# Power Electronics Questions and Answers – Thyristor Chopper Circuits-2

This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Thyristor Chopper Circuits-2”.

1. For the below given voltage commutation circuit, find the peak value of voltage across the freewheeling diode FD. a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

Explanation: The load current then flows through Vs-C-TA-Load. Hence, Vo = Vs + (capacitor voltage). As the capacitor is already charged to a voltage Vs. Vo = Voltage across FD = 2Vs.

2. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. The SCR turn-off time is 20 μs. Calculate the value of capacitor (C) required for the commutating circuit.
a) 400 μF
b) 40 nF
c) 40 μF
d) 0.4 mF

Explanation: Capacitor current = C dv/dt
For constant load current Io, the above expression can be
Io = C x (Vs/tc) or C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 Farads.

3. Identify the below given circuit. a) current commutated chopper
b) voltage commutated chopper

Explanation: Diode in anti-parallel with the main SCR. Hence (a).

4. A voltage commutated chopper feeds power to an electric car (battery-powered). The battery voltage is 60 V. Starting current is 60 A and the SCR turn-off time is 20 μs. Calculate the value of inductance (L) required for the commutating circuit.
a) 400 μH
b) 40 nH
c) 40 μH
d) 0.4 mH

Explanation: L = (Vs/Io)2 x C
C = (tc x Io)/Vs
C = ( 40 x 60 x 10-6)/60 = 40 x 10-6 F
L = (60/60)2 x 40 x 10-6 = 40 uH.

5. In the below given circuit, at the instant T1 the main SCR is triggered, the voltage across the load terminal is a) Vs
b) 0
c) 2Vs
d) Vs + Ldi/dt

Explanation: When the main SCR is gated, the commutating circuit remains inactive. Hence the load is directly connected to Vs through the SCR. Considering SCR as an ideal switch Vo = Vs.
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6. In current commutated copper circuit
a) the capacitor cannot charge on no load
b) the auxiliary thyristor is turned on automatically
c) the auxiliary thyristor is naturally commutated
d) the commutation is not reliable

Explanation: It is naturally commutated as its commutating current passes through zero value in the ringing circuit formed by L and C.

7. A current commutated chopper is fed from a source of 230 V. Its commutating components are L = 20 μH and C = 50 μF. Find the value of peak commutating current.
a) 233 A
b) 59 A
c) 363 A
d) 198 A

Explanation: I = V √(C/L) = 230 x √(50/20) = 363.66 A.

8. A voltage commutated chopper has the following parameters:
Vs = 220 V. Commutation circuit: L = 20 μH and C = 50 μF.
Find the effective on period for a constant load current of 80 A and Ton = 800 μsecs.
a) 1.075 μs
b) 10.75 ms
c) 1075 μs
d) 1.075 sec

Explanation: The effective on period Ton is given by
Ton + 2Vs.C/Io.

9. 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.
a) 137.5 μs
b) 1075 μs
c) 49.67 μs
d) 200 μs

Explanation: C = (tc x Io)/Vs
tc = C.Vs/I = 137.5 μsecs.

10. The load commutated chopper circuit consists of
a) two thyristors and one commutating capacitor
b) four thyristors and one commutating capacitor
c) two thyristors and two commutating capacitors
d) four thyristors and two commutating capacitors

Explanation: It consists of 4 SCRs and one capacitor. 4 SCRs forming 2 pairs of 2 SCRs conduct alternately.

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