Induction motor is one of the motors used most commonly in many applications. As it runs at a speed less than the synchronous speed it is called as asynchronous motor.
Define Synchronous speed:
In a rotary machine the speed of rotation of the magnetic field is called the Synchronous speed and it depends on the frequency and number of poles of the machine.
It is said that the induction motor’s speed is less than the synchronous speed because the rotating magnetic field generated in the stator will produce flux with the rotor and this is the reason behind the rotor’s rotation. The rotor will never reach to the speed of the magnetic field that is synchronous speed because of the lag of flux current in the rotor with flux current in the stator.
There are two different types of induction motor and it is one the basis of input supply they are: single phase induction motor and three phase induction motor.
Single phase induction motor is not a self-starting motor but the latter is!
Working Concept of Induction Motor
For making a machine to rotate there is a need for double excitation. For instance if you are considering a DC motor, one supply will be given to the stator and the other supply will be given to the rotor with the help of brush arrangement. It is really interesting to know the work of an induction motor because there is one supply in need. Flux will get generated in the coil due to the flow of current in the coil while giving supply to the stator winding. For creating a short circuit in the rotor itself the rotor winding is arranged accordingly. According to Faraday’s law of electromagnetic induction current will initiate to flow in the coil of the rotor as the rotor coils are short circuited. During the time of current flow another flux will get generated in the rotor. There are two flux one the rotor and the other is stator; the rotor flux lags with respect to the stator flux. Due to this, the blades will feel a twisting which will create the blades to shift. This will create a torque and will make the rotor to rotate in the direction of rotating magnetic field. So the speed of the rotor depends on the input supply which can be adjusted and this is concept of the induction motor.
Types Induction Motor
Single Phase Induction Motor
Split Phase introduction motor
Capacitor start induction motor
Capacitor start capacitor run induction motor
Shaded pole induction motor
Three Phase Induction Motor
Squirrel cage induction motor
Slip ring induction motor
Earlier we have mentioned that single phase is not self-starting whereas three phase is self-starting. So let us see what is self-starting?
When the device automates running without any exterior power to the device, then it is known as self-starting. For instance the fan starts to spin when you put on the switch. Point to be observed that fan used at home is single phase induction motor which is naturally not self-starting.
Reason for Three Phase Induction Motor is Self-Starting
In this system there are three individual phase lines with a difference of 120 degrees. The rotor moves because of this difference in phase in the rotating magnetic field. For example if we consider three phases a, b, and c; if a is magnetized, the rotor will move towards it; and if b is magnetized the rotor will shift towards b and it is the same for c. So this causes motion in the rotor.
Reason behind Single Phase Induction Motor is not Self-Starting
The most interesting thing over here is it has only one phase but still it makes the rotor to rotate. Here we need to know why it is not self-starting and how to overcome it. As we all know that AC supply is sinusoidal wave and it generates uniform magnetic field and as a result there will be no torque and thus the rotor will not rotate. This issue has been set by making the stator winding into two winding, one is main winding and another is auxiliary winding and a series fixed capacitor with the auxiliary winding. This will produce a phase difference and a torque thereby rotating the rotor. We can make out that the rotor does not rotate when the capacitor is turned off from the device but if we give a force with hand it will begin to rotate. So this is the reason of using capacitor in the single phase induction motor.