The motor, in ASCII art

Posted on July 6, 2008

25


A fellow tinkerer has contacted me showing his own project of in-wheel motor.

His design is of Axial Flux with double rotor, and he shows me a series of details and shares a few enhancements over my original axial flux single rotor design.

I found this event quite gratifying; it’s the kind of confirmation I like to see that the smartest people on the planet are always the first to openly share what they know and have, in hope that their contribution will feed progress. Everybody wins. 😉

But meta-remarks aside, I deeply enjoyed the simplicity and elegance with which he showed me his design: using ASCII art. It was very clean and explicit. And it also made me think that it’s been a long time since I posted the design I’m working on (and since people are still not sure if I’m working on an Axial or Radial Flux motor, I think it’s time to make things clearer).

So, here it goes. My model is a Radial Flux motor, with double Rotor. And it is designed to fit outside the brake drum, with the rotor fixed between the wheel hub and the brake drum, and the stator fixed to the suspension arm. This is a view in cut, along the axle of the wheel. On the left is the outside of the wheel, and on the right the inside (suspension).

               r                         r
               rrrrrrrrrrrrrrrrrrrrrrrrrrr
               r
               r RRRRRRRRRRRRRR
              rr R MMMMMMMMM
              rr R
              rr R  CCSSSSSSSSSCC
              rr R  SSSSSSSSSSSSSSS
              rr R  CCSSSSSSSSSCC S
              rr R               S
              rr R MMMMMMMMM  S
              rr RRRRRRRRRRRRRRRR S
              rr R                S
              rrrR ddddddddddd s  S
  rotor       rrrRdddd      dd s  S
  bolts -->   rrrRdddd      dd sssSsss
  here        rrrRdddddddddddd sssSsss <--- stator bolts here
              rrrR  ooo    ooo sssSsss
              rrrR  aaaaaaaaaaaaaaaaaa
 (Exterior)   rrrR  aaaaaaaaaaaaaaaaaa    (Interior)
              rrrR  ooo    ooo sssSsss
              rrrRdddddddddddd sssSsss
              rrrRdddd      dd sssSsss
              rrrRdddd      dd s  S
              rrrR ddddddddddd s  S
              rr R                S
              rr RRRRRRRRRRRRRRRR S
              rr R MMMMMMMMM  S
              rr R               S
              rr R  CCSSSSSSSSSCC S
              rr R  SSSSSSSSSSSSSSS
              rr R  CCSSSSSSSSSCC               rr R
              rr R MMMMMMMMM
               r RRRRRRRRRRRRRR
               r
               rrrrrrrrrrrrrrrrrrrrrrrrrrr
               r                         r

Already existing components in the car (small cap letters):
"r" - light-alloy wheel rim;
"d" - brake drum;
"o" - bearings;
"a" - axle;
"s" - suspension arm;

New components to add so to build a motor into the wheel:
"R" - rotor (stainless steel for disc plate, soft iron for magnetic core);
"M" - neodymium-iron-boron magnets;
"C" - coil windings of stator;
"S" - stator (stainless steel for disc plate, soft iron for magnetic core);

There it is, in all its non-scaled glory. 😉 The red parts group is the rotor I’m building, and the blue parts group is the stator.

This fellow tinkerer, named John Bass, sent me his design explaining that the “1 ton force problem” was resolved by using a double rotor. I had thought of that too, at the time, but that disposition creates another problem: assembly. Such a motor (axial flux, double rotor) is impossible to separate into 2 parts (rotor and stator) without undoing the rotor itself. This is something I didn’t want for my design; I wanted something easy to assemble and disassemble into the wheel, something that integrates 100% into the wheel and car without modifications and also allows for easy maintenance of the brakes, just the way they are now. That’s why I dropped the axial flux in favour of the radial flux: this way I can use double rotor layers and still easily remove the rotor to allow access to the brake drum.

Another difference between his design and mine is that I put the motor outside the brake, whereas he put the motor on the inside and the brake (disc) on the outside. I prefer my design, because this way I can get more torque from the same electric power (the mechanical arm – the radius – is longer).

Anyway, I’m still running the batch simulations… I already found 2 good candidates, 1 Halbach variant with 36 poles and very thin core capable of 950Nm, and another normal variant with 60 poles and thick iron core capable of 1700Nm, both at 100A per phase. Now, there are a lot of approximations incorporated into these simulations, so a better analysis will be done later. But now I just want to find out the best geometries, and this is good enough.

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