HYSTERESIS MOTOR DEFINITIONS

HYSTERESIS MOTOR 

This is the synchronous motor which does not require any d.c. excitation to the rotor and it uses non projected poles.It consists of a stator which carries main and auxiliary windings so as to produce rotating magnetic field. The stator can also be shaded pole type. The rotor is smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity.This requires selecting a material with high hysteresis loop area. The rotor does not carry any winding.The construction is shown in the Fig. 7.4 (a) while nature of hysteresis loop required for rotor material.

(a) Cross-sectional view of hysteresis motor   (b) Hysteresis loop for rotor material

•     When stator is energized, it produces rotating magnetic field. The main and both the winding must be supplied continuously at start as well as in running core as to maintain the rotating magnetic field.

•     This field induces poles in the hysteresis phenomenon is dominant for the rotor material chosen and due to which pole axis lag behind the axis of rotating magnetic field. Due to this, rotor poles get a towards the moving stator field poles.

•     Thus rotor gets subjected to torque called hysteresis torque. This torque is constant at all speeds. When the stator field axis moves forward to high retentivity the rotor pole strength remains maintained.
  
  
  

•     So higher the r higher is the hysteresis torque. Initially rotor starts rotating due to combined effect of hysteresis torque as well a due to eddy currents induced in the rotor. Once the speed is near about the synchronous stator pulls rotor into synchronism.

•     In such case, as relative motion between stator field rotor vanishes, so the torque due to eddy currents vanishes.     Only hysteresis t present which keeps rotor running at synchronous speed. The high retentivity ensures continuous magnetic locking between stator and rotor.

•     Due to principle of magnetic I the motor either rotates at synchronous speed or not at all.

MATHEMATICAL ANALYSIS

•     The eddy current loss in the machines is given by,

•     We know the relation between rotor frequency f and supply frequency f,

•     The torque due to eddy currents is given by

•     So when rotor rotates at synchronous speed, the slip becomes zero and torque eddy current component vanishes. It only helps at start.

•     The hysteresis loss is given by,

•     The corresponding torque is given by,

TORQUE-SPEED CHARACTERISTICS

•     The starting torque and running torque is almost equal in this type of motor.

•     As stator carries mainly the two windings its direction can be reversed by interchanging the terminals of either main winding or auxiliary winding.
  
  

•     The torque-speed characteristic is as shown in the Fig. 7.5.

 

 

                                Fig. 7.5 Torque characteristics of hysteresis motor

•     As seen from the characteristics torque at start is almost same throughout the operation of the motor.

    Advantages

•     The advantages of this motor are:

•     As rotor has no teeth, no winding, there are no mechanical vibrations.

•     Due to absence of vibrations, the operation is quiet and noiseless.

•     Suitability to accelerate high inertia loads

•     Possibility of multi speed operation by employing gear train.

    Applications

•     Due to noiseless operation it is used in sound recording instruments, sound producing equipments, high quality record players, tape recorders, electric clocks, teleprinters, timing devices etc.