Summary of a Bearingless Motor with Passive Electrodynamic Axial Suspension

Either active control or passive stabilization can be used to achieve non-contact magnetic sus-pension in magnetic bearings and bearingless motors. As the rotational speed is increased, the d-axis current becomes harder to regulate, thus motivating the research of full passive suspension. Permanent magnet bearings and reluctance forces cannot stabilize all Degrees Of Freedom (DOF) by themselves, therefore, complementary passive methods are required to achieve full passive suspension in bearingless motors. Among several passive suspension techniques, electrodynamic suspension is chosen for this machine, because the same coils that generate motor torque can be used for passive stabilization and it does not require superconductors with a dedicated cooling system. Passive electrodynamic suspension in rotating machines has been already explored in several structures. It was applied to both passive electrodynamic bearings [1] and bearingless motors [2]–[4]. This paper reports an improvement of a previously reported bearingless motor with passive axial suspension through figure-eight coils previously presented in an IEEE IAS cosponsored conference paper [4]. It suffered from low axial force due to the low radial flux density. This electrodynamic suspension principle on axial direction is similar to previously published ma-chines [2], [3]. In this paper, magnetic flux density and, consequently, axial electrodynamic force at low rotational speed are significantly increased when compared to the previous model [4]. A prototype machine is built to confirm the axial force characteristics.