Electrical Interview Questions with Answers

Electrical Machines Viva Questions and Answers: Part-1

2016-12-22T07:46:00.001-08:00

Electrical Machines Viva Questions and Answers

[1] How to reverse the direction of rotation of dc motor?
To reverse direction of rotation of dc motor, either direction of main field produced by the field winding is reversed or direction of the current passing through the armature is reversed.
By changing the direction of current passing through the field winding,the direction of the main field can be reversed.

[2] What are the functions of yoke? what is the choice material for the yoke?

It serves the purpose of outer most cover of the dc machine. so that the insulating material get protected from the harmful atmospheric environmental like noise, dust and various gases like SO2, acidic fumes etc.
It provides mechanical support to the poles.
It forms a part of the magnetic circuit.
It provides a path of low reluctance for magnetic flux.
It is prepared by using cast iron. For large machines, rolled steel, cast steel, silicon steel is used which provides high permeability.

[3] Why generators are operated in parallel?

The generators are operated in parallel to get,

Continuity and reliability of service without interruption
Maintenance and repair will be easy
Efficient operation of each generator
Increase in plant capacity

[4] Why pole shoe has been given a specific shape?

It is necessary that maximum area of the armature comes across the flux produced by the field winding.
Pole shoe enlarges the area of armature core to come across the flux, which is necessary to produce larger induced emf. To achieve this, pole shoe has been given a particular shape.

[5] What are the basic components required for generator action to exits?
A generating action requires following basic components to exits,

The conductor or a coil
The flux
The relative motion between conductor and flux

Thanks for reading...

Electrical Circuit Theorems for Circuit Analysis

2015-12-25T23:01:00.000-08:00

Electrical Circuit Theorems:

To solve problems in electrical networks, there are number circuit theorems have been developed like
(1) Kirchhoff's laws
(2) Superposition theorem
(3) Thevenin theorem
(4) Norton Theorem
(5) The maximum power transfer theorem

In this post we have listed all these theorems. We believe that this post will serve as a ready reference for you.

Kirchhoff's laws:
(a) Current Law:
At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction, i.e. I =0

(b) Voltage Law:
In any closed loop in a network, the algebraic sum of the voltage drops (i.e.products of current and resistance) taken around the loop is equal to the resultant e.m.f. acting in that loop.

Superposition Theorem:
In any network made up of linear resistances and containing more than one source of EMF, the resultant current flowing in any branch is the algebraic sum of the currents that would flow in that branch if each source was considered separately, all other sources being replaced at that time by their respective internal resistances.

Thevenin Theorem:
The current in any branch of a network is that which would result if an EMF. equal to the potential differential across a break made in the branch, were introduced into the branch, all other EMFs being removed and represented by the internal resistances of the sources.

Norton Theorem:
The current that flows in any branch of a network is the same as that which would flow in the branch if it were connected across a source of electrical energy, the short-circuit current of which is equal to the current that would flow in a short-circuit across the branch, and the internal resistance of which is equal to the resistance which appears across the open-circuited branch terminals.

The Maximum Power Transfer Theorem:
The power transferred from a supply source to a load is at its maximum when the resistance of the load is equal to the internal resistance of the source

Thanks for reading... Please leave your comments..

Power Electronics Interview Questions and Answers: Part-4

2015-04-01T09:30:00.001-07:00

Power Electronics Interview Questions with Answers:

Here, we had collected Various Power Electronics and Drives related Interview questions, Viva Questions. We are appending the new Q & A whenever we are getting from readers.
Some of the questions are asked to Experienced candidates based on their previous experience. If it is not related to your field, just ignore it.

[Q] What is AC voltage controller?
It is defined as the power electronics converter which converts the fixed AC voltage into a variable AC voltage without change in the frequency.

