Question

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

I had been asked this question in a national level competition.

The doubt is from 3-Phase Bridge Inverters-2 topic in portion Inverters of Power Electronics

Answer 1

The correct answer is (c) VRN = VBN = – Vs/3, VYN = 2Vs/3

For 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.