Design of Slotless Single-Drive Bearingless Permanent Magnet Motor for High-Speed Applications

For electrical motors, three translational (x-y-z) and two tilting (theta(x)-theta(y)) motions of the rotor must be restricted. Ball bearings are commonly used for motion restrictions; however, they cannot be applied to high-speed motors with large DN values. The rotor in this study has aerostatic bearing support in radial (x-y) and tilting (theta(x)-theta(y)) motions. However, if an additional magnetic bearing/actuator is applied to the motor for axial motion control (z), the motor size and shaft length increase. To avoid this problem, a novel bearingless motor structure with a magnetically integrated bearing function was developed in this study. The structure combines the functions of both torque production (theta(z)) and axial force generation (z) in one unit. It uses only a set of three-phase slotless windings and three-phase voltage source inverter. Such bearingless motors are referred to as single-drive bearingless motors. The d-axis and q-axis currents regulate the axial suspension force and torque, respectively. This paper presents the structure, operation principle, 3D-finiete element calculation results, and experimental results. The fabricated prototype bearingless motor with a shaft diameter of 30 mm can actively control the axial motion of the rotor at 60,000 rpm, that is, with a DN value of 1,800,000.