What Are Hysteresis Motors?

Hysteresis motors are single-phase synchronous AC motors that develop torque through the magnetic hysteresis properties of a hardened ferromagnetic rotor, rather than through wound windings or salient-pole reluctance. Because the rotor acquires and retains magnetic polarization as it is swept by the rotating stator field, the motor produces a continuous and constant torque from standstill all the way up to synchronous speed, after which it locks into synchronism and runs without slip. This smooth, stepless acceleration distinguishes hysteresis motors from other small synchronous motor types and makes them well suited to applications that require constant-speed operation with minimal vibration and noise.

The motor draws on principles from electrical machine theory, materials science, and magnetics. Stator construction is essentially identical to that of a conventional single-phase induction motor, typically using a shaded-pole or capacitor-start winding to establish a rotating magnetic field. The distinguishing design is entirely in the rotor: a solid cylinder of high-hysteresis-loss magnetic alloy mounted on a non-magnetic shaft arbor, with no slots, conductors, or windings of any kind.

Operating Principle

When the stator is energized, the rotating magnetic field induces poles in the rotor material. Because the rotor alloy has high coercivity, the induced polarization lags the driving field by a phase angle proportional to the hysteresis loop area of the rotor material. This angular lag between the rotor pole axis and the stator field axis produces a sustained torque called hysteresis torque. Unlike reluctance torque, which depends on rotor position relative to field alignment, hysteresis torque is constant at every speed from zero to synchronous. At synchronous speed the rotor locks in, the induced poles become permanently oriented, and the motor continues to run as a permanent-magnet synchronous machine. An IEEE Xplore conference paper on hunting in high-speed hysteresis motors examines how the rotor material properties influence stability at elevated operating frequencies, a factor relevant to high-precision spindle applications.

Rotor Materials and Design

The rotor cylinder must combine high remanence, high coercivity, and adequate electrical resistivity to limit eddy current losses. Historically, hardened cobalt steel alloys with cobalt content in the range of 35 percent were the standard choice, providing B-H loop areas large enough to produce commercially useful torque densities. Alnico and chrome-steel alloys have also been used. The solid cylindrical form is essential: the absence of slots eliminates the tooth-ripple flux variation that produces vibration in slotted rotors, which is the primary reason hysteresis motors achieve their characteristically quiet operation. Because the rotor carries no winding or electrical connections, the construction is mechanically simple and highly reliable. The Bodine Electric overview of synchronous AC motor types provides a practical comparison of hysteresis and reluctance motor design tradeoffs for precision motion applications.

Performance Characteristics

The constant torque-speed characteristic of a hysteresis motor means it can self-start under full load and synchronize any inertia it can accelerate, without the need for auxiliary starting arrangements beyond the stator winding configuration. Speed accuracy in steady-state synchronous operation is determined entirely by the supply frequency, which makes the motor directly replaceable by a frequency-controlled drive for variable-speed precision applications. The Electrical4U reference on hysteresis motor working principles notes that the absence of rotor teeth also eliminates cogging torque, which is the periodic detent force that causes positional inaccuracy in switched and slotted motor designs.

Applications

Hysteresis motors have applications in a wide range of disciplines, including:

• Audio and video turntable drives requiring constant and precise rotational speed
• Electric clock mechanisms and timing instruments
• Laboratory centrifuges and precision spindles
• Medical and dental instrument drives where quiet operation is essential
• Tape transport systems and precision positioning mechanisms