Design and implementation of a radial magnetic bearing with permanent magnet bias

A new yoke winding for a radial magnetic bearing with permanent magnet bias flux is investigated and shown to reduce the copper loss and copper cost, by 32% in the prototype built. Magnetic circuit analysis neglecting fringing was found to give a poor estimate of the permanent magnet bias flux and 3D finite element analysis (FEA) would be needed to model it. However, 2D FEA was sufficient to model the field in the plane of the stator laminations, providing that the correct bias flux was imposed as a boundary condition. Solutions for the axial magnetic vector potential component A_z were obtained, with a discontinuity prescribed across a cut from the inner to outer domain boundary. A small prototype was built and tested on a shaft whose other end was supported by a ball bearing race with free angular movement. For each of the horizontal and vertical directions, closed loop control was applied with Hall elements providing the shaft position signal. A lead-lag controller, designed using Matlab Simulink and implemented in Turbo Pascal 6, levitated the rotor.