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Power Electronics MCQ Questions & Answers

Power Electronics MCQs : This section focuses on the "Power Electronics". These Multiple Choice Questions (MCQs) should be practiced to improve the Power Electronics skills required for various interviews (campus interview, walk-in interview, company interview), placement, entrance exam and other competitive examinations.




Question 1

In a circuit using a full wave converter (M - 2 connection) the PIV of each thyristor is 400 V. For the same output voltage and fully controlled bridge converter, PIV will be

A. 400 V
B. 800 V
C. 200 V
D. 2002 V

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Question 2

The terminals of a power BJT are called

A. emitter, base, collector
B. emitter, base, drain
C. source, base, drain
D. source, base, collector

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Question 3

For a 12 pulse operation, the two 6 pulse units are fed by

A. Y - Y and Y - Y transformers
B. Δ - Δ and Δ - Δ transformers
C. Y - Y and Y - Δ transformers
D. Y - Δ and Y - Δ transformers

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Question 4

The amount of dielectric heating is inversely proportional to frequency.

A. TRUE
B. FALSE

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Question 5

The terminals of a power MOSFET are called

A. emitter, base, collector
B. source, gate, drain
C. source, base, drain
D. emitter, gate, drain

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Question 6

A thyristor has a maximum allowable junction temperature of 120°C and the ambient temperature is 40°C. If thermal resistance is 1.6° C/W, the maximum allowable internal power dissipation is

A. 20 W
B. 50 W
C. 92 W
D. 128 W

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Question 7

For a BJT a = 0.98, then β =

A. 0.02
B. 0.5
C. 4.9
D. 49

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Question 8

A cycloconverter can be

A. step down
B. step up
C. step down or step up
D. none of the above

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Question 9

Assertion (A): A TRIAC is a bidirectional SCR.Reason (R): A TRIAC is a four layers 3 terminal device and can conduct in both directions.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 10

A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is Is and rms value of load current in Irms. Then

A. Is = Irms irrespective of the value of firing angle
B. Is = 0.5 Irms irrespective of the value of firing angle
C. Is = Irms if firing angle is less than 30°
D. Is = Irms if firing angle is more than 30°

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Question 11

In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally

A. equal
B. different
C. equal or different
D. different but sometimes equal

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Question 12

The total number of leads in reverse conducting thyristor is

A. 2
B. 3
C. 1
D. 4

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Question 13

Assertion (A): Power transistor has lower switching time than SCR Reason (R): Power transistor has greater switching time than SCR

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 14

A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be

A. about 350 rpm
B. about 250 rpm
C. about 175 rpm
D. about 125 rpm

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Question 15

A thyristor has a turn on time of 6 μs. If the anode circuit is inductive, the turn on time will be

A. 6 μs
B. less than 6μs
C. more than 6 μs
D. either 6 μs or less

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Question 16

Assertion (A): If the duty cycle of chopper is low the current would not become discontinuousReason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 17

If duty cycle is a and the load is a resistance R, the rms value of output voltage is

A. aV
B. a V
C. a2V

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Question 18

A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current

A. are similar
B. are not similar
C. may be similar or dissimilar
D. are similar only if firing angle is zero

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Question 19

Thyristors are not suitable for logic circuits.

A. TRUE
B. FALSE

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Question 20

In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is

A. 333.33
B. 250
C. 500
D. 1000

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Question 21

Assertion (A): Inverters and choppers use fast switching thyristors Reason (R): Fast switching SCR has low turn off time.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 22

Two thyristor of same rating and same specifications

A. will have equal turn on and turn off periods
B. will have equal turn on but unequal turn off periods
C. may have equal or unequal turn on and turn off periods
D. will have unequal turn on and turn off periods

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Question 23

In single phase half wave regulator, the average current over one full cycle

A. is always positive
B. may be positive or negative
C. may be negative
D. is always negative

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Question 24

In a 3 phase fully controlled converter the firing frequency is

A. 3 times the line frequency
B. 6 times the line frequency
C. 9 times the line frequency
D. 12 times the line frequency

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Question 25

If Vdc1 is the dc output voltage in half wave controlled rectifier circuit feeding resistive load and Vdc2 is the dc output voltage in full wave controlled rectifier circuit (M - 2 connection) feeding resistive load, then

A. Vdc1 = 0.5 Vdc2 for all values of firing angle
B. Vdc1 = Vdc2 for all values of firing angle
C. Vdc1 ≤ 0.5 Vdc2 for all values of firing angle
D. Vdc1 ≥ 0.5 Vdc2 for all values of firing angle

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Question 26

The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)

A. limits charging current
B. limits discharging current
C. limits both charging and discharging currents
D. none of the above

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Question 27

In a multiphase chopper, all choppers operate together.

A. TRUE
B. FALSE

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Question 28

A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is

A. 4
B. 5
C. 6
D. 8

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Question 29

A semi converter operates in two quadrants.

A. TRUE
B. FALSE

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Question 30

The phenomenon of overlap in converter operation due to

A. source resistance
B. source inductance
C. both source resistance and source inductance
D. high value of firing angle

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Question 31

Current derating may be necessary when thyristors are operating in

A. series
B. parallel
C. series or parallel
D. none of the above

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Question 32

Assertion (A): An ideal rectifier should have unity harmonic factorReason (R): An ideal rectifier should have unity displacement factor.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 33

Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.

A. TRUE
B. FALSE

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Question 34

In a thyristor the gate current is increased, then

A. anode current will increase
B. anode current will decrease
C. anode current will remain constant
D. anode current may increase or decrease

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Question 35

In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is

A. p - a
B. p
C. p + a
D. p - 2a

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Question 36

A single phase half wave controlled rectifier circuit has a free wheeling diode. The load is a combination of R and L. The firing angle is a. The period of conduction of SCR and free wheeling diode respectively are

A. a < ωt < p and p < ωt < (2p + a)
B. a < ωt < p and p < ωt < 2p
C. a < ωt < (p + a) and (p + a) < ωt < 2p
D. a < ωt < p and p < ωt < (2p - a)

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Question 37

A chopper

A. converts constant voltage dc into ac and then into variable voltage dc
B. converts constant voltage dc into variable voltage dc directly
C. converts ac of one frequency into ac of another frequency
D. converts ac to dc

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Question 38

The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by

A. forward characteristics of thyristors
B. reverse recovery characteristics of thyristors
C. both forward and reverse recovery characteristics of thyristors
D. none of the above

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Question 39

An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is

A. 16% maximum
B. 40% of maximum
C. 60% of maximum
D. 84% of maximum

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Question 40

The number of leads in an SCR are :

A. 2
B. 3
C. 4
D. 5

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Question 41

An SCR is triggered at 40° in the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80°, the average anode current is likely to be

A. 50 A
B. 25 A
C. less than 50 A but more than 25 A
D. less than 25 A

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Question 42

A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about

A. 127 V
B. 120 V
C. 110 V
D. 100 V

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Question 43

The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, v becomes

A. about 100 times
B. about 10 times
C. about 3 times
D. about twice

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Question 44

Assertion (A): A transistor requires a continuous base signal for conduction but a thyristor requires a gate pulse.Reason (R): Transistor find widespread application in power electronic circuits.

A. Both A and R are correct and R is correct explanation of A
B. Both A and R correct but R is not correct explanation of A
C. A is correct but R is wrong
D. A is wrong but R is correct

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Question 45

A semiconverter feeding an RLE load operates in

A. first quadrant only
B. first and second quadrants
C. first and fourth quadrants
D. first and third quadrants

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