# Power Electronics Questions and Answers – 3-Phase Bridge Inverters-2

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses 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

Explanation: 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

Explanation: 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

Explanation: 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. VRN = VBN = Vs/3.
VYN = -2Vs/3.

4. What are the phase voltages when T6, T1 and T2 are conducting? Consider a star connected R load. a) VYN = VBN = Vs/3, VRN = 0
b) VYN = VRN = – Vs/3, VBN = 2Vs/3
c) VRN = VBN = – Vs/3, VYN = 2Vs/3
d) VYN = VBN = – Vs/3, VRN = 2Vs/3

Explanation: 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.
VYN = VBN = -Vs/3.
VRN = 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) VYN = VBN = VRN = 0
b) VYN = VRN = Vs/3, VBN = – 2Vs/3
c) VRN = VBN = – Vs/3, VYN = 2Vs/3
d) VYN = VBN = Vs/3, VRN = – 2Vs/3

Explanation: 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.
VRN = VYN = Vs/3.
VBN = -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

Explanation: 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 VYB, when T6, T1 and T2 are conducting. Consider a star connected R load. a) Vs
b) Vs/3
c) 0
d) 2Vs/3

Explanation: 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.
VYN = VBN = -Vs/3.
VYB = VYN – VBN = (-Vs/3) – (-Vs/3) = -Vs/3 + Vs/3 = 0.

8. Find VRY, when T6, T1 and T2 are conducting. Consider a star connected R load. a) Vs
b) Vs/3
c) 0
d) 2Vs/3

Explanation: 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.
VYN = VBN = -Vs/3.
VRN = 2Vs/3
VRY = VRN – VYN = (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

Explanation: 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.
VRN = VBN = -Vs/3.
VYN = 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

Explanation: 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.
VRN = VBN = -Vs/3.
VYN = 2Vs/3.

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