Electrical Objective Book

2013-14

ELECTRICAL OBJECTIVE QUESTION

MADE BY:-

P.M.MEVADA

P.M.MEVADA

E L E C T R I C A L I N T E R V I E W Q U E S T I O N & A N S W E R

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TABLE OF CONTENTS

SUBJECT PAGE NO. CHAPTER-1. BASIC ELECTRICAL ENGINEERING 1 CHAPTER-2. ELECTRICAL MACHINE 14

2.1 ALTERNATOR 14 2.2 D.C. MACHINES 15 2.3 D.C. MOTOR 16 2.4 - INDUCTION MOTORS 17 2.5 SYNCHRONOUS MOTOR 18 2.6 GENERAL ELECTRICAL MACHINE 19

CHAPTER-3. TRANSFORMERS 30 CHAPTER-4. POWER SYSTEM 40 CHAPTER-5. POWER ELECTRONICS 45 CHAPTER-6. OP-AMP 61 CHAPTER-7. MEASUREMENTS & INSTRUMENTATION 66 CHAPTER-8. BASIC ELECTRONICS 72 CHAPTER-9. CONTROL SYSTEM 87 CHAPTER-10. ELECTRICAL CIRCUIT THEORY 95

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CHAPTER -1. BASIC ELECTRICAL ENGINEERING

[1] Which of the following can vary with ac, but never with dc? A. Power B. Voltage C. Frequency D. Amplitude

[2] If all other factors are kept constant, the inductance of the air-core coil is not affected by A. The frequency B. The number of turns C. The diameter of the coil D. The length of the coil

[3] The length of time between a point in one cycle and the same point in the next cycle of an ac wave is the A. Frequency B. Magnitude C. Period D. Polarity

[4] Capacitance acts to store electrical energy as A. Current B. Voltage C. A magnetic field D. An electric field

[5] On a spectrum analyzer, a ac signal having only one frequency component looks like A. A single pip B. A sine wave C. A square wave D. A sawtooth wave

[6] A ferromagnetic core is place in the inductor to A. Increase the current carrying capacity B. Increase the inductance C. Limit the current D. Reduce the inductance

[7] The period of a ac wave, in seconds is A. The same as the frequency in hertz

B. Not related to the frequency in any way C. Equal to 1 divided by the frequency in hertz D. Equal to the peak amplitude in volts divided by the frequency in hertz

[8] An inductor works by A. Charging a piece of wire B. Storing energy as a magnetic field C. Choking off dc D. Introducing resistance into a field

[9] As capacitor plate area increases A. The capacitance increases B. The capacitance decreases C. The capacitance does not change D. The current handling ability decreases

[10] In a small inductance A. Energy is stored and released slowly B. The current flow is always large C. The current flow is always slow D. Energy is stored and released quickly

[11] In ac circuit, the ratio of kW/kVA is A. power factor B. form factor

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C. load factor D. diversity factor [12] Suppose that two ac waves have the same frequency but differ in phase by exactly 1/20 of a cycle. What is the phase difference between these two waves?

A. 18 deg B. 20 deg C. 36 deg

D. 5.73 deg [13] The unit of inductance is

A. Mho B. hendry C. ohm D. farad [14] What is the angular frequency of the ac signal having a frequency of 1770Hz

A. 1770 rad/s B. 11,120 rad/s C. 282 rad/s D. Data insufficient

[15] Thevenin's equivalent circuit consists of A. series combination of Rth, Eth, Rl B. series combination of Rth, Eth C. parallel combination of Rth, Eth, Rl D. parallel combination of Rth, Eth

[16] The frequency of DC supply is A. 16 2/3 Hz B. 60 Hz C. 50 Hz D. 0 Hz [17] Load factor is defined as the ratio of A. max demand/ average demand B. average demand/ connected load C. average demand/ max. demand D. connected load/ max demand [18] Q factor of an inductive coil is given by

A. 2 fL/R B. 2 fr/R C. R/Z D. lr/l

[19] The rms value of sinusoidal 100V peak to peak is A. 100 V B. 50/2 V C. 50 V D. 100/2 V [20] Which of the following bulbs will have the least resistance?

A. 220V, 60W B. 220V, 100W C. 115V, 60W D. 115V, 100W

[21] The wire is stretched to double its length, then its resistance R will become

A. r/2

B. 4r C. 2r D. r/4

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[22] The power drawn by the circuit whose input is 20KVA and p.f is 0.8 lagging A. 12 B. 20 C. 16 D. 8

[23] The combined resistance of two equal resistors connected in parallel is equal to A. One half the resistance of one resistor B. twice the resistance of one resistor C. four times the resistance of one resistor D. one fourth the resistance of one resistor

[24] The superposition theorem is applicable to circuits only having ............ elements A. non-linear B. passive C. resistive D. linear bilateral [25] The Q-factor of a coil is given by A. its power factor B. Ratio of max. energy stored & energy dissipated per cycle. C. reciprocal of its power factor D. ratio R/Z [26] The power factor at resonance in RLC circuit is

A. zero B. unity C. 0.5 lagging D. 0.5 leading

[27] Cells are connected in parallel to A. Increase the available voltage B. Reduce the cost of wiring C. Increase the available current D. Reduce the time required to fully charge them after use

[28] the power factor of a purely resistive circuit is A. zero B. unity C. lagging D. leading

[29] Which of the following has minimum running cost? A. Hydro electric station B. nuclear power station C. thermal power station D.diesel power station

[30] The color code of a resistor 2.7Kohm with the tolerance of 10% is A. red, violet, red and silver B. red, violet, yellow and gold C. red, violet, orange, silver D. red, violet, red,gold

[31] In a 117Vrms utility circuit, the peak-to-peak voltage is approximately A. 82.7 V B. 165 V C. 234 V D. 331 V

[32] Watt hour is the unit of A. Electric power B. Electric capacity C. Electric energy D. Electric charge

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[33] In India,electrical power is transmitted by A. 1-phase ac system B. 3-wire dc system C. 3-phase 3-wire ac system D. 2 wire dc system

[34] If two perfect sine waves have the same frequency and the same phase, the composite wave A. Is a sine wave with an amplitude equal to the difference between the amplitude of the waves B. Is a sine wave with an amplitude equal to the sum of the amplitudes of the two original waves C. Is not a sine wave, but has the same frequency as the two input waves D. Has zero amplitude (ie, it does not exist) because the two input waves cancel each other out.

[35] In a ac circuit the product of voltage and current is known as A. Power B. Real Power C. Resistive power D. Apparent power

[36] The advantage of AC over DC in utility applications is A. AC is easier to transform from one voltage to another B. AC is transmitted with lower loss in wires C. AC can be easily obtained from dc generators D. AC can be generated with less dangerous by products

[37] A network that does not have either voltage or current source is called ......network A. Active B. Passive C. Resistive D. Dummy

[38] The sixth harmonic of a ac wave whose period is 1 Milli second has a frequency of A. 0.006Hz B. 167.0 Hz C. 7.0 kHz D. 6 kHz

[39] A degree of phase represents A. 6.28 cycles B. 57.3 cycles C. 1/60 of a cycle D. 1/360 of a cycle

[40] A battery is source of A. DC voltage B. 1 phase AC voltage C. 3 phase ac voltage D. ac or dc voltage

[41] The power plant having highest efficiency is

A. Hydro B. Thermal C. Nuclear D. Diesel

[42] As the spacing between plates in a capacitor is made smaller A. The capacitance increases B. The capacitance decreases C. The capacitance does not change D. The resistance increases

[43] The relationship between frequency and time period of ac waveform is A. f = T B. f = 1/ T2

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C. f = 1/T D. f = T2

[44] The five 0.050uF capacitors are connected in parallel. The equivalent capacitance is A. 0.010 uf B. 0.25 uf C. 0.50uf D. 0.025uf

[45] The current gain of a bipolar transistor drops at high frequencies because of A. Transistor capacitance B. High current effects in the base C. Parasitic inductive elements D. the early effect

[46] If two perfect sine waves have the same frequency and the same amplitude, but are in opposite phase, the composite wave

A. Has twice the amplitude of either input wave alone B. Has half the amplitude of either input wave alone C. Is complex, but has the same frequency as the originals. D. Has zero amplitude ( that is, it does not exist) because the two input waves cancel each other out.

