Three-phase AC voltage regulator: How many thyristors are typically used (assume a standard three-phase, bidirectional control of each phase)?

Introduction / Context:
In a three-phase AC regulator, each phase must be controlled for both positive and negative half-cycles. The common implementation places antiparallel SCR pairs per phase. Knowing the device count is basic to topology understanding and cost/complexity estimation.

Given Data / Assumptions:

• Three independent phases feeding a load (or three single-phase loads).
• Bidirectional control per phase requires antiparallel devices.
• No common return path device sharing assumed.

Concept / Approach:

Each phase needs two controlled unidirectional switches in antiparallel so current of either polarity can be phase-controlled. For three phases, total SCRs = 3 phases * 2 SCRs/phase = 6 SCRs.

Step-by-Step Solution:

Verification / Alternative check:

Standard three-phase AC regulator schematics universally show six SCRs (or TRIACs for lower power), confirming the count.

Why Other Options Are Wrong:

• 3 thyristors: insufficient; only one per phase cannot control both polarities.
• 9 or 12: represent other converter topologies or parallel strings, not the basic regulator.

Common Pitfalls:

Confusing with 6-pulse or 12-pulse rectifiers; here we are not rectifying but regulating AC per phase.

Final Answer:

6 thyristors