Hysteresis Motor: Working Principle & Applications

What is Hysteresis Motor?

A hysteresis motor is a self-starting synchronous motor whose smooth magnetic rotor develops torque through hysteresis coupling with the rotating stator field. The energy represented by the rotor material’s B-H loop appears as hysteresis losses while the rotor slips. Small motors are often single-phase, but polyphase versions also exist. The active rotor is a hard ferromagnetic material mounted on a non-magnetic hub or support.

Hysteresis Motor Construction

Construction varies, but a small single-phase design can contain these parts:

1. A laminated stator core
2. Main and auxiliary single-phase stator windings
3. A smooth hysteresis-alloy rotor sleeve
4. A shaft and non-magnetic rotor support
5. A phase-shifting element, such as an auxiliary winding, capacitor or shading coil, according to the design

The stator creates the rotating field and the rotor provides hysteresis coupling:

• Stator: The stator of a hysteresis motor uses phase-displaced currents or a polyphase supply to produce a rotating field at synchronous speed. A single-phase motor can use main and auxiliary windings, a capacitor or shaded poles. The starting method and winding arrangement are design-specific.
• Rotor: The Rotor of hysteresis motor is a smooth sleeve or disc made from a heat-treated magnetic alloy with high coercivity and useful remanence. Its wide hysteresis loop permits the rotor magnetisation to lag the rotating field and produce torque.
  The active rotor normally has no windings, slots or teeth. A non-magnetic hub mechanically connects the magnetic sleeve to the shaft.
  Material resistivity and rotor geometry influence eddy current loss and any associated induction torque.

Working Principle of Hysteresis Motor

A hysteresis motor accelerates from rest under hysteresis torque and may also receive induction torque from rotor eddy currents, depending on its construction. Unlike a normal single phase induction motor, it pulls into synchronism and then runs without fundamental-frequency slip while the load remains below its stability limit.

At the Starting Condition

• When energised, the stator windings produce a rotating magnetic field at synchronous speed.
• In a permanent-split-capacitor or comparable single-phase design, the main and auxiliary circuits maintain the rotating magnetic field during starting and running. Other designs use different phase-producing arrangements.
• At standstill, the rotating field repeatedly magnetises the rotor. The magnetisation lags the applied field because of hysteresis, creating starting torque.
• Rotor conductivity can also allow eddy currents and induction torque during acceleration. Their size depends on the alloy and geometry; hysteresis torque does not require a squirrel-cage winding.
• Below synchronous speed, the rotor experiences the field at slip frequency and continues to accelerate if developed torque exceeds load and loss torque.
• The motor is therefore asynchronous during acceleration, but calling it an induction motor is only appropriate when eddy-current torque makes a material contribution.

At Steady State Running Condition

• As the rotor approaches synchronous speed, magnetic coupling pulls it into step with the rotating field if the load and inertia are within the motor’s pull-in capability.
• At ideal synchronism, the rotor and fundamental rotating field have no relative speed. Fundamental-frequency induction torque then falls to zero, although harmonics and non-ideal fields can still cause rotor loss.
• The stator field establishes a magnetic pole pattern in the hard magnetic rotor. At synchronism that pattern holds a fixed position relative to the rotating field, so the rotor behaves like an induced permanent-magnet pattern.
• Coercivity, remanence, rotor volume and air-gap flux density influence hysteresis torque. Below synchronism the idealised torque is approximately independent of speed for a fixed field, while at synchronism the magnetic pattern remains locked to the stator field.
• The area enclosed by the material’s B-H loop represents hysteretic energy per unit volume for one complete magnetisation cycle. Actual rotor loss and torque also depend on field distribution and material behaviour.
• At light load, the rotor magnetic axis follows the stator field with a small coupling angle. A non-zero angle supplies bearing, windage and load torque.
• As load torque rises, the coupling angle increases and the motor approaches its pull-out limit.
• The rotor magnetic axis lags the stator rotating magnetic field by coupling angle δ_h. This angular displacement produces synchronous torque.
• Increasing load increases the lag toward δ_max. Beyond the available pull-out torque, the rotor loses synchronism.
• Within the stable range, the rotor magnetic pattern remains coupled to the moving stator field and the shaft runs at synchronous speed.

