Split phase induction motor :
There are two windings used in split phase induction motor. It is a type of single-phase induction motor. There are two winding one is called main winding and second is called auxiliary winding or starting winding. Main winding is inductive in nature & auxiliary or starting winding is highly resistive in nature, this is because we have to create a phase difference in between these two windings. Since there are two windings there will be two current flow one in main winding and second in starting winding. In main winding current flow is Im and in starting winding current flow is Ist. Starting winding is connected in parallel with main winding and there is a centrifugal switch in series with starting winding. When the motor is in off state to turn on the motor we give single phase supply to it.
The centrifugal switch is closed initially and current flow in both main winding and the auxiliary winding. Since the main winding is inductive and starting winding is highly resistive it creates a phase difference which rotates the rotor. When rotor approaches the speed 75 to 80% of synchronous speed the centrifugal switch is open or turned off, By turning off centrifugal switch it disconnects the starting winding and then rotor rotates or runs only on main winding.
The main winding current (Im) & starting winding current (Ist) splitted from each other by angle “α”. The main winding is inductive so the current Im lags the voltage by ϕm. Starting winding current (Ist) is almost in phase with voltage V since the starting winding is highly resistive.
Tst is the starting torque which is proportional to split angle α. But the split phase induction motor have poor starting torque. By changing the terminals of either starting winding or main winding we can change the direction of the rotating magnetic field which will also reverse the direction of rotation of the motor. Since split-phase induction motor have low starting torque it is used for small fans, blowers, centrifugal pumps, grinders etc.