A novel magnetically levitated interior permanent magnet slice motor

Abstract

A magnetically levitated motor, also known as a bearingless motor, combines the function of a magnetic bearing and motor to both levitate and rotate a rotor. This enables contact-free operation, which is advantageous in applications which require low friction, long operational lifetime, and high purity or cleanliness. In this thesis, we present the design, construction, and testing of a novel magnetically levitated interior permanent magnet slice motor. This design is targeted for use as a blood pump in extracorporeal life support (ECLS) applications. A magnetically levitated blood pump reduces the risk of blood damage that frequently occurs at the blood seal in a conventional pump due to frictional heat generation.

We have designed and constructed a bearingless motor prototype system that consists of a novel segmented dipole interior permanent magnet (IPM) slice rotor, a bearingless motor stator based on a prior design, a position sensing system, a control system, and a user interface. The segmented dipole IPM rotor contains a unique pattern of interior permanent magnets arranged to generate a dipole air gap flux pattern. The magnets are encapsulated within an electrical steel rotor structure. This simple design provides balanced force and torque capacities as compared to prior art designs and alternate topologies. In addition to the segmented dipole IPM design, we also analyze several other bearingless IPM rotor design concepts and present comparisons of their predicted performance. The sensing system is used to provide rotor angle and radial position estimates for force commutation, torque commutation, and closed-loop radial suspension feedback control.

This system utilizes an array of Hall elements to sense the rotor's rotation angle along with differential pairs of optical sensors to sense the rotor's radial position. We also process the Hall element signals to produce estimates of the rotor's axial and tilt motions. While not required for commutation or control, these additional estimates are useful for characterizing the passively stable dynamics of the slice motor. We also perform tests to experimentally characterize the bearingless motor system performance. In these experiments, we demonstrate stable levitation and open-loop rotation of the segmented dipole IPM rotor. The system achieves a maximum rotor speed of 6156 RPM with no load in air. The system also exhibits asymmetric and rotor-angle-dependent suspension dynamics, achieving a minimum unity gain loop crossover frequency of 117 Hz.

The sensing system achieves 0.17 [mu]m RMS radial position resolution at a 15.6 kHz bandwidth and 0.015 degree RMS angular resolution at a 1.17 kHz bandwidth. Given these results, the segmented dipole IPM slice motor shows promise for ECLS applications as well as other applications which require a non-contact solution. The Hall element-based sensing system also shows promise for future use in prototype bearingless motor systems to provide both angular position estimates and diagnostic estimates of the rotor tilt and axial motions.