• With zero fundamental slip, synchronous hysteresis coupling supplies the average load torque. The motor speed is then fixed by supply frequency and pole count until overload causes pull-out.

What is Hysteresis Power Loss, P_h in Hysteresis Motor?

A simplified empirical expression for rotor hysteresis loss is:

where
f_r is the flux-reversal frequency seen by the rotor in Hz,
B_max is the peak flux density used by the model in T,
P_h is hysteresis heat loss in W, and
k_h is a material and geometry coefficient. The exponent and coefficient must come from data for the actual alloy, waveform and operating range.

What is the Equation of Hysteresis Torque in the Hysteresis Motor?

In the idealised model, loss power is proportional to slip frequency, so dividing by relative angular speed gives an approximately speed-independent hysteresis torque below synchronism. Real torque changes with flux density, temperature, harmonics, material response and saturation.

What is the Torque-Speed Characteristic of Hysteresis Motor?

The Torque-speed characteristics of hysteresis motor are shown below.
Ideal hysteresis torque is nearly constant from standstill to synchronous speed for a fixed applied field. The motor can pull a load into synchronism only if its accelerating torque and time are sufficient for the load inertia and opposing torque.
At synchronous speed, stable operation appears as a vertical segment from light load up to pull-out torque.

What is the Speed-Torque Characteristics?

The second speed-torque plot shows the same transition to synchronous operation.
During acceleration, hysteresis torque is approximately constant and design-dependent eddy-current torque may add to it. At synchronous speed, average fundamental induction torque vanishes and hysteresis coupling carries the load. Torque is not unlimited; excessive load causes pull-out.

What is the Starting Torque of the Hysteresis Motor?

Let Φ_S represent stator flux.
Let Φ_r represent the rotor’s magnetic flux. Because of hysteresis, Φ_r lags Φ_S by angle α.
The simplified interaction model gives starting torque as:

K is a proportionality constant that depends on motor geometry, winding and unit conventions.

Types of Hysteresis Motors

Hysteresis motors can be grouped by rotor shape and dominant air-gap flux direction:

1. Cylindrical hysteresis motors: A cylindrical rotor sleeve operates in a radial air gap.
2. Disk hysteresis motors: A flat disc or annular rotor operates with axial or combined flux.
3. Circumferential-Field hysteresis motor: The dominant rotor magnetisation and flux path are circumferential. A practical non-magnetic support has low but finite relative permeability.
4. Axial-Field hysteresis motor: The working flux crosses the air gap mainly along the shaft axis. Magnetic back iron has high but finite permeability.

Advantages of Hysteresis Motor

The main advantages of hysteresis motor follow from its smooth rotor and synchronous operation:

• A toothless, winding-free rotor avoids rotor electrical connections and reduces cogging torque
• Smooth torque and low cogging suit applications that need quiet operation
• Nearly uniform accelerating torque can pull suitable inertia loads into synchronism
• Gearing or a different supply frequency and pole arrangement can provide other output speeds

Disadvantages of Hysteresis Motor

The disadvantages of hysteresis motor include:

• A Hysteresis motor generally provides less output for a given frame than an induction motor; the ratio depends on each design and cannot be fixed at one-quarter
• Lower efficiency than many modern motor and drive alternatives
• Limited pull-in and pull-out torque for the motor size
• Low power factor in many line-fed designs
• Commercial use is concentrated in small fixed-speed and specialist motors

Applications of Hysteresis Motor

Hysteresis motors suit low-noise drives that need speed tied closely to supply frequency, including:

1. Acoustic and sound-producing equipment
2. Sound-recording instruments
3. High-quality record players
4. Timing mechanisms
5. Electric clocks
6. Legacy teleprinters and chart drives