Axial flux machines drives: a new viable solution for electric cars

Abstract

For electric car propulsion systems, the wheel motor is an application that requires electrical machine shape flexibility, compactness, robustness, high efficiency and high torque. The axial flux machine is an interesting solution, where the motor is directly coupled or inside the driven wheel. In this paper, axial flux induction and synchronous machines as wheel motor application are presented, and some considerations for each motor type are drawn by the authors. The structure with two rotors seems to be a very promising solution for both induction and synchronous machines. The twin rotor structures allow saving of the motor active material: stator winding copper or stator core iron. Thus the machines can have higher power density and efficiency in comparison with two individual motors. In addition, for the induction case, the two rotors can rotate at different speeds, and thus the motor can act as a mechanical differential, even if supplied by a single invertor. The axial flux PM motor with two rotors is very compact and can be integrated inside the wheel. Since the identification of a unique drive solution for any type of electric vehicle, the authors focus the attention on the small electric city cars to be used in European towns where the environment pollution problems are quite heavy.