[Q] What is inverter? What are the types of Inverter?
A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.
Inverters are broadly classified into
Voltage Source Inverter (VSI)
Current Source Inverter (CSI)

[Q] What is chopper?
It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to get the variable dc output voltage from a constant dc input voltage.
Chopper basics, types and principle of operation is discussed in a separate page. Click here to know more about Choppers

Step down chopper:
In this circuit, the average output voltage [V_O = D . V_S] is less than the input supply voltage. It is also known as Buck converter.

Step up chopper:
It is also known as Boost converter. Here the average output voltage [V_O = V_S / 1 - D] is more than the input supply voltage.

[Q] What is load commutation?
In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

[Q] What is meant by pulse triggered devices?
As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard triggering.
Ex. Thyristor, GTO

[Q] What is level-sensitive devices? give some examples
Some of the level sensitive devices are
MOSFET
IGBT
MCT
IGCT
In order to keep these devices in the ON state condition, we have to apply gate current/voltage continuously to these devices.

[Q] What is meant by GTO?
It is a three terminal, four layer PNPN Power Semiconductor device that can be turned on by a positive gate current and can be turned off by a reverse gate current.

[Q] What are the types of GTO?

Asymmetrical GTO
Symmetrical GTO

[Q] What does 10V AC mean? Is it the RMS voltage or Peak voltage or Average voltage?
In general, AC voltages and currents are mentioned in RMS values only. It is sensible to compare with steady DC voltages and currents.
For example 230V AC supply means, 230Vrms AC Supply.

[Q] When we have to connect the SCRs in a parallel manner?
To meet the high current demand we will connect the SCRs in a parallel manner.
Similarly to meet the high voltage demand we have to connect the SCRs in series.

[Q] Define Gate Charge in MOSFET?
The gate charge is defined as the amount of charge required for the device during turn-on and turn-off time. It is the most important parameter we have to consider while selecting the MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver can charge or discharge the input gate charge.

More Power Electronics Interview Questions:
Power Electronics Interview Questions: Set-1
Power Electronics Technical Interview Questions

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Anna University Previous Year Question Papers for EEE 4th Sem Electrical Machines-I Paper

2014-12-14T09:16:00.001-08:00

Anna University Previous Year Question Papers For EEE 4th sem

[1] Define the term synchronous speed [Dec-2003]
For synchronous machines there exists a fixed relationship between number of poles P, frequency (f) and the speed of the machine. The speed of the synchronous machine for the given number of poles and the rated frequency is called the synchronous speed mentioned as NS.

[2] Write an expression for synchronous speed (Dec-2004)
An expression for synchronous speed is NS = 120f/P.
Where
f = frequency
P = No of poles of the machine.

[3] What does speed voltage mean? (Dec-2007)
When the magnetic flux is constant as well as stationary and the coil rotates to cut the flux then EMF gets induced due to relative speed between flux and coil. This EMF is called speed EMF, rotational EMF or dynamically induced EMF.

[4] Mention the factors on which hysteresis loss depends (Dec-2008)
The hysteresis loss is directly proportional to the area under the hysteresis curve ie area of the hysteresis loop.
It is directly proportional to frequency ie number of cycles of magnetization per second.
It is directly proportional to volume of the material.

[5] Distinguish between statically induced and dynamically induced EMF (Dec - 2010, 2011, 2009)
How is EMF induced dynamically ( May-2010)
An induced EMF which is due to physical movement of coil, conductor with respect to flux or movement of magnet with respect to stationary coil, conductor is called dynamically induced EMF or motional induced EMF.
The change in flux lines with respect to coil can be achieved without physically moving the coil or the magnet. Such induced emf in a coil which is without physical movement of coil or a magnet is called statically induced EMF.

[6] Define torque (May-2010)
A turning or a twisting force about an axis is called as torque.