[47] Three capacitors are connected in series having capacitance of 8uF, 32uF, 16uF, the total capacitance is A. 32/7 uF B. 7.32 uF C. 56 uF D. 32 uF

[48] Which one of the following does not affect the power output available from a particular ac generator? A. The strength of the magnet B. The number of turns in the coil C. The type of natural energy source used D. The speed of rotation of the coil or magnet

[49] Which of the following are active components A. resistor and inductor B. Diode, BJT, FET C. Opamp, BJT, thermionic triode D. Capacitor, Inductor

[50] The energy required to charge a 10 uF capacitor to 100V is (a) 0.10 J (b) 0.05 J (c) 5 X 10-9 J (d) 10 X 10-9 J

[51] The current in a inductor changes from 0 to 200mA in 4ms and induces a voltage of 100mV. The value of inductor is

(a) 2mH (b) 0.5mH (c) 8mH (d) 4mH

[52] A battery has a short-circuit current of 30A and an open circuit voltage of 24V. If the battery is connected to an electric bulb of resistance 2 ?, the power dissipated by the bulb is

(a) 80W (b) 1800W (c) 112.5W (d) 228W

[53] Which of the following is ferromagnetic material? a) Copper b) Palladium c) Silver

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d) Cobalt [54] A complex wave form made up frequency components 1 Hz, 3 Hz, 5 Hz, 7 Hz and 9 Hz. Its fundamental frequency is

a) 9 Hz b) 12.5 Hz c) 1 Hz d) Indeterminate

[55] If a 175V dc source were connected in series with the utility mains from a standard wall outlet, the result would be

A. Smooth dc at a constant voltage B. Pure ac with equal peak voltages C. Ac with one peak voltage greater than the other D. Fluctuating dc

[56] The main advantage of air as a dielectric material for capacitors is A. Has a high dielectric constant B. Is not physically dense C. Has low loss D. Allows for large capacitance in a small volume

[57] Which of the following is a polarized capacitor? A. Paper B. Mica C. Inter electrode D. Electrolytic

[58] In a 117Vrms utility circuit, the positive peak voltage is approximately A. 82.7V B. 165V C. 234V D. 331V

[59] If a capacitor has a negative temperature coefficient then A. Its capacitance decreases as the temperature rises B. Its capacitance increases as the temperature rises C. Its capacitance does not change with temperature D. It will not work if the temperature is below freezing

[60] Inductors in series, assuming there is no mutual inductance, combine A. Like resistors in parallel B. Like resistors in series C. Like batteries in series with opposite polarities D. In a way unlike any other type of component

[61] The five 0.050 F capacitors are connected in series. The equivalent capacitance is A. 0.010 f B. 0.25 f C. 0.50 f D. 0.025 f

[62] A material with a high dielectric constant A. Acts to increase capacitance per unit volume B. Acts to decrease capacitance per unit volume C. Has no effect on capacitance D. Causes a capacitor to become polarized

[63] In a perfect sine wave, the peak to peak amplitude is equal to A. Half the peak amplitude B. The peak amplitude C. 1.414 times the peak amplitude D. Twice the peak amplitude

[64] A triangular wave exhibits A. An instantaneous rise and a defined decay

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B. A defined rise and an instantaneous decay C. A defined rise and a defined decay, and the two are equal D. An instantaneous rise and an instantaneous decay

[65] The colour bands on a carbon composition resistor occur in the sequence: yellow, violet, yellow and silver. Its resistance is

(a) 470 K 47 K (b) 470 K 23.5 % (c) 47 K 10 % (d) 47 K 5 %

[66] If 120C of charge passes through an electric conductor in 60 sec, the current in the conductor is (a) 0.5A (b) 2A (c) 3.33mA (d) 0.3mA

[67] The energy required to move 120 coulomb through 3V is (a) 25mJ (b) 360J (c) 40J (d) 2.78mJ

[68] The current carrying capacity of a 1W, 4M resistor used in radio receiver is (a) 0.5kA (b) 2kA (c) 2mA (d) 0.5mA

[69] A capacitor is charged by a constant current of 2mA and results in a voltage increase of 12V in a 10 sec interval. The value of capacitance is

(a) 0.75mF (b) 1.33mF (c) 0.6mF (d) 1.67mF

[70] Three phase ac supply A. Has sawtooth waves that add together in phase B. Consists of three sine waves in different phases C. Is a sine wave with exactly three harmonics D. Is of interest only to physicists

[71] Power is defined as A. the rate at which current flows in a circuit B. the product of voltage and resistance in a circuit C. the rate at which energy is radiated or dissipated D. the accumulation of energy over time E. the amount of heat generated in a circuit.

[72] At the exact moment a 60Hz ac sine wave is at its positive peak voltage, the instantaneous rate of change in the voltage is

A. Large and positive B. Small and negative C. Large and negative D. Small and negative E. Zero

[73] A flute sounds different than a violin, even if the two instruments are played at the same pitch, because of a difference in the

A. Phase B. Chamber length C. Frequency D. Waveform E. Bias

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[74] A common-collector transistor circuit is often used A. to provide high gain and sensitivity over a wide range of frequencies B. to match a high impedance to a low impedance C. as a high-fidelity audio power amplifier D. as an oscillator at microwaves frequencies E. as the rectifier in a dc power supply.

[75] The internal conductance of an ammeter is generally A. Low B. Directly proportional to the current C. Inversely proportional to the current D. High E. Any value. It doest matter

[76] A steady magnetic field can be produced by A. a straight wire carrying a constant direct current. B. a loop of wire carrying a constant direct current. C. a coil of wire carrying a constant direct current. D. a constant-intensity stream of protons in free space, moving in a straight line. E. any of the above.

[77] The output wave of a common-gate amplifier circuit with a pure sine-wave input A. is in phase with the input wave. B. lags the input wave by 90 of phase. C. leads the input wave by 90 of phase. D. is 180 out of phase with the input wave. E. is inverted with respect to the input wave.

[78] Which of the following capacitor types is polarized? A. Electrolytic B. Paper C. Ceramic D. Mica E. Air variable

[79] when a non zero net reactance exists in an ac circuit, the apparent power is A. Zero B. Equal to the true power C. Less than the true power D. Greater than the true power E. Infinite

[80] The gauss is a unit of (a) Charge carrier flow speed. (b) Magnetic flux density. (c) Electrostatic field strength. (d) Electromagnetic field intensity. (e) Electrical charge quantity.

Ans:B. Exp: In SI system the unit is tesla(T). In CGS system the unit is Gauss [81] An average responding rectifier type electronic ac voltmeter has its scale calibrated in terms of the rms value of a sine wave, when a square wave voltage of peak magnitude 100V is measured using this voltmeter then the reading indicated by the meter, will be ?

a) 111V b) 100V c) 90.09V d) 70.7V

[82] A house served by a 220V supply light, is protected by a 9-Ampere fuse. The maximum number of 60 W bulbs in parallel that can be turned on is

a) 11 b) 33

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c) 22 d) 44

[83] Silicon has a preference in IC technology because (a) it is an indirect semiconductor (b) it is a covalent semiconductor (c) it is an elemental semiconductor (d) of the availability of nature oxide SiO

[84] Three equal resistance of magnitude 5 Ohm each are connected in delta. The resistance between any two pair of terminals of the delta will be

(a) 5 Ohm (b) 5/3 Ohm (c) 10/3 Ohm (d) 3/5 Ohm

[85] The R.M.S. value of a half wave rectified sinusoidal alternating current with peak value Im is (a) Im / 1 (b) Im / v2 (c) Im / 2 (d) Im v3

[86] The unit which indicates the rate at which energy is expended? A. Watt

B. Ampere-hour

C. Coulomb

D. Volt

[87] The peak voltage in an ac wave is always A. greater than the average voltage B. less than the average voltage C. greater than or equal to average voltage D. less than or equal to the average voltage

[88] When an electrical charge exists but there is no flow of current, the charge is said to be A. ionizing B. electronic C. Static D. Atomic

[89] As the number of turns in a coil that carries ac increases without limit, the current in the coil will A. Eventually become very large B. Stay the same C. Decrease, approaching zero D. Be stored in the core material

[90] As the number of turns in a coil increases, the reactance at a constant frequency A. Increases B. Decreases C. Stays the same D. Is stored in the core material

[91] In an RL circuit, as the ratio of inductive reactance to resistance (XL/R) decreases, the phase angle A. Increases B. Decreases C. Stays the same D. Becomes alternately positive and negative

[92] The best filter for a power supply is A. a capacitor in series B. a choke in series C. a capacitor in series and a choke in parallel D. a capacitor in parallel and a choke in series

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[93] Voltage regulation can be achieved by a zener diode connected in A. parallel with the filter output, forward biased B. parallel with the filter output, reverse biased C. series with the filter output, forward biased D. series with the filter output, reverse biased

[94] A current surge takes place when a power supply is first turned on because A. the transformer core is suddenly magnetized B. the diodes suddenly start to conduct C. the filter capacitor must be initially charged D. arching takes place in the power switch

[95] A dc electromagnet A. has constant polarity B. Requires an air core C. Cannot be used to permanently magnetize anything D. Does not attract or repel permanent magnet

[96] In a multi-stage RC coupled amplifier the coupling capacitor______________ A) Limits the low frequency response B) Limits the high frequency response C) Does not affect the frequency response D) Block the DC component without affecting the frequency response

[97] It is required to measure temperature in the range of 13000 deg C to 15000 deg c. The most suitable thermocouple to be used as a transducer would be?

a) chromel - constantan b) Iron - constantan c) chromel - alumel d) platinum- rhodium

[98] Telemetering is a method of? a) Counting pulses sent over long distances b) Transmitting pictures from one place to another c) Transmitting information concerning a process over a distance d) None

