250+ TOP MCQs on 3-Phase Bridge Inverters-2 and Answers

Power Electronics Multiple Choice Questions on “3-Phase Bridge Inverters-2”.

1. The conducting SCRs from 180 to 240 degrees would be

A. 6, 1, 2
B. 2, 3, 4
C. 3, 4, 5
D. 5, 6, 1
Answer: B
Clarification: 180 to 240 degrees i.e. step IV.

2. What is the peak value of phase voltage in case of 3-phase VSI with 180° mode. The supply side consists of a constant dc voltage source of Vs.
A. Vs
B. 3Vs/2
C. 2Vs/3
D. 3Vs
Answer: C
Clarification: Apply KVL to the equivalent circuit of the inverter by closing and opening the proper switches. The phase voltage is a stepped sine-wave with peak value 2Vs/3.

3. What is the R phase voltage when T1, T6 and T5 are conducting? Consider a star connected R load.

A. Vs
B. 2Vs/3
C. Vs/3
D. –Vs/3
Answer: C
Clarification: They conducting SCRs can be represented as closed switches. The load terminals R and B are connected to the positive bus but the terminal Y is connected to the negative bus. V_RN = V_BN = Vs/3.
V_YN = -2Vs/3.

4. What are the phase voltages when T6, T1 and T2 are conducting? Consider a star connected R load.

A. V_YN = V_BN = Vs/3, V_RN = 0
B. V_YN = V_RN = – Vs/3, V_BN = 2Vs/3
C. V_RN = V_BN = – Vs/3, V_YN = 2Vs/3
D. V_YN = V_BN = – Vs/3, V_RN = 2Vs/3
Answer: C
Clarification: When 6, 1 and 2 are conducting and the others are off. The conducting SCRs can be represented as closed switches. The load terminals Y and B are connected to the negative bus but the terminal R is connected to the positive bus. Supply voltage is Vs.
V_YN = V_BN = -Vs/3.
V_RN = 2Vs/3.
It should be noted that at any time, the summation of these phase voltages must be zero.

5. What are the phase voltages when T1, T2 and T3 are conducting? Consider a star connected R load.

A. V_YN = V_BN = V_RN = 0
B. V_YN = V_RN = Vs/3, V_BN = – 2Vs/3
C. V_RN = V_BN = – Vs/3, V_YN = 2Vs/3
D. V_YN = V_BN = Vs/3, V_RN = – 2Vs/3
Answer: B
Clarification: When 6, 1 and 2 are conducting and the others are off. They conducting SCRs can be represented as closed switches. The load terminals R and Y are connected to the positive bus and the terminal B is connected to the nrgative bus. Supply voltage is Vs.
V_RN = V_YN = Vs/3.
V_BN = -2Vs/3.
It should be noted that at any time, the summation of these phase voltages must be zero.

6. What is the maximum line voltage value in case of a three-phase VSI in 180° mode?
A. 2Vs
B. Vs
C. 3Vs
D. 2Vs/3
Answer: B
Clarification: The line voltage waveform has a peak value of Vs. Any line voltage value can be found by just adding the two phase voltage value.
e.g. Vab = Vrn – Vbn
Vab = (2Vs/3) – (-Vs/3) = Vs.

7. Find V_YB, when T6, T1 and T2 are conducting. Consider a star connected R load.

A. Vs
B. Vs/3
C. 0
D. 2Vs/3
Answer: C
Clarification: When 6, 1 and 2 are conducting and the others are off. They conducting SCRs can be represented as closed switches. The load terminals Y and B are connected to the negative bus but the terminal R is connected to the positive bus. Supply voltage is Vs.
V_YN = V_BN = -Vs/3.
V_YB = V_YN – V_BN = (-Vs/3) – (-Vs/3) = -Vs/3 + Vs/3 = 0.

8. Find V_RY, when T6, T1 and T2 are conducting. Consider a star connected R load.

A. Vs
B. Vs/3
C. 0
D. 2Vs/3
Answer: A
Clarification: When 6, 1 and 2 are conducting and the others are off. They conducting SCRs can be represented as closed switches. The load terminals Y and B are connected to the negative bus but the terminal R is connected to the positive bus. Supply voltage is Vs.
V_YN = V_BN = -Vs/3.
V_RN = 2Vs/3
V_RY = V_RN – V_YN = (2Vs/3) – (-Vs/3) = 3Vs/3 = Vs.

9. What is the Y phase voltage, when T2, T3 and T4 are conducting?

A. 0
B. Vs/3
C. 2Vs/3
D. -Vs/3
Answer: C
Clarification: Construct the equivalent circuit, considering the conducting devices as S.C and the non-conducting devices as open circuit.
When 2, 3 and 4 are conducting and the others are off. They conducting SCRs can be represented as closed switches. The load terminals R and B are connected to the negative bus but the terminal Y is connected to the positive bus. Supply voltage is Vs.
V_RN = V_BN = -Vs/3.
V_YN = 2Vs/3.

10. What is the voltage between the B phase and the neutral, when T2, T3 and T4 are conducting?

A. -2Vs/3
B. Vs/3
C. 2Vs/3
D. -Vs/3
Answer: D
Clarification: Construct the equivalent circuit, considering the conducting devices as S.C and the non-conducting devices as open circuit.
When 2, 3 and 4 are conducting and the others are off. They conducting SCRs can be represented as closed switches. The load terminals R and B are connected to the negative bus but the terminal Y is connected to the positive bus. Supply voltage is Vs.
V_RN = V_BN = -Vs/3.
V_YN = 2Vs/3.