This type of motor has single phase stator winding called main winding. In addition to this, stator carries one more winding called auxiliary winding or starting winding. The auxiliary winding carries a series resistance such that its impedance is highly resistive in nature. The main winding is inductive in nature.
Let Im = Current through main winding
and Ist = Current through auxiliary winding
As main winding is inductive, current Im lags voltage by V by a large angle ?m while Ist is almost in phase in V as auxiliary winding is highly resistive. Thus three exists a phase difference of ? between the two currents and hence between the two fluxes produced by the two currents. This is shown in the Fig.1(c). The resultant of these two fluxes is a rotating magnetic field. Due to this, the starting torque, which acts only in one direction is produced.
|Fig. 1 Split phase induction motor|
The auxiliary winding has a centrifugal switch in series with it. When motor gather a speed upto 75 to 80% of the synchronous speed, centrifugal switch gets opened mechanically and in running condition auxiliary winding remains out of the circuit. So motor runs only stator winding. So auxiliary winding is designed for short time use while the main winding is designed for continuous use. As the current Im and are splitted from each other by angle ‘? ‘ at start, the motor is commonly called split phase motor.
The torque-speed characteristics of split phase motors is shown in the Fig.2.
The starting torque Tst is proportional to the split angle ‘? ‘ but split phase motors give poor starting torque which is 125 to 150% of full load torque.
The direction of rotation of this motor can be reversed by reversing the terminals of either main winding or auxiliary winding. This changes the direction of rotating magnetic field which in turn changes the direction of rotation of the motor.
These motors have low starting current and moderate starting torque. These are used for easily started loads like fans, blowers, grinders, centrifugal pumps, washing machines, oil burners, office equipments etc. These are available in the range of 1/120 to 1/2 kW.