[99] A dc to dc converter having an efficiency of 80% is delivering 16W to a load. If the converter is generating an output of 200V from an input source of 20V, then the current drawn from the source will be?

a) 0.1A b) 0.5A c) 1.0A d) 10.0A

[100] The location of lighting arrestor is? a) Near the transformer b) Near the circuit breaker c) Away from the transformer d) None

[101] Which of the following is not a general characteristic of ac wave? A. The wave shape is identical for each cycle B. The polarity reverses periodically C. The electrons always flow in the same direction D. There is a definite frequency

[102] All sine waves A. Have similar general appearance B. Have instantaneous rise and fall times C. Are in the same phase as cosine waves D. Rise instantly, but decay slowly

[103] The derivative of a sine wave A. Is shifted in phase by cycle from the sine wave B. Is the rate of change in the instantaneous value

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C. Has instantaneous rise and decay times D. Rise instantly, but decays slowly

[104] A phase difference of 180deg in the circular motion model of a sine wave represents A. revolution B. revolution C. A full revolution D. Two full revolution

[105] We can add or subtract a .degrees of phase to or from a wave, and end up with exactly the same wave again

A. 90 B. 180 C. 270 D. 360

[106] A wave has a frequency of 300kHz. One complete cycle takes..seconds A. 1.3000 B. 0.00333 C. 1/3000 D. 3.33 X 10-6

[107] A wave has a frequency of 440Hz, how long does it take for 10: of a cycle to occur? A. 0.00273s B. 0.0000273s C. 0.0000631s D. 0.00000631s

[108] One waveform has a peak value of 3V and the other has peak value of 5V. These two waves are in phase coincidence. The resultant has voltages of

A. 8Vpk, in phase with the composites B. 2 Vpk, in phase with the composites C. 8Vpk, in phase opposition with respect to the composites D. 2 Vpk, in phase opposition with respect to the composites

[109] Two pure sine waves that differ in phase by 180: can be considered to A. Be offset by two full cycles B. Be in phase opposition C. Be separated by less than cycle D. Have a frequency of cycle

[110] A BJT has A. three PN junctions B. three semiconductor layers C. two N type layers around a P type layer D. a low avalanche voltage

[111]Consider the following statements: [IES2010]

Piezoelectric materials serve as

1.A source of ultrasonic waves

2.When electric field is applied, the mechanical dimensions of the substances are not at all altered.

3.Converts electrical energy to mechanical and vice versa

4.Converts thermal energy to electrical energy

Which of the above statements is/are correct?

A.1 only

B.2 and 3 only

C.1 and 3 only

D.1,2,3 and 4

[112]The current through the 2K resistance in the circuit shown is [GATE2009]

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A.0 mA B.1 mA C.2 mA D.6 mA

[113]Out of the following plant categories 1.Nuclear 2.Run-of-river 3.Pump storage 4.Diesel

the base load power plants are [GATE2009] A.1 and 2 B. 2 and 3

C.1,2 and 4 D.1, 3,and 4

[114]The following circuit has a source voltage Vs as shown in the graph. The current through the circuit is also shown

The element connected between a and b could be [GATE 2009]

[115]The following circuit has R= 10k , C=10F. The input voltage is a sinusoid at 50Hz with an rms value of 10V. Under ideal conditions, the current is from the source is [GATE 2009]

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A. 10 mA leading by 90 B. 20 mA leading by 90 C. 10 mA leading by 90 D. 10mA lagging by 90

Ans: A [116]In the figure shown, all elements used are ideal. For time t Pout for both transformer and emitter follower C. Pin < Pout for transformer and Pin = Pout for emitter follower D. Pin = Pout for transformer and Pin < Pout for emitter follower

Ans: None of the above [118]The equivalent capacitance of the input loop of the circuit shown is [GATE 2009]

A. 2F B. 100F C. 200F D. 4F

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CHAPTER-2. ELECTRICAL MACHINE

2.1 - ALTERNATOR [1] Squirrel cage bars placed in the rotor pole faces of an alternator help reduce hunting A. above synchronous speed only B. below synchronous speed only C. above and blow synchronous speeds both D. none of the above [2] The stationary alternator should not be connected to live bus-bars because it A. is likely to run as synchronous motor B. will get short - circuited C. will decrease bus - bar voltage though momentarily D. will disturb generated emf's of other alternators connected in parallel. [3] With a unity load p.f, the effect of armature reaction on the main field flux of an alternator is A. distortional B. magnetising C. demagnetising D. nominal [4] At lagging loads, armature reaction in an alternator is A. cross-magnetising B. demagnetising C. non-effective D. magnetising [5] The frequency of voltage generated by an alternator having 4 poles and rotating at 1800 rpm is A. 60 Hz B. 7200 Hz C. 120 Hz D. 450 Hz [6] The main disadvantages of using short pitch winding in alternators is that it A. reduces harmonics in the generated voltage B. reduces the total voltage around the armature coils C. produces asymmetry in the three phase windings D. increases Cu of end connections. [7] Zero power factor method of an alternator is used to find its A. efficiency B. voltage regulation C. armature resistance D. synchronous impedance [8] Armature reaction in an alternator mainly affects A. rotor speed B. terminal voltage per phase C. frequency of armature current D. generated voltage per phase [9]The effect of increasing air gap length in the induction motor will increase the A. power factor B. speed C. magnetising current D. air gap flux [10] The principle of operation of a 3 phase induction motor is most similar to that of a A. synchronous motor B. repulsion start induction motor C. transformer with a shorted secondary D. capacitor start, induction run motor

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2.2 D.C. MACHINES [1] Voltage equation of a dc motor is

A. V = Eb + IaRa B. Eb = V + IaRa C. V = Eb/IaRa D. V = Eb + Ia2Ra

[2] Both Hopkinson's test and Field test A. Require two electrically coupled series motors B. Need two similar mechanically coupled motors C. Use negligible power D. Are regenerative tests

[3] which of the following motor has the constant speed? A. Series motor B. Shunt motor C. Cumulatively compound motor D. All of the above

[4] The usual test to find the efficiency of the traction motor is A. Field's test B. Retardation test C. Hopkinson's test D. Swinburn's test

[5] A DC series motor is best for driving A. Lathes B. Cranes and hoists C. Shears and punches D. Machine tools

[6] Retardation test on a dc shunt motor is used for finding A. Stray loss B. Copper loss C. Friction loss D. Iron loss

[7] In a DC series motor increasing the load current will A. Decrease the speed B. Increase the speed C. Better commutation D. Increase the back emf

[8] One of the main advantage of the swinburn's test is A. It is applicable both shunt and compound motors B. It needs one running test C. It is very economical and convenient D. It ignores any charge in iron loss

[9] the main disadvantage of hopkinson's test for finding efficiency of shunt dc motors is that it A. Requires full load power B. Ignores any change in iron loss C. Needs one motor and one generator D. Requires two identical shunt machines

[10] The most economical method of finding no losses of a large dc shunt motor is A. Hopkinson's test B. Swinburn's test C. Retardation test D. Field's test

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2.3 D.C. MOTOR [1] The basic requirement of a dc armature winding is that it must be

A. a closed one B. a lap winding C. a wave winding D. either b or c

[2] The sole purpose of a commutator in a dc generator is to A. increase output voltage B. reduce sparking at brushes C. provide smoother output D. convert the induced ac into dc

[3] In small DC machines, armature slots are sometimes not made axial but are skewed, results in A. quieter operation B. slight decrease in losses C. saving of copper D. both a and b

[4] The critical resistance of the dc generator is the resistance of A. armature B. field C. load D. brushes

[5] In a dc generator, the generator, the generated emf is directly proportional to the A. field current B. pole flux C. number of armature parallel paths D. number of dummy coils

[6] The commutation process in a dc generator basically involves A. passage of current from moving armature to a stationary load B. reversal of current in an armature coil as it crosses MNA C. conversion of ac to dc D. suppression of reactance voltage

[7] The essential condition for stable parallel operation of two dc generators having similar characteristics is that they should have

A. same kilowatt output ratings B. dropping voltage characteristics C. same percentage regulation D. same no load and full load speed

[8] An ideal dc generator has .......... voltage regulation. A. low B. zero C. positive D. negative

[9] Which generator has poorest voltage regulation A. series B. shunt C. compound D. high

[10] The voltage regulation of an over compound dc generator is always A. Positive B. negative C. zero D. high

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2.4 - INDUCTION MOTORS [1] The crawling in an induction motor is caused by

A. improper design of the machine B. low voltage supply C. high loads D. harmonics developed in the motor Ans = D

[2] The speed of an induction motor A. decreases too much with the increase of load B. increase with the increase of load C. decreases slightly with the increase of load D. remains constant with the increase of load

[3] The effect of increasing the length of the air gap in an induction motor will increase A. power factor B. speed C. magnetising current D. air-gap flux

[4] the difference between the synchronous speed and the actual speed of an induction motor is known as A. Regulation B. back lash C. slip D. lag

