Power-Electronic-Controlled Electromagnetic Axial Bearings for High-Speed Electrical Machines: Status Review and Design Example

Contactless, electromagnetic bearings, controlled through power-electronic converters, are a good option for high-speed electrical machines. In addition to radial forces, the shaft of a machine also experiences axial forces due to reasons which are briefly indicated in this paper. This paper reviews the present status of power-electronic-controlled electromagnetic thrust (or axial) bearing (ETB) and reports a design example of the same. Being contactless, these bearings can operate at high speeds with a long life. Their lubrication-free operation makes them suitable for operation in vacuum, contamination-free or hazardous environments. This paper discusses the construction, operation, magnetic circuit analysis, and force-current characteristics of the ETB. Differential control of ETB is reviewed, including the overall control structure and dynamic models for position and current control. Electromagnetic design of an ETB to axially support a rotor of diameter 36 mm, delivering a maximum axial force of 1,600 N, is presented. The design steps include preliminary selection of major dimensions using design equations, followed by design iterations to achieve the specified performance. The final design is evaluated using 2-D and 3-D finite element analyses. Detailed modelling and controller design are reported for the ETB designed, whose performance is verified through simulations. Limitations of the present technologies and possible future research directions in various aspects of ETB are highlighted.