# 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
b) end of the load current, end of the load voltage
c) start of the load current, start of the load 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
b) Load voltage is maximum
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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