[5] Rotating magnetic field is produced in A.... A. single - phase induction motor B. three phase induction motor C. dc series motor D. ac series motor

[6] The stator core of the induction motor is made of A. Laminated cast iron B. Mild steel C. Silicon steel stampings D. Soft wood

[7] Star- delta starter of an induction motor A. Inserts resistance in rotor circuit B. Inserts resistance in stator circuit C. Applies reduced voltage to rotor D. Applies reduced voltage to stator

[8] The starting torque of a 1-phase induction motor is A. High B. Medium C. Low D. Zero

[9] The thrust developed by a linear induction motor depends on A. Synchronous speed B. Rotor input C. Number of poles D. both A and B

[10]A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical load. The torque-speed characteristics of the motor (solid curve) and of the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B? [GATE 2009]

A.A is stable B is unstable B.A is unstable B is stable C.Both are stable D.Both are unstable

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2.5 SYNCHRONOUS MOTOR [1] In a synchronous motor, damper winding is provided to

A. Stabilize rotor motion B. Suppress rotor oscillations C. Develop necessary starting torque D. Both B and C

[2] Synchronous capacitor is A. An ordinary static capacitor bank B. An over excited synchronous motor driving mechanical load C. An over excited synchronous motor running without mechanical load D. None of the above

[3] A synchronous machine is called as doubly excited machine because A. It can be over excited

B. It has two sets of rotor poles C. Both its rotor and stator are excited D. It needs twice the normal exciting current

[4]If the field of a synchronous motor is under excited, the power factor will be A. Lagging

B. Leading C. Unity D. More than unity

[5] The direction of rotation of a synchronous motor can be reversed by reversing A. Current to the field winding B. Supply phase sequence C. Polarity of rotor poles D. None of the above

[6]A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity pf. On changing the excitation only, the armature current will have A. leading pf with under-excitation

B. leading pf with over excitation C. lagging pf with over excitation D. no change of pf

[7] The maximum value of torque angle a in a synchronous motor is.......degrees electrical A. 45

B. 90 C. between 45 and 90 D. below 60

[8] The angle between the synchronous rotating stator flux and rotor poles of a synchronous motor is A. Synchronizing angle

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B. Torque angle C. Power factor angle D. Slip angle

[9] In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop A. synchronous motor torque

B. dc motor torque C. induction motor torque D. induction generator torque

[10] When load on a synchronous motor is increased its armature current is increased provided it is A. normally excited

B. over excited C. under exciter D. all of the above

2.6 GENERAL ELECTRICAL MACHINE [1]A field excitation of 20A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 2000V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200A is [GATE2009]

A.1V B.10V C.100V D.1000V

[2] Figure shows the extended view of a 2 pole dc machine with 10 armature conductors.Normal brush positions are shown by A and B, placed at the interpolar axis. If the brushes are now shifted, in the direction of rotation, to A' and B' as shown, the voltage waveform VA'B' will resemble. [GATE]

Ans: A

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[3]A 220V,50Hz,single-phase induction motor has the following connection diagram and winding orientations shown.MM' is the axis of the main stator winding(M1M2) and AA' is that of the auxiliary winding (A1A2). Directions of the winding axes indicate direction of flux when currents in the windings are in the directions shown. Parameters of each winding are indicateD. When switch S is closed, the motor [GATE]

A. rotates clockwise B. rotates anticlockwise C. does not rotate D. rotates momentarily and comes to a halt

[4] The electromagnetic torque Te of a drive, and its connected load torque Tl are as shown below. Out of the operating points A,B, C and D, the stable ones are [GATE 2007]

Ans:C [5] In a transformer, zero voltage regulation at full load is [GATE 2007] A. Not possible B. Possible at unity Power factor load C. Possible at leading Power factor load D. Possible at lagging Power factor load [6] The DC motor, which can provide zero speed regulation at full load without any controller is [GATE 2007]

A. Series B. Shunt C. Cumulative Compound D. Differential Compound

[7] A single phase 10kVA, 50 Hz transformer with 1kV primary winding draws 0.5A and 55W, at rated voltage and frequency, on no loaD. A second transformer has a core with all its linear 2 times the corresponding dimensions of the first transformer. The core material and lamination thickness are the same in both transformers. The primary windings of both the transformers have the same number of turns. If a rated voltage of 2kV at 50Hz is applied to the primary of the second transformer, then the no load current and power, respectively, are [GATE2012] A. 0.7 A,77.8W B. 0.7A,155.6 W C. 1A,110W D. 1A,220W

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[8] The locked rotor current in a 3-phase, star connected 15kW, 4-pole, 230V, 50Hz induction motor at rated conditions is 50A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236V, 57Hz supply is [GATE2012] A. 58.5A B. 45.0A C. 45.7A D. 55.6A [9] A 220V, 15kW,1000rpm shunt motor with armature resistance of 0.25, has a rated line current of 68A and a rated field current of 2.2A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8A and a field current of 1.8A is [GATE2012] A. 18.18% increase B. 18.18% decrease C. 36.36% increase D. 36.36% decrease [10] In 8 - pole wave connected motor armature, the number of parallel paths are

A. 8 B. 4 C. 2 D. 1

[11] A salient pole synchronous motor is running at no loaD. Its field current is switched off. The motor will (A) come to stop. (B) continue to run at synchronous speeD. (C) continue to run at a speed slightly more than the synchronous speeD. (D) continue to run at a speed slightly less than the synchronous speeD.

[12] The emf induced in the primary of a transformer (A) is in phase with the flux.

(B) lags behind the flux by 90 degree. (C) leads the flux by 90 degree. (D) is in phase opposition to that of flux.

[13]The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about (A) 50 Hz.

(B) 20 Hz. (C) 2 Hz. (D) Zero.

[14] The two windings of a transformer is (A) conductively linked.

(B) inductively linked. (C) not linked at all. (D) electrically linked.

[15] The D.C. series motor should always be started with load because (A) at no load, it will rotate at dangerously high speeD. (B) it will fail to start. (C) it will not develop high starting torque. (D) all are true.

[16] Out of the following methods of heating the one which is independent of supply frequency is

(A) electric arc heating (B) induction heating (C) electric resistance heating (D) dielectric heating

[17] In a stepper motor the angular displacement (A) can be precisely controlled. (B) it cannot be readily interfaced with micro computer based controller. (C) the angular displacement cannot be precisely controlled.

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(D) it cannot be used for positioning of work tables and tools in NC machines. [18] The generation voltage is usually

(A) between 11 KV and 33 KV. (B) between 132 KV and 400 KV. (C) between 400 KV and 700 KV. (D) None of the above.

[19] When a synchronous motor is running at synchronous speed, the damper winding produces (A) damping torque. (B) eddy current torque. (C) torque aiding the developed torque. (D) no torque.

[20] A hysteresis motor (A) is not a self-starting motor. (B) is a constant speed motor. (C) needs dc excitation. (D) can not be run in reverse speed.

[21] If a transformer primary is energized from a square wave voltage source, its output voltage will be (A) A square wave. (B) A sine wave. (C) A triangular wave. (D) A pulse wave.

[22] The power factor of a squirrel cage induction motor is (A) low at light load only. (B) low at heavy load only. (C) low at light and heavy load both. (D) low at rated load only.

[23] In a D.C. machine, the armature mmf is (A) stationary w.r.t. armature. (B) rotating w.r.t. field. (C) stationary w.r.t. field. (D) rotating w.r.t. brushes.

[24] In a transformer the voltage regulation will be zero when it operates at (A) unity p.f. (B) leading p.f. (C) lagging p.f. (D) zero p.f. leading.

[25] The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be

(A) 3.6 V. (B) 2.5 V. (C) 3.0 V. (D) 6.0 V.

[26] The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a

(A) dc machine. (B) 3 phase induction machine. (C) synchronous machine. (D) single phase induction machine.

Ans: All are correct [27] The current from the stator of an alternator is taken out to the external load circuit through

(A) slip rings. (B) commutator segments. (C) solid connections. (D) carbon brushes.

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[28] A motor which can conveniently be operated at lagging as well as leading power factors is the (A) squirrel cage induction motor. (B) wound rotor induction motor. (C) synchronous motor. (D) DC shunt motor.

[29] The most suitable servomotor for low power applications is (A) a dc series motor. (B) a dc shunt motor. (C) an ac two-phase induction motor. (D) an ac series motor.

[30] The size of a conductor used in power cables depends on the (A) operating voltage. (B) power factor. (C) current to be carried. (D) type of insulation used.

[31]The size of the feeder is determined primarily by (A) the current it is required to carry. (B) the percent variation of voltage in the feeder. (C) the voltage across the feeder. (D) the distance of transmission.

[32] The emf induced in the primary of a transformer (A) is in phase with the flux. (B) lags behind the flux by 90 degree. (C) leads the flux by 90 degree. (D) is in phase opposition to that of flux.

