In a Hysteresis Motor, the rotor must have

• A. Resistivity
• B. Retentivity
• C. Susceptibility
• D. None of these

Answer: (B)

The rotor of the Hysteresis Motor is the smooth cylindrical type which is made up of hard magnetic material like chrome steel or alnico for high retentivity (it is the capacity of an object to retain magnetism after the action of the magnetizing force has ceased. This requires selecting a material with high hysteresis loop area. The rotor does not carry any winding. The stator construction is either split phase type or shaded pole type. 
 
The motor starts rotating due to eddy current and hysteresis torque developed on the rotor At synchronous speed, there is no induced emf in the rotor, as the stator synchronously rotating field and the rotor is stationary with respect to each other.
In the absence of induced eddy current, the torque due to eddy current is zero. At synchronous speed, the rotor torque is only due to the hysteresis effect.
When the rotor rotates at synchronous speed, the stator revolving field flux induces Poles on the rotor. Due to the hysteresis effect, the rotor polarities linger an instant after the stators