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

**Answer: A**
The output of a two-pulse converter consists of two pulses in one cycle. The output frequency of the two pulse converters 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

**Answer: C**
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 a 3-Φ Fully controlled rectifier calculate the average value of the voltage if the supply is 400 V and the firing angle is 15°.

A. 521.2 V

B. 522 V

C. 523 V

D. 524 V

**Answer: B**
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 a 3-Φ Fully controlled rectifier if the firing angle value is 70°.

A. .32

B. .38

C. .31

D. .33

**Answer: A**
The value of the input power factor for a 3-Φ Fully controlled rectifier is

.95cos(70°)=.32.

The input power factor is a product of the distortion factor and displacement factor.

5. Calculate the value of THD value for a 3-Φ Fully controlled rectifier.

A. 48.43 %

B. 47.25 %

C. 39.26 %

D. 31 %

**Answer: D**
The value of the distortion factor is .95. The value of THD value for a 3-Φ Fully controlled rectifier is

√(1÷.95)2-1=31 %.

THD measures the amount of harmonic distortion.

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6. In a 3-Φ semi-controlled rectifier calculate the average value of the voltage if the supply is 440 V and the firing angle is 22°.

A. 571.5 V

B. 572.8 V

C. 548.3 V

D. 524.1 V

**Answer: B**
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 the supply frequency is 60 Hz.

A. 8.8 msec

B. 7.4 msec

C. 10.1 msec

D. 6.5 msec

**Answer: C**
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 the supply frequency is 50 Hz.

A. 3.8 msec

B. 5.2 msec

C. 9.3 msec

D. 8.7 msec

**Answer: A**
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

**Answer: A**
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 the Crest factor if Vpeak=12 V and Vr.m.s=24 V.

A. .2

B. .3

C. .4

D. .5

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