POWER ELECTRONICS AND DRIVES PART 20

POWER ELECTRONICS AND DRIVES PART 20

1. If a step up chopper’s switch is always kept off then (ideally)
a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs

Answer: c

2. If a step up chopper’s switch is always kept open then (ideally)
a) Vo = 0
b) Vo = ∞
c) Vo = Vs
d) Vo > Vs
 

Answer: b

3. Find the average value of output voltage of a basic step-down chopper, with duty cycle = α and load = R Ω.
  

a) I = Vs x α
b) I = (Vs x α)/R
c) I = 0
d) I = Vs/R
 

Answer: b

4. For the below given circuit, find the output current at the instant of commutation.
 
a) Vo/R
b) Vs/R
c) 0
d) α/2
 

Answer: b

5. For a step-down chopper, find the rms value of output voltage. Let α be the duty cycle and Vs be the input voltage.
a) α x Vs
b) Vs/α
c) √α x Vs
d) Vs/2
 

Answer: c

6. A step down chopper is operated at 240V at duty cycle of 75%. Find the value of RMS switch (IGBT/MOSFET) current. Take R = 10 Ω.
a) 2.07 A
b) 200 mA
c) 1.58 A
d) 2.4 A
 

Answer: a

7. Find the expression for effective input resistance of a step down chopper. With R load and duty cycle = α.
a) R x α
b) R/2
c) 0
d) R/α
 

Answer: d

8. A step-up chopper has input voltage of 220 V and output voltage of 660 V. If the conducting time of the IGBT based chopper is 100 μs, compute Toff width of the output voltage pulse.
a) 100 μs
b) 150 μs
c) 50 μs
d) Insufficient data
 

Answer: c

9. For a step-up chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned
 

Answer: a

10. For a step-down chopper, when the duty cycle is increased the average value of the output voltage
a) increases
b) decreases
c) remains the same
d) none of the mentioned
 

Answer: a