Basic Electronics

The Audio-frequency Power Amplifier

2012-10-20T05:30:00.001-07:00

Introduction to Audio Power Amplifier

In this tutorial we shall study vacuum-tube amplifier circuits which give a large output voltage of substantially the same waveform as that of the input voltage, accompanied by a large curr ent of like waveform, so that there is considerable power delivered to the load, rfhe operation really is a power-conversion process, as in most cases all of the output power derives from the plate d-c source, under the control of the signal voltage applied to the grid. In general the treatment can often be made similar to that used for voltage amplifiers, except that usually the operation is not linear and therefore graphical or experimental procedures give most accurate results.

Except for details, a power tube resembles a voltage-amplifier tube. Power tubes vary in size depending upon the amount of output power desired and the efficiency of operation. For a small amount of output power a voltage tube may be used. The tubes with the higher power ratings need larger anodes to dissipate by radiation the heat derived from the energy of the impinging electrons, without making too high a temp erature necessary. For tubes of very high power rating the heat may be carried away by convection by an air or a water stream. A second requirement for a power tube is a sufficiently large cathode to furnish the necessary electron emission. This also means that a relatively large amount of heating power will be needed. Small power tubes use oxide- coated cathodes, while those of medium power employ thoriated tungsten, which will withstand somewhat more ionic bombardment. High-power tubes with their very high voltage drops use pure-tungsten filamentary cathodes. Tubes of small and moderate ratings may be triodes, tetrodes, or pentodes, but for the very high power ratings there is a problem of cooli ng the screen grid of the latter two kinds, which makes the triode usually preferred.

In making a choice of tube, the class of operation, which is related to the efficiency, must be taken into account. If the very best quality of output is required, as in some measurements circuits, it is well to use class A1 operation as this renders the output most free of distortion. In this connection it may also be advisable to choose a tube of higher power rating than actually required, so that the grid drive may be made small compared with the maximum allowable and thus decrease the distortion.

Feedback Amplifiers

2012-10-20T05:17:00.000-07:00

Distortion and Stability in output

It is well known that the output of an amplifier does not have a voltage waveform exactly a replica of the input signal. Because of the presence of inductance and capacitance in the tubes and circuits, the various frequency components of the signal are affected by different amplifications and phase shifts. Also because of nonlinear action of the tube itself, the output voltage is not exactly prop optional to the input voltage. Again, if one or more of the bias voltages E0, Eb0, etc., varies,

the operating point shifts and the resulting changes in , r, and Urn affect the amplification. The latter effect is sometimes useful as a volume control, or it may be the basis for a method of changing the waveform of the output in an easily controlled manner. The latter process, called modulation, sometimes takes place when not wanted, as, f or example, when a poorly filtered plate-supply voltage is used. Whether this occurs in an a-f amplifier or in an r-f amplifier, the result may be noticeable in a loudspeaker as hum. The change in gain with change in operating point may be very objectionable for another reason:

If a battery-operated amplifier has a certain normal gain, this gain may increase upon renewal or recharge of the batteries. This may not seem serious at first thought, but suppose we consider a long-distance telephone system. The energy given by the microphone alone may be sufficient for satisfactory local operation, but it normally needs building up by repeater (amplifier) units two or more times every 100 miles of line. Since these repeaters are distant from one another, they operate independently and a rise in gain for one is not automatically compensated for by a lower gain in the next. Hence a distant repeater may find itself with a signal far too weak or maybe too strong. What is needed for each repeater is a stabilizer which will make the amplification independent of small changes in operating biases.

Interference in Feedback Amplifier Output

In addition to the distortion and modulation products discussed in the preceding article, the output of an amplifier contains interference and noise components which have frequencies that are unrelated to the input signal. Plate-supply hum in a-f amplifiers may be considered as interference. This hum may be removed by more comp lete filtering, or it may be eliminated by the feedback processes discussed in this chapter. Other interference sources will now be considered.

