A voltage commutated chopper has the following parameters: Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF. Find the turn-off time for the main SCR with a constant load current of 80 A and Ton = 800 μsecs.

To find the turn-off time for the main SCR in a voltage commutated chopper, we can use the formula for turn-off time ((t_{off})):

[

t_{off} = frac{L cdot I}{V_{C}} + frac{C cdot V_{C}}{I}

]

Where:

– (L) is the inductance in henries (H), so convert (20 mu H = 20 times 10^{-6} H)

– (C) is the capacitance in farads (F), so (C = 50 mu F = 50 times 10^{-6} F)

– (I) is the current in amperes (A), here it’s (80 A)

– (V_{C}) is the voltage across the capacitor; assuming ideal conditions, this would equal the supply voltage (V_s) in this situation.

Let’s substitute the numerical values:

1. Calculate (V_{C}):

– Let’s assume (V_C = V_s = 220 V).

2. Using the formula:

– (t_{off} = frac{20 times 10^{-6} cdot 80}{220} + frac{50 times 10^{-6} cdot 220}{80})

Now calculating each term:

– First term: