Development of electrical machines for hostile environment

Abstract

This thesis focuses on improving the electromagnetic performance of switched reluctance machines (SRM) without compromising their mechanical advantages to fulfil the demanding requirements of more-electric aircraft. Literature review reveals the advantages of sinusoidal excited mutually coupled (MC) SRM windings in boosting the power density and reducing the torque ripple. Three winding configurations to implement sinusoidal excitation are studied via finite element analysis (FEA). Among the three winding arrangements, short-pitched MCSRM is found to produce the most advantageous features thanks to the shortest flux path. The concept of short flux path is then taken further to the four-phase mutually coupled SRM with dc-biased sinusoidal current to produce unidirectional flux flow. It is discovered that under such winding arrangement, the four-phase SRM can be operated akin to the traditional permanent magnet synchronous machines with interdepended control over the excitation, direct and quadrature axis current components. Via FEA, it is validated that the power factor and torque ripple are further improved compared to the three-phase SRM designs. The mathematic model of the proposed four-phase MCSRM is also derived and validated with FEA for the benefit of future digital controller development. Computational fluid dynamic simulation is conducted to validated the cooling effectiveness of the water-based indirect cooling and oil-based direct cooling, where the latter depicts considerably lower winding temperatures. A novel closed-slot concept is proposed in SRM to enable winding in-slot cooling with minimal impact on the electromagnetic performances. The influences from the closed-slot structure are evaluated via FEA for both the conventional average torque control as well as the proposed dc-biased sinusoidal control to validate the power density and torque ripple improvement. Lastly, the closed slot four-phase SRM design is prototyped and validated against the FEA results. Methodologies to extract the key inductance components from the hardware results are also explained. The hardware results confirm the FEA results and the analytical predictions in the tested range, which justifies the machine performance in actual engine environments.