A 200 V RMS single-phase AC supply feeds a resistive load through a single-phase full-wave rectifier using thyristors, with phase control. Which of the following is closest to the average output voltage at a firing angle of 60°?

Answer: C. About 135 V

With a resistive load the current falls to zero when the supply voltage reaches zero and the thyristor turns off, so the output in each half cycle is the sine wave from α to π. The average is Vd = (2√2/π) × V × (1 + cos α)/2 = 0.90 × 200 × (1 + 0.5)/2 ≈ 135 V. At α = 0 it equals that of diode full-wave rectification, about 180 V. With a strongly inductive load whose current never stops, the average becomes 0.90 × V × cos α ≈ 90 V, so note that the formula depends on the load.

• A. This is the half-wave phase-control value (√2/π) × V × (1 + cos α)/2 ≈ 68 V.
• B. This is 0.90 × V × cos α, which holds for an inductive load with continuous current. It does not apply to a resistive load.
• C. Correct. 0.90 × 200 × 1.5 ÷ 2 ≈ 135 V.