Scratchbuild Hub Motor

A friend at MITERS handed me a pile of electric motor stators (the fancy-shaped, laminated, magnetic steel cores at the heart of many electric motors) and I knew immediately that I was going to build a hub motor.

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It turns out building a hub motor isn’t as terribly hard as it seems. The basic idea is to build a brushless outrunner (the type of motor you see on RC airplanes) with really strong sides and bearings. Thanks to the magic of MITERS, I had access to bearings, raw aluminum, steel tube, super-strong magnets, wire, and a stator.

I started out with a massive steel pipe (it needs to be steel for the magnetic properties). The inner diameter of this pipe was too small, so I had to lathe some metal off of the inside walls.

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After a very long time (steel is very slow to machine compared to aluminum), I had removed enough steel that the magnets and the stator fit together in the tube. I used the lathe to cut the piece off of the end of the tube.

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It’s beautiful!

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Here it is, next to the stator and bearings:

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The next step was to glue magnets to the inside of the tube.

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I decided to do a test-fit of all the magnets, and it turned out to be a lifesaver! I put all the magnets into the tube, and at the end, counted. 13 magnets. Precisely 1 less than necessary for a functioning motor! Having caught this potential disaster, I put the tube back in the lathe to make room for one more magnet.

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After making it the correct size for 14 magnets, I began the magnet attachment process.

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I used one magnet sitting on the bench to test the polarity of each magnet before putting it into the motor. Having one backwards causes the motor not to run! One thing to note about super-strong magnets near other super-strong magnets is that they can really jump to each other. I almost pinched the skin of my fingers a few times, which would have been absolute pain. Eventually, I had all the magnets glued into the motor.

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Next, the motor needed endcaps. Thankfully, these do not need any fancy magnetic properties, so I could use aluminum! I spun an endcap on the lathe that would fit partially inside the steel pipe, with a lip around the edge to keep the cap in place.

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Test fitting:

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That endcap was a very snug press-fit, and would not be going anywhere. Once the endcap was pressed in, I could lathe a space for the bearing and axle.

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Boom! Beautiful, concentric bