Induction Motor
Under Induction Motor
One of the most common electrical motor used in most application which is known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than synchronous speed. In this ,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 no. 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
three phase induction motor
single phase induction motor is not a self starting motor which we will discuss and three phase induction motor is self starting motor. Now in general we need to give two supply i.e. double excitation to make a machine to rotate .
Working Principle of Induction Motor
But in induction motor we give 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 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 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 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 the ac supply and speed can be controlled by varying the input supply, This is the working principle of an induction motor of either type,
Types Of 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
Why is three phase induction motor is self starting ?
In three phase system, there are three single phase line with 120 degree phase difference, So the rotation magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a ,b,c, when phase a is magnetized, the rotor will move towards the phase a winding, in the next movement phase b will get magnetized and it will attract the motor and than phase c. So the rotor will continue to rotate.
Why single phase induction motor is not self starting ?
It will be having only one phase still it makes the rotor to rotate, so it is quite interesting. Before that we need to know why single phase induction motor is not self starting motor and how the problem is overcome. We know that ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly distributed stator winding. Since pulsating magnetic field can be assumed as two oppositely rotating magnetic fields, there will be no resulting torque produce at the starting and due to this motor does not run. After giving the motor supply, if rotor is made to rotate in either direction by external force, then the motor will start to run. This problem has been solved by making the stator winding into two winding main winding and auxiliary winding , a capacitor is fixed in series with the auxiliary winding. This will make a phase difference when current will flow through the both coils. When there will be phase difference , the rotor will generate a starting torque and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor is disconnected from the motor but if we rotate with hand it will start to rotate. So this is the reason of using capacitor in the single phase induction motor,
How to reverse single phase and three phase motor ?
In single phase winding motor rotation can change by reversing the capacitor terminals in the winding, but in three phase terminals RYB is forward direction and make it RBY is reverse direction .