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A Text Book Of Electrical Technology Vol.1+2+3+4 By BL Theraja PDF Free Download

A Text Book Of Electrical Technology By BL Theraja  and AK Theraja

Now you can read electrical technology by bl theraja and ak theraja pdf  by downloading into your mobile/PC in pdf format.All the concepts related to machines are clearly explained with neat & attractive sketches. This book contains the very basic knowledge on motors and generators. A Text Book Of Electrical Technology By BL Theraja & AK Theraja regared as best book for beginners.

 BL Theraja and Ak Theraja together wrote 4 volumes on electrical technology they are given below:

A Text Book Of Electrical Technology Volume.1 By BL Theraja & AK Theraja PDF Free Download
A Text Book Of Electrical Technology Volume.2 By BL Theraja & AK Theraja PDF Free Download
A Text Book Of Electrical Technology Volume.3 By BL Theraja & AK Theraja PDF Free Download
A Text Book Of Electrical Technology Volume.4 By BL Theraja & AK Theraja PDF Free Download

A Text Book Of Electrical Technology By BL Theraja  and AK Theraja PDF Free Download

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Unijunction Transistor (UJT) With Operation & Applications

What Is Unijunction Transistor (UJT) ?

The unijunction transistor, abbreviated UJT, is a three-terminal, single-junction device. The basic UJT and its variations are essentially latching switches whose operation is similar to the four-layer diode, the most significant difference being that the UJT’s switching voltage can be easily varied by the circuit designer. Like the four-layer diode, the UJT is always operated as a switch and finds most frequent applications in oscillators, timing circuits and SCR/TRIAC trigger circuits.

Unijunction Transistor (UJT) Operation

A typical UJT structure, pictured in figure, consists of a lightly doped, N-type silicon bar provided with ohmic contacts at each end. The two end connections are called base-l , designated B, and base-2, B2. A small, heavily doped P-region is alloyed into one side of the bar closer to 82. This P-region is the UJT emitter E, and forms a P-N junction with the bar.
An interbase resistance, RBB, exists between B1 and B2. It is typically between 4 kΩ and 10kΩ, and can easily be measured with an ohmeter with the emitter open. RBB is essentially the resistance of the N-type bar. This interbase resistance can be broken up into two resistances, the resistance from B1 to emitter, called RB1 and resistance from B2 to emitter, called RB2. Since the emitter is closer to B2, the value of RB1 is greater than RB2 (typically 4.2 kΩ versus 2.8 kΩ).

The operation of the UJT can better be explained with the aid of an equivalent circuit.The UJT’s circuit symbol and its equivalent circuit are shown in below. The diode represents the P-N junction between the emitter and the base-bar (point x). The arrow through RB1, indicates that it is variable since during nonnal operation it may typically range from 4 kΩ down to 10 Ω.

The essence of UJT operation can be stated as follows:

(a) When the emitter diode is reverse biased, only a very small emitter current flows. Under this condition, RB1 is at its normal high-value (typically 4 kΩ). This is the UJT’s “off" state.
(b) When the emitter diode becomes forward biased, RB1 drops to a very low value (reason to be explained later) so that the total resistance between E and BI becomes very low, allowing emitter current to flow readily. This is the “on” state.

Circuit Operation of Uni junction Transistor (UJT) 

The UJT is normally operated with both B2 and E biased positive relative to B1 as shown in below figure. B1 is always the UJT reference terminal and all voltages are measured relative to B1. The VBB source is generally fixed and provides a constant voltage from B2 to B1. The VEE source is generally a variable voltage and is considered the input to the circuit. Very often, VEE is not a source but a voltage across a capacitor.
We will analyze the UJT circuit operation with the aid of the UJT equivalent circuit, shown inside the dotted lines in Fig.(a). We will also utilize the UJT emitter-base-1 VE-IE curve shown in Fig.(b). The curve represents the variation of emitter current  IE, with emitter-base-1 voltage, VE, at a constant B2-B1 voltage. The important points on the curve are labelled, and typical values are given in parentheses.

