17 Tim Woolmer | In conversation E-Mobility Engineering | July/August 2024 they were quite loss-prone, but they improved so much that by the end of my PhD, the best powders could work about as well as standard electric steel.” One constraint on SMCs’ critical manufacturability advantage over laminations was the force it took to press them, and finding a press capable of making parts wider than 10 cm was difficult. So, Woolmer had to design his own topology of motor segments, by which a stator and rotor could be assembled from pressed SMC parts. His supervisor pointed him towards axial-flux motors, suspecting that their shape and dynamics would work better with a segmented topology than radial-flux machines. Subsequent r&d led to Woolmer’s concept for what today remains the fundamental design philosophy of YASA’s motors, which can only be made with SMCs, but met all the criteria he felt were critical to future success in EVs, including light weight, power efficiency and cost-effective mass producibility. “I stumbled upon this concept early in my PhD, so I got three years to build motors and inverters, and try them in cars. They were far from perfect, because I was, after all, just a PhD student without the massive resources of a real manufacturer, but, as stated in the final report, we achieved around 40-50 kW of continuous power from a 12 kg motor. “That meant around 3-4 kW/kg in an e-motor in 2009, compared with around 0.6 kW/kg in a Toyota Prius’ motor, so we saw there was a huge difference this technology could make in the market, and we gained some seed money from the university to optimise our prototype into something more akin to a proper product. We ended up calling that product the YASA 750, and we still sell it today, 15 years later.” Power in motion Following these early designs, YASA went on to numerous successful collaborations across motorsport and commercial automotive. One was the CX75 Concept hybrid-electric sportscar, developed by Jaguar in partnership with Williams Advanced Engineering (now WAE). This concept HEV combined a relatively small, four-cylinder Cosworth engine with two electric motors: one rearward-mounted on the engine shaft for boost power and the other on the front axle. “Their motor was the best in production at the time – a 50 kg, 400 Nm, American-made machine, which worked at 7500 rpm,” says Woolmer. “That was the spec they wanted, and we came back telling them we could do something similar, but weighing 20 kg. “There were big challenges along the way, because the YASA 750 had been relatively high speed, low torque – about 2000 rpm nominal – and stresses increase with the square of speed in e-machines, but we soon demonstrated that the magnetics worked, and we could deliver the torque Jaguar wanted.” Most critically, YASA went on to iterate six versions of the Jaguar project motor over the subsequent seven to eight months, solving problems and hitting new benchmarks along the way, thereby proving its capacity for Company (the company is featured in our Dossier on p20), as part of a wider consortium that included other rising names in the EV world, such as Riversimple (see Issue 10, summer 2021), whose technical director, Hugo Spowers, had a significant influence on Woolmer. “I have huge respect for Hugo, because he comes from motorsport, and so understands holistic system design, weight distribution and other engineering disciplines vital for EVs,” Woolmer says. “I was less enthused about hydrogen than he was, but I absolutely embraced his thinking around lightweighting and whole system design, centred around his ‘Mass Decompounding’ principle. If you can make one subcomponent lighter, say, dial back 100 kg on the motor, you make a lighter EV, which means you can use a lighter brake, suspension, chassis and battery pack. And, taking weight out of all those means you can switch to a lighterstill motor again, because you don’t need as much torque to propel the vehicle.” With that principle in mind, Woolmer soon found himself undertaking his PhD project, focusing on traction systems for upcoming EVs and HEVs. Qualities he was particularly interested in were how they had been designed to reduce weight, and to increase both efficiency and manufacturability. Softly, softly “That got me looking into electric motor structures, topologies, and components that could reduce their weight and size for automotive applications. I was especially interested in a material called SMC [soft magnetic composite], made by Hoganas in Sweden,” Woolmer says. “Making e-machines with stacked, electric steel laminations limits you to 2D shapes, but SMC powder is effectively electric steel – just steel with a lamination-type coating around each piece of powder – and you can press it in complex ways to make 3D shapes. Early SMCs weren’t great, and in the 90s Soft magnetic composites are a critical part of how YASA maximises the power-to-weight ratio of its e-motors (Image courtesy of YASA)
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