On the Mechanical Design of High-Power High Speed Radial Flux Permanent Magnet Electric Propulsion Machines for Aerospace Applications

Abstract

To combat the effects of climate change, more industries are beginning to move away from fossil fuels and towards green energy via electrification. This process is currently underway in the aerospace industry. The demand for more power-dense machines grows as manufacturers look to increase the capabilities of electric machines for use in propulsion applications within all-electric aircraft. Recent advances and research into aerospace electrification show that high-speed radial flux permanent magnet synchronous machines have the potential to power small electric aircraft and air taxis. However, issues arise when considering the high-speed nature of this type of motor topology. The immense centrifugal force that acts upon the rotating assembly, which is compounded with thermal expansion and interference fits, substantially increases the rotor components' stress, strain, and deformation. It is also prone to vibrational failure as a result of shaft whirling and resonance. This thesis will first review the structure and operation of electric machines. Electric machine topologies and architectures are briefly explained. A review of the current state of-the-art electric machines used in aerospace applications will also be discussed to provide background on what trends exist in terms of the power density and speed of high-power motors. This thesis details the design process of two high-speed, high-power radial flux permanent magnet propulsion machines. The first motor is a 20,000 RPM 150 kW motor. The features and mechanical design considerations of Motor A will be thoroughly explored. The second motor is a 20,000 RPM 1 MW motor introduced in this thesis. The second motor will only consider the rotating assembly in its analysis due to its significance in determining the power density and safety from multiple failure modes, such as magnet retention failure and vibrational failure. Optimization is not the express goal of this motor, but rather a detailed explanation of how to iterate and improve upon the mechanical design using various results, such as critical speed, eigenfrequencies, strain energy density, stress, strain, deformation, nodal forces, and force reactions. Evaluation of the rotating assembly design and possible improvements are summarized in the conclusions.