Instantaneous negative output in a 3-phase full converter For a three-phase fully controlled bridge converter, under what condition can the instantaneous DC output voltage become negative (ignoring overlap), even though average output may be positive?

Introduction / Context:
Three-phase fully controlled converters select the highest instantaneous line-to-line source voltage pairs to appear across the DC terminals. While the average output voltage depends on the firing angle α, the instantaneous output can dip below zero for certain α, a key point when assessing commutation and freewheeling needs.

Given Data / Assumptions:

• Six-pulse three-phase bridge, ideal supply, no overlap.
• Firing angle α referenced to natural commutation instants.
• Examining instantaneous v_o(t), not only the average V_dc.

Concept / Approach:
As α increases, the conduction shifts later in each 60° interval. Beyond α ≈ 60°, the selected line-to-line segment applied to the DC side can become negative for a portion of the cycle, creating negative instantaneous output voltage even when the average remains positive if α is below 90°.

Step-by-Step Solution:
For α ≤ 60°, each 60° segment applied is largely positive with respect to the DC polarity.For α > 60°, some applied line-to-line segments straddle zero crossings, producing intervals where v_o(t) < 0.Thus, the instantaneous output can be negative when α exceeds 60°.

Verification / Alternative check:
Standard converter waveforms show negative notches appearing in v_o(t) when α > 60°, while the average V_dc remains proportional to cos α and stays positive until α reaches 90°.

Why Other Options Are Wrong:
Less than 60°: Instantaneous negative portions are absent in the idealized no-overlap case.
Less than 90° or more than 90°: These statements do not pinpoint the correct onset; negative instantaneous values begin just beyond 60°.

Common Pitfalls:
Confusing average negativity (α > 90°) with instantaneous negativity (α > 60°). These are distinct phenomena.

Final Answer:
firing angle is more than 60°