Friday, April 10, 2015

Multiple Choice Questions with Answers on Power Electronics and Drives: Part15

Multiple Choice Questions with Answers on Power Electronics and Drives

[Q-1] Silicon based rectifiers are preferred than germanium based rectifiers because
(a) Si is available easily compared to Ge
(b) Only Si has a stable off state
(c) Ge is very temperature sensitive
(d) Si only has the characteristics α1 + α2 < 1 at low collector currents and reaches 1 at high currents
Which of the above statements are true?
Options:
1. A, B, D
2. B, D
3. B Only
4. D Only
5. None of the above options

[Q-2] In a triac,
(a) The triggering pulse to main terminal 1 should be of the same polarity as the anode potential between MT1 and MT2
(b) The triggering pulse should be of opposite polarity to that of anode potential
(c) When the triggering pulse is positive and anode is positive, it is operating in the first quadrant
(d) When the triggering pulse is negative and anode is negative its sensitivity is highest.
Which of the above statements are true?
Options:
1. A, D
2. A, C
3. C, D
4. A, C, D
5. None of the above options


[Q-3] In a single phase full wave SCR circuit with R, L load
(a) Power is delivered to the source for firing angle of less than 90°
(b) The scr changes from inverter to converter at α = 90°
(c) The negative dc voltage is maximum at α = 180°
(d) To turn off the scr, the maximum delay angle must be less than 180°
Which of the above statements are true?
Options:
1. C, D
2. C Only
3. D Only
4. A , B
5. None of the above options


[Q-4] While comparing triac and scr,
(a) Both are unidirectional devices
(b) Triac requires more current for turn on than scr at a particular voltages
(c) A triac has less time for turn off than SCR
(d) Both are available with comparable voltage and current ratings
Which of the above statements are true?
Options:
1. A, C
2. B, C
3. A, B
4. D Only
5. None of the above options


[Q-5] Which of the following statements are true
(a) If the SCR, even with proper gate excitation and anode-cathode voltage does not conduct for a particular load resistance, then it would be necessary to decrease the load resistance to turn ON the SCR
(b) The SCR would be turned OFF by voltage reversal of the applied anode-cathode ac supply of frequencies up to 30kHz
(c) If the gate current of the SCR is increased, then the forward breakdown voltage will decrease
Options
1. A, B, C
2. B, C
3. A , C
4. A, B
5. None of the above options


[Q-6] Which of the following statements are true
When gate triggering is employed, a SCR can withstand higher values of di/dt, if the
1. Gate current is increased
2. Rate of rise of gate current is increased
3. Gate current is increased
4. Rate of rise of gate current is decreased
OPTIONS:
(a) 3, 4
(b) 1, 4
(c) 2, 3
(d) 1, 2
(e) None of the above options


[Q-7] In a SCR based converter, the free wheeling diode is used to
(a) Add to the conduction current of thyristors
(b) Oppose the SCR conduction
(c) Conduct current during the OFF period of the SCR
(d) Protect the SCR by providing a shunt path

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10:52 AM