When Mercedes purchased UK-based YASA last year, it opened the door for axial flux electric motors to find a place in mass produced production vehicles. The axial flux motor is the brain child of Tim Woolmer, who came up with the idea while a Ph.D candidate at Oxford University.
“I was five weeks into my Ph.D at Oxford University, when I suddenly realized there was a much better way to build an electric machine called a torus axial-flux motor. The insight I had was that by removing the motor’s stator yoke, and splitting it into segments, we could reduce the motor’s weight — and yet at the same time improve its torque, power density, efficiency and manufacturability, making it potentially transformative within the then nascent electrification industry.”
What’s so great about an axial flux motor? Several things. It is lighter and smaller than the radial flux motors that power most electric vehicles today, yet has up to four times the torque output. Simply put, it does more with less and is more efficient as well. The one area where it does not excel is in ultimate top speed, which is of little interest to the vast majority of motorists.
YASA Explains The Axial Flux Motor
On its website, YASA explains the core advantage of an axial flux motor is that the spinning rotor has a larger diameter because it turns alongside the stator rather than inside it. Torque equates to force multiplied by radius. With an axial flux design, there’s a larger radius than in a conventional radial motor, which translates into more torque for the same force. This means for the same permanent magnet and copper winding provision, you get more torque.
The YASA motor topology also removes the stator yoke, which reduces the stator iron mass by up to 80%. This innovation provides a significant power density advantage and a 5% to 10% range benefit over the radial electric motors on which most of today’s mass market electric vehicles depend.
YASA axial flux electric motors feature a proprietary Yokeless And Segmented Armature — which is where the name of the company comes from. Its armature windings consist of separate segments ideally suited to mass manufacture with minimal application engineering.
The Right Motor For The Right Application
YASA axial flux motors are being used in ultra high performance vehicles such as the Ferrari SF 90 and 296 GTB and the Koenigsegg Regera. Rolls Royce is also using them for its experimental electric airplane, the Spirit Of Innovation. The Ferrari experience highlights the trade-offs manufacturers make to achieve the level of performance required in different models.
Ferrari uses two radial motors to power the front wheels of the SF 90 in order to get ultimate top speed, while using an axial motor on the rear axle for maximum acceleration. For the 296 GTB, handling was deemed more important, so only a lighter axial motor was used between the engine and transmission. “It’s just a matter of what kind of driving experience you want to design for your customers with a specific engine,” Davide Ferrara, the head of electric motors at Ferrari tells Bloomberg Hyperdrive. “Different voices make sweet notes.”
In July of last year, Mercedes announced it had acquired YASA for an undisclosed sum and would put its motors in AMG models slated to go into production in 2025. “If you look at the history of automotive generally, the auto companies have wanted to have the engine, their core technology, in-house,” Woolmer says. “The batteries, the motors, this is their core technology now. They recognize the importance of having long term differentiation in these spaces, so they have to bring it in-house.”
The most important aspect of axial motors, according to Malte Jaensch, professor of sustainable mobile drivetrains at the TUM School of Engineering and Design in Munich, is that their smaller size could allow carmakers to put one motor on each wheel, which isn’t feasible with radial motors. (Note: It’s not clear whether Jaensch is talking about in-wheel motors.) That in turn would permit torque vectoring that provides precise control over how much power the motors send to each individual wheel for improved traction and cornering ability.
Direct Oil Cooling
Electric motors can get hot, especially when operating at maximum power. YASA says direct oil cooling allows its motors to achieve higher continuous power levels. A 200 kW peak power radial motor, run continuously, might typically give 50% of peak power between 80 and 100 kW, as a result of thermal limitations, the company says. In contrast, a 200 kW YASA motor runs continuously at 150 kW thanks to the improved high thermal contact cooling oil offers. Consumers are demanding more performance from their electric vehicles. Automakers who can offer their customers higher performance levels over sustained periods of time will have a significant market advantage.
The gates of history turn on tiny hinges. Jaguar Land Rover was the first automaker to express an interest in using YASA’s axial flux motors for its proposed CX 75 hybrid sports car, but decided to pass on the opportunity for economic reasons. Later, when it introduced its I-Pace electric SUV, it used motors designed in-house. Then in July of last year, Mercedes purchased YASA specifically to fulfill the need for high performance electric vehicles from its AMG division.
“The power to weight ratio is really a record number and much better than conventional motors,” Markus Schaefer, Mercedes’s chief technology officer, tells Bloomberg Hyperdrive about the company’s upcoming AMG electric vehicle platform. “It will make use of the small size of the motor.”
Previously, in the internal combustion engine era, more performance meant engines with a greater displacement, more cylinders, dual overhead camshafts, automatic cam timing adjustments, electronic fuel injection, electronic ignition systems, turbochargers, superchargers, or exotic fuels like nitromethane. In the electric car era, high performance will come from batteries and motors that can convert a thousand amps or more of electricity into forward motion as quickly as possible, consistently, and with absolute rock solid reliability.
It took a century for the automotive industry to perfect the internal combustion engine. We can expect a similar, although much shorter, trajectory for EV powertrains. The future’s so bright, we gotta wear shades!
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