Assertion–Reason on AC regulators Assertion (A): An AC voltage regulator uses natural (line) commutation. Reason (R): An AC regulator can be used for static on-load tap changing of a transformer.

Introduction / Context:
AC regulators (phase-controlled AC controllers) vary RMS voltage by delaying SCR firing on each half-cycle. Their turn-off mechanism uses the natural current zero crossing of the AC source.

Given Data / Assumptions:

• Standard single- or three-phase AC regulator using antiparallel SCRs or TRIACs.
• Transformer tap changing with electronic switching is a valid application.

Concept / Approach:
Natural commutation occurs when the alternating source forces current through zero, removing holding current so the device turns off. This is independent of whether the application is tap changing or general voltage control.

Step-by-Step Solution:
Assess A: True—AC regulators rely on line current zero crossing for turn-off.Assess R: True—AC regulators can implement static on-load tap changing.Causality: The ability to do tap changing does not explain why commutation is natural. Hence, R is not the correct explanation of A.

Verification / Alternative check:
Any phase-controlled load (no DC link) naturally commutates at source zero crossing regardless of the load type.

Why Other Options Are Wrong:

• (a) Incorrect because R is not the reason for A.
• (c) and (d) wrongly negate a correct statement.
• (e) Both are not wrong.

Common Pitfalls:

• Confusing forced commutation (DC converters) with natural commutation (AC regulators).

Final Answer:
Both A and R correct but R is not correct explanation of A