Discussion
A three-phase voltage source inverter is connected with input DC voltage of 400 V. It is first operated in \(180^\circ\) conduction mode and then in \(120^\circ\) conduction mode. The ratio of maximum output phase voltage between \(180^\circ\) to \(120^\circ\) conduction mode is:
Answer: A
In a three phase bridge inverter the maximum value of the phase voltage depends on the conduction mode. In the \(180^\circ\) conduction mode the phase voltage is a six step wave with levels of \(\pm V_{dc}/3\) and \(\pm 2V_{dc}/3\), so its maximum is \(2V_{dc}/3\). In the \(120^\circ\) mode, the phase voltage takes the values of \(0\) and \(\pm V_{dc}/2\), so its maximum is \(V_{dc}/2\).
\[ \frac{(2/3)V_{dc}}{(1/2)V_{dc}} = \frac{4}{3} \]
The input voltage of 400 V cancels out in the ratio. The answer is 4/3, option A.
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