# Power Electronics Questions and Answers – AC Voltage Controllers-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “AC Voltage Controllers-2”.

1. In the below shown circuit, the diode conducts for a) 90°
b) > 90°
c) < 90°
d) 0°

Explanation: The diode conducts from π to 2π in the negative half cycle. If it were a non resistive load, the diode would conduct for less than 90°, as the inductor would force conduct the SCR for some time.

2. The SCR T1 is fired at an angle of α, and the supply Vs = Vm sinωt. Find the average value of the output voltage. a) (Vm/2π) (cosα + )
b) (Vm/2π) (cosα)
c) (Vm/2π) (cosα – 1)
d) Vm

Explanation: Vo = (1/2π) ∫Vm sinωt d(ωt). Where the integration would be run from α to 2π as the conduction takes place from α to 2π.
Vo = (Vm/2π) (cosα – 1).

3. The below given output voltage waveform can be obtained by a a) half wave ac voltage controller
b) full wave ac voltage controller
c) half wave controller with firing angle = 0° for T1
d) full wave controller with firing angle = 0° for both T1 and T2

Explanation: As the positive half is chopped off due to some value of firing angle, the firing angle is not equal to zero. As the negative half is a half sine wave, either it is a full wave controller with firing angle for T2 set to zero or it is a half wave controller with a Thyristor (T1) and a diode.

4. In the below shown AC converter circuit with firing angle = α for both the devices, T2 will conduct from a) α to π
b) π + α to 2π
c) π to 2π
d) α to 2π

Explanation: T2 is triggered at π + α, it conducts from π + α to 2π after which it is line commutated.

5. The below given output voltage waveform can be obtained by a a) half wave ac voltage controller
b) full wave ac voltage controller
c) full wave inverter circuit
d) none of the mentioned

Explanation: As the control is in both the directions or cycles, it is a full wave ac voltage controller circuit.
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6. In the integral cycle control method of ac voltage controller
a) the average power delivered to the load is controlled
b) the instantaneous power delivered to the load is controlled
c) the frequency of output voltage is controlled
d) none of the mentioned

Explanation: In integral cycle control, power is delivered for m cycles and not delivered for n cycles. Hence, the average value of the power delivered is controlled by manipulating m and n.

7. In the integral cycle control of ac voltage controller, is the load is on for n cycles and off for m cycles, then the periodicity is given by? Consider the output is sinusoidal.
a) m/2π(m+n)
b) n/2π(m+n)
c) m/π(m+n)
d) n/π(m+n)

Explanation: Over a complete cycle of 2π x (on cycles + off cycles) the power is delivered for n cycles.
Hence, P = n/2π(m+n).

8. If k is the duty cycles of the controller, then the rms value of the output voltage in case of a integral cycle control circuit will be?
Consider the input to be sinusoidal with peak value Vm and rms value Vs.
a) Vs x k
b) Vs/k
c) Vs x √k
d) Vs

Explanation: Vrms = [ (k/2π) x ∫Vm2 sin⁡2 ωt d(ωt) ] 1/2
Where the integral runs from 0 to 2π.
Vrms = (Vm/√2) k = Vs x √k.

9. Find the power delivered to the load in the integral cycle control method of ac voltage control, having a sine input of Vs, R load and duty cycle = k. a) Vs2/R
b) k.Vs2/R
c) √k .Vs2/R
d) 0

Explanation: Output voltage = Vs x √k
P = (Vs x √k)2/R.

10. In the integral cycle control method with duty cycle = k and maximum load current = Im. Find the value of average SCR current. a) Im/k.π
b) Im
c) k.Im
d) k.Im/π

Explanation: As each SCR conducts for π radians during each cycle of n on cycles, the average value of SCR current is
I = (k/2π) ∫Im sin ωt d(ωt). Where the integration is from 0 to π.

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