Split Phase Induction Motor is a type of A.C. single phase motor. It has two windings placed at 90° to each other. These are starting winding and main windings. The main winding has low resistance and high inductance. It is wound with thick conductor having less number of turns then the starting winding. It is embedded in the slots. The starting winding has more resistance and low inductance. It is done with thin conductor having more number of turns or some times equal. It is done on the upper side of the running winding.
Fig. 1. Split phase motor.
The electrical angular displacement between these two windings is 90° electrical as shown in Fig. 1. The currents of both windings differ in value and phase. The current of starting winding Is is near to the voltage vector because of more resistance and current of main winding Im is towards lagging side because of more inductance. The main winding is connected across the line where the starting winding only for the starting period. The starting winding is disconnected from the mains by the centrifugal switch.
Fig. 2. Vector diagram.
Working of Split Phase Induction Motor
When the motor is connected across the supply the starting winding because of more resistance draws current at a phase difference near to the voltage vector. The main winding because of more inductance draws current on the lagging side, away from the voltage vector as shown in Fig. 2. There are two currents, resulting two magnetic fields which produces the magnetic field of rotating in nature. Hence the motor starts. As the motor attains its 75% of the synchronous speed, the centrifugal switch disconnect the starting winding and now motor is running on main winding only.
Characteristics of Split Phase Induction Motor
Torque
The starting torque is low. It is about 1.3 to 1.7 the full load torque. The torque increases with the increasing of load as shown in Fig. 21.6.
Current
The starting current is approximately five times the full load current. Without load, current is minimum and increases with the increasing of load.
Speed
The speed at no load is approximately equal to the Synchronous speed and slightly falls down with the increasing of road.
Power factor
The power factor is low i.e. 0.6 to 0.8 lagging.
Efficiency
The efficiency of the motor is low as compared with the three phase induction motors.