Meachom Bridge Oscillator

2012-10-20T04:14:00.001-07:00

Meachom Bridge Oscillator with circuit

The Meacham bridge oscillator, illustrated in figure 1460, provides the greatest frequency stability of any vacuum-tube oscillator yet devised, but the region of maximum frequency stability is limited to the lower frequencies because of the increased effect of the 8cray circuit capacitances when the frequency becomes greater than a few hundred kilocycles per second. The oscillator is of the crystal-stabilized type employing tuned circuits. At frequencies above 1000 kv the effect of the stray capacitance is sufficient to reduce the stability to a point where little is to be gained by the use of the Meacham
circuit. The oscillator is principally employed with GT-cut crystals in frequency standards, to generate frequencies of 100 kc. In figure 1-160, it can be seen that if the bridge were perfectly balanced there would be no excitation voltage. At the start of oscillations, the ratio of R1 (practically equal to the series-arm R of the crystal unit) to R2 is smaller than the ratio of the R3 to R4. But R4 is a thermistor it is the resistance of a tungsten lamp which sharply increases in value as the temperature rises. (A semiconductor such as carbon, silicon, or german ium can be used, in which case the resistnce will decrease with temperature. The negative tempera ture coefficient of the semiconductor is generally larger than the positive coefficient of tungsten, but the semiconductor thermistor is more expensive and is much more difficult to duplicate because of its great sensitivity to impurities.) As oscillations build up, the current through R4 increases to a point where the heat generated from the power losses raises the the thermistor temperature, and hence the resistance, to a point where the bridge is almost balanced. Equilibrium is reached when the imbalance of the bridge is just sufficient to supp ly heat to the thermistor at the same rate at which it escapes. For maximum amplitude stabili ty, the mbient temperature of R4 should not be permitted to vary over a wide range. Normally, the tungsten lamp will heat to a dull red of approxim ately 600 degrees centigrade. A variation of over 100 degrees in the ambient temperature could have a significant effect on the equilibrium power losses in the thermistor, if extreme precision were des ired. The operating temperature of the lamp is very low compared with the rated temperature, and consequently the lamp can be expected to last indefinitely. The oscillator should be designed and adjusted so that the phase shift occurs entirely in the bridge. That is, the tube should operate into a pure resistance,

so that the instant the plate current is maximum the peak transformer voltages should occur with polarities as indicated in figure 1-160. Tran8forme rs having powdered-i ron, toroidal cores can provide a coefficient of coupling very close to unity in the low-frequency range. The following analysis of the frequency stability and the activity stability of the Meacham oscillator, except for minor deviations and extensions, has been guided by the postulates and basic considerations as presented by W. A. Edson.*

Crystal Oscillators

2012-10-20T04:09:00.000-07:00

Oscillators with Crystals Having Two Sets of Electrodes

The original crystal oscillator devised by Dr. Nicolson, as well as a number of the earlier crystal oscillators tested by Dr. Cady, employed crystals with, effectively, two pairs of electrodes. The basic circuit is shown in figure 1-156. The re quired phase inversion of, the amplifier output voltage is provided by the crystal unit operating at a mode for which the polarities of the plate and grid terminals with respect to ground are 180 degrees out of phase. The circuit shown operates the crystal unit very

near its series-resonance frequency. In practice, a capacitor is normally connected between crystal and ground, so that the circuit is more commonly employed for parallel- mode tested crystal units. Still, it is not without some JiCefl8e that we classify this type of oscillator as a parallel-mode type. The crystals most applic able for this class of circuit are the very-low- frequency elements of the X group, which vibrate in lengthwise extensional or flexural modes. The electrode connections that permit the desired phase inversion depend upon the particular crystal element. Assume that electrodes numbers I and 3 are on one side of the crystal, and that 2 and 4 are on the opposite side, as indicated in figure 1-156 (A). For a flexure element, such as element N, where electrodes 1 and 3 parallel each other down the length of the crystal, as shown in figure 1456 (B), the flexure mode is excited when the potential across 1 and 2 is oppositely polarized to that across 3 and 4. If the same electrode arrangement is to be used to excite an extentional mode (or the flexural mode of the duplex element J) the polarities of the two sets of electrodes must be in phase. In this case, the connections of one set of electrodes should be reversed in the circuit shown in figure 1456(A). For example, plates 2 and 3 should be connected to ground and plate 4 should be connected to the grid, if the proper pha8e inversion is to be obtained. A crystal having the two sets of electrodes at opposite ends of the crys tal, as shown in figure 1-156(C), would be driven at the second harmonic of the length extensional mode (or of the fiexural mode of a duplex ta, if connected as shown in figure 1-156(A). Greater stability and a smaller crystal are possible for a given frequency by operating at the fundamental mode. To permit this, if the crystal unit is plated as shown in figure 1456(C), the connections of one pair of electrodes should be the reverse of those shown in figure 1-156(A). If it can be assumed that the current in the grid circuit is negligible compared with the crystal current between term inals I and 2, and if the stray capacitance bet ween the two sets of electrodes is ignored, the equivalent circuit between terminals 1 and 2 will appear approximately as shown in figure 1-156 (D). L, C, and C0 represent the parameters of a fully plated crystal. A more exact analysis of this type of crystal unit can be found in the book “Electromechanical Transducers and Wave Filt ers” by W. P. Mason, D. Van Nostrand Co.