[8] What are the three types of basic rotating electric machines? (May-2011)
DC machines
Induction Machines
Synchronous Machines

[9] What are the causes of core loss? what are the components of core loss?
When a core is subjected to an alternating flux then it undergoes the cycles of magnetisation and demagnetisation. This produces hysteresis effect which causes hysteresis loss in the core.
Similarly core is under the influence of the changing flux and under such condition according to the Faraday's law of electromagnetic induction, EMF gets induced in the core. Such currents in the core which are due to induced emf in the core are called as eddy current loss. Thus eddy current and hysteresis are the two components of the core loss.

Special motors (Permanent Magnet Synchronous Motor) Interview Questions

2014-12-01T09:16:00.001-08:00

Special Machines (Permanent Magnet Synchronous Motor) Interview Questions

[1] What is the PMSM?
In medium and small size synchronous motors, DC field can be produced by permanent magnets. Such kind of motors are called as PMSM

[2] What are the advantages and disadvantages of PMSM
Advantages

Elimination of field copper loss
Higher power density
Lower rotor inertia
Robust construction
Disadvantages

Power rating low
Motor power factor can not be controlled because of field excitation cannot be controlled

[3] What are the classification of PMSM
Based on construction
Surface mounted PMSM
Interior mounted PMSM
Based on the nature of voltage induced
Sinusoidally excited
Trapezoidally excited

[4] Give some applications of PMSM
Robots
Machine tools

[5] Define torque angle
The angle between excitation emf and supply voltage. This angle is called torque angle or load or power angle .

[6] What are the different classifications of synchronous motor?
(i) Wound field synchronous motor
Cylindrical rotor construction
Salient pole rotor construction
(ii) Permanent-Magnet Synchronous Motors
Surface mounted (Projecting Type and Inset Type)
Interior Buried Type
(iii) Synchronous Reluctance Motors
(iv) Hysteresis motors

[7] What are the different modes employed to achieve variable frequency control in synchronous motors?

Separate or true synchronous mode
Self controlled mode

[8] Why a Synchronous Motor without damper winding is used, in the drive system employing a Cycloconverter?
A Synchronous Motor without the damper winding is used, because the damper winding reduces the inductance of the machine, and therefore is ability to filter out harmonics in the output voltage of Cycloconverter. Since the drive operates in self-controlled mode, the damper winding is not needed for its converter.

Interview questions on more power electronics topics are covered in other posts... Please leave your comments below...

Electric Energy Generation Utilisation Interview Questions with Answers

2014-11-29T07:35:00.001-08:00

Electric Energy Generation Utilisation Interview Questions & Answers

If you are preparing for a technical interview for a lighting systems related products(LED, Lamps etc) company, then you have landed in a correct page.

This post will serve to refresh the Lighting concepts, illuminations, related laws and important terms.

Here we had collected some of frequently asked questions related to illumination.

In addition to that we had added Anna university previous years questions on EEE 8th sem paper Electric Energy Generation Utilisation [Paper code EE1451].

[1] Define luminous flux

It is defined as the total quantity of light energy emitted per second from a luminous body.

It is represented by symbol F and is measured in lumens.
The conception of luminous flux helps us to specify the output and efficiency of a given light source.

[2] What is meant by luminance?

It is defined as the luminous per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.

[3] What are the laws of illumination?

Law of Inverse squares

Illumination at a point is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.

If a source of light which emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere.
If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius.

Lamber's cosine law:

The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the normal to the surface at that point.

[4] What is meant by candle power?

It 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 CP

Cp = lumens /ω

[5] Define MHCP

The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power

[6] What are all the sources of light?

As per the principle of operation the light sources may be grouped as follows

Arc lamps
High temperature Lamps
Gaseous Discharge Lamps
Fluorescent type Lamps

[7] Define utilisation factor

It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp

utilisation factor = [Total lumens reaching the working plane] / [Total lumens given out by the lamp]

[8] Name the various photometer heads

Bunsen head or Grease spot photometer
Lummer-brodhun photometer head

There are two types of lummer broadhun heads

Equality oc Brightness type photometer head
Contrast type photometer head

[9] What is polar curve?

