Category: Kitmotter!

Build your own electric something-or-other!

Kitmotter 0002 Design Files

chorl2 Comments

Now that I’ve put a few miles on Kitmotter 0002 in the form of Johnscooter, I’m more confident in releasing the design so people can build their own versions or modify it.

Ride testing has let me measure Johnscooter’s propulsion efficiency at about 29 Wh/mi. This is a little worse than RazEr’s 25 Wh/mi, but besides having the World’s Trashiest Bearings, the fact that Johnscooter is operating almost entirely in 25A constant-current mode (due to the current-limiting nature of the motor controllers) means I’m basically full-throttling it everywhere. RazEr and RazEr Rev had a more “open loop” voltage drive which let me draw brief bursts of current to get over obstacles at a higher speed where Johnscooter would slowly crawl through the same.

The motor itself has been holding up fairly well for being made of particleboard (more properly, hardboard, which I suppose is rather plastic-y as wood goes). The new stator hub has been holding up with no issues so far, and it was the most worrisome part of the current design.

It has a few dings on it from actual use on sidewalks and bike lanes, and I never did drill those through-holes for the wheel mounting flange. So the wheel is loose on the motor a little, but when rider weigh is pressing on everything there’s nothing that feels strange.

The trashy 5/8″ bearings have loosened up substantially over time – both in seal drag and in…. radial runout tolerances. I can now grab the motor and wiggle it very slightly, and the greatest contributor to the noise spectrum when riding is those bearings. If anyone else builds this, remember that even though I have the Trashy McMaster Bearings listed in the Bill of Materials, real precision flanged 5/8″ bearings should still be seeked out if possible.

Speaking of BOMs, the files are located here and this is what the ZIP file contains:

  • Original design files in Autodesk Inventor 2012 format. Unfortunately I’m not quite so motivated as to provide generic/open source models for everything.
  • Premade DXFs of all motor mechanical parts ready to cut on your favorite 2D Rapid Proto gadget. The materials (e.g. “0250_al” meaning .25″ aluminum) are recommendations, but the rotor rings MUST BE STEEL. Otherwise, feel free to cut the 1/8″ spacer rings and 1/4″ endcaps from polycarbonate for that clear case-mod motor effect or aluminum for extra strength.
  • Basic bill of materials with most hardware and mechanical parts. The tools I used to carve out the wheel center are also included in the BOM. There’s things that are missing from the BOM because they are variable with each design, including the magnet wire, output leads/termination methods, and optional Hall Effect sensors.

There’s some fab considerations which I’d like to share for this particular design, as they might make the motor more durable:

  • Please, please make the endcaps from something which is not wood. Pleeeeeease. (There’s nothing that wrong with it mechancially, I suppose, but just don’t ride through rain).
  • The two 1/4″ discs can be made from four 1/8″ discs – this is in fact what my endcaps above are – if it would make it easier and cheaper to fabricate. For instance, I ordered a single panel of 4 discs plus the “0125” parts from Big Blue Saw.
  • The stator hub is designed to be a loose fit with a standard non-kerf-compensating laser cutter. Waterjet cutters and BBS’ laser services all offset the kerf such that parts come out “true” to dimension. Parts made using this method will be tighter-fitting into the stator and shaft as a result and might need filing or sanding.
  • My final winding is 30 turns (+/- 2) of 9-strand-parallel 28 gauge magnet wire. This is equivalent in area of copper to a single 18.5 gauge wire. Dual stranded 22 gauge wire is also workable and 22ga is probably easier to find. This results in an intrinsic Kt (V/rad/s, or volts generated per rad/s of rotational speed & Nm/A, torque per ampere of phase current) of 0.18 – the Kt will vary a little depending on exactly what drive profile is used (trapezoidal, sinusoidal, sensored vs. sensorless, poor vs. well-timed sensors…). The final line-to-line resistance was 0.31 ohms.
  • Magnet wire is cheaper on Amazon and eBay, but here’s a McMaster roll of 22 gauge.
  • Drilling the wheel flange holes through the tire is probably a good idea. Wood/sheet metal screws, with coarse threads, should be used to fasten wheel to the flange.
  • Really, really crank down the #2-56 axial tie rods (but don’t strip the threads!). #2 hardware will pretty much fail in shear instantly if they’re loose and your weight is on them, which is why I tried to use so many tie rods to compensate. The more these are tightened, the more static friction exists between the layers of the motor, so the more weight they can take from the wheel without shearing the screws.

I expect to see more examples of this construction in the near future. Remember, buildable without big machine tools!

