# Electric Drives Questions and Answers – Solid-State Switching Circuits – Three Phase, Half-Wave, AC/DC Conversion for Resistive Loads

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Solid-State Switching Circuits – Three Phase, Half-Wave, AC/DC Conversion for Resistive Loads”.

1. Calculate the Vo(avg) for the 3-Φ phase Half-wave uncontrolled rectifier if the supply value is 440 V.
a) 297.25 V
b) 298.15 V
c) 298.11 V
d) 300.15 V

Explanation: The Vo(avg) for the 3-Φ phase Half-wave uncontrolled rectifier is 3Vml÷2π. The value of output voltage is 3Vml÷2π=3×√2×440÷6.28=297.25 V.

2. Calculate the pulse number if the supply frequency is 2π and the output frequency is 2π÷3.
a) 4
b) 5
c) 6
d) 3

Explanation: The pulse number can be calculated using the ratio of input frequency to the output frequency. The value of pulse number (P) is 2π÷(2π÷3)=3. It is a three-pulse converter or 3-Φ phase Half-wave uncontrolled rectifier.

3. Calculate the average value of current for the 3-Φ phase Half-wave uncontrolled rectifier if the supply value is 400 V and resistive load value is 5 Ω.
a) 54.04 A
b) 57.26 A
c) 51.64 A
d) 58.15 A

Explanation: The Io(avg) for the 3-Φ phase Half-wave uncontrolled rectifier is 3Vml÷2πR. The value of output voltage is 3Vml÷2πR=3×√2×440÷6.28×5=54.04 A.

4. In 3-Φ Fully controlled rectifier calculate the average value of the voltage if the supply is 400 V and firing angle is 45°.
a) 381.15 V
b) 382.16 V
c) 383.19 V
d) 384.25 V

Explanation: In 3-Φ Fully controlled rectifier, the average value of the voltage is 3Vml(cos(∝))÷π=3×400×√2(cos(45°))÷3.14=382.16 V.

5. Calculate the Vo(r.m.s) (Highly inductive load) for the 3-Φ phase Half-wave uncontrolled rectifier if the supply value is 441 V.
a) 297.25 V
b) 298.15 V
c) 298.11 V
d) 300.15 V

Explanation: The Vo(r.m.s) for the 3-Φ phase Half-wave uncontrolled rectifier is 3Vml÷2π. The value of output voltage is 3Vml÷2π=3×√2×441÷6.28=297.25 V.
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6. Calculate the Vo(avg) for the 3-Φ phase Half-wave controlled rectifier if the supply value is 364 V and the value of firing angle is 2°.
a) 247.62 V
b) 245.76 V
c) 214.26 V
d) 233.26 V

Explanation: The Vo(avg) for the 3-Φ phase Half-wave controlled rectifier is 3Vmlcos(α)÷2π. The value of output voltage is 3Vmlcos(α)÷2π=3×√2×364×(.99)÷6.28=245.76 V.

7. Calculate the value of the Input power factor for 3-Φ Fully controlled rectifier if the firing angle value is 16°.
a) .96
b) .91
c) .93
d) .94

Explanation: The value of the input power factor for 3-Φ Fully controlled rectifier is .95cos(16°)=.91. The input power factor is a product of distortion factor and displacement factor.

8. In 3-Φ Semi-controlled rectifier calculate the average value of the voltage if the supply is 299 V and firing angle is 56.15°.
a) 311.26 V
b) 304.26 V
c) 314.51 V
d) 312.45 V

Explanation: In 3-Φ Semi-controlled rectifier, the average value of the voltage is 3Vml(1+cos(∝))÷2π=3×299×√2(1+cos(56.15°))÷6.28=314.51 V.

9. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 13° and supply frequency is 49.5 Hz.
a) 730.2 msec
b) 740.8 msec
c) 754.5 msec
d) 755.1 msec

Explanation: The circuit turn-off time for 3-Φ Fully controlled rectifier is (240°-α)÷ω. The value of circuit turn-off time for ∝ < 60° is (240°-13°)÷6.28×49.5=730.2 msec.

10. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 170° and supply frequency is 50.5 Hz.
a) 32.4 msec
b) 35.2 msec
c) 39.3 msec
d) 31.5 msec

Explanation: The circuit turn-off time for 3-Φ Fully controlled rectifier is (180°-α)÷Ω. The value of circuit turn-off time for ∝ ≥ 60° is (180°-170°)÷6.28×50.5=31.5 msec.

11. Calculate the form factor if V(r.m.s)=45 V, V(avg)=15 V.
a) 2
b) 5
c) 3
d) 8

Explanation: The form factor is defined as the ratio of the Vr.m.s÷Vavg=45÷15=3. The r.m.s value is three times of average value Vavg.

12. Calculate the Vo(avg) for the 3-Φ phase Half-wave controlled rectifier if the supply value is 480 V and the value of firing angle is 92°.
a) 264.02 V
b) 487.26 V
c) 858.26 V
d) 248.25 V

Explanation: The Vo(r.m.s) for the 3-Φ phase Half-wave controlled rectifier is 3Vm(1+cos(α))÷2π. The value of output voltage is 3Vm(1+cos(α+30))÷2π=3×480×√3×√2(1+cos(92))÷2π=264.02 V.

13. Full form of SCR is ____________
a) Silicon controlled rectifier
b) State-controlled rectifier
c) State cover rectifier
d) State-controlled reset

Explanation: SCR stands for silicon controlled rectifier. It is a semi-controlled, bipolar, uni-directional switch. It is a high power rating than other power electronic devices.

14. Full form of TRIAC is __________
a) Triode for Alternating current
b) Tri for Alternating current
c) Triode for Alternating counter
d) Tri for Alternating counters

Explanation: TRIAC stands for Triode for Alternating current. It is a bipolar, bidirectional switch. It is used in fan regulators to control the speed of fans.

15. TRIAC is a unipolar switch.
a) True
b) False

Explanation: TRIAC stands for Triode for Alternating current. It is a bipolar, bidirectional switch. It is used in fan regulators to control the speed of fans.

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