# Power Electronics Questions and Answers – Thyristor Chopper Circuits-3

This set of tough Power Electronics Multiple Choice Questions(MCQs) focuses on “Thyristor Chopper Circuits-3”.

1. Find the expression for peak capacitor voltage in case of a current commutated chopper circuit.
a) Io √(L/C)
b) Vs + √(L/C)
c) Vs + Io √(L/C)
d) Zero

Explanation: 1/2 CVc2 = 1/2 LIo2
Vc = Io √(L/C)
Vc (peak) = Vs + Io √(L/C).

2. Identify the below given chopper circuit.

a) Type D chopper
b) Type C chopper with commutating capacitor
d) Voltage commutated step-up chopper

Explanation: Load commutated chopper uses 4 SCRs and one commuting capacitor. As there is no L in the circuit, it is a step down chopper.

3. In the below given commutated chopper circuit, __ and __ SCRs act together in pairs.

a) T1-T2, T3-T4
b) T1-T3, T2-T4
c) T1-T4, T2-T4
d) T1, T2

Explanation: T1-T2 and T3-T4 act together as one pair for conducting the load current alternately.

4. When T1 and T2 are conducting, which components act as the commutating components?

a) T3 and T4
b) All the SCRs
c) T3, T4 and C
d) T3, T4, C and FD

Explanation: Commutating components are the one which will help the main SCRs T1 and T2 in this case turn off safely. FD is not a commutating component, it only freewheels the current during inductive loads.

5. What is the voltage across the load when T1 and T2 are gated after charging the capacitor to Vs?

a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

Explanation: The main SCRs T1 and T2 are conducting, and the path is Vs –T1 – C – T2 – Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = 2Vs.
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6. What is the expression for design of commutation capacitance in case of a load commutated chopper circuit?
a) (I x Ton)/Vs
b) Cannot be determined
c) (I x Ton)/2Vs
d) (I x T)/Vs

Explanation: Total change in the voltage is 2xVs in time Ton.
I = C x (2Vs/ Ton)
C = (IxTon)/2Vs.

7. Which type of commutating circuit will have highest switching losses for same rating of all the components?
a) current commutated chopper
b) voltage commutated chopper
d) forced commutated chopper

Explanation: Load commutated choppers have 4 SCRs which require frequent turning on and turning off which increases the switching losses.

8. The speed of a separately excited dc motor is controlled below base speed by using a type-A SCR based chopper. The supply voltage is 220 V dc and the armature resistance is 0.5 Ω. Calculate the minimum value of firing angle if the motor drives a constant torque load requiring an average armature current of 30 A.
a) 1/2
b) 3/44
c) 59/46
d) 7/12

Explanation: For a motor, Vo = E + Ir
The minimum possible speed = 0. This gives the motor counter emf E = 0.
Therefore, Vo = αVs = Ir
α x 220 = 30 x 0.5
α = 15/220 = 3/44.

9. For the step down chopper shown in the figure below, Vs = 100 V and the duty cycle of the switch is 0.8. The load is sufficiently inductive so that the load current is ripple free. The average current through the diode (D) under steady state is

a) 2 A
b) 1.6 A
c) 8 A
d) 6.4 A

Explanation:
The diode is active only during Toff.
If Ton = 0.8 then Toff = 0.2
Output voltage Vo = 0.8 x 100 = 80 V
When D is active during Toff, consider inductor as a short circuit thus, Id = (VoxToff)/R = 80 x 0.2/10 = 1.6 A.

10. A chopper circuit is fed from an input voltage of 20 V delivers a power of 16 watts. If the chopper efficiency is 80 %, then find the value of input current.
a) 0.64 A
b) 1 A
c) 0.8 A
d) 1.25 A

Explanation: 0.8 = ouput/input
Input power = 16/0.8
16/0.8 = V x I
V = 20 V. Hence, I = 1 A.

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