An Analysis of Radial Forces and a Rotor Position Control Method of Reluctance Type Bearingless Motors

Abstract

Magnetic bearings have many advantages of maintenance free, oil free and active dumping. Although these bearings are suitable for high speed machines, an increase in the shaft length of a motor with two magnetic bearings causes low critical speeds. A bearingless motor is a combined motor with a magnetic bearing, so the shaft length can be short. Radial force for position control is produced with superimposed (n±2)-pole flux on n-pole motor flux. Therefore, the radial force may be influenced with variations in motor current. If so, the radial control system may be unstable depending on load conditions.
In this paper, a model of a 5-axis controlled bearingless motor is shown. It is explained with the model that the transfer characteristics of radial position control have variations depending on the motor current. To control radial positions at any motor operations, a decoupling control method is proposed. The effectiveness of the proposed control system is confirmed in experiments.