When single-phase AC supply is given to stator winding. It produces alternating flux.i.e. which alternates along one space axis only. It is not synchronously revolving (or rotating) flux, as in case of 3 phase stator winding, the fed cannot produce rotation. Hence single phase induction motor is not self-starting.
To overcome this problem and to make the motor self-starting, it is temporarily converted into two-phase motor during starting. For this purpose, the stator of 1 phase motor is provided with extra winding known as starting winding in addition to the main winding. These two winding are placed across the single phase supply.
This winding is so arranged that the phase difference between the currents in the two stator winding is very large. hence motor behaves like a two-phase motor. The two current produces a revolving flux and this makes the 1 ∅ motor self-starting.