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A three-phase, fully controlled thyristor bridge converter is used as a line-commutated inverter to feed 50 kW power at 420 V DC to a three-phase, 415 V (line), 50 Hz AC mains. Consider the DC link current to be constant. The RMS current of each thyristor is

1. 119.05 A
2. 79.37 A
3. 68.73 A
4. 39.68 A

Correct answer: 68.73 A

Solution

The DC current is $I_d=P/V_d=50000/420\approx 119.05$ A. In a three-phase fully controlled bridge with continuous current, each thyristor carries current for 120°; hence its RMS current is $I_d/\sqrt{3}\approx 68.73$ A.

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