The GATE Grind

GATE 2020 EE – Question 22

Power Electronics · Voltage and Current commutated Thyristor based converters · 1 mark · Multiple choice

Thyristor $T_1$ is triggered at an angle $\alpha$ (in degree), and $T_2$ at angle $180^\circ+\alpha$, in each cycle of the sinusoidal input voltage. Assume both thyristors to be ideal. To control the load power over the range 0 to 2 kW, the minimum range of variation in $\alpha$ is:

  1. $0^\circ$ to $60^\circ$
  2. $0^\circ$ to $120^\circ$
  3. $60^\circ$ to $120^\circ$
  4. $60^\circ$ to $180^\circ$

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (D) $60^\circ$ to $180^\circ$

Explanation

The load is $10\angle-60^\circ\ \Omega$, so full conduction gives $P=\frac{200^2}{10}\cos60^\circ=2$ kW, which is reached when $\alpha$ equals the load angle, $60^\circ$. The thyristor controller can no longer conduct for $\alpha=180^\circ$, where the power is zero. So $\alpha$ must vary from $60^\circ$ to $180^\circ$.