There is a subdued hum in the emerging world of the EV: the sound of several types of electric motor vying for top podium position within the industry.
“It is similar to the computer debate in the early 1980s before the IBM PC became the industry standard,” said Ian Foley, Managing Director of Equipmake, a specialist in the design and development of innovative electric powertrains. “Now, we have many different electric motor types, and they come with different manufacturing philosophies. It is essential to look at the whole system—motor, battery, inverter, gearbox and controls—not just motors in isolation. The lowest cost solution for all of this will win!”
And that, he hopes, will be his company’s spoke motor, designated APM200. He claims that it has already demonstrated such a high level of efficiency that it is the most torque and power dense (kW per kg) of any other types of permanent magnet motors for EVs. It also saves cost and weight, adds Foley.
The motor weighs about 49 kg (108 lb), can run at 10,000rpm, and has peak power and torque respectively of 220 kW (295 hp) and 450 N·m (332 lb·ft). Cost saving is helped by using cheaper grade neodymium iron boron (NdFeB) magnets. The motor incorporates an integral 5.5:1 epicyclic gearbox so the output shaft of the gearbox can be connected directly to the wheel hub. Equipmake also has developed an inverter specific to the APM200. It incorporates Power Electronic technology with silicon carbide diodes combined with IGBTs (Insulated Gate Bipolar Transistors) to improve power capability and allow it to run at high switching frequencies.
Keep cool, keep cool
Cooling is key to electric motor performance; the cooler its magnets, the longer it can run at maximum power. But a must is to do that at the right price and fulfil required reliability and efficient manufacturing criteria.
“Those criteria can be achieved by the spoke configuration,” explains Foley. “A conventional interior permanent magnet motor has its magnets arranged in a very shallow V shape in laminations round the circumference of the rotor; the spoke has them arranged perpendicularly like the spokes of a wheel on the surface of the aluminum rotor, enabling us to get coolants (water/glycol at 60deg C) very close to the magnets. That is not possible in a conventional configuration, which has the magnets sitting in the lamination. Because one end of the magnet is positioned in an aluminum hub, we can get a coolant path very close to it and so conduct out heat. A spoke motor is not as straightforward to manufacture as a regular design but we are confident that we have come up with a usable production version.”
Foley said the key to achieving a solution to meet volume production lay in the spoke motor’s detail design, including the way laminations fit into the hub: “Basically it is a forging that is machined during production. The fact that we can cool it so effectively means we can get the great strength needed. By keeping the aluminum below 100o C, we can use cheaper magnets, with no loss of performance, reliability or longevity. Thermal engineering is one big differentiator between types of EV motors.”
Spoke is a known topology, albeit not in the auto industry, explains Foley. But Equipmake’s involvement with the UK extreme-performance HIPERCAR (High Performance Carbon Reduction) project scheduled for production by Ariel in 2020 (in partnership with Delta Motorsport) generated research that led to the development of the spoke configuration and what Foley describes as “the most torque dense motor, which has been proven fundamentally and is leading to further development."
Its evolution has taken around three years, he said: “Design, cooling and manufacturing challenges had to be overcome. We have done that, not only for HIPERCAR but also for a bus application, working with an Argentian partner using two APM200 motors. The bus project mates the motors with Semikron SKA1 inverters and AESC batteries. We expect to produce about 2000 motors a year by 2020 and ramp up from there.”
Equipmake’s spoke motor benefits from using aluminum for the hub with steel laminations because the material’s properties, notably fatigue and life, are known and able to provide the design life requirements.
The APM 200’s stator is described as “completely conventional." A validation program which includes life testing is now under way.
All about cost
Foley is very well aware of the financial aspect of fresh to automotive technologies: “Everything in the auto industry is about cost; I know very well that engineers are very good at dreaming up great ideas that sometimes cost too much! But having the architecture that gets the most out of the magnets is what matters. We carried out an analytical comparison of spoke technology with a major OEM’s EV conventional rotor and found we could get the same torque with 25% fewer (and cheaper) magnets.”
The spoke motor incorporates what Foley calls “cooking grade” magnets because keeping the temperature of the motor down also brings down costs: “It is not necessary to use rare earth neodymium magnets that work at up to 200o C; we are operating at half that.”
He has not released any specific figures for the Equipmake spoke motor with regard to its contribution to increased vehicle range.
Learning from flywheels, racing
Foley is a highly-experienced engineer and technology innovator. He spent the early part of his career (1980s and 90s) with a variety of F1 teams, working with Lotus F1 on active suspension for several years and was involved with Lotus’ GT endurance team. He formed Equipmake in the late 1990s and one of its first projects was the production of a paddleshift gearbox used at Le Mans by many competing cars.
In the 2000s he began work on a new type of high-speed electric flywheel, which found success in 2009 with Williams F1 in its Kinetic Energy Recovery System (KERS). The technology had many applications beyond F1. As a result, a new company was formed, Williams Hybrid Power, of which Foley was managing director.
Williams Hybrid Power developed electric powertrains for commercial vehicles including buses and trains and subsequently was bought by GKN.Continue reading »