Phase Controlled Rectifier MCQ

1. Consider the following statements:

  1. The voltage developed across the OFF switches of the half-bridge converter is the maximum dc link voltage.
  2. In the full-bridge converter, the voltage across the primary of the transformer is the dc link voltage.
  3. The voltage developed across the OFF switches of the full-bridge converter is half the maximum dc link voltage.

Which of the above statements are correct?

  1. i and iii only
  2. i, ii and iii
  3. ii and iii only
  4. i and ii only
Answer. d

2. A freewheeling diode in phase-controlled rectifiers

  1. enables inverter operation
  2. is responsible for additional reactive power
  3. improves the line power factor
  4. is responsible for additional harmonics
Answer. c

3. A single-phase full-wave rectifier is constructed using thyristors. If the peak value of the sinusoidal input voltage is Vm and the delay angle is π/3 radian, then the average value of output voltage is

  1. 0.48 Vm
  2. 0.32 Vm
  3. 0.71 Vm
  4. 0.54 Vm
Answer. b

4. For a firing angle of 60° in respect of a single-phase half-wave controlled rectifier with resistive load supplied from a 230 V (rms), 50 Hz supply, the average output voltage would be

  1. 77.5 V
  2. 52 V
  3. 155 V
  4. 104 V
Answer. a

5. A line cumulated phase-controlled inverter is operating at its inverter limit. There can be a commutation failure if

  1. the frequency decreases
  2. the voltage increases
  3. the frequency increases
  4. both voltage and frequency change such that v/f is constant
Answer. c

6. Consider the following statements:

Phase controlled converters at small values of output voltage have

  1. Large harmonics in utility system
  2. Poor power factor
  3. High efficiency
  4. Notches in line voltage waveform

Which of the above statements are correct?

  1. i and ii only
  2. i, ii and iv
  3. ii, iii and iv
  4. i and iv only
Answer. b

7. A single phase full converter feeds power to RLE load with R= 10 Ω, L = 10 mH and E = 50 V, the ac source voltage is 230 V, 50 Hz. For continuous conduction, what is the average value of load current for firing angle delay of 60°?

  1. 6 A
  2. 4.63 A
  3. 5.35 A
  4. 6.5 A
Answer. c

8. For a single-phase a.c. to d.c. controlled rectifier to operate in regenerative mode, which of the following conditions should be satisfied?

  1. Half-controlled bridge, α < 90°, source of e.m.f. in load
  2. Half-controlled bridge, α > 90°, source of e.m.f. in load
  3. Full-controlled bridge, α > 90°, source of e.m.f. in load
  4. Full-controlled bridge, α < 90°, source of e.m.f. in load
Answer. c

9.  In a single phase full wave controlled bridge rectifier, minimum output voltage and maximum output voltage are obtained at which conduction angles?

  1. 0°, 180° respectively
  2. 180°, 0° respectively
  3. 0°, 0° respectively
  4. 180°, 180° respectively
Answer. a

10. A fully controlled line commutated converter functions as an inverter when firing angle (α) is in the range

  1. 0° – 90°
  2. 90° – 180°
  3. 90° – 180° only when there is a suitable d.c. source in the load
  4. 90° – 180° only when it supplies a back e.m.f load
Answer. d

11. For the same voltage output, which one of the following has larger peak inverse voltage of the thyristor?

  1. Single-phase full-wave centre tapped circuit
  2. Single-phase full-wave bridge circuit
  3. Three-phase full-wave bridge circuit
  4. Three-phase full-wave centre tapped circuit
Answer. d

12. When the firing angle α of a single phase fully controlled rectifier feeding constant d.c. current into the load is 30°, what is the displacement factor of the rectifier?

  1. 1
  2. 0.5
  3. 1.73
  4. 0.865
Answer. d

13. When fed from a fully controlled rectifier, a d.c. motor, driving an active load, can operate in

  1. forward motoring and reverse braking mode
  2. forward motoring and forward braking mode
  3. reverse motoring and reverse braking mode
  4. reverse motoring and forward braking mode
Answer. b

14. A single-phase full-bridge converter with a free-wheeling diode feeds an inductive load. The load resistance is 15.53 Ω and it has a large inductance providing constant and ripple-free d.c. current. Input to converter is from an ideal 230 V, 50 Hz single-phase source. For a firing delay angle of 60°, the average value of diode current is

  1. 8.1655 A
  2. 10 A
  3. 3.33 A
  4. 5.774 A
Answer. c

15. In a thyristor-controlled rectifier, the firing angle of thyristor is to be controlled in the range of

  1. 0° to 180°
  2. 0° to 90°
  3. 90° to 270°
  4. 90° to 180°
Answer. a

16. The frequency of ripple in the output voltage of a three-phase controlled bridge rectifier depends on

  1. load inductance
  2. firing angle
  3. supply frequency
  4. load resistance
Answer. c

17. In a 3-phase controlled bridge rectifier, the maximum conduction of each thyristor is

  1. 60°
  2. 90°
  3. 120°
  4. 150°
Answer. c

18. A 3-phase converter feeds a pure resistance load at a firing angle of α = 60°. The average value of current flowing in the load is 10 A. If a very large inductance is connected in the load circuit, then the

  1. average value of current will remain as 10 A
  2. average value of current will become greater than 10 A
  3. average value of current will become less than 10 A
  4. trend of variation of current cannot be predicted unless the exact value of the inductance connected is known
Answer. a

19. A 3-phase thyristor controlled rectifier is feeding a purely resistive load on the DC side. The firing angle of the 6 thyristors of the converter is 90°. AC side is connected to a balanced 3-phase supply. Considering only the fundamental of the input current, active power P and reactive power Q estimated at the AC side of the rectifier are

  1. P ≠ 0, Q = 0
  2. P = 0, Q ≠ 0
  3. P ≠ 0, Q ≠ 0
  4. P = 0, Q = 0
Answer. c

20. What is the maximum output voltage of a 3-phase bridge rectifier supplied with line voltage of 440 V?

  1. 616 V
  2. 594 V
  3. 396 V
  4. 528 V
Answer. b
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