Kitmotter 0002 Rework and Rewind

Sunday, July 22nd, 2012 @ 17:38 | Kitmotter!, Project Build Reports

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.



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    3 Responses to “Kitmotter 0002 Rework and Rewind”

    1. Jules Says:

      I wonder what kind of efficiency gain you would see if your rotor/flux ring was made of low carbon steel instead of MDF.

    2. the chuxxor Says:

      Oh, the rotor itself is actually steel. Only the side plates are MDF (and now the stator hub).

    3. Jules Says:

      Oh, OK, that makes a lot more sense. I love the design, by the way.