Figure 1-157 shows a practical oscillator design employing crystal units having two sets of electrodes. Although the electrode connections shown for CR would indicate that the plate and

Miller Oscillator with Cricuit

2012-10-20T04:01:00.000-07:00

1428. The Miller oscillator is the crystal equivalent of a Hartley oscillator in which no mutual inductance exists between the plate-to cathode and gridt o.cithode inductances. ( See figure 1-140.) The Miller oscillator has an average frequency deviation of approximately 1.5 times that of the Pierce circuit. The plate circuit must appear inductive in order that the correct phase shift will be produced in E, the plate r-f voltage, to compensate for the

resistance in the feed-back arm, since this resist ance prevents the necessary 180.-degree phse rotation of the equivalent generator voltage of the amplifier from occurring entirely in the feedback circuit. The effective load capacitance into which the crystal unit operates is, approximately,

• —

where Z1)g and Z, are both considered as unsigned magnitudes, and the various symbols correspond to those in figure 1-140. Since the load capacitance is a function of the frequency, a Miller oscillator cannot be operated at more than one frequency and still present the same load capacitance to each crystal unit except by providing for an adjustment of the circuit parameters. In spite of its greater frequency instability and lack of circuit simplicity as compared with the Pierce circuit, the Miller design is the one most widely used in crystal oscill ators. The reason for this popularity is the greater output that can be obtained for the same crystal drive level. En either the Pierce or the Miller basic circuit, the output cnnot exceed the voltage across Zpg, the largest single impedance in the plate tank circuit. In the Pierce circuit, the maximum voltage is thus the maximum permissible across the crystal unit; in the Miller circuit the maximum voltage is (k + 1) times the maximum permissible vo1tige across the crystal unit, where k is the gain of the stage, equal to Ep/Eg. This gain, theoretically (not practically), can approach the mu of the tube as a limit when the load impedance, ZL, is large comp ared with R. Thus, the use of a Miller circuit permits a saving of one amplifier stage.

1-329. The feed-back capacitance of the Miller circ uit is, normally, simply the plate-to-grid inter- electrode capacitance of the tube. It cannot, of course, be less than this unless an inductive shunt is connected between the plate and grid. When a pentode is used, it is usually necessary to insert a small feed-back capacitance on the order of a few micromicrofarads. The waveform in the output is improved by the use of a tuned tank circuit having a low L/C ratio in place of L. The plate tank must be tuned to a frequency above the oscillator freq uency, in order that the tank impedance will app ear inductive. Such an arrangement also ensures a large effective L of high Q. A vgiriable capaci tance in the plate tank facilitates adjustments to obtain the correct load capacitance for the crystal unit.

Principles of Rectification

2012-10-20T03:50:00.001-07:00

BasicPrinciple of Rectification

Rectifiers change ac into pulsating dc by eliminating the nega Live half’cycles or alternating the ac voltage. Only a series of sinewave pulsations of positive polarity remain. An ideal rectifier may be thought of as a switch that closes a load circuit whenever the polarity of the alternating voltage is positive, and opens the circuit whenever the alternating voltage is of negative polarity. A switch
operating with such synchronism would have effectively zero resistance for that half of the time when the circuit is closed during positive half-cycles and infinite resistance for the other

half of the time when the circuit is open cycles. Practical rectifiers do not attain this to it. Some vacuum tube rectifiers may resistance during the nonconducting inte but the resistance during the conducting during negative half goal, but come close have almost infinite (back resistance),

interval (forward resistance) is never zero or even constant. Regardless of their app roach to the ideal, all rectifiers must provide a oneway path for electric current; that is, conduction must take place primarily in one direction only. This is called unilateral conduction, or a unidirectional characteristic.

Basic Electronic Components

2012-03-09T05:28:00.003-08:00

The point of this tutorial is to provide the reader with a essential concept of the physical structure of individual electronic components.

Resistors

A resistor, as the name tells us, is electronic device resists (or opposes) current flow in a circuit.
The characteristic of opposing current flow can be applied for many purposes. Hence, resistors are on of the most common discrete components found in electronic equipment (the word discrete is used in electronics to notify non-integrated, or standing alone device). There are two major categories of Resistors:fixed and adjustable.

