Split Phase Induction Motor



The Split Phase Motor is also known as a Resistance Start Motor. The stator of a split-phase induction motor is provided with a starting (or auxiliary) winding in addition to the main winding (M) as shown in below diagram.

Split Phase Induction Motor circuit diagram


The two windings are connected in parallel across the supply. Thus, the same supply voltage V is applied across them.


The two windings are so designed that the starting winding has high resistance and relatively small reactance while the main winding has relatively low resistance and large reactance.


Due to the inductive nature, the current through main winding lags the supply voltage by a large angle and due to the resistive nature of the starting winding, the current through it is almost in phase with the voltage as shown.

Split Phase Induction Motor phasor diagram


Thus, there exists a phase difference between the currents and thereby phase difference between the fluxes produced by these currents. The resultant of these two fluxes produce rotating magnetic field and hence, starting torque.


The starting torque (Tst) is given by,
Tst = K ImIs sinα
Where, K is constant whose value depends on the motor design.

Usually, α is small (250 to 300) , sinα is small and therefore, split-phase induction motors give poor starting torque. Thus, they are used for low starting torque applications such as blowers fans, washing machines, grinders etc.

When the motor reaches 75% to 80% of synchronous speed, the centrifugal switch (speed sensitive switch), connected in series with the starting winding, opens and thereby starting winding is disconnected from the circuit. The motor continues to rotate till it reaches its normal speed.


The direction of rotation of split phase induction motor can be reversed by  reversing the line connection of either the main winding or the auxiliary winding but not both.


Torque-speed Characteristics

Split Phase Induction Motor torque-speed characteristics


The starting torque of a split-phase induction motor is about 1.5 times the full load torque. N0 is the speed at which centrifugal switch opens.

Motor follows main+auxiliary winding characteristics till N0 . From N0  to NS it follows main winding alone characteristics. The complete characteristics is shown in bold line.


Related :-

(1) Split phase Induction motor