[33] The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a

(A) dc machine. (B) 3 phase induction machine. (C) synchronous machine. (D) single phase induction machine. Ans: all options are correct

[34]As the voltage of transmission increases, the volume of conductor (A) increases. (B) does not change. (C) decreases. (D) increases proportionately. HINT:-Due to skin effect

[35]In a 3-phase synchronous motor (A) the speed of stator MMF is always more than that of rotor MMF. (B) the speed of stator MMF is always less than that of rotor MMF. (C) the speed of stator MMF is synchronous speed while that of rotor MMF is zero. (D) rotor and stator MMF are stationary with respect to each other. HINT:-Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction.

[36]An alternator is delivering rated current at rated voltage and 0.8 power-factor lagging case. If it is required to deliver rated current at rated voltage and 0.8 power-factor leading, the required excitation will be

(A) less. (B) more. (C) more or less. (D) the same. HINT:-Over excitation gives leading power factor and under excitation gives lagging p.f .

[37] Out of the following methods of heating the one which is independent of supply frequency is (A) electric arc heating

(B) induction heating

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(C) electric resistance heating (D) dielectric heating

[38]In a capacitor start single-phase induction motor, the capacitor is connected (A) in series with main winding. (B) in series with auxiliary winding. (C) in series with both the windings. (D) in parallel with auxiliary winding. HINT:-To make single phase motor self start. We split the phases at 90 degree. Hence, motor behaves like a two phase motor.

[39]A synchro has (A) a 3-phase winding on rotor and a single-phase winding on stator. (B) a 3-phase winding on stator and a commutator winding on rotor. (C) a 3-phase winding on stator and a single-phase winding on rotor. (D) a single-phase winding on stator and a commutator winding on rotor.

HINT:-The basic synchro unit called a synchro transmitter. Its construction similar to that of a Three phase alternator.

[40] A ceiling fan uses (A) split-phase motor. (B) capacitor start and capacitor run motor. (C) universal motor. (D) capacitor start motor. HNIT:-To give starting torque and to maintain speed.

[41] The torque-speed characteristics of an A.C. operated universal motor has a ______characteristic and it______ be started under no-load condition.

(A) inverse, can (B) nearly inverse, can (C) inverse, cannot (D) nearly inverse, cannot

HINT:-N direct proportional to 1/ T [42] In the heating process of the ________type a simple method of temperature control is possible by means of a special alloy which loses its magnetic properties at a particular high temperature and regains them when cooled to a temperature below this value.

(A) Indirect induction over (B) core type induction furnace (C) coreless induction furnace (D) high frequency eddy current

HINT:-Magnetic property of alloy changes with change of the temperature and Heat is produced due to eddy current = Square(i) * R and i proportional to square(f)

[43] In order to reduce the harmful effects of harmonics on the A.C. side of a high voltage D.C. transmission system ______are provideD.

(A) synchronous condensers (B) shunt capacitors (C) shunt filters (D) static compensators

HINT:- Xc= 1/C

[44]An A.C. tachometer is just a ________with one phase excited from the carrier frequency. (A) two-phase A.C. servomotor (B) two-phase induction motor (C) A.C. operated universal motor (D) hybrid stepper motor.

HINT:-It is a special purpose machine. It's stator coil can be energized by electronically switched current.

[45] The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz

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(A) 8 (B) 4 (C) 6 (D) 2

HINT:- f = N P/ 120 = 1000* 6/120 = 50 Hz

Rotor frequency fr=s * f = 0.04 * 50= 2.0 Hz [46] The speed-torque characteristics of a DC series motor are approximately similar to those of the _________motor.

(A) universal (B) synchronous (C) DC shunt (D) two-phase HINT:-Universal motor has same characteristics as DC series motor It is known as an A.c series motor.

[47]In case of a universal motor, torque pulsation is minimized by _________. (A) load inertia (B) rotor inertia (C) both rotor and load inertia (D) none of the above

[48] A hysteresis motor (A) is not a self-starting motor. (B) is a constant speed motor. (C) needs dc excitation. (D) can not be run in reverse speed.

[49] The most suitable servomotor for low power applications is (A) a dc series motor. (B) a dc shunt motor. (C) an ac two-phase induction motor. (D) an ac series motor.

[50] The size of a conductor used in power cables depends on the (A) operating voltage. (B) power factor. (C) current to be carried. (D) type of insulation used.

[51]A stepper motor is (A) a dc motor. (B) a single-phase ac motor. (C) a multi-phase motor. (D) a two phase motor.

HINT:-Stepper motor works on 1-phase-ON or 2-phase ON modes of operation [52] A motor which can conveniently be operated at lagging as well as leading power factors is the

(A) squirrel cage induction motor. (B) wound rotor induction motor. (C) synchronous motor. (D) DC shunt motor.

[53] The D.C. series motor should always be started with load because (A) at no load, it will rotate at dangerously high speed. (B) it will fail to start. (C) it will not develop high starting torque. (D) all are true.

[54] The sheath is used in cable to (A) provide strength to the cable. (B) provide proper insulation. (C) prevent the moisture from entering the cable.

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(D) avoid chances of rust on strands. HINT:-The sheath in underground cable is provided to give mechanical strength.

[55] The drive motor used in a mixer-grinder is a (A) dc motor. (B) induction motor. (C) synchronous motor. (D) universal motor. HINT:-The universal motor is suitable for AC & DC both supply systems.

[56] A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is

(A) 1500 r.p.m. (B) 1425 r.p.m. (C) 25 r.p.m. (D) 75 r.p.m. HINT:-NS = 120f /P = 120 x 50 /4 =1500rpm N = NS ( 1-s) = 1500 (1-0.05) = 1425 Relative speed = 1500 1425 = 75 rpm

[57] The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be

(A) 3.6 V. (B) 2.5 V. (C) 3.0 V. (D) 6.0 V.

[58] The current from the stator of an alternator is taken out to the external load circuit throu (A) slip rings. (B) commutator segments. (C) solid connections. (D) carbon brushes.

[59] In a 3 phase induction motor the maximum torque (A) is proportional to rotor resistance r2 .

(B) does not depend on r2 . (C) is proportional to square root of r2 . (D) is proportional to square of r2 .

[60] In a D.C. machine, the armature mmf is (A) stationary w.r.t. armature.

(B) rotating w.r.t. field. (C) stationary w.r.t. field. (D) rotating w.r.t. brushes.

[61] In a transformer the voltage regulation will be zero when it operates at (A) unity p.f. (B) leading p.f. (C) lagging p.f. (D) zero p.f. leading.

[62] In a stepper motor the angular displacement (A) can be precisely controlled.

(B) it cannot be readily interfaced with micro computer based controller. (C) the angular displacement cannot be precisely controlled. (D) it cannot be used for positioning of work tables and tools in NC machines.

[63] The power factor of a squirrel cage induction motor is (A) low at light load only. (B) low at heavy load only. (C) low at light and heavy load both. (D) low at rated load only.

[64] The generation voltage in India is usually

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(A) between 11 KV and 33 KV. (B) between 132 KV and 400 KV. (C) between 400 KV and 700 KV. (D) None of the above.

[65] When a synchronous motor is running at synchronous speed, the damper winding produces (A) damping torque. (B) eddy current torque. (C) torque aiding the developed torque. (D) no torque.

[66]If a transformer primary is energised from a square wave voltage source, its output voltage will be (A) A square wave. (B) A sine wave. (C) A triangular wave. (D) A pulse wave.

[67] A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will (A) come to stop. (B) continue to run at synchronous speed. (C) continue to run at a speed slightly more than the synchronous speed. (D) continue to run at a speed slightly less than the synchronous speed.

[68] The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about (A) 50 Hz. (B) 20 Hz. (C) 2 Hz. (D) Zero.

[69] The speed of a dc motor can be controlled by varying A. its flux per pole B. resistance of armature circuit C. applied voltage D. all of the above

[70] Regarding Ward-Leonard system of speed control which statement is false? A. It is usually used where wide and very sensitive speed control is required B. It is used for motors having ratings from 750kW to 4000Kw C. Capital outlay involved in the system is right since it uses two extra machines D. It gives a speed range of 10:1 but in one direction only E. It has low overall efficiency especially at light loads

[71] In a DC motor, unidirectional torque is produced with the help of A. Brushes B. Commutator C. End-plates D. Both a and b

[72] The counter emf of dc motor A. Often exceeds the supply voltage B. Aids the applied voltage C. Helps in energy conversion D. Regulates its armature voltage

[73] The Eb/V ratio of a dc motor is an indication of its A. Efficiency B. Speed regulation C. Starting torque D. Running Torque

[74] The induced emf in the armature conductors of a dc motor is A. Sinusoidal B. Trapezoidal C. Rectangular

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D. Alternating [75] A dc motor can be looked upon as dc generator with the power flow

A. Reduced B. Reversed C. Increased D. Modified

[76] A series motor is best suited for driving A. Lathes B. Cranes and hoists C. Shears and punches D. Machine tools

[77]The Ta/Ia graph of a dc series motor is a A. Parabola from no load to overload B. Straight line throughout C. Parabola throughout D. Parabola upto full load and a straight line at overloads

