Hysteresis Motor

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 to select a material with high hysteresis loop area. The rotor does not carry any winding. The construction is shown in the Fig. 9.3.1 (a) while nature of hysteresis loop required for rotor material is shown in the Fig. 9.3.1 (b). 

When stator is energised, it produces rotating magnetic field. The main and auxiliary, both the windings must be supplied continuously at start as well as in running condition so as to maintain the rotating magnetic field. This field induces poles in the rotor. The hysteresis phenomenon is dominant for the rotor material chosen and due to which rotor pole axis lag behind the axis of rotating magnetic field. Due to this, rotor poles get attracted 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, due to high retentivity the rotor pole strength remains maintained. So higher the retentivity, higher is the hysteresis torque.

Initially rotor starts rotating due to combined effect of hysteresis torque as well as torque due to eddy currents induced in the rotor. Once the speed is near about the synchronous, the stator pulls rotor into synchronism. In such case, as relative motion between stator field and rotor vanishes, so the torque due to eddy currents vanishes. Only hysteresis torque is present which keeps rotor running at synchronous speed. The high retentivity, ensures the continuous magnetic locking between stator and rotor. Due to principle of magnetic locking, the motor either rotates at synchronous speed or not at all.

1. Mathematical Analysis

The eddy current loss in the machines is given by,

P_e = K_e f²₂ B²

where          K_e = Eddy current constant, f₂ = Frequency of eddy currents,

B = Flux density

We know the relation between rotor frequency f₂ and supply frequency f₁

F₂ = sf₁

where          s = Slip

P_e = K_e s² f²₁ B²

The torque due to eddy currents is given by

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

The hysteresis loss is given by,

P_h = K_h f₂ B^1.6 = K_h sf₁ B^1.6

The corresponding torque is given by,

where   T_h = P_h / s w_s K = Constant

Key Point Thus the hysteresis torque component is constant at all the rotor speeds.

2. 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 characteristics is as shown in the Fig. 9.3.2.

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

3. Advantages

The advantages of this motor are :

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

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

3) Suitability to accelerate high inertia loads.

4) Possibility of multispeed operation by employing gear train.

4. 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.

Review Questions

1. Write a note on hysteresis motor.

2. With neat sketches explain the salient features of hysteresis motor.

3. Explain the construction and working of hysteresis motor.