Split Phase Motors

A single phase induction motor is not self starting.  This is because the ac supply creates a pulsating magnetic field in the stator core and not a rotating magnetic field which is required for an induction motor to be self starting.

Many methods are used to start a single phase motor.  One such method is the split phase method.  The split phase motor has a main winding in the stator.  In addition to this, it has a starting winding wound in the stator.  The starting winding is connected parallel to the main winding.  When the supply to the windings.  The starting winding is displaced 90 degrees from the main winding.

This is because the starting winding has a higher resistance and occupies less space in the stator.  It is wound over a few slots in the stator and is usually placed above the main winding in the slots. The main winding has many turns and thus has a higher reactance.  Thus it lags behind the starting winding. 

Split phase motors are used where low to moderate starting torque is sufficient.  They are designed up to 1/3 hp. 

The starting winding is only used when the motor is starting.  The winding is connected through a centrifugal switch.  When sufficient rpm is reached, the centrifugal switch operates and isolates the winding.