MCQs on Power Electronics Part-2

Power Electronics

1. At reduced voltage and reduced frequency in ac drives: 

1. Current gets reduced
2. pf is improved under maximum torque conditions
3. The maximum torque is reduced 
4. The maximum torque occurs at a higher value of slip

Choose the correct code from the given options:

a. 1,2,3 and 4

b. 2 and 4

c. 1 and 3

d. 2,3 and 4

2. At lower speeds of a motor or at higher firing angle, power factor of the system is______.

a. Unaffected

b. Deteriorated

c. Improved

d. May improve or deteriorate

3. For an ac voltage controller fed induction motor drive negotiating a load whose torque requirement does not vary with speed_____.

a. The fundamental component of current drawn from the supply decreases as speed is reduced

b. The fundamental component of current drawn from supply is independent of speed

c. The fundamental component of current drawn from the supply increases as speed is reduced

d. None of the above

4. Which of the following option is not correct about an ac drive compared to a dc drive?

a. AC drives are less expensive

b. Power converters for AC drives generate harmonic in the supply system and load circuit

c. AC motors are lighter in weight

d. Power converters for AC drives are simple

5. A delta-connected induction motor being fed by a 3-phase ac to dc inverter and operated in constant v/f control mode requires during starting a _____.

a. Auto-transformer starter

b. DOL starter

c. star-delta starter

d. None of the above

6. As SCR circuit supplies power to a dc motor. The armature current of the motor will be ______.

a. Exponential rise and decay

b. Sinusoidal 

c. Constant

d. None of the above

7. In ac motor control, the ratio of voltage-to-frequency is maintained at constant value______.

a. To maximise the current drawn from the supply to provide torque

b. To provide maximum pull-out torque

c. To make maximum use of magnetic circuits

d. To make minimum use of magnetic circuits

8. The phase controlled rectifiers used in speed control of dc motors converts fixed ac supply voltage into______ output voltage.

a. variable dc

b. Variable ac

c. Full rectified dc

d. variable frequency ac

9. The speed of dc motor can be controlled by using.

a. Thyristors

b. Diodes

c. Transistors

d. Either diodes or thyristors

10. In converter control of induction motor's speed control, a d.c. link is use. This is to_____.

a. Trigger the thyristor of the inverter

b. Vary the slip

c. Provide necessary d.c. voltage to inverter

d. Provide a constant peak flux density to the air-gap and core insulation

  

11. In cycloconverters speed control of an induction motor, cycloconverter frequency_____.

a. Is always less and about one-third of supply frequency

b. Can be higher or lower than supply frequency

c. Is higher than the supply frequency

d. Is equal to that of supply frequency

12. A fully controlled thyristor bridge drives a d.c. motor. The system is capable of_____.

a. Only motoring in forward direction, no braking

b. Motoring in forward direction and braking in reverse direction

c. Only motoring in both directions, no braking

d. Motoring and braking in both directions

13. UPS find application in ______.

a. Electric traction

b. Computer power supply

c. Battery powered vehicle

d. HVDC

14. SMPS are superior to linear power supplies in respect of______.

a. Efficiency and regulation

b. Noise and cost

c. Size and efficiency

d. Regulation and noise

15. Consider the following statements :

Switched mode power supplies are preferred over the continuous types, because these are: 

1. Suitable for low power circuits
2. More efficient
3. Suitable for use in both ac and dc
4. Suitable for high-power circuits

of these, the correct statements are:

a. 2 and 3

b. 1 and 2

c. 1 and 3

d. 2 and 4