Hybrid Active–Passive Actuation Approach of Passively Levitated Thrust Self-Bearing Machines

Passively levitated thrust self-bearing machines with combined armature windings have recently been introduced. These machines reach higher reliability and compactness while being lower cost than their actively suspended counterparts. However, they result in small axial restoring force and stiffness at low speeds since the axial guidance relies on passive phenomena arising from induced currents. The axial destabilizing stiffness produced by the permanent magnet centering bearings as well as the axial load, therefore, prevent the rotor from levitating below a threshold speed. This article introduces and develops a solution to this problem based on a hybrid active–passive actuation approach of these thrust self-bearing machines. The approach consists of regulating the rotor axial position through the direct-axis component of the currents flowing in half of the combined winding until reaching the threshold speed beyond which passive axial suspension can be achieved. Since the active operation solely requires the addition of an axial position sensor, the benefits related to passive levitation are only slightly affected. Extensive experimental investigations corroborate the force and torque model derived for the active operation and highlight that the transitions between suspension modes have a limited impact on the rotor axial position and spin speed, validating the proposed hybrid active–passive actuation approach.