core or moderator slows down the moving neutrons to achieve more effective splitting of the nuclei.

Fuels used in reactors have some component of 235U. Natural uranium is sometimes used although the energy density is considerably less than for enriched uranium. The basic reactor consists of the fuel in the form of rods or pellets situated in an environment (moderator) which will slow down the neutrons and fission products and in which the heat is evolved. The moderator can be light or heavy water or graphite. Also situated in the moderator are movable rods which absorb neutrons and hence exert control over the fission process. In some reactors the cooling fluid is pumped through channels to absorb the heat, which is then transferred to a secondary loop in which steam is produced for the turbine. In water reactors the moderator itself forms the heat-exchange fluid.

A number of versions of the reactor have been used with different coolants and types of fissile fuel. In Britain the first generation of nuclear power stations used Magnox reactors in which natural uranium in the form of metal rods was enclosed in magnesium-alloy cans. The fuel cans were placed in a structure or core of pure graphite made up of bricks (called the moderator). This graphite core slowed down the neutrons to the correct range of velocities in order to provide the maximum number of collisions. The fission process was controlled by the insertion of control rods made of neutron-absorbing material; the number and position of these rods controlled the heat output of the reactor. Heat was removed from the graphite via carbon dioxide gas pumped through vertical ducts in the core. This heat was then transferred to water to form steam via a heat exchanger. Once the steam had passed through the high-pressure turbine it was returned to the heat exchanger for reheating, as in a coal- or oil-fired boiler.

A reactor similar to the Magnox is the advanced gas-cooled reactor (AGR) which is still in use in Britain but now coming towards the end of its service life. A reinforced-concrete, steel-lined pressure vessel contains the reactor and heat exchanger. Enriched uranium dioxide fuel in pellet form, encased in stainless steel cans, is used; a number of cans are fitted into steel fitments within a graphite tube to form a cylindrical fuel element which is placed in a vertical channel in the core. Depending on reactor station up to eight fuel elements are held in place one above the other by a tie bar. Carbon dioxide gas, at a higher pressure than in the Magnox type, removes the heat. The control rods are made of boron steel. Spent fuel elements when removed from the core are stored in a special chamber and lowered into a pond of water where they remain until the level of radioactivity has decreased sufficiently for them to be removed from the station and disassembled.

In the USA and many other countries pressurized-water and boiling-water reactors are used. In the pressurized-water type the water is pumped through the reactor and acts as a coolant and moderator, the water being heated to 315 C at around 150 bar pressure. At this temperature and pressure the water leaves the reactor at below boiling point to a heat exchanger where a second hydraulic circuit feeds steam to the turbine. The fuel is in the form of pellets of uranium dioxide in bundles of zirconium alloy.

The boiling-water reactor was developed later than the pressurized-water type. Inside the reactor, heat is transferred to boiling water at a pressure of 75 bar (1100 p.s.i.). Schematic diagrams of these reactors are shown in Figures 1.7 and 1.8. The ratio of pressurized-water reactors to boiling-water reactors throughout the world is around 60/40%.

Both pressurized- and boiling-water reactors use light water.1 The practical pressure limit for the pressurized-water reactor is about 160 bar (2300 p.s.i.), which limits its efficiency to about 30%. However, the design is relatively straightforward and experience has shown this type of reactor to be stable and dependable. In the boiling-water reactor the efficiency of heat removal is improved by use of the latent heat of evaporation. The steam produced flows directly to the turbine, causing possible problems of radioactivity in the turbine. The fuel for both light-water reactors is uranium enriched to 3–4% 235U. Boiling-water reactors are probably the cheapest to construct; however, they have a more complicated fuel make up with different enrichment levels within each pin. The steam produced is saturated and requires wet-steam turbines. A further type of water reactor is the heavy-water CANDU type developed by Canada. Its operation and construction are similar to the light-water variety but this design uses naturally occurring, un-enriched or slightly enriched uranium.

Concerns over the availability of future supplies of uranium led to the construction of a number of prototype breeder reactors. In addition to heat, these reactors produce significant new fissile material. However, their cost, together with the technical and environmental challenges of breeder reactors, led to most of these programmes being abandoned and it is now generally considered that supplies of uranium are adequate for the foreseeable future.

Error Detection and Correction Codes

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Error Detection and Correction Codes

When we talk about digital systems, be it a digital computer or a digital communication set-up, the issue of error detection and correction is of great practical significance. Errors creep into the bit stream owing to noise or other impairments during the course of its transmission from the transmitter to the receiver. Any such error, if not detected and subsequently corrected, can be disastrous, as digital systems are sensitive to errors and tend to malfunction if the bit error rate is more than a certain threshold level. Error detection and correction, as we will see below, involves the addition of extra bits, called check bits, to the information-carrying bit stream to give the resulting bit sequence a unique characteristic that helps in detection and localization of errors. These additional bits are also called redundant bits as they do not carry any information. While the addition of redundant bits helps in achieving the goal of making transmission of information from one place to another error free or reliable, it also makes it inefficient. In this section, we will examine some common error detection and correction codes.

Parity Code

A parity bit is an extra bit added to a string of data bits in order to detect any error that might have crept into it while it was being stored or processed and moved from one place to another in a digital system.

We have an even parity, where the added bit is such that the total number of ls in the data bit string becomes even, and an odd parity, where the added bit makes the total number of ls in the data bit string odd. This added bit could be a ‘0’ or a ‘1’. As an example, if we have to add an even parity bit to 01000001 (the eight-bit ASCII code for ‘A’), it will be a ‘0’ and the number will become 001000001. If we have to add an odd parity bit to the same number, it will be a ‘l’ and the number will become 101000001. The odd parity bit is a complement of the even parity bit. The most common convention is to use even parity, that is, the total number of 1s in the bit stream, including the parity bit, is even. The parity check can be made at different points to look for any possible single-bit error, as it would disturb the parity. This simple parity code suffers from two limitations. Firstly, it cannot detect the error if the number of bits having undergone a change is even. Although the number of bits in error being equal to or greater than 4 is a very rare occurrence, the addition of a single parity cannot be used to detect two-bit errors, which is a distinct possibility in data storage media such as magnetic tapes. Secondly, the single-bit parity code cannot be used to localize or identify the error bit even if one bit is in error. There are several codes that provide self-single-bit error detection and correction mechanisms, and these are discussed below.

Repetition Code

The repetition code makes use of repetitive transmission of each data bit in the bit stream. In the case of threefold repetition, ‘1’ and ‘0’ would be transmitted as ‘111’ and ‘000’ respectively. If, in the received data bit stream, bits are examined in groups of three bits, the occurrence of an error can be detected. In the case of single-bit errors, ‘1’ would be received as 011 or 101 or 110 instead of 111, and a ‘0’ would be received as 100 or 010 or 001 instead of 000. In both cases, the code becomes self-correcting if the bit in the majority is taken as the correct bit. There are various forms in which the data are sent using the repetition code. Usually, the data bit stream is broken into blocks of bits, and then each block of data is sent some predetermined number of times. For example, if we want to send eight-bit data given by 11011001, it may be broken into two blocks of four bits each. In the case of threefold repetition, the transmitted data bit stream would be 110111011101100110011001. However, such a repetition code where the bit or block of bits is repeated 3 times is not capable of correcting two-bit errors, although it can detect the occurrence of error. For this, we have to increase the number of times each bit in the bit stream needs to be repeated. For example, by repeating each data bit 5 times, we can detect and correct all two-bit errors. The repetition code is highly inefficient and the information throughput drops rapidly as we increase the number of times each data bit needs to be repeated to build error detection and correction capability.

Cyclic Redundancy Check Code

Cyclic redundancy check (CRC) codes provide a reasonably high level of protection at low redundancy level. The cycle code for a given data word is generated as follows. The data word is first appended by a number of 0s equal to the number of check bits to be added. This new data bit sequence is then divided by a special binary word whose length equals n + 1, n being the number of check bits to be added. The remainder obtained as a result of modulo-2 division is then added to the dividend bit sequence to get the cyclic code. The code word so generated is completely divisible by the divisor used in the generation of the code. Thus, when the received code word is again divided by the same divisor, an error-free reception should lead to an all ‘0’ remainder. A nonzero remainder is indicative of the presence of errors.

The probability of error detection depends upon the number of check bits, n, used to construct the cyclic code. It is 100 % for single-bit and two-bit errors. It is also 100 % when an odd number of bits are in error and the error bursts have a length less than n + 1. The probability of detection reduces to 1 – (1/2)n−1 for an error burst length equal to n + 1, and to 1 – (1/2)n for an error burst length greater than n + 1.

Hamming Code

We have seen, in the case of the error detection and correction codes described above, how an increase in the number of redundant bits added to message bits can enhance the capability of the code to detect and correct errors. If we have a sufficient number of redundant bits, and if these bits can be arranged such that different error bits produce different error results, then it should be possible not only to detect the error bit but also to identify its location. In fact, the addition of redundant bits alters the ‘distance’ code parameter, which has come to be known as the Hamming distance. The Hamming distance is nothing but the number of bit disagreements between two code words. For example, the addition of single-bit parity results in a code with a Hamming distance of at least 2. The smallest Hamming distance in the case of a threefold repetition code would be 3. Hamming noticed that an increase in distance enhanced the code’s ability to detect and correct errors. Hamming’s code was therefore an attempt at increasing the Hamming distance and at the same time having as high an information throughput rate as possible.

The algorithm for writing the generalized Hamming code is as follows:

The generalized form of code is P1P2D1P3D2D3D4P4D5D6D7D8D9D10D11P5 , where P and D respectively represent parity and data bits.
We can see from the generalized form of the code that all bit positions that are powers of 2 (positions 1, 2, 4, 8, 16, ) are used as parity bits.
All other bit positions (positions 3, 5, 6, 7, 9, 10, 11, ) are used to encode data.
Each parity bit is allotted a group of bits from the data bits in the code word, and the value of the parity bit (0 or 1) is used to give it certain parity.
Groups are formed by first checking N− 1 bits and then alternately skipping and checking N bits following the parity bit. Here, N is the position of the parity bit; 1 for P1, 2 for P2, 4 for P3, 8 for P4 and so on. For example, for the generalized form of code given above, various groups of bits formed with different parity bits would be P1D1D2D4D5, P2D1D3D4D6D7, P3D2D3D4D8D9 , P4D5D6D7D8D9D10D11 and so on. To illustrate the formation of groups further, let us examine the group corresponding to parity bit P3. Now, the position of P3 is at number 4. In order to form the group, we check the first three bits (N− 1 = 3) and then follow it up by alternately skipping and checking four bits (N = 4).

The Hamming code is capable of correcting single-bit errors on messages of any length. Although the Hamming code can detect two-bit errors, it cannot give the error locations. The number of parity bits required to be transmitted along with the message, however, depends upon the message length, as shown above. The number of parity bits n required to encode m message bits is the smallest integer that satisfies the condition (2n – n > m.

The most commonly used Hamming code is the one that has a code word length of seven bits with four message bits and three parity bits. It is also referred to as the Hamming (7, 4) code. The code word sequence for this code is written as P1P2D1P3D2D3D4, with P1, P2 and P3 being the parity bits and D1, D2, D3 and D4 being the data bits. We will illustrate step by step the process of writing the Hamming code for a certain group of message bits and then the process of detection and identification of error bits with the help of an example. We will write the Hamming code for the four-bit message 0110 representing numeral ‘6’. The process of writing the code is illustrated in Table 2.9, with even parity. Thus, the Hamming code for 0110 is 1100110. Let us assume that the data bit D1 gets corrupted in the transmission channel. The received code in that case is 1110110. In order to detect the error, the parity is checked for the three parity relations mentioned above. During the parity check operation at the receiving end, three additional bits X, Y and Z are generated by checking the parity status of P1D1D2D4, P2D1D3D4 and P3D2D3D4 respectively. These bits are a ‘0’ if the parity status is okay, and a ‘1’ if it is disturbed. In that case, ZYX gives the position of the bit that needs correction. The process can be best explained with the help of an example. Examination of the first parity relation gives X =1 as the even parity is disturbed. The second parity relation yields Y = 1 as the even parity is disturbed here too. Examination of the third relation gives Z = 0 as the even parity is maintained. Thus, the bit that is in error is positioned at 011 which is the binary equivalent of ‘3’. This implies that the third bit from the MSB needs to be corrected. After correcting the third bit, the received message becomes 1100110 which is the correct code.

Number Representation in Binary

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Number Representation in Binary

Different formats used for binary representation of both positive and negative decimal numbers include the sign-bit magnitude method, the 1’s complement method and the 2’s complement method.

Sign-Bit Magnitude

In the sign-bit magnitude representation of positive and negative decimal numbers, the MSB represents the ‘sign’, with a ‘0’ denoting a plus sign and a ‘1’ denoting a minus sign. The remaining bits represent the magnitude. In eight-bit representation, while MSB represents the sign, the remaining seven bits represent the magnitude. For example, the eight-bit representation of +9 would be 00001001, and that for −9 would be 10001001. An n−bit binary representation can be used to represent decimal numbers in the range of −(2n−1 − 1) to +(2n−1 − 1). That is, eight-bit representation can be used to represent decimal numbers in the range from −127 to +127 using the sign-bit magnitude format.

1’s Complement

In the 1’s complement format, the positive numbers remain unchanged. The negative numbers are obtained by taking the 1’s complement of the positive counterparts. For example, +9 will be represented as 00001001 in eight-bit notation, and −9 will be represented as 11110110, which is the 1’s complement of 00001001. Again, n-bit notation can be used to represent numbers in the range from −(2n−1 − 1) to +(2n−1 − 1) using the 1’s complement format. The eight-bit representation of the 1’s complement format can be used to represent decimal numbers in the range from −127 to +127.

2’s Complement

In the 2’s complement representation of binary numbers, the MSB represents the sign, with a ‘0’

used for a plus sign and a ‘1’ used for a minus sign. The remaining bits are used for representing

magnitude. Positive magnitudes are represented in the same way as in the case of sign-bit or 1’s

complement representation. Negative magnitudes are represented by the 2’s complement of their

positive counterparts. For example, +9 would be represented as 00001001, and −9 would be written

as 11110111. Please note that, if the 2’s complement of the magnitude of +9 gives a magnitude of −9,

then the reverse process will also be true, i.e. the 2’s complement of the magnitude of −9 will give a

magnitude of +9. The n-bit notation of the 2’s complement format can be used to represent all decimal

numbers in the range from +(2n−1 − 1) to −(2n−1. The 2’s complement format is very popular as it is

very easy to generate the 2’s complement of a binary number and also because arithmetic operations

are relatively easier to perform when the numbers are represented in the 2’s complement format.

