Most electric vehicles use cylindrical radial-flux electric motors, but there’s a movement toward the disc-type axial-flux motors (also called pancake motors due to their shape) and for good reason. Mercedes-Benz acquired YASA, a British concern that designs axial-flux motors, just last year, and displayed such a motor in its 2023 Vision One-Eleven concept car.
Audi has clearly been paying attention, and its recent filing with the European Patent Office suggests it’s been refining its own axial-flux tech. The patent describes a stator design for axial-flux motors that has radial cooling channels directly in the cores of the individual stator windings in a bid to improve cooling performance, efficiency, and power output.
Why Axial Flux?
Axial-flux motors already star in some sports cars such as the McLaren Artura plug-in hybrid electric vehicle (PHEV). The motor weighs only 34 pounds – about half the weight of the radial-flux motor that was used in the P1. The Ferrari SF90 Stradale PHEV also uses an axial-flux motor. In an axial-flux motor, torque increases with the cube of the rotor’s diameter, whereas only a quadratic increase is obtained in a radial-flux motor.
Compared to a radial-flux motor, an axial-flux motor:
- Is 15-20% the size, making it easier to fit and mount in a vehicle
- Is about half the weight
- Has a higher power density
- Has reduced cooling requirements, and so requires a smaller and lighter cooling system
Improving Cooling
Audi aims to improve the efficiency of the motor’s cooling system even further by redesigning the stator and integrating radial cooling channels directly into the core elements of the stator windings, thereby improving cooling and eliminating the need for separate cooling elements. This allows more efficient cooling, more compact packaging thanks to even smaller motor sizes, better efficiency, and higher power densities.
Similar solutions have been proposed before, but Audi’s patent is unique in the way it integrates the cooling channels into the overall motor architecture, and thereby address two issues at once – size and cooling performance. This is likely to prove beneficial as more automakers adopt axial-flow motors due to their obvious advantages, arguably the most important of which is reducing the weight of electric vehicles.
