Design and Optimization of a Radial Magnetic Bearing Considering Unbalanced Magnetic Pull Effects for Magnetically Suspended Compressor

In magnetic suspended centrifugal compressors, the unbalanced magnetic pull (UMP) may cause saturation on power amplifier currents driving the coils of the active magnetic bearing (AMB), eventually affecting the stability of the rotor. This article addresses the issue on the stable operation of the AMB-rotor system subject to the UMP. A mathematical model of the UMP with uneven magnetization of permanent magnet (PM) and eccentricity is proposed for a surface-mounted PM synchronous motor supported by magnetic bearings. Based on this model, an improved radial magnetic bearing (RMB) with control and bias separated coil structure is designed and optimized to decrease the compensation current used to balance the UMP. First, a mathematical model containing the unevenness of the magnetization and eccentricity is established to estimate the UMP. Next, the compensation current for the UMP is analyzed based on the UMP model. Then, with the aim of minimizing the compensation current, the proposed RMB is optimized under the constraints of the starting current, the magnetic bearing volume, and the coil current density. The compensation current of the optimized RMB is reduced by 25%, compared with that of the original RMB. Finally, the experimental results on a 132-kW magnetic suspended centrifugal compressor, equipped with the optimized RMB, show the effectiveness of the modeling and optimization design.