Discussion
For a 3-phase semi-converter, if the firing angle is \(80^\circ\), then the free-wheeling diode conducts for ______.
Answer: D
In a three phase semi-converter, each SCR carries current for 120 degrees at most, and the free wheeling diode starts to conduct when the output voltage tries to go negative, which happens only for firing angles above \(60^\circ\). For \(\alpha > 60^\circ\) the diode conducts for \(\alpha - 60^\circ\) in each of the three intervals of the supply cycle.
\[ \alpha - 60^\circ = 80^\circ - 60^\circ = 20^\circ \]
The conduction of the diode is zero for \(\alpha \le 60^\circ\), so 60 degrees and 90 degrees cannot be the answers, and 40 degrees would correspond to \(\alpha = 100^\circ\). The answer is \(20^\circ\), option D.
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