Optimal Number of Stator Poles for Compact Active Radial Magnetic Bearings

Abstract

We present a method for finding the optimal number of stator poles for active radial magnetic bearings that minimizes the stator outside diameter. We use magnetic circuit analysis to determine the number of turns of wire to generate the worst case load capacity within limits of coil currents and flux densities. Using the analysis, we developed six types of magnetic bearing for a given value of journal diameter. We found that 3-pole bearings yield the smallest outside diameter among the six types of bearing for a journal diameter less than 50 mm; however, all the bearings have almost the same outside diameter for a journal diameter larger than 200 mm. For an infinite-length bearing, the stator diameter is a linear function of the product of numbers of poles and coil turns. We applied a linear controller design method to a heteropolar 3-pole magnetic bearing that has nonlinear coupling between the orthogonal components of bearing force. With this controller, we successfully levitated a slender-rotor system and rotated it at 3571 rpm.This paper treats the optimal number of stator poles for active radial magnetic bearings in a sense of minimizing the stator outside diameter. The magnetic circuit analysis is used to determine the number of turns of wire to generate the worst case load capacity within limits of coil currents and flux densities. Owing to the analysis, six types of magnetic bearing are designed for a given value of journal diameter. The designed stator outside diameters show that 3-pole bearings are the smallest among the six types of bearing for a smaller value of journal diameter than 50 mm, and, however, all the bearing have almost the same outside diameter when a journal diameter is larger than 200 mm. For an infinite-length bearing, the stator diameter is a linear function of the product of numbers of poles and coil turns. A linear controller design method is applied to a heteropolar 3-pole magnetic bearing that has nonlinear coupling between the orthogonal components of bearing force. A slender-rotor system is successfully levitated by the designed controller and rotated at the speed of 3571 rpm.