Hysteresis motor is a type of synchronous motor that operates by the principle of hysteresis loss. The rotor of this motor is made up of a ferromagnetic material with high retentivity, which means it can retain a large amount of magnetism even in the absence of an external magnetic field.

Explanation:

• Retentivity is a characteristic of a magnetic material that determines the amount of magnetism it can retain. In hysteresis motor, the rotor must have high retentivity so that it can maintain a strong magnetic field even in the absence of an external magnetic field.

• The stator of the hysteresis motor produces a rotating magnetic field, which induces a magnetic field in the rotor. Due to the high retentivity of the rotor material, the induced magnetic field in the rotor lags behind the rotating magnetic field of the stator.

• This lagging causes the rotor to rotate in the direction of the rotating magnetic field of the stator. The hysteresis motor does not require any external excitation, and it operates on the principle of hysteresis loss.

• The rotor of the hysteresis motor also requires low resistivity to reduce the power loss due to eddy currents. However, susceptibility is not a relevant characteristic for the hysteresis motor rotor.

Conclusion:

Therefore, from the above explanation, we can conclude that in a hysteresis motor, the rotor must have high retentivity to maintain a strong magnetic field even in the absence of an external magnetic field.