The past few decades have seen a rapid sales increase and technological development of electric vehicles (EVs). As the key part of the electrical powertrain systems, the traction machine drive systems in modern EVs are composed of voltage source inverters (VSI) and electric machines. In this thesis, multiphase VSIs are studied and designed to achieve volume reductions when compared with existing 3-phase benchmark VSIs. Different existing switching strategies for arbitrary phase number multiphase VSIs are investigated resulting in an understanding of best practice and a newly proposed switching strategy. Thus, the first contribution of this thesis is switching strategies that support subsequent investigations and experimental validation. DC-link capacitor and heat sink are two bulkiest components in VSIs and hence it is more efficient to decrease their volumes to achieve the compactness improvement. The investigation methodology and procedure for arbitrary phase number VSI DC-link capacitor requirements, i.e. capacitance and RMS current ratings, are firstly developed. Increased phase number decreases the DC-link capacitor requirements and hence the VSI volume significantly. Throughout this analysis, the connected multiphase machine is considered appropriately, though no electric machine design is described in the thesis. While other authors have studied DC-link current ripple, this thesis qualifies and quantifies the system benefits. This is the second contribution. Multiphase VSIs thermal models are built and their respective thermal performances studied and evaluated against a reference 3-phase benchmark VSI. The power loss deviation among different semiconductor dies is lower or even eliminated in the multiphase VSIs. Furthermore, the multiphase integrated design VSIs have a significant heat sink volume reduction when compared to the 3-phase benchmark VSI. This study and concluding benefits are the third contribution. Finally, comparative test validations are made on an experimental set-up designed to illustrate the benefits of a 9-phase against a reference 3-phase system. Here, the test hardware and implementation are carefully designed to representatively illustrate performance benefits.

Doctor of Philosophy (PhD)

Thesis