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

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “3-Phase Bridge Inverters-3”.

1. The 120° mode of operation of a three phase bridge inverter requires ___________ number of steps.
a) 2
b) 4
c) 6
d) 8

Explanation: Like the 180 mode, the 120° mode also requires six steps, each of 60° duration.

2. In case of the 120° mode of operation, __________ devices conduct at a time.
a) 2
b) 3
c) 4
d) none of the mentioned

Explanation: Unlike the 180° mode, in the 120° mode one 2 devices are conducting at a time as each conduct for 120°.

3. Safe commutation can be achieved in case of the ____________ operating mode.
a) 180°
b) 120°
c) 360°
d) none of the mentioned

Explanation: In the 120 mode of operation, there is a 60° interval between turning off of T1 and turning on of T4 (or any two SCRs belonging to the same leg), hence SCRs are commutated safely.

4. What is the R phase voltage when only T1 and T2 are conducting from 60° to 120°. Consider star connected R load.

a) Vs
b) Vs/2
c) –Vs/2
d) 0

Explanation: As only 2 SCRs are conducting at a time, it is 120 mode.
When T1 and T2 are conducting, R phase is connected to the positive bus through the T1 SCR and B phase is connected to the negative bus through the T2 SCR.
VRN = Vs/2
VYN = -Vs/2.

5. If T1 is gated at 0 °, T3 and T5 will start conducting at _______ and _________ respectively.
a) 180°, 270°
b) 120°, 240°
c) 180°, 300°
d) 240°, 360°

Explanation: As T1 is gated at 0, T3 will be gated after 120° and T5 after 120+120 = 240°.

 Devices T1 T1 0 T4 T4 0 T1 T1 0 T6 0 T3 T3 0 T6 T6 0 T3 0 T2 T2 0 T5 T5 0 T2 T2

6. What is the B phase voltage when only T1 and T2 are conducting from 60° to 120°. Consider star connected R load.

a) Vs
b) Vs/2
c) –Vs/2
d) 0

Explanation: When T1 and T2 are conducting, R phase is connected to the positive bus through the T1 SCR and B phase is connected to the negative bus through the T2 SCR. Considering that the load is a balanced R load.

 Devices T1 T1 0 T4 T4 0 T1 T1 0 T6 0 T3 T3 0 T6 T6 0 T3 0 T2 T2 0 T5 T5 0 T2 T2

VRN = Vs/2
VBN = -Vs/2.

7. What is the R phase voltage and Y phase voltage when only T3 and T4 are conducting? Consider a star connected balanced R load.

a) Vs, -Vs
b) Vs/2, -Vs/2
c) –Vs/2, Vs/2
d) –Vs, Vs

Explanation: When T3 and T4 are conducting, R phase is connected to the negative bus through the T4 SCR and the Y phase is connected to the positive bus through the T3 SCR. Considering that the load is a balanced R load.

 Devices T1 T1 0 T4 T4 0 T1 T1 0 T6 0 T3 T3 0 T6 T6 0 T3 0 T2 T2 0 T5 T5 0 T2 T2

VYN = Vs/2
VRN = -Vs/2.

8. If the SCR T1 is gated at 0°, then for 120° mode of operation, from ωt = 240° to 300° __________ devices would conduct.

a) T3, T4
b) T4, T5
c) T1, T6
d) T5, T6

Explanation:

 Devices T1 T1 0 T4 T4 0 T1 T1 0 T6 0 T3 T3 0 T6 T6 0 T3 0 T2 T2 0 T5 T5 0 T2 T2

ωt = 240° to 300°, would form the 5th step.

9. Find the line voltage VYR when only T3 and T4 are conducting? Consider a star connected balanced R load.

a) 2Vs/3
b) Vs/2
c) Vs
d) –Vs

Explanation: When T3 and T4 are conducting, R phase is connected to the negative bus through the T4 SCR and the Y phase is connected to the positive bus through the T3 SCR. Considering that the load is a balanced R load.
VYN = Vs/2
VRN = -Vs/2.
VYR = VYN – VRN = Vs.

10. The peak value of the line voltage in case of 120° mode of operation of a three-phase bridge inverter is
a) Vs/2
b) 3Vs/2
c) Vs/√2
d) Vs

Explanation: The peak value for 120° mode is Vs. The line voltage waveform is a sine wave with a peak value of Vs ( = supply voltage).

Sanfoundry Global Education & Learning Series – Power Electronics.

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