High fidelity mechanical and loss modelling of an interior permanent magnet traction machine for electrical vehicles

High-performance permanent magnet electrical machines have become the leading machine technology in the electric vehicle market due to their combination of high efficiency and high-power density. However, their design optimisation involves complex and often strongly coupled mechanical, electromagnetic and thermal behaviour. Of the many possible topologies of permanent magnet machines, interior (IPM) machines have become the favoured machine type as they offer advantages in field weakening, a contribution from reluctance torque and the ability to retain the magnets within the rotor core without the need in many cases for a separate containment sleeve. However, the trade-off between electromagnetic and mechanical performance is especially important in IPMs because of the use of thin bridge-sections within the rotor core. This thesis reports on detailed design study into the mechanical and electromagnetic optimisation of an 8-pole, 100kW IPM machine with a base speed of 4,000rpm and an extended speed range up to 12,000rpm and makes extensive use of structural and electromagnetic finite element analysis to identify a preferred design. The other aspect of IPM performance which is investigated in this thesis is the influence of high frequency converter switching on the iron loss in the machine. An analysis methodology is developed and applied to an IPM machine with combines a SIMULINK model with pre-calculated finite element characteristics of the machine to predict detailed localised element-by-element flux density variations in the cores of an IPM machine which includes realistic representation of switching events. The effect of current ripple and the grounding of the star-point is investigated. These high frequency flux density waveforms are then used as the basis for estimating the effect of high frequency current ripple on iron loss. This aspect includes a detailed investigation of the limitations of different analytical and numerical models for solving the diffusion equation with 3D eddy current finite element simulations providing a baseline against which to test various models. This aspect of the research results in a time-stepped finite difference representation of 1D eddy current flow in laminations and is applied at full machine level as post-processing tool. The thesis concludes with some experimental measurements of core loss with switching ripple which demonstrates the value of including lamination level eddy current effects in loss predictions.