Phase Controlled Rectifier MCQ

21. Which one of the following is the correct statement?

In a two quadrant converter working in the 1st and 2nd quadrants

  1. load current and load voltage are always positive
  2. load current is always negative
  3. load current can be positive or negative
  4. load current and load voltage are always negative
Answer
Answer. c

22. A three pulse converter is feeding a purely resistive load. What is the value of firing delay angle α, which dictates the boundary between continuous and discontinuous mode of current conduction?

  1. α = 0°
  2. α = 30°
  3. α = 60°
  4. α = 150°
Answer
Answer. b

23. A 6-phase bridge-converter feeds a purely resistive load. The delay angle α is measured from the point of natural-commutation. The effective control of voltage can be obtained when α lies in the range

  1. 0 ≤ α ≤ 120°
  2. 0 ≤ α ≤ 105°
  3. 0 ≤ α ≤ 180°
  4. 0 ≤ α ≤ 150°
Answer
Answer. a

24. The average output of a semi-converter connected to a 120 V, 50 Hz supply and firing angle of π/2 is

  1. 54.02 V
  2. 56.02 V
  3. 108.04 V
  4. Zero
Answer
Answer. a

25. A three-phase semi-converter feeds the armature of a separately excited dc motor supplying a non zero load torque. For steady-state operation, the motor current is found to assume zero value in certain instances of time. At such instances, the armature voltage

  1. is equal to the instantaneous value of ac voltage
  2. is equal to the instantaneous value of motor back emf
  3. assumes an arbitrary value
  4. becomes zero
Answer
Answer. b

26. In a three-phase semi-converter, if firing angle is less than or equal to 60°, then the duration of conduction of each thyristor and diode would be respectively

  1. 90° and 30°
  2. 60° and 60°
  3. 180° and 180°
  4. 120° and 120°
Answer
Answer. d

27. In a 3-phase semiconverter, for firing angle less than or equal to 60°, freewheeling diode conducts for

  1. 60°
  2. 90°
  3. 0°
  4. 30°
Answer
Answer. c

28. In a single-phase semiconverter with discontinuous conduction and extinction angle β < π, freewheeling action takes place for

  1. α
  2. α – β
  3. β – π
  4. 0°
Answer
Answer. d

29. A converter which can operate both in 3-pulse and 6-pulse modes is a

  1. 1-phase full converter
  2. 3-phase half-wave converter
  3. 3-phase semiconverter
  4. 3-phase full converter
Answer
Answer. c

30. In order to simplify the design of a converter transformer, the two converters in a dual converter should be connected using

  1. Series connection
  2. Cross-connection
  3. Direct anti-parallel connection
  4. Anti-parallel connection
Answer
Answer. c

31. In a dual converter, the circulating current

  1. allows smooth reversal of load current, but increases the response time
  2. allows smooth reversal of load current with improved speed of response
  3. does not allow smooth reversal of load current, but reduces the response time
  4. flows if there is no interconnecting inductor
Answer
Answer. b

32. The circulating current inductor is required in a dual converter to

  1. Improve the p.f.
  2. Smoothen the waveform of circulating current
  3. Limit the circulating current
  4. Increase the circulating current
Answer
Answer. c
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