Introduction to Number Systems in Digital Electronics

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Introduction to Number Systems

We will begin our discussion on various number systems by briefly describing the parameters that are common to all number systems. An understanding of these parameters and their relevance to number systems is fundamental to the understanding of how various systems operate. Different characteristics that define a number system include the number of independent digits used in the number system, the place values of the different digits constituting the number and the maximum numbers that can be written with the given number of digits. Among the three characteristic parameters, the most fundamental is the number of independent digits or symbols used in the number system. It is known as the radix or base of the number system. The decimal number system with which we are all so familiar can be said to have a radix of 10 as it has 10 independent digits, i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. Similarly, the binary number system with only two independent digits, 0 and 1, is a radix-2 number system. The octal and hexadecimal number systems have a radix (or base) of 8 and 16 respectively. We will see in the following sections that the radix of the number system also determines the other two characteristics. The place values of different digits in the integer part of the number are given by r0, r1, r2, r3 and so on, starting with the digit adjacent to the radix point. For the fractional part, these are r−1, r−2, r−3 and so on, again starting with the digit next to the radix point. Here, r is the radix of the number system. Also, maximum numbers that can be written with n digits in a given number system are equal to rn.

Decimal Number System

The decimal number system is a radix-10 number system and therefore has 10 different digits or symbols. These are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. All higher numbers after ‘9’ are represented in terms of these 10 digits only. The process of writing higher-order numbers after ‘9’ consists in writing the second digit (i.e. ‘1’) first, followed by the other digits, one by one, to obtain the next 10 numbers from ‘10’ to ‘19’. The next 10 numbers from ‘20’ to ‘29’ are obtained by writing the third digit (i.e. ‘2’) first, followed by digits ‘0’ to ‘9’, one by one. The process continues until we have exhausted all possible two-digit combinations and reached ‘99’. Then we begin with three-digit combinations. The first three-digit number consists of the lowest two-digit number followed by ‘0’ (i.e. 100), and the process goes on endlessly.

The place values of different digits in a mixed decimal number, starting from the decimal point, are 100, 101, 102 and so on (for the integer part) and 10−1, 10−2, 10−3 and so on (for the fractional part).

The value or magnitude of a given decimal number can be expressed as the sum of the various digits

multiplied by their place values or weights.

As an illustration, in the case of the decimal number 3586.265, the integer part (i.e. 3586) can be

expressed as

3586 = 6×100 +8×101 +5×102 +3×103 = 6+80+500+3000 = 3586

and the fractional part can be expressed as

265 = 2×10−1 +6×10−2 +5×10−3 = 02+006+0005 = 0265

We have seen that the place values are a function of the radix of the concerned number system and

the position of the digits. We will also discover in subsequent sections that the concept of each digit

having a place value depending upon the position of the digit and the radix of the number system is

equally valid for the other more relevant number systems.

Binary Number System

The binary number system is a radix-2 number system with ‘0’ and ‘1’ as the two independent digits.

All larger binary numbers are represented in terms of ‘0’ and ‘1’. The procedure for writing higherorder binary numbers after ‘1’ is similar to the one explained in the case of the decimal number system.

For example, the first 16 numbers in the binary number system would be 0, 1, 10, 11, 100, 101, 110,

111, 1000, 1001, 1010, 1011, 1100, 1101, 1110 and 1111. The next number after 1111 is 10000, which

is the lowest binary number with five digits. This also proves the point made earlier that a maximum

of only 16 (= 24 numbers could be written with four digits. Starting from the binary point, the place

values of different digits in a mixed binary number are 20, 21, 22 and so on (for the integer part) and

2−1, 2−2, 2−3 and so on (for the fractional part).

Octal Number System

The octal number system has a radix of 8 and therefore has eight distinct digits. All higher-order numbers are expressed as a combination of these on the same pattern as the one followed in the case of the binary and decimal number systems described in Sections 1.3 and 1.4. The independent digits are 0, 1, 2, 3, 4, 5, 6 and 7. The next 10 numbers that follow ‘7’, for example, would be 10, 11, 12, 13, 14, 15, 16, 17, 20 and 21. In fact, if we omit all the numbers containing the digits 8 or 9, or both, from the decimal number system, we end up with an octal number system. The place values for the different digits in the octal number system are 80, 81, 82 and so on (for the integer part) and 8−1, 8−2, 8−3 and so on (for the fractional part).

Hexadecimal Number System

The hexadecimal number system is a radix-16 number system and its 16 basic digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. The place values or weights of different digits in a mixed hexadecimal number are 160, 161, 162 and so on (for the integer part) and 16−1, 16−2, 16−3 and so on (for the fractional part). The decimal equivalent of A, B, C, D, E and F are 10, 11, 12, 13, 14 and 15 respectively, for obvious reasons.

The hexadecimal number system provides a condensed way of representing large binary numbers stored and processed inside the computer. One such example is in representing addresses of different memory locations. Let us assume that a machine has 64K of memory. Such a memory has 64K (= 216 = 65 536) memory locations and needs 65 536 different addresses. These addresses can be designated as 0 to 65 535 in the decimal number system and 00000000 00000000 to 11111111 11111111 in the binary number system. The decimal number system is not used in computers and the binary notation here appears too cumbersome and inconvenient to handle. In the hexadecimal number system, 65 536 different addresses can be expressed with four digits from 0000 to FFFF. Similarly, the contents of the memory when represented in hexadecimal form are very convenient to handle.

Number Systems – Some Common Terms

In this section we will describe some commonly used terms with reference to different number systems.

Binary Number System

Bit is an abbreviation of the term ‘binary digit’ and is the smallest unit of information. It is either ‘0’ or ‘1’. A byte is a string of eight bits. The byte is the basic unit of data operated upon as a single unit in computers. A computer word is again a string of bits whose size, called the ‘word length’ or ‘word size’, is fixed for a specified computer, although it may vary from computer to computer. The word length may equal one byte, two bytes, four bytes or be even larger.

The 1’s complement of a binary number is obtained by complementing all its bits, i.e. by replacing 0s with 1s and 1s with 0s. For example, the 1’s complement of (10010110)2 is (01101001)2. The 2’s complement of a binary number is obtained by adding ‘1’ to its 1’s complement. The 2’s complement of (10010110)2 is (01101010)2.

Decimal Number System

Corresponding to the 1’s and 2’s complements in the binary system, in the decimal number system we have the 9’s and 10’s complements. The 9’s complement of a given decimal number is obtained by subtracting each digit from 9. For example, the 9’s complement of (2496)10 would be (7503)10. The 10’s complement is obtained by adding ‘1’ to the 9’s complement. The 10’s complement of (2496)10 is (7504)10.

Octal Number System

In the octal number system, we have the 7’s and 8’s complements. The 7’s complement of a given octal number is obtained by subtracting each octal digit from 7. For example, the 7’s complement of (562)8 would be (215)8. The 8’s complement is obtained by adding ‘1’ to the 7’s complement. The 8’s complement of (562)8 would be (216)8.

Hexadecimal Number System

The 15’s and 16’s complements are defined with respect to the hexadecimal number system. The 15’s complement is obtained by subtracting each hex digit from 15. For example, the 15’s complement of (3BF)16 would be (C40)16. The 16’s complement is obtained by adding ‘1’ to the 15’s complement. The 16’s complement of (2AE)16 would be (D52)16.

Microprocessor architecture: to know 8085 architecture

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Microprocessor architecture

8085 is pronounced as "eighty-eighty-five" "eighty-eighty-five" microprocessor. It is an 8-bit microprocessor designed by Intel in 1977 using NMOS technology.

It has the following configuration −

8-bit data bus
16-bit address bus, which can address up to 64KB
A 16-bit program counter
A 16-bit stack pointer
Six 8-bit registers arranged in pairs: BC, DE, HL
Requires +5V supply to operate at 3.2 MHZ single-phase clock

It is used in washing machines, microwave ovens, mobile phones, etc.

8085 Microprocessor – Functional Units

8085 consists of the following functional units −

Accumulator

It is an 8-bit register used to perform arithmetic, arithmetic, logical, logical, I/O & LOAD/ST LOAD/STORE operations. operations. It is connected to internal data bus & ALU.

Arithmetic and logic unit

As the name suggests, suggests, it performs arithmetic and logical logical operations like Addition, Addition, Subtraction, Subtraction, AND, OR, etc. on 8-bit data.

General-purpose register

There are 6 general-purpose registers in the 8085 processor, i.e. B, C, D, E, H & L. Each register can

hold 8-bit data. These registers can work in pairs to hold 16-bit data and their pairing combination combination is like B-C, D-E & H-L.

Program counter

It is a 16-bit register used to store the memory address location of the next instruction to be

executed. executed. Microprocessor increments the program whenever an instruction is being executed, executed so that the program counterpoints to the memory address of the next instruction that is going to be executed.

Stack pointer

It is also a 16-bit register that works like a stack, which is always incremented/decremented incremented/decremented by 2 during push & pop operations.

Temporary register

It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.

Flag register

It is an 8-bit register having five 1-bit flip-flops, flip-flops, which holds either 0 or 1 depending upon the result stored in the accumulator.

These are the set of 5 flip-flops −

Sign (S)
Zero (Z)
Auxiliary Carry (AC)
Parity (P)
Carry (C)

Its bit position is shown in the following table −

Instruction register and decoder

It is an 8-bit register. When an instruction is fetched from memory then it is stored in the Instruction register. The instruction decoder decodes the information present in the Instruction register.

Timing and control unit

It provides timing and control signals to the microprocessor to perform operations. operations. Following are the timing and control signals, which control external and internal circuits −

Control Signals: READY, RD’, WR’, ALE
Status Signals: S0, S1, IO/M’
DMA Signals: HOLD, HLDA
RESET Signals: RESET IN, RESET OUT

Interrupt control

As the name suggests it controls the interrupts during a process. process. When a microprocessor is

executing the main program and whenever an interrupt occurs, occurs, the microprocessor shifts the control from the main program to process the incoming incoming request. request. After the request is completed, completed, the control goes back to the main program.

There are 5 interrupt signals in the 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.

Serial Input/output control

It controls the serial data communication by using these two instructions: instructions: SID (Serial (Serial input data) and SOD (Serial output data).

Address buffer and address-data buffer

The content stored in the stack pointer and program counter is loaded into the address buffer and

address-data address-data buffer to communicate with the CPU. The memory and I/O chips are connected to these buses; the CPU can exchange the desired data with the memory and I/O chips.

Address bus and data bus

The data bus carries the data to be stored. stored. It is bidirectional, whereas the address bus carries the location to where it should be stored and it is unidirectional.It is used to transfer the data & Address Address I/O devices

MCQ OF THREE PHASE INDUCTION MOTORS

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1. Which of the following component is usually fabricated out of silicon steel ?

(a) Bearings

(b) Shaft

(d) None of the above

Ans: c

2. The frame of an induction motor is usually made of

(a) silicon steel (b) cast iron (c) aluminium (d) bronze Ans: b

3. The shaft of an induction motor is made of (a) stiff (b) flexible (c) hollow (d) any of the above Ans: a

4. The shaft of an induction motor is made of (a) high speed steel (b) stainless steel (c) carbon steel (d) cast iron Ans: c

5. In an induction motor, no-load the slip is generally (a) less than 1% (b) 1.5% (c) 2% (d) 4% Ans: a

6. In medium sized induction motors, the slip is generally around (a) 0.04% (b) 0.4% (c) 4% (d) 14% Ans: c

7. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to (a) reduce windage losses (b) reduce eddy currents (c) reduce accumulation of dirt and dust (d) reduce magnetic hum Ans: d

8. In case the air gap in an induction motor is increased (a) the magnetising current of the rotor will decrease (b) the power factor will decrease (c) speed of motor will increase (d) the windage losses will increase Ans: b

9. Slip rings are usually made of (a) copper (b) carbon (c) phospor bronze (d) aluminium Ans: c

10. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be (a) 200 Hz (b) 50 Hz (c) 2 Hz (d) 0.2 Hz Ans: c

11. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be (a) Ns (b) s.N, (c) (l-s)Ns (d) (Ns-l)s Ans: c

12. The efficiency of an induction motor can be expected to be nearly (a) 60 to 90% (b) 80 to 90% (c) 95 to 98% (d) 99% Ans: b

13. The number of slip rings on a squirrel cage induction motor is usually (a) two (b) three (c) four (d) none Ans: d

14. The starting torque of a squirrel-cage induction motor is (a) low (b) negligible (c) same as full-load torque (d) slightly more than full-load torque Ans: a

15. A double squirrel-cage induction motor has (a) two rotors moving in oppsite direction (b) two parallel windings in stator (c) two parallel windings in rotor (d) two series windings in stator Ans: c

16. Star-delta starting of motors is not possible in case of (a) single phase motors (b) variable speed motors (c) low horse power motors (d) high speed motors Ans: a

17. The term 'cogging' is associated with (a) three phase transformers (b) compound generators (c) D.C. series motors (d) induction motors Ans: d

18. In case of the induction motors the torque is (a) inversely proportional to (Vslip) (b) directly proportional to (slip)2 (c) inversely proportional to slip (d) directly proportional to slip Ans: d

19. An induction motor with 1000 r.p.m. speed will have (a) 8 poles (b) 6 poles (c) 4 poles (d) 2 poles Ans: b

20. The good power factor of an induction motor can be achieved if the average flux density in the air gap is (a) absent (b) small (c) large (d) infinity Ans: b

21. An induction motor is identical to (a) D.C. compound motor (b) D.C. series motor (c) synchronous motor (d) asynchronous motor Ans: d

22. The injected e.m.f. in the rotor of induction motor must have (a) zero frequency (b) the same frequency as the slip fre-quency (c) the same phase as the rotor e.m.f. (d) high value for the satisfactory speed control Ans: b

23. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ? (a) By changing the number of stator poles (b) Rotor rheostat control (c) By operating two motors in cascade (d) By injecting e.m.f. in the rotor circuit Ans: a

24. The crawling in the induction motor is caused by (a) low voltage supply (b) high loads (c) harmonics develped in the motor (d) improper design of the machine (e) none of the above Ans: c

25. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type (a) star connected only (b) delta connected only (c) (a) and (b) both (d) none of the above Ans: c

