Tuesday, July 17, 2018

A Radical Improvement in Electric Motors

Chances are, if you're like me, you've taken apart a motor at some point in your life.  I've thought every kid who had a slot car took apart a motor at least once.  At the least, you've probably seen inside a motor.  Either AC or DC as a broad brush idea doesn't matter, the coils of wire were likely wound something like this view of the stator (non-moving) portion of a motor.

Because of the way current and magnetic fields are always at right angles to each other, these are called radial flux motors.  The magnetic flux is perpendicular to the axis of the motor, pointing in or out radially to the motor.  Unless you work on the leading edge of motors, this is probably all you've ever seen. 

As is often the case, there's another approach to building motors that hasn't worked out as well in a practical sense in the past.  This alternative is called axial flux motors, and it's being advanced by a Belgian company called Magnax as covered in Machine Design news this week.  As the name implies, the magnetic flux is parallel to the axis of the motor.  Again, the concept isn't new, but there have been manufacturing cost and product design issues that have kept the approach in marginal roles in the market.  Magnax is saying they've improved on some of the known drawbacks to the method.  The axial flux approach, literally thinking at right angles to conventional motors, is leading to massive improvements in power density by reducing the size (and therefore weight) of the motors.
Magnax says the key to the high power density is its motors’ yokeless AF design with two rotors at each side of the machine. The yokeless AF motors have a shorter flux path, and permanent magnets are farther away from the axis, resulting in greater efficiency and leverage around the central axis.

Furthermore, thanks to the axial flux design, very little copper is wasted on overhanging loops on the windings. The motors have zero overhang; that is, 100% of the windings are active.
The motors are being targeted where the higher power density is the most sought after: electric vehicles; not just cars, but including motorized scooters and other mobility devices.  The same approach can be used in generators where they can reduce the weight of the generator in critical uses.  They're targeting wind turbines.  
According to the company, a 100-kW axial-flux permanent-magnet (AFPM) generator for a mid-size wind turbine offers the following benefits:
  • Increased efficiency (+96% efficiency, up to 97% for larger generators).
  • Reduced length (5X to 8X shorter than traditional wind-turbine drive trains).
  • Reduced mass (2X to 5X lighter than geared or traditional geared direct-drive generators).
  • Reduced resources requirement (½ to ⅓ of materials required vs. traditional RF direct-drive geneators, which also results in lower costs). [RF = radial flux.  SiG]
I think the head to head comparison to an electric car motor is rather dramatic.  Magnax compares one of their motors to the motor BMW puts in their I3 electric car.

Note the 4x advantage in power vs. weight, which comes directly from the weight reduction.  The smaller motor weighs less, offers more peak power and the same torque as the motor BMW is using. The motor isn't a big contributor to vehicle weight, but every pound taken off the vehicle weight helps the range.

One of the things that strikes me as cool is that their design is easily scalable, and easy to parallel for more power output.  Look at this comparison to another generator technology.   That's three of their identical generator "slices" paralleled to increase output. 

The MD article goes into lots of details on the comparison of radial flux (RF) to axial flux (AF) motors.  I've just barely excerpted highlights to whet your interest.


  1. indyjonesouthereJuly 18, 2018 at 1:13 AM

    The weight and power would make a bigger difference in electric motor driven sailplanes...for an equal weight you could carry more battery storage.

  2. Hmmm, more power in smaller place, sounds like a perfect application for naval ships and especially submarines. Or electric wheelchairs. Hmmm...

    1. I remember reading about this type of motor in 1987 being tested in an electric torpedo.

  3. Interesting comments. You're thinking right along with the sources.

    Magnax mentions electric planes and drones, while the Machine Design article mentions the electric wheelchairs.

    Neither mentioned submarines, but better efficiency never hurt anything.

  4. Magnax has been hyping the heck out of this, but windmill generators have been built this way for 40 years. The design is easier to build for a home machinist (hint-hint) than the radial flux motor/generator.

    I was fooling around with designing wheelhub motors this way in 1991. Hippies were building windmill generators out of whatever garbage they had laying around (Volvo front disk hubs were popular cores) since at least the early 70's.

    Don't fall for their hype. There's nothing new about this, except the Magnax marketing department.

    1. Thanks. You know I always appreciate feedback from people who know the field, unlike me.

    2. Don't get me wrong, it's a great design, and it's a lot of fun to mess around with getting the most out of a motor.

      Short story: I was impressed by the original Honda Insight, because it used a motor around the flywheel to augment rather than replace the IC engine. It seemed obvious that this was a great way to modulate out the torque pulses so that the little 3-cylinder would be smooth as silk. But, no! I learned many years later (when somebody discovered this notion in small yard tools) that Honda never did this – even though it would have taken only a little software. What a shame. The car might have sold better with that as a feature.

      I remain convinced that concurrent operation is a better approach than either a serial or a parallel hybrid system.

  5. As I was pondering crazy ideas involving axial-flux motors, I half-remembered something.
    It's many a year since I disassembled a 3.5" floppy drive, but... didn't some of them have axial-flux motors, with the windings printed on the PC board and magnets embedded in the flywheel?

    1. Good one. I haven't had a floppy drive to open in ages, but that kinda sounds familiar.

      Now I have to go look up the difference between Axial flux motors and whatever those were.