Fixed resistors are by far the most common form of resistors.

Adjustable resistors are called potentiometers or they are also known as rheostats.

Capacitors

Next to resistors, we talk about capacitors, these are probably the most common component in electronics field. Capacitors are manufactured in a large range variety of shapes and sizes. In most, they are either flat, disk-shaped components, or sometimes they are tubular in shape.

Capacitors differ in size from almost microscopic to about the same diameter, and twice the height, of a big coffee cup. Capacitors are famous two-lead devices, sometimes the capacitors resembls with a fixed resistors which is without the color bands.

Inductors

Inductors are studied in two main classifications:one is transformers and other is chokes (or coils). Transformers are designed for power conversion applications, referred to as power transformers or filament transformers, and these are usually heavier, blockshaped components consisting of two or more than two coils of wire that arewound around stiff rectangular wafers of iron.

Diodes

Next to transforemers are Diodes, aDiodes are two-lead semiconductor devices with tubular bodies that are similar to fixed resistors. Diodes bodies are usually black or clear, having a single-colored band close to one end. Diode bridges are actually four diodes that are encased in square or round housings. They contains four leads, or connection terminals, two of which will be denoted with a horizontal S symbol (sine-wave symbol), one will be marked “_” and the other will be“_.”

Transistors

Transistors are an important electronic component that arethree-lead semiconductor devices manufactured in a wide range of shapes and sizes to accommodate such design parameters as power dissipation, breakdown voltages, and cost-benefit. Some power transistors have an oval-shaped body with containing 2 mounting holes on either side (this is commonly called as TO-3 or TO-5 case styleWhen you see first, it appears that it has have two leads, but, in fact, the body itself is the third (collector) lead.

The Hidden Cameras for Security

2011-12-25T05:59:00.000-08:00

The CCTV digital camera, hidden cam, spy digital camera, secret digital camera or house movie monitoring digital camera is a digital camera that is hidden as another item. These little digital cameras have remarkable quality, yet are little enough to cover up in a radio, smoke detectors, reflection or many other familiar things that would be in a room. Hidden Cameras can be used not only in the property, but over the counter or industrially as video digital cameras.

Today a hidden cameras can record movie for watching later or connected to the world wide web for watching from anywhere in the world at real-time. To get connected to the world wide web you simply need a little system called a USB DVR - Digital Video Recording unit. This system joins directly to your digital camera and your computer to make your movie available online. An IP Camera does not require a USB DVR, but is hard to cover because of its size.

Frequency Domain Curve-Fitting FDCF

2011-12-21T03:01:00.000-08:00

Frequency domain curve-fitting is useful technique to fit a TFM very close into the observed FRF data. FDCF is a two-step procedure, which includes the model structure selection and model parameter optimization.

The first step is to parameterize the TFM in some special forms. Two forms of parameterizing are introduced in the following: the matrix fraction (MF) parameterization and the polynomial matrix (PM) parameterization. Parameterizing is always a critical step in the identification due to the reason that it will generally lead to quite different parameter optimization algorithms, in addition to this, it results in model properties.

This tutorial clarify that for the MF form, the parameters can be optimized by the help of linear least squares (LLS) solutions. As for the PM parameterization, you have alternative to some nonlinear techniques; specifically. The PM parameterization offers more flexibility in the context that it allows the designer to specify certain properties of the identified model (e.g., allows the designer to fix zeros in any input/output channels).

Direct Current Density

2011-12-21T02:56:00.000-08:00

Current is known as the flow of charge. By inheritance, the direction of this flow is with the movement of positive charge. The amount of charge δQ flowing through (that is, perpendicular) to a surface, in time δt is given by the formula

δQ = Iδt, where I stands for the current.

In the limit of infinite simally small time increments, the current I through the surface can be defined as:

Magneto-statics is a field theory and, consequently, the quantities of interest are usually distributed throughout space. As such, the volume current density, j(x, y, z) is frequently employed. If we want current density in terms of the charge carriers, the current density is given by the formula;

where N stands for the number of charge carriers per unit volume (i.e., it is the number density), q is the charge, and v is the average (i.e., drift) velocity of the electrons.

