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
Vavg = ∫ Vm Sin ωt d(ωt) where the integration runs from α to π
Vavg = 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