After nearly a decade in development, Belgian startup Magnax claims it has developed an ultra-high power, lightweight, compact axial flux electric motor with performance figures that blow away everything in the conventional radial flux world. Crucially, it says it's worked out how to manufacture them too.
It might not have the romance of combustion engine tuning, but it seems a bit of a battle is brewing to develop the kinds of high-performance motors that will power the electric cars, motorcycles, aircraft and industrial equipment of the future.
A week ago we wrote about Equipmake's spoke motor design, which allows it to pump out some 9 kilowatts per kilogram with exceptional cooling and continuous power production ability.
To put that kind of power production in perspective, the ludicrously fast 193-horsepower BMW S1000RR superbike of 2011 – a power-to-weight beast that would slay just about anything in the automotive world – has a lightweight motor that makes a puny 2.4 kilowatts per kilogram. So 9 kW/kg is no joke.
Which makes this axial flux, direct drive motor from Belgian company Magnax a real eyebrow-raiser. Magnax claims it makes a peak power no less than 15 kW/kg, with the ability to produce sustained power at around 7.5 kW/kg. To bring that back to the motorcycle example, if you built a Magnax motor that weighed as much as the BMW superbike's engine, you'd have yourself a 603-horse powertrain that could produce bursts of up to 1206 horsepower for short periods before overheating and needing to take it easy for a bit.
Obviously, that's a silly example, but these kinds of ultra-lightweight motors could do significant work towards offsetting the large weight figures of today's heavy lithium battery packs in electric vehicles. And until automotive-grade battery density takes a significant leap forward as it's been promising to for several years now, weight will continue to be a serious issue for e-mobility.
Benefits and drawbacks of Axial Flux Designs
While the vast majority of electric motors currently in circulation are radial flux designs, Magnax claims the key to the high power density it's achieving is the direct drive axial flux design used in all its motors, which uses a stator disc sandwiched between two rotor discs with small air gaps in between. Yokeless axial flux motors, the company claims, have a number of advantages if implemented properly. The flux path is shorter, and the magnets further away from the axis, leading to greater efficiency and leverage around the central axis.
What's more, the axial flux design allows Magnax to waste very little copper on overhanging loops on the windings. Magnax's motors have zero overhang; 100 percent of the windings are active, where the company claims radial flux motors can sometimes have up to 50 percent of their copper inactive, adding extra resistance and causing heat build-up. Magnax uses a rectangular-section copper wire in its windings to give the highest possible density. And the motors are much thinner than radial flux machines, meaning that you can stack them easily to work in parallel.
There are, of course, difficulties when it comes to building axial flux motors – otherwise everyone would be making them. Powerful magnetic forces acting between the rotor and stator discs tend to make it very difficult to keep the air gap between them uniform. If they start to wobble or bend, the discs can start rubbing against one another, leading to bearing damage at best, and rapid, spectacular unscheduled disassembly at worst.
Magnax claims it addresses this in its yokeless axial flux design by having two rotor discs that constantly put equal and opposing forces onto the stator disc. The rotors are connected directly to one another via a shaft ring, so the magnetic forces cancel each other out, and the internal bearing doesn't have to deal with them.
Cooling is key with any high powered electric motor that's expected to do consistent work, and axial flux designs tend to suffer in this regard, since their stator windings are sandwiched between the rotor discs, making it hard to get heat out. Magnax claims its motor designs cool well, as the windings are in direct contact with the outer aluminum casing, allowing decent heat transfer.
It seems to be working. Continuous power figures for the Magnax motor come out at 50 percent of what it can make at its peak, which is pretty decent but not in the realm of the best-cooled radial flux motors. The Equipmake motor, for example, can continuously make nearly 70 percent of its peak power, suggesting superior cooling.
One further challenge comes with manufacturing, as the stator discs can be particularly hard to get right, and even harder to build in an automated high volume process. So when they do get built, they're hand-made and highly expensive as a result. Magnax claims to have cracked this problem too, with a number of "proprietary solutions" that allow it to scale and build these things cost-effectively.
The Magnax motor is highly scalable, ranging in size from 15 centimetres (~6 inches) right up to discs 5.4 metres (~13 feet) in diameter and beyond. They can be slotted in next to one another to run in parallel, and they can run either as direct drive or through a gearbox if you're willing to accept the efficiency losses involved.
Magnax is pitching them at electric cars and motorcycles, aircraft rotors, and as large-diameter, high torque, low RPM solutions for wind power, hydroelectric and wave power generation.
At the end of the day, field testing in the automotive and industrial worlds will be the proof of this pudding, but if this is truly a high-power, long-life, well-cooled, high-efficiency, low-maintenance axial flux motor, Magnax could be poised to make some serious waves.
The company has spent some nine years getting its tech together after a proof of concept was originally built at the University of Ghent in 2009. Now, it claims to have working prototypes and a manufacturing methodology sorted out. With a bit of luck, the rubber will hit the road soon and we can see if this truly is the electric motor of the future.
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