# Power Electronics Questions and Answers – Three Phase Converters-4

This set of Basic Power Electronics Interview questions and answers focuses on “Three-Phase Converters-4”.

1. In a 3-phase full wave converter, if V is the maximum value of line voltage at the input, then each SCR is subjected to a peak negative voltage of
a) V
b) 3V
c) √3V
d) V/2

Explanation: PIV = √3 Vml in case of a semi-converter.

2. In a 3-phase semi-converter, firing angle is less than 60°, as such each SCR and diode conduct respectively for __________ (in degrees)
a) 60, 60
b) 90, 30
c) 120, 120
d) 180, 180

Explanation: Each will conduct for 120° per cycle whatever the firing angle is.

3. The effect of source inductance on the performance of a 3-phase controlled converter is to
a) increase the average load voltage
b) reduce the average load voltage
c) make the load current continuous
d) remove ripples from the load current

Explanation: It reduces the average value of the output voltage by introducing a overlap delay μ.

4. Which of the following converter circuits would require a neutral point?
a) 3-phase semi-converter
b) 3-phase full converter
c) 3-phase full converter with freewheeling diode
d) 3-phase half wave converter

Explanation: Half wave converter would require delta-star transformer, the secondary winding star connection requires a neutral point.

5. The range of firing angle for a 3-phase, 3-pulse converter feeding a resistive load is __________ (in degrees).
a) 0 to 180
b) 0 to 150
c) 30 to 150
d) 30 to 180

Explanation: Firing angle for a 3-phase, 3-pulse converter feeding a resistive load is 0 to 150 degrees.
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6. A 3-phase bridge converter is given a three-phase supply in the phase sequence R-Y-B. Let the neutral to R phase voltage be Vm sinωt. The first SCR (connected to R phase) is fired at an angle of 15°. What is the maximum value at the output terminals at this instant?
a) Vm/√2
b) Vm
c) 1.5 Vm
d) 3 Vm

Explanation: In case of a 3 phase bridge converter, the maximum value of voltage at the output terminal is always 1.5 Vm.

7. The PIV experienced by each SCRs in M-3 converter is __________ times that in a 3-phase full converter having the same output voltage.
a) 0.5
b) 1
c) 2
d) 1.5

Explanation: In case of M-3 type of connection, the devices have to handle more peak inverse voltage (PIV) than the 3-phase full converter which has 6 SCRs.

8. Each SCR of a 3-phase 6-pulse converter conducts for
a) 120 degrees
b) 60 degrees
c) 180 degrees
d) 360 degrees

Explanation: A 3-phase 6-pulse converter is nothing but the 3-pulse full controlled converter using 6 devices each conducting for 120°.

9. A 3-phase full converter has an average output voltage of 365 V for zero degree firing and resistive load. For a firing angle of 90 degree, the output voltage would be
a) 125 V
b) 569 V
c) 365 V
d) zero

Explanation: Cos 90 = 0.

10. Semi-converters are
d) none of the mentioned

Explanation: Semi-converters are single quadrant converters, because the voltage and current can only be both positive due to the diodes connected.

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