On hunting in hysteresis motors and new damping techniques

"Hunting," or small random excursions of the rotor, in synchronous motors may cause objectionable time-displacement errors in video headwheel drives and other precision scanning systems. The idealized model of the hunting motor is seen to be a simple second-order linear system. Hysteresis motors behave very much like classical synchronous motors, except that the motor magnetic pole definition varies slightly with torque. During these excursions, there is some natural damping comprised of a linear eddy current effect and a nonlinear hysteresis effect. The nonlinear effect causes the oscillatory to vary inversely with angular motion. A qualitative theory is offered to explain this. New damping techniques have been developed using only electrical components, and include both active and passive circuits. All of them, however, operate without need of explicit motion detection. In an active damping circuit developed for a satellite video recorder, a reduction in time-displacement error of an order of magnitude has been achieved. No loss of motor torque capacity or efficiency is incurred.