For my Electric Razor project I needed to build an in-hub brushless motor. These are not the kinds of things that can be bought, so I had to make it, and it comprised of the bulk of the fabrication and machining that was required for the project. Below is a rendering of the finished motor, or how it would look if all the parts were machined in what my physics teacher would call, Machining Space (an annex of the idealist world of Physics Space).
The motor is composed of a few primary components. There is the Magnet can, a cylinder of mild steel which will hold the magnets, as well as provide a flux path for them. It is also the main structural component of the motor, as it acts as the hub on which the tire will be affixed. The magnet can also has 6 holes drilled into the ends which were then tapped with a 10-24 thread so that the endplates can be affixed. The endplates will hold the magnet can concentric with the stator, and are be secured to the magnet can with 6 socket head screws.They are made of vanilla grade 6061 aluminum. The bearings will be press fit into the endplates. After some searching I decided on some 1 3/8 OD bearings from McMaster, which were a good compromise between size (if they are too thick then the motor will not fit on the back of the scooter) and having enough strength to absorb all the shocks, bumps and bounces that they will encounter every time the wheel bounces off a crack or falls into a pothole. The odd surface finish you see below is due to the fact that everything here was made on a rotary table, or (in the case of the magnet can) very carefully centered and then bored out using a boring head.
The shaft is made out of some alloy steel, and will hold the stator inside of the magnet can and allow the wires to pass out of the motor. I decided to pass the wires through the center of the shaft, by drilling a cross hole radially through the shaft, which allows the wires to pass from the stator into the center of the shaft, and then out of the motor. The surface finish is not what it could be due to the fact that this was made on a bit of a ghettolathe, that is, since I don’t have a real metal lathe, I mounted it in the spindle of my milling machine, then used the fine feed and the x-axis of the table to move a single point lathe tool around to turn it down to size.
The stator was bought from goBrushless, and is there largest, at 65mm in diameter, and with 18 slots. It is epoxy coated which makes it easier to wind since if the stator is not insulated it needs auxiliary insulation because otherwise the edges will cut into the the magnet wire and cause a short. The stator has a half round keyway cut into its ID, but since there isn’t any convenient way to put a setscrew into the assembly, the stator is Locktited to the shaft.
The stator is wound with 21 gauge magnet wire. Each slot got 30 turns, in 2 layers, 15 turns each. Since there are 18 slots that gives me 6 slots per phase. Since thicker magnet wire would have been too much trouble to wind, each phase was wound with 2 sets of 3 adjacent slots, which gave 2 sets of 90 turns in parallel for each. I have got to say that winding this stator was pretty darn painful to the fingers, seeing as I had to pull tight 540 individual turns with my hands, and after about 200, they start to get really sore. I also found that clamping the stator, whether it be in a vice, or with some clamps and a workbench (my choice) it makes it a lot easier to get the windings tight and even than if you try to hold the stator in your hand.
The magnet can was populated with 80, 5x3x15mm N50 Neodymium magnets from supermagnetman.com. They were first held in place with super glue, and then bonded more permanently with epoxy.
Finally here is the completed motor with (a couple) of the cap screws in, holding the end plates on. Just as a little machining tip, to get the holes just right, it helps to use a transfer punch after having drilled the end caps to get the tap holes in the right place on the end of the magnet can, especially if your mill does not have a DRO.
I do not yet have a brushless motor controller (RC airplane variety) so I have not been able to test the motor, though through manually turning it I can tell it turns smoothly, with no binding, and with a moderate amount of cogging (from the magnets).