Electric Traction Machine Design for an E-RWD Unit

Abstract: Since the first generation of the Toyota Prius was introduced in December 1997, the number of Hybrid Electric Vehicles (HEVs) and pure Electric Vehicles (EVs) available in the market has increased substantially. The growing competition existent puts high demands on the electric system as well as the rest of the vehicle. As a consequence, substantial design effort is devoted to optimize both at system and component level, with respect to different parameters such as fuel efficiency, power density, cost and reliability. As bidirectional energy converters between electric and mechanical energy, electrical machines play an essential role inside electric or hybrid drivelines. The performance of the electrical machine has a high impact on the overall traction system, and therefore a thorough machine design is crucial for a successful electric driveline. This thesis describes in detail the design process of an electric traction machine for a particular HEV application, an Electric Rear Wheel Drive (E-RWD) unit. The evolution of the design is presented, analyzing the effect of the different driving forces at the different stages. Throughout the thesis it is seen that a modest change on the initial specifications set can lead to a significantly different solution. For this reason, a minimum set of initial specifications is proposed, together with a new design methodology based on the description of the load profile in the form of driving cycles. The manufacturing of a prototype and the experimental validation of the design are also covered in this work. The proposed traction machine is tested standalone, as well as integrated in the E-RWD unit onboard the vehicle. From the experience gained both from the design and the experimental testing phases, it appears that the performance of the machine is usually limited by its thermal capabilities. Improving the cooling conditions allows to operate the machine at higher power levels, which has a direct influence on the outcome of the design process. A way to enhance the cooling capabilities for conventional radial flux machines is investigated. Moreover, a machine design with a completely novel cooling concept based on direct cooling of laminated stator windings is presented, and its thermal characteristics are extensively analyzed.