On the Design of Electric Traction Machines : Design and Analysis of an Interior Permanent Magnet Synchronous Machine for Heavy Commercial Vehicles

University dissertation from Department of Biomedical Engineering, Lund university

Abstract: Recent years have proven the benefits of electrifying the road bound vehicle fleet. With new components entering, the general understanding as well as the components as such needs to be improved. Focus in the thesis is on the design of an electric machine based on specifications of requirements for a commercial heavy vehicle such as a truck or a bus. One strict requirement is that the machine has to fit in the vehicle without compromising the performance. Besides limitations on the size, this affects the power density and hence efficiency and cooling. Another characteristic of a traction machine is the difference between peak operation and average or continuous loading. Within the automotive sector, cost is also an important factor. Prior to the design work, pre-studies are used to acquire good understanding of the intended applications. The result is a space claim of ∅220 mm times 400 mm and a peak power of 180 kW. By designing the machine with a top speed of almost five times that of a conventional heavy duty engine, the required power levels are reached with less torque. As torque is proportional to size, the power demand is reached with a smaller and hence also less expensive machine. The design work is done in a two dimensional finite element environment partly developed at the division at Lund University. Main focus is on the limited space claim and requested peak power. Cooling is done with oil directed to the active parts of the machine. Prototype testing proves the machine to be capable of propelling a heavy commercial vehicle. Some in depth studies are also done on torque ripple in the skewed machine and on mapping of the losses.The thesis presents the thorough work on setting the requirements, designing, prototyping and testing an interior permanent magnet machine intended for propulsion of heavy commercial vehicles. Improvements implemented in the design tool is verified with measurements. A deeper study on the torque output from the skewed machine shows a load dependant influence with larger impact in the field weakening region. It is also found larger than expected from the analytical expression in relevant text books. The losses are mapped with main focus on the speed dependant parts. A review of how manufacturing processes and machine controls affect the iron losses is presented. The iron loss model is adapted based on test results. Losses in the windings and in the rotor are included in the study as well.