Strong Eddy-Current Applications of Permanent Magnets

Abstract

Small eddy-current brakes for kwh meters have a practically linear torque-speed characteristic, because the induced current increases proportionally to the low relative speed between eddy-current disk and magnetic poles. In bigger designs a considerable reaction of the induced-current field shifts the eddy-current paths in the direction of movement, so that the electrodynamic force between current and magnetic poles deviates from the linear relation, reaches a maximum at a critical speed, and decreases asymptotically to zero at infinite speed, whereas the dissipated power becomes a maximum. Some investigations were made into the behavior of permanent magnets with a water-cooled setup with p.m. rotor and an air-cooled device with p.m. stator, both of cylindrical shape with 11.8 in. effective diameter and an axial length of 5.5 in. and 6 in., respectively. Maximum torques of 410 ft-lb and 304 ft-lb were obtained. Pole pieces may sometimes increase the static flux, but do not always improve the dynamic behavior. The ideal solution might be obtained with a high-induction magnet without pole pieces, but the induction in available materials is limited by the demagnetizing effect on the free pole top. Possible applications could be found in a retarder for heavy trucks in mountainous districts and in a flexible coupling, slipping with decreasing torque when overloaded.