Three-phase full converter with firing angle α < 60° – instantaneous DC output polarity In a 3-phase full converter (line-commutated) supplying a DC load, if the firing angle is less than 60°, how does the instantaneous DC output voltage behave?

Introduction / Context:The instantaneous output of a 3-phase full converter is formed from segments of line-to-line voltages selected by controlled thyristors. The firing angle α sets when each device conducts and thus determines polarity and average value of the output voltage.

Given Data / Assumptions:

• Line-commutated 3-phase full converter.
• Firing angle α < 60°.
• Typical RL load with sufficiently continuous current.

Concept / Approach:

For α in the rectification range (0° to < 90°), the converter selects the most positive line-to-line voltages at appropriate intervals, producing a unidirectional (positive) instantaneous DC voltage. Negative instantaneous segments arise as α increases well beyond 60–90° or under inverter operation (α > 90°).

Step-by-Step Solution:

Verification / Alternative check:

Waveforms from standard texts show unipolar positive DC for α below about 60°–90°, with average Vdc proportional to cos α and still positive for α < 90°.

Why Other Options Are Wrong:

• “Both positive and negative parts” occurs near inversion (α large) or with discontinuous current.
• “May have both” is too vague for α < 60° under stated assumptions.
• “Negative part if load is inductive” is incorrect; inductance sustains current but does not flip instantaneous polarity at small α.

Common Pitfalls:

Confusing average negative voltage (α > 90°) with instantaneous polarity at small firing angles.

Final Answer:

will have positive part only