Design of Three Phase Induction Motor MCQ PDF

21. In case of induction motors the ratio of L/t for minimum cost is taken as

(a) 1.0
(b) 1.2 to 2.0
(c) 0.6 to 2.0
(d) 2.0 to 3.5

22. In case of Induction motors, the ratio of length to pole pitch (L/t) for good efficiency is taken as

(a) 1.0
(b) 1.2 to 2
(c) 1.5
(d) 0.6 to 2

23. In case of induction motors L/t for good power factor is taken as

(a) 1.0 to 1.25
(b) 1.2 to 2.0
(c) 1.5
(d) 2.0 to 3.5

24. For small induction motors, the ratio of length to pole pitch is taken as

(a) 0.25
(b) 0.6
(c) 1.0
(d) 1.5

25. Peripheral speed of rotor of a normal induction motor should be

(a) 10 m/s.
(b) 20 m/s.
(c) 30 m/s.
(d) 80 m/s.

26. For a 3-phase 50 Hz 4 pole induction motor having a rotor diameter of 35 cm, the peripheral speed is

(a) 20.1 m/s
(b) 24.2 m/s
(c) 26.9 m/s
(d) 32.2 m/s [A.M.I.E. 2006]

27. In case of slip-ring induction motor, the full load rotor mmf is taken as ………percent of stator mmf.

(a) 85
(b) 65
(c) 45
(d) 95

28. In induction motors semi-closed slots give

(a) better overload capacity.
(b) less noise.
(c) low pf.
(d) none of the above.

29. Closed slots are preferred for…………size induction motors.

(a) large
(b) medium
(c) small

30. In induction motors, phase reactance with the increase in number of slots per pole per phase.

(a) decreases
(b) increases
(c) remains unaffected
(d) none of the above

31. In the design of induction motors, normally the number of slots per pole per phase is taken as

(a) two.
(b) three.
(c) three to five.
(d) eight.

32. In induction motors provided with open slots

(a) exciting current is high.
(b) power factor is low.
(c) exciting current is low.
(d) power factor is high.
(e) both (a) and (b).

33. In an induction motor the best power factor can be achieved when the diameter D in terms of length L and number of poles P is equal to

(a) L
(b) 1.35 P√L
(c) √(PL)
(d) P√L

34. For a 3-phase, 4-pole induction motor, D²L = 0.005 m³. Then internal diameter and gross length should be about

(a) 0.23 m and 0.1 m.
(b) 0.18 in and 0.15 m.
(c) 0.19 m and 0.13 m.
(d) 0.13 m and 0.19 m.

35. Air gap of a poly-phase induction motor is kept small to

(a) reduce the possibility of crawling.
(b) reduce the noise.
(c) reduce magnetizing current.
(d) obtain high starting torque.

36. If an induction motor is designed with lesser air gap, then the motor will have

(a) better cooling.
(b) better overload capacity.
(c) better power factor.
(d) lower power factor.

37. The length of air gap in an induction motor is given by

(a) 0.2 + √(2DL) mm.
(b) 0.2 + 2√(DL) mm.
(c) 2√(DL) mm.
(d) 0.2 + √(DL) mm.

38. In induction motors, the length of air gap is kept as small as practically possible. This is to achieve

(a) high overload capacity.
(b) low unbalanced magnetic null.
(c) high power factor.
(d) less noise.

39. A 3-phase squirrel cage induction motor is designed to have rotor copper bars. If instead of using copper, aluminium is used for rotor bars

(a) maximum torque will remain unaffected.
(b) starting torque will be 1.62 times of that with the use of copper.
(c) output of the machine will be 78 percent of that with the use of copper.
(d) full load slip will be 1.62 times that with copper conductors.
(e) all of the above.

40. The number of stator slots should……….be equal to the number of rotor slots.

(a) never
(b) always
(c) sometimes
(d) none of these