Single phase induction motor is not self starting motor.
To overcome this drawback, single phase motor is converted into a two-phase motor during starting period.
This is achieved by adding an additional winding called starting winding, along with the main or running winding.
They are connected across the single phase AC supply and the circuit diagram is shown in figure.
The main winding of the induction motor has low resistance and high reactance. The starting winding and running winding are spaced 90° electrically apart and in parallel to each other.This 90 degree phase difference is achieved by directly making the starting winding with low reactant by embedding the winding deeply in the core.
Resistance of the starting winding is increase using high resistance. Copper wire for winding for connecting high resistance are in series with it.
Give the supply by closing the switch S.
The current 'Im' is drawn by the main winding legs voltage by a very large angle as shown in the phasor diagram.
The current 'Is' drawn by the starting winding legs voltage by a small angle.
The phase difference between these two currents is alpha.
These two current produce magnetic fluxes.
The phase difference between these two flux is also Alpha that is nearly 90°.
The resultant of these two flux is a rotating magnetic field.
This rotating magnetic field starts to rotate the motor.
A centrifugal switch is connected in series with the starting winding.
It disconnects the starting winding, once the motor reaches 70 to 80% of full load speed.
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