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- Question
A dc chopper V = 500 V feeds an electric fraction dc series motor. The chopping frequency is 400 Hz. The series inductance required to limit the maximum current swing to 10 A is
Options- A. 20 mH
- B. about 31 mH
- C. about 51 mH
- D. about 92 mH
- Correct Answer
- about 31 mH
Explanation
.
Power Electronics problems
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- 1. In a single phase dual converter, a1, and a2 are firing angles of the two converters. Then
Options- A. a1 - a2 =
- B. a1 + a2 = p
- C. a1 - a2 = p
- D. a1 + a2 =
Discuss
Correct Answer: a1 + a2 = p
Explanation:
Since one converter is rectifying and the other is invertingVdc1 = -Vdc2
or a1 + a2 = p.
- 2. In a single phase full wave ac regulator, the firing angles in the two half cycles
Options- A. are always equal
- B. are sometimes equal
- C. are never equal
- D. may be equal or unequal Discuss
Correct Answer: are always equal
- 3. In an SCR, the anode current consists of
Options- A. only holes
- B. only electrons
- C. both electrons and holes
- D. either electrons or holes Discuss
Correct Answer: both electrons and holes
Explanation:
Both electrons and holes contribute to current.- 4. The waveshape of output voltage of half bridge inverter is
Options- A. sinusoidal
- B. square
- C. triangular
- D. either (a) or (b) Discuss
Correct Answer: square
- 5. In a dual converter both converters work as rectifiers.
Options- A. True
- B. False Discuss
Correct Answer: False
Explanation:
One operators as rectifier and the other as inverter.- 6. Between which points does UJT exhibit negative resistance characteristics
Options- A. between peak and valley points
- B. before peak point
- C. after peak point
- D. after valley point Discuss
Correct Answer: between peak and valley points
- 7. The gate current required to turn on an SCR is
Options- A. few amperes
- B. a few mA
- C. almost equal to anode current
- D. about 50% of anode current Discuss
Correct Answer: a few mA
Explanation:
Only a very small gate current is required to trigger SCR into conduction.- 8. In a semi converter feeding a resistive load, the firing angle is a. Each thyristor conducts for
Options- A. p radians
- B. (p - a) radians
- C. (p + a) radians
- D. (p - 0.5 a) radians Discuss
Correct Answer: (p - a) radians
Explanation:
The conduction is from a to p.- 9. An SCR has a turn on time of 4 ?s. The gate pulse should have a duration of about
Options- A. 2 ?s
- B. 5 ?s
- C. 20 ?s
- D. 50 ?s Discuss
Correct Answer: 5 ?s
Explanation:
Duration of gate pulse should be slightly more than turn time of SCR.- 10. A 3 phase semiconverter when supplying a certain load has a rectification efficiency of 0.6 and transformer utilisation factor of 0.35. When a 3 phase full converter is supplying the same load
Options- A. rectification efficiency will be more than 0.6 and TUF will be more than 0.35
- B. rectification efficiency will be more than 0.6 but TUF will be less than 0.35
- C. rectification efficiency will be less than 0.6 and TUF will be less than 0.35
- D. rectification efficiency will be less than 0.6 but TUF will be more than 0.35 Discuss
Correct Answer: rectification efficiency will be more than 0.6 and TUF will be more than 0.35
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