[78] When load is removed, .......motor will run at the highest speed. A. Shunt B. Cumulative compound C. Differential compound D. Series

[79] The power factor of a squirrel cage induction motor is A. Low at light loads only B. Low at heavy loads only C. Low at light and heavy loads both D. Low at rated load only

[80] The synchronous speed of a linear induction motor does not depend on A. Width of pole pitch B. Number of poles C. Supply frequency D. Any of the above

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CHAPTER-3. TRANSFORMERS [1] A Buchholz relay can be installed on

A. Auto-transformers B. Air-cooled transformers C. Welding transformers D. Oil cooled transformers

[2] The chemical used in breather is A. Asbestos fibre B. Silica sand C. Sodium chloride D. Silica gel

[3] Material used for construction of transformer core is usually A. 0.4mm to 0.5 mm

B. 4mm to 5 mm C. 14mm to 15 mm D. 25mm to 40 mm

[4] Helical coils can be used on A. Low voltage side of high kVA transformers B. High frequency transformers C. High voltage side of small capacity transformers D. High voltage side of high kVA rating transformers

[5] The transformer ratings are usually expressed in terms of A. Volts B. Amperes C. kW D. kVA

[6] The value of flux involved in the emf equation of a transformer is A. rms value B. rms value C. Maximum value D. Instantaneous value

[7] The main advantage of auto transformer over a two winding transformer is A. Hysteresis losses are reduced B. Saving in winding material C. Copper losses are negligible D. Eddy losses are totally eliminated

[8] During short circuit test iron losses are negligible because A. The current on secondary side is negligible B. The voltage on secondary side does not vary C. The voltage applied on primary side is low D. Full-load current is not supplied to the transformer

[9] Which of the following properties is not necessarily desirable in the material for transformer core? A. Mechanical strength B. Low hysteresis loss C. High thermal conductivity D. High permeability

[10] The main reason for generation of harmonics in a transformer could be A. Fluctuating load B. Poor insulation C. Mechanical vibrations D. Saturation of core

[11] A transformer transforms

A. frequency

B. voltage

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C. current D. voltage and current

[12] Which is not the basic element of the transformer? A. core B. primary winding C. secondary winding D. mutual flux

[13] In an ideal transformer A. windings have no resistance B. core has no losses C. core has infinite permeability D. all of the above

[14] The frequency of the secondary voltage of a transformer will be..... A. less than frequency of the primary voltage B. equal to the primary voltage C. greater than the frequency of the primary voltage D. much greater than the frequency of the primary voltage

[15] The efficiency of a transformer is maximum when A. It runs at half full load B. it runs at full load C. its Cu loss equal iron loss D. it runs over load

[16] A step-up transformer increases A. voltage B. current C. power D. frequency

[17] Which type of loss is not common to transformer and rotating machines? A. Eddy current loss B. Copper loss C. Hysteresis loss D. Windage loss

[18] The transformer core is laminated to A. Reduce the copper losses B. Reduce the core losses C. Reduce the eddy current losses D. None of the above

[19] Transformer cores are built up from laminations rather than from solid metal so that A. Oil penetrates the core more easily B. Eddy current loss is reduced C. Less lamination is required for the windings D. Turn ratio is higher than voltage ratio

[20] the main purpose of using core in a transformer is to A. Decrease iron losses B. prevent eddy current loss C. eliminate magnetic hysteresis D. decrease reluctance of the common magnetic circuit

[21] The ordinary two winding transformer's primary and secondary windings always have A. different no of turns B. same size of copper wire C. a common magnetic circuit D. separate magnetic circuits

[22] No load test on a transformer is carried out to find A. copper loss B. magnetising current

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C. magnetising current and no load loss D. efficiency of the transformer

[23] Transformers are rated in KVA instead of KW because A. load power factor is often not known B. KVA is fixed whereas KW depends on load power factor C. Total transformer loss depends on volt ampere D. It has become customary

[24] Transformer cores are laminated in order to A. simplify its construction B. minimise eddy current loss C. reduce cost D. reduce hysteresis loss

[25] A step up transformer increases A. voltage B. current C. power D. frequency

[26] In a two winding transformer, the primary and secondary induced emf E1 & E2 are always A. equal in magnitude B. anti phase with each other C. in phase with each other D. determined by load on transformer secondary

[27] In a transformer, the leakage flux of each winding is proportional to the current in that winding because A. Ohm's law applies to magnetic circuits B. Leakage paths do no saturate C. the two windings are electrically isolated D. mutual flux is confined to the core

[28] In a two winding transformer, the emf /turn in secondary winding is always......the induced emf power turn in primary

A. equal to k times B. equal to 1/k times C. equal D. greater than

[29] In transformer terminology, the ratio 20:1 indicates that A. there are 20 turns on primary one turn on secondary B. secondary voltage is 1/20th of the primary C. primary current is 20 times greater than the secondary current D. for every 20 turns on primary, there is one turn on secondary

[30] In performing the short circuit test of a transformer A. high voltage is usually short circuited B. low voltage side is usually short circuited C. any side is short circuited with preference D. none of the above

Common Data for Questions 1 and 2: [GATE 2009] The star-delta transformer shown above is excited on the star side with a balanced,4-wire,3-phase,sinusoidal voltage supply of rated magnitude. The transformer is under no load condition.

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[31] With both S1 and S2 open, the core flux waveform will be [GATE 2009]

A. a sinusoidal at fundamental frequency B. flat-topped with third harmonic C. peaky with third-harmonic D. none of these

[32] With S2 closed and S1 open, the current waveform in the delta winding will be [GATE 2009] A. a sinusoidal at fundamental frequency B. flat-topped with third harmonic C. only third-harmonic D. none of these

Common Data for Questions 3 and 4: The circuit diagram shows a two-winding, losses transformer with no leakage flux, excited from a current source,i(t),whose waveform is also shown. The transformer has a magnetizing inductance of 400/ mH.

[33] The peak voltage across A and B, with S open is [GATE 2009]

A. 400/ V B. 800 V C. 4000/ V D. 800/ V

[34] If the waveform of i(t) is changed to i(t)= 10 sin(100t)A, the peak voltage across A and B with S closed is [GATE 2009]

A. 400V B. 240V C. 320V D. 160V

Statement for Linked Answer Questions 5 and 6: [GATE 2009]

The figure above shows coils 1 and 2, with dot markings as shown, having 4000 and 6000 turns respectively. Both the coils have a rated current of 25A. Coil 1 is excited with single phase, 400V, 50Hz supply.

[35] The coils are to be connected to obtain a single phase, 400/1000V, auto-transformer to drive a load of 10kVA. Which of the options given should be exercised to realize the required auto-transformer? [GATE 2009]

A. Connect A and D; Common B B. Connect B and D; Common C

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C. Connect A and C; Common B D. Connect A and C; Common D

[36] In the autotransformer obtained in Question 57, the current in each coil is [GATE 2009] A. Coil-1 is 25A and Coil-2 is 10A B. Coil-1 is 10A and Coil-2 is 25A C. Coil-1 is 10A and Coil-2 is 15A D. Coil-1 is 15A and Coil-2 is 10A

[37]The single phase, 50Hz iron core transformer in the circuit has both the vertical arms of cross sectional area 20cm2 and both the horizontal arms of cross sectional area 10cm2. If the two windings shown were wound instead on opposite horizontal arms, the mutual inductance will [GATE 2009]

A.Double B.Remain same C.Be halved D.Become one quarter

Ans:- [38] In a transformer the energy is conveyed from primary to secondary?

A. Through cooling coil

B. Through air

C. By the flux

D. None of the above

[39] Which loss is not common between a transformer and rotating machines?

