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