Question

Figure shows a chopper. The device $S_1$ is the main switching device. $S_2$ is the auxilliary commutation device. $S_1$ is rated for 400 V, 60 A. $S_2$ is rated for 400 V, 30 A. The load current is 20 A. The main device operates with duty ratio of 0.5. The peak current through $S_1$ is

Answer 1

Initially main thyristor $\left(S_1\right)$ and auxiliary thyristor $\left(S_2\right)$ are off and capacitor is assumed charged to voltage $V_s$ with upper plate positive. When $S_1$ is turned on, source voltage $V_s$ is applied across load and load current $I_0$ begins to flow which is assumed to remain constant. With $S_1$ ON another osciliatory circuit consisting of C,$S_1$, and L is formed where the capacitor current is given by
$i_c=V_s\sqrt{\frac{C}{L}}sin{\omega }_{0}t$
So current through $S_1$,
$I_{s1}=I_0+i_c=I_0+V_s\sqrt{\frac{C}{L}}sin\omega t$
Peak value of
$I_{s1}=I_0+V_s\sqrt{\frac{C}{L}}$
$=20+200\sqrt{\frac{2\times {10}^{-6}}{200\times {10}^{-6}}}=40A$