250+ TOP MCQs on Choppers-4 and Answers

Power Electronics Quiz on “Choppers-4”.

1. For a step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of average output voltage for a duty cycle of 0.4.

A. 230 V
B. 92 V
C. 90 V
D. 91.2 V
Answer: D
Clarification: The voltage drop across the switch occurs only when it is on.
Vo = (Vs – 2) x Duty cycle = 91.2 V.

2. For the step-down chopper shown in the figure below, dc source voltage = 230 V and the voltage drop across the chopper = 2 V. Find the value of rms output voltage for a duty cycle of 0.4.

A. 91.2 V
B. 230 V
C. 144.2 V
D. 145 V
Answer: C
Clarification: The voltage drop across the switch will occurs only when it is on.
Vo (rms) = √0.4 x (230 – 2) = 144.2 V.

3. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power delivered to the load of R = 10 Ω. Duty cycle = 40 %. Take voltage drop at the switch to be 2 V.

A. 1 kW
B. 2.08 kW
C. 569 W
D. 207 W
Answer: B
Clarification:
Vo (rms) = √0.4 x (230 – 2) = 144.2 V
P = 144.2²/10 = 2079.3 W = 2.08 kW.

4. For the step-down chopper shown in the figure below, dc source voltage = 230 V. Find the power input to the chopper If load of R = 10 Ω is connected. Duty cycle = 40 %.

A. 2300 W
B. 2097 W
C. 2560 W
D. 5290 W
Answer: B
Clarification: Power input to the chopper is Vs . Io
Vs = 230 V
Vo = (Vs – 2) x Duty cycle = 91.2 V
Io = Vo/R = 91.2/10
P_i = Vs.Io = 2097.6 W.

5. The below given chopper circuit is that of a

A. step-up chopper
B. step-down chopper
C. step-up/step-down chopper
D. none of the mentioned
Answer: C
Clarification: It is a step-up/step-down chopper. By controlling the duty cycle it can be made to behave like a step-up or a step down chopper.

6. The expression for a step-up/step-down chopper with α as the duty cycle and Vs as the dc input voltage is
A. Vs/1 – α
B. α x Vs
C. Vs (α/1-α)
D. Vs (α/1+α)
Answer: C
Clarification: A step-up, step-down chopper can behave as a step up chopper for α > 0.5 and step –down chopper for α < 0.5.

7. For a step-up/step-down chopper, if the duty cycle > 0.5 then
A. Vo = Vs
B. Vo < Vs
C. Vo > Vs
D. None of the mentioned
Answer: C
Clarification: Vo = Vs (α/1-α)
For α is greater than 0.5, the chopper behaves as a step-up chopper. Hence, Vo > Vs.

8. A step down chopper has Vs = 230 V and R = 10 Ω. For a duty cycle of 0.4, the power taken by the chopper is 2097 Watts. Find the chopper efficiency. Take the voltage drop across the chopper switch as 2 V.
A. 98 %
B. 89.96 %
C. 99.14 %
D. 96.54 %
Answer: C
Clarification: Vo (rms) = √0.4 x (230 – 2) = 144.2 V
Po = 144.2²/10 = 2079.3 Watts
Pi = 2097 Watts
η = Po/Pi = 99.14 %.

9. A step down chopper has input dc voltage of 220 V and R = 10 Ω in series with L = 65 mH. If the load current varies linearly between 11 A and 17 A, then find the duty cycle α.
A. 1
B. 0.4
C. 0.6
D. 0.7
Answer: D
Clarification: Average load current = 11+17 / 2 = 14A
Average load voltage = IxR = 14×10 = 140V
But, Vo = α x Vs
Therefore, α x 220 = 140, α = 0.7.

10. For a step-up/step-down chopper, if α (duty cycle) = 0.5 then
A. Vo = Vs
B. Vo < Vs
C. Vo > Vs
D. none of the mentioned
Answer: A
Clarification:
Vo = Vs (α/1-α)
For α = 0.5
Vo = Vs x (0.5/0.5).

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