# Power Electronics Questions and Answers – Choppers-4

This set of Power Electronics Quiz focuses 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

Explanation: 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

Explanation: 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

Explanation:
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

Explanation: 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
Pi = 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

Explanation: 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)

Explanation: 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

Explanation: 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 %

Explanation: 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

Explanation: 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

Explanation:
Vo = Vs (α/1-α)
For α = 0.5
Vo = Vs x (0.5/0.5).

Sanfoundry Global Education & Learning Series – Power Electronics.

To practice all areas of Power Electronics for quizzes, here is complete set of 1000+ Multiple Choice Questions and Answers. 