Hysteresis motor is a single-phase motor whose operation depends upon the hysteresis effect i.e. magnetization produced in a ferromagnetic material lags behind the magnetizing force.
Construction of Hysteresis Motor
It consists of:
- a stator using permanent capacitor type construction. Both the windings (i.e. main as well as starting winding) remain connected in the circuit during running operation as well as at the start. The value of capacitance is so adjusted as to result in a flux revolving at synchronous speed.
- a rotor consisting of a smooth cylinder of magnetically hard steel, without winding or teeth.
Operation of Hysteresis Motor
- When the stator is energized from a single-phase supply, a synchronously-revolving field (assumed in the anticlockwise direction) is produced due to split-phase operation.
- The revolving stator flux magnetizes the rotor. Due to hysteresis effect, the axis of magnetization of the rotor will lag behind the axis of stator field by hysteresis lag angle α as shown in the figure. If the rotor is stationary, the starting torque produced is given by:
- After starting, the rotor rotates at synchronous speed with a uniform torque. Because φr and φs are now constant. When the rotor accelerates, the lag angle α remains constant since the angle α depends merely upon the hysteresis loop of the rotor and is independent of the rate at which the loop is traversed. The motor, therefore, develops constant torque from the standstill to synchronous speed.
- After reaching synchronism, the motor continues to run at synchronous speed and adjusts to run at synchronous speed and adjusts its torque angle so as to develop the torque required by the load.
Ts α φsφrsinα
Where, φs = stator flux
φr = rotor flux
Characteristics of Hysteresis Motor
- A hysteresis motor can synchronize any load which it can accelerate, no matter how great the inertia. It is because the torque is uniform from standstill to synchronous speed.
- Hysteresis motor produces very smooth and soundless rotation.
- The rotor takes on the same number of poles as the stator field. Thus by changing the number of stator poles through pole-changing connection, we can get a set of synchronous speeds for the motor.
Applications of Hysteresis Motor
Due to quiet operation and ability to derive high-inertia loads, hysteresis motors are used for timing devices and precision audio-equipment.
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