Split Phase Induction Motor

Split Phase Induction Motor AU : May-08,18, Dec.-08, 11, 13

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     I_m = Current through main winding

and    I_st = Current through auxiliary winding

As main winding is inductive, current I_m lags voltage V by a large angle ϕ_m while I_st is almost in phase with V as auxiliary winding is highly resistive. Thus there exists a phase difference of ɑ between the two currents and hence between the two fluxes produced by the two currents. This is shown in the Fig. 8.6.1 (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.

The auxiliary winding has a centrifugal switch in series with it. When motor gathers 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 rims only on stator winding. So auxiliary winding is designed for short time use while the main winding is designed for continuous use. As the current I_m and I_st are splitted from each other by angle 'ɑ' at start, the motor is commonly called split phase motor.

The torque-speed characteristics of split phase motors is       shown in the Fig. 8.6.2.

The starting torque T_st is proportional to the split angle 'a', but split phase motors give poor starting torque which is 125 to 150 % of the 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.

1. Applications

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 1/20 to 1/2 kW.

Review Question

1. Describe the principle of operation of single phase split phase type induction motor alongwith torque-speed characteristics. Draw the circuit and phasor diagram. State its applications. AU : May-08,18, Dec.-08, 11, 13, Marks 8