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- Question
A single phase semiconverter is feeding a highly inductive load. The circuit has a freewheeling diode across the load and firing angle is a. During one cycle of input voltage, the total duration of conduction of freewheeling diode is
Options- A. a
- B. 2 a
- C. 4 a
- D. 0.5 a
- Correct Answer
- 2 a
ExplanationFreewheeling diode conducts for a° in each half cycle.
Power Electronics problems
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- 1. A single phase half wave controlled rectifier circuit has an R-L load. A freewheeling diode is also in the circuit. When freewheeling diode is conducting the SCR
Options- A. is forward biased
- B. is reverse biased
- C. may be forward biased or reverse biased
- D. forward biased initially but reverse biased afterwards Discuss
Correct Answer: is reverse biased
Explanation:
Free wheeling diode is forward biased only when SCR is reverse biased.- 2. In an SCR, the anode current is controlled by
Options- A. gate current only
- B. external circuit only
- C. both gate current and external circuit
- D. none of the above Discuss
Correct Answer: external circuit only
Explanation:
Once SCR has been triggered the gate has no control. Only external resistance controls the anode current.- 3. Which of these commutation methods uses an auxiliary SCR?
Options- A. Class A
- B. Class B
- C. Class C
- D. Class D Discuss
Correct Answer: Class C
Explanation:
Class C commutation uses an auxiliary SCR and it is also called auxiliary commutation.- 4. In 180° mode of operation of a 3 phase bridge inverter, two thyristors conduct at one time.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
In 180° mode three thyristors conduct simultaneously.- 5. If ton and toff are the turn on and turn off times of a thyristor, then
Options- A. ton and toff are almost equal
- B. ton >> toff
- C. toff >> ton
- D. ton is about twice toff Discuss
Correct Answer: toff >> ton
Explanation:
Turn on time is less than 3 ?s while turn off time is between 10-300 ?s.- 6. A DIAC has two terminals, anode and cathode.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
They are designated as T1 and T2 because diac can conduct in both directions.- 7. The rated current of a thyristor is 40 A. The holding current will be about :
Options- A. 20 A
- B. 1 A
- C. 100 mA
- D. 10 mA Discuss
Correct Answer: 10 mA
Explanation:
The holding current is much less then 1% of rated current.- 8. Induction heating systems require low frequency inverters.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
It uses high frequency inverter.- 9. A static induction thyristor
Options- A. is a majority carrier device
- B. is a minority carrier device
- C. either a majority carrier or minority carrier device
- D. has a total number of 2 leads Discuss
Correct Answer: is a minority carrier device
Explanation:
It can be turned on by positive gate pulse and turned off by negative gate pulse. It is minority carrier device. It can be manufactured in high current and high voltage ratings and its on state voltage drops is low.- 10. UJT can be used for
Options- A. firing an SCR
- B. turning off an SCR
- C. both firing and turning off an SCR
- D. none of the above Discuss
Correct Answer: firing an SCR
Explanation:
UJT relaxation oscillator is used to provide gate pulses to SCR for triggering.
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