# Power Electronics Questions and Answers – Single Phase FW AC-DC-5

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) for campus interviews focuses on “Single-Phase FW AC-DC-5”.

1. In the circuit shown below, find the expression of average output voltage if due to some faults the SCR T3 gets permanently open circuited. a) 2Vm/π cosα
b) Vm/π cosα
c) Vm/√π
d) Zero

Explanation: If one of the device gets O.C the circuit would behave just like a half-wave rectifier. Only Two SCRs T1 & T2 will operate, even T4 is useless in this case.
Vo = 1/2π x [ ∫ Vm sinωt d(ωt) ] where the integration would run from α to π+α.

2. A single phase full converter, feeds power to a RLE load with R = 6 Ω and E = 60 V. The supply voltage is 230 V AC and the firing angle is 50°. Find the power delivered to the battery.
a) 542 W
b) 100 W
c) 960 W
d) 730 W

Explanation: Vo = 2Vm/π cosα = 133.084 V
I = Vo – E/R = 133.084-60 / 6 = 12.181 A
Power delivered = E x I = 60 x 12.181 = 730.86 Watts.

3. In a single phase full converter with resistive load and firing angle α, the load current is
a) zero at α, π+α, …
b) remains zero for duration α
c) Vm/R sinα at α, π+α, …
d) remains zero for duration π-α

Explanation: The current starts to flow from α, π+α, …

4. In a single phase semi converter with resistive load and a firing angle α, each SCR and freewheeling diode would conduct for
a) α, 0°
b) π-α, α
c) π+α, α
d) π-α, 0°

Explanation: FD does not come into the picture for resistive loads.

5. A single phase full converter has average & peak voltage values of 133 V and 325 V respectively. Find the value of the firing angle.
a) 40°
b) 140°
c) 50°
d) 130°

Explanation: Vm = 325V
Vo = 2Vm/π cosα = 133 V.
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6. A freewheeling diode placed across a RL load provides
a) fast turn-on time
b) slow turn-off time
c) poor utilization factor
d) better power factor

Explanation: FD improves efficiency by providing freewheeling action which increase the power delivered to the load.

7. A single phase full converter has discontinues load current. The converter is supplying a dc shunt motor (R-L-E) load. When the current falls to zero the output voltage is
a) Zero
b) Vo
c) Vm
d) E

Explanation: As the load is RLE output voltage will always exists even if the current goes to zero due to E.

8. A single phase full converter has discontinues load current. The converter is supplying a DC shunt motor (R-L-E) load. When the current falls to zero a) none of the devices conduct
b) all of the devices conduct
c) only one device conducts
d) only one pair of devices conducts

Explanation: The current falls to zero when the L has discharged completely and the devices are not yet fired.

9. For the circuit shown below to act as a line commutated inverter a) 180°>α>90° & E is removed
b) 180°>α>90° & E is reversed
c) 180°>α<90° & E is removed
d) 180°>α<90° & E is reversed

Explanation: For inverter operation the firing angle should be above 180 and E should be reversed.

10. If in the below given converter configuration, any 3 of the 4 SCRs are replaced by diodes
a) the average output voltage would increase
b) the average output voltage would decrease
c) the average output voltage would not change
d) the circuit would malfunction

Explanation: If diodes are put in place of any 3 SCRs the conduction angle would increase and hence the voltage.

Sanfoundry Global Education & Learning Series – Power Electronics.

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