Question

In the class D commutation circuit shown below, $V_s=230V,L=20\mu HandC=40\mu F$. For a constant load current of 120 A, circuit turn-off times of main and auxiliary thyristors are respectively

• A. $67.76\mu sand43.44\mu s$
• B. $43.44\mu sand67.76\mu s$
• C. $44.43\mu sand76.67\mu s$
• D. $76.67\mu sand44.43\mu s$

Answer 1

The correct option is D $76.67\mu sand44.43\mu s$


Circuit turn-off time for main thyristor is
$t_c=\frac{C{V}_s}{I_0}=40\times {10}^{-6}\left(\frac{230}{120}\right)=76.67\mu s$

${\omega }_0=\frac{1}{\sqrt{LC}}=\frac{{10}^6}{\sqrt{20\times 40}}=\frac{{10}^6}{\sqrt{800}}$

Circuit turn-off time for auxiliary thyristor is
$t_{c1}=\frac{\pi }{2{\omega }_0}=\frac{\pi \sqrt{800}}{2\times {10}^6}=44.43\mu s$