Electro-Magnetic Bearings with Power Electronic Control for High-Speed Rotating Machines: A Review

This paper reviews electromagnetic bearings with power electronic control for high-speed machinery, which are termed as Active Magnetic Bearings (AMB). AMB is a contact-less type bearing which uses magnetic force to support the rotor. Due to its contact-less operation, AMB has less wear and tear, lower losses and longer life than the conventional ball bearings, and hence suitable for high speed applications. Further, AMBs are suitable for harsh operating conditions (e.g. extreme temperatures, extreme pressures, corrosive environment) as well as for clean environments (e.g. food processing and pharmaceutical industries). In spite of its advantages, its non-linear and inherently unstable characteristics make its control quite complicated. Due to recent advancements in fast switching power devices, high switching frequency power converter, high bandwidth current control, nonlinear control strategies, advanced digital controllers and sensors, AMBs have become promising for high-speed aerospace, industrial and energy applications. This paper contains a brief tutorial on the operating principle, structure, analysis and control of AMB along with a detailed survey on the various electromagnetic configurations, power converters, sensors and control techniques for AMB. Further, a design example of an 8-pole AMB for a load capacity of 180 N is presented. The performance of the AMB is evaluated using simplified magnetic circuit analysis and also using Finite Element (FE) analysis, which brings out the effect of magnetic saturation on the bearing performance. A few possible and necessary improvements to the state-of-the-art AMB are indicated.