An air-cooled YASA motor for in-wheel electric vehicle applications

In-wheel motors present a range of opportunities for innovation in electric vehicle design as the torque produced at each of two or four wheels can be controlled individually. A high-aspect ratio (large radius, short axial length) motor is required to fit within the wheel. Due to its location, liquid cooling of the in-wheel motor is difficult and undesirable, but a high power density is required to reduce the mass—which is particularly important as it is unsprung—and fit the space envelope. Furthermore, a high-torque density is required to eliminate the need for a gearbox. These constraints create a real challenge for the design of a machine for this application. An axial field machine using a yokeless and segmented armature (YASA) topology is designed to fit these requirements as such a machine has clear advantages when considering the high-aspect ratio. A soft magnetic composite material is utilized to carry the flux in its nonplanar path without incurring excessive losses or requiring a lamination design, which is difficult and expensive to manufacture. A novel cooling arrangement involving heat-spreading elements on each armature segment is employed to improve heat dissipation and, hence, power density. The design, analysis, manufacturing, and testing of the motor is described in this article to verify the concept against the requirements outlined earlier.