# Power Electronics Questions and Answers – Single-Phase HW AC-DC-3

This set of Power Electronics Multiple Choice Questions and Answers for freshers focuses on “Single-Phase HW AC-DC-3”.
1. Choose the correct statement with respect to the below given circuit.

a) The load current can be negative
b) The load voltage can never be negative
c) The load voltage can never be zero
d) The load voltage can never be positive

Explanation: The voltage cannot be negative due to the FD (freewheeling diode or commutating diode so connected).

2. In the below given circuit, the FD (Freewheeling diode) is forward biased at ωt =

a) 0
b) α
c) π
d) 2π

Explanation: It is forward biased at π by the conducting SCR & the current starts to through the FD & Load.

3. In the below given circuit, when the commutating diode or FD is conducting than the

a) SCR has reverse bias voltage and the load current is zero
b) SCR has reverse bias voltage and the load current is positive
c) SCR has forward bias voltage and the load current is zero
d) SCR has forward bias voltage and the load current is positive

Explanation: When the FD is forward biased at π by the conducting SCR & the current starts to through the FD & load.

4. The output voltage waveform of the below given circuit would be the same that obtained from a

d) full-wave RL load with freewheeling diode

Explanation: The wave from will be like a half wave diode rectifier circuit. Which is the same as that obtained from a half-wave R load circuit and the above given circuit.

5. In a single-phase half-wave circuit with RL load and a freewheeling diode, the load voltage during the freewheeling period will be
a) zero
b) positive
c) negative
d) positive than negative

Explanation: The FD short circuits the load and voltage across a short circuit would be = 0.

6. In a single-phase half-wave circuit with RL load and a freewheeling diode, the freewheeling period is
a) 0 to π
b) α to π+α
c) π to 2π+α
d) π/2 to 2π-α

Explanation: Freewheeling period is the one in which the FD diode conducts.
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7. A single-phase half wave rectifier with a FD is supplied by Vs = 240 V, AC with a load R = 10 Ω, L = 0.5 mH and a firing angle α = 30°. Find the average value of the load voltage.
a) 50 V
b) 100 V
c) 150 V
d) 200 V

Explanation: Vo = (Vm/2π) x (1+cosα)
Vm = √2Vs.

8. A single-phase HW rectifier with a FD is supplied by Vs = 240 V, 50 Hz, AC with a load R = 10 Ω, L = 0.5 mH and a firing angle α = 30°. Find the average value of the load current.
a) 10 A
b) 0.063 A
c) 6.3 A
d) 0.1 A

Explanation: Vo = (Vm/2π) x (1+cosα)
Vm = √2Vs
Io = Vm/R (Due to the FD).

9. A single phase half-wave controlled rectifier has 400 sin314t as the input voltage and R as the load. For a firing angle of 60°,the average output voltage is
a) 200/π
b) 300/π
c) 100/π
d) 400/π

Explanation: Vo = (Vm/2π) x (1+cosα) = 400/2π x (1+cos60) = 300/π.

10. Choose the incorrect statement with respect to the use of FD in half-wave circuits.
a) Input pf is improved
b) Load current waveform is improved
c) It prevents the load voltage from becoming negative
d) Reduces the reverse voltage (PIV) faced by the SCR

Explanation: PIV is unaffected with the use of FD (freewheeling diodes).

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