Natural (line) commutation applicability Natural commutation of thyristors can be used effectively in:

Introduction / Context:
Commutation refers to turning a thyristor off after it has latched on. Natural (line) commutation exploits the inherent current zero crossings in AC systems, allowing the device to recover its blocking ability without auxiliary circuits.

Given Data / Assumptions:

• Thyristors are line-commutated devices in rectifiers and AC controllers.
• DC circuits do not have natural current zeros unless artificially created.

Concept / Approach:
In AC circuits, the sinusoidal source forces current through zero every half-cycle. If the thyristor current falls to zero and a reverse voltage is applied for the recovery interval, the SCR regains blocking capability automatically. In DC circuits, current does not reverse or go to zero by itself; forced commutation (capacitors, auxiliary switches) or natural load current interruption is required.

Step-by-Step Solution:
Identify requirement: current must go to zero and a reverse voltage must be applied for the turn-off time.AC satisfies this naturally each half-cycle → natural commutation works.DC lacks natural zeros → cannot rely on line commutation.

Verification / Alternative check:
Rectifier bridges and AC regulators using SCRs are line-commutated; DC choppers/inverters using SCRs require forced commutation schemes.

Why Other Options Are Wrong:

• DC only: incorrect because no inherent zero-crossing exists.
• Both AC and DC: only true if DC has additional forced commutation, which is not “natural”.
• None: incorrect since AC clearly allows it.

Common Pitfalls:

• Assuming current ripple in DC equals true zero-crossing; it does not guarantee recovery.

Final Answer:
AC circuits only