power electronics Mcqs
1. The average value of the output voltage in a step down dc chopper is given by__________?
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A. V 0 = D V s

B. V 0 = V s

C. V 0 = V s / D

D. V 0 = V s / ( 1 D )

2. The carrier which are commonly used in constant frequency PWM is______________?
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A. Sawtooth carrier

B. All of these

C. Triangle carrier

D. Inverted sawtooth carrier

3. The control method used for PWM dc dc converter is______________?
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A. Hysteric control

B. All of these

C. Voltage mode control

D. Current mode control

4. A rectifier with an external low pass filter is an example of______________?
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A. Direct switch matrix circuits

B. Embedded converters

C. All of these

D. Indirect switch matrix circuits

5. In a full bridge VSI, in order to avoid the short circuit across the DC bus and the undefined AC output voltage condition, the modulating technique should ensure that___________?
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A. Bottom switch of each leg is on at any instant

B. Top switch of each leg is on at any instant

C. None of these

D. Either A. or B.
6. Which semiconductor device acts like a diode and two transistor?
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A. UJT

B. Diac

C. SCR

D. Triac
7. The typical time of rising time lies between______________?
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A. 1 4 s

B. 90 100 s

C. 10 20 s

D. 40 60 s

8. For a buck converter to reduce the conduction losses in diode
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A. A high on resistance switch can be added in series

B. A low on resistance switch can be added in series

C. A high on resistance switch can be added in parallel

D. A low on resistance switch can be added in parallel

9. An SCR is considered to be a semi controlled device because_______?
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A. it can be turned OFF but not ON with a gate pulse.

B. it conducts only during one half cycle of an alternating current wave.

C. it can be turned ON but not OFF with a gate pulse.

D. it can be turned ON only during one half cycle of an AC.

10. How can we protect SCR from thermal conditions _____________?
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A. Using equalizing circuit

B. Use of snubber circuit.

C. Using heat sink.

D. Using CB and fuse.

11. A crowbar is a circuit which is used to protect a_____________?
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A. voltage sensitive load from excessive dc power supply output voltages

B. current sensitive load from excessive dc power supply output voltages

C. voltage sensitive load from excessive ac power supply output voltages

D. current sensitive load from excessive ac power supply output voltages

12. In BJT, the forward biased base emitter junction inject holes from base to emitter, the holes_______?
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A. Do not contribute to the collector current

B. Contribute to the collector current

C. Only (b) and (c)

D. Result in net current flow component into the base

E. Only (a) and (b)
13. The function of snubber circuit connected across the SCR is to___________?
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A. Decrease di / dt.

B. Increase dV / dt.

C. Decrease dV / dt.

D. Suppress dV / dt.
14. An ideal switch is___________?
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A. Does not carry any current in any direction when it is off

B. All of these

C. Carry current in any direction when it is on

D. Lossless

15. If a shunt motor is started with its field winding open then?
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A. It will rotate at dangerously high speed

B. It will rotate at the same speed as that with its field winding closed

C. It will rotate at less speed as that with its field winding closed

D. None of these

16. An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be_____?
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A. 1500 A.

B. 6000 A.

C. 2121.32 A.

D. 4242.64 A.

17. A TRIAC can be turned on with______________?
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A. Either (a) or (b)

B. None of these

C. Negative voltage at the gate terminal

D. Positive voltage at the gate terminal

18. Which of the following is used in heat sink_____________?
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A. Carbon

B. Aluminium

C. Iron

D. ilver

19. The input current waveform of a bridge controlled rectifier when the load is perfectly filtered is___________?
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A. Saw tooth wave

B. Trapezoidal wave

C. Sine wave

D. Square wave
20. Advantages of Cuk converter is / are_____________?
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A. None of these

B. Low stress on switch

C. Low stress on capacitor

D. Large number of reactive component

21. Which of the following is disadvantage of fast recovery diodes?
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A. Doping is carried out

B. Recovery is only 50 s

C. Recovery is only 5 s

D. None of these

22. When a large surge current of very short duration flows through a thyristor then which one of the following device will operate to protect the thyristor ___________?
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A. Snubber circuit

B. Voltage clamping device

C. Fast acting current limiting device (FACL fuse)

D. CB

23. n a phase controlled 3 phase ac voltage controller, the harmonic present is__________?
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A. 2nd

B. 7th

C. 3rd

D. 5th

24. Dynamic equalizing circuit is used for________________?
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A. equal division of voltage across each thyristor in parallel

B. equal division of current through each thyristor in parallel

C. equal division of current through each thyristor in series

D. equal division of voltage across each thyristor
25. Which statement is true ?
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A. Reverse recovery time ( trr ) > gate recovery time (tgr)

