Power Electronics Question and Answers – Single-Phase HW AC-DC-2 and Answers

Power Electronics Interview Questions and Answers on “Single-Phase HW AC-DC-2”.

1. For a certain SCR configuration, the below shown waveform is obtained. Find the value of the average output voltage with α = 30° & Vs = 240 V.

A. 50.27 V
B. 100.8 V
C. 140 V
D. 120 V
Answer: B
Clarification: Vm = √2 Vs = 339.41 V
V_avg = ∫ Vm Sin ωt d(ωt) where the integration runs from α to π
V_avg = Vm/2π (1+cos α).

2. A single-phase half wave circuit has Vs = 230 V with a R load of 100 Ω. Find the average load current at α = 30°.
A. 1.45 A
B. 0.57 A
C. 0.96 A
D. 2.3 A
Answer: C
Clarification: Io = Vo/R
Vo = (Vm/2π) x (1+cosα).

3. A single phase 230 V, 1 kW heater is connected across a 1-phase 230 V, 50 Hz supply through an SCR. The firing angle of the SCR is adjusted to give a rms voltage of 155 V. Find the power absorbed by the heater element.
A. 250 watts
B. 1 kW
C. 454 watts
D. 378 watts
Answer: C
Clarification: R (Heater element resistance) = 230²/1kW = 52.9 Ω
Now, P = Vrms²/R = 155²/52.9.

4. For a single phase half-wave thyristor circuit with R load, the power delivered to the resistive load is
A. (average load voltage) x (average load current)
B. (rms supply voltage)²/R
C. (rms load voltage)²/R
D. (average load voltage)/R
Answer: C
Clarification: P = I².R
V = IR
I = V/R
Hence, P = V²/R.

5. For a single phase half-wave thyristor circuit with R load, the input power factor is given by
A. rms source voltage/total rms line current
B. rms input power/power delivered to the load
C. cos α
D. power delivered to load/input VA
Answer: D
Clarification: (p.f)input = power delivered to load/input VA.

6. For the below shown circuit,

The load voltage waveform from ωt = π to ωt = π+α = β (extinction angle) is
A. gradually rising
B. going from positive to negative
C. gradually decaying
D. zero
Answer: C
Clarification: In this case, the Inductor L is force conducting the SCR & supply current to the source the current is gradually decaying as the inductor is discharging.

7.For the below shown circuit,

The SCR is gated at α. At ωt = π
A. load voltage & load current are both zero
B. only load voltage is zero
C. only load current is zero
D. both are non zero & non negative
Answer: B
Clarification: At π the L force conducts the SCR & current starts gradually decaying whereas at π the voltage is zero as it goes from positive to negative at π.

8. In case of a single-phase half-wave circuit with RL load, with firing angle α and extinction angle β, the conduction angle γ can be written as
A. γ = β+α
B. γ = β-α
C. γ = β/α
D. γ = α/β
Answer: B
Clarification: Extinction angle is some angle > π at which load current reduces to zero. It depends upon the inductor value.

9. For a half-wave single phase thyristor rectifier circuit with any type of load the _______ + _________ waves must ideally give the supply voltage waveform.

A. load voltage, load current
B. load voltage, thyristor voltage
C. supply current, load voltage
D. supply current, thyristor voltage
Answer: B
Clarification: The supply voltage either appears across at the thyristor or the load.

10. In case of a single-phase half-wave circuit with RL load, with firing angle α and extinction angle β, the thyristor is reversed biased from
A. β to α
B. β to 2π+α
C. β to 2π
D. β to 2β
Answer: C
Clarification: As the load current goes to zero from β, the SCR is reversed biased or in the OFF state till it is turned on at 2π again by the positive AC cycle.

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