Classic industrial cycloconverters (for large low-speed drives) typically rely on which commutation principle during normal operation?

Introduction / Context:
Cycloconverters directly synthesize low-frequency AC from a higher-frequency AC source. Thyristor bridges are gated such that the supply itself turns devices off at current zero (line commutation).

Given Data / Assumptions:

• Traditional thyristor-based cycloconverter system.
• Applications: large drives requiring low-frequency, high-torque.
• No intermediate DC link.

Concept / Approach:
Line (natural) commutation uses the AC source voltage to reverse-bias and turn off the conducting thyristors as current naturally goes through zero each half cycle. This is different from forced commutation, which uses external L–C or active circuits.

Step-by-Step Solution:
1) Identify device family: thyristors without self-turn-off.2) Recognize that in cycloconverters, source voltage crosses zero, enabling natural turn-off.3) Therefore, commutation is predominantly natural.
Verification / Alternative check:
Reference diagrams show anti-parallel controlled rectifier groups relying on supply zero-crossings for current reversal and device turn-off.

Why Other Options Are Wrong:

• Forced commutation: used in choppers/inverters, not typical in classic cycloconverters.
• Either/both simultaneously: not standard architecture.
• Resonant-only: incorrect for mainstream cycloconverter designs.

Common Pitfalls:

• Confusing cycloconverters with force-commutated PWM inverters.
• Assuming all thyristor systems need commutation networks—line commutation suffices here.

Final Answer:
Natural (line) commutation (Option A).