# Power Electronics Question and Answers – Single-Phase HW AC-DC-1

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Single Phase HW AC-DC-1”.

1. A voltage source Vs = Vm sinωt is connected in series with a resistance R and an SCR. At some firing angle delay of α a positive gate pulse is applied to the SCR which turns on the SCR. Considering ideal conditions, at the instant α the voltage at the resistor terminals Vo
a) falls to zero
b) falls to –Vm sin α
c) rises to Vm sin α
d) rises to Vm sin ωt

Explanation: At the firing angle, the SCR turns on and thus the resistor or load R is connected to the source.

2. The average output voltage is maximum when SCR is triggered at ωt =
a) π
b) 0
c) π/2
d) π/4

Explanation: The sooner the conduction starts higher the average power. Though practically a device cannot be triggered at exactly zero degrees.

3. In the below given circuit, the SCR is triggered at a certain firing angle α. The load current falls to zero at ωt =
a) α
b) α/2
c) π
d) π/2

Explanation: The SCR is naturally commutated at 90°.

4. In the method of phase control, the phase relationship between ___ & ___ is controlled by varying the firing angle
a) supply current, supply voltage

Explanation: By varying the firing angle, the load current and load voltage can be controlled.

5. In a single phase half-wave thyristor circuit with R load & Vs=Vm sinωt, the maximum value of the load current can be given by
a) 2Vm/R
b) Vs/R
c) Vm/2
d) Vs/2

Explanation: Vm is the peak value of the load as well as supply voltage. I = Vm/R.
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6. Below shown ideal circuit is triggered at a firing angle of α. The voltage across the SCR is zero from ωt = a) 0 to α
b) 0 to π
c) α to 2π
d) α to π

Explanation: Before the SCR is triggered all of the supply voltage appears across it that is from 0 to α, at α the device is gated and it starts to conduct acting like a S.C (ideally). Hence voltage across it is zero from α to π. At π the SCR is again naturally commutated due to the reversal of the AC supply & a negative voltage appears across it.

7. Below shown ideal circuit is triggered at a firing angle of α. From ωt = π to 2π+α a) SCR voltage is zero
c) SCR voltage is –Vmsinα
d) SCR acts like a short circuit

Explanation: From ωt = π to 2π+α, negative cycle is active and the SCR experiences revered biased voltage by Vs.

8. For a single phase thyristor circuit with R load & firing angle α, the conduction angle can be given by
a) π+α
b) 2π+α
c) π-α
d) α

Explanation: SCR is fired at α, hence it conducts from α till it is commutated by the reversal of the AC supply i.e at π radians.

9. For the below given circuit with firing angle α & angular frequency of ω, the circuit turn-off time in secs can be given by a) α/π
b) π/α
c) ω/π
d) π/ω

Explanation: The circuit turns of at ωt = π as it is a half wave circuit.
Hence toff = π/ω.

10. A single-phase half-wave thyristor circuit with R load is triggered at an angle of α = 0°. As such, the maximum value of the average output voltage would be given by
Consider Vs = Vm sinωt.
a) Vm
b) 2Vm/π
c) Vm/π
d) Vm/α

Explanation: For the whole cycle of 2π, the R load would be active only for half of the cycle. Hence, Vo = Vm/π.

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