It has only one phase still it makes the rotor to rotate, so it is quite interesting. Before that, we need to know why a single phase induction motor is not a self-starting motor and how we overcome the problem. We know that the AC supply is a sinusoidal wave and it produces a pulsating magnetic field in the uniformly distributed stator winding.
Since we can assume the pulsating magnetic field as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting, and hence the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by an external force, then the motor will start to run. We can solve this problem by making the stator winding into two winding – one is the main winding, and another is auxiliary winding.
We connect one capacitor in series with the auxiliary winding. The capacitor will make a phase difference when current flows through both coils. When there is a 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 gets disconnected from the motor, but if we rotate with the hand, it will start rotating. That is why we use a capacitor in the single-phase induction motor.
Due to the various advantages of an induction motor, there is a wide range of applications of an induction motor. One of their biggest advantages is their high efficiency – which can go as high as 97%. The main disadvantage of an induction motor is that the speed of the motor varies with the applied load.
The direction of rotation of induction motor can easily be changed by changing the phase sequence of three-phase supply, i.e., if RYB is in a forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor, but in a single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.
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