Inherently decoupled magnetic suspension in homopolar-type bearingless motors

The authors have proposed bearingless motors, that is, magnetic bearings combined with motors in the same stator. It is possible to reduce their shaft length compared with a conventional motor with magnetic bearings, and to achieve higher rotational speed. Bearingless motors generate radial force by adding n +/- 2-pole flux on n-pole motor flux to make the flux distribution unbalanced. Several types of bearingless motors have already been proposed, and most of them require variations of motor flux to be taken into account to maintain stable radial position control. In this paper, homopolar-type bearingless motors are proposed and their radial force is analyzed. The rotor of a homopolar motor has two cores with salient poles, and each core is magnetized in a single pole. The important characteristics that the radial force does not vary in accordance with the rotor angle or the motor torque current are derived from analysis. It is confirmed by experiments that the radial position can be controlled nearly independently from the rotational angle and torque.