Hysteresis Motor

 HYSTERESIS MOTOR

Hysteresis motor is defined as a synchronous motor that is having cylindrical rotor and works on hysteresis losses induced in the rotor of hardened steel with high retentivity. It is a single phase motor and its rotor is made of ferromagnetic material with non magnetic support over the shaft.

It consists of,

 1.     Stator

2.     Single phase stator winding

3.     Rotor

4.     Shaft

5.     Shading coil

 Operation of a Hysteresis Motor:

• When supply is given applied to the stator, a rotating magnetic field is produced. This magnetic field magnetizes the rotor ring and induces pole within it. Due to the hysteresis loss in the rotor, the induced rotor flux lags behind the rotating stator flux. The angle δ between the stator magnetic field B_S and the rotor magnetic field B_R is responsible for the production of the torque. The angle δ depends on the shape of the hysteresis loop and not on the frequency.

• Thus, the value of Coercive force and residual flux density of the magnetic material should be large. The ideal material would have a rectangular hysteresis loop as shown by loop 1 in the hysteresis loop figure. The stator magnetic field produces Eddy currents in the rotor. As a result, they produce their own magnetic field.

• The eddy current loss is given by the equation shown below.

Where,

§  k_e is a constant

§  f₂ is the eddy current frequency

§  B is the flux density

 we know that,

zWhere, s is the slip and f1 is the frequency of the stator.

Therefore,

The torque is given by the equation shown below.
Now, the torque due to hysteresis loss is given by the equation shown below.

The Torque due to hysteresis is given as

• From the equation (1) it is clear that the torque is proportional to the slip. Therefore, as the speed of the rotor increases the value of Ʈ_e decreases. As the speed of the motor reaches synchronous speed, the slip becomes zero and torque also become zero.

• As the electromagnet torque is developed by the motor is because of the hysteresis loss and remains constant at all rotor speed until the breakdown torque. At the synchronous speed, the eddy current torque is zero and only torque due to hysteresis loss is present.

The speed torque curve of the motor:

• Curve 1 is the ideal curve, and the curve 2 is the practical hysteresis motor curve. The torque-speed characteristic of the hysteresis motor is different from an induction motor. Since, at the synchronous speed, the torque developed by an induction motor becomes zero, whereas in the hysteresis motor the torque is constant at all the speed even at the synchronous speed. Thus, from the curve, it is seen that the locked rotor, starting and pull out torque is equal.

• The noise level of the hysteresis motor is very low as compared to the induction motor because it operates at a constant speed and its rotor is smooth. This type of motor is smoothest running, quietest single phase motor and is used for quality sound reproduction equipment like record players, tape recorders, etc. It is also employed in electric clocks and other timing devices.

  Types of Hysteresis Motors:

There are various types of hysteresis motor by construction. They are

1.     Cylindrical hysteresis motors:It has cylindrical rotor.

2.     Disk hysteresis motors:It has annular ring shaped rotor.

3.     Circumferential :field hysteresis motor: It has rotor supported by a ring of non magnetic material with zero magnetic permeability.

4.     Axial – field hysteresis motor: It has rotor supported by a ring of magnetic material with infinite magnetic permeability.

Advantages of Use of Hysteresis Motor:

• ·         As no teeth and no winding in rotor, no mechanical vibrations take place during its operation.
• ·         Its operation is quiet and noiseless as there is no vibration.
• ·         It is suitable to accelerate inertia loads.
• ·         Multi speed operation can be achieved by employing gear train.

 Disadvantages of use of Hysteresis Motor:

• ·         Hysteresis motor has poor output that is one – quarter of output of an induction motor with same dimension.

• ·         Low efficiency

• ·         Low torque.

• ·         Low power factor

• ·         This type of motor is available in very small size only.

Applications of Hysteresis Motors:

They are widely used in

1.     Sound producing equipments,

2.     Sound recording instruments,

3.     High quality record players,

4.     Timing devices

5.     Electric clocks,

6.     Teleprinters.