# Power Electronics Questions and Answers – Force-Commutated Inverters

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Force-Commutated Inverters”.

1. In voltage fed thyristor inverters __________ commutation is required.
a) load
b) forced
c) self
d) any commutation technique can be used
View Answer

Answer: b
Explanation: In VSI, the input voltage source Vs keeps the SCRs always forward biased. Hence, forced commutation becomes essential.

2. The McMurray circuit is a
a) commutation circuit
b) force commutated VSI
c) self commutated VSI
d) none of the mentioned
View Answer

Answer: b
Explanation: McMurray is a popularyly used forced commutated VSI circuit.

3. Forced commutation requires
a) a precharged inductor
b) a precharged capacitor
c) an overdamped RLC load
d) a very high frequency ac source
View Answer

Answer: b
Explanation: Forced commutation requires a pre charged capacitor of correct polarity to turn-off the earlier conducting SCR.
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4. Which elements are not present in the original McMurray inverter but are present in the modified McMurray inverters?
a) Two auxiliary thyristors and the di/dt inductor
b) Two auxiliary diodes and the damping resistor
c) One auxiliary SCR and one auxiliary diode
d) None of the mentioned
View Answer

Answer: b
Explanation: In the original inverter circuit given by McMurray the elements DA1, DA2 and the damping resistor Rd were not present.

5. Let Im be the maximum load current and Vm be the minimum supply voltage value. Than the expression for the design of commutation circuit parameters in a single-phase modified McMurray half-bridge inverter is given by

a) (1.5 x Im)/Vm
b) Im/(1.5 x Vm)
c) Vm/(1.5 x Im)
d) (1.5 x Vm)/Im
View Answer

Answer: c
Explanation: The design is carried out on the basis of the worst operating conditions which consist of the minimum supply voltage Vm and the maximum load current. (c) gives the empirical formula for calculation of the L and C commutation circuit parameters.
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6. The number of diodes, SCRs and other components in full-bridge inverter McMurray inverter is ____________ of those in half-bridge McMurray inverter.
a) same
b) double
c) three times
d) none of the mentioned
View Answer

Answer: b
Explanation: The modified McMurray full-bridge requires twice the components that required in the half-wave type.

7. In the single-phase modified McMurray full-bridge inverter, for commutating the main SCRs T1 and T2
a) The capacitor is charged
b) TA1 is triggered
c) The commutation circuit is switched on
d) TA1 and TA2 are triggered
View Answer

Answer: d
Explanation: TA1 and TA2 are triggered together, which then turn-off the main SCRs. TA1 and TA2 are the auxiliary SCRs in the commutation circuit of the McMurray inverter.
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8. How many diodes are there in total in the single-phase, modified McMurray full-bridge inverter?
a) 4
b) 6
c) 8
d) 10
View Answer

Answer: c
Explanation: 4 belong to the main full-bridge circuit and 4 in the commutation circuit (2 on both the sides).

9. The McMurray-Bedford half-bridge inverter requires
a) 4 SCRs, 2 diodes, 2 capacitors and 2 inductors
b) 4 SCRs, 4 diodes, 2 capacitors and 2 inductors
c) 2 SCRs, 4 diodes, 2 capacitors and 1 inductor
d) 2 SCRs, 2 diodes, 2 capacitors and 2 inductors
View Answer

Answer: d
Explanation: McMurray-Bedford type requires less number of SCRs and diodes as compared to the McMurray type, however the number of capacitors and inductors required is the same.
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10. The single-phase McMurray-Bedford type bridge inverter is a/an
a) auxiliary-commutated inverter
b) complementary-commutated inverter
c) supplementary-commutated inverter
d) none of the mentioned
View Answer

Answer: b
Explanation: If one SCR gets turned on, the other conducting SCR gets turned off. This type of commutation is called as complementary commutation.

Sanfoundry Global Education & Learning Series – Power Electronics.

To practice all areas of Power Electronics, here is complete set of 1000+ Multiple Choice Questions and Answers.

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