The stator winding of a single phase induction motor is connected to single phase AC supply. Then a magnetic field is developed in the stator whose axis is always along the axis of stator windings. With alternating current in the fixed stator coil the mmf wave is stationary in space but pulsates in magnitude and varies sinusoidally with time.
Due to the transformer action, currents are induced in the rotor conductors. The direction of the current is to oppose the stator mmf.
Thus the axis of rotor mmf wave coincides with the axis of stator mmf wave. Therefore the torque angle is zero and no starting torque is developed in the motor. However if rotor is initially given a starting torque by some means, the motor will pick up the speed and continue to rotate in the same direction. Thus the single phase induction motor is not a self starting motor. The starting torque can be produced by some external arrangement.
Starting of single phase induction motor
The starting method of single phase induction motor is very simple. An auxiliary winding in the stator is provided in addition to the main winding. Then the induction motor starts as a two phase motor.
The main winding axis and auxiliary winding axis are displaced by 90 electrical degrees. The impedances of the windings differ and currents in the main and auxiliary winding are phase shifted from each other. As a result of this a rotating stator field is produced and the rotor rotates.
When the motor speed is about 75% of synchronous speed, the auxiliary winding is disconnected from the circuit. This is done by connecting a centrifugal switch in the auxiliary winding which is used for starting purpose only. That is why it is called starting winding. Under running condition, a single phase induction motor can develop torque only with main winding. That is why it is called running winding.
Types of Single Phase Induction Motor
The single phase induction motors can be classified according to the phase difference produced between the currents in the main and auxiliary windings. The classifications are
- Split-phase motors
- Capacitor – start motors
- Capacitor-run motors
- Capacitor – start and – run motors
- Shaded – pole motors
- See More : Three phase induction motor
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- See More : Working principle of a transformer
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What are the different types of single phase motor?
Single phase induction motors, Single phase synchronous motors, Single phase series (or universal) motors