Difficulty: Medium
Correct Answer: less than 60°
Explanation:
Introduction / Context:
Three-phase semiconverters (half-controlled bridges) often include a freewheeling diode (FWD) to provide a path for inductive load current when the converter’s instantaneous output voltage goes negative. The timing of freewheeling depends on the firing angle α and the phase voltages.
Given Data / Assumptions:
Concept / Approach:
In a semiconverter, the instantaneous converter output can become negative during certain intervals. If α is small (close to the natural commutation instant), the load current still needs a positive path; the FWD provides this by clamping the load voltage near zero and circulating current internally. As α increases, the negative portions shrink and beyond a threshold (≈60°) freewheeling ceases to be needed under ideal assumptions.
Step-by-Step Solution:
For α < 60°, intervals exist where the converter’s line-to-line selection would impose reverse voltage at the output.The FWD conducts during those intervals, maintaining current continuity and preventing negative output voltage across the inductive load.At α ≥ 60°, the converter’s selection keeps the output from going negative in the same way; freewheeling becomes unnecessary in the idealized analysis.
Verification / Alternative check:
Standard waveforms in power electronics texts show freewheeling conduction lobes for small α in semiconverters and vanishing FWD conduction beyond ≈60°.
Why Other Options Are Wrong:
Equal to / more than 60° / more than 90°: Opposite to the established condition.
Zero only: Too restrictive; freewheeling exists for a range below 60°.
Common Pitfalls:
Assuming freewheeling is always present; it depends on α and the intersection of phase voltages.
Final Answer:
less than 60°
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