In addition to this, a current density through an open surface “S” is related to the current with the help of the formula:

Structure and Working Liquid-Filled Transformers

2011-12-21T02:50:00.000-08:00

A liquid-immersed transformer is composed of a magnetic core and coil assembly immersed in a fluid, which is normally a mineral oil. The fluid must have firslty good heat transfer characteristics and secondly electrical insulating characteristics.
An benefit of a using liquid-immersed transformer is that it allows us compact design. In normal practice, transformers that are above 10 MVA and/or 34.5 kV are liquid-filled. Since oils are signifcantly flammable, the liquid-immersed transformers must be installed with adequate care and precautions according to the flammability of oil. This aspect of liquid filled transformers has put forward challenges for indoor applications.

Mineral oil used in transformer insulation systems degrades if it exposed to oxygen for a long period of time. In addition to this, mineral oil has a fairly large thermal expansion co-efficient.

When talking about the disadvantages liquid transformer, several systems are applied for the purpose of minimizing degradation. Liquid-filled transformers normally protect insulating oil with one of 3 types of preservation systems:

· sealed tank system,

· nitrogen blanked, or

· conservator.

Power Systems Steady-State Analysis

2011-12-21T02:48:00.000-08:00

In steady-state analysis of power systems, any transients from disturbances are considered to have settled down, and the system state is considered as unchanging. Particularly, system loads including transmission system losses, are closely matched with power generation so that the system frequency is becomes constant.

The primary concern during steady-state-analysis may be is economic operation of the system; reliability is also an important factor as the system must be operated to avoid out as disturbances occur. The primary analysis tool for steady-state operation in power systems is the so-called power flow analysis, in this analysis the voltages and power flow through the system are figured out. This important analysis is widely used for the operation and feasibility studies and it is also helpful throughout the system at both the high & low transmission voltages.

There are 3 mjor componets of power systems sub components. these are generation, transmission and distribution, and load. The transmission and distribution are comprised of

· power transformers,

· transmission lines,

· capacitors,

· reactors,and

· protection devices.

The most of the generation is produced by the help of synchronous generators. Where as the Loads consist of a great number and a diverse assortment of devices, for the supply to home appliances and providing lighting to heavy industrial equipment and sophisticated and sensitive electronic units. As such, modeling the overall effect is a challenging aspect in power system analysis.

Description and Use of Transformers

2011-12-21T02:46:00.000-08:00

A transformer is a static device and it is commonly used all over the world for transforming electric energy from one circuit to another magnetically (that is, by induction rather than by way of conduction). usually, this transformation is accomplished b/w circuits of different voltages but having same frequency

. A power transformer will have a magnetic core surrounded by two or more windings. These windings carefully insulated from each other & from the ground. The windings are linked together in a manner to achieve the requires voltage transformation ratio. The overall assembly of core and coils is generally insulated and it is cooled by immersion in mineral oil or other suitable liquid that is stored in an enclosing tank. Connection to the windings is done usually by using the insulated bushings.

In a todays utility, electrical energy may subject to 4or 5 voltage transformations b/w the generation site and to the point where utilization takes place. As such, a given system possesses approximately 5times the kilovoltamperes [kVA] of transformers in comparison to that of generators.

State-Space System Realization SSSR

2011-12-21T02:44:00.000-08:00

System realization is a method to find out an internal state-space description for a system given with an external description, normally its TFM or impulse response.

The name clearly reflects the reality that if a state-space description is available, an electronic circuit can be constructed in a clear-cut manner to realize the system response. There is a great deal of literature discussing on this subject both from a system theoretical point of view and from a practical system identification point of view.

A well-developed method in the second category, the eigensystem realization algorithm (ERA), is chosen to desing and make a model in the state space form. Below presented are the formulas that are applied to generate the Markov parameters from the TFM, which are the starting point for the ERA procedure.

Objectives of Power System and Operation and Control

2011-12-21T01:54:00.000-08:00

The main objective of power system operation is providing power to consumers meeting strict tolerances on voltage magnitude and frequency. consequently,
the operation control problems logically segregate into the control of voltage magnitudes or the voltage control issues and the system frequency control or the problems related to frequency control. As a power system is an interconnected system, large system spread over a galobally wide network, operation of the large system is complex. The controls are built to exploit the inherent timescale and structural properties of the system.

For your information, the automatic frequency control installed in the North American electric power grid was the first instance of a successful performance of a large-scale network-based control scheme. The frequency control systems face two subproblems. First, cost- benefit balance; there is need to determine optimal values of generations that results into minimization of the total generation costs while meeting the load demands. This problem is referred to as the economic dispatch problem.

The gap between total active power generation and the total active power consumption lead to frequency drifting. Because the load fluctuations themselves are random, it is almost impossible to 100 percent match the total supply with the power consumption demands at all times. Therefore, the system frequency will have a propensity to drift around on its own.

