Consider the following statements about features of a hysteresis motor and select the suitable combination of correct choices.

a. Its hysteresis torque remains practically constant from locked rotor to synchronous speed.

b. Hysteresis motor is able to accelerate at any load.

c. It operates quietly and does not suffer from magnetic pulsations.

d. Hysteresis motor is subject to mechanical vibrations.

The correct answer is option 3): a, b and c are correct

Concept:

• A hysteresis motor is a single-phase synchronous motor whose operating principle is based on the effect of magnetic hysteresis. According to magnetic hysteresis, the magnetic flux density in a ferromagnetic material lags behind the magnetising force.

Construction of Hysteresis Motor

A hysteresis motor consists of a stator and a rotor

• The stator of the hysteresis motor has a main winding along with an auxiliary or starting winding. When the stator winding is fed from a single-phase supply, it produces a synchronously revolving magnetic field.
• The revolving magnetic field is accomplished by using a permanent split capacitor type construction of the motor. As a result, both the starting and the main windings of the motor remain connected in the motor circuit during running as well as at starting.
• The value of the capacitor is so adjusted that it results in a flux revolving at synchronous speed. The rotor of the hysteresis motor consists of a smooth cylinder of magnetically hard steel, without winding or rotor teeth.

Operation of Hysteresis Motor​

• When a single-phase AC supply is connected to the stator winding of the hysteresis motor, a synchronously revolving magnetic field (assumed in a counter-clockwise direction) is produced. This revolving magnetic field produced by the stator magnetises the rotor. Due to the effect of magnetic hysteresis, the magnetization axis of the rotor will lag behind the stator field axis by hysteresis lag angle (θ).
• When the rotor is stationary, then the starting torque produced in the motor is given by,

Where

φ_𝑠 is the stator flux

φ_𝑟 is the rotor flux.

φ_𝑠 and φ_r are constant.

• When the rotor accelerates, the hysteresis lag angle (θ) remains constant, since the lag angle (θ) depends only upon the hysteresis loop of the rotor and is independent of the rate at which the loop is traversed. Therefore, the motor develops a uniform torque from a standstill to synchronous speed.
• Once the synchronism is reached, the motor continues to run at synchronous speed and adjusts its hysteresis lag angle so as to develop the torque required by the load.
• A hysteresis motor can synchronise any load which it can accelerate, no matter how great the inertia of the load is. A hysteresis motor is inherently quiet and produces smooth rotation of the load. In a hysteresis motor, the number of rotor poles is equal to the number of stator poles
• Due to the small rotor, the motor operates quietly and is not involved in magnetic pulsation.
• No mechanical vibration due to the absence of teeth and winding in the rotor. so option d is wrong