In most lamps or sources of light the luminous intensity is not the same in all directions.
If the luminous intensity, ie, the candle power is measured in a horizontal plane about a vertical axis and a curve is plotter between candle power and the angular position, a curve obtained is called as horizontal polar curve.
The luminous intensity in all the directions can be represented by polar curves.
If the luminous intensity in a vertical plane is plotted against position, a curve known as vertical polar curve.

[10] Define space-height ratio

It is defined as the ratio of horizontal distance between adjacent lamps and height of their mountings.

Space-height ratio = [Horizontal distance between two adjacent lamps] / [Mounting height of lamps above working plane]

Thanks for reading .... Please leave your comments below....

Anna University Previous Year Questions on Transformers

2014-10-18T19:20:00.000-07:00

Anna University Previous Year Questions on Transformers

In this post, we collected some of the questions on transformers (semester-III) asked in anna university previous year semester exams. We are sure that they will help you to prepare for your technical interview.

[1] Why transformers rated in KVA instead of KW?
The copper loss (I2R) in the transformer depends on the current I through the winding while the iron or core loss depends on the voltage V as frequency of operation is constant. None of these losses depend on the power factor (cosφ) of the load. Hence losses decide the temperature rise and hence the rating of the transformer. As losses depends on V and I only, the rating of the transformer is specified as a product of these two parameters VxI. Thus the transformer rating is in kVA and not in kW.

[2] Why is the efficiency of transformers more than that of other rotating machines?
There are no moving parts in transformer hence the friction and mechanical losses are absent in transformer. Hence efficiency of the transformer is more than of other rotating machines.

[3] Define leakage inductance
Inductance offered by the winding due to the leakage flux associated with it is called leakage inductance. It is the ratio of leakage flux linkages with the winding to the current passing through the winding.

[4] What material is used in the transformer core?
High grade silicon steel laminations are used for the construction of the core.

[5] What is meant by leakage flux?
Part of the primary flux as well as the secondary flux completes the path through air and links with the respecting winding only. Such a flux is called leakage flux. Leakage fluxes link with the respective windings only and not to both the windings.

[6] Define voltage regulation of a transformer?
The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load ie, rating load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

[7] What is an ideal transformer?
A transformer is said to be ideal if it satisfies following properties:

It has no losses
Its windings have zero resistance
Leakage flux is zero ie, 100% flux produced by primary links with the secondary
Permeability of core is so high that negligible current is required to establish the flux in it.

Alternators related Interview Questions with Answers

2014-06-23T10:10:00.001-07:00

Frequently Asked Interview Questions on Alternators
In this post we had given few Alternators related Interview questions with solutions. We believe that it will be helpful for your interview preparation. Please leave your comments in comments section.

[Q] What is meant by synchronising the alternators?
The process of connecting two or more alternators in parallel for supplying a common load is called synchronising.

[Q] What are the conditions to be satisfied for proper synchronising of alternators?
(i) The terminal voltage of the incoming alternator must be the same as bus-bar voltage.
(ii) The speed of the incoming alternator must be such that its frequency equals to the bus-bar frequency
(iii) The phase sequence of the incoming alternator must be the same as that of the other alternators or busbars.

[Q] What are the advantages of Parallel operation of alternators?
(i) It ensures continuity of power supply to consumers in case of breakdown of an alternator in an generation station.
(ii) This is economical and improves the efficiency of the generating station.
(iii) When the demand of power increases, new alternators can be installed to operate in parallel.
(iv) It is not possible to built single large size alternator to meet the requirement.

[Q] What are types of rotors used in alternators?
There are two types of rotors used in alternators namely
(i) Salient pole rotor
(ii) Smooth cylindrical type rotor

[Q] What are the advantages of providing damper winding?
The damper winding is useful in preventing the hunting ( momentary speed fluctuations) in generators.
The damper winding also used to maintain balanced 3 phase voltage under unbalanced load conditions.

