Requirements for Full Passive Suspension on a Bearingless Motor with Electrodynamic Axial Stabilization and Radial Permanent Magnet Bearings

This paper investigates a bearingless motor with full passive suspension. The axial direction is stabilized by an electrodynamic force generated by a specific coil configuration called a figure-eight coil. Radial directions and tilting angles are stabilized by passive permanent magnet bearings. Since the electrodynamic force increases with rotational speed, it must overcome a certain minimum threshold speed to compensate the rotor weight and the unstable axial force caused by the permanent magnet bearing. A weight relief structure with a pair of permanent magnets in attractive configuration is designed to reduce the axial load by the rotor weight. This weight relief structure reduces the balance axial position at steady state. With a smaller axial displacement, the suspension current, and, consequently, the copper losses at rated operation are reduced. An external inductor is connected in series with the coils, resulting in enhanced electrodynamic stiffness at low rotational speed. Through theoretical equations derived in a previously published manuscript, an appropriate external inductor is chosen. Forces in the radial permanent magnet bearings and weight relief structures are calculated through Finite Element Analysis. The steady state balance position and the minimum rotational speed required for full passive suspension are estimated through the theoretical equation. It is found that both the external inductance and the weight relief structure improve the axial stiffness performance for rotational speeds below 5000 rpm.