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Hysteresis motor construction and principle

Short answer

A hysteresis motor has a smooth hard-steel rotor with high retentivity. The rotating field magnetizes it, and hysteresis lag produces constant torque from standstill to synchronism; at synchronous speed it locks as a permanent-magnet rotor. It runs silently and is used in clocks, tape recorders, and timing devices.

Describe the construction and working principle of hysteresis motor. (Write a short note on hysteresis motor.)

A hysteresis motor is a single-phase synchronous motor whose rotor is a smooth cylinder of hard magnetic material; it develops torque due to hysteresis in the rotor, and runs at synchronous speed silently.

Construction

  • Stator: split-phase, capacitor or shaded-pole winding that produces a rotating field.
  • Rotor: a smooth cylinder of high-retentivity hard steel (e.g. cobalt or chrome steel) with large hysteresis loop, mounted on a non-magnetic arbour. No teeth, no windings.
   stator (rotating field)
   _____________________
  |  ________________   |
  | | hard steel ring|  |
  | |  (no slots)    |  |
  | |_____arbour_____|  |
  |_____________________|

Working principle

  1. The rotating stator field magnetises the rotor. Because of hysteresis, rotor magnetisation lags the stator field by a hysteresis angle α\alpha; this lag produces a torque (hysteresis torque) that is constant from standstill to synchronous speed.
  2. Eddy currents in the rotor also add torque below synchronous speed.
  3. At synchronous speed, the rotor becomes permanently magnetised and runs locked to the field as a permanent-magnet synchronous motor.

Features and applications

  • Smooth, quiet, vibration-free; constant torque during run-up; can synchronise any load it can accelerate.
  • Used in electric clocks, tape recorders, record players and timing devices.

✓ Answer checked by Claude Sonnet 5.5

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