Wednesday, March 25, 2015

Power Electronics Objective Type Questions with Answers: Part-14

Power Electronics Objective Questions and Answers

[Q-1] When an inductance is inserted in the load circuit of SCR
(a) The turn on time of SCR is increased
(b) Output voltage is reduced for the same firing angle
(c) Conduction continues even after reversal of phase of input voltage
(d) A free wheeling diode is connected in such circuits
Which of the above statements are true?
(1) a, d
(2) b, c, d
(3) a, b, c,d
(4) c, d
Answer: 3

[Q-2] Snubber circuit is used to limit the rate of
(a) Rise of current
(b) Conduction period
(c) Rise of voltage across SCR
(d) None of the above
Answer:C

[Q-3] The provision of a free wheeling diode across an inductive load is
(a) To restore conduction angle on phase
(b) To avoid negative reversal voltage drop
(c) To reduce the PRV
(d) None of the above
Answer:A

[Q-4] While working in series operation, equalising circuits are added across each SCR to provide uniform
(a) Current distribution
(b) Firing of SCRs
(c) Voltage distribution
(d) None of the above
Answer:C

[Q-5] When the SCR conducts, the forward voltage drop
(a) Is 0.7V
(b) Is 1 to 1.5V
(c) Increases slightly with load current
(d) Remains constant with load current
Which of the above statements are true?
(1) a only
(2) b, c
(3) d only
(4) a, c
Answer: 2

[Q-6] The latching current of a SCR is 18mA. Its holding current will be
(a) 6mA
(b) 18mA
(c) 54mA
(d) 12mA
Answer:A
Exp: Latching Current = 3 x Holding Current

[Q-7] The turn off time is longer than turn on time because
(a) The anode and cathode junctions get reverse biased while gate junction is still forward biased
(b) There is flow of reverse current
(c) The gate pulse has been removed
(d) The forward break over voltage is high.
Which of the above statements are true?
(1) b, c
(2) c, d
(3) a, b
(4) a, b, c, d
Answer: 3

[Q-8] The thyristor will turn on faster with
(a) Pulse signal applied to the gate terminal of the SCR
(b) Continuous signal applied to the gate terminal of the SCR
(c) Both are same
(d) Pulse signal but with minimum duration
Which of the above statements are true?
(1) a only
(2) b only
(3) a, d
(4) none of the above
Answer: 3

[Q-9] In an SCR,
(a) The holding current is less than latching current
(b) The holding current is greater than latching current
(c) The two currents are equal
(d) The latching current is about 3 times the holding current
Which of the above statements are true?
(1) a only
(2) d only
(3) a, c, d
(4) a, d
(5) b only
Answer: 4

[Q-10] When a positive voltage is applied to the gate of a reverse biased SCR
(a) It injects more electrons into junction J1
(b) It increases reverse leakage current into anode
(c) Heating of junction is unaffected
(d) Failure of junctions occurs due to thermal runaway
Which of the above statements are true?
(1) b only
(2) a, b ,d
(3) b, d
(4) d only
Answer: 3

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