[Q] What are the various methods to determine the voltage regulation of the large alternators?
In case of small machines, the regulation can be found by direct loading.
For large alternators, to find the voltage regulation indirect methods are used. They are
(i)Synchronous Impedance Method.
(ii) The Ampere-turn method.
(iii) Zero power factor or Pointer Method.

[Q] What is the basic principle of alternators?
Alternators is noting but a AC generators. They operate on the fundamental principle of electromagnetic induction as dc generators.
ie, when the rotor rotates, the emf is induced in the stator.

[Q] Which type of rotor is suitable for low speed alternators? salient pole type or cylindrical type rotor?
Salient pole type alternators are suitable for low and medium speed alternators. It has large number of projecting poles. It has large diameters and short axial lengths.

[Q] What is meant by turbo alternators?
High speed alternators are called as Turbo alternators.
As it runs at very high speed, salient pole rotors are not used. Smooth cylindrical type rotor is suitable for turbo alternators.

Please leave your comments below...That is highly appreciated...

Basic Electrical Engineering Interview Questions Answers 1

2014-06-15T02:41:00.001-07:00

Basic Electrical Engineering Interview Questions Answers 1

[Q] Define Power and Energy?
The rate at which work is done in an electric circuit is known as Electric power.
Power = Voltage X Current
The basic unit for electric power is Watts
The instrument used to measure the power is known as watt meter.

The total amount of work done in an electric circuit is called as electric energy.
Energy = Power X Time
the basic unit is Joule or watt-sec
The practical unit is Kilo watt-hour
The instrument used to measure the energy is known as Energy meter.

[ Like this you have to explain elaborately in the interviews. The interviewer will expect detailed answer from the candidate.
If he feels that you are deviating too much, then he will stop you and ask another question. Please don't answer in one line. Add related points
where ever possible]

[Q] In our house are we monitoring/measuring power or energy?
Energy. It is done by energy meter.

[Q] what is meant by cycle in ac waveform? what is period? Define frequency?
One complete set of change in value is known as waveform.
If you any basic question on AC waveform, then first draw an ac waveform and explain with reference to the waveform.

[Q] Explain about various AC Values?
Instantaneous value:
Instantaneous values are the values of the alternating quantities at any instant of time. They are represented by small letters like i, e etc.
Peak value:
The largest value reached in a half cycle is called the peak value or the maximum value or the amplitude of the waveform. Such values are represented by Vm, Im etc.
Peak-to-peak value is the difference between the maximum and minimum values in a cycle.
Average value:
The average or mean value of a symmetrical alternating quantity (such as a sine wave), is the average value measured over a half cycle (since over a complete cycle the average value is zero). For a sine wave, average value = 0.637 times maximum value
RMS Value:
The effective value of an alternating current is that current which will produce the same heating effect as an equivalent direct current. The effective value is called the root mean square (rms) value and whenever an alternating quantity is given, it is assumed to be the rms value.

[Q] What is form factor and peak factor?
The ratio of rms value to average value of an ac quantity is called form factor.
Form Factor = RMS value/ Average value
The ratio of maximum value to the RMS value of an ac quantity is called as peak factor.
Peak Factor = Maximum Value/RMS value

Induction Motor Interview Questions: Part 1

2014-06-04T10:05:00.002-07:00

Basic Interview Questions on Induction Motors:

[Q] How does the Induction motor work? (OR) Why does the Rotor rotate?

When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up.
The flux passes through the air-gap and sweeps past the rotor surface thus it cuts the rotor conductors.
Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday's laws of electromagnetic induction.
The frequency of the induced emf is the same as the supply frequency.
Its magnitude is proportional to the relative velocity between the flux and the conductors.
Its direction will be as per Fleming's right hand rule.
Since the rotor conductors form a closed circuit the rotor current is produced.
This current's direction will oppose the very cause producing it ( as per Len's law)
Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.
Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.

