Assertion–Reason on converter quadrants and cost Assertion (A): A fully controlled bridge converter can operate in the first and fourth quadrants. Reason (R): A semi-converter is cheaper than a full converter.

Introduction / Context:
Quadrant operation describes capability for power flow direction and voltage/current polarities. Fully controlled bridges (using four thyristors) can perform rectification and inversion, enabling operation in multiple quadrants. Semi-converters (mixed diodes/thyristors) are cheaper but have reduced controllability.

Given Data / Assumptions:

• Fully controlled bridge with appropriate firing control and load that can return power (e.g., DC motor).
• Sufficient source conditions for inversion (back emf > DC link, proper firing angles).
• Semi-converter uses two SCRs and two diodes, reducing component cost.

Concept / Approach:

A fully controlled bridge can deliver power to the load (Quadrant I) and, when firing angle exceeds 90° with suitable load back-emf, it can operate in inversion to return power (Quadrant IV with positive voltage but negative current direction relative to the source). The reason statement concerns cost, which is true but does not explain why the fully controlled bridge achieves two-quadrant operation (that is due to controllability of all four devices).

Step-by-Step Solution:

Verification / Alternative check:

Drive literature shows inversion mode with α > 90° for fully controlled bridges; semi-converters cannot fully reverse power flow due to diodes blocking inversion paths.

Why Other Options Are Wrong:

Claiming R explains A is incorrect; saying A is wrong contradicts standard converter behavior.

Common Pitfalls:

Equating cost with capability; assuming semi-converters can invert power without additional hardware.

Final Answer:

Both A and R are correct but R is not correct explanation of A