ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

3 Phase Induction Motor - Introduction

•Converts electrical power into mechanical

power

•Energy transfer by means of electromagnetic

induction

• Industrial & Commercial Applications Pumping Systems

Refrigeration Systems

Compressors

Fans & Blowers

Industrial Drives

Rotating Magnetic Field

• 3 phase supply is given to a balanced 3 phase winding.

• 3 alternating magnetic fluxes displaced mutually at 120o is produced.

240)t(sinφφ

120)t(sinφφ

tsinφφ

maxB

maxY

maxR

Considering different instances of time:

(i) At wt = 0o

The instantaneous fluxes are

The resultant magnetic flux is 1.5 times peak value and

Acting at 90o to axis of reference.

maxB

maxY

R

φ866.0φ

φ866.0φ

0φ

(ii) At wt = 60o

The instantaneous fluxes are

The resultant magnetic flux is

1.5 times peak value and acting

at 30o to axis of reference.

i.e., rotated by 60o w.r.t. previous angle

0φ

φ866.0φ

φ866.0φ

B

maxY

maxR

Similarly if various instances are considered upto 360o, it is seen that the resultant always has a magnitude of 1.5 times peak value and pointing in a direction at the angle of consideration.

For every 360o, the resultant magnetic field completes one rotation.

Inference:When 3 phase currents flow in a balanced 3 phase winding, a

rotating magnetic field is created which has constant magnitude, but rotates in synchronism with supply frequency.

This speed of rotating magnetic field is called SYNCHRONOUS SPEED

P

fNS

120

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Classification

• Construction

Squirrel Cage, Slip Ring

• Connection Stator : Star, Delta

Rotor : Cage, Wound

• Application Constant Power, Constant Torque, Constant Speed,

Variable Torque - Variable Speed

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Cross Sectional View

Terminal Box

Outer Frame

Stator

Cage Rotor

Shaft

Ball Bearings

Supporting Base

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Construction – Stator

• Stator frame: Cast Iron, Mechanical Support to stator core• Stator core: Stack of cylindrical steel laminations• Stator Slots: Inner periphery, Windings

Slots

Stator Conductors

Laminations

Outer frame

Stator Core

Stator Slots

Stator windings

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Construction – Rotor

• Types on the basis of rotor

construction Squirrel Cage Rotor

Slip Ring Rotor

• Cylindrical Laminated core

• Slots cutout on outer periphery

• Conductors placed in slots

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Construction – Squirrel cage Rotor

• Skewed arrangement

• Copper or Aluminum Bars

• Conductors shorted by end

rings

• Closed rotor circuit

Rotor bars

End rings

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal

Construction – Slip ring rotor

• One end of Rotor conductors connected to each other

(Star)

Rotor

External resistance

Shaft

• Other ends connected to slip rings• Slip rings in contact with brushes

• Brushes connected to external resistance

Slip ringsRotor

Conductors

Brush

Representation

ELE 101 / 102 Basic Electrical Technology Dept of E & E, MIT Manipal L1 – 15

3Ø Supply

Stator Core

Stator SlotsStator conductors

Rotor conductorsRotor SlotsRotor CoreShaft