Induction motors

One of the most common electrical motor used in most applications which is known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than its synchronous speed.

Here we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it depends upon the frequency and number poles of the machine. An induction motor always runs at a speed less than synchronous speed because the rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed.


There are basically two types of induction motor that depend upon the input supply – single phase induction motor and three phase induction motor. Single phase induction motor is not a self-starting motor and three phase induction motor a self-starting motor.

Single Phase Induction Motor

1.Split phase induction motor

2.Capacitor start induction motor

3.Capacitor start capacitor run induction motor

4.Shaded pole induction motor

Three Phase Induction Motor

1.Squirrel cage induction motor

2.Slip ring induction motor

Working Principle of Induction Motor: –

It is very simple, from the name itself we can understand that induction process is involved. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday’s law of electromagnetic induction, current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging w.r.t to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type – single and three phase. Various part of Induction motors is as shown in below dig.


Induction motor basically consists of a stator and a rotor. Stator can be considered as primary of transformer and rotor as secondary of transformer which has closed circuit.

Rotor receives electric power by means of mutual induction through air gap like in transformer where its secondary receives power by mutual induction through iron core. Infect an induction motor can be treated as a rotating transformer in which primary winding is stationary and secondary is free to rotate.

Stator:An induction motor or 3 phase induction motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. Basic dig.of stator are as shown in below
The windings are placed on the stampings which are of 3 types:
Types of stampings

When the 3 phase stator winding is provided with 3phase supply there is a creation of magnetic flux which is of constant magnitude and runs at synchronous speed(Ns=120f/p). This revolving flux induces an emf in the rotor by mutual induction. The no. of poles in stator depends on the requirement of speed N α 1/p.

N is speed and p= No. of poles

Rotor: -The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the winding and magnetic fields which produces a torque around the rotor’s axis.Basic dig. Of rotor are as shown in fig.No.-2

Rotor of squirrel cage Rotor of slip ring/Phase wound motor
It is most widely used because of its simple and rugged construction It is has complicated construction
High efficiency Low efficiency
Low or moderate starting torque High starting torque
The rotors bars are short circuited at both the ends with the help of end rings Armature conductors are attached to the slip
We cannot connect external resistance We can add external resistance
Starting torque is not adjustable Starting torque is adjustable because we can connect resistance accordingly
Brushes are not used Brushes and slip is used


Applications of induction Motor: –

  • Domestic Fans
  • Lathe Machines
  • Blowers
  • Vacuum


  • Requires minimum maintenance
  • High efficiency
  • Low cost
  • Reliable
  • Simple and rugged construction

Disadvantages: –

  • Speed cannot be varied without sacrificing the efficiency
  • Speed decreases with increase in load

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