Mechanical Design of Permanent Magnet Rotors for High-Speed Electric Machines

With an ever-increasing drive towards greener energy and a reduction in emissions, high-speed electric machines (HSEMs) have an important role to play. Heavy-duty vehicle usage is growing, and electrification remains difficult, therefore there is a potentially large market for HSEMs in the form of electric assisted turbochargers (EATs).

The surface-mounted permanent magnet (SPM) rotor configuration is most suited to high-speed applications. However, as the magnets are sintered, they have low tensile strength and become a critical factor for rotor durability. Therefore, the stress analysis for SPM rotors must be accurate, however the existing literature is limited. Theoretical analysis on three-cylinder SPM rotors is unexplored for the plane strain and generalise plane strain (GPS) approaches, while it is vastly under-explored for the plane stress approach.

The aim of this project was to develop an efficient methodology of accurately predicting high-speed SPM rotor stresses to enable the selection of an optimal rotor design based on a variety of design parameters. To achieve this, an accurate closed form analysis for SPM rotor stresses was developed. The accuracy of the theoretical analysis was then verified via finite element analysis (FEA) and experimental testing. Once verified, the theoretical equations were used to develop an optimisation tool for the design of SPM rotors.

For three-cylinder rotors, the novel development of the GPS theory was shown to be the most accurate approach when compared to the FEA simulations. Utilising digital image correlation (DIC), test results were extracted from the rotor surface aligning with the FEA predictions and adding validity to the accuracy of the GPS theory. The results culminated in the development of an efficient multi-criteria optimisation tool based on the GPS theory that met the project aim.

Significant contributions to knowledge were made through the derivation of novel theoretical stress theory and the application of axisymmetric FEA rotor modelling, enabling important axial changes to be analysed. The verification testing methodologies provide a novel technique of extracting results directly from an SPM rotor, typically not attempted in the literature. The optimisation tool is a substantial contribution to existing knowledge through its implementation of novel theory and the ability to provide users with an efficient method of obtaining accurate, optimal SPM rotor designs.