Construction and Operation:
The stator of a hysteresis motor carries the main winding as well as an auxiliary winding.
A capacitor is permanently connected in series with auxiliary winding to have the phase split.
When the stator is supplied with a single-phase supply it gives rise to a revolving magnetic field.
The rotor is a solid hard steel cylinder and it does not carry any cage or a winding.
As the rotor is made up of solid steel, a large hysteresis loss takes place in it.
Due to the phenomenon of hysteresis the rotor flux lags behind the stator flux.
Interaction between stator and rotor fluxes creates torque and the motor rotates.
Fig. shows the sectional view of a hysteresis motor.
The rotor material chosen must be such that it has high retentivity.
Such material is hard steel or aluminum-nickel-cobalt (Alnico) with iron. The area of the hysteresis loop is large
The rotor pole tries to catch the rotating magnetic field in the stator. This torque depends upon the retentivity of the material.
At start, torque is due to currents induced in the solid rotor and hysteresis effect.
But when the motor is pulled into synchronism. then-current induced in rotor becomes zero. Only hysteresis torque remains.
Torque speed characteristic is as shown in Fig.
It is seen that starting torque remains constant. The direction of rotation can be reversed by reversing the direction of the rotating magnetic field.
For this purpose, either main winding or auxiliary winding connections has to be reversed.
As there is no winding on the rotor and since the rotor has no teeth or projecting poles (as in reluctance motor) there are no mechanical vibrations.
Hence this motor gives a silent noiseless operation.
Applications:
Also starting torque is comparatively high than other motors. It is therefore used in
1. tape recorders,
2. record players,
3. all timing devices,
4. Precision audio equipment.