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Electronics Engineering Objective Questions { Power Semiconductor Devices and Their Applications }

211. Consider the following statements regarding speed control of induction motors by means of external rotor resistors : 1. Reduction in speed is accompanied by reduced efficiency. 2With a large a resistance circuit, the speed would vary considerably with variation in torque. 3. The method is very complicated. The Disadvantage of such a method of speed control would include
A. 1 and 3
B. 2 and 3
C. 1 and 3
D. 1,2 and 3
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212. A 3- pulse converter feeds a pur resistive load at a firing angle of a = 600. teh average value of current flowing in the load is 10 A. If a very large inductance is connected in the load circuit, then the average value of current will remain as
B.average value of current will become greater than 10 A
C.average value of current will become less than WA
D. trend of variation of current cannot be predicted unless the exact value of the inductance connected is known.
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213. In a 3-phase rectifier circuit, thyristor number 1, 2 and 3 are connected respectively to R, Y and B phases of the star-connected transformer secondary, When the current is being commutated from thyristor No. 1 to No. 2, the effect of the transformer leakage and the ac system inductance will be such that it will
A. prolong the conduction in No. 1 and delay the turn-on of No. 2 correspondingly
B.stop the conduction in No. 1 at the scheduled time, but delay the turn-on of No. 2
C.produce conduction in both No. 1 and No. 2 in parallel for an overlapping period through a transient.
D.double the voltage output through a commutation transient.
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214.  A single-phase voltage source square-wave inverter feeds pure inductive load. The waveform of the load current will be
A. sinusoidal
B. rectangular
C. trapezoidal
D. triangular
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215. When a thyristor is negatively biased
A. all the three junctions are negatively biased
B. outer junctiona are positively biased and the inner junction is negatively biased
C. outer junctions are negatively biased and the inner junction is positively biased
D. the junction near the anode is negatively biased and the one near the cathode is positively biased
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216. For an SCR, di/dt protection is achieved through the use of
A. R in series with SCR
B. L in series with SCR
C. RL in series with SCR
D. RLC in series with SCR
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217. A chopper operating at a fixed frequency is feeding an R-L load. As the duty ratio of the chopper is increased from 25% to 75%, the ripple in the load current
A. remains constant
B. decreases, reaches a minimum at 50% duty ratio and then increases
C. increases, reaches a maximum at 50% duty ratio and then decreases
D. keeps on increasing as the duty ratio is increased
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