In North American grids, there locates a central closed-loop controller that maintains system-wide power flows and regulate frequency to maintain the system within tight tolerance levels. It is also responsible for maintaining economically dispatched generations.

Equivalent distributed Circuit

2011-12-20T01:50:00.000-08:00

We can easily symbolized TEM mode in a transmission line by the equivalent circuit, shown in dia4.7 below, for a length Az. Physically, it is said to be distributed circuit because the circuit parts are not lumped at separate locations as in a conventional low-frequency circuit rather they are distributed uniformly along the length of the line. Thus, we can define the elements on per-unit-length basis.

The series resistance per unit length, R, is the resultant resistance of all the conductors connected in the line,& it accounts for power indulgence in the conductors. As current flow along the conductor's surface is accompanied by a circumferential magnetic field.

The connection between magnetic flux with the current produces a combined inductance per unit length, L, which takes in into account the flux linkages both internal and external to the conductors. Positive and negative charges of equal magnitude are implimented on the 2 conductors.

This charge separation produces a potential b/w between the conductors, which accounts for the capacitance per unit length, C. The shunt conductance per unit length, G, accounts for the power dissipation in the non-ideal dielectric medium b/w the conductors.

Use of Kirchhoff's voltage and current laws to the circuit path to the system of coupled differential equations for the phasor voltage & current in the limit

Metal-Oxide-Silicon Capacitor

2011-12-20T01:23:00.000-08:00

The metal-oxide-silicon (MOS) capacitor locates in the center of the complementary metal-oxide-silicon (CMOS) technology. MOSFETs place reliance on the very high quality of the interface between SiO2, the standard gate dielectric, and silicon (Si). This tutorials provides the detailed understanding of the MOS capacitor which is helpful in understanding the MOSFETs

The easy to understand structure diagram of the MOS capacitor is shown in diagram below. The structure of the MOS capacitor is similar to a parallel plate capacitor in which the bottom electrode is replaced by a semiconductor. When Si is applied as the substrate material, the electrode at the top is referred to as the gate, is typically made of polycrystalline silicon (polysilicon). In addition to this the dielectric is thermally grown silicon dioxide. In MOS context, the substrate is normally known as the body

Electronic Filters

2011-12-20T01:16:00.000-08:00

In electronics the Electrical filters are circuits that are designed to shape the magnitude as well as the phase spectrum of an input signal in order to generate a desired response at the output.

Therefore, we can define a frequency range as the passband where the input signal should be transmitted through the filter undistorted and unattenuated (or amplified to some extent); stopband filters require transmission to be blocked or the signal to be at least highly attenuated. In the frequency domain, the transmission characteristic is described by the transfer function which is given below:

Electron and Hole Velocities and Mobilities

2011-12-20T01:12:00.000-08:00

The Electrons and holes performs a chaotic random thermal motion. The average kinetic energy of thermal motion per one electron is given by

3kBT/2

where T stands for temperature in degrees Kelvin and kB represents Boltzmann constant. The electron thermal velocity, Vthn, is found by equating the electron kinetic energy to 3kBT/2:

Thermal velocities Vth~ and Vthp tells us about the average magnitudes of electron and hole velocities induced by the thermal motion. Since the directions of this thermal motion are random, the same amount of carriers crosses any device cross-section in any direction so that the electric current is become equal to 0 level.

In an electric field, the electrons and holes gain an extra drift velocity caused by the electric field, which is superimposed on the chaotic thermally induced velocities. The drift velocity of the electrons and holes In a uniform semiconductor and the drift velocity of the electrons and holes in a weak electric field, F, vn and vp, are proportional to the electric field:

State Variables for Electrical Circuits

2011-12-20T01:06:00.000-08:00

As we know that only capacitors and inductors are capable of storing energy in a circuit, and so only the variables related with them are able to place some influence the future condition of the circuit. The voltages from corner to corner of the capacitors and the currents through the inductors may serve as state variables.
If loops are not exclusively made up of capacitors and voltage sources, then the voltages across all the capacitors are independent variables and may be taken as state variables. In a similar way, if there are no sets of inductors and current sources that separate the circuit into two or more parts, then the currents associated with the inductors are independent variables and may be taken as state variables.

Variable Speed Drives

2011-12-03T01:46:00.000-08:00

INTRODUCTION

Variable speed drives are very useful in conjunction with supply &return fans & cooling tower fans, In addition to this these are also very helpful in virtually any type of centrifugal pump. In addition, centrifugal chillers are a leading applications. Engineers consider Speed Control and recognize such systems primarily for its energy savings benefits.

