Single-phase full-wave AC voltage regulator feeding an R–L load The minimum firing angle α can be set to zero degrees. Is this statement correct?

Introduction / Context:
Single-phase full-wave AC voltage regulators (two antiparallel SCRs) control the RMS voltage applied to a load by delaying turn-on within each half-cycle. For R–L loads, designers often ask whether firing exactly at the zero crossing (α = 0°) is allowed.

Given Data / Assumptions:

• Two SCRs in antiparallel (full-wave regulator).
• Supply is sinusoidal; load is R–L.
• Idealized commutation; finite load inductance may extend current beyond 180°.

Concept / Approach:
The firing angle α is measured from the start of each half-cycle. There is no prohibition against triggering at α = 0°; doing so simply applies the full sinewave at that instant. With inductive loads, current may persist past the natural current zero due to stored energy, but that does not forbid α = 0°—it affects extinction behavior instead.

Step-by-Step Solution:
At α = 0°, gate pulses are applied at each zero crossing.SCRs conduct from near the start of the half-cycle; inductor current lags the voltage.Natural commutation occurs at the subsequent current zero; the next device then takes over.

Verification / Alternative check:
AC controllers routinely demonstrate α ranging from 0° up to a topology-dependent maximum; α = 0° corresponds to the maximum output RMS voltage.

Why Other Options Are Wrong:

• False: contradicts standard operation.
• Conditions like “only with unity power factor” or “only with a free-wheeling diode” are unnecessary in full-wave AC control.

Common Pitfalls:

• Confusing rectifiers (DC output) with AC regulators; free-wheeling diodes are typical in DC converters, not AC regulators.
• Believing inductance forbids α = 0°; it only affects extinction timing.

Final Answer:
True