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
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) 6, 1, 2
b) 2, 3, 4
c) 3, 4, 5
d) 5, 6, 1
View Answer

Answer: b
Explanation: 180 to 240 degrees i.e. step IV.
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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
View Answer

Answer: c
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.
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) Vs
b) 2Vs/3
c) Vs/3
d) –Vs/3
View Answer

Answer: c
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.
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
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
View Answer

Answer: c
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.
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
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
View Answer

Answer: b
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.
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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
View Answer

Answer: b
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.
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) Vs
b) Vs/3
c) 0
d) 2Vs/3
View Answer

Answer: c
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.
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) Vs
b) Vs/3
c) 0
d) 2Vs/3
View Answer

Answer: a
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?
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) 0
b) Vs/3
c) 2Vs/3
d) -Vs/3
View Answer

Answer: c
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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10. What is the voltage between the B phase and the neutral, when T2, T3 and T4 are conducting?
power-electronics-questions-answers-3-phase-bridge-inverters-2-q1
a) -2Vs/3
b) Vs/3
c) 2Vs/3
d) -Vs/3
View Answer

Answer: d
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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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn