POWER ELECTRONICS MCQ PART 1

POWER ELECTRONICS MCQ PART 1

POWER ELECTRONICS PART ONE

Instructions: Click the answer button to see the correct answer.

1. The typical value of SCR for modern alternator is

1. 1.5.
2. 1.0.
3. 0.5.
4. 1.2.

Answer: c

POWER ELECTRONICS MCQ PART 1

2. An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. One cycle surge current will be

1. 4242.64 A.
2. 2121.32 A.
3. 6000 A.
4. 1500 A.

Answer: b

3. An SCR is considered to be a semi controlled device because

1. it can be turned OFF but not ON with a gate pulse.
2. it can be turned ON but not OFF with a gate pulse.
3. it conducts only during one half cycle of an alternating current wave.
4. it can be turned ON only during one half cycle of an AC.

Answer: b

4. A single phase one pulse controlled circuit has a resistance R and counter emf E load 400 sin(314 t) as the source voltage. For a load counter emf of 200 V, the range of firing angle control is

1. 30° to 150°.
2. 60° to 180°.
3. 30° to 180°.
4. 60° to 120°.

Answer: a

POWER ELECTRONICS MCQ PART 1

5. A single phase full bridge inverter can operated in load commutation mode in case load consist of

1. RLC overdamped.
2. RLC underdamped.
3. RL.
4. RLC critically damped.

Answer: b

6. A step up chopper has input voltage 110 V and output voltage 150 V. The value of duty cycle is

1. 0.27.
2. 0.32.
3. 0.45.
4. 0.26

Answer: d

7. Which statement is true for latching current ?

1. It is related to turn on process of the device.
2. It is related to turn off process of the device.
3. all of these
4. It is related to conduction process of device.

Answer: a

POWER ELECTRONICS MCQ PART 1

8. If holding current of a thyristor is 2 mA then latching current should be

1. 0.004 A.
2. 0.009 A.
3. 0.002 A.
4. 0.01 A.

Answer: a

9. In reverse blocking mode of a thyristor

1. junction J2 is in reverse bias and J1, J3 is in forward bias.
2. junction J3 is in forward bias and J1, J2 in reverse bias.
3. junction J1 and J2 is in forward bias and J3 is in reverse bias.
4. junction J1, J3 is in reverse bias and J2 is in forward bias.

Answer: d

POWER ELECTRONICS MCQ PART 1

10. In forward blocking mode of a thyristor

1. Junction J1, J3 is in reverse bias and J2 is in forward bias.
2. junction J2 is in reverse bias and J1, J3 is in forward bias.
3. junction J3 is in forward bias and J1, J2 is in reverse bias.
4. Junction J1 and J2 is in forward bias and J3 is in reverse bias.

Answer: b

11. Leakage current flows through the thyristor in

1. forward blocking mode.
2. reverse blocking mode.
3. forward conduction mode.
4. both forward and reverse blocking mode.

Answer: d

POWER ELECTRONICS MCQ PART 1

12. Which triggering is the most reliable?

1. Thermal triggering.
2. Forward voltage triggering.
3. Gate triggering.
4. dV / dt triggering.

Answer: c

13. Light triggering mainly used in

1. low – voltage direct current transmission.
2. high voltage direct current transmission.
3. all of these.
4. medium voltage direct current transmission.

Answer: b

14. In a P1N1P2N2 thyristor which layer is less doped ?

1. P1
2. N1
3. P2
4. N2

Answer: b

POWER ELECTRONICS MCQ PART 1

15. Let of a thyristor Vc1, Vc2, Vc3 are forward break over voltage for gate current Ig1, Ig2, Ig3 respectively. Then

1. Vc1 > Vc2 > Vc3 when Ig1 > Ig2 > Ig3.
2. All of these
3. Vc1 = Vc2 = Vc3 any value of Ig.
4. Vc1 > Vc2 > Vc3 when Ig1 < Ig2 < Ig3.

Answer: d

16. SCR will be turned off when anode current is

1. both < latching current but greater than holding current and gate signal is 0 & less than holding current.
2. < latching current but greater than holding current and gate signal is present.
3. less than holding current.
4. < latching current but greater than holding current and gate signal is 0.

Answer: a

17. After proper turn on of thyristor

1. none of these
2. gate signal is always present.
3. gate signal must be removed.
4. gate signal should present but can be removed.

