Thursday, March 4, 2010

LEAF Motor: Assembly Box of Doom + Back EMF

If you're just tuning in, the LEAF (Less Epic Axial Flux) Motor is the end result of a long and unnecessarily complicated process that started when I decided I would buy a set of 16 neodymium iron boron wedge magnets for $74, figuring I would make a motor out of them some day.

Well now I have two sets of 16 magnets, and I want to put them together with less than 3/4" between them. This translates to roughly 500lbs of force trying to smash them, and anything between them, together. This calls for the fabrication of a very strong box from which to make a giant motor puller. Here's the bottom of the box:

It's only purpose in this world is to hold the lower rotor disk securely. The top of the box is much more interesting:

This is the "puller," with three 5/16"-18 threaded rods that screw into the top rotor hub. The rods in tension do the work of lifting or holding against the axial force, sending it to the side of the box in compression. All together, it looks something like this:

The motor puller box is designed with about a 3x safety factor on all the forces, but along with the box, there are some rules. Rule Number One is never put appendages between the disks. They are always separated by the puller and then the entire top of the box is removed by lifting from the sides before any work is done on either disk. Rule Number Two: whenever the disks are separated, a block of wood between them acts as a failsafe in case the box or rotor disk breaks.

So that takes care of one of the things over which I was losing sleep. The other, winding, turns out to be not that hard at all with the flat magnet wire I picked up from Alpha-Core. Now all I need is a few solid free hours in which to do it. (Actually, probably like a full free day.) But anyway, I managed to do a single winding. I basically half-ass the 40-turn winding, then force it to be the right shape with my winding press:

Patent Pending.

Okay, I will put a little more effort into the real winding. But for now I just wanted to get to the magical single-winding back EMF test that I have so many times claimed is actually a useful thing to do. This time, I'm sure it's legit. With a coreless motor, there is no incomplete magnetic circuit to worry about. The no-load B-field between the disk looks the same with or without the windings. So with one winding's measured back EMF, I can extrapolate the performance of the entire motor. (You probably still don't believe me...I don't blame you.)

Anyway, I put the winding in the stator and put the stator in the box:

(Notice the nice new countersunk non-magnetic stainless screws?)

And stupidly I did not take any pictures of the whole thing together. But to my great relief it actually spins, the magnets no longer grind against the screws, it is centered, and the gap is...well it's not very uniform. It sort-of favors one side. I think it might need a little persuasion toward the other side. But it doesn't seem like the 1/r^2 problem...the dual bearings on each rotor disk seem to take care of that. But anyway mechanical stuff can be poked and shimmed and squeezed into compliance. What about the fundamental premise that this thing can actually transform power?

This time, I didn't need the go-kart to spin it up. (No cogging torque, and no violent shaking of the entire test bench.) I used a smaller but equally unorthodox prime mover: a cordless drill with a hole saw attached to it. The hole saw dug into the rotor hub nicely and got it up to about 600rpm, plenty fast enough to capture a back EMF waveform on the scope:


That is much more sinusoidal compared to the steel core version. Why? It's still a concentrated winding with non-skewed magnets. Well, probably a few reasons: The magnetic interaction is more distributed, not focused to the edges of the stator teeth by steel. It's also interacting with field that is relatively far away from the magnets, where it starts to blend together smoothly from North to South instead of having sharp transitions.

As for the fundamental motor constant, the amplitude of the back EMF as a function of frequency (speed), I get about 9.5V(rms) per 1,000RPM. Since four of these windings would be in parallel, that is essentially the phase voltage of the motor. That's a fairly nice number, since in the practical speed range for a home-built motor (3,000RPM?) it falls into the 72V battery category. Through the magic of power conservation, I can come up with a torque constant for the motor as well: 0.27Nm/A(rms). This is...well it's all pretty much dead on with the predictions. (Wow, even the shape of the back EMF is correct...)

Lastly, I measured the winding resistance for a single 40-turn winding to be 0.127Ω. Since there would be four in parallel, the phase resistance is actually one quarter of that, or 0.032Ω. This starts to give me a feel for the thermal performance of the motor. Altogether, the motor constant and resistance are fairly similar to the array of small EV motors (Etek, Mars PMAC, PERM, etc.). The LEAF, which is the only coreless motor in this lot, will likely have a slightly higher torque constant, but also a higher resistance and lower thermal mass for heat sinking. So we'll see how it compares in continuous and peak ratings. As always, it will depend a lot on the cooling solution.

More winding ahead!

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