# Power Electronics Questions and Answers – SCR Commutation Techniques-2

This set of Power Electronics Questions & Answers for entrance exams focuses on “SCR Commutation Techniques-2”.

1. For the circuit shown in the figure below,

C = 4 pF
L = 16 μH
Vs = 200V
The capacitor voltage after the SCR is self commutated is
a) -100 V
b) -200 V
c) -400 V
d) 0 V

Explanation: It is simply the negative of Vs as it is a series circuit & after turn off the whole voltage appears across the thyristor.

2. For the circuit shown below,

The maximum value of current through thyristors T1 & TA can be given by
a) Vs/R, Vs√C/L
b) Vs √C/L, Vs/R
c) Vs/R + LC, Vs
d) Vs/LC, Vs/R

Explanation: I1 = (Vs/R)
T2 = (Vs√C/L).

3. For the circuit shown in the figure below,

C = 4 pF
L = 16 μH
Vs = 200 V
The peak thyristor current is
a) 800 A
b) 400 A
c) 100 A
d) 50 A

Explanation: The peak SCR current is Vs/2.

4. For the circuit shown in the figure below, ___ type of commutation would take place for the SCR (THY1)

a) line
c) forced
d) external-pulse

Explanation: The given figure is that of a class B commutation or resonant-pulse commutation, which is a type of forced commutation technique.

5. For the circuit shown in the figure below,

R1 = 50 Ω, R2 = 100 Ω, Vs = 100 V. At the time of turn-on of the SCR, initial thyristor current is
a) 0 A
b) 10 A
c) 5 A
d) 3.5 A

Explanation: Io = Vs x [(2/R1)+(1/R2)].
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6. Natural commutation of an SCR takes place when
a) voltage across the device becomes negative
b) voltage across the device becomes positive
c) gate current becomes zero
d) anode current becomes zero

Explanation: Anode current (load current) becomes zero and turns off the device, hence the name line commutation.

7. ___________ commutation is usually used in phase-controlled rectifiers
a) line
c) forced
d) external-pulse

Explanation: Line commuataion is used in converters.

8. Parallel-capacitor commutation is
a) line commutation
c) forced commutation
d) external-pulse commutation

Explanation: Parallel capacitor is another name for forced commutation.

9. Class E commutation is a/an
a) line commutation technique
c) forced commutation technique
d) external-pulse commutation technique

Explanation: As an external source is used it is a external-pulse commutation technique.

10. Below is the circuit of a resonant-pulse commutation, the maximum voltage to which the capacitor C charges is

a) Vs
b) Vs/2
c) 2
d) √2Vs

Explanation: The capacitor charges from 0 to Vs with the right side plate positive.

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