The “Off ” state If we neglect the diode fora moment, we can see in Fig.(a) that RB1 and RB2 form a voltage divider that produces a voltage Vx, from point x relative to ground.
Where η (the greek letter “eta") is the internal UJT voltage divider ratio RB1/RBB and is called the intrinsic stand of ratio.

Values of η typically range from 0.5 to 0.8 but are relatively constant for a given UJT.

The voltage at point x is the voltage on the N-side of the P-N junction. The VEE source is applied to the emitter which is the P-side. Thus, the emitter diode will be reverse-biased as long as VEE is less than Vx This is the “off” state and is shown on the VE-IE curve as being a very low current region. In the “off" state, then, we can say that the UJT has a very high resistance between E and B1, and IE is usually a negligible reverse leakage current. With no IE, the drop across RE is zero and the emitter voltage, VE, equals the source-voltage.

The UJT "off " state, as shown on the VE-IE curve, actually extends to the point where the emitter voltage exceeds Vx by the diode threshold voltage, VD, which is needed to produce forward current through the diode. The emitter voltage and this point, P, is called the peak-point voltage, VP, and is given by

VP= Vx + VD= ηVBB+ VD 

where VD is typically 0.5 V. For example, if η = 0.65 and VBB= 20V, then VP=13.5 V. Clearly, VP will vary as VBB varies.

The “On " state As VEE increases, the UJT stays “off” until VE approaches the peak-point value VP, then things begin to happen. As VE approaches Vp, the P-N junction becomes forward biased and begins to conduct in the opposite direction.
Note on the VE-IE curve that IE becomes positive near the peak point P. When VE exactly equals VP, the emitter current equals IP, the peak-point current. At this point, holes from the heavily doped emitter are injected into the N-type bar, specially into the B1 region. The bar, which is lightly doped, offers very little chance for these holes to recombine. As such, the lower half of the bar becomes replete with additional current carriers (holes) and its resistance RB1, is drastically reduced. The decrease in RB1 causes Vx to drop. This drop in turn causes the diode to become more forward biased, and IE increases even further. The larger IE injects more holes into B1 further reducing RB1, and so on. When this regenerative or snowballing process ends, RB1. has dropped to a very small value (2-25 Ω) and IE can become very large, limited mainly by external resistance RE.

The UJT operation has switched to the low-voltage. high-current region of its VE- IE curve. The slope of this “on” region is very steep, indicating a low resistance. In this region, the emitter voltage VE, will be relatively small, typically 2V, and remains fairly constant as IE is increased up to its maximum rated value,IE(sat). Thus, once the UJT is “on,” increasing VEE will serve to increase IE while VE remains around 2V.

Turning “Off” the UJT Once it is “on,” the UJT‘s emitter current depends mainly on VEE and RE. As VEE decreases, IE will decrease along the “on” portion of the VE - IE curve. When IE decreases to point V, the valley point, the emitter current is equal to IV, the valley current, which is essentially the holding current needed to keep the UJT “on”. When 15 is decreased below IV, the UJT turns “off" and its operation rapidly switches back to the “off” region of its VE - IE curve,where IE = 0 and VE - VEE. The valley current is the counterpart of the holding current in PNPN devices, and generally ranges between 1 and 10 mA.

Applications of UJT:

Unijunction transistors are used extensively in oscillator, pulse and voltage sensing circuits. Some of the important applications of UJT are discussed below :
(i) UJT relaxation oscillator.
(ii) Overvoltage detector.
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Principle & Working of Buck Converter ( Step-Down Chopper )

In the previous articles we have seen basic operation and principle of a chopper.In this tutorial we will see Principle of Step-Down Chopper (Buck-Converter) with a neat sketch of circuit diagram

Don't Know What Is Chopper ? Click Here To Read It Now..

