# Power Electronics Questions and Answers – SCR Commutation Techniques-1

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “SCR Commutation Techniques-1”.

1. The thyristor turn-off requires that the anode current
a) falls below the holding current
b) falls below the latching current
c) rises above the holding current
d) rises above the latching current

Explanation: For effective turn-off of the SCR the anode current must fall below the holding current value.

2. In case of class A type commutation or load commutation with low value of R load the
a) L is connected across R
b) L-C is connected across R
c) L is connected in series with R
d) L-C is connected in series with R

Explanation: In case of Class A commutation the requirement is that the circuit should be an under-damped RLC circuit.

3. The class A commutation or load commutation is possible in case of
a) dc circuits only
b) ac circuits only
c) both DC and AC circuits
d) none of the above mentioned

Explanation: The nature of the circuit should be such that when energized from the source, current must a a natural tendency to decay to zero for load commutation to occur in a SCR circuit.

4. In case of class B commutation or resonant-pulse commutation with L = 5 μH and C = 20 μC with initial voltage across the capacitor (Vs) = 230 V. Find the peak value of resonant current.
a) 560 A
b) 460 A
c) 360 A
d) 260 A

Explanation: Ip = Vs x √C/L.

5. In case of class B commutation or resonant-pulse commutation with L = 5 μH and C = 20 μC with the initial voltage across the capacitor (Vs) = 230 V. Find the conduction time for auxiliary thyristor.
a) 0.23 μs
b) 6.57 μ
c) 31.41 μs
d) 56 μs

Explanation: Ip = Vs x √C/L
ω = 1/√LC
t = π/ω.

6. An SCR is connected in series with L = 5 mH and C = 20 μF. Find the resonant frequency of the circuit.

Explanation: ω = 1/√LC.

7. The type of commutation when the load is commutated by transferring its load current to another incoming thyristor is
a) class A or load commutation
b) class B or resonant commutation
c) class C or complementary commutation
d) class D or impulse commutation

Explanation: In the Class C type commutation also called as complementary commutation the load is commutated by transferring the current th another device.

8. The type of commutation in which the pulse to turn off the SCR is obtained by separate voltage source is
a) class B commutation
b) class C commutation
c) class D commutation
d) class E commutation

Explanation: In class E commutation, another voltage source is used. It is also called as external pulse commutation.

9. The natural reversal of ac supply voltage commutates the SCR in case of
a) forced commutation
b) only line commutation
c) only natural commutation
d) both line & natural commutation

Explanation: Both line and natural commuataion are used in converters.

10. ___________ commutation technique is commonly employed in series inverters.
a) line
c) forced
d) external-pulse

Explanation: Load commuation is used in inverter in which L and C are connected in series with the load or C in parallel with the load such that overall load circuit is under damped.

Sanfoundry Global Education & Learning Series – Power Electronics.

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