26. The torque developed in the cage induction motor with autostarter is (a) k/torque with direct switching (6) K x torque with direct switching (c) K2 x torque with direct switching (d) k2/torque with direct switching Ans: c

27. When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered (a) in series (b) in parallel (c) in series-parallel (d) in parallel with stator Ans: b

28. It is advisable to avoid line-starting of induction motor and use starter because (a) motor takes five to seven times its full load current (b) it will pick-up very high speed and may go out of step (c) it will run in reverse direction (d) starting torque is very high Ans: a

29. Stepless speed control of induction motor is possible by which of the following methods ? (a) e.m.f. injection in rotor eueuit (b) Changing the number of poles (c) Cascade operation (d) None of the above Ans: b

30. Rotor rheostat control method of speed control is used for (a) squirrel-cage induction motors only (b) slip ring induction motors only (c) both (a) and (b) (d) none of the above Ans: b

31. In the circle diagram for induction motor, the diameter of the circle represents (a) slip (b) rotor current (c) running torque (d) line voltage Ans: b

32. For which motor the speed can be controlled from rotor side ? (a) Squirrel-cage induction motor (b) Slip-ring induction motor (c) Both (a) and (b) (d) None of the above Ans: b

33. If any two phases for an induction motor are interchanged (a) the motor will run in reverse direction (b) the motor will run at reduced speed (c) the motor will not run (d) the motor will burn Ans: a

34. An induction motor is (a) self-starting with zero torque (b) self-starting with high torque (c) self-starting with low torque (d) non-self starting Ans: c

35. The maximum torque in an induction motor depends on (a) frequency (b) rotor inductive reactance (c) square of supply voltage (d) all of the above Ans: d

36. In three-phase squirrel-cage induction motors (a) rotor conductor ends are short-circuited through slip rings (b) rotor conductors are short-circuited through end rings (c) rotor conductors are kept open (d) rotor conductors are connected to insulation Ans: b

37. In a three-phase induction motor, the number of poles in the rotor winding is always (a) zero (b) more than the number of poles in stator (c) less than number of poles in stator (d) equal to number of poles in stator Ans: d

38. DOL starting of induction motors is usually restricted to (a) low horsepower motors (b) variable speed motors (c) high horsepower motors (d) high speed motors Ans: a

39. The speed of a squirrel-cage induction motor can be controlled by all of the following except (a) changing supply frequency (b) changing number of poles (c) changing winding resistance (d) reducing supply voltage Ans: c

40. The 'crawling" in an induction motor is caused by (a) high loads (6) low voltage supply (c) improper design of machine (d) harmonics developed in the motor Ans: d

41. The power factor of an induction motor under no-load conditions will be closer to (a) 0.2 lagging (b) 0.2 leading (c) 0.5 leading (d) unity Ans: a

42. The 'cogging' of an induction motor can be avoided by (a) proper ventilation (b) using DOL starter (c) auto-transformer starter (d) having number of rotor slots more or less than the number of stator slots (not equal) Ans: d

43. If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as (a) humming (b) hunting (c) crawling (d) cogging Ans: c

44. Slip of an induction motor is negative when (a) magnetic field and rotor rotate in opposite direction (b) rotor speed is less than the syn-chronous speed of the field and are in the same direction (c) rotor speed is more than the syn-chronous speed of the field and are in the same direction (d) none of the above Ans: c

45. Size of a high speed motor as compared to low speed motorfor the same H.P. will be (a) bigger (b) smaller (c) same (d) any of the above Ans: b

46. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor (a) will continue running burning its one phase (b) will continue running burning its two phases (c) will stop and carry heavy current causing permanent damage to its winding (d) will continue running without any harm to the winding Ans: a

47. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor (a) will continue running burning its one phase (b) will continue running burning its two phase (c) will stop and carry heavy current causing permanent damage to its winding (d) will continue running without any harm to the winding Ans: c

48. Low voltage at motor terminals is due to (a) inadequate motor wiring (b) poorely regulated power supply (c) any one of the above (d) none of the above Ans: c

49. In an induction motor the relationship between stator slots and rotor slots is that (a) stator slots are equal to rotor slots (b) stator slots are exact multiple of rotor slots (c) stator slots are not exact multiple of rotor slots (d) none of the above Ans: c

50. Slip ring motor is recommended where (a) speed control is required (6) frequent starting, stopping and reversing is required (c) high starting torque is needed (d) all above features are required Ans: d

51. As load on an induction motor goes on increasing (a) its power factor goes on decreasing (b) its power factor remains constant (c) its power factor goes on increasing even after full load (d) its power factor goes on increasing upto full load and then it falls again Ans: d

52. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move (a) in the opposite direction as the direction of the rotating field (b) in the same direction as the direction of the field (c) in any direction depending upon phase squence of supply Ans: b

53. It is advisable to avoid line starting of induction motor and use starter because (a) it will run in reverse direction (b) it will pick up very high speed and may go out of step (c) motor takes five to seven times its fullload current (d) starting torque is very high Ans: c

54. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines (a) D.C. series motor (b) D.C. shunt motor (c) universal motor (d) none of the above Ans: b

55. Which type of bearing is provided in small induction motors to support the rotor shaft ? (a) Ball bearings (b) Cast iron bearings (c) Bush bearings (d) None of the above Ans: a

56. A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will (a) stall after sometime (b) stall immediately (c) continue to run at lower speed without damage (d) get heated and subsequently get damaged Ans: d

57. 5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ? (a) 730 r.p.m. (b) 960 r.p.m. (c) 1440 r.p.m. (d) 2880 r.p.m. Ans: a

58. A 3-phase slip ring motor has (a) double cage rotor (6) wound rotor (c) short-circuited rotor (d) any of the above Ans: b

59. The starting torque of a 3-phase squirrel cage induction motor is (a) twice the full load torque (b) 1.5 times the full load torque (c) equal to full load torque Ans: b

60. Short-circuit test on an induction motor cannot be used to determine (a) windage losses (b) copper losses (c) transformation ratio (d) power scale of circle diagram Ans: a

61. In a three-phase induction motor (a) iron losses in stator will be negligible as compared to that in rotor (6) iron losses in motor will be neg¬ligible as compared to that in rotor (c) iron losses in stator will be less than that in rotor (d) iron losses in stator will be more than that in rotor Ans: d

62. In case of 3-phase induction motors, plugging means (a) pulling the motor directly on line without a starter (b) locking of rotor due to harmonics (c) starting the motor on load which is more than the rated load (d) interchanging two supply phases for quick stopping Ans: d

63. Which is of the following data is required to draw the circle diagram for an induction motor ? (a) Block rotor test only (b) No load test only (c) Block rotor test and no-load test (d) Block rotor test, no-load test and stator resistance test Ans: d

64. In three-phase induction motors sometimes copper bars are placed deep in the rotor to (a) improve starting torque (b) reduce copper losses (c) improve efficiency (d) improve power factor Ans: a

65. In a three-phase induction motor (a) power factor at starting is high as compared to that while running (b) power factor at starting is low as compared to that while running (c) power factor at starting in the same as that while running Ans: b

66. The vafcie of transformation ratio of an induction motor can be found by (a) open-circuit test only (b) short-circuit test only (c) stator resistance test (d) none of the above Ans: b

67. The power scale of circle diagram of an induction motor can be found from (a) stator resistance test (6) no-load test only (c) short-circuit test only (d) noue of the above Ans: c

68. The shape of the torque/slip curve of induction motor is (a) parabola (b) hyperbola (c) rectangular parabola (d) straigth line Ans: c

69. A change of 4% of supply voltage to an induction motor will produce a change of appromimately (a) 4% in the rotor torque (b) 8% in the rotor torque (c) 12% in the rotor torque (d) 16% in the rotor torque Ans: d

70. The stating torque of the slip ring induction motor can be increased by adding (a) external inductance to the rotor (b) external resistance to the rotor (c) external capacitance to the rotor (d) both resistance and inductance to rotor Ans: b

71. A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be (a) 0.01 (b) 0.02 (c) 0.03 (d) 0.04 Ans: d

72. The complete circle diagram of induetion motor can be drawn with the help of data found from (a) noload test (6) blocked rotor test (c) stator resistance test (d) all of the above Ans: d

73. In the squirrel-cage induction motor the rotor slots are usually given slight skew (a) to reduce the magnetic hum and locking tendency of the rotor (b) to increase the tensile strength of the rotor bars (c) to ensure easy fabrication (d) none of the above Ans: a

74. The torque of a rotor in an induction motor under running condition is maximum (a) at the unit value of slip (b) at the zero value of slip (c) at the value of the slip which makes rotor reactance per phase equal to the resistance per phase (d) at the value of the slip which makes the rotor reactance half of the rotor Ans: c

75. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ? (a) The slip of the motor will be 5% (b) The rotor will not run (c) The rotor will run at very high speed (d) The torque produced will be very large Ans: b

76. The circle diagram for an induction motor cannot be used to determine (a) efficiency (b) power factor (c) frequency (d) output Ans: a

77. Blocked rotor test on induction motors is used to find out (a) leakage reactance (b) power factor on short circuit (c) short-circuit current under rated voltage (d) all of the above Ans: d

78. Lubricant used for ball bearing is usually (a) graphite (b) grease (c) mineral oil (d) molasses Ans: b

79. An induction motor can run at synchronous speed when (a) it is run on load (b) it is run in reverse direction (c) it is run on voltage higher than the rated voltage (d) e.m.f. is injected in the rotor circuit Ans: d

80. Which motor is preferred for use in mines where explosive gases exist ? (a) Air motor (b) Induction motor (c) D.C. shunt motor (d) Synchronous motor Ans: a

81. The torque developed by a 3-phase induction motor least depends on (a) rotor current (b) rotor power factor (c) rotor e.m.f. (d) shaft diameter Ans: d

82. In an induction motor if air-gap is increased (a) the power factor will be low (b) windage losses will be more (c) bearing friction will reduce (d) copper loss will reduce In an induction motor Ans: a

83. In induction motor, percentage slip depends on (a) supply frequency (b) supply voltage (c) copper losses in motor (d) none of the above Ans: c

84. When /?2 is tne rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running condi-tions will be (a) sR2X2 = 1 (b) sR2 = X2 (c) R2 = sX2 id) R2 = s2X2 Ans: c

85. In case of a double cage induction motor, the inner cage has (a) high inductance arid low resistance (b) low inductance and high resistance (c) low inductance and low resistance (d) high inductance and high resis¬tance Ans: a

86. The low power factor of induction motor is due to (a) rotor leakage reactance (b) stator reactance (c) the reactive lagging magnetizing current necessary to generate the magnetic flux (d) all of the above Ans: d

87. Insertion of reactance in the rotor circuit (a) reduces starting torque as well as maximum torque (b) increases starting torque as well as maximum torque (c) increases starting torque but maxi-mum torque remains unchanged (d) increases starting torque but maxi-mum torque decreases Ans: a

88. Insertion of resistance in the rotcir of an induction motor to develop a given torque (a) decreases the rotor current (b) increases the rotor current (c) rotor current becomes zero (d) rotor current rernains same Ans: d

89. For driving high inertia loods best type of induction motor suggested is (a) slip ring type (b) squirrel cage type (c) any of the above (d) none of the above Ans: a

90. Temperature of the stator winding of a three phase induction motor is obtained by (a) resistance rise method (b) thermometer method (c) embedded temperature method (d) all above methods Ans: d

91. The purpose of using short-circuit gear is (a) to short circuit the rotor at slip rings (b) to short circuit the starting resis¬tances in the starter (c) to short circuit the stator phase of motor to form star (d) none of the above Ans: a

92. In a squirrel cage motor the induced e.m.f. is (a) dependent on the shaft loading (b) dependent on the number of slots (c) slip times the stand still e.m.f. induced in the rotor (d) none of the above Ans: c

93. Less maintenance troubles are experienced in case of (a) slip ring induction motor (6) squirrel cage induction motor (c) both (a) and (b) (d) none of the above Ans: b

94. A squirrel cage induction motor is not selected when (a) initial cost is the main consideration (b) maintenance cost is to be kept low (c) higher starting torque is the main consideration (d) all above considerations are involved Ans: c

95. Reduced voltage starter can be used with (a) slip ring motor only but not with squirrel cage induction motor (b) squirrel cage induction motor only but not with slip ring motor (c) squirrel cage as well as slip ring induction motor (d) none of the above Ans: c

96. Slip ring motor is preferred over squirrel cage induction motor where (a) high starting torque is required (b) load torque is heavy (c) heavy pull out torque is required (d) all of the above Ans: a

97. In a star-delta starter of an induction motor (a) resistance is inserted in the stator (b) reduced voltage is applied to the stator (c) resistance is inserted in the rotor (d) applied voltage per1 stator phase is 57.7% of the line voltage Ans: d

98. The torque of an induction motor is (a) directly proportional to slip (b) inversely proportional to slip (c) proportional to the square of the slip (d) none of the above Ans: a

99. The rotor of an induction motor runs at (a) synchronous speed (b) below synchronous speed (c) above synchronous speed (d) any of the above Ans: b

100. The starting torque of a three phase induction motor can be increased by (a) increasing slip (b) increasing current (c) both (a) and (b) (d) none of the above Ans: c

101. Insertion of resistance in the stator of an induction motor (a) increases the load torque (b) decreases the starting torque (c) increases the starting torque (d) none of the above Ans: b

TRANSFORMERS OBJECTIVE TYPE QUESTION

2021-11-30T16:43:00.002+05:30

2. TRANSFORMERS

1. Which of the following does not change in a transformer ? (a) Current (b) Voltage (c) Frequency (d) All of the above Ans: c

2. In a transformer the energy is conveyed from primary to secondary (a) through cooling coil (b) through air (c) by the flux (d) none of the above Ans: c

3. A transformer core is laminated to (a) reduce hysteresis loss (b) reduce eddy current losses (c) reduce copper losses (d) reduce all above losses Ans: b

4. The degree of mechanical vibrations produced by the laminations of a transformer depends on (a) tightness of clamping (b) gauge of laminations (c) size of laminations (d) all of the above Ans: d

5. The no-load current drawn by transformer is usually what per cent of the full-load current ? (a) 0.2 to 0.5 per cent (b) 2 to 5 per cent (c) 12 to 15 per cent (d) 20 to 30 per cent Ans: b

6. The path of a magnetic flux in a transformer should have (a) high resistance (b) high reluctance (c) low resistance (d) low reluctance Ans: d

