Investigation of DC-Link voltage and temperature variations on EV traction system design

DC-link voltage and temperature variations are critical issues when designing an electric vehicle (EV) traction system. However, systems are generally reported at fixed voltage and temperature and may not, therefore, be fully specified when considering the variation of these parameters over full vehicle operating extremes. This paper presents an assessment of power-train options based on the Nissan Leaf vehicle, which is taken as a benchmark system providing experimental validation of the study results. The Nissan Leaf traction machine is evaluated and performance assessed by considering dc-link voltage and temperature variations typical of an automotive application, showing that the system lacks performance as battery state of charge decreases. An alternative traction machine design is proposed to satisfy the target performance. The vehicle power-train is then modified with the inclusion of a dc/dc converter between the vehicle battery and dc-link to maintain the traction system dc-link voltage near constant. A supercapacitor system is also considered for improved system voltage management. The trade-offs for the redesigned systems are discussed in terms of electronic and machine packaging, and mitigation of faulted operation at high speeds.