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- Question
In the below figure, the average value of load current is 15 A. The load is highly inductive. The peak value of thyristor current is
Options- A. 15 A
- B. 152 A
- C.
- D. 30 A
- Correct Answer
- 15 A
ExplanationBecause of highly inductive load, output current is nearly constant.
Power Electronics problems
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- 1. When thyristors are connected in series and parallel, it may be necessary to have :
Options- A. current derating
- B. voltage derating
- C. both current and voltage derating
- D. none of the above Discuss
Correct Answer: both current and voltage derating
Explanation:
Thyristors in series do not share the voltage equally and require voltage derating. Thyristors in parallel need current derating.- 2. A high frequency series inverter consists of
Options- A. a number of series inverters in series
- B. a number of series inverters in parallel
- C. a number of parallel inverters in parallel
- D. a number of parallel inverters in series Discuss
Correct Answer: a number of series inverters in parallel
Explanation:
A high frequency series inverter has many series inverters in parallel. Only one inverter operates at one time and they operate in fixed sequence.- 3. An SBS can conduct is one direction only.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
It is a bilateral switch.- 4. The average load current supplied by a thyristor depends on
Options- A. firing angle
- B. firing frequency
- C. magnitude of gate current
- D. all of the above Discuss
Correct Answer: firing angle
Explanation:
The firing angle controls the period of conduction and hence average load current.- 5. A single phase series converter is used for
Options- A. high voltage output
- B. high current output
- C. high voltage and high current output
- D. improving the circuit efficiency Discuss
Correct Answer: high voltage output
Explanation:
Series connection increases the voltage.- 6. In a multiphase chopper, the different choppers operate in
Options- A. series and all simultaneously
- B. parallel and one at a time
- C. parallel and simultaneously
- D. either (b) or (c) Discuss
Correct Answer: parallel and one at a time
Explanation:
Only one chopper operates at one time.- 7. In dielectric heating, the thyristor circuits consist of
Options- A. rectifier-chopper combination
- B. controlled rectifier
- C. ac regulator
- D. rectifier-inverter combination Discuss
Correct Answer: rectifier-inverter combination
Explanation:
Input ac is rectified and then converted to high frequency ac.- 8. In a 3 phase semiconverter, the firing frequency is
Options- A. equal to line frequency
- B. 3 times the line frequency
- C. 6 times the line frequency
- D. 12 times the line frequency Discuss
Correct Answer: 3 times the line frequency
Explanation:
Since the circuit has 3 thyristors, firing frequency is three times the line frequency.- 9. A single phase fully controlled bridge rectifier is fed by a source having inductance. The output voltage during overlap period is
Options- A. Vm
- B. 0.5 Vm
- C. zero
- D. 0.5 Vm - voltage drop across inductance Discuss
Correct Answer: zero
Explanation:
During the period of overlap all the four thyristors conduct and load is effectively short-circuited. Output voltage is zero.- 10. A thyristor has its gate current zero. To start conduction the forward voltage must exceed
Options- A. about 0.5 V
- B. forward breakover voltage
- C. on state voltage
- D. peak working off state forward voltage Discuss
Correct Answer: forward breakover voltage
Explanation:
With zero gate current, conduction can be started only if forward voltage exceeds breakover voltage.
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