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.

   Answerc

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.

   Answerb

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.

   Answerb

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°.

   Answera

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.

   Answerb

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

   Answerd

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.

   Answera

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.

   Answera

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.

   Answerd

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.

    Answerb

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.

    Answerd

12. Which triggering is the most reliable?
    1. Thermal triggering.
    2. Forward voltage triggering.
    3. Gate triggering.
    4. dV / dt triggering.

    Answerc

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.

    Answerb

14. In a P1N1P2N2 thyristor which layer is less doped ?
    1. P1
    2. N1
    3. P2
    4. N2

    Answerb

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.

    Answerd

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.

    Answera

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.

    Answerc

18. 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.

    Answerb

19. 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.

    Answerc

20. Thyristor can be protected from over voltages by using
    1. voltage clamping device.
    2. heat sink.
    3. snubber circuit.
    4. fuse.

    Answera

21. Example of a voltage clamping device
    1. aluminium block.
    2. metal oxide varistor.
    3. snubber circuit.
    4. fast acting fuse.

    Answerb

22. Under over voltage condition impedance offered by the voltage clamping device is
    1. low.
    2. infinity.
    3. moderate.
    4. high.

    Answera

23. Under normal operating condition voltage clamping device offers impedance of
    1. low value.
    2. moderate value.
    3. zero value.
    4. high value.

    Answerd

24. What is used to protect the SCR from over current ?
    1. CB and fuse.
    2. Heat sink.
    3. Voltage clamping device.
    4. Snubber circuit.

    Answera

25. 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

    Answera

26. 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.

    Answera

27. 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.

    Answerd

28. Which of the following is used in heat sink?
    1. Carbon.
    2. Aluminium.
    3. Silver.
    4. Iron.

    Answerb

29. Materials used in heat sink should have
    1. large surface area.
    2. All of these.
    3. high thermal conductivity.
    4. high melting point.

    Answerb

30. What is used to protect a thyristor from high di / dt conditions?
    1. Inductor.
    2. Snubber circuit.
    3. Voltage clamping device.
    4. Fuse.

    Answera

31. 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.

    Answera

32. What happen due to high di / dt?
    1. None of these.
    2. Insulation failure.
    3. Local hot spot.
    4. Breakdown of junction.

    Answerc

33. What may happen high dV / dt?
    1. Breakdown of J2 junction.
    2. Anyone of these.
    3. Both
    4. Unwanted turn ON.

    Answerc

34. 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.

    Answerb

35. Snubber circuit is used with SCR
    1. in parallel.
    2. in series.
    3. either series or parallel.
    4. anti parallel.

    Answera

36. 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.

    Answera

37. 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.

    Answera

38. 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.

    Answerd

39. Maximum power loss occurs during
    1. rise time.
    2. spread time.
    3. all.
    4. delay time.

    Answera

40. The typical time of rising time lies between
    1. 40 – 60 µs.
    2. 1 – 4 µs.
    3. 10 – 20 µs.
    4. 90 – 100 µs.

    Answerb

41. 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.

    Answerb

42. During reverse recovery time
    1. charge carrier of junction J3 is swept out.
    2. charge carrier of junction J1&J3 is swept out.
    3. charge carrier of junction J1 is swept out.
    4. charge carrier of junction J2 recombined.

    Answerb

43. During gate recovery time
    1. charge carriers of J3 junction is removed.
    2. charge carrier of J1 junction removed.
    3. charge carriers of J2 junction is swept out.
    4. charge carriers of J2 junction recombined.

    Answerd

44. Typical range of thyristor turn OFF time is
    1. 3 – 10 µs.
    2. 3 – 50 µs.
    3. 3 – 100 µs.
    4. 3 – 500 µs.

    Answerc

45. Which statement is true ?
    1. All of these.
    2. Reverse recovery time ( trr ) > gate recovery time (tgr).
    3. Device turn OFF time ( tq ) > reverse recover time (trr).
    4. Circuit turn OFF time > device turn OFF time ( tq ).

    Answera

46. ON state voltage drop across SCR lie between the range
    1. 0 – 0.5 V.
    2. 1.5 – 2 V.
    3. 1 – 1.5 V.
    4. 0.5 – 1 V.

    Answerc

47. SCRs are used in series to meet
    1. high voltage demand
    2. low current demand.
    3. high current demand.
    4. low voltage demand.

    Answera

48. By which one of the following we can measure the reliability of a string?
    1. Factor of safety.
    2. String efficient.
    3. Derating factor.
    4. Reliability factor.

    Answerc

49. 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
    1. 6, 4.
    2. 4, 6.
    3. 5, 7.
    4. 7, 5.

    Answerd

50. 60
    thyrsistors are connected in series and parallel to form a 10 KV and
    5.5 KA switch. Each thyristor is rated for 1.2 KV, 1 KA. The no. of
    parallel path are 6. The efficiency of the switch is
    1. 83.3 %.
    2. 91.6 %.
    3. 76.3 %.
    4. 90.9 %

    Answerc

Copyright (C) 2016 by (ALL EXAM REVIEW ADDA)