VARIABLE SPEED DRIVES

We can find large number of variable speed drive systems that are available in the market and that have been widely in use for control of fan or pump speed. These include following

· hydroviscous,

· eddy current clutch,

· direct current, and

· Variable frequency converters.

The distinctive advantage of the adjustable frequency drive is that we may use the standard AC motors. This implies that equipments such as cooling towers can be effectively converted to automatic control without making any change in the mechanical components and major design.

Examples of Variable speed derives is induction motor.

The variable frequency drives (VFD), which play role of interface b/w the AC power supply & the induction motor, should attain the following fundamental requirements:

1. they should have the ability of adjusting the frequency according to the desired output speed.

2. they should be able to regulate the output voltage in order to maintain a constant air-gap flux in the constant torque region.

3. it should have feature to supply a rated current without any delay i.e. on continuous basis at any given rate of frequency.

Types of Multivibrators

2011-11-17T04:12:00.000-08:00

The Multivibrator circuits are classified in 3 general categories.

1- Astable multivibrators.

Astable multivibrators circuits contain no stable state but still keep changing from one state to the other, and so the named as multivibrator. There useful application is as clocks or oscillators. These are also used as a 555 as an astable multivibrator.

2- Bistable Multovibrators.

These circuits can be easily induced to move from 1 state to another, and can remain in either state permanently after the input signals have been removed. They remain thus stable in both of their allowed states. The more commonly used term for bi-stable multi-vibrator is flip-flop.

3- Mono-stable Multi-vibrators.

These circuits exist only in a single stable state. They can only be forced to move out of their stable state by a trigger pulse, but they return to their original state after a very limited period of time. The main use for mono stables is to produce pulses of known duration from triggering pulses of shorter, longer, or of variable duration. Mono-stables are widely applied to generate ‘gate’ signals for counter circuits. They are generally considered to be hybrid analog/digital chips in that the digital output is normally determined by means of the RC time constant of an external analog circuit linked to the chip.

Another term used for mono-stable multi-vibrator is 1-shot, since it is a device that ‘shoots’ only once (that is issues an output pulse) each time it receives an input signal. Among mono-stables, the 555 timer is the top choice for pulse widths ranging from milliseconds to hours, & for applications in which the pulse width have to be stable to better than 0.05%.

In case of digital designs, mono-stables such as the model 74121 are favored for pulse widths ranging from about 40 ns to 10 ms, & will operate up to tens of seconds, but we can not predict their pulse widths as predictable or stable.

The WRIG Phase Coordinate Model

2011-11-04T01:33:00.000-07:00

The WRIG is furnished with laminated stator & rotor cores with uniform slots in which 3-phase windings are placed (Fig 2.1 below). Usually, the rotor winding is connected to copper slip-rings.

Brushes on the stator gather (or transmit) the rotor currents from (to) the rotor-side static power converter. For the time being, the slip-ring–brush system resistances are lumped into rotor phase resistances,

& the converter is replaced by an ideal voltage source.

As a result, the main flux self-inductances of various stator &, respectively, rotor phases are not dependent on rotor position.

The stator–rotor phase main flux mutual inductances, on the other hand, vary sinusoidally with rotor position ө_er. The mutual inductances between stator phases are also independent of rotor position, as the air gap is principally uniform. The same idea is applicable for mutual inductances between rotor phases. When mentioning stator & rotor phase & leakage inductances, all phase circuit parameters are included, with the exception of parameters to account for core losses (fundamental & stray-load core losses). Winding stray-load losses are mainly caused by frequency effects in the windings & may be recorded for in the phase resistance formula. Thus the phase coordinate model of WRIG is clear-cut:

Eq (2.1)

The stator equations are written in stator coordinates, & the rotor equations are written in rotor coordinates, which clarify the absence of motion-induced voltages. Generator mode association of voltage signs for both stator & rotor is evident. So, delivered electric powers are of +ve value. We can easily translate Equation 2.1 into matrix form as below:

Automotive Claw-Pole- Rotor Generator Systems

2011-11-04T00:57:00.000-07:00

Enhancing comfort & safety in cars, trucks, & buses driven by combustion engines require more installed electric power on board. As of now, the claw-pole-rotor generator is the only category of automotive generator used in industry, with whole powers per unit up to 5 kW and speeds up to 18,000 rpm

A solid rotor claw-pole structure possessing ring-shaped single direct current (DC) excitation coil, though supplied via slip-rings & brushes from the battery on board, has proven to be simple & reliable, with low cost, low volume and low excitation power loss. In common, the claw-pole-rotor generator is a 3-phase generator with 3 or 6 slots per pole & with 12, 14, 16, 18 poles, & a diode full power rectifier.