Answer: c

POWER ELECTRONICS MCQ PART 1

17. Gate circuit or triggering circuit of a thyristor is

1. magnetic circuit.
2. lower power circuit.
3. high power circuit.
4. may be low power or high power circuit.

Answer: b

18. The latching current of SCR is 20 mA. Its holding current will be

1. 40 mA.
2. 60 mA.
3. 10 mA.
4. 23 mA.

Answer: c

19. Thyristor can be protected from over voltages by using

1. voltage clamping device.
2. heat sink.
3. snubber circuit.
4. fuse.

Answer: a

POWER ELECTRONICS MCQ PART 1

20. Example of a voltage clamping device

1. aluminium block.
2. metal oxide varistor.
3. snubber circuit.
4. fast acting fuse.

Answer: b

21. Under over voltage condition impedance offered by the voltage clamping device is

1. low.
2. infinity.
3. moderate.
4. high.

Answer: a

POWER ELECTRONICS MCQ PART 1

22. Under normal operating condition voltage clamping device offers impedance of

1. low value.
2. moderate value.
3. zero value.
4. high value.

Answer: d

23. What is used to protect the SCR from over current ?

1. CB and fuse.
2. Heat sink.
3. Voltage clamping device.
4. Snubber circuit.

Answer: a

24. CB used for over current protection of thyristor operates when the fault current is

1. of long period.
2. of short duration.
3. neither
4. both

Answer: a

POWER ELECTRONICS MCQ PART 1

25. 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 ?

1. Fast acting current limiting device (FACL fuse).
2. Voltage clamping device.
3. Snubber circuit.
4. CB.

Answer: a

26. How can we protect SCR from thermal conditions ?

1. Using CB and fuse.
2. Use of snubber circuit.
3. Using equalizing circuit.
4. Using heat sink.

Answer: d

POWER ELECTRONICS MCQ PART 1

27. Which of the following is used in heat sink?

1. Carbon.
2. Aluminium.
3. Silver.
4. Iron.

Answer: b

28. Materials used in heat sink should have

1. large surface area.
2. All of these.
3. high thermal conductivity.
4. high melting point.

Answer: b

POWER ELECTRONICS MCQ PART 1

29. What is used to protect a thyristor from high di / dt conditions?

1. Inductor.
2. Snubber circuit.
3. Voltage clamping device.
4. Fuse.

Answer: a

30. Which of the following is used in SCR to protect from high dV / dt?

1. Snubber circuit.
2. Fuse.
3. Equalizing circuit.
4. Circuit breaker.

Answer: a

POWER ELECTRONICS MCQ PART 1

31. What happen due to high di / dt?

1. None of these.
2. Insulation failure.
3. Local hot spot.
4. Breakdown of junction.

Answer: c

32. What may happen high dV / dt?

1. Breakdown of J2 junction.
2. Anyone of these.
3. Both
4. Unwanted turn ON.

Answer: c

33. Why resistor is used in Snubber circuit ?

1. all of these
2. To minimize the discharging current.
3. To minimize the loss
4. To minimize the charging current.

Answer: b

POWER ELECTRONICS MCQ PART 1

34. Snubber circuit is used with SCR

1. in parallel.
2. in series.
3. either series or parallel.
4. anti parallel.

Answer: a

35. Delay time is defined by the interval when

1. all of these.
2. anode current reaches 10 % from forward leakage current.
3. anode voltage drops from 100 % to 90 % of its actual value.
4. gate current increases from 90 % to 100 % of its final value.

Answer: a

36. Rise time is defined by the interval when

1. both
2. anode voltage drops from 90 % to 10 % of its initial value.
3. none of these
4. anode current rises 10 % to 90 % of its final value.

Answer: a

POWER ELECTRONICS MCQ PART 1

37. Spread time is defined as the interval during which

1. anode current rises from 90 % to its final value.
2. none of these
3. anode current rises from 10 % to 90 % of its final value.
4. anode voltage drops from 10 % of its initial value to zero.

Answer: d

38. Maximum power loss occurs during

1. rise time.
2. spread time.
3. all.
4. delay time.

Answer: a

POWER ELECTRONICS MCQ PART 1

39. The typical time of rising time lies between

1. 40 – 60 µs.
2. 1 – 4 µs.
3. 10 – 20 µs.
4. 90 – 100 µs.

Answer: b

40. During which time maximum conduction spreading take place in the thyristor during turn ON?

1. Same for every case.
2. Spread time.
3. Delay time.
4. Rise time.

Answer: b