7. No-load on a transformer is carried out to determine (a) copper loss (b) magnetising current (c) magnetising current and loss (d) efficiency of the transformer Ans: c

8. The dielectric strength of transformer oil is expected to be (a) lkV (b) 33 kV (c) 100 kV (d) 330 kV Ans: b

9. Sumpner's test is conducted on trans-formers to determine (a) temperature (b) stray losses (c) all-day efficiency (d) none of the above Ans: a

10. The permissible flux density in case of cold rolled grain oriented steel is around (a) 1.7 Wb/m2 (b) 2.7 Wb/m2 (c) 3.7 Wb/m2 (d) 4.7 Wb/m2 Ans: a

11. The efficiency of a transformer will be maximum when (a) copper losses = hysteresis losses (b) hysteresis losses = eddy current losses (c) eddy current losses = copper losses (d) copper losses = iron losses Ans: d

12. No-load current in a transformer (a) lags behind the voltage by about 75° (b) leads the voltage by about 75° (c) lags behind the voltage by about 15° (d) leads the voltage by about 15° Ans: a

13. The purpose of providing an iron core in a transformer is to (a) provide support to windings (b) reduce hysteresis loss (c) decrease the reluctance of the magnetic path (d) reduce eddy current losses Ans: c

14. Which of the following is not a part of transformer installation ? (a) Conservator (b) Breather (c) Buchholz relay (d) Exciter Ans: d

15. While conducting short-circuit test on a transformer the following side is short circuited (a) High voltage side (b) Low voltage side (c) Primary side (d) Secondary side Ans: b

16. In the transformer following winding has got more cross-sectional area (a) Low voltage winding (b) High voltage winding (c) Primary winding (d) Secondary winding Ans: a

17. A transformer transforms (a) voltage (b) current (c) power (d) frequency Ans: c

18. A transformer cannot raise or lower the voltage of a D.C. supply because (a) there is no need to change the D.C. voltage (b) a D.C. circuit has more losses (c) Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero (d) none of the above Ans: c

19. Primary winding of a transformer (a) is always a low voltage winding (b) is always a high voltage winding (c) could either be a low voltage or high voltage winding (d) none of the above Ans: c

20. Which winding in a transformer has more number of turns ? (a) Low voltage winding (b) High voltage winding (c) Primary winding (d) Secondary winding Ans: b

21. Efficiency of a power transformer is of the order of (a) 100 per cent (b) 98 per cent (c) 50 per cent (d) 25 per cent Ans: b

22. In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are (a) friction and windage losses (b) copper losses (c) hysteresis and eddy current losses (d) none of the above Ans: c

23. A common method of cooling a power transformer is (a) natural air cooling (b) air blast cooling (c) oil cooling (d) any of the above Ans: c

24. The no load current in a transformer lags behind the applied voltage by an angle of about (a) 180° (b) 120" (c) 90° (d) 75° Ans: d

25. In a transformer routine efficiency depends upon (a) supply frequency (b) load current (c) power factor of load (d) both (b) and (c) Ans: d

26. In the transformer the function of a conservator is to (a) provide fresh air for cooling the transformer (b) supply cooling oil to transformer in time of need (c) protect the transformer from damage when oil expends due to heating (d) none of the above Ans: c

27. Natural oil cooling is used for transformers upto a rating of (a) 3000 kVA (b) 1000 kVA (c) 500 kVA (d) 250 kVA Ans: a

28. Power transformers are designed to have maximum efficiency at (a) nearly full load (b) 70% full load (c) 50% full load (d) no load Ans: a

29. The maximum efficiency of a distribution transformer is (a) at no load (b) at 50% full load (c) at 80% full load (d) at full load Ans: b

30. Transformer breaths in when (a) load on it increases (b) load on it decreases (c) load remains constant (d) none of the above Ans: b

31. No-load current of a transformer has (a) has high magnitude and low power factor (b) has high magnitude and high power factor (c) has small magnitude and high power factor (d) has small magnitude and low power factor Ans: d

32. Spacers are provided between adjacent coils (a) to provide free passage to the cool¬ing oil (b) to insulate the coils from each other (c) both (a) and (b) (d) none of the above Ans: a

33. Greater the secondary leakage flux (a) less will be the secondary induced e.m.f. (b) less will be the primary induced e.m.f. (c) less will be the primary terminal voltage (d) none of the above Ans: a

34. The purpose of providing iron core in a step-up transformer is (a) to provide coupling between primary and secondary (b) to increase the magnitude of mutual flux (c) to decrease the magnitude of mag-netizing current (d) to provide all above features Ans: c

35. The power transformer is a constant (a) voltage device (b) current device (c) power device (d) main flux device Ans: d

36. Two transformers operating in parallel will share the load depending upon their (a) leakage reactance (b) per unit impedance (c) efficiencies (d) ratings Ans: b

37. If R2 is the resistance of secondary winding of the transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be (a) R2/VK (b) R2IK2 (c) R22!K2 (d) R22/K Ans: b

38. What will happen if the transformers working in parallel are not connected with regard to polarity ? (a) The power factor of the two trans-formers will be different from the power factor of common load (b) Incorrect polarity will result in dead short circuit (c) The transformers will not share load in proportion to their kVA ratings (d) none of the above Ans: b

39. Ifthe percentage impedances of the two transformers working in parallel are different, then (a) transformers will be overheated (b) power factors of both the trans-formers will be same (c) parallel operation will be not possible (d) parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load Ans: d

40. In a transformer the tappings are generally provided on (a) primary side (b) secondary side (c) low voltage side (d) high voltage side Ans: c

41. The use of higher flux density in the transformer design (a) reduces weight per kVA (6) reduces iron losses (c) reduces copper losses (d) increases part load efficiency Ans: a

42. The chemical used in breather for transformer should have the quality of (a) ionizing air (b) absorbing moisture (c) cleansing the transformer oil (d) cooling the transformer oil. Ans: b

43. The chemical used in breather is (a) asbestos fibre (b) silica sand (c) sodium chloride (d) silica gel Ans: d

44. An ideal transformer has infinite values of primary and secondary inductances. The statement is (a) true (b) false Ans: b

45. The transformer ratings are usually expressed in terms of (a) volts (b) amperes (c) kW (d) kVA Ans: d

46. The noise resulting from vibrations of laminations set by magnetic forces, is termed as (a) magnetostrication (b) boo (c) hum (d) zoom Ans: c

47. Hysteresis loss in a transformer varies as CBmax = maximum flux density) (a) Bmax (b) Bmax1-6 (C) Bmax1-83 (d) B max Ans: b

48. Material used for construction of transformer core is usually (a) wood (b) copper (c) aluminium (d) silicon steel Ans: d

49. The thickness of laminations used in a transformer is usually (a) 0.4 mm to 0.5 mm (b) 4 mm to 5 mm (c) 14 mm to 15 mm (d) 25 mm to 40 mm Ans: a

50. The function of conservator in a transformer is (a) to project against'internal fault (b) to reduce copper as well as core losses (c) to cool the transformer oil (d) to take care of the expansion and contraction of transformer oil due to variation of temperature of sur-roundings Ans: d

51. The highest voltage for transmitting electrical power in India is (a) 33 kV. (6) 66 kV (c) 132 kV (d) 400 kV Ans: d

52. In a transformer the resistance between its primary and secondary is (a) zero (b) 1 ohm (c) 1000 ohms (d) infinite Ans: d

53. A transformer oil must be free from (a) sludge (b) odour (c) gases (d) moisture Ans: d

54. A Buchholz relay can be installed on (a) auto-transformers (b) air-cooled transformers (c) welding transformers (d) oil cooled transformers Ans: d

55. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds (a) 50°C (b) 80°C (c) 100°C (d) 150°C Ans: d

56. The main reason for generation of harmonics in a transformer could be (a) fluctuating load (b) poor insulation (c) mechanical vibrations (d) saturation of core Ans: d

57. Distribution transformers are generally designed for maximum efficiency around (a) 90% load (b) zero load (c) 25% load (d) 50% load Ans: d

58. Which of the following property is not necessarily desirable in the material for transformer core ? (a) Mechanical strength (6) Low hysteresis loss (c) High thermal conductivity (d) High permeability Ans: c

59. Star/star transformers work satisfactorily when (a) load is unbalanced only (b) load is balanced only (c) on balanced as well as unbalanced loads (d) none of the above Ans: b

60. Delta/star transformer works satisfactorily when (a) load is balanced only (b) load is unbalanced only (c) on balanced as well as unbalanced loads (d) none of the above Ans: c

61. Buchholz's relay gives warning and protection against (a) electrical fault inside the transformer itself (b) electrical fault outside the transformer in outgoing feeder (c) for both outside and inside faults (d) none of the above Ans: a

62. The magnetising current of a transformer is usually small because it has (a) small air gap (b) large leakage flux (c) laminated silicon steel core (d) fewer rotating parts Ans: a

63. Which of the following does not change in an ordinary transformer ? (a) Frequency (b) Voltage (c) Current (d) Any of the above Ans: a

64. Which of the following properties is not necessarily desirable for the material for transformer core ? (a) Low hysteresis loss (b) High permeability (c) High thermal conductivity (d) Adequate mechanical strength Ans: c

65. The leakage flux in a transformer depends upon (a) load current (b) load current and voltage (c) load current, voltage and frequency (d) load current, voltage, frequency and power factor Ans: a

66. The path of the magnetic flux in transformer should have (a) high reluctance (b) low reactance (c) high resistance (d) low resistance Ans: b

67. Noise level test in a transformer is a (a) special test (b) routine test (c) type test (d) none of the above Ans: c

68. Which of the foJIowing is not a routine test on transformers ? (a) Core insulation voltage test (b) Impedance test (c) Radio interference test (d) Polarity test Ans: c

69. A transformer can have zero voltage regulation at (a) leading power factor (b) lagging power factor (c) unity power factor (d) zero power factor Ans: a

70. Helical coils can be used on (a) low voltage side of high kVA trans¬formers (b) high frequency transformers (c) high voltage side of small capacity transformers (d) high voltage side of high kVA rating transformers Ans: a

71. Harmonics in transformer result in (a) increased core losses (b) increased I2R losses (c) magnetic interference with communication circuits (d) all of the above Ans: d

72. The core used in high frequency transformer is usually (a) copper core (b) cost iron core (c) air core (d) mild steel core Ans: c

73. The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be (a) 6400 W (b) 1600 W (c) 800 W (d) 400 W Ans: d 1.

74. The value of flux involved m the e.m.f. equation of a transformer is (a) average value (b) r.m.s. value (c) maximum value (d) instantaneous value Ans: c L.

75. Silicon steel used in laminations mainly reduces (a) hysteresis loss (b) eddy current losses (c) copper losses (d) all of the above Ans: a

76. Which winding of the transformer has less cross-sectional area ? (a) Primary winding (b) Secondary winding (c) Low voltage winding (d) High voltage winding Ans: d

77. Power transformers are generally designed to have maximum efficiency around (a) no-load (b) half-load (c) near full-load (d) 10% overload Ans: c .

78. Which of the following is the main advantage of an auto-transformer over a two winding transformer ? (a) Hysteresis losses are reduced (b) Saving in winding material (c) Copper losses are negligible (d) Eddy losses are totally eliminated Ans: b

79. During short circuit test iron losses are negligible because (a) the current on secondary side is negligible (b) the voltage on secondary side does not vary (c) the voltage applied on primary side is low (d) full-load current is not supplied to the transformer Ans: c

80. Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in (a) short-circuiting of the secondaries (b) power factor of one of the trans¬formers is leading while that of the other lagging (c) transformers having higher copper losses will have negligible core losses (d) loading of the transformers not in proportion to their kVA ratings Ans: d

81. The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer (a) Conservator (b) Breather (c) Bushings (d) Buchholz relay Ans: a

82. An ideal transformer is one which has (a) no losses and magnetic leakage (b) interleaved primary and secondary windings (c) a common core for its primary and secondary windings (d) core of stainless steel and winding of pure copper metal (e) none of the above Ans: a

83. When a given transformer is run at its rated voltage but reduced frequency, its (a) flux density remains unaffected (b) iron losses are reduced (c) core flux density is reduced (d) core flux density is increased Ans: d

84. In an actual transformer the iron loss remains practically constant from noload to fullload because (a) value of transformation ratio remains constant (b) permeability of transformer core remains constant (c) core flux remains practically constant (d) primary voltage remains constant (c) secondary voltage remains constant Ans: c

85. An ideal transformer will have maximum efficiency at a load such that (a) copper loss = iron loss (b) copper loss < iron loss (c) copper loss > iron loss (d) none of the above Ans: a

86. If the supply frequency to the transformer is increased,"the iron loss will (a) not change (b) decrease (c) increase (d) any of the above Ans: c

87. Negative voltage regulation is indicative that the load is (a) capacitive only (b) inductive only (c) inductive or resistive (d) none of the above Ans: a 88. Iron loss of a transformer can be measured by (a) low power factor wattmeter (b) unity power factor wattmeter (c) frequency meter (d) any type of wattmeter Ans: a

89. When secondary of a current transformer is open-circuited its iron core will be (a) hot because of heavy iron losses taking place in it due to high flux density (b) hot because primary will carry heavy current (c) cool as there is no secondary current (d) none of above will happen Ans: a

90. The transformer laminations are insulated from each other by (a) mica strip (6) thin coat of varnish (c) paper (d) any of the above Ans: b

91. Which type of winding is used in 3phase shell-type transformer ? (a) Circular type (b) Sandwich type (c) Cylindrical type (d) Rectangular type Ans: b

92. During open circuit test of a transformer (a) primary is supplied rated voltage (b) primary is supplied full-load current (c) primary is supplied current at reduced voltage (d) primary is supplied rated kVA Ans: a

93. Open circuit test on transformers is conducted to determine (a) hysteresis losses (b) copper losses (c) core losses (d) eddy current losses Ans: c

94. Short circuit test on transformers is conducted to determine (a) hysteresis losses (b) copper losses (c) core losses (d) eddy current losses Ans: b

95. For the parallel operation of single phase transformers it is necessary that they should have (a) same efficiency (b) same polarity (c) same kVA rating (d) same number of turns on the secondary side. Ans: b

96. The transformer oil should have _____ volatility and _____ viscosity. (a) low,low (b) high,high (c) low,high (d) high,low Ans: a

97. The function of breather in a transformer is (a) to provide oxygen inside the tank (b) to cool the coils during reduced load (c) to cool the transformer oil (d) to arrest flow of moisture when outside air enters the transformer Ans: d

