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- Question
A 3 kV circuit uses SCR of 800 V rating. If derating is 25%, the number of SCRs in series is
Options- A. 4
- B. 5
- C. 6
- D. 8
- Correct Answer
- 5
Explanation
.
Power Electronics problems
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- 1. A single phase half wave rectifier is feeding a resistive load. When a = 0, dc output voltage is 127 V. When a = 30°, the dc output voltage will be about
Options- A. 127 V
- B. 120 V
- C. 110 V
- D. 100 V Discuss
Correct Answer: 120 V
Explanation:
Using Eq
= 0.933. Hence Vdc for 30° firing angle = 0.933 x 127 = ? 120 V.
- 2. An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is
Options- A. 16% maximum
- B. 40% of maximum
- C. 60% of maximum
- D. 84% of maximum Discuss
Correct Answer: 40% of maximum
Explanation:
Vl = V a. Therefore Vl2 - V2 (a). Heating is proportional to a.- 3. The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by
Options- A. forward characteristics of thyristors
- B. reverse recovery characteristics of thyristors
- C. both forward and reverse recovery characteristics of thyristors
- D. none of the above Discuss
Correct Answer: reverse recovery characteristics of thyristors
Explanation:
Value of C is proportional to maximum difference between reverse charge of SCRs in series.- 4. In a multiphase chopper, all choppers operate together.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
They operate in a sequence.- 5. A thyristor has a turn on time of 6 ?s. If the anode circuit is inductive, the turn on time will be
Options- A. 6 ?s
- B. less than 6?s
- C. more than 6 ?s
- D. either 6 ?s or less Discuss
Correct Answer: more than 6 ?s
Explanation:
Inductive load increases the turn on time.- 6. 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.- 7. 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.- 8. 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.- 9. 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 .- 10. 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.
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