# Power Electronics Questions and Answers – Chopper Control Strategies

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Chopper Control Strategies”.

1. In case of TRC (Time Ratio Control), _________ is varied
a) duty cycle
b) firing angle
c) supply frequency
d) supply voltage magnitude

Explanation: In case of TRC, α (Duty cycle) is varied to change the average value of output voltage.

2. In constant frequency TRC or pulse width modulation scheme, ________ is varied.
a) Vs
b) Ton
c) T
d) f

Explanation: In pulse width modulation scheme also known as constant frequency time ratio control, the on-time Ton is varied keeping total time T constant. Toff also varies automatically with Ton to keep Ton constant.

3. In case of variable frequency system __________ is varied
a) T
b) Ton
c) Toff
d) supply frequency

Explanation: In frequency modulation scheme, the frequency (f) hence T (1/f) is varied by either keeping Ton constant or keeping Toff constant.

4. In pulse width modulation scheme, _________ is kept constant.
a) Vs
b) Ton
c) T
d) Toff

Explanation: In pulse width modulation scheme also known as constant frequency time ratio control, the on-time Ton is varied keeping total time T constant. Toff also varies automatically with Ton to keep Ton constant.

5. In case of a constant frequency system, Ton = (1/4)T. If the chopping frequency 2 kHz, find the value of Toff.
a) (1/8) ms
b) (3/8) ms
c) (1/8) μs
d) (3/8) μs

Explanation: Ton = (1/4) T
T = 1/2kHz = 0.5 ms
Ton = T/4
Therefore, Toff = T – Ton = T – T/4 = 3T/4 = 3/8 msec.

6. In case of frequency modulation system, ________ is kept constant.
a) T
b) Ton
c) Toff
d) Either Ton or Toff

Explanation: In frequency modulation scheme, the frequency (f) hence T (1/f) is varied by either keeping Ton constant or keeping Toff constant.

7. The control strategy in which on and off time is guided by the pervious set of values of a certain parameter is called as
a) time ratio control
b) pulse width modulation
c) current limit control
d) constant frequency scheme

Explanation: In current ratio control, the on and off time is guided by the pervious set of value of load current.

8. In the current limit control method, the chopper is switched off when
a) load current reaches the lower limit
b) load current reaches the upper limit
c) load current falls to zero
d) none of the mentioned

Explanation: In CLC, when the current reaches the upper limit the chopper is switched off and current starts to decay. When current reaches a predefined lower limit, the chopper is again switched on and current starts to rise, the process is thus again repeated.

9. Which of the following mentioned control strategy/strategies would require a feedback loop?
a) pwm
b) constant frequency system
c) current limit control
d) none of the mentioned

Explanation: CLC would require a feedback loop as it is required to measure the values of current constantly before switching on or off the chopper.

10. In the current limit control method, when the load current reaches a predefined lower value, then
a) the chopper is switched off
b) the chopper is switched on
c) the source voltage is removed
d) load voltage goes to zero

Explanation: In CLC, when the current reaches the upper limit the chopper is switched off and current starts to decay. When current reaches a predefined lower limit, the chopper is again switched on and current starts to rise, the process is thus again repeated.

Sanfoundry Global Education & Learning Series – Power Electronics.

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