# Electric Drives Questions and Answers – Solid State Controlled Drives – AC Motor Systems

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Solid State Controlled Drives – AC Motor Systems”.

1. Calculate the output frequency for the two-pulse converter if the supply frequency is 20 Hz.
a) 40 Hz
b) 20 Hz
c) 60 Hz
d) 90 Hz

Explanation: The output of a two-pulse converter consists of two pulses in one cycle. The output frequency of the two pulse converter is 2×supply frequency=2×20=40 Hz.

2. Calculate the pulse number if the supply frequency is 2π and the output frequency is π÷3.
a) 4
b) 2
c) 6
d) 8

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÷π÷3=6. It is a six-pulse converter.

3. In 3-Φ Fully controlled rectifier calculate the average value of the voltage if the supply is 400 V and firing angle is 15°.
a) 521.2 V
b) 522 V
c) 523 V
d) 524 V

Explanation: In 3-Φ Fully controlled rectifier, the average value of the voltage is 3Vml(cos(∝))÷π=3×400×√2(cos(15°))÷3.14=522 V.

4. Calculate the value of the Input power factor for 3-Φ Fully controlled rectifier if the firing angle value is 70°.
a) .32
b) .38
c) .31
d) .33

Explanation: The value of the input power factor for 3-Φ Fully controlled rectifier is .95cos(70°)=.32. The input power factor is a product of distortion factor and displacement factor.

5. Calculate the value of THD value for 3-Φ Fully controlled rectifier.
a) 48.43 %
b) 47.25 %
c) 39.26 %
d) 31 %

Explanation: The value of the distortion factor is .95. The value of THD value for 3-Φ Fully controlled rectifier is √(1÷.95)2-1=31 %. THD measures the amount of harmonic distortion.

6. In 3-Φ Semi-controlled rectifier calculate the average value of the voltage if the supply is 440 V and firing angle is 22°.
a) 571.5 V
b) 572.8 V
c) 548.3 V
d) 524.1 V

Explanation: In 3-Φ Semi-controlled rectifier, the average value of the voltage is 3Vml(1+cos(∝))÷2π=3×440×√2(1+cos(22°))÷6.28=572.8 V.

7. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 20° and supply frequency is 60 Hz.
a) 8.8 msec
b) 7.4 msec
c) 10.1 msec
d) 6.5 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°-20°)÷6.28×60=10.1 msec.

8. Calculate the circuit turn-off time for 3-Φ Fully controlled rectifier if the firing angle is 110° and supply frequency is 50 Hz.
a) 3.8 msec
b) 5.2 msec
c) 9.3 msec
d) 8.7 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°-110°)÷6.28×50=3.8 msec.

9. Calculate peak-peak voltage if Vmax=80 V and Vmin=20 V.
a) 60 V
b) 50 V
c) 70 V
d) 10 V

Explanation: Peak-Peak voltage is equal to the difference between the maximum and minimum voltage. It is mathematically represented as Vp-p=Vmax-Vmin=80-20=60 V.

10. Calculate the value of Crest factor if Vpeak=12 V and Vr.m.s=24 V.
a) .2
b) .3
c) .4
d) .5

Explanation: The value of the crest factor is Vpeak÷Vr.m.s=12÷24=.5. It signifies the peak value is .5 times than the r.m.s value.

11. Calculate the output voltage of the Buck converter if the supply voltage is 11 V and duty cycle value is .4.
a) 4.4 V
b) 2.2 V
c) 4.8 V
d) 6.4 V

Explanation: The output voltage of the buck converter is Vo = Vin×(D)=11×.4=4.4 V. The value of the duty cycle is less than one which makes the Vo < Vin. The buck converter is used to step down the voltage.

12. Calculate the output voltage of the Boost converter if the supply voltage is 8 V and duty cycle value is .6.
a) 40 V
b) 20 V
c) 48 V
d) 51 V

Explanation: The output voltage of the boost converter is Vo = Vin ÷ (1-D) = 8÷.4 = 20 V. The value of the duty cycle is less than one which makes the Vo > Vin as denominator value decreases and becomes less than one. The boost converter is used to step up the voltage.

13. Calculate the output voltage of the Buck-Boost converter if the supply voltage is 78 V and duty cycle value is .1.
a) 7.2 V
b) 4.5 V
c) 8.6 V
d) 5.1 V

Explanation: The output voltage of the buck-boost converter is Vo = D×Vin ÷ (1-D)=.1(78)÷.9=8.6 V. It can step up and step down the voltage depending upon the value of the duty cycle. If the value of the duty cycle is less than .5 it will work as a buck converter and for duty cycle greater than .5 it will work as a boost converter.

14. The principle of Boost converter can be applied for the regenerative braking.
a) True
b) False

Explanation: The Buck converter is used in motoring mode but a Boost converter can operate only braking mode because the characteristics are in the second quadrant only.

15. The unit of angular acceleration is Joule.
a) True
b) False 