98. The secondary winding of which of the following transformers is always kept closed ? (a) Step-up transformer (b) Step-down transformer (c) Potential transformer (d) Current transformer Ans: d

99. The size of a transformer core will depend on (a) frequency (b) area of the core (c) flux density of the core material (d) (a) and (b) both Ans: d

100. N atural air coo ling is generally restricted for transformers up to (a) 1.5 MVA (b) 5 MVA (c) 15 MVA (d) 50 MVA Ans: a

101. A shell-type transformer has (a) high eddy current losses (b) reduced magnetic leakage (c) negligibly hysteresis losses (d) none of the above Ans: b

102. A transformer can have regulation closer to zero (a) on full-load (b) on overload (c) on leading power factor (d) on zero power factor Ans: c

103. A transformer transforms (a) voltage (b) current (c) current and voltage (d) power Ans: d

104. Which of the following is not the standard voltage for power supply in India ? (a) llkV (b) 33kV (c) 66 kV (d) 122 kV Ans: d

105. Reduction in core losses and increase in permeability are obtained with transformer employing (a) core built-up of laminations of cold rolled grain oriented steel (b) core built-up of laminations of hot rolled sheet (c) either of the above (d) none of the above Ans: a

106. In a power or distribution transformer about 10 per cent end turns are heavily insulated (a) to withstand the high voltage drop due to line surge produced by the shunting capacitance of the end turns (b) to absorb the line surge voltage and save the winding of transformer from damage (c) to reflect the line surge and save the winding of a transformer from damage (d) none of the above Ans: a

107. For given applied voltage, with the increase in frequency of the applied voltage (a) eddy current loss will decrease (b) eddy current loss will increase (c) eddy current loss will remain unchanged (d) none of the above Ans: c

108. Losses which occur in rotating electric machines and do not occur in trans formers are (a) friction and windage losses (b) magnetic losses (c) hysteresis and eddy current losses (d) copper losses Ans: a

109. In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are (a) hysteresis and eddy current losses (b) friction and windage losses (c) copper losses (d) none of the above Ans: a

110. Which of the following statements regarding an idel single-phase transformer having a turn ratio of 1 : 2 and drawing a current of 10 A from 200 V A.C. supply is incorrect ? (a) Its secondary current is 5 A (b) Its secondary voltage is 400 V (c) Its rating is 2 kVA (d) Its secondary current is 20 A (e) It is a step-up transformer Ans: d

111 The secondary of a current transformer is always short-circuited under operating conditions because it (a) avoids core saturation and high voltage induction (b) is safe to human beings (c) protects the primary circuit (d) none of the above Ans: a

112. In a transformer the resistance between its primary and secondary should be (a) zero (b) 10 Q (c) 1000 Q (d) infinity Ans: d

113. A good voltage regulation of a transformer means (a) output voltage fluctuation from no load to full load is least (b) output voltage fluctuation with power factor is least (c) difference between primary and secondary voltage is least (d) difference between primary and secondary voltage is maximum Ans: a

114. For a transformer, operating at constant load current, maximum efficiency will occur at (a) 0.8 leading power factor (b) 0.8 lagging power factor (c) zero power factor (d) unity power factor Ans: d

115. Which of the following protection is normally not provided on small distribution transformers ? (a) Overfluxing protection (b) Buchholz relay (c) Overcurrent protection (d) All of the above Ans: b

116. Which of the following acts as a protection against high voltage surges due to lightning and switching ? (a) Horn gaps (b) Thermal overload relays (c) Breather (d) Conservator Ans: a

117. The efficiency of two identical transformers under load conditions can be determined by (a) short-circuit test (b) back-to-back test (c) open circuit test (d) any of the above Ans: b

118. Which of the following insulating materials can withstand the highest temperature safely ? (a) Cellulose (b) Asbestos (c) Mica (d) Glass fibre Ans: c

119. Which of the following parts of a transformer is visible from outside ?(a) Bushings (b) Core (c) Primary winding (d) Secondary winding Ans: a

120. The noise produced by a transformer is termed as (a) zoom (b) hum (c) ringing (d) buzz Ans: b

121. Which of the following loss in a transformer is zero even at full load ? (a) Core loss (b) Friction loss (c) Eddy current loss (d) Hysteresis loss Ans: b

122. Which of the following is the most likely source of harmonics in a transformer ? (a) poor insulation (b) Overload (c) loose connections (d) Core saturation Ans: d

123. If a transformer is continuously operated the maximum temperature rise will occur in (a) core (b) windings (c) tank (d) any of the above Ans: b

124. The hum in a transformer is mainly attributed to (a) load changes (b) oil in the transformer (c) magnetostriction (d) mechanical vibrations Ans: c

125. The maximum load that a power transformer can carry is limited by its (a) temperature rise (b) dielectric strength of oil (c) voltage ratio (d) copper loss Ans: c

126. The efficiency of a transformer, under heavy loads, is comparatively low because (a) copper loss becomes high in proportion to the output (b) iron loss is increased considerably (c) voltage drop both in primary and secondary becomes large (d) secondary output is much less as compared to primary input Ans: a

127. An open-circuit test on a transformer is conducted primarily to measure (a) insulation resistance (b) copper loss (c) core loss (d) total loss (e) efficiency (f) none of the above Ans: c

128. A no-load test is performed on a transformer to determine (a) core loss (b) copper loss (c) efficiency (d) magnetising current (e) magnetising current and loss Ans: e

129. The voltage transformation ratio of a transformer is equal to the ratio of (a) primary turns to secondary turns (b) secondary current to primary current (c) secondary induced e.m.f. to primary induced e.m.f. (d) secondary terminal voltage to primary applied voltage Ans: c

130. Part of the transformer which is most subject to damage from overheating is (a) iron core (b) copper winding (c) winding insulation (d) frame or case (e) transformer tank Ans: c

131. If a transformer is switched on to a voltage more than the rated voltage (a) its power factor will deteriorate (b) its power factor will increase (c) its power factor will remain unaffected (d) its power factor will be zero Ans: a

132. Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is (a) approximately equal to one (b) less than one (c) great than one (d) none of the above Ans: a

133. Minimum voltage regulation occurs when the power factor of the load is (a) unity (b) lagging (c) leading (d) zero Ans: c

134. In a step-down transformer, there is a change of 15 A in the load current. This results in change of supply current of (a) less than 15 A (b) more than 15 A (c) 15 A (d) none of the above Ans: a

135. The efficiencies of transformers compared with that of electric motors of the same power are (a) about the same (6) much smaller (c) much higher (d) somewhat smaller (e) none of the above Ans: c

DC MOTOR Objective type question

2021-11-27T19:09:00.000+05:30

DC MOTOR

1. No-load speed of which of the following motor will be highest ?

(a) Shunt motor

(b) Series motor

(d) Differentiate compound motor

Ans: b

2. The direction of rotation of a D.C. series motor can be changed by

(a) interchanging supply terminals

(b) interchanging field terminals

(d) None of the above

Ans: b

3. Which of the following application requires high starting torque ?

(a) Lathe machine

(b) Centrifugal pump

(d) Air blower

Ans: c

4. If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?

(a) Series motor

(b) Shunt motor

(d) Cumulative compound motor

Ans: a

5. Which D.C. motor will be preferred for machine tools ?

(a) Series motor

(b) Shunt motor

(d) Differential compound motor

Ans: b

6. Differentially compound D.C. motors can find applications requiring

(a) high starting torque

(b) low starting torque

(d) frequent on-off cycles

Ans: b

7. Which D.C. motor is preferred for elevators ?

(a) Shunt motor

(b) Series motor

(d) Cumulative compound motor

Ans: d

8. According to Fleming's left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of

(a) current in the conductor

aovtaat of conductor

(d) none of the above

Ans: a

9. If the field of a D.C. shunt motor gets opened while motor is running

(a) the speed of motor will be reduced %

(b) the armature current will reduce

(d) the motor will continue to nuvat constant speed

Ans: c

10. Starters are used with D.C. motors because

(a) these motors have high starting torque

(b) these motors are not self-starting

(d) to restrict armature current as there is no back e.m.f. while starting

Ans: d

11. In D.C. shunt motors as load is reduced

(a) the speed will increase abruptly

(b) the speed will increase in proportion to reduction in load

(d) the speed will reduce

Ans: c

12. A D.C. series motor is that which

(a) has its field winding consisting of thick wire and less turns

(b) has a poor torque

(d) has almost constant speed

Ans: a

13. For starting a D.C. motor a starter is required because

(a) it limits the speed of the motor

(b) it limits the starting current to a safe value

(d) none of the above

Ans: b

14. The type of D.C. motor used for shears and punches is

(a) shunt motor

(b) series motor

(d) cumulative compound D.C. motor

Ans: d

15. If a D.C. motor is connected across the A.C. supply it will

(a) run at normal speed

(b) not run

(d) burn due to heat produced in the field winding by .eddy currents

Ans: d

16. To get the speed of D.C, motor below the normal without wastage of electrical energy is used.

(a) Ward Leonard control

(b) rheostatic control

(d) none of the above method

Ans: a

17. When two D.C. series motors are connected in parallel, the resultant speed is

(a) more than the normal speed

(b) loss than the normal speed

(d) zero

Ans: c

18. The speed of a D.C. shunt motor more than its full-load speed can be obtained by

(a) decreasing the field current

(b) increasing the field current

(d) increasing the armature current

Ans: a

19. In a D.C. shunt motor, speed is

(a) independent of armature current

(b) directly proportional to the armature current

(d) inversely proportional to the armature current

Ans: a

20. A direct on line starter is used: for starting motors

(a) iip to 5 H.P.

(b) up to 10 H.P.

(d) up to 20 H.P.

Ans: a

21. What will happen if the back e.m.f. of a D.C. motor vanishes suddenly?

(a) The motor will stop

(b) The motor will continue to run

(d) The motor will run noisy

Ans: c

22. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

(a) back e.m.f. is equal to armature drop

(b) armature drop is negligible

(d) flux is practically constant in D.C. shunt motors

Ans: d

23. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be

(a) almost negligible

(b) rated full-load current

(d) more than full-load current

Ans: d

24. These days D.C. motors are widely used in

(a) pumping sets

(b) air compressors

(d) machine shops

Ans: c

25. By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?

(a) Frame

(b) Shaft

(d) Stator

Ans: c

26. In which of the following applications D.C. series motor is invariably tried?

(a) Starter for a car

(b) Drive for a water pump

(d) Motor operation in A.C. or D.C.

Ans: a

27. In D.C. machines fractional pitch winding is used (a) to improve cooling (b) to reduce copper losses (c) to increase the generated e.m.f. (d) to reduce the sparking Ans: d

28. A three point starter is considered suitable for

(a) shunt motors (b) shunt as well as compound motors (c) shunt, compound and series motors (d) all D.C. motors Ans: b

29. In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be (a) 100% (b) around 90% (c) anywhere between 75% and 90% (d) less than 50% Ans: d

30. The ratio of starting torque to full-load torque is least in case of (a) series motors (b) shunt motors (c) compound motors (d) none of the above Ans: b

32. In D.C. motor which of the following can sustain the maximum temperature rise? (a) Slip rings (b) Commutator (c) Field winding (d) Armature winding Ans: c

33. Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ? (a) Lenz's law (b) Faraday's law (c) Coloumb's law (d) Fleming's left-hand rule Ans: d

34. Which of the following load normally needs starting torque more than the rated torque? (a) Blowers (b) Conveyors (c) Air compressors (d) Centrifugal pumps Ans: b

35. The starting resistance of a D.C. motor is generally (a) low (b) around 500 Q (c) 1000 Q (d) infinitely large Ans: a

36. The speed of a D.C. series motor is (a) proportional to the armature current (b) proportional to the square of the armature current (c) proportional to field current (d) inversely proportional to the armature current Ans: d

37. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to (a) 100% of the previous value (b) 50% of the previous value (c) 25% of the previous value (d) 10% of the previous value (e) none of the above Ans: c

38. The current drawn by the armature of D.C. motor is directly proportional to (a) the torque required (b) the speed of the motor (c) the voltage across the terminals (d) none of the above Ans: a

39. The power mentioned on the name plate of an electric motor indicates (a) the power drawn in kW (b) the power drawn in kVA (c) the gross power (d) the output power available at the shaft Ans: d

40. Which D.C. motor has got maximum self loading property? (a) Series motor (b) Shunt motor (c) Cumulatively compounded 'motor (d) Differentially compounded motor Ans: d

41. Which D.C. motor will be suitable alongwith flywheel for intermittent light and heavy loads? (a) Series motor (b) Shunt motor (c) Cumulatively compounded motor (d) Differentially compounded motor Ans: c

42. If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens (a) nothing will happen to th£ motor (b) this will make armature to take heavy current, possibly burning it (c) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (d) motor will run at very slow speed Ans: c

43. D.C. series motors are used (a) where load is constant (b) where load changes frequently (c) where constant operating speed is needed (d) in none of the above situations. Ans: d

44. For the same H.P. rating and full load speed, following motor has poor starting torque (a) shunt (b) series (c) differentially compounded (d) cumulativelyc'ompounded Ans: c

45. In case of conductively compensated D.C. series motors, the compensating winding is provided (a) as separately wound unit (6) in parallel with armature winding (c) in series with armature winding (d) in parallel with field winding Ans: c

46. Sparking at the commutator of a D.C. motor may result in (a) damage to commutator segments (b) damage to commutator insulation (c) increased power consumption (d) all of the above Ans: d

47. Which of the following motor is preferred for operation in highly explosive atmosphere ? (a) Series motor (b) Shunt motor (c) Air motor (d) Battery operated motor Ans: c

48. If the supply voltage for a D.C. motor is increased, which of the following will decrease ? (a) Starting torque (b) Operating speed (c) Full-load current (d) All of the above Ans: c

49. Which one of the following is not the function of pole shoes in a D.C. machine ? (a) To reduce eddy current loss (b) To support the field coils (c) To spread out flux for better unifor-mity (d) To reduce the reluctance of the mag-netic path Ans: a

50. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is (a) 4.0 (b) 2.0 (c) 1.0 (d) 0.5 Ans: d

51. The condition for maximum power in case of D.C. motor is (a) back e.m.f. = 2 x supply voltage (b) back e.m.f. = | x supply voltage (c) supply voltage = | x back e.m.f. (d) supply voltage = back e.m.f. Ans: b

52. For which of the following applications a D.C. motor is preferred over an A.C. motor ? (a) Low speed operation (b) High speed operation (c) Variable speed operation (d) Fixed speed operation Ans: c

