Modeling and Control Strategies of a Novel Axial Hybrid Magnetic Bearing for Flywheel Energy Storage System

This article presents modeling and control strategies of a novel axial hybrid magnetic bearing (AHMB) for household flywheel energy storage system (FESS). The AHMB combines a passive permanent magnet (PM) magnetic bearing (MB) and an axial active MB in one unit, thus can offer benefits such as compactness of the structure, high load capacity, and low power consumption. A coupled magnetic field-circuit method is proposed for modeling this bearing. Compared with the regular equivalent magnetic-circuit (EMC) method, it introduces special concerns of the complex air-gap field, calculated from the conformal transformation of the air-gap geometry, to take into account the effect of flux fringing and leakage. A low-cost Hall sensor is used to replace the expensive Eddy-current position sensor to measure the changes in air gap flux density caused by the rotor axial displacement. Therefore, a novel flux density feedback control is adopted instead of the traditional position feedback control to directly stabilize the magnetically suspended flywheel. A prototype of this bearing is designed and fabricated, which is tested in a small-scale FESS. The simulation results and the experimental results demonstrate that the proposed model provides better accuracy than the traditional EMC and the flux-density feedback control strategy based on a low-cost Hall sensor can achieve a consistent performance with the traditional position feedback control strategy.