# Power Electronics Questions and Answers – Three Phase Converters-1

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Three Phase Converter-1”.

1. A three-phase, three-pulse, M-3 type controlled converter uses ____________ number of SCRs.
a) 1
b) 2
c) 3
d) 4

Explanation: It uses three SCRs with a three-phase transformer. M-3 type 3-pulse converters are not practically used.

2. A three-phase, three-pulse, M-3 type controlled converter has firing angle for one of the SCRs set as 15°. This SCR would start conducting at
a) 0°
b) 15°
c) 30°
d) 45°

Explanation: In a three phase controller, the actually conduction starts at 30° + α. Hence, ωt = 30+15 = 45°.

3. In a three-phase, three-pulse, M-3 type controlled converter T1 starts to conduct at 30 + n°. At what angles do T2 and T3 start to conduct? Assume that the conduction sequence is T1-T2-T3.
a) 2n° and 3n°
b) 150 + n° and 270 + n°
c) n° each
d) 30 + n° and 60 + n°

Explanation: In three-phase three pulse converter the conduction can start only after 30°. As each SCR conducts from 120°, T2 would conduct on 30+120+n° = 150+n° and so on.

4. A three-phase three pulse type controlled converter is constructed using 3 SCR devices. The circuit is supplying an R load with α < 30°. As such, each SCR device would conduct for
a) 60° each cycle
b) 120° each cycle
c) 180° each cycle
d) 360° each cycle

Explanation: Each conduct for 120° per cycle is the firing angle is less than 30°. 120 x 3 = 360°.

5. Find the expression for average output voltage at R for the below given configuration. Take firing angle as α = 15°, transformer ratio as 1:1:1 and Vmp as the maximum value of phase voltage at the supply.

a) (3Vmp/2π) x cosα
b) (3√3Vmp/2π) x cosα
c) (3√3Vmp/2π) x sinα
d) (3Vmp/2π) x sinα

Explanation: The circuit is that of a three-pulse M-3 connection. The firing angle is less than 30°. Therefore, each device conducts for an angle of 120°.
Vo = 3 x [ 1/2π ∫ Vmp sin⁡ωt d(ωt) ] Where, the integration runs from α+π/6 to α+5π/6.
Vo = (3√3/2π) x Vmp x cosα.

6. In the circuit shown below, SCR T1 conducts first. If T1 is fired at an angle of α > 30°, then T1 would conduct from

a) α to 180°
b) 30 + α to 180°
c) 30 + α to 150°
d) 30 + α to 120°

Explanation: When firing angle is more than 30°, T1 would conduct from 30 + α to 180°. Irrespective of the firing angle, T1 will be turned on at 180° because it conducts first which means it is connected to the R phase and the phase sequence is R-Y-B. As R starts at 0° its value is 0 at 180° which reverse biases the SCR T1.

7. Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage.

a) (3√3Vmp/2π) x cosα
b) (3√3Vmp/2π) x (1+cosα)
c) (3√3Vmp/2π) x [1+cos(30+α)].
d) (3√3Vmp/2π) x [3+cos(30+α)].

Explanation: Vo = 3 x [ 1/2π ∫ Vmp sin⁡ωt d(ωt) ] Where, the integration runs from α+π/6 to π. Because conduction takes place from 30 + α to 180° for T1 and than the waveform is symmetrical for all other SCRs.
Vo = (3√3/2π) x Vmp x [1+cos(30+α)].

8. A three-phase three-pulse converter would operate as a line commutated inverter when
a) 30° < α < 60°
b) 90° < α <180°
c) 90° > α
d) it can never operate as a line commutated inverter

Explanation: The output voltage is proportional to cosine of the firing angle α. If α goes above 90° then the output voltage is negative, meaning that it is operating as an inverter.

9. A three-phase M-3 converter is operated from a 3-phase, 230 V, 50 Hz supply with load resistance R = 10 Ω. Find the value of firing angle if an average output voltage of 50% of the maximum possible output voltage is required.
Hint: α > 30°.
a) 92.7°
b) 67.7°
c) 45°
d) 75.7°

Explanation: We need , Vo = 0.5 Vom.
α>30° hence we use the equation Vo = (3√3/2π) x Vmp x [1+cos(30+α)] √3Vmp = Vml = √2×230
Therefore, Vo = (3/2π) x √3 Vml x [1+cos(30+α)] = 0.5Vom
(1/√3) x [1+cos(30+α)] = Vo x 2π/3Vml = Vo/Vom = 1/2
α = 67.7°.

10. A three-phase half-wave controlled converter is fed from a 3-phase, 400 V source and is connected to a load which takes a constant current of 36 A. Find, the value of average output voltage and average current rating of SCR for a firing angle of 30°.
a) 234 V, 36 A
b) 234 V, 12 A
c) 135 V, 36 A
d) 135 V, 12 A

Explanation: Vo = (3√3/2π) x Vmp x cosα = 233.874 V.
Ia = Io/3 = 12 A.

Sanfoundry Global Education & Learning Series – Power Electronics.

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