Three-phase semiconverter (half-controlled) output continuity Beyond what firing angle does the DC output voltage become discontinuous (gaps appear) for a resistive/weakly inductive load?

Introduction / Context:
The three-phase semiconverter (one quadrant, half-controlled) uses three controlled devices and three diodes. Its output voltage waveform quality and continuity depend strongly on the firing angle α and load smoothing. Designers must know where natural gaps begin to appear so that filter sizing and performance expectations are realistic.

Given Data / Assumptions:

• Three-phase semiconverter feeding a load with limited smoothing.
• Ideal devices, negligible overlap for concept.
• Focus on onset of discontinuity as α increases.

Concept / Approach:

In a semiconverter, each device pair supplies segments of the line-to-line wave. As α increases, the conduction intervals shrink relative to the 60° spacing of line EMFs. When α exceeds 60°, there are intervals where no device pair can sustain positive DC voltage, leading to notches or zero output spans—i.e., discontinuity in the average DC voltage waveform for lightly filtered or resistive loads.

Step-by-Step Solution:

Verification / Alternative check:

Phase diagram timing confirms the loss of continuous transfer when the trigger is delayed beyond the 60° inter-phase spacing typical of a three-phase system.

Why Other Options Are Wrong:

30° is too low; 90° and 120° overstate the continuity range for a semiconverter with weak smoothing.

Common Pitfalls:

Confusing semiconverters with fully controlled bridges; assuming large output inductance which can maintain current continuity even when voltage has gaps.

Final Answer:

α > 60°