Question

A voltage commutated chopper as shown in the figure is used to control the speed of the dc motor. Given: $T_{on}=800\mu sec,T=200\mu sec,$ For a constant load current of 80 A, capacitor voltage at 150 ms after $T_A$ is triggered, is

• A. 240 V
• B. 40 V
• C. -20 V
• D. 20 V

Answer 1

The correct option is D 20 V

Capacitor voltage before $T_A$ is triggered is equal to ($-V_s)$. After $T_A$ is triggered, capacitor begins to charge from $(-V_s).$ Therefore, capacitor voltage after $T_A$ is triggered, is given by
$V_C=\frac{I_{0}t}{C}-V_s$
$=\frac{80\times 150\times {10}^{-6}}{50\times {10}^{-6}}-220$
$=20V$