250+ TOP MCQs on Thyristor Chopper Circuits–1 and Answers

Power Electronics Multiple Choice Questions on “Thyristor Chopper Circuits – 1”.

1. The process of commutating a SCR by applying a reverse voltage to an SCR through a previously charged capacitor is called as
A. capacitor commutation
B. forced commutation
C. voltage commutation
D. current commutation
Answer: C
Clarification: It is called as voltage commutation, which is a type of forced commutation.

2. In case of current commutation of SCR
A. a diode is connected in series with the main SCR
B. a diode is connected in parallel with the main SCR
C. a diode is connected in anti-parallel with the main SCR
D. none of the mentioned
Answer: C
Clarification: The voltage drop across the anti parallel connected SCR reverse biases the main SCR.

3. Below given circuit is a

A. current commutated chopper
B. voltage commutated chopper
C. load commutated chopper
D. none of the mentioned
Answer: B
Clarification: It is a voltage commutated chopper or impulse-commutated chopper. If the diode was directly in parallel with the main SCR it would be a current commutated chopper circuit.

4. In the below shown voltage commutated chopper circuit, the chopper operation can only start when

A. t1 is gated
B. the capacitor is charged
C. the diode is forward biased
D. the capacitor is dis-charged
Answer: B
Clarification: The chopper can only start (effectively) when the C in the commutating circuit is charged first.

5. In a voltage commutated chopper circuit, the capacitor C can be charged to Vs by

A. gating T1
B. gating TA
C. closing the switch
D. both gating TA and closing the switch
Answer: D
Clarification: When switch is closed, the C gets charged through Vs, C, Switch. When TA is triggered C gets charged through Vs, C, TA and the load.

6. In the below given circuit, when the main thyristor T1 is triggered after charging the capacitor, the commutation current flows through the path

A. Vs – T1 – Load – Vs
B. S – C – T1 – Load – S
C. C – T1 – L – D – C
D. C – T1 – TA – C
Answer: C
Clarification: The commutation current is nothing but the capacitor current.

7. In the below given circuit, when the main SCR T1 is commutated at t₂, the load voltage is

A. Vs
B. 0
C. 2Vs
D. Vs + Ldi/dt
Answer: C
Clarification: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1. The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage).
As the capacitor is already charged to a voltage Vs. Vo = 2Vs.

8. In a voltage commutated SCR, the main SCR is commutated by
A. triggering the auxiliary SCR
B. sending a negative pulse to the main SCR
C. removing the supply Vs
D. none of the mentioned
Answer: A
Clarification: The main SCR T1 is commutated by the triggering of the TA which then makes the capacitor revere bias the main SCR T1.

9. Which type of commutation circuit does not work on no load?
A. Voltage commutation
B. Current commutation
C. Both voltage and current commutation
D. None of the mentioned
Answer: A
Clarification: This is because the capacitor won’t be able to charge on no load.

10. What is the expression for the value of the capacitor in the voltage commutation circuit? Let tc be the circuit turn off time and Io be the constant load current.
A. Io x (dv/dt)
B. Vs/Io
C. (tc x Io)/Vs
D. none of the mentioned
Answer: C
Clarification: Capacitor current = C dv/dt
For constant load current Io, the above expression can be
Io = C x (Vs/tC. or C = (tc x Io)/Vs.