A. Eddy current loss

B. Copper loss

C. Windage loss

D. Hysteresis loss

[40] No load test on a transformer is carried out to find

A. Copper loss

B. magnetising current

C. Magnetising current and loss

D. Efficiency of the transformer

[41] Sumpner's test is conducted on transformers to find

A. Temperature

B. Stray losses

C. All-day efficiency

D. None of the above

[42] The efficiency of a transformer will be maximum when

A. copper losses = hysteresis losses

B. Hysteresis losses = eddy current losses

C. Eddy current losses = copper losses

D. Copper losses = iron losses

[43] The purpose of providing an iron core in a transformer is to

A. Provide support to windings

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B. Reduce hysteresis loss

C. decrease the reluctance of the magnetic path

D. reduce eddy current losses

[44] The highest voltage for transmitting electrical power in India is

A. 33kV

B. 66kV

C. 132kV

D. 400kV

[45] The function of conservator in a transformer is

A. To protect against internal fault

B. To reduce copper as well as core losses

C. To cool the transformer oil

D. To take care of the expansion and contraction of transformer oil due to variation of

temperature of surroundings

[46] The core used in high frequency transformer is usually

A. Copper core

B. Cost iron core

C. Air core

D.Mid steel core

[47] Cross over windings are used in

A. Low voltage side of high kVA rating transformers

B. Current transformers

C. High voltage side of high kVA rating transformers

D. High voltage side of low kVA rating transformers

[48] During light loads, the transformer efficiency is low because

A. Secondary output is low B. Transformer losses are high C. Fixed loss is high in proportion to the output D. Cu loss is small

[49] The open circuit test is carried out in a transformer to find the A. Cu loss B. Core loss C. Total loss D. Insulation resistance

[50] The equivalent resistance of the primary of the transformer having K = 5, R1 = 0.1 ohm when referred to secondary becomes .....ohm

A. 0.5 B. 0.02 C. 0.004 D. 2.5

[51] A transformer has negative voltage regulation when its load power factor is A. Zero B. Unity C. Leading D. Lagging

[52] A transformer has 1000 primary turns, connected to 250V ac supply. To get 400V secondary voltage the no of secondary turns should be

A. 1600 B. 250 C. 400 D. 1250

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[53] The iron loss of the transformer is negligible during short circuit test because A. The entire input is just sufficient to meet Cu losses only B. Flux produced is a small fraction of the normal flux C. Iron core becomes fully saturated D. Supply frequency is held constant

[54] In operating a 400hz transformer at 50Hz A. Only voltage is reduced in the same proportion as the frequency B. Only KVA rating is reduced in the same proportion as the frequency C. Both voltage and KVA rating are reduced in the same proportion as frequency D. None of the above

[55] When a 400Hz transformer is operated at 50Hz its KVA rating is A. Reduced to 1/8 B. Increased 8 times C. Unaffected D. Increased 64 times

[56] Which of the following does not change in a transformer? A. Current B. Voltage C. Frequency D. All of the above

[57] In a transformer the resistance between its primary and secondary is A. Zero B. 1 C. 1000 D. Infinite

[58] If a pump motor is run on 2/3rd of its supply voltage, it will A. Continue to deliver same power B. burn C. stall D. continue to run at lower speed

[59] An ideal transformer has infinite values of primary and secondary inductances. The statement is A. True B. False

[60] The noise resulting from vibrations of laminations set by magnetic force, is termed as A. magnetostriction B. boo C. hum D. zoom

[61] The degree of mechanical vibrations produced by the laminations of a transformers depends on A. Tightness of clamping B. Gauge of laminations C. Size of laminations D. All of the above

[62] The no-load current drawn by transformer is usually........percent of the full-load current? A. 0.2 to 0.5 % B. 2 to 5% C. 12 to 15 % D. 20 to 30 %

[63] The dielectric strength of transformer oil is expected to be A. 1kV B. 33kV C. 100kV D. 330 kV

[64] During the short-circuit test on a small transformer the frequency is increased from 50-200Hz. The copper losses will increase by a factor of

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A. 16 B. 4 C. 1 D. 1/4

[65] No-load current in a transformer A. Lags behind the voltage by about 75 deg B. leads the voltage by about 75 C. lags behind the voltage by about 15 D. leads the voltage by about 15

[66] While conducting short-circuit test on a transformer the following side is short circuited A. High voltage side B. low voltage side C. primary side D. secondary side

[67] The chemical used in breather for transformer should have the quality of A. Ionizing air B. Absorbing moisture C. Cleansing the transformer oil D. Cooling the transformer oil

[68] The path of a magnetic flux in a transformer should have A. High resistance B. High reluctance C. Low resistance D. Low reluctance

[69] In case there are burrs on the edges of the laminations of the transformer, it is likely to result in A. Vibrations B. Noise C. Higher eddy currents loss D. Higher hysteresis loss

[70] Silicon steel used in laminations mainly reduces A. Hysteresis loss B. Eddy current losses C. Copper loss D. All of the above

[71] High frequency transformers sometimes make use of ferrite cores because it has A. High specific gravity B. High resistance C. High hysteresis D. low permeability

[72] Harmonics in transformer result in A. Increased core losses B. Increased I2R losses C. Magnetic interference with communication circuits D. All of the above

[73] The full load copper loss of a transformer is 1600W. At half-load the copper loss will be A. 6400W B. 1600W C. 800W D. 400W

[74] Power transformers are generally designed to have maximum efficiency around A. No load B. Half load C. Near full load D. 10% overload

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[75] Two transformers are connected in parallel. These transformers do not have equal percentage impedance which results

A. Short-circuiting of the secondaries B. Power factor of one of the transformers is leading while that of the other lagging C. Transformers having higher copper losses will have negligible core losses D. Loading of the transformers not in proportion to their kVA ratings.

[76] The changes in volume of transformer cooling oil due to variation of atmospeheric temperature during day and night is taken care of by which part of transformer?

A. Conservator B. Breather C. Bushings D. Buchholz relay

[77] The transformer laminations are insulated from each other by A. Mica strip B. Thin coat of varnish C. Paper D. Any of the above

[78] Which type of winding is used in 3 phase shell type transformer? A. Circular type B. Sandwich type C. Cylindrical type D. Rectangular type

[79] During open circuit test of a transformer A. Primary is supplied rated voltage B. Primary is supplied full load current C. Primary is supplied current at reduced voltage D. Primary is supplied rated kVA

[80] Which of the following is not standard voltage for power supply in India A. 11kV B. 33kV C. 66 kV D. 122 kV

[81] A transformer core is laminated to

A. Reduce hysteresis loss

B. Reduce eddy current losses

C. Reduce copper losses

D. Reduce all above losses

[82] While conducting short-circuit test on a transformer the following side is short-circuited A. High voltage side B. Low voltage side C. Primary side D. Secondary side

[83] Distribution transformers are generally designed for maximum efficiency around A. 90% load B. Zero load C. 25% load D. 50% load

[84] A transformer oil must be free from A. Sludge B. Odour C. Gases D. Moisture

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[85] The use of higher flux density in the transformer design A. Reduces weight per KVA B. Reduces iron losses C. Reduces copper losses D. Increases part load efficiency

[86] In a transformer the toppings are generally provided on A. Primary side B. Secondary side C. Low voltage side D. High voltage side

[87] Which of the following is not a part of transformer installation? A. Conservator B. Breather C. Buchholz relay D. Exciter

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CHAPTER-4. POWER SYSTEM

[1]Three generators are feeding a load of 100MW.The details of the generators are Rating(MW) Efficiency() Regulation(p.u.) on 100MVA base Generator-1 100 20 0.02 Generator-2 100 30 0.04 Generator-3 100 40 0.03 In the event of increased load power demand, which of the following will happen? [GATE 2009]

A. All the generators will share equal power B.Generator 3 will share more power compared to Generator 1 C.Generator 1 will share more power compared to Generator 2 D.Generator 2 will share more power compared to Generator 3 Ans: None of the above

[2]A 500MW, 21kV, 50Hz, 3-phase, 2-pole synchronous generator having a rated p.f. =0.9, has a moment of inertia of 27.5 x 103 kg-m2. The inertia constant (H) will be [GATE 2009]

A.2.44 s B.2.71s C.4.88s D.5.42s

[3]For a fixed value of complex power flow in a transmission line having a sending end voltage V, the real power loss will be proportional to [GATE 2009]

A.V B.V2 C.1/V2 D.1/V

[4]How many 200W/220V incandescent lamps connected in series would consume the same total power as a single 100W/220V incandescent lamps? [GATE 2009]

A.Not possible B.4 C.3 D.2

[5]Match the items in list I with the items in list II and select the correct answer using the codes given below the lists [GATE 2009] LIST 1 LIST2 TO USE A.improve power factor 1.shunt reactor B.reduce the current ripples 2.shunt capacitor C.increase the power flow in line 3.series capacitor D.reduce the ferranti effect 4.series reactor

A. a2,b3,c4,d1 B. a2,b4,c3,d1 C. a4,b3,c1,d2 D. a4,b1,c3,d2

[6]Match the items in list I with the items in list II and select the correct answer using the codes given below the lists [GATE 2009] LIST 1 LIST2 Type of Transmission Line Type of Distance Relay Preferred A.short line 1.Ohm Relay

B.Medium line 2.Reactance Relay C.Long line 3.Mho Relay

A. a2,b1,c3 B. a3,b2,c1 C. a1,b2,c3 D. a1,b3,c2

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[7] For the system shown below, SD1 and SD2 are complex power demands at bus 1 and bus 2 respectively. If |V2|=1pu, the VAR rating of the capacitor (QG2) connected at bus 2 is [GATE2012]

A. 0.2 pu B. 0.268 C. 0.312 D. 0.4pu [8] A cylindrical rotor generator delivers 0.5 pu power in the steady-state to an infinite bus through a transmission line of reactance 0.5 pu. The generator no-load voltage is 1.5 pu and the infinite voltage is 1.5 pu. The inertia constant of the generator is 5MW-s/MV and the generator reactance is 1 pu.The critical clearing angle, in degrees, for a three-phase dead short circuit fault at the generator terminal is [GATE2012] A. 53.5 B. 60.2 C. 70.8 D. 79.6 [9] Power is transfered from system A to system B by an HVDC link as shown in the figure.If the voltages

VAB and VCD are as indicated in the figure, and I>0,then

A. VAB0,VCD>0,VAB>VCD C. VAB>0,VCD>0,VAB0,VCD

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[11] Consider two buses connected by an impedance of (0+j5). The bus 1 voltage is 10030 V, and bus 2 voltage is 1000 V. The real and reactive power supplied by bus 1, respectively are A. 1000W,268Var B. -1000W,-134Var C. 276.9W,-56.7Var D. -276.9W,56.7Var [12] A three-phase, 33kV oil circuit breaker is rated 1200A, 2000MVA, 3s. The symmetrical breaking current is A. 1200A B. 3600A C. 35kA D. 104.8kA [13] Consider a stator winding of an alternator with an internal high-resistance ground fault.The currents under the fault condition are as shown in the figure.The winding is protected using a differential current scheme with current transformers of ratio 400/5A as shown. The current through the operating coil is

A. 0.17875A B. 0.2A C. 0.375A D. 60kA [14] A 50Hz synchronous generator is initially connected to a long lossless transmission line which is open circuited at the receiving enD.With the field voltage held constant, the generator is disconnected from the transmission line.Which of the following may be said about the steady state terminal voltage and field current of the generator?

