Power Electronics problems
- 1. The terminals of a power MOSFET are called
Options- A. emitter, base, collector
- B. source, gate, drain
- C. source, base, drain
- D. emitter, gate, drain Discuss
Correct Answer: source, gate, drain
Explanation:
In a MOSFET terminals are source, gate, drain irrespective of rating.
- 2. In a 3 phase fully controlled converter the firing frequency is
Options- A. 3 times the line frequency
- B. 6 times the line frequency
- C. 9 times the line frequency
- D. 12 times the line frequency Discuss
Correct Answer: 6 times the line frequency
Explanation:
It is a 6 pulse converter.
- 3. In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
Options- A. equal
- B. different
- C. equal or different
- D. different but sometimes equal Discuss
Correct Answer: equal
Explanation:
Firing angles are kept equal to get symmetrical output voltage.
- 4. The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)
Options- A. limits charging current
- B. limits discharging current
- C. limits both charging and discharging currents
- D. none of the above Discuss
Correct Answer: limits discharging current
Explanation:
Discharge current flows through Re .
- 5. A dc separately excited motor has constant field current. The armature is fed from a single phase supply through a full converter. When a = 0, speed is 500 rpm. If a = 45°, the speed is likely to be
Options- A. about 350 rpm
- B. about 250 rpm
- C. about 175 rpm
- D. about 125 rpm Discuss
Correct Answer: about 125 rpm
Explanation:
Average output voltage ? cos a and therefore reduced to 0.707 of the value for a = 0. Speed is nearly proportional to voltage.
- 6. Two thyristor of same rating and same specifications
Options- A. will have equal turn on and turn off periods
- B. will have equal turn on but unequal turn off periods
- C. may have equal or unequal turn on and turn off periods
- D. will have unequal turn on and turn off periods Discuss
Correct Answer: may have equal or unequal turn on and turn off periods
Explanation:
Turn on and turn off times of similar SCRs may not be same.
- 7. In single phase half wave regulator, the average current over one full cycle
Options- A. is always positive
- B. may be positive or negative
- C. may be negative
- D. is always negative Discuss
Correct Answer: is always negative
Explanation:
The input current during positive half cycle is less than that during negative half cycle.
- 8. Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
They are suitable for both dc and ac circuit breakers.
- 9. If duty cycle is a and the load is a resistance R, the rms value of output voltage is
Options- A. aV
- B. a V
- C. a2V
- D.
Discuss
Correct Answer: a V
Explanation:
RMS output voltage
VL = a V.
- 10. The amount of dielectric heating is inversely proportional to frequency.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
It is proportional to frequency.
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