53. In D.C. machines the residual magnetism is of the order of (a) 2 to 3 per cent (6) 10 to 15 per cent (c) 20 to 25 per cent (d) 50 to 75 per cent Ans: a

54. Which D.C. motor is generally preferred for cranes and hoists ? (a) Series motor (b) Shunt motor (c) Cumulatively compounded motor (d) Differentially compounded motor Ans: a

55. Three point starter can be used for (a) series motor only (b) shunt motor only (c) compound motor only (d) both shunt and compound motor Ans: d

56. Sparking, is discouraged in a D.C. motor because (a) it increases the input power con-sumption (b) commutator gets damaged (c) both (a) and (b) (d) none of the above Ans: b

57. Speed control by Ward Leonard method gives uniform speed variation (a) in one direction (b) in both directions (c) below normal speed only (d) above normal speed only. Ans: b

58. Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be (a) level compounded (b) under compounded (c) cumulatively compounded (d) differentially compounded Ans: c

59. Following motor is used where high starting torque and wide speed range control is required. (a) Single phase capacitor start (b) Induction motor (c) Synchronous motor (d) D.C. motor (e) None of the above Ans: d

60. In a differentially compounded D.C. motor, if shunt field suddenly opens (a) the motor will first stop and then run in opposite direction as series motor (b) the motor will work as series motor and run at slow speed in the same direction (c) the motor will work as series motor and run at high speed in the same direction (d) the motor will not work and come to stop Ans: a

61. Which of the following motor has the poorest speed regulation ? (a) Shunt motor (b) Series motor (c) Differential compound motor (d) Cumulative compound motor Ans: b

62. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of (a) D.C. series motor (b) D.C. shunt motor (c) induction motor (d) all of above motors Ans: a

63. As -the load is increased the speed of D.C. shunt motor will (a) reduce slightly (b) increase slightly (c) increase proportionately (d) remains unchanged Ans: a

64. The armature torque of the D.C. shunt motor is proportional to (a) field flux only (b) armature current only (c) both (a) and (b) (d) none of the above Ans: b

65. Which of the following method of speed control of D.C. machine will offer minimum efficiency ? (a) Voltage control method (b) Field control method (c) Armature control method (d) All above methods Ans: c

66. Usually wide and sensitive speed control is desired in case of (a) centrifugal pumps (b) elevators (c) steel rolling mills (d) colliery winders Ans: d

67. The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is (a) 2.36% (6) 4.76% (c) 6.77% (d) 8.84% Ans: b

68. The armature voltage control of D.C. motor provides (a) constant torque drive (b) constant voltage drive (c) constant current drive (d) none of the above Ans: a

69. As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit (a) a resistance is connected in series with armature (b) a resistance is connected parallel to the armature (c) armature is temporarily open circuited (d) a high value resistor is connected across the field winding Ans: a

70. The speed of a D.C. shunt motor can be increased by (a) increasing the resistance in armature circuit (b) increasing the resistance in field circuit (c) reducing the resistance in the field circuit (d) reducing the resistance in the armature circuit Ans: b

71. If I2 be the armature current, then speed of a D.C. shunt motor is (a) independent of Ia (b) proportional to la (c) varies as (Ia) (d) varies as la Ans: a

72. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will (a) remain unchanged (6) reduce to one-fourth value (c) increase four folds (d) be doubled Ans: a

73. At the instant of starting when a D.C. motor is put on supply, it behaves like (a) a highly resistive circuit (6) a low resistance circuit (c) a capacitive circuit (d) none of the above Ans: b

74. The speed of a D.C. motor can be varied by varying (a) field current (b) applied voltage (c) resistance in series with armature (d) any of the above Ans: d

75. Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ? (a) Low cost (b) Wide speed range (c) Stability (d) High starting torque. Ans: a

76. For a D.C. shunt motor if the excitation is changed (a) torque will remain constant (b) torque will change but power will remain constant (c) torque and power both will change (d) torque, power and speed, all will change Ans: b

77. Which motor has the poorest speed control? (a) Differentially compounded motor (b) Cumulatively compounded motor (c) Shunt motor (d) Series motor Ans: d

78. The plugging gives the (a) zero torque braking (b) smallest torque braking (c) highest torque braking (d) none of the above Ans: c

79. The armature voltage control of D.C. motor provides (a) constant voltage drive (b) constant current drive (c) constant torque drive (d) none of the above Ans: c

80. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor (a) of lower H.P. should be selected (6) of higher H.P. should be selected (c) can be used for 50°C ambient temperature also (d) is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor Ans: d

81. If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking (o) regenerative (b) plugging (c) dynamic braking (d) none of the above (e) any of the above Ans: b

82. Which of the following motors one will choose to drive the rotary compressor ? (a) D.C. shunt motor (b) D.C. series motor (c) Universal motor (d) Synchronous motor Ans: d

83. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will (a) increase (b) decrease (c) remain same (d) become zero Ans: a

84. Why are the D.C. motors preferred for traction applications ? (a) Torque and speed are inversely proportional to armature current (b) Torque is proportional to armature current (c) Torque is proportional to square root of armature current (d) The speed is inversely proportional to the torque and the torque is proportional to square of armature current Ans: d

85. Which of the following motors is usually used in house-hold refrigerators ? (a) D.C. shunt motor (b) D.C. series motor (c) Single phase induction motor (split phase start or induction run motor) (d) Reluctance motor (e) Synchronous motor Ans: c

86. Which of the following motors is most suitable for signalling devices and many kinds of timers ? (a) D.C. shunt motor (b) D.C. series motor (c) Induction motor (d) Reluctance motor Ans: d

87. Which motor should not be started on no-load ? (a) Series motor (b) Shunt motor (c) Cumulatively compounded motor (d) Differentially compounded motor. Ans: a

88. Ward-Leonard control is basically a (a) voltage control method (b) field divertor method (c) field control method (d) armature resistance control method Ans: a

89. For constant torque drive which speed control method is preferred ? (a) Field control (b) Armature voltage control (c) Shunt armature control (d) Mechanical loading system Ans: b

90. In Ward-Leonard control the lower limit of speed is imposed by (a) residual magnetism of the generator (b) core losses of motor (c) mechanical losses of motor and gen¬erator together (d) all of the above Ans: a

91. The main disadvantage of the Ward-Leonard control method is (a) high initial cost (b) high maintenance cost (c) low efficiency at Hght loads (d) all of the above Ans: d

92. Regenerative method of braking is based on that (a) back e.m.f. is less than the applied voltage (b) back e.m.f. is equal to the applied voltage (c) back e.m.f. of rotor is more than the applied voltage (d) none of the above Ans: b

93. The hysteresis loss in a D.C. machine least depends on (a) Frequency of magnetic reversals (b) Maximum value of flux density (c) Volume and grade of iron (d) Rate of flow of ventilating air Ans: d

94. In a D.C. generator all of the following could be the effects of iron losses except (a) Loss of efficiency (b) Excessive heating of core (c) Increase in terminal voltage (d) Rise in temperature of ventilating air Ans: c

95. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ? (a) hysteresis loss (b) field copper loss (c) armature copper loss (d) eddy current loss Ans: c

96. Which of the following loss in a D.C. generator varies significantly with the load current ? (a) Field copper loss (b) Windage loss (c) Armature copper loss (d) None of the above Ans: c

97. Torque developed by a D.C. motor depends upon (a) magnetic field (b) active length of the conductor (c) current flow through the conductors (d) number of conductors (e) radius of armature (f) all above factors Ans: f

98. D.C. shunt motors are used for driving (a) trains (b) cranes (c) hoists (d) machine tools Ans: d

99. In a manual shunt motor starter (a) over load relay is connected in series and no volt relay in parallel with the load (6) over load relay is connected in paral¬lel and no volt relay in series with the load (c) over load relay and no volt relay are both connected in series with the load (d) over load relay and no volt relay are both connected in parallel with the load Ans: a

100. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ? (a) Providing laminations in armature core (b) Providing laminations in stator (c) Using non-magnetic material for frame (d) Using material of low hysteresis co-efficient for armature core material Ans: d

101. Which of the following loss in a D.C. generator is dissipated in the form of heat? (a) Mechanical loss (b) Core loss (c) Copper loss (d) All of the above Ans: d

102. Which of the following losses are significantly reduced by laminating the core of a D.C. generator ? (a) Hysteresis losses (b) Eddy current losses (c) Copper losses (d) Windage losses Ans: b

103. The total losses in a well designed D.C. generator of 10 kW will be nearly (a) 100 W (b) 500 W (c) 1000 W (d) 1500 W Ans: b

104. The condition for maximum efficiency for a D.C. generator is (a) eddy current losses = stray losses (b) hysteresis losses = eddy current losses (c) copper losses = 0 (d) variable losses = constant losses Ans: d

105. D.C. generators are normally designed for maximum efficiency around (a) full-load (b) rated r.p.m. (c) rated voltage (d) all of the above Ans: a

106. In a D.C. generator, the iron losses mainly take place in (a) yoke (b) commutator (c) armature conductors (d) armature rotor Ans: d

107. D.C. generators are installed near the load centres to reduce (a) iron losses (b) line losses (c) sparking (d) corona losses Ans: b

108. The purpose of retardation test on D.C. shunt machines is to find out (a) stray losses (b) eddy current losses (c) field copper losses (d) windage losses Ans: a

109. Which of the following tests will be suitable for testing two similar D.C. series motors of large capacity ? (a) Swinburne's test (b) Hopkinson's test (c) Field test (d) Brake test Ans: c

110. Hopkinson's test on D.C. machines is conducted at (a) no-load (b) part load (c) full-load (d) overload Ans: c

111. During rheostat braking of D.C. series motors (a) motor is run as a generator (b) motor is reversed in direction (c) motor is run at reduced speed Ans: a

112. For which types of D.C. motor, dynamic braking is generally used ? (a) Shunt motors (b) Series motors (c) Compound motors (d) All of the above Ans: d

113. Which method of braking is generally used in elevators ? (a) Plugging (b) Regenerative braking (c) Rheostatic braking (d) None of the above Ans: a

114. In variable speed motor (a) a stronger commutating field is needed at low speed than at high speed (b) a weaker commutating field is needed at low speed than at high speed (c) same commutating field is needed at low speed than at high speed (d) none of the above is correct Ans: b

115. When the armature of a D.C. motor rotates, e.m.f. induced is (a) self-induced e.m.f. (b) mutually induced e.m.f. (c) back e.m.f. (d) none of the above Ans: c

116. Where D.C. motor of H.P. 12 or more requires frequent starting, stopping, reversing and speed control (a) drum type controller is used (b) three point starter is used (c) four point starter is used (d) all above can be used Ans: a

117. If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens (a) this will make armature to take heavy current, possibly burning it (6) this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses (c) nothing will happen to motor (d) motor will come to stop Ans: a

118. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the (a) series motor (b) shunt motor (c) compound motor (d) any of the above Ans: a

119. D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this (a) series motor (b) shunt motor (c) compound motor (d) any of the above Ans: c

120. Which D.C. motor has got maximum self relieving property ? (a) Series motor (6) Shunt motor (c) Cumulatively compounded motor (d) Differentially compounded motor Ans: a

121. In the D.C. motor the iron losses occur in (a) the field (b) the armature (c) the brushes (d) the commutator Ans: b

122. The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by (a) reducing the field current (b) decreasing the armature current (c) increasing the armature current (d) increasing the excitation current (e) none of the above methods Ans: a

123. One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven (a) runs as a generator (b) does not run as a generator (c) also runs as a motor comes to stop after sometime Ans: a

DC GENERATORS OBJECTIVE TYPE QUESTIONS (MCQ)

2021-08-07T02:52:00.006+05:30

OBJECTIVE TYPE QUESTIONS

1. A. DC GENERATORS

1. Laminations of core are generally made of

(a) case iron
(b) carbon
(c) silicon steel
(d) stainless steel
Ans: c

2. Which of the following could be lamina-proximately the thickness of laminations of a D.C. machine?

(a) 0.005 mm
(b) 0.05 mm
(c) 0.5 m
(d) 5 m
Ans: c

3. The armature of D.C. generator is laminated to

(a) reduce the bulk
(b) provide the bulk
(c) insulate the core
(d) reduce eddy current loss
Ans: d

4. The resistance of armature winding depends on

(a) length of conductor
(b) cross-sectional area of the conductor
(c) number of conductors
(d) all of the above
Ans: d

5. The field coils of D.C. generator are usually made of

(a) mica
(b) copper
(c) cast iron
(d) carbon
Ans: b

6. The commutator segments are connected to the armature conductors by means of

(a) copper lugs
(b) resistance wires
(c) insulation pads
(d) brazing
Ans: a

7. In a commutator

(a) copper is harder than mica
(b) mica and copper are equally hard
(c) mica is harder than copper
(d) none of the above
Ans: c

8. In D.C. generators the pole shoes are fastened to the pole core by

(a) rivets
(b) countersunk screws
(c) brazing
(d) welding
Ans: b

9. According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of

(a) motion of conductor
(b) lines of force
(c) either of the above
(d) none of the above
Ans: b

10. Fleming's right-hand rule regarding direction of induced e.m.f., correlates

(a) magnetic flux, direction of current flow, and resultant force
(b) magnetic flux, direction of motion, and the direction of e.m.f. induced
(c) magnetic field strength, induced voltage, and current
(d) magnetic flux, direction of force, and direction of motion of a conductor
Ans: b

11. While applying Fleming's right-hand rule to And the direction of induced e.m.f., the thumb points towards

(a) direction of induced e.m.f.
(b) direction of flux
(c) direction of motion of the conductor if forefinger points in the direction of generating e.m.f.
(d) direction of motion of conductor, if forefinger points along the lines of flux
Ans: d

12. The bearings used to support the rotor shafts are generally

(a) ball bearings
(b) bush bearings
(c) magnetic bearmgs
(d) needle bearings
Ans: a

13. In D.C. generators, the cause of rapid brush wear maybe

(a) severe sparking
(b) rough commutator surface
(c) imperfect contact
(d) any of the above
Ans: d

14. In lap winding, the number of brushes is always

(a) double the number of poles
(b) same as the number of poles
(c) half the number of poles
(d) two
Ans: b

15. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?

