LEAF Motor: Spin-Up

The Less Epic Axial Flux motor, a thought exercise a bit over five weeks ago, is now a spinning physical prototype. (Okay, I cheated a little since the rotor disks were already made for the original Epic Axial Flux motor.) Most of what remained to be fabricated was the new coreless stator. Although the entire motor did get a bit of a redesign. Here is the exploded view of the latest design:

Click for full-res.

And, the collapsed version:

  Click for full-res.

The only new thing since last post is the "outrunner outrigger." It connects the mounting structure to one side of the motor shaft, which is not rotating. This completes a structural loop, giving the shaft more stiffness than if it were just connected to the central stator structure. The extra weight it adds is offset by the ability to take significantly more abuse from the chain load on that side of the rotor. This is a minor design change, though. What about the windings?

There's the first six, done in about three hours.

Compared to the ones in the scooter motors, these windings are a breeze to make, for a number of reasons. First, the flat magnet wire is absolutely wonderful. It packs nicely and stays where you put it. But, it only really works because I can remove individual winding inserts and wind in free space. This means instead of looping wire through slots, I simply spin the entire insert, spooling wire onto it under tension. It's not trivial, but it's not very hard either. I would place that in the "win" category for this motor.

All the windings, a two-day job.

With all 12 windings in place, and plenty of room in the center of the stator for interconnections, the job is easily finished. I decided to connect all four winding sets (12 divided by 3-phase) in parallel, and to wire the parallel sets in delta. This will give the lowest-voltage, highest-current motor possible with these windings. Before you yell at me for that being the most inefficient configuration: To first order, it can produce the same power per unit dissipation in the copper. And even in this lowest-voltage configuration, the motor still takes about 50V to get to 3,000RPM. When I get a controller that can handle more than 48V, I'll re-wire it. For now, I'm eager to get on with testing, so I put it together:

Kit Bot FTW. I'll replace the outrigger later.

It's not set up for sensored control yet, so I needed to borrow a high-current sensorless controller. Luckily, I know exactly where to find one. Cold Arbor, a combat robot with a brushless cold saw, happens to live nearby and also happened to have pre-cut and stripped wires courtesy of a particularly violent match. So, with a kick-start to get the sensorless controller going, it spins up:

This test went to 1,500rpm. (Not 3,000 as the tachometer seems to say.) That's at 22V using BLDC (square wave) control. This falls right into the expected range for the delta-wound, 40-turn, 4-in-parallel winding set. It seems to draw a good deal of no-load current at this speed (15A). I think it's too early to say whether this is a function of circulating current in the delta winding, eddy currents, bearing losses (and in what proportions). The next step will be to set it up on my sinusoidal controller with rotor position sensors and see how it fares then. Since this is the target control scheme anyway, I will defer judgment for now and move on to smaller and better things...