Kitmotter 0002 Rework and Rewind

chorl3 Comments

Last time I left off with Kitmotter 0002 suffering from melty-hub syndrome; during test riding, the stator had gotten so hot as to melt its sintered nylon hub partially, causing it to lose grip on the shaft and shear my wire insulation off, causing a short. It was clear that I would have to replace the hub with something more durable and high-temperature.

As much as I would love to include all of the common rapid prototyping / digital fabrication processes on one device, and have a hub 3D printed out of stainless steel, there are cost and practicality concerns. Since half of Kitmotter is made of MDF already, I elected to continue using the material for now. If I could come up with a hub that works when made of MDF, then it should also work when made of aluminum.

This is the layered hub design. Luckily, having to stick to Z-axis thickness quanta of 1/8″ means that this hub is exactly 1″ long, so a few different materials can be used to finely tune costs. I added more ‘hub features’ which fit the HP Laserjet 8000 motor’s stator, as well as a deeper keyway. This hub will be assembled on the shaft and glued in place first, then the stator slipped over it and glued again. With a multi-piece design like this, I didn’t want to risk relying on a press fit (especially since the shaft is a stock chunk of keyed drive shaft and has no ‘placement features’), hence the added mechanical coupling.

I elected to laser this one in-house for quick results. I got a little off on the tolerances, so this is going to end up a little tight, but that will be fixed in the design files before I release them.

Pressing out the old stator hub, which was totally melted in place, required a little attention to not damage the windings. I used an arbor press with a giant wad of shop towels under the copper windings for the initial shove, then just grabbed it and wiggled the assembly apart. You can clearly see there how the normally powdery surface of the SLS nylon part has been melted.

The outside tolerances were correct, so the stator slipped on properly. I used ultra-thin CA glue to retain it – the stuff wicks into the thin and porous MDF gap very well. A higher temperature solution would be coating both stator bore and hub in epoxy, but this was quicker.

It turns out that the new hub’s projections to grip the stator’s “keyways” did not play well with how the phase windings crossed the stator, so I had to cut the crossing strands and solder in jumper wires.

I repaired the damaged heat shrink insulation (with more heat shrink) and tossed it back onto Johnscooter. I was able to get around campus with it without the motor displaying signs of overheating (though the shaft did get very hot). This thing is still really underpowered.  With Kitmotter 0002 coming in at a dismal 0.41 ohms line-to-line, it means I was (again) losing half of every watt put into the motor as heat.

making it betur

As I mentioned before, the winding on Kitmotter 0002 was kind of bullshit – like Kitmotter 0001, I made it quickly without any intention of it actually producing torque. There’s an immense amount of open space still in those slots. 30 turns of my Chibikart hex-28 gauge produced a reasonable torque per amp, but it came at the cost of high resistance – i.e. not that many amps to be had. For motors of the same size, a higher resistance motor tends to heat up more and run slower just due to the I*R penalty of no-load current draw coursing through it. It also won’t produce as much stall torque because the R limits the stall current.

Well that’s ass. If hex-28 worked, then nona-28 should also work:

I ordered 3 more reels of magnet wire for my “Hobbykinging Rig” that I made for winding the Chibikart motors. 9 28 gauge strands in parallel are equivalent to a roughly “18.5 gauge” winding in terms of “circular mils” of copper. It should reduce my resistance by a third, putting me more in the .28 ohms range. I hope.

That’s much more like it. See those filled slots? 30 turns on this core is at the edge of sanity (and windings falling out sideways – the bad version of “Hobbkinging” you don’t want to experience in your motor). Maybe I could have done 31 or 32 if I pulled harder, but 30 turns is on par with the previous winding. The same torque per amp, but less resistance, yields a motor with more maximum output power. Unfortunately the resistance turned out to be more like 0.31 ohms. I suspect the extra is due to the “end turns” effect, where successive layers of windings need to travel around the end of the coil for a longer distance to reach the active length of the stator again. For this reason, busbar windings and holy-shit-how-did-you-wind-that Crazy German Guy 12 gauge magnet wire windings win over Hobbykinging, still. But hey, reduction of resistance by about 25%.

The stator pictured above is another RH7-1260 stator I had (it’s not the one inside Kitmotter 0002). I performed a “stator swap” after finishing this winding. The result is that Johnscooter doesn’t necessarily have more torque to launch with, since the Jasontroller current limits to about 25 amps, but it can keep accelerating for longer because the lessened resistance means the motor’s back-EMF can keep building (accelerating) further before the sum of it and the I*R voltage drop equals the supply voltage from the controller.

Once Kitmotter 0002 gets a few more miles on it, I intend to upload the design files to my site (the Useful Stuff section really needs more love); both 3D models and ready-to-cut DXFs will be provided.