(a) Lap winding
(b) Wave winding
(c) Either of (a) and (b) above
(d) Depends on other features of design
Ans: b

16. In a four-pole D.C. machine

(a) all the four poles are north poles
(b) alternate poles are north and south
(c) all the four poles are south poles
(d) two north poles follow two south poles
Ans: b

17. Copper brushes in D.C. machine are used

(a) where low voltage and high currents are involved
(b) where high voltage and small currents are involved
(c) in both of the above cases
(d) in none of the above cases
Ans: a

18. A separately excited generator as compared to a self-excited generator

(a) is amenable to better voltage control
(b) is more stable
(c) has exciting current independent of load current
(d) has all above features
Ans: d

19. In case of D.C. machines, mechanical losses are a primary function of

(a) current
(b) voltage
(c) speed
(d) none of above
Ans: c

20. Iron losses in a D.C. machine are independent of variations in

(a) speed
(b) load
(c) voltage
(d) speed and voltage
Ans: b

21. In D.C. generators, current to the external circuit from armature is given through

(a) commutator
(b) solid connection
(c) slip rings
(d) none of above
Ans: a

23. Brushes of D.C. machines are made of

(a) carbon
(b) soft copper
(c) hard copper
(d) all of above
Ans: a

22. Satisfactory commutation of D.C. machines requires

(a) brushes should be of proper grade and size
(b) brushes should smoothly run in the holders
(c) smooth, concentric commutator properly undercut
(d) all of the above
Ans: d

24. If B is the flux density, I the length of conductor, and v is the velocity of conductor, then induced e.m.f. is given by

(a)Blv
(b)Blv2
(c)Bl2v
(d)Bl2v2
Ans: a

25. In case of a 4-pole D.C. generator provided with a two-layer lap winding with sixteen coils, the pole pitch will be

(a) 4
(b) 8
(c) 16
(d) 32
Ans: b

26. The material for commutator brushes is generally

(a) mica
(b) copper
(c) cast iron
(d) carbon
Ans: d

27. The insulating material used between the commutator segments is normally

(a) graphite
(b) paper
(c) mica
(d) insulating varnish
Ans: c

28. In D.C. generators, the brushes on commutator remain in contact with conductors which

(a) lie under south pole
(b) lie under north pole
(c) lie under an interpolar region
(d) are farthest from the poles
Ans: c

29. If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be

(a) demagnetization only
(b) cross magnetization as well as magnetization
(c) crossmagnetisation as well as demagnetising
(d) cross magnetization only
Ans: c

30. Armature reaction of an unsaturated D.C. machine is

(a) crossmagnetising
(b) demagnetising
(c) magnetizing
(d) none of above
Ans: a

31. D.C. generators are connected to the busbars or disconnected from them only under the floating condition

(a) to avoid sudden loading of the prime-mover
(b) to avoid mechanical jerk to the shaft
(c) to avoid burning of switch contacts
(d) all above
Ans: d

32. Eddy currents are induced in the pole shoes of a D.C. machine due to

(a) oscillating magnetic field
(b) pulsating magnetic flux
(c) relative rotation between field and armature
(d) all above
Ans: c

33. In a D.C. machine, short-circuited field coil will result in

(a) the odour of burning insulation (b) unbalanced magnetic pull producing vibrations (c) reduction of generated voltage for which excitation has to be increased to maintain the voltage (d) all above Ans:

34. Equilizer rings are required in case armature is

(a) wave wound (b) lap wound (c) delta wound (d) duplex wound Ans: b

35. Welding generator will have

(a) lap winding (b) wave winding (c) delta winding (d) duplex wave winding Ans: a

36. In case of D.C. machine winding, number of commutator segments is equal to

(a) number of armature coils (b) number of armature coil sides (c) number of armature conductors (d) number of armature turns Ans: a

37. For a D.C. machines laboratory following type of D.C. supply will be suitable

(a) rotary converter (b) mercury is rectifier (c) induction motor D.C. generator set (d) synchronous motor D.C. generator set Ans: c

38. The function of pole shoes in the case of D.C. machine is

(a) to reduce the reluctance of the magnetic path (b) to spread out the flux to achieve uniform flux density (c) to support the field coil (d) to discharge all the above functions Ans: d

39. In the case of lap winding resultant pitch is

(a) multiplication of front and back pitches (b) division of front pitch by back pitch (c) a sum of front and back pitches (d) difference of front and back pitches Ans: d

40. A D.C. welding generator has

(a) lap winding (b) wave moving (c) duplex winding (d) any of the above
Ans: a

41. Which of the following statement about D.C. generators is false?

(a) Compensating winding in a D.C. machine helps in commutation
(b) In a D. C. generator interpoles winding is connected in series with the armature winding
(c) Back pitch and front pitch are both odd and approximately equal to the pole pitch
(d) Equalizing bus bars are used with parallel running of D.C. shunt generators
Ans: d

42. The demagnetizing component of armature reaction in a D.C. generator

(a) reduces generator e.m.f. (b) increases armature speed (c) reduces interpoles flux density (d) results in sparking trouble
Ans: a

43. Magnetic field in a D.C. generator is produced by

(a) electromagnets (b) permanent magnets (c) both (a) and (b) (d) none of the above
Ans: a

44. The number of brushes in a commutator depends on

(a) speed of armature (b) type of winding (c) voltage (d) amount of current to be collected
Ans: d

45. Compensating windings are used in D.C. generators

(a) mainly to reduce the eddy currents by providing local short-circuits (b) to provide a path for the circulation of cooling air (c) to neutralize the cross-magnetizing effect of the armature reaction (d) none of the above Ans: c

46. Which of the following components of a D.C, generator plays a vital role in providing direct current of a D.C. generator?

(a) Dummy coils (b) Commutator (c) Eyebolt (d) Equilizer rings Ans: b

47. In a D.C. generator the ripples in the direct e.m.f. generated are reduced by

(a) using conductor of annealed copper (b) using commutator with a large number of segments (c) using carbon brushes of superior quality (d) using equalizer rings
Ans: c

48. In D.C. generators, lap winding is used for

(a) high voltage, high current (b) low voltage, high current (c) high voltage, low current (d) low voltage, low current
Ans: b

49. Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generator A and B will be

(a) 2 : 3 (b) 3 : 1 (c) 3 : 2 (d) 1 : 3
Ans: b

50. The voltage drop for which of the following types of brush can be expected to be least?

(a) Graphite brushes (b) Carbon brushes (c) Metal graphite brushes (d) None of the above
Ans: c

51. The e.m.f. generated by a shunt-wound D.C. generator is. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be

(a) E/2 (b) 2E (c) slightly less than E (d) E
Ans: b

52. In a D.C. generator the actual flux distribution depends upon

(a) size of air gap (b) shape of the pole shoe (c) clearance between tips of the ad¬jacent pole shoes (d) all of the above Ans:

53. The armature core of a D.C. generator is usually made of

(a) silicon steel (b) copper (c) non-ferrous material (d) cast-iron Ans: a

54. Open circuited armature coil of a D.C. machine is

(a) identified by the scarring of the commutator segment to which open-circuited coil is connected (b) indicated by a spark completely around the commutator (c) both (a) and (b) (d) none of the above Ans: c

55. In a D.C. machine, fractional pitch winding is used

(a) to increase the generated voltage (b) to reduce sparking (c) to save the copper because of shorter end connections (d) due to (b) and (c) above Ans:

56. For the parallel operation of two or more D.C. compound generators, we should ensure that

(a) voltage of the incoming generator should be same as that of bus bar (b) polarity of incoming generator should be same as that of bus bar (c) all the series fields should be run in parallel by means of an equalizer connection (d) series fields of all generators should be either on positive side or negative side of the armature (e) all conditions mentioned above should be satisfied Ans: d

57. D.C. series generator is used

(a) to supply traction load (b) to supply industrial load at constant voltage (c) voltage at the toad end of the feeder (d) for none of the above purpose Ans: c

58. Following D.C. generator will be in a position to build up without any residual magnetism in the poles

(a) series generator (b) shunt generator (c) compound generator (d) self-excited generator Ans: d

59. Interpole flux should be sufficient to

(a) neutralize the commutating self-induced e.m.f. (b) neutralise the armature reaction flux (c) neutralize both the armature reaction flux as well as commutating e.m.f. induced in the coil (d) perform none of the above functions Ans: c

60. D.C. generator generally preferred for charging automobile batteries is

(a) series generator (b) shunt generator (c) long shunt compound generator (d) any of the above Ans: c

61. In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when

(a) r.p.m. is more than 300 (b) r.p.m. is less than 300 (c) number of poles is 4 (d) number of poles is 2 Ans: d

62. Permeance is the reciprocal of

(a) flux density (b) reluctance (c) ampere-turns (d) resistance Ans: b

63. In D.C. generators the polarity of the interpoles

(a) is the same as that of the main pole ahead (b) is the same as that of the immediately preceding pole (c) is opposite to that of the main pole ahead (d) is neutral as these poles do not play part in generating e.m.f. Ans: a

64. The e.m.f. generated in a D.C. generator is directly proportional to

(a) flux/pole (b) speed of armature (c) number of poles (d) all of the above Ans: b

65. In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when

(a) there is no load on the generator (b) the generator runs on full load (c) the generator runs on overload (d) the generator runs at designed speed Ans: a

66. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralized by all of the following except

(a) interpoles (b) dummy coils (c) compensating winding (d) shifting of axis of brushes Ans: b

67. In D.C. generators on no-load, the air gap flux distribution in space is

(a) sinusoidal (b) triangular (c) pulsating (d) flat-topped Ans: d

68. A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly

(a) 150 V (b) 175 V (c) 240 V (d) 290 V Ans: c

69. The purpose of providing dummy coils in a generator is

(a) to reduce eddy current losses (b) to enhance flux density (c) to amplify voltage (d) to provide mechanical balance for the rotor Ans: d

70. In a shunt generator the voltage build-up is generally restricted by

(a) speed limitation (b) armature heating (c) insulation restrictions (d) saturation of iron Ans:

71. If a D.C. generator fails to build up the probable cause could not be

(a) imperfect brush contact (b) field resistance less than the critical resistance (c) no residual magnetism in the generator (d) faulty shunt connections tending to reduce the residual magnetism Ans: b

72. Flashing the field of D.C. generator means

(a) neutralizing residual magnetism (b) creating residual magnetism by a D.C. source (c) making the magnetic losses of forces parallel (d) increasing flux density by adding extra turns of windings on poles Ans: b

73. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be

(a) 640 V (b) 620 V (c) 600 V (d) 580 V Ans: d

74. In a D.C. generator the critical resistance refers to the resistance of

(a) brushes (b) field (c) armature (d) load Ans: b

75. To achieve sparkless commutation brushes of a D.C. generator are rocked ahead so as to bring them

(a) just ahead of magnetic neutral axis (b) in magnetic neutral axis (c) just behind the magnetic neutral axis Ans: a

76. Armature coil is short-circuited by brushes when it lies

(a) along neutral axis (b) along field axis (c) in any of the above positions (d) in none of the above positions Ans: a

77. A cumulatively compounded long shunt generator when operating as a motor would be

(a) cumulatively compounded long shunt (b) differentially compounded long shunt (c) cumulatively compounded short shunt (d) differentially compounded short shunt Ans: b

78. To avoid formation of grooves in the commutator of a D.C. machine

(a) the brushes of opposite polarity should track each other (b) the brushes of same polarity should track each other (c) brush position has no effect on the commutator grooving Ans: a

79. The following constitute short-circuit in the armature winding.

(a) Insulation failure between two com-mutator bars (6) Insulation failure between two turns of a coil (c) Two or more turns of the same coil getting grounded (d) All of the above Ans: d

80. The rapid wear of brushes takes place due to

(a) abrasion from dust (b) excessive spring pressure (c) rough commutator bars (d) high mica insulation between com-mutation bars (e) all of the above factors Ans: e

81. Number of tappings for each equalizer ring is equal to

(a) number of pole pairs (b) number of poles (c) number of parallel paths (d) number of commutator segments Ans: a

82. A D.C. generator can be considered as

(a) rectifier (b) prime-mover (c) rotating amplifier (d) power pump Ans: c

83. In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilized is called

(a) rotor (b) stator (c) field (d) armature Ans: d

84. In a D.C. machine stray loss is the sum of

(a) total copper loss and mechanical loss (b) armature copper loss and iron loss (c) shunt field copper loss and mechanical loss (d) iron loss and mechanical loss Ans: d

85. Lap winding is composed of

(a) any even number of conductors (b) any odd number of conductors (c) that even number which is exact multiple of poles + 2 (d) that even number which is exact multiple of poles Ans: a

86. In a D.C. generator in case the resistance of the field winding is increased, then output voltage will

(a) increase (b) decrease (c) remain unaffected (d) fluctuate heavily Ans: b

87. An exciter for a turbo generator is a

(a) separately excited generator (b) shunt generator (c) series generator (d) compound generator Ans: b

88. In case of a flat compounded generator

(a) voltage generated is less than the rated voltage (b) generated voltage is proportional to the load on the generator (c) voltage remains constant irrespec¬tive of the load (d) speed varies in proportion to the load on the generator Ans: c

89. Which of the following generator will have negligible terminal voltage while running on no-load?

(a) Series generator (b) Shunt generator (c) Compound generator (d) Separately excited generator Ans: a

90. Which of the following D.C. generators will be in a position to build up without any residual magnetism in the poles ?

(a) Series generator (b) Shunt generator (c) Compound generator (d) None of the above Ans: d

91. In over compounded generator, full load terminal voltage is

(a) almost zero (b) less than no-load terminal voltage (c) more than no-load terminal voltage (d) equal to no-load terminal voltage Ans: c

92. In a level compounded D.C. generator, full load terminal voltage is

(a) negligibly low (b) equal to no-load terminal voltage (c) more than no-load terminal voltage (d) less than no-load terminal voltage Ans: b

93. The terminal voltage of a D.C. shunt generator drops on load because of all of the following reasons except

(a) armature reaction (b) armature resistance drop (c) field weakening due to armature reaction and armature (d) commutation Ans: d

94. In a D.C. generator

(a) external resistance = internal char-acteristic - armature reaction (b) internal characteristic = magnetisation characteristic - ohmic drop (c) external characteristic = magnetisation characteristic - ohmic drop - armature reaction (d) magnetisation characteristic = external characteristic Ans: c

95. A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armature voltage wave

(a) will be zero (b) will be of 5 Hz (c) willbeof5xiVHz (d) will be of v Hz 5 Ans: b

96. A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be

(a) zero (b) about 2 V (c) about 50 V (d) 220 V Ans: b

97. In a separately excited generator supplying rated load the armature reaction,

(a) is always present (b) is always absent (c) maybe sometimes present (d) none of the above Ans: a

98. If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be

(a) zero (b) small (c) the same as rated voltage (d) high Ans: a

99. Armature reaction in a generator result in

(a) demagnetization of leading pole tip and magnetization of trailing pole tip (b) demagnetization of trailing pole tip and magnetization of leading pole tip (c) demagnetizing the center of all poles (d) magnetizing the center of all poles Ans: a

100. Following energized winding of a D.C. machine should not be opened as it would produce a high inductive voltage which may be dangerous to personnel and may cause its own insulation failure.

