GATE 2020 EE – Question 61
A single-phase, full-bridge, fully controlled thyristor rectifier feeds a load comprising a $10\ \Omega$ resistance in series with a very large inductance. The rectifier is fed from an ideal 230 V, 50 Hz sinusoidal source through cables which have negligible internal resistance and a total inductance of 2.28 mH. If the thyristors are triggered at an angle $\alpha=45^\circ$, the commutation overlap angle in degree (rounded off to 2 decimal places) is ________.
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Correct answer: 4.51 to 5.10
Explanation
With source reactance $X_s=2\pi\times50\times2.28\text{ mH}=0.716\ \Omega$, the output is $V_d=\frac{2\sqrt2V_s}{\pi}\cos\alpha-\frac{2X_s}{\pi}I_d$ with $I_d=\frac{V_d}{10}$, which gives $V_d=140$ V and $I_d=14$ A. The overlap satisfies $\cos\alpha-\cos(\alpha+\mu)=\frac{2X_sI_d}{\sqrt2V_s}=0.0616$, so $\cos(\alpha+\mu)=0.6455$, $\alpha+\mu=49.8^\circ$ and $\mu\approx4.8^\circ$.