Buck-Converter ( Step-Down Chopper ) Principle & Working

In general, d.c. chopper consists of power semiconductor devices (SCR, BJT, power MOSFET, IGBT, GTO, MCT, etc., which works as a switch), input d.c. power supply, elements (R, L, C, etc.) and output load. (Refer below figure). The average output voltage across the load is controlled by varying on-period and off-period (or duty cycle) of the switch.
Buck Converter ( Step-Down Chopper ) Circuit Diagram
Buck Converter ( Step-Down Chopper ) Circuit Diagram

Buck-Converter ( Step-Down Chopper )  Operation

A commutation circuitry is required for SCR based chopper circuit. Therefore, in general, gate commutation devices based choppers have replaced the SCR based choppers. However, for high voltage and high-current applications, SCR based choppers are used. The variations in on and off periods of the switch provides an output voltage with an adjustable average value. The power-diode (DP) operates in freewheeling mode to provide a path to load-current when switch (S) is OFF. The smoothing inductor filters out the ripples in the load current. Switch S is kept conducting for period Ton and is blocked for period Toff. The chopped load voltage waveform is shown in figure.
Buck Converter ( Step-Down Chopper ) Output
Buck Converter ( Step-Down Chopper ) Output
During the period Ton, when the chopper is on, the supply terminals are connected to the load, terminals. During the interval Toff, when the chopper is off, load current flows through the freewheeling diode DF. As a result, load terminals are short circuited by DF and load voltage is therefore, zero during Toff. In this manner, a chopped dc. voltage is produced at the load terminals.

From output of buck converter figure, the average load-voltage E0 is given by
From Eq 1, it is obvious that the output voltage varies linearly with the duty-cycle. It is therefore possible to control the output voltage in the range zero to Edc.
If the switch S is a transistor, the base-current will control the ON and OFF period of the transistor switch. If the switch is GTO thyristor, a positive gate pulse will tum-it ON and a negative gate pulse will turn it OFF. If the switch is an SCR, a commutation circuit is required to turn it OFF.
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What Is Chopper ? Classification Of DC To DC Power Converters/Choppers

What Is Called A DC Chopper ? 

A d.c. chopper is a static device (switch) used to obtain variable d.c. voltage from a source of constant d.c. voltage.
In the right below figure you can see circuit diagram of a chopper. Therefore, chopper may be thought of as d.c equivalent of an ac. transformer since they behave in an identical manner. Besides, the saving in power, the dc.chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control, and, for many applications, lower cost, than motor-generator sets or gas tubes approaches.

Solid-state choppers due to various advantages are widely used in trolley cars, battery-operated vehicles, traction-motor control, control of a large number of d.c. motors from a common d.c. bus with a considerable improvement of power factor, control of induction motors, marine hoists, forklift trucks and mine haulers. The objective of this chapter is to discuss the basic principles of chopper operation and more common types of chopper configuration circuits.

Classification Of  Choppers or DC To DC Power Converters

DC choppers can be classified as:

(A) According to the Input/Output Voltage Levels
(i) Step-down chopper: The output voltage is less than the input
(ii) Step-up chopper: The output voltage is greater than the input

(B) According to the Directions of Output Voltage and Current
(i) Class A (type A) chopper
(ii) Class B (type B) chopper
(iii) Class C (type C) chopper
(iv) Class D (type D) chopper
(v) Class E (type B) chopper
The voltage and current directions for above classes are shown in below figure.


(C) According to Circuit Operation
(i) First-quadrant chopper: The output voltage and both must be positive.(Type A).
(ii) Two-quadrant chopper: The output voltage is positive and current can be positive or negative (class-C) or the output current is positive and the voltage can be positive or negative (class-D).
(iii) Four-quadrant chopper: The output voltage and current both can be positive or negative (class-E).

(D) According to Commutation Method
(i) Voltage-commutated choppers
(ii) Current-cornmutated choppers
(iii) Load-commutated choppers
(iv) Impulse-commutated choppers

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Differences Between Instrument and Power Transformers

In this article differences between power transformer and instrument transformer are discussed.Current transformer and Potential transformers come under instrument transformers.

