Analysis of the three pole radial magnetic bearing

The three pole magnetic bearing has recently gained interest within the magnetic suspension research community due to its simple structure that relies on only three coils and ability to be operated by three phase power electronic converters. However, the three pole bearing is substantially more complicated to control. Control strategies based on linearized current models result in parasitic coupling between x and y radial forces, yielding significant force vector errors. Research initiatives have developed distinct three-pole geometries with varying capabilities and operating principles. This paper reviews the various research initiatives on the three-pole bearing, develops a generalized framework for considering the forces of any three-pole bearing configuration, exercises this framework to provide insights into fundamental design topology choices, and proposes a method to calculate exact control currents and an optimal bias field. The paper shows that using the exact control current calculations with the optimal bias field yield a force density improvement of up to 15.5% as compared to using conventional bias levels. Experimental validation of the force calculation framework is provided.