Hysteresis synchronous motor

Reference notes

The hysteresis synchronous motor has a smooth cylindrical rotor of HARD magnetic material driven by a conventional rotating stator field. Torque comes from the rotor's magnetization M lagging the stator H-field by a fixed hysteresis angle α — independent of speed. Result: flat torque-speed curve, exact synchronous lock-in, and very smooth quiet rotation. Use Next → to walk through the construction, mechanism, operating phases, and where this niche motor is still used.

Construction

• Stator — conventional 3-φ distributed winding (or shaded-pole / capacitor-start for 1-φ models). Standard induction-motor stator.
• Rotor — smooth cylindrical bar of HARD magnetic material (high coercivity): chrome-cobalt steel (Vicalloy), Alnico, or special vacuum-melted alloys.
• NO laminations (would break up the hysteresis effect), NO salient poles, NO windings, NO cage.
• Power range: fractional horsepower (mW to ~1 kW practical maximum).

Torque mechanism

When the rotor is subjected to a rotating stator field, its magnetization vector M traces a small loop in the rotor's B-H plane. Because the material has wide hysteresis, M always LAGS the applied H-field by a fixed angle α determined by the loop shape:

α is INDEPENDENT of speed → torque is independent of speed → flat T-s curve.

Two operating phases

1. Starting / accelerating (s > 0): two torque components.
   • Hysteresis torque T_hyst — constant, independent of slip.
   • Eddy-current torque T_eddy — proportional to slip s, similar to induction motor action.
2. Synchronous lock-in (s = 0): eddy-current torque drops to zero. Hysteresis torque remains. The rotor magnetizes to a steady pattern with fixed lag α relative to the rotating stator field. Rotor runs at exactly synchronous speed n_s.

Unique among synchronous motors: pull-in torque = pull-out torque (no distinct breakdown). The motor doesn't snap into sync — it transitions smoothly.

Properties

• Self-starting — no auxiliary winding or capacitor needed (constant T from rest).
• Locks at exact synchronous speed — ideal for timekeeping when mains frequency is stable.
• Very smooth rotation — no cogging (smooth rotor), no slot harmonics (uniform rotor), no salient features.
• Quiet operation — minimal acoustic noise.
• Low efficiency — 5-30 %. The hysteresis loss IS the operating mechanism, so significant power is lost as rotor heating.
• Poor power factor — 0.3–0.5 typical.

Applications

• Precision analog clocks and timers — exact synchronism with 50/60 Hz mains makes for perfect timekeeping. Industrial control-panel clocks, aviation timers.
• Tape recorder and turntable capstan drive — Studer, Revox, EMT professional audio recorders used hysteresis motors for jitter-free playback.
• Gyroscope rotors in inertial navigation (historical) — smooth rotation and exact reference speed.
• Small fans, electric shavers, dental tools — sealed simple drive.
• Synchronous-drive timers on industrial equipment.

Why it's becoming niche

BLDC motors with electronic drives match the hysteresis motor's smoothness and exact-speed properties at much higher efficiency (~85 % vs ~20 %) and with variable speed via VFD. Quartz timing replaced hysteresis-synchronous clocks in the 1980s. MEMS gyroscopes replaced spinning gyroscope rotors. Modern usage is mostly legacy / specialty / audiophile niches.

Comparison snapshot