Operating quadrants of a fully controlled bridge converter A line-commutated, fully controlled bridge feeding an inductive DC load (current unidirectional) can operate in which voltage–current quadrants?

Introduction / Context:
Quadrant capability describes the signs of voltage and current at the DC output. A fully controlled bridge (all thyristors) can both rectify and, when firing angle exceeds 90°, invert power back to AC, provided the DC current remains in the same direction due to inductance.

Given Data / Assumptions:

• Unidirectional DC current (due to series inductance and device orientation).
• Firing angle α variable from 0° to > 90°.
• Line-commutated operation (no forced commutation).

Concept / Approach:
With α < 90°, the average DC voltage Vdc is positive, current is positive → Quadrant I (V > 0, I > 0). With α > 90°, the average DC voltage becomes negative while current remains positive (still flowing into the load due to continuity) → power flows from DC side back to AC (inversion), which is Quadrant II (V < 0, I > 0).

Step-by-Step Solution:
Use Vdc ∝ cos α for a full bridge.For 0° ≤ α < 90°, cos α > 0 ⇒ Vdc > 0, I > 0 → Quadrant I.For 90° < α < 180°, cos α < 0 ⇒ Vdc < 0, but current remains positive → Quadrant II.

Verification / Alternative check:
Drive texts on DC motor control show two-quadrant operation (I and II) with a single fully controlled bridge and continuous current.

Why Other Options Are Wrong:

• (c) Quadrant III requires negative current, which this topology does not provide.
• (d) Quadrant IV also needs negative current; again not available with line-commutated bridge and passive series L.
• (a) Understates the inversion capability.

Common Pitfalls:

• Confusing motor torque reversal (which may need current reversal) with electrical inversion (voltage reversal at same current direction).

Final Answer:
1 and 2