Thermal and mechanical analyses of high-speed permanent-magnet electrical machines

In the thesis, methods for the thermal and mechanical analyses of high-speed PM electrical machines are presented and implemented. The first method implemented for the thermal analysis is a combined 2D-3D numerical method. The thermal and turbulent properties of the flow, such as the temperature rise in the flow and the coefficients of thermal convection, are estimated using a 2D multiphysics method that couples CFD with heat-transfer equations. The detailed distribution of the temperature rise in the whole solid domain of the machine is determined using a 3D numerical heat-transfer method. The temperature rises in the machine are also estimated with the traditional thermal-network method, which uses a totally different approach to the heat-transfer analysis. The methods used for the mechanical analysis of the machine include finite-element rotordynamics modelling of the rotor for estimation of the critical speeds and the shapes of the bending modes and also analytical estimation of the stress in the retaining sleeve. The implemented methods are used for the comparative thermal and mechanical analyses of three different high-speed PM rotor constructions. The first type of rotor construction is retained with a carbon-fibre sleeve and uses a shield for eddy currents made of aluminium. The second rotor construction is retained with a retaining sleeve made from the alloy Ti-6%Al-6%V-2%Sn and the sleeve of the third rotor construction is made from the alloy Ti-2.5%Cu. The last two rotor constructions do not have separate eddy-current shields. The comparative analysis shows that the rotor with a carbon-fibre sleeve and an aluminium eddy-current shield shows the best thermal properties. The rotor with a retaining sleeve made of the titanium alloy Ti-6%Al-6%V-2%Sn offers promising thermal properties because the critical temperatures in the rotor are not exceeded. Additionally, the same rotor construction provides the best rotordynamics properties when compared to the other rotor constructions. The rotor construction retained with a sleeve made of the alloy Ti-2.5%Cu is inferior from the thermal and mechanical points of view when compared with the previous rotor constructions. The methods used for the thermal and mechanical analyses are also used for the determination of the maximum power limits for high-speed PM electrical machines for air-compressor applications. For that purpose, five high-speed PM electrical machines for the speeds of 20,000 rpm, 40,000 rpm, 60,000 rpm, 80,000 rpm, and 100,000 rpm are designed in order to determine their maximum mechanical powers. The electromagnetic, thermal, and mechanical designs of each machine are performed simultaneously and all the critical values of the thermal and mechanical design constraints are considered. The obtained maximum power limit defines the speed-power region of safe operation of the high-speed PM electrical machines intended for compressor applications.