In practice some arrangement is provided in the single phase induction motors so as that the stator flux produced becomes rotating type rather than the alternating type, which rotates in particular direction only. So torque produced due to such rotating magnetic field is unidirectional as there is no oppositely directed torque present. Hence under the influence of rotating magnetic field in one direction, the induction motor becomes self starting. It rotates in same direction as that of rotating magnetic field. Thus depending upon the methods of producing rotating stator magnetic flux, the single phase induction motors are classified as,
- Split phase induction motor
- Capacitor start induction motor
- Capacitor start capacitor run induction motor
- Shaded pole induction motor
To produce rotating magnetic field, it is necessary to have minimum two alternating fluxes having a phase difference between the two. The interaction of such two fluxes produces a resultant flux which is rotating magnetic flux, rotating in space in one particular direction. So an attempt is made in all the single phase induction motors to produce an additional flux other than stator flux, which has a certain phase difference with respect to stator flux.
Such two fluxes are shown in the Fig. 1 having phase difference of between them.
More the phase difference angle ?, more is starting torque produced. Thus production of rotating magnetic field at start is important to make the single phase induction motors self starting. Once the motor starts, then another flux ?2 may be removed and motor can continue to rotate under influence of stator flux or main flux alone.
Let us see how the rotating magnetic field is produced in various types of single phase induction motors.