The document discusses 3-phase induction motors. It describes how a rotating magnetic field is produced in the stator by 3-phase currents, which induces currents in the rotor and causes it to rotate. It discusses the construction of squirrel cage and wound rotors. It also covers key concepts like slip, torque production, equivalent circuits, power flow, and torque-speed characteristics of 3-phase induction motors.

1. 3-Phase Induction Motor
SYl: Principle, Construction, Rotating magnetic field, slip,
torque, char, power stages, Eq ckt, Vector Diagram, Motor Tests,
Starters, Speed Control, Enclosures and Motor Classes

2. Syl: Double field revolving theory, Torque-slip curve, Eq Ckt, Making
1- Phase motor self starting, capacitors starts and run motor,
Universal motor

3. Basic Concepts
• Faraday’s law of EMI
• Lentz’s law
• 3 Phase supply of Generator

10. 3-Phase Induction Motor
With AC system universally adopted, use of AC motor has been widened
considerably .
Available with bewildering multiplicity of types.

11. Induction Motor

12. General Principle
DC motor receives power by conduction ( brush, Commutator)
but an induction motor receives power by induction like a 2
winding transformer, So it is treated as Rotating Transformer

13. 2-Winding Transformer

14. Construction

15. A number of stampings slotted to receive winding
Carries 3 phase windings fed from 3 phase supply.
Wound for definite number of poles, P=2n,
n = no of slots /pole/phase.
Stator windings when supplied with 3 phase currents
produces a magnetic flux which is of constant magnitude
but rotates at synchronous speed Ns

20. Rotor
 Squirrel Cage rotor(Squirrel cage motor)
 Phase wound rotor ( Wound Motor)

21. Squirrel Cage is 90% of rotors.
Cylindrical laminated core with parallel slots for carrying
rotor conductors. Conductors are not bars but heavy
copper bar or Al or Alloys, Bars are welded to short
circuited end rings.
Not possible to add any external resistance ( for Starting
purposes)

22. Rotor Construction

23. Rotor ( Phase wound)
Rotor has 3 phase double layered distributed winding
consists of coil.
Other three terminals are bought out together and
connected to slip-rings mounted with shaft and
brushes.
3 brushes are connected to 3 phase star rheostat for
starting and speed control by means of Metal collar.
Brushes are automatically shifted to reduce frictional
losses and wear and tear.
Under normal running rotor is short circuited like Sq
cage motor

24. Rotor ( Phase wound)

25. Parts of motor

26. Parts of Motor

27. Rotating Magnetic Field (2 Φ)

28. Rotating Magnetic Field (2 Φ) cont’d

29. Rotating Magnetic Field (3 Φ)

30. Rotating Magnetic Field (3 Φ) cont’d

31. Rotating Magnetic Field (3 Φ) cont’d

32. Rotating Magnetic Field (3 Φ) cont’d

33. When the 3-phase stator windings, are fed by a 3-phase supply then a magnetic flux
of constant magnitude, but rotating at synchronous speed, is set up.
The flux passes through the air-gap, sweeps past the rotor surface and so cuts the
rotor conductors which, as yet, are stationary.
Due to the relative speed between the rotating flux and the stationary conductors,
an e.m.f. is induced in the latter, according to Faraday’s laws of electro-magnetic-
induction. The frequency of the induced e.m.f. is the same as the supply frequency.
Its magnitude is proportional to the relative velocity between the flux and the
conductors
and its direction is given by Fleming’s Right-hand rule.
Since the rotor bars or conductors form a closed circuit, rotor current is produced
whose direction, as given by Lenz’s law, is such as to oppose the very cause producing
it.
In this case, the cause which produces the rotor current is the relative velocity
between the rotating flux of the stator and the stationary rotor conductors. Hence, to
reduce the relative speed, the rotor starts running in the same direction as that of
the flux and tries to catch up with the rotating flux.

36. Slip and Frequency of rotor current
Frequency of rotor current:
Slip:

37. Example problem

38. Example Problem

39. Power Developed in DC motor

40. Torque

41. Torque and Rotor p.f.

42. Torque and Rotor p.f.( Rotor inductive)

44. Example Prob on Starting Torque

47. Maximum Torque during Running

48. Example problem on running torque

49. Relation between Torque and Slip

50. Torque and Speed curve

51. Shape of Torque Speed Curve

52. Plugging of Induction motor

53. Induction motor running as generator

54. Induction motor as Generator ( cont’d)

55. Complete Torque Speed curve

56. Power Stages of Induction Motor

57. Power Flow in Induction Motor

58. Power Flow of IM

59. Example problem on Power Flow of IM

60. Example problem on Power Flow

61. Induction Motor as Transformer

62. Induction motor as Transformer

63. Equivalent Circuit

64. Approximate Equivalent Circuit

65. Maximum Power Output

66. Example Problem