Design of Lorentz force-type integrated motor-bearing system using permanent magnets and concentrated windings

Abstract

The integrated motor-bearing system integrates both functions of active magnetic bearing (AMB) and electric motor into a unit. It can realize a compact mechanical structure, yet preserving inherent advantages of AMB. In return, however, the layout and analysis of its magnetic field and the controller design become a little complex because the motoring and levitation of the rotor has to be simultaneously controlled. Lorentz force is bi-directional and holds good linear property, by virtue of which the analysis and design of the system can be made easy. In addition, it can be obtained even if no ferrite core is employed, so that it is possible to avoid such troubles as iron loss, magnetic saturation, hysteresis, eddy current effect, etc. In recent years, several Lorentz force-type integrated motor-bearing systems were introduced, but the development of them were limited to specific conditions. In this work, a generalized approach to generation of torque and radial force in integrated motor-bearing systems using Lorentz forces is presented. The constraints for arrangement of permanent magnets and distribution of concentrated coil windings are derived in the disk-type and cylindrical type configuration. The difference between the number of permanent magnets on the rotor and the magnetic poles for motoring has to be an integer multiple of the number of windings. The phase difference of position control current is determined by the rule that the number of poles for position control should be more or less than that of the motoring poles by two. The radial control force is independent of the motoring torque and the rotational speed of the rotor, not requiring a complicated demodulation scheme. Some feasible combinations of design parameters satisfying the constraints are listed. A dual disk rotor structure with a coreless stator for the Lorentz force-type integrated motor-bearing system is proposed, which can realize no negative stiffness in the radial direction. The m...