[Q] What is the general working principle of Induction motor?

The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor.
In DC motors the electrical power is conducted directly to the armature through brushes and commutator.
Thus the DC motor can be called as conduction motor. But in case of AC motors, the rotor receives electric power, not by conduction but by induction.
This is exactly in the same way as the secondary of two winding transformer receives its power from the primary.
That is why such motors are known as induction motors.
Thus an induction motor is also known as rotating transformer ( ie, one in which primary winding is stationary and the secondary is free to rotate)

[Q] What is the advantage of skewed stator slots in the rotor of Induction motors?

In the induction motor design, the rotor slots are purposely made with a slight skew arrangement. It will not be parallel to the shaft.

This is for the purpose of reducing magnetic locking or reducing magnetic attraction between stator and rotor teeth.

In addition to that this arrangement will help to reduce the magnetic hum and noise.

[Q] What is meant by cogging in the Induction motor? How to prevent the cogging?

When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to each other, since this corresponds to minimum reluctance position. In such case the rotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging.

This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

[Q] What are the various methods of measuring slip?

1. By actual measurement of rotor speed

2. By measurement of rotor frequency

3. Stroboscopic method

[Q] What are the various methods of speed control in three phase induction motors?

(i) Control from stator side

1. By changing the supply frequency

2. By changing the number of stator poles

3. By changing the supply voltage

(ii) Control from rotor side

1. By inserting resistance in rotor circuit

2. By various ways of cascade connection

3. By injecting EMFs in the rotor circuit.

[Q] What is meant by crawing in the induction motor?

In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.

Power Electronics Technical Interview Questions [Ametek Aerospace]

2013-08-24T08:33:00.000-07:00

Power Electronics Technical Interview Questions asked in Ametek Aerospace

One of our friend shared his interview experience. The position is for Senior Power Electronics Engineer with 5 year experience. We believe that it will be usefull for your interview preparation...

The following questins were asked in the interview...

[1] How to choose a micro controller?

[2] What is the difference between a LDO and a Linear regulator?

[3] What is the difference between I2C and SPI?

[4] What is the difference between logic analyzer and a oscilloscope?

[5] On what crises do we select components/Selection of a capacitor?

[6] Opamps/ADC/DAC how do you interface them externally /which ADC/DAC you have been used and why do you selected them/resolution ?

[7] What are usages and issues with J-Tag debugger?

[8] Any experience in Automation?

[9] Emi/Emc testing and how do you conduct them?

[10] What is the ESD protection how do you select any component to be ESD protected?

[11] Experience with other protocols Explain?

[12] Selection of capacitor, inductor?

[13] Frequency required and interfacing with other peripherals?

[14] Mixed signal interfacing circuits?

[15] Emc/Emi testing for immunity and radiation testing process and how do you conduct them and where it has been done in your product..

[16] FMEA analysis?

[17] Power supply design for single phase for Dc-Dc converter?

[18] DFT,DFM Analysis,reliability,RPN number preference

[19] Production manufacturing issues?

[20] Design process in HW/SW?

[21] How do you conduct design reviews?

[22] PCB manufacturing And placement analysis?

[23] How do you come to know that a micro controller is programmable and what are the issues that occurred while debugging?

Apart from the technical questions, our friend was faced with the usual interview questions like:

[1] Tell me about yourself?

[2] Carrier experience and Roles?

[3] Past and present experience?

[4] Kind of projects handled?

[5] Preferred area of working

[6] Why are you looking for a change?

[7] Willing to relocate to Bangalore?

[8] Roles and kind of job to be handled

About work experience and clients working for in the present organization

Roles and reporting personalities

Projects description.

Please share your interview experience with our readers....

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Please don't just read and go... Please leave your comments...Your comments are highly appreciated...