Differences Between Instrument and Power Transformers

Power Transformers Instrument Transformers
1. Mainly used to change voltage levels in
a power system.
1. Mainly used to extend the ranges of the
instruments while measuring parameters like
voltage,current,power etc.
2. They are required to transform huge amount
of power to the load.
2. They are required to transform very small power
as their loads are generally delicate moving elements
of the instruments.
3. They can be used to step up or step down the
3. They are basically step down transformers and
used along with devices such as protective
relays,indicators etc.
4. The exciting current is a small fraction of the
secondary winding of load current.
4. As the load it self is small, the exciting current is
of the order of the secondary winding.
5. The cost is main consideration in the design
while efficiency and regulations are secondary
5. Accuracy is the main consideration while designing
to keep ratio and phase angle errors minimum.
Cost is the second consideration.
6. As they handle large power, the heat
dissipation is the major consideration and
cooling method is necessary.
6. The power output is very small as loads are light.
Hence heating is not severe.
7. The limitation on the load is due to temperature
7. Accuracy is the main load limitation factor and not
the temperature rise.
8. Example : Distribution transformer used in
8. Example : Current transformer and Potential

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Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

Differences Between Moving Coil (MC) And Moving Iron (MI) InstrumentsLet us have a comparison between two major type of electrical measuring instruments they are moving coil (MC) and moving iron (MI) instruments.In this article all major differences between moving iron and  moving coil   instruments are discussed.When you go viva or interview most often question on electrical measurements is what is the difference between mi and mc type instruments?.In this article you will get answer to all differences between MI & MC typ instruments.

Differences Between Moving Coil (MC) And Moving Iron (MI) Instruments

M.C Instruments M.I Instruments
1. MC type instruments are more accurate. 1. MI type are less accurate than MC type.
2. Manufacturing cost is high. 2. Cheap in cost.
3. Reading scale is uniformly distributed. 3. Non-uniform scale
(scale cramped at beginning and finishing)
4. Very sensitive in construction & for input. 4. Robust in construction.
5. Low power consumption 5. Slightly high power consumption.
6. Eddy current damping is used. 6. Air friction damping is used.
7. Can be used only for D.C measurements. 7. Can be used for A.C as well as for D.C
8. Controlling torqure is provided by spring. 8. Controlling torque is provided by
gravity or spring
9. Deflection proportional to current.( θ α I ). 9. Deflection proportional to square of current.
( θ α I² ).
10. Errors are set due to aging of control
springs,permanent magnet (i.e. No Hysteresis loss)
10. Errors are set due to hysteresis and stray fields.
(i.e. hysteresis loss takes place).

In this comparison between MC and MI Instruments we shared top points.

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Torque Equation Of Moving Iron Instruments

In previous tutorial on Moving Iron Instrument Operation construction & working principle was discussed. In this post moving iron instrument torque equation will be derived.

Torque Equation of Moving Iron Instruments

READ HERE Moving Iron Instrument Working Operation CLICK HERE 

Consider a small increment in current supplied to the coil of the instrument. due to this current let dθ be the deflection under the deflecting torque Td. Due to such deflection, some mechanical work will be done.
Mechanical Work = Td .dθ
There will be a change in the energy stored i the magnetic field due to the change in inductance. This is because the vane tries to occupy the position of minimum reluctance. The inductance is inversely proportional to the reluctance of  the magnetic circuit of coil.

Let   I = initial current
L = instrument inductance
θ = deflection
dI = increase in current
dθ = change in deflection
dL = change in inductance

In order to effect an increment dL in the current, there must be an increase in the applied voltage given by,

e = d(L*I)/dt
   = I * dL/dt + T * dI/dt     as both I and L are changing.

The electrical energy supplied is given by 

eIdt = { I * dL/dt + T * dI/dt }Idt
       =I²dL + ILdI
The stored energy increases from 1/2*(LI²) to 1/2*[(L+dL)(I+dI)²]
Hence the change in stored energy is given by,
1/2*[(L+dL)(I+dI)²] - 1/2*(LI²)
Neglecting higher order terms,this becomes ILdI +  1/2 * I² dL
The energy supplied in nothing but increase in stored energy plus the energy required for mechanical work done.

