Chopper Control Strategies.pdf

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

View Answer Answer: b 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 View Answer Answer: d  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 View Answer Answer: c  Explanation: In current ratio control, the on and off  time is guided by the previous 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 View Answer Answer: b  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 View Answer Answer: c  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 View Answer Answer: b  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.