What is a Hysteresis Motor : Construction, Working & Its Applications

A motor is an electrical device where the input is given in the electrical form like current or voltage and the output obtained is in the mechanical form like torque or force. Electric motors are classified into two types namely DC motors like Brushless & Brushed and AC motors like synchronous AC motor and Asynchronous AC motor. Synchronous motors are classified into two types like Nonexcited (Reluctance & Hysteresis) and Direct Current Excited. Asynchronous AC motors are Induction and Commutator. A hysteresis motor is a subclassification of synchronous motor, these motors are mainly used in a noiseless operating environment with constant speed. Few of the application of hysteresis motor is a sound recording and sound-producing experiments like electric clocks, tape recorders, record players, etc.

What is Hysteresis Motor?

Definition: A hysteresis motor works on the principle of hysteresis losses (it is a loss occurred due to magnetization and demagnetization of the material depending on the direction of flow of current). It can be operated either using a single phase or three phases and in a noiseless operating environment, it maintains a constant speed. The torque generated in the motor is due to the hysteresis and eddy current which is induced due to stator winding. There are 4 types of hysteresis motor they are

• Cylindrical type
• Disk type
• Circumferential-Field type
• Axial-Field type

Constructional Feature of Hysteresis Motor

The main parts of hysteresis motor are stator and rotor, the stator is similar to single-phase or three-phase (using three phases balanced winding) motor. Where single-phase motor is classified into two types shaded pole type and permanent split capacity type.

• The advantage of shaded pole type motor is that it occupies less area and it requires less cost, but the disadvantage is that the torque generated is not uniform causing noisy operation.
• By using a split capacitive type rotor a balance two-phase supply is provided, which generates uniform torque with noiseless operation. But the disadvantage of this is that it occupies more area and the cost is high.

The rotor is made up of hysteresis material, containing a number of hysteresis rings (made up of hard chrome or cobalt or steel) which has a very large hysteresis loop. It is used to reduce the eddy current losses. Since it has a larger weight to overcome this disadvantage we use a non-magnetic material (also known as a spider) made up of aluminum, which is present at the center portion of the motor. The main advantage of this non-magnetic material is that it lightens the rotor weight, by improving the speed of the motor and reduces the value of inertia.

Working Principle of Hysteresis Motor

Hysteresis motor starts like a single-phase induction motor and runs like a synchronous motor, it can be observed from the following conditions.

Starting Condition

When an AC supply is provided to the stator, a magnetic field is generated both on main and auxiliary windings of the motor is of the constant rotating magnetic field. Initially, rotors start with eddy current torque and then reach hysteresis torque. Once it reaches synchronization the stator makes the rotor into synchronism where the torque due to eddy current is zero.

Steady State Running Condition

At steady-state running condition (or synchronous condition) the stator induces poles on the rotor, where the hysteresis effect produced in the circuit will make the rotor flux lag behind the stator flux at an angle α. Where α is the angle between stator and rotor magnetic fields (BS and BR). Hence the rotor experiences attraction towards the rotating stator, with a torque called hysteresis torque, which does not depend on the speed of the rotor (higher the residual magnetism, higher is the hysteresis torque). The presence of high retentivity allows the motor to operate either with synchronous speed or operates normally.

Equation of Hysteresis Torque in the Hysteresis Motor

Eddy current equation is given as

P_e = k_e f₂² B²  ……… 1

Where

k_e = constant

f₂ =frequency of eddy current

B = flux density

We know that f₂ = sf₁ ……….2

S = slip, f1 = stator frequency

Therefore P_e = k_e s² f₁² B²  ..……..3

Torque equation is given by

Ґ_e = p_em/ s w_s  …….4

Ґ_e = k^’ s ………5

Where torque is inversely proportional to slip, which means as rotor speed increases the value of torque decreases and also if the motor speed reaches synchronous speed the slip and torque become zero.

Where k’ = k_e f₁² B² / w_s  = constant

Hysteresis Power Loss and Ph in Hysteresis Motor

Hysteresis loss is given by

P_h = k_h f₂ B^1.6 ……….6

Or

P_h = k_h sf₁ B^1.6 …..….7

The torque due to hysteresis is given by

Ґ_h = p_h / s w_s  = k_h f₁ B^1.6 / w_s = k’’ = constant………..8

We can observe from the above equation that if the torque which is developed due to hysteresis loss remains constant until the torque reaches the breakdown point, and at synchronous speed, the torque becomes zero.

Ph in Hysteresis Motor

The hysteresis losses generated in the motor are directly proportional to the area of cross-section under the hysteresis curve. Where these losses are dissipated in the form of heat. The losses can be derived from the following equations,

The dissipated energy in the rotor is given as

W = N_s E_h       (E_h  = hysteresis loss per revolution)………9

 Where power is dissipated in the form of heat which is given by

 P_h = W/t = N_s E_h / 60 …………10

The mechanical power which drives rotor is given by

P_h = 2Π N_s T_h   / 60  ……11

On equating both the power we get

2Π N_s T_h   / 60 = N_s E_h / 60 ………12

T_h = rotors exerted torque [N-m] ; E_h = hysteresis energy.

Torque-Speed Characteristic of Hysteresis Motor

Torque-Speed characteristic of hysteresis motor can be explained using the following graph, where the x-axis represents torque and the y-axis represents speed.

• The torque (starting and running) generated in this motor is approximately the same.
• The torque generated by the hysteresis motor at synchronous speed is constant.
• The rotor, starting torque and pull out torque are equal at this condition. Hence the motor operates noiselessly with constant speed.

Advantages

The following are the advantages of the hysteresis motor

• Absence of mechanical vibrations
• It operates noiselessly
• Mainly suitable for accelerating inertia loads

Disadvantages

The following are the disadvantages of hysteresis motor

• The output obtained is ¼ times of induction motor
• Small in size
• Torque is less

Applications

The following are the applications of hysteresis motor

• Record players
• Electric clocks
• Timing devices, etc.

FAQs

1). What are hysteresis losses?

It is a loss occurred due to magnetization and demagnetization of the material depending on the direction of the flow of current.

2). What is Schrage motor?

A Schrage motor is a polyphase commutator motor whose characteristics are shunted, where the rotor has two windings, one connected to supply and other to the commutator.

3). What causes hysteresis?

It is caused due to magnetization and demagnetization of the material depending on the direction of the flow of current.

4). What is a synchronous reluctance motor?

It is an AC synchronous motor which converts electrical power into mechanical power

5). What is the principle of hysteresis motor?

A hysteresis motor works on the principle of hysteresis losses (it is a loss occurred due to magnetization and demagnetization of the material depending on the direction of flow of current).

A motor is an electrical device that converts electrical energy into mechanical energy. This article overviews on synchronous hysteresis motor which works on the principle of hysteresis loss. The torque generated remains constant before reaching synchronous speed and becomes zero after reaching synchronous speed. The hysteresis losses is the area underneath the B-H curve. The torque (starting and running) generated in this motor is approximately the same. The main advantage is that it operates soundlessly.