(a) Series field (b) Compensating field (c) Inter pole field (d) Shunt field Ans: d

101. Wave winding is composed of

(a) any even number of conductors (b) any odd number of conductors (c) that even number which is exact multiple of poles + 2 (d) that even number which is exact multiple of poles Ans: c

102. The critical resistance of the D.C. generator is the resistance of

(a) field (b) brushes (c) armature (d) load Ans: a

103. When two D.C. series generators are running in parallel, an equalizer bar is used

(a) to increase the speed and hence generated e.m.f. (b) to increase the series flux (c) so that two similar machines will pass approximately equal currents to the load (d) to reduce the combined effect of armature reaction of both machines Ans: c

104. Which of the following generating machine will offer constant voltage on all loads?

(a) Self-excited generator (b) Separately excited generator (c) Level compounded generator. (d) All of the above Ans: c

105. Which of the following generators will be preferred if they are required to be run in parallel?

(a) Shunt generators (b) Series generators (c) Compound generators (d) None of the above Ans: a

106. Two generators are running in parallel. One of the generators may run as a motor for which of the following reasons?

(a) The direction of that generator is reversed (b) The speed of that generator is increased (c) The field of that generator is weakened (d) That generator takes a large share of loads Ans: d

107. A D.C. generator works on the principle of

(a) Lenz's law (b) Ohm's law (c) Faraday's law of electromagnetic induction (d) none of the above Ans: c

108. A series generator can self-excite

(a) only if the load current is zero (b) only if the load current is not zero (c) irrespective of the value of load current (d) none of the above Ans: b

109. A shunt generator can self-excite

(a) only if the resistance of the field circuit is less than critical value (b) only if the resistance of the field circuit is greater than critical value (c) irrespective of the value of the resistance in the field circuit Ans: a

110. The terminal voltage of a series generator is 150 V when the load current is 5 A. If the load current is increased to 10 A, the terminal voltage will be

(a) 150 V (b) less than 150 V (c) greater than 150 V (d) none of the above Ans: c

111. The open-circuit voltage of a compound generator is 250 V. At full load the terminal voltage

(a) will be less than 250 V (b) will always be 250 V (c) maybe greater or less than 250 V (d) none of the above Ans: c

112. Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohms run in parallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their bus bar voltage will be

(a) 270 V (b) 267.5 V (c) 265 V (d) 257.4 V Ans: b

113. The essential condition for parallel operation of two D.C. generators is that they have '

(a) same kW rating (b) the same operation r.p.m. (c) the same drooping voltage characteristics (d) same percentage regulation Ans: c

114. When two D.C. generators are running in parallel an equalizer bar is used

(a) to increase the series flux (b) to increase the generated e.m.f. (c) to reduce the combined effect of armature reaction of both the machines (d) so that the two identical machines will pass approximately equal currents to the load Ans: d

115. With a D.C. generator which of the following regulation is preferred?

(a) 100% regulation (b) infinite regulation (c) 50% regulation (d) 1% regulation Ans: d

116. Which generator would you prefer for feeding long D.C. transmission lines?

(a) Series generator (b) Shunt generator (c) Over compound generator (d) Flat compound generator Ans: c

117. In a D.C. generator, the critical resistance can be increased by

(a) increasing its field resistance (b) decreasing its field resistance (c) increasing its speed (d) decreasing its speed Ans: c

118. The number of armature parallel paths in a two-pole D.C. generator having duplex lap winding is

(a) 2 (b) 4 (c) 6 (d) 8 Ans: b

119. For both lap and wave windings, there are as many commutator bars as the number of

(a) slots (b) armature conductors (c) winding elements (d) poles Ans: c

120. The series field of a short-shunt D.C. generator is excited by

(a) external current
(b) armature current
(c) shunt current
(d) load current
Ans: d

121. As a result of armature reaction, the reduction in the total mutual air gap flux in a D.C. generator is approximately

(a) 40 percent
(b) 25 percent
(c) 10 percent
(d) 5 percent
Ans: d

122. Shunt generators are most suited for stable parallel operation because of their

(a) rising voltage characteristics
(b) identical voltage characteristics
(c) drooping voltage characteristics
(d) linear voltage characteristics
Ans: c

123. The main factor which leads to unstable parallel operation of flat and over compounded generators are

(a) their rising voltage characteristics
(b) unequal number of turns in their series field windings
(c) unequal speed regulation of their prime-movers
(d) unequal series field resistances
Ans: a

124. If a self-excited D.C. generator after being installed fails to build upon its first trial run, the first thing to do is to

(a) reverse the field connections
(b) increase the field resistance
(c) increase the speed of prime-mover
(d) check armature insulation resis¬tance
Ans: a

Introduction to Transformer

2021-06-24T05:43:00.007+05:30

Introduction to Transformer

Michael Faraday propounded the principle of electromagnetic induction in 1831. It states that a voltage appears across the terminals of an electric coil when the flux linked with the same changes. The magnitude of the induced voltage is proportional to the rate of change of the flux linkages. This finding forms the basis for many magneto-electric machines. The earliest use of this phenomenon was in the development of induction coils. These coils were used to generate high voltage pulses to ignite the explosive charges in the mines. As the d.c. the power system was in use at that time, very little of transformer principle was made use of. In the d.c. supply system the generating station and the load center have to be necessarily close to each other due to the requirement of economic transmission of power. Also the d.c. generators cannot be scaled up due to the limitations of the commutator. This made the world look for other efficient methods for bulk power generation and transmission. During the second half of the 19th century the alternators, transformers, and induction motors were invented. These machines work on alternating power supply. The role of the transformers became obvious. The transformer which consisted of two electric circuits linked by a common magnetic circuit helped the voltage and current levels to be changed keeping the power invariant. The efficiency of such conversion was extremely high. Thus one could choose a moderate voltage for the generation of a.c. power, a high voltage for the transmission of this power over long distances, and finally use a small and safe operating voltage at the user end. All these are made possible by transformers. The a.c. power systems thus got well established.

Transformers can link two or more electric circuits. In its simple form, two electric circuits can be linked by a magnetic circuit, one of the electric coils is used for the creation of a time-varying magnetic field. The second coil which is made to link this field has an induced voltage in the same. The magnitude of the induced emf is decided by the number of turns used in each coil. Thus the voltage level can be increased or decreased by changing the number of turns. This excitation winding is called a primary and the output winding is called a secondary. As a magnetic medium forms the link between the primary and the secondary windings there is no conductive connection between the two electric circuits. The transformer thus provides electric isolation between the two circuits. The frequency on the two sides will be the same. As there is no change in the nature of the power, the resulting machine is called a ‘transformer’ and not a ‘converter’. The electric power at one voltage/current level is only ‘transformed’ into electric power, at the same frequency, to another voltage/current level.

Even though most of the large-power transformers can be found in the power systems, the use of the transformers is not limited to the power systems. The use of the principle of transformers is universal. Transformers can be found operating in the frequency range starting from a few hertz going up to several megahertz. Power ratings vary from a few milliwatts to several hundreds of megawatts. The use of the transformers is so widespread that it is virtually impossible to think of a large power system without transformers. Demand for electric power generation doubles every decade in a developing country. For every MVA of generation, the installed capacity of transformers grows by about 7MVA. These figures show the indispensable nature of power transformers.

Bussed Architecture, Address Bus, Data Bus, Control Bus, Computer Language

2021-06-24T05:35:00.004+05:30

Bussed Architecture

The basic components of a microcomputer, as discussed earlier, are:

CPU
Program memory
Data memory
Output ports
Input ports
Clock generator.

The clock generator generates the appropriate clock pulses for the synchronized operation of different components of microcomputer. The clock generator is on-chip in Intel 8085A microprocessor. Now, the question comes, how the microprocessor is connected to other components - memory and I/O ports. One possibility is that all the memory chips and ports are connected separately to CPU as shown in fig.

In this case, large no of address lines, signal lines, input and output lines are required from the CPU. The size of the CPU increases much if all the components are to be simultaneously controlled. The capability of expanding the system by adding more components will be limited. If a CPU is fabricated on a chip such that it provides interface to N devices, adding one more device will be impossible. The system becomes too complex, therefore, bussed architecture is used to connect component to the microprocessor.

The Bussed Architecture for Microprocessor

The first question is what is a ‘Bus’? Bus is a group of parallel lines that connect two or more devices. It carries information in bits. Whenever processor (CPU) needs to access any memory or I/O device of the microcomputer system, it does so by setting up signals on the address bus to identify the appropriate circuit. Data may be transferred by means of data bus, in required direction between the device and the processor. Signals on the control bus serve a number of purposes such as control the transfer of data direction.

Consider a situation where there are N devices connected to one single data bus as shown in fig.1.3. Some of them are input and some are output devices. Let us consider, device ‘1’ wants to transfer data to device ‘2’ using this line. This transfer of data can be performed provided:

Device ‘1’ knows when to output data such that device ‘2’ is in a position to receive the data. This can be easily ensured if device ‘2’ has some means of signally device ‘1’ to output data.
Other than device ‘1’, no device outputs data on that signal line during this period. Device ‘1’ should be the only device driving the signal line at this time.
Other than device ‘2’, no device should accept the data from the data bus.

If the above conditions can be ensured, the same signal line can be shared by all N devices to transfer data between any two of the devices. The signal lines that are shared by a number of devices are referred to as the bus.

Normal gates are not suited for driving the bus in a bussed architecture. This is because such a gate will always be outputting either logic ‘1’ or logic ‘0’ it will not be possible to satisfy condition ‘b’. Therefore, tri-state buffers are used for driving a bus.

The Microcomputer Bus

The microcomputer contains three buses which carry all the address, data and control information involved in program execution. These buses connect the microprocessor to other elements - memory and I/O devices so that transfer of information between the microprocessor & any of the elements can take place.

Address Bus

In a microcomputer system, it is the CPU, which is the heart of the system, decides what action is to be taken in the system. Therefore, processor always selects the device for data transfer by putting the address of the device on the address bus. On address bus, information (address) flow takes place only in one direction, i.e., from the microprocessor to the memory or I/O devices. Therefore, this is called unidirectional address bus. The processor uses the address bus to identify an I/O device or memory. In the case of memory, this address also identifies the particular memory location inside the memory.

In 8085A processor, this bus is typically 16 bit long (A0 to A15). The CPU can generate 2 16 or 65,536 different addresses on this bus. A memory location or an I/O device can be represented by each one of these addresses.

When the microprocessor wants to transfer information between itself and a certain memory location on I/O devices, it generates the 16-bit address (in 8085A processor) from an internal register on its 16 address pins, which then appear on the address bus. These address bits are decoded to determine the desired memory location on I/O devices. The decoding process normally requires logic circuit (decoders) in the microcomputer system. The logic circuit decodes the address to decide which I/O device or memory location is required to be involved in any data transfer operation. If this decoding uniquely identifies only one port or memory location, then only data transfer takes place.

Data Bus

A set of data lines (8 in 8085A processor)) referred to as the data bus is shared by number of devices to transfer data between microprocessor and peripherals. Care must be taken that at a time only one device should output data on the data bus, the other devices which can output data meet be in high-Z condition. The data can flow in both directions, i.e., to or from the microprocessor. Therefore, this is called bidirectional data bus (BDB). In some microprocessors, the data pins are also used to send other information such as address bits in addition to data. This means that the data pins are time shared or multiplexed. In Intel 8085A microprocessor lower 8-bits of the address (A7-A0) are time-multiplexed with the 8-bit data (D7-D0) and, therefore, this bus is called AD bus (AD7-AD0).

Control Bus

The control bus is comprised of various single lines that carry control signals. These signals are used to synchronize the operation of the individual microcomputer elements. The microprocessor uses these signals for every operation it performs, like reading or writing a memory location or I/O device. These signals are also used to identify a memory location or an I/O device, e.g., RD̅̅̅̅̅, WR ̅̅̅̅̅, IO/M̅. Some of the signals of the control bus are issued by the processor and some of the signals are received by the processor. Therefore, the control bus is called bidirectional control bus (BCB). The difference between BDB and BCB is that in BDB all data lines are either in input mode or in output mode whereas in BCB the direction of signal flow on a line is fixed.

A microprocessor performs the function of the central processing unit. The microprocessor in combination with memory, I/O & a clock is essentially a microcomputer.

Computer Language

Each machine has its own set of instructions based on the design of its microprocessor. To communicate with the computer one must give instruction in binary language or machine language the form in which it is stored in memory, i.e, as patterns of 1s & 0s. Since it is difficult for most users to write programs in machine language, computers manufactured have developed English like words to represent the binary instructions of a microprocessor. e.g. ADD, SUB or JMP etc. Users can write programs, called assembly language ALU CPU ROM RAM I/P O/P AB BCB BDB Address Bus Data Bus Control Bus Control Unit Reg. Unit Clock programs (ALP), using these words called mnemonics. However, since the microprocessor can only executes the bit patterns of machine language instructions, the assembly language program must be converted to machine codes. This conversion can be carried out by hand, but this procedure is also time consumes and error prone. Special programs are available for each type of microprocessor that converts their assembly language programs to the equivalent machine codes. These programs are called assemblers and are run either on a microcomputer or minicomputer.

Because an assembly language is specific to a given machine, programs written in assembly language are not transferable from one machine to another. To circumvent this limitation, such general purpose languages as BASIC, FORTRAN, PASCAL, PL/M, C, have been devised, a program written in these languages are called high level languages (HLL). The programmes written in HLL are converted to machine language by another program called compiler or interpreter.

High–level languages do have same limitations in processor applications. The machine codes produced by compiler may be less efficient than that of the optimum equivalent ALP, increased memory requirement may not be important in view of cheap memory chips but increased executions time may be unacceptable in time critical applications. It is then desirable to write time critical parts of the program in assembly language. Further many peripheral device dependent operations may have to be programmed in ALP as such operation is often not supported by high level language.