Induction motor [ asynchronous motor ] :
We called induction Motor as asynchronous Motor because it runs at a speed less than synchronous speed.
Note : 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.
Note : induction motor always runs at a speed less than synchronous speed.
Question : Why Induction Motor runs at a speed less than synchronous speed ?
Answer : 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 :
1- single phase induction motor [ NOT a self starting motor ]
2- 3-Phase induction motor.[ self-starting motor :starts running automatically without any external force to the machine ]
In general we need to give two supply i.e. double excitation to make a machine to rotate. For example if we consider a DC motor, we will give one supply to
the stator and another to the rotor through brush arrangement.
- Working Principle of Induction Motor
But in induction motor we give only one supply, so it is really interesting to know that how it works. It is very simple, from the name itself we can understand that there is induction process occurred. 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 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.
Types Induction 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