B. Device turn OFF time ( tq ) > reverse recover time (trr)

C. All of these

D. Circuit turn OFF time > device turn OFF time ( tq )

26. Under over voltage condition impedance offered by the voltage clamping device is__________?
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A. moderate

B. low

C. infinity

D. High

27. A string of n parallel SCRs is operated at 72 KA, the rating of each SCR is 1 KA. If derating factor of the string is 0.1. value of n will be___________?
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A. 60

B. 70

C. 80

D. 90

28. A combination of synchronized leading edge and trailing edge modulation has also been used to control a____________?
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A. Boost single phase power factor converter

B. A buck dc dc converter to reduce ripple in the intermediate dc bus capacitor

C. Both A. and B.

D. None f these

29. Which triggering is the most reliable____________?
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A. Thermal triggering

B. Forward voltage triggering

C. Gate triggering

D. dV / dt triggering

30. Electrical power output in a D.C. generator is equal to_____________?
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A. Mechanical power input iron and friction losses copper losses

B. Electrical power developed in armature iron and copper losses

C. Electrical power developed in armature copper losses

D. Mechanical power input iron and friction losses

31. The most suited gate pulses given to the AC regulator with R L load can be in the form of____________?
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A. None of these

B. Continuous signal

C. A train of pulses

D. Large isolating pulse transformer

32. COOLMOS device can be used in application up to power range of___________?
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A. 500 VA

B. 1 KVA

C. 2 KVA

D. 100 KVA

33. LISN is a device used to measure conducted emissions. LISN stands for_____________?
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A. Line integrated stored network

B. Line impedance stabilization network

C. Line integrated stabilization network

D. Laser integrated stabilization networking

34. Unipolar modulation is generally used in____________?
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A. DC DC converters

B. AC AC converters

C. AC DC converters

D. DC AC converters

35. Thyristor is nothing but a____________?
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A. Controlled switch

B. Controlled transistor

C. Amplifier with higher gain

D. Amplifier with large current gain

36. The reverse recovery time of diode is trr = 3 Is and the rate off all of the diode current is di/dt = 30 A/Is. The storage charge current QRR is___________?
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A. 140 Is

B. 130 Is

C. 135 Is

D. 145 Is

37. The Graetz bridge makes excellent use of ______________?
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A. SCR

B. Power transformer

C. Potential transformer

D. Current transformer

38. In constant frequency PWM, at perturbation the amplitude of the sinusoidal component is a___________?
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A. Linear function

B. Non linear function

C. None of these

D. Constant function

39. In commutation PAC stands for_______?
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A. Phase and commutation

B. Phase angle commutation

C. Phase angle converter

D. Permanent angle converter

40. A thyristor string is made of a no. of SCR connected in series and parallel. The string have volume and current of 11 KV and 4 KA. The voltage and current rating of available SCRs are 1800 V and 1000 A. For a string efficiency of 90 % let the number of SCRs in series and parallel are a and b respectively. Then the value of a and b will be_____________?
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A. 4, 6

B. 5, 7

C. 7, 5

D. 6, 4

41. For similar carrier and modulating signals, the line current used in CSI is__________?
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A. Identical to phase voltage in CSI

B. Identical to line current in VSI

C. Identical to line voltage in a VSI

D. Identical to phase voltage in VSI

42. In a 3 phase VSI out of eight valid states, the number of valid states that produce zero ac line voltages is/are_________?
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A. two

B. One

C. Four

D. Three

43. IGBT combines the advantages of_____________?
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A. SITs and MOSFETs

B. BJTs and SITs

C. BJTs and MOSFETs

D. None of these

44. If energy is taken from the AC side of the inverter and sends it back into the DC side, then it is known as___________?
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A. None of these

B. Motoring mode operation

C. Braking mode operation

D. Regenerative mode operation
45. Power electronics essentially deals with control of a.c. power at____________?
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A. 50 Hz frequency

B. Frequencies above 1000 kHz

C. Frequencies less than 10 Hz

D. Frequencies above 20 kHz

46. A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is_____?
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A. 0.27

B. 0.45

C. 0.32

D. 0.67

47. If the gate voltage of an SCR is removed, then the_______________?
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A. cathode current increases

B. anode current increases

C. anode current does not decrease at all

D. anode current decreases

48. A GTO can be turned on by applying___________?
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A. Positive gate signal

B. Positive drain signal

C. None of these

D. Positive source signal

49. With gate open, the maximum anode current at which SCR is turned off from ON condition is called____________?
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A. peak reverse voltage

B. breakdown voltage

C. latching current

D. holding current
50. The conduction losses in IGBT is____________?
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A. Equal to that of MOSFET

B. More than that of MOSFET

C. Lower than that of MOSFET

D. Equal to that of BJT