Power Semiconductor Devices and Their Applications MCQs

Question 1

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

Accepted Answer

1 and 3

Answer

2 and 3

Answer

1,2 and 3

Question 2

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

Accepted Answer

10A

Answer

average value of current will become greater than 10 A

Answer

average value of current will become less than WA

Answer

trend of variation of current cannot be predicted unless the exact value of the inductance connected is known.

Question 3

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

Accepted Answer

produce conduction in both No. 1 and No. 2 in parallel for an overlapping period through a transient.

Answer

prolong the conduction in No. 1 and delay the turn-on of No. 2 correspondingly

Answer

stop the conduction in No. 1 at the scheduled time, but delay the turn-on of No. 2

Answer

double the voltage output through a commutation transient.

Question 4

When a thyristor is negatively biased

Accepted Answer

outer junctions are negatively biased and the inner junction is positively biased

Answer

all the three junctions are negatively biased

Answer

outer junctiona are positively biased and the inner junction is negatively biased

Answer

the junction near the anode is negatively biased and the one near the cathode is positively biased

Question 5

For an SCR, di/dt protection is achieved through the use of

Accepted Answer

L in series with SCR

Answer

R in series with SCR

Answer

RL in series with SCR

Answer

RLC in series with SCR

Question 6

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

Accepted Answer

remains constant

Answer

decreases, reaches a minimum at 50% duty ratio and then increases

Answer

increases, reaches a maximum at 50% duty ratio and then decreases

Answer

keeps on increasing as the duty ratio is increased

Question 7

A zener diode works on the principle of

Accepted Answer

tunnelling of charge carriers across the junction

Answer

thermionic emission

Answer

diffusion of charge carriers across the junction

Answer

hoping of charge carriers across the junction

Question 8

When a line commutated converter operates in the inverter mode

Accepted Answer

it delivers both real and reactive power to the A.C. supply

Answer

it draws both real andreactive power from the A.C. supply

Answer

it draws reactive power from the A.C. supply

Question 9

The speed and torque of induction motors can be varied by which of the following means : 1. Stator voltage control 2. Rotor voltage control 3. Frequency control

Accepted Answer

1,2 and 3

Answer

1 only

Answer

2 only

Answer

1 and 2 only