A. The magnitude of terminal voltage decreases,and the field current does not change B. The magnitude of terminal voltage increases,and the field current does not change C. The magnitude of terminal voltage increases,and the field current increases D. The magnitude of terminal voltage does not change,and the field current decreases Ans: none of the above [15] For enhancing the power transmission in a long EHV transmission line, the most preferred method is to connect a A. series inductive compensator in the line B. shunt inductive compensator at the receiving end C. series capacitive compensator in the line D. shunt capacitive compensator at the sending end

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[16] Consider a three-phase,50Hz,11kV distribution system.Each of the conductors is suspended by an insulator string having two identical porcelain insulators.The self capacitance of the insulator is 5 times the shunt capacitance between the link and the ground,as shown in the figure.The voltage across the two insulators is

A. e1=3.74kV,e2=2.61kV B. e1=3.46kV,e2=2.89kV C. e1=6.0kV,e2=4.23kV D. e1=5.5kV,e2=5.5kV [17] Consider a three-core, three-phase,50Hz,11kV cable whose conductors are denoted as R,Y and B in the figure.The inter-phase capacitance(C1) between each pair of conductors is 0.2F and the capacitance between each line conductor and the sheath is 0.4F.The per-phase charging current is

A. 2.0A B. 2.4A C. 2.7A D. 3.5A [18] For the power system shown in the figure below,the specifications of the components are the following: G1: 25kV,100MVA,X=9% G2: 25'kV,100MVA,X=9% T1: 25kV/220kV,90MVA,X=12% T2: 220kV/25kV,90MVA,X=12% Line1: 220kV, X=150 ohms.

Choose 25kV as the base voltage at the generator G1 and 200MVA as the MVA base.The impedance diagram is.....Options A,B,C,D are given below

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[A]

[B]

[C]

[D] Ans:B

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CHAPTER-5. POWER ELECTRONICS

[1] A 240V, 50Hz supply feeds a highly inductive load of 50 Ohm resistance through a half controlled thyristor bridge. When the firing angle =45, the load power is

A. 418 W B. 512 W C. 367 W D. 128 W

Exp: Vav = (Vm/) (1 + cos) = [(2*240)/() ) ( 1 + cos45)] = 184.4 V Iav = Vav / R = 184.4/50 =3.69A

= 3.69 * [(180 -45)/180] = 3.2A P = 3.2 * 3.2 *50 = 512 W .................................................................................................................................................................................................................................. [2] A D.C. to D.C. chopper operates from a 48 V battery source into a resistive load of 24Ohm. The frequency of the chopper is set to 250Hz. When chopper on-time is 1 ms the load power is

A. 6W B. 12W C. 24W D. 48W

Exp: Vavg = V* f * Ton = 48 x 250 x (10 - 3) = 12V Iav = Vav/R = 12/24 = 0.5A Vrms= V* Square root (Ton) * f = 48 * Square root (0.25) = 24V Irms = Vrms/R = 24/24 = 1A P = Irms * Irms * R = 1 * 24 = 24W .................................................................................................................................................................................................................................. [3] A thyristor half wave controlled converter has a supply voltage of 240V at 50Hz and a load resistance of 100 Ohm. when the firing delay angle is 30 the average value of load current is

A. 126A B. 2.4A C. 126mA D. 24 A

Exp:We know the output wave form of the half wave rectifier For any delay angle alpha, the average load voltage is given by

solving,

substituting the values in the above equation, Vav = (2*240) / (2) * [ 1 + cos30 ] = 100.8 V Iav = Vav/R = 100.8/100 = 126 mA .................................................................................................................................................................................................................................. [4] A full-wave fully controlled bridge has a highly inductive load with a resistance of 55 Ohm, and a supply of 110V at 50Hz. The value of load power for a firing angle =75 is

A. 10W B. 11W C. 10.5W

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D. 10.9W Exp: Vav = [2Vm/()]cos] = [(2 *(2*110)/ 3.14 ] * cos 75 = 99 cos 75 = 25.6V Iav = Vav / R = 25.6/55 = 0.446A = Irms P = Irms * Irms * R = 0.446 * 0.446 * 55 =10.9W .................................................................................................................................................................................................................................. [5] A D.C. to D.C. chopper operates from a 48 V source with a resistive load of 24Ohm. The chopper frequency is 250Hz. When Ton= 3 ms, the rms current is

A. 1.5A B. 15mA C. 1.73A D. 173mA

Exp: Vav = V *f * Ton = 48 * 250 * 3 *(10 - 3) = 36V Iav = Vav/R = 36/24 = 1.5 A Vrms = V* Square root (Ton) * f = 48 * 0.75 = 41.6V Irms = Vrms/R = 41.6/24 = 1.73A .................................................................................................................................................................................................................................. [6] A 240V, 50Hz supply feeds a highly inductive load of 50 Ohm resistance through a thyristor full control bridge. when the firing angle = 45, load power is A. 456 W

B. 466 W C. 732 W D. 120 W

Exp: Vav = (2Vm/) * cos = [(2 * 339)/3.14] cos 45 = 152.6V Iav = Vav/R = 152.6 / 50 = 3.05A = Irms P = Square of Irms * R = 3.04 * 3.04 * 50 = 466W .................................................................................................................................................................................................................................. [7] Thermal runaway is not possible in FET because as the temperature of FET increases

A. The mobility decreases B. The transconductance increases C. The drain current increases D. None of the above

[8] The ripple frequency from a full wave rectifier is A. twice that from a half wave circuit B. the same as that from a half wave circuit C. half that from a half wave circuit D. 1/4 that from a half wave circuit

[9] In a full-wave rectifier using two ideal diodes,Vdc & Vm are the dc & peak values of the voltage respectively across a resistive loaD. If PIV is the peak inverse voltage of the diode,then the appropriate relationships for the rectifier is

A. Vdc=Vm/,PIV=2Vm B. Vdc=2Vm/,PIV=2Vm C. Vdc=2Vm/,PIV=Vm D. Vdc=Vm/,PIV=Vm

[10] The output of a rectifier circuit without filter is

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A. 50 Hz AC B. smooth DC C. pulsating DC D. 60 hz AC

[11] An advantage of full wave bridge rectification is A. it uses the whole transformer secondary for the entire ac input cycle B. it costs less than other rectifier types C. it cuts off half of the ac wave cycle D. it never needs a filter

[12] The best rectifier circuit for the power supply designed to provide high power at low voltage is A. half wave arrangement B. full wave, center tap arrangement C. quarter wave arrangement D. voltage doubler arrangement

[13] If a half wave rectifier is used with 165Vpk AC input, the effective dc output voltage is A. considerably less than 165V B. slightly less than 165V C. exactly 165V D. slightly more than 165V

[14] If a full wave bridge circuit is used with a transformer whose secondary provides 50Vrms, the peak voltage that occurs across the diodes in the reverse direction is approximately

A. 50 Vpk B. 70 Vpk C. 100 Vpk D. 140 Vpk

[15] The main disadvantage of voltage doubler power supply circuit is A. Excessive current B. Excessive voltage C. Insufficient rectification D. Poor regulation under heavy loads

[16] A source follower using an FET usually has a voltage gain which is A. Greater than +100 B. Slightly less than unity but positive C. Exactly unity but negative D. About -10

[17] The average output voltage(Vdc) of the full wave diode bridge rectifier is A. Vm/2 B. 2Vm/pi C. 3Vm/pi D. 4Vm/pi

[18] The typical ratio of latching current to holding current in a 20A thyristor is [GATE -2011] A. 5.0 B. 2.0 C. 1.0 D. 0.5

[19] A half controlled single phase bridge rectifier is supplying an R-L loaD. It is operated at a firing angle (alpha) and load current is continuous. The fraction of cycle that the freewheeling diode conducts is [GATE -2011]

Ans: D

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