Discussion
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- Question
A single phase electric heater uses phase controlled thyristors. The firing angle is 90°. The degree of heating is
Options- A. 25% of maximum
- B. 50% of maximum
- C. 75% of maximum
- D. none of the above
- Correct Answer
- 50% of maximum
ExplanationConduction occurs only during 50% of wave.
Power Electronics problems
Search Results
- 1. Present day HVDC converters are all
Options- A. 3 pulse converters
- B. 6 pulse converters
- C. 12 pulse converters
- D. either 6 pulse or 12 converters Discuss
Correct Answer: 12 pulse converters
Explanation:
To reduce ripple 12 pulse converters are used.- 2. Assertion (A): A cycloconverter may be line commutated or forced commutated
Reason (R): A step up cycloconverter requires forced commutation.
Options- A. Both A and R are correct and R is correct explanation of A
- B. Both A and R correct but R is not correct explanation of A
- C. A is correct but R is wrong
- D. A is wrong but R is correct Discuss
Correct Answer: Both A and R are correct and R is correct explanation of A
- 3. A 3 phase induction motor is fed by a 3 phase ac regulator to change the stator voltage. The variation in speed will be more if
Options- A. load torque is constant
- B. load torque increases with increase in speed
- C. load torque decreases with increase in speed
- D. either (a) or (b) Discuss
Correct Answer: load torque increases with increase in speed
Explanation:
If load torque is constant, the change in speed is very small.- 4. A cycloconverter cannot give a frequency higher than input frequency.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
Step up cycloconverter can give a frequency higher than input frequency.- 5. A single phase full wave regulator feeds R-L load. The best gating signal is
Options- A. short duration pulses
- B. long duration pulses
- C. pulse train
- D. either (a) or (b) Discuss
Correct Answer: pulse train
Explanation:
To ensure proper triggering pulse train in necessary because load is R - L.- 6. In a single phase full bridge converter (B - 2 connection), the input voltage is v = Vm sin ?t. The output dc voltage for resistive load
Options- A.
- B.
- C.
- D.
Discuss
Correct Answer:
Explanation:
SCRs conduct from a to p in each half cycle.- 7. On load tap changer using thyristors can have a maximum of two taps.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
Any number of taps are possible.- 8. In controlled heating circuits, thyristors act as
Options- A. rectifier
- B. inverter
- C. ac regulator
- D. chopper Discuss
Correct Answer: ac regulator
Explanation:
The output voltage is controlled ac.- 9. A single phase full converter gives maximum and minimum output voltage at firing angles of
Options- A. 0 and 180° respectively
- B. 180° and 0° respectively
- C. 0 and 90° respectively
- D. 90° and 0° respectively Discuss
Correct Answer: 0 and 180° respectively
Explanation:
Output voltage is maximum if a = 0 and is minimum if a = 180°.- 10. A Shockley diode can be used
Options- A. in place of SCR
- B. as a trigger switch for SCR
- C. in place of UJT
- D. both in place of SCR and as a trigger switch for SCR Discuss
Correct Answer: as a trigger switch for SCR
Explanation:
It is a four layered PNPN device without a gate terminal. Its characteristics is similar to that of SCR with zero gate current. It is acting as a trigger switch for Thyristor.
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