As a benchmark, Shane's scooter generates 134N of traction force, so 7.5N isn't even in the ballpark. That being said, the motor does spin so it wasn't a complete waste of time. The whole point of this project was to design and build a hub-motor that was smaller than what was currently available on the market. The stator laminations were ordered from China, while the axle was cut from 6061 bar-stock by a local CNC shop. Next, the lamination's were clamped together and secured to the axle with extra-strength epoxy, along with the winding inserts which were laser-cut from a high-temperature teflon synthetic.
The bearings, axle-hardware, and driver were off-the-shelf. In order to get the number of turns from the design calculation and to make commutation simpler I wound every other stator tooth. This is probably a bit wasteful and definitely doesn't take full advantage of the stator iron, but was the only way to get the 16 gauge magnet wire to fit.
The rotor was constructed from a set of lamination's from Protolam, which were TIG welded together in a custom fixture. My original plan was to use a piece of round 1018 steel as the back-iron, but I couldn't find a nice piece of tube that was the right size, and I would have had to machine down a piece of solid, which would not be fun. Also, I wasn't sure how I was going to properly space the magnets on a piece of smooth round. The lamination's, though more expensive, have a better B-H curve and made assembly a snap. The magnet's were custom ordered from a factory in China, they were the highest strength Neodymium that the company offered at that time (Nd48's I think).
The same epoxy used on the stator core was used to glue in the magnets. The endcaps were turned down on a Clausing 10" lathe at the local hackerspace. The bearing races had to be within a few thousands tolerance, which admittedly took a few tries to achieve. The stator and spoke holes were drilled on a Bridgeport with a DRO with built-in bolt-hole pattern functionality, this saved a lot of time.
Next, the endcaps were heated with an acetylene torch and the bearings and driver ratchet dropped, providing a healthy interference fit as they cooled. 3 120° spaced hall sensor's were attached to the coils to provide commutation signals. With all the parts in-hand the motor was assembled and ready to test.
The video below shows the motor being driven with the e3PH controller at 32V. The controller applies sine-wave voltage on the q-axis at each sensor interrupt, and interpolates based on previous speed in between transitions. There's no 'vector-control' going on here, so with voltage on the q-axis the current vector is slightly lagging the optimal location.
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