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.
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.
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
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
