Design and Unbiased Control of Nine-Pole Radial Magnetic Bearing

Abstract

Typical radial active magnetic bearings are structurally symmetric. For example, an eight-pole bearing uses two opposing pairs to control one axis by winding the pair in series. The magnetic force produced by an active magnetic bearing is quadratically proportional to coil currents and inversely proportional to the square of the gap between the bearing and the journal. Bias linearization is widely used to linearize the relationship of coil currents to the magnetic force. However, the bias currents increase ohmic losses and require a larger than necessary capacity of power amplifiers to supply the sum of bias and control currents. Unbiased control of symmetric bearings has the critical issue of slew-rate limiting. Unbiased control of unsymmetrical bearings can eliminate the need for bias currents while avoiding slew-rate singularity except in extreme cases. Although a generalized inversion of the force–current relationship of unbiased unsymmetrical bearings has been proposed previously, no experimental validation is reported. The main objective of this research is to implement the unbiased control strategy and show that exactly the same linear control strategy for eight-pole symmetric bearings can be applied to nine-pole unsymmetrical bearings on industry-scale compressor test rigs. Two test rigs are built: one with eight-pole symmetric bearings and another with nine-pole unsymmetrical bearings. Linear control algorithms are designed and applied. Both control algorithms are linear and consist of lead filters and notch filters. The test results show that the linear control design used for unsymmetrical bearings can achieve the same level of stability that the symmetric bearings provide, satisfying the sensitivity criterion specified by ISO 14839-3.