3-phase induction motors. Part – 3

21. As compared to DOL starting, a cage induction motor with star-delta starting shall have

A. more starting torque

B. more starting time

C. reduced starting current

D. smoother acceleration

22. An induction motor has synchronous speed of 1500 rpm. What will be the slip when it is running at a speed of 1450 rpm?

A. +3.33 %

B. -3.33 %

C. +3 %

D. -3 %

23. Which of the following losses are negligible in blocked rotor test?

A. Iron loss

B. Mechanical loss

C. Copper loss

D. both (A) and (B)

24. A soft starter reduces

A. Starting time

B. Starting current

C. Starting power

D. Full load power

25. In an induction motor, if air gap is increased its

A. Power factor will be reduce

B. Space will reduce

C. Efficiency will improve

D. Breakdown torque will reduce

26. A 3-phase induction motor is

A. Essentially a constant speed motor

B. A variable speed motor

C. Very costly

D. Not easily maintainable

27. For higher efficiency of 3-phase induction motor, the slip should be

A. Large

B. Very large

C. As small as possible

D. 1

28. The purpose of the starter is to

A. Limit the starting current

B. Limit the speed

C. Protect against over voltage

D. Produce back EMF

29. A 3-phase induction motor is running at 2% slip. If the input to rotor is 1000W, then mechanical power developed by the motor is

A. 20W

B. 980W

C. 500W

D. 200W

30. A 3-phase induction motor draws a current of 50A from mains when started by direct switching. If an auto transformer with 60% tapping is used starting, the current drawn from the mains will be

A. 50 A

B. 18 A

C. 36 A

D. 83.3 A

3-phase induction motors. Part – 2

11. In induction motor the air gap flux density is usually kept low so as to

A. Improve efficiency

B. Improve power factor

C. Reduce machine cost

D. None of the above

12. The medium size induction motor having maximum efficiency while operating at

A. 75% of full load

B. Half load

C. 25% of full load

D. Any load

13. The frequency of rotor current at standstill is equal to

A. Zero

B. 2f

C. f.

D. Sf.

14. A slip-ring induction motor is recommended for applications requiring

A. High starting torque

B. Variable speed operation

C. Frequent starting, stopping and reversing operations

D. All of the above features

15. The maximum torque in an induction motor depends to

A. Frequency

B. Rotor inductive reactance

C. Square of supply voltage

D. All of the above

16. Rotor of an induction motor runs at

A. Synchronous speed

B. Below synchronous speed

C. Above synchronous speed

D. Any of the above

17. An induction motor is

A. Self starting with zero torque

B. Self starting with high torque

C. Self starting with low torque

D. Non self starting

18. Slip ring induction motors are employed only for

A. Speed control

B. High starting torque

C. Both (A) and (B)

D. None of these

19. A motor having a power factor of 0.8 absorbs in active power of 1200W the reactive power drawn from the supply is ______ VAR.

A. 130

B. 900

C. 1500

D. 600

20. If V/f speed control of 3-phase induction motor, with increase in frequency, the slip at which maximum torque occurs

A. increase

B. decrease

C. either of these

D. none of the above 

3-phase induction motors. Part – 1

1. In induction motor Ns is the synchronous speed and S slip, then actual running speed of an induction motor will be

A. Ns

B. S∙Ns

C. (1-S)Ns

D. (Ns-1)S

2. The torque developed in an induction motor is nearly proportional to

A. 1/V

B. V

C. V²

D. 1/V²

3. In case the air gap in an induction motor is increased

A. The magnetizing current of the rotor is increased

B. The power factor will decrease

C. Speed of motor will increase

D. The winding losses will increase

4. The crawling in an induction motor is caused by

A. High loads

B. Improve design of machine

C. Low voltage supply

D. Harmonics developed in the motor

5. In order to achieve good power factor of an induction motor, the average flux density in air gap should be,

A. small

B. large

C. infinity

D. more than 1.5 Wb/m²

6. What are the expectation from an ideal motor starter?

A. Limitation of starting current

B. Improvement of starting torque

C. Smooth starting of motor

D. All of the above

7. In case of induction motors, the full load slip is generally,

A. 0.3 to 0.5%

B. 20 to 50%

C. 30 to 50%

D. 3 to 5%

8. The slip speed of an induction motor depends upon

A. Armature current

B. Eddy currents

C. Core losses

D. Mechanical load

9. In squirrel cage induction motor, the rotor slots are usually given slight skew in order to

A. Reduce magnetic hum

B. Reduce accumulation of dirt and dust

C. Reduce eddy currents

D. Reduce windage losses

10.  A 3–phase 6-pole 50Hz induction motor is running at 5% slip, what will be the speed of motor?

A. 850 rpm

B. 950 rpm

C. 900 rpm

D. 1000 rpm