Electrical Machine are Electromechanical Energy conversion devices.
Electrical Motor converts Electrical Energy into Mechanical Energy and electrical
generators are vice versa.
An induction motor works on transforming action.
The stator works as the primary while the rotor works as the secondary.
It is also called asynchronous motor.
Most of the industrial load are inductive, especially 3- phase induction motor are
widely used in industrial applications for continuous operation.
A three-phase induction motor
consists of two major parts:
1. A Stator: It is the stationary part of
2. A Rotor: It is the rotating part of the
6. Stator has three main parts:
Outer Frame – It is the outer body of the motor.
It protects the inner part of the machine.
Stator Core – Built up of high grade Silicon Steel.
Carries the alternating magnetic field.
Stator Winding – Has a three phase winding.
There are two types of rotors which are employed in 3 – phase induction motor.
Squirrel Cage Rotor.
Phase Wound/ Slip Ring Rotor.
8. Squirrel Cage Rotor
It consists of a laminated cylindrical core having
semi closed circular slots at the outer periphery.
Copper or aluminium bar conductors are placed in
these slots and short circuited at each end by
copper or aluminium rings called short circuiting
The rotor winding is permanently short circuited
and it is not possible to add any external
9. Phase Wound/ Slip Ring Rotor
Consists of a laminated core having semi closed
slots at the outer periphery and carries a 3-phase
The rotor is wound for the same number of poles as
that of stator.
The three finish terminals are connected together
forming a star point and the three star terminals are
connected to three slip rings fixed on the shaft.
11. Rotating Magnetic Field
When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase
current will flow in the windings, which also will induced 3 phase flux in the stator.
These flux will rotate at a speed called a synchronous speed (Ns). The flux is called
as rotating magnetic field.
Synchronous Speed: Speed of rotating flux
12. Working Principle
When 3 phase supply is given to the
resulting current generates a magnetic
According to Faraday’s law, Emf is
induced in the Copper bar and due to
this, current flows in the rotor.
The direction of the rotor can be given
by Lenz law.
Induction motor rotor always rotate at a speed less than synchronous speed.
The difference between the flux (Ns) and the rotor Speed (N) is called Slip.
% Slip =
(Ns – N)
Slip Speed = (Ns – N)
The construction of motor is very simple and robust.
The working of an induction motor is very simple.
It can operate in any environment condition.
The efficiency of the motor is very high.
The maintenance of an induction motor is less compared to other motors.
During light load condition, the power factor is very less and it draws more
current. So, the copper loss is more which reduce the efficiency during light load
The induction motor is a constant speed motor. For the application where variable
speed requires, this motor is not used.
Speed control of this motor is difficult.
The induction motor has a high starting inrush current. This causes a reduction in
voltage at the time of starting.