Rotor Designs in Pyrolytic Graphite Bearingless Motor for Increased Passive Radial Stiffness

This paper proposes a novel rotor design of diamagnetic bearingless motors. The rotor material is pyrolytic graphite (PG) so that five degrees of freedom of the rotor are passively stabilized above a permanent magnet array. The stator consists of two components of a Halbach permanent magnet array and an axial-flux stator core with twelve teeth holding the eight-pole three-phase winding. Passive levitation forces are generated between the permanent magnet unit and the PG rotor. However, these stiffness are low compared to typical passive magnetic bearings, in particular, the radial stiffness is seriously low. In this paper, a new rotor design is proposed to improve the radial stiffness, and the proposed PG rotor is fabricated and tested. In experiments, it is demonstrated that the radial stiffness is increased by 2.45 times. In addition, the proposed rotor can improve maximum rotational speed by 43% because a critical speed is changed to high-speed region.