All components of an electric propulsion system – the motor, battery pack and inverter, in particular – are interrelated and optimized for a system function. Still, there are significant trade-offs in cost and what’s best for sustainability when developing today’s e-drive systems, according to David Fulton, director of rotating electric machines, PowerDrive Systems at BorgWarner Inc.
“The dominant design for motors today is probably the worst for sustainability in terms of KPIs [key performance indicators] as well as highest in cost. But it serves the greater good of the system [by enabling] the lowest cost for the battery pack and inverter,” Fulton said at the 2023 SAE COMVEC conference, during his presentation on next-gen motor technology for commercial vehicles.
The majority of current motors are rare-earth-magnet types on the rotor side, he said. These designs use heavy rare earth (HRE) elements that allow them to be used at high temperatures without demagnetizing. “This is a concern because of how short the supply is globally for these, and they virtually all are found in China or near China,” Fulton said. “This is going to be a short-term supply problem very soon. So, heavy rare earths are going to be the first to be taken off the table.”
Dysprosium and terbium are two HREs commonly used as additives in neodymium (Nd) magnets. Though light rare earths such as Nd are more plentiful than lead, for example, higher BEV volumes and increased wind power – not to mention unpredictable shifts in geopolitics – likely will drive prices higher, Fulton said.
“Rare earths in general are well known to have high environmental impacts,” he said. “They might be only 3% of the mass fraction of the motor but over 20% on the carbon footprint, because of all that has to happen to process those materials.” The recyclability of rare earth metals also poses a challenge – it’s difficult and not particularly cost-effective.
“People would rather dig them out the ground and make more new magnets than recycle,” Fulton said. “If we did something like grade-to-grade recycling – as we separate plastics into categories and recycle those – and just change the dimensions of the magnet in the future but not what they’re made of, that could help enormously. But the problem with that, of course, is at end-of-life for these motors, is that magnet grade going to be competitive? My hope is that we find entirely different magnets altogether that are much lower cost to recycle by that point in time.”
Rotor development for next-gen motors
Different types of rotors that are more sustainable have been gaining traction in the automotive industry. Copper induction rotors offer higher conductivity and greater efficiency than aluminum induction, according to Fulton. Wound rotors essentially replace the magnets with electromagnets. “That means we have to provide current to the rotor and need a special excitation board in the inverter to do that,” Fulton explained. “That’s not a problem.”
Another option is non-HRE rotors. Several automotive OEMs have moved in this direction. Fulton referenced General Motors and its Chevy Volt hybrid, which uses ferrite magnets, as one example. But there’s a catch with these higher-sustainability solutions: higher system costs.
“Smart engineering” – a term repeatedly used at the SAE COMVEC conference – is necessary to find cost-neutral solutions that satisfy performance and sustainability goals. System optimization tools and lifecycle analysis will be key to evaluating the relevant metrics and managing trade-offs, according to Fulton. “Having the right kind of toolset to be able to not only [determine] multiphysics optimization, which we’ve done for years, but also to look at cost models and lifecycle analysis for each of these components to weigh that all out is important,” he said.
Material standardization can help with costs as well. “But it’s only relevant in areas where materials are relatively mature,” Fulton said. “I think rare earth magnets are probably in that camp. Batteries, not so much.”
An alternative, less-than-ideal approach to smart engineering is “brute force adaptation,” Fulton said, comparing a baseline HRE magnet used in one of BorgWarner’s commercial-vehicle motors to a redesigned zero-HRE magnet. “This [baseline] is very lean on the use of heavy rare earths – as lean as you can get today using the diffusion processes. If we used the best possible non-heavy rare earth magnet for sustainability reasons, we’re redesigning it to get the same function – the same performance, same maximum speed rating and same protection against demagnetization,” Fulton explained.
“But when we do this, we’re handicapped right from the beginning by losing half the coercivity of this magnet material at high temperature. As a result, to protect the magnet against demagnetization you see how much thicker it becomes [see slide], and that’s the outcome of brute force adaptation,” he said. “So even though the cost-per-kilogram is lower, the cost of the magnet is quite a bit higher, over 50% higher. And the magnet is, of course, a significant fraction of the bill-of-material cost of these permanent magnet motors.”
Fulton said engineers can take a step back and employ the five whys technique to determine new solutions. “Why does the magnet have to tolerate such high temperatures? Why can’t we deal with something that is not so high temperature? Of course, that begs the question of the cost of the cooling system. So, there are other ways to look at this that helps us to be more creative and not adapt with a brute-force mentality.”
Engineers also must address the need for shorter charge times and the impact that has on motors. “For instance, 1200-volt [batteries] is something that’s been looked at; I’ve heard some of our heavy-duty customers talking about it as a way to get to faster charge times,” Fulton said. “But this is going to drive the need for thicker insulation materials in our motors. Because of creepage and clearance, it has a power-density penalty. When you have more insulation, you have less room for copper – as a result, your losses go up in the motor. For the same range, that’s going to have a slight battery-size penalty associated with it.”Continue reading »