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

1. In a 3 phase M-6 controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.
b) 30 degrees
d) 120 degrees

Explanation: Each SCR conducts for 60° or 2π/6 radians. 6 x 60 = 360°.

2. In a 3 phase, 12-pulse controlled converter for continuous conduction mode, each SCR conducts for __________ per cycle.
b) 60 degrees
d) 12 degrees

Explanation: Each SCR conducts for 30° or π/6 radians. 12 x 30 = 360°.

3. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation. Find the fundamental component of the source current amplitude.
a) 11.03
b) 2.205
c) 11.03 sin 45
d) 46.98

Explanation: Put n = 1 (fundamental component) and the rest of the given values in the above given equation.

4. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. Find the DF (distortion factor).
a) 1.414
b) 0
c) 0.707
d) 0.569

Explanation: DF = cosα = cos45 = 0.707
Note that ripple free current does not mean that DF = 0. It means the current is continues in magnitude and direction, current will always contain harmonics (distortions) whether it is rippled or ripple free.

5. What is the relationship between DF, CDF and PF?
a) PF = CDF = DF
b) PF = CDF/DF
c) PF = DF/CDF
d) PF = CDF x DF

Explanation: PF (power factor) = (distortion factor) x (current distortion factor).
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6. The commutation period when both incoming and outgoing SCRs are conducting due to source inductance is called as the
a) conduction delay
b) overlap period
c) one on one period
d) distorting angle

Explanation: Due to source inductance, SCRs cannot start and stop conducting immediately, hence a time occurs when both incoming and outgoing SCRs are conducting together. This delay is called as commutation angle or overlap period (μ).

7. A 3-phase full converter delivers a ripple free load current of 10 A with a firing angle delay of 45°. The input voltage is 3-phase, 400 V, 50 Hz. The source current is given by the following relation. Find the value of 2nd harmonic source current amplitutue.
a) 11.25 A
b) 0.256 A
c) 2.69 sin (ωt – 45) A
d) 0 A

Explanation: 2nd harmonics are absent in a 3-phase full converter, it has only odd number of harmonics i.e. 3rd, 5th etc.

8. In a p-pulse converter, each SCR conducts for (per cycle)
b) p degrees

Explanation: In a p-pulse controlled converter, each device conducts for p radians. p-pulse converter would have p devices. Hence, 2π/p x p = 2π = 360°.

9. Which of the below mentioned converter can operate in both 3-pulse and 6-pulse modes?
a) 3-phase half wave controller
b) 3-phase full converter
c) 3-phase semi-converter
d) None of the mentioned

Explanation: Semi-converters can operate in 6-pulse modes gating the SCRs at appropriate intervals.

10. A M-6 controlled converter or 6-pulse half-wave controlled converter is obtained by using a transformer having
a) a double delta connected secondary winding
b) a double star connected secondary winding
c) a double delta connected primary winding
d) 6-windings on both primary and secondary sides

Explanation: M-6 requires a transformer having a delta connected primary and a double star connected secondary such that 6 SCRs are connected to it on the secondary side.

Sanfoundry Global Education & Learning Series – Power Electronics.

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