Its major demerit is the rather large losses (due to low efficiency), around just 50% when operating at full power & high speed. Generating electricity on board with such high losses is no longer acceptable as the electric power requirements per vehicle increase. Enhancements to the claw-pole-rotor (or Lundell) generator design for better efficiency at higher powers per unit are currently under insistent investigation by both industry & academia, & encouraging results were recently published. A cross-section of a typical industrial claw-pole-rotor generator is shown in diagram 6.1. It includes the following main components:

1. Uniformly slotted laminated stator iron core

2. Three-phase alternating current (AC) winding: typically one layer with q = 1, 2 slots per pole per phase, star or delta connection of phases

3. Claw-pole rotor made of solid iron parts that surround the ring-shaped DC-fed excitation (single) coil

4. Copper slip-rings with low voltage drop brushes to transfer power to the DC excitation coil on the rotor

5. Bearings & an end frame made of 2 sides - the slip-ring side & the drive-end side; the generator is driven by the internal combustion engine (ICE) by means of a belt transmission The Lundell generator AC output is rectified through a 3- or 4-leg diode rectifier & connected dc batteries are used, but 42 Vdc batteries are now used as a new standard for automotive application loads. The diodes D1 to D6 (D8) serve the full-power output rectification & are designed for the maximum power of the generator. For huge units (for trucks, etc.), 3 elementary diodes in parallel are mounted on radiator semi-legs to fulfill the requirement of rather high current levels involved (28 V_dc batteries are representative for large vehicles).

Permanent Magnet Synchronous Generator Systems

2011-11-04T00:28:00.000-07:00

By the term permanent magnet (PM) synchronous generators (SGs), we imply here radial or axial air gap PM brushless generators with distributed (q greater than 1) or concentrated (q ≤ 1) windings & rectangular or sinusoidal current control having surface PM or interior PM (IPM) rotors.

A PMSG’s output voltage amplitude & frequency both are proportional to speed. In case of constant speed prime mover applications, PMSGs might carry out voltage self-regulation by appropriate design; i.e., inset or interior PM pole rotors. Small speed variation (from ±10 to 15%) may be suitable for diode rectified loads with series capacitors & voltage self-regulation. However, the majority applications require operation at variable speed, and, in this case, constant output voltage vs. load, be it direct current (DC) or alternating current (AC), requires full static power conversion & close-loop control.

The Versatile mobile generator sets (gen-sets) employ variable speed for fuel savings, & PMSGs by means of full power electronics control can provide high torque density, low losses, & multiple outputs (DC and AC at fifty [60] Hz or 400 Hz, single phase or 3 phase).

A high efficiency, high active power to max out kilovolt ampere (kVA) ratio permits for reasonable power converter costs that offset the additional costs of PMs as compared to SRGs (switched reluctance generators) or induction generators (IGs) for the same level of speed.

Fly-wheel batteries with high kilowatts per kilogram (kW/kg), good kilo-watt hours per kilogram (kWh/kg), & long life, also utilize super-high-speed PMSGs with 4-quadrant P & Q control. They are proposed for energy storage on vehicles & spacecraft & for power systems backup.

Diverse as they may appear, these applications are accommodated by only a small number of practical PMSGs classified as follows:

1. With radial airgap (cylindrical rotor)

2. With axial airgap (disk rotor)

3. With distributed stator windings (q > 1)

4. With concentrated windings (q ≤ 1)

5. With surface PM rotors

6. With interior or inset PM rotors

7. With rectangular current control

8. With sinusoidal current control

9. In terms of loads, they are classified as follows:

10. With passive AC load

11. With DC load

12. With controlled AC voltage and frequency at variable speed

The super-high-speed PM generators are different in rotor construction, which requires a mechanical shell against centrifugal forces & a copper shield (damper) for reducing rotor losses. Also, at high fundamental frequency (above 1 kHz), stator skin effect & control imply special solutions for reducing machine & static converter losses & overall costs. As in nearly all PMSG surfaces PM rotors are used, the latter will be provided the most attention.

A full analytical field model is introduced & thoroughly checked through finite element method (FEM) field and torque production analysis. Loss models for generator steady-state circuit modeling are brought in for rectangular & for sinusoidal current control.