I²dL + ILdI = ILdI + 1/2*(I²)dL +Td.
Td.dθ =  1/2( I².dL )

Td = 1/2  I²dL/dθ

While the controlling torque is given by,
Tc = Kθ
where K = spring constant 
Kθ = 1/2  I²dL/dθ
θ = 1/2  I²dL/dθ * 1/K      under equilibrium 

Thus the deflection is proportional to the square of the current through the coil. And the instrument gives square law response.

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Moving Iron Instrument Working Operation

Moving Iron Instruments or MI Instruments

In our previous article we have discussed PMMC Instrument Working Opeartion. In this tutorial on Moving Iron Instrument Working Operation we go through the construction & basic principle of MI type instrument.  

The moving iron instruments are classified as:
i) Moving iron attraction type instruments
ii) Moving iron repulsion type instruments

Moving Iron Attraction Type Instrument Working Operation

Moving Iron Instrument Working Principle : The basic working principle of these instruments is very simple that a soft iron piece if brought near magnet gets attracted by the magnet.

The construction of the attraction type moving iron instrument is shown in the below figure.

lt consists of a fixed coil C and moving iron piece D. The oil is flat and has narrow slot like opening. The moving iron is a flat disc which is eccentrically mounted on the spindle. The spindle is supported between the jewel bearings. The spindle carries a pointer which moves over a graduated scale.The number of turns of the fixed coil are dependent on the range of the instrument. For passing large current through the coil only few turns are required.

The controlling torque is provided by the springs but gravity control may also be used for vertically mounted panel type instruments.

The damping torque is provided by the air friction. A light aluminium piston is attached to the moving system. it moves in a fixed chamber. The chamber is closed at one end. it can also be provided with the help of vane attached to the moving system.

The operating magnetic field in moving iron instruments is very weak. Hence eddy current damping is not used since it requires a permanent magnet which would affect or distort the operating field.This is the reason Why why eddy current damping is not used in moving iron instrument.

Moving Iron Repulsion Type Instrument

Moving iron repulsion Type instruments have two vanes inside the coil. the one is fixed and other is movable. When the current flows in the coil, both the vans are magnetized with like polarities induced on the same side. Hence due to the repulsion of like polarities, there is a force of repulsion between the two vanes causing the movement of the moving vane. The repulsion type instruments are the most commonly used instruments.

The two different designs of repulsion type instruments are:
i) Radial vane type and
ii) Co-axial vane type

Radial Vane Emulsion Typo Instrument

Below shows the radial vane repulsion type instrument. Out of the other moving iron mechanisms, this is the moat sensitive and has most linear scale.

The two vanes are radial strips of iron. The fixed vane is attached to the coil. The movable vane is attached to the spindle and suspended in the induction field of the coil. The needle of the instrument is attached to this vane.

Even-though the current through the coil is alternating, there is always repulsion between the like poles of the fixed and the movable vane. Hence the deflection of the pointer is always in the same direction. The deflection is effectively proportional to the actual current and hence the scale is calibrated directly to read amperes or volts. The The calibration is accurate only for the frequency for which it is designed because the impedance is different for different frequencies.

Concentric Vane Repulsion Type Instrument

Figure shows the concentric vane repulsion type instrument. The instrument has two concentric vanes. One is attached to the coil frame rigidly while the other can rotate co-axially inside the stationary vane.

Both the vanes are magnetized to the same polarity due to the current in the coil. Thus the movable vane rotates under the repulsive force. As the movable vane is attached to the pivoted shaft, the repulsion results in a rotation of the shaft. The pointer deflection is proportional to the current in the coil. The concentric vane type instrument is moderately sensitive and the deflection is proportional to the square of the current through coil. Thus the instrument said to have square low response. Thus the scale of the instrument is non-uniform in nature. Thus whatever may be the direction of the current in the coil, the deflection in the moving iron instruments is in the same direction. Hence moving iron instruments can be used for both a.c. and d.c. measurements. Due to square low response, the scale of the moving iron instrument is non-uniform.

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