TCS Campus Interview Questions Answers

2012-11-03T18:30:00.000-07:00

These questions were asked in TCS campus interview at Govt. College of Engineering, Tirunelveli.

1. Tell me about yourself

2. About TCS

3. As a electrical engineer, why software company

4. Draw AND gate

5. what is difference between latch and flipflop

A flip-flop continuously checks its inputs and correspondingly changes its output only at times determined by clocking signal.

A latch is a device which continuously checks all its input and correspondingly changes its output, independent of the determined by clocking signal.

6. Types of flipflop:

There are basically four main types of flip-flops: SR, D, JK, and T.

7. Do you know C

8. Are you willing to travel for onsite

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Thanks for reading. Please leave your comments below. Please share your interview experiences. We will post it with answers which will be useful for our readers.

Goodrich Interview: Exp.-2010

2012-11-03T16:27:00.000-07:00

These questions are asked in the technical interview conducted in Goodrich, Bangalore for the post of Design Engineer with 2 year experience. As it is for the experienced engineers, we would have to explain as much as possible about the topic. We did not answer the non technical questions here.

A1: Question about previous exp.

A2: Question about currently working project.

[1] Explain the amplifier operation of OP-AMP?
The golden rules of OP-AMP are:

1. Vout = A. Delta Vin [ A = Gain, Delta Vin = V+ - V-]

2. V-- < Vout < V++

3. Zout = 0 , Zin+ = Zin- = Infinity

The opamp will work as inverting amplifier and non- inverting amplifier based on the supply applied to its terminals.

Non Inverting Amplifier:

Inverting Amplifier:

[2]Interrupted and Asked to derive the equation...

Current i = (Vin-Vout) / (Rin+Rf)

=( Vin - Vx)/Rin = (Vout - Vx)/Rf

Vx = virtual ground =the node at inverting input terminal =0

[3] Interrupted and asked what is virtual ground?

Explained.

then i = Vin/Rin - Vx/Rin = Vout/Rf- Vout/Rf

Rearranging we will get the final equation.

[4] Explain how transistor acts as a switch?

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region.

This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its "fully-OFF" (cut-off) and "fully-ON" (saturation) regions as shown below.

A3: Why do you want such a high salary which is well above the market average?

......

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Thanks for reading. Please leave your comments below. Please share your interview experiences. We will post it with answers which will be useful for our readers.

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Off Campus Interview for a Fresher

2012-11-03T16:24:00.000-07:00

These questions are given by one of our reader. He shared his experience. Please share yours...It is more informative if you mention the company name etc....

What is the Scope of Electrical Engineering?

[ This question is mostly asked to check your communication. Speak casually. Tell the importance of electrical and electronics engineering course, what are the topics to be covered in the course, what are the job prospects etc.]

Answer: Electrical Engineering has a very broad scope with the science and technology which involves electricity, electronics, and electromagnetism in order to design, construct, and maintain products, services, and information systems.

Electrical Engineering is a branch that deals with the field associated with power transmission and motor control.

Electrical engineering is one of the core fields of engineering, so it is always evergreen in terms of job prospects.

Electrical Engineering is the heart of whole engineering discipline.

Nowadays, we see Electronics playing an important role in the implementation and control of Electrical Machines. Keeping this fact file in mind the course have transformed from core electrical Engineering into a Fusion of Electronics and Electrical and has been renamed as Electronics and Electrical engineering.

Electricity has become the basic need for survival, globally. From household to industrial plants, communication and satellite navigation system, electronic equipment, computers etc., all require electricity.

Electrical engineering thus deals with study and application of electrical systems for use in these different environments. It equips you with the knowledge of transmission and generation of electrical power, electrical circuit design, electronics, instrumentation, control system, understanding electrical and electronic networks etc.

The course also covers the study of electronic devices and circuits involved in measurement, instrumentation, control and protection of electrical equipments and conversion systems. Concept of computer and recent applications of computer based systems in design, analysis and efficient operation of power system, maintaining quality and security, also included in the course.

What does '230V AC Supply' really mean? Is it the RMS or peak voltage?

If the peak value is meant it should be clearly stated, otherwise assume it is the RMS value. The supply is 230V AC means 230V RMS, so the peak voltage of the mains is about 320V.

What does the multimeter in volt meter mode show? Is it the RMS or peak voltage?

If the multimeter is in voltmeter mode and when it connects to AC circuits it shows the RMS value of the voltage or current. When it connects to varying DC signals it will show RMS value only.

How does Induction Motor work?

Induction motors are also known as rotating transformer.( Primary winding is stator and secondary winding is rotor).

When the 3 phase stator windings are fed by 3 phase supply a magnetic flux of constant magnitude, rotating at synchronous speed is generated. These flux passes through the air gap, sweeps past the rotor surface, cuts stationary rotor conductors.

According to Faraday's laws of electro-magnetic induction, an emf is induced in the rotor. This emf frequency is same as supply frequency.

As rotor conductors form a closed circuit, the induced emf produces a current. It's direction will oppose the cause producing it ( according to lenz's law).

In this case, the cause is relative velocity between the rotating flux of the stator and stationary rotor conductors. So to reduce the relative speed, the rotor starts to run in the same direction of the stator flux.

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Power Electronics Graduate Trainee Interview [BGR Energy]

2012-11-03T16:15:00.000-07:00

These questions were asked in Power Electronics Graduate Trainee Interview conducted by BGR Energy Systems Company at MIT Chennai.

[1] Draw and Explain the SCR V-I characteristic?

Vak = +ve & Vg = 0
(i) When a positive voltage is applied to anode with respect to cathode, the junctions J1 and J3 are forward biased, but the junction J2 is reverse biased.

(ii) The SCR is in its forward blocking state. At this time the Gate signal is not applied.

(iii) As shown in figure a small amount of forward leakage current is flows through the device.

Vak = + & Vg = +ve
(i) When the small amount of positive voltage is applied, while positive voltage is applied to anode with respect to cathode, the junction J3 becomes forward biased.

(ii) Thus the SCR conducts a large value of forward current with small voltage drop. With the application of gate signal the SCR changed from forward blocking state to forward conducting state. It is called as latching.Without gate signal it happens at forward breakdown voltage (Vfbd).

(iii) When the gate signal value is increased, the latching happens for low Vak voltages as mentioned in the figure. In the presence of forward current (i.e. after the thyristor is turned on by a suitable gate voltage) it will not turn off even after the gate voltage has been removed.

(iv) The thyristor will only turn off when the forward current drops below holding current. The holding current is defined as the minimum current required to hold the SCR in the forward conduction state.

Vak = -ve

(i) When a negative voltage is applied to anode with respect to cathode, the junctions J1 and J3 are reversed biased, but the junction J2 is forward biased. The SCR is in its reverse blocking state.

(ii) As shown in figure a small amount of reverse leakage current flows through the device.

If the applied voltage is more than reverse breakdown voltage, the device will collapse and the large amount of current flows through it.

[2] Explain Buck topology

(i) When the switch is ON condition (ie, T_ON), the input provides energy to inductor and output.
(ii) The difference between the input and output voltage is applied to inductor as shown in the below waveform.
(iii) When the switch is turned off, the inductor current will flow in the same direction to load. It is inductor property to maintain the current flow in the same direction.
(iv) The freewheeling diode D1 is forward biased at this time and completes the inductor current path.

The output of the buck converter with continuous output current is

Vout = D*V_IN

D = Duty cycle = T_ON / [T_ON + T_OFF]

[3] Compare MOSFET and BJT

BJT MOSFET

More Power handling Capability Less Power handling capability

Low switching speed Fast switching speed

High On state resistance Low on state resistance

Has second breakdown voltage problem No second breakdown voltage problem