3-phase full-wave AC regulator with star-connected resistive load A three-phase full-wave AC voltage regulator feeds a star-connected purely resistive load. If the firing angle is 75°, how many thyristors conduct simultaneously (at a given instant)?

Introduction / Context:
Three-phase full-wave AC regulators control the RMS line-to-load voltage by phase-angle firing. The instantaneous number of conducting devices affects input current shape, harmonic content, and device current stress.

Given Data / Assumptions:

• 3-phase, star-connected resistive load (no neutral).
• Full-wave regulator using anti-parallel SCR pairs per phase (six SCRs total).
• Firing angle α = 75°, steady-state operation.

Concept / Approach:

For a resistive 3-phase load without neutral, conduction occurs in phases where the fired SCRs connect the appropriate line potentials to maintain KCL at the isolated star point. For small α (typically α ≤ 60°), three devices may conduct. When α exceeds 60°, conduction intervals shrink and the star point potential adjusts such that only two SCRs conduct at a time over most instants.

Step-by-Step Solution:

Verification / Alternative check:

Phase-plane and star-point potential analyses for α > 60° show two-device conduction is dominant for resistive star loads.

Why Other Options Are Wrong:

1 device cannot satisfy KCL without neutral; 3 devices are typical only for α ≤ 60°; 4 devices do not occur in this configuration with a star-connected resistive load.

Common Pitfalls:

Assuming three-device conduction independent of α, or confusing delta vs. star load behavior.

Final Answer:

2