Torque Show: How YASA’s high-efficiency axial-flux motors are extending EV range

July 6, 2021

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YASA’s CTO and founder, Dr Tim Woolmer

If a tech scale-up is to be judged by the company it keeps, the Oxford-based electric motor and controller manufacturer YASA has to be a major-league company of interest for vehicle OEMs in the EV industry. The reason? YASA already numbers supercar makers like Ferrari amongst its customers for its revolutionary breed of high-power-density, highly efficient axial-flux electric motors.

But it’s not all about luxury, high performance EVs: YASA, in its next growth phase, is planning a second factory. A new high volume factory to make hundreds of thousands of electric motors and inverters for the premium EV market. At the same time, the company is moving into aerospace, partnering with Rolls Royce on a record-breaking aviation electrification project as well as mainstream airframers.

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So how is YASA winning hearts and minds across both the automotive and aerospace industries? To find out, Paul Marks spoke to YASA’s CTO and founder, Dr Tim Woolmer.

Paul Marks: How has a relatively early-stage company like YASA, which is only a decade old, managed to acquire so many high profile manufacturers as customers? What exactly is the innovation that has attracted them?

Tim Woolmer: It’s actually based on a eureka moment I had about five weeks into my PhD, when I realised 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 [see render] I could see some very significant chances to 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.

Derivation of the YASA topology from the NS-Torus Axial Flux Motor

Derivation of the YASA topology from the NS-Torus Axial Flux Motor

PM: What advantages does your axial-flux motor design offer OEMs over the incumbent radial technology?

TW: The core advantage of an axial flux motor is the spinning rotor has a larger diameter, because it’s turning alongside the stator, rather than inside it. And because torque equates to force multiplied by radius, and you’re working at a larger radius, you get more torque for the same force. So for the same permanent magnet and copper winding provision, you get more torque for free, essentially. This gives you a theoretical 20% benefit. However, the YASA topology also removes the stator yoke (removing 60-80% of the stator iron mass). This gives more like a 50% benefit when compared to radial-flux electric motors  the like of which are used most ubiquitously in today’s 1st generation mass market EV’s. We are now making great progress in disrupting that.

PM: So the YASA design improves on both radial-flux motors and conventional axial-flux motors, is that right?

TW: Exactly. Moving from radial designs to our yokeless, segmented design provides a 50% improvement. However, when we apply our direct oil cooling solution and winding technique, we generally see a three-fold improvement in power density.

PM: This power boost sounds hugely applicable to all road EVs, where range is everything. So why the headline-grabbing relationship with Ferrari?

TW: It’s a peculiarity of the automotive industry: to get an innovative new product into any kind of volume, your factories need to demonstrate a huge production track record and they need to be making a product that’s proven to work on quality and cost. Basically, the only way into the automotive industry is in through the top, through low-volume luxury brands.

PM: Is that why Tesla started at the sporty end?

TW: Yes. When Nissan and others tried to make a regular mass market EV, and have had limited success, Tesla realised that the only way in is through the top. So it decided to make the best electric vehicle it could, build the brand, and then build bigger factories, take costs out and get volumes up. And YASA has mirrored that journey, with high-performance OEM customers including Ferrari – and now the technology is proven, we’re entering the premium car market.

YASA motor efficiency

PM: What aspects of your technology are you trying to improve? Is there a “Moore’s Law” of sorts for axial-flux motors?

TW: We constantly nudge up the numbers on motor speed and power density, following our technology road map. Typically, our motors have run at slightly lower speeds than radial machines, but that’s something we’re working on: we’ll soon be at 14,000rpm and 16,000rpm after that, which is very competitive with radial machines. The very first motor we made ten years ago at Oxford University had power density of about 1kW/kg, and our Ferrari motor now runs at 14kW/kg, almost three times Tesla’s 5kW/kg. And our aerospace motors are reaching 20-25kW/kg levels, which is a massive improvement over the 10-to-15 year period we’ve been developing this.

PM: Is power density the most critical metric in this space?

TW: It’s important but as OEMs move to pure EV’s, it’s efficiency that’s becoming the most important characteristic, the figure that really drives the EV’s range. And we’re pushing our YASA motors up to average efficiencies between 96% and 97% over standard WLTP drive cycles, whereas competitive products struggle to get much above 90%. And that has a direct impact on range: some OEMs tell us we’re giving them a 5-10% range advantage against their competitors.

PM: How does the yokeless, segmented format achieve that range boost?

TW: Material differences: we’re a whole lot lighter than radial machines. The laminations in a radial flux machine, for a typical automotive motor, might incorporate 20-25kg of iron. In a YASA motor of similar rating, we’ll only be using 3-4kg of iron. And it’s iron losses that dominate over the WLTP cycle, because a powerful 150kW or 200kW motor is being used at very light loads. So efficiency is a fundamental unique selling point of our technology.

PM: What kind of cost savings does this technology lead to for EV makers?

TW: There’s an interesting sequence of knock-on effects here. If we can offer a 5%-10% range benefit, that could translate to a 5%-10% battery size reduction, perhaps lightening the EV by 50kg. But it actually grows to something bigger than that, because of the mass compounding effect: lightening the battery means you might end up losing another 50kg somewhere else on the vehicle, because, suddenly, you need a smaller motor because the car’s got lighter, and then, just as suddenly, you need fewer electronics modules and the cooling systems all shrink too. This compounding effect has pretty much a 1:1 ratio in EVs. So if our motor is say 50kg lighter, and we save maybe another 50kg on the battery pack, that 100kg saving turns into a 200kg compound saving, which starts to get quite significant in adding further range benefit.

PM: Talking of cooling systems, why are YASA motors oil cooled? What is the advantage to OEMs of doing that?

TW:  Oil is one of the reasons YASA motors’ achieve higher continuous power. A 200kW peak-power radial motor, run continuously, might give 50% of peak power, say between 80 and 100kW, as a result of thermal limitations. A 200kW YASA motor will run continuously at 150kW thanks to the improved high-thermal-contact cooling oil offers. And as oil is an insulator, it’s way safer than water cooling, too: even if a 1000-volt potential is attached to it, touching the coolant if it leaks does not risk electric shock. As for the OEM’s, EV running costs and environmental advantages are inherent. However, as is evident from early adoption trends, consumers are also demanding more performance and fun from their EV’s. An EV with a higher continuous rating, particularly in the premium vehicle segment, is going to be a highly attractive and competitive feature.

PM: Some of the most exciting research you’re involved in beyond EVs is the projection of your technology into aerospace. What is YASA doing in that arena?

TW: In terms of electrification maturity, aviation is roughly where the automotive market was ten to fifteen years ago. We’re working on the ACCEL project with Rolls Royce which is aiming to break the electric aircraft speed record, flying a single-motor zero-emissions plane at 300mph with a powertrain developed around a stack of YASA motors. Basically, in aviation, we’re looking to effectively double the power density of our technology compared to what we are doing in automotive. Our technology scales so well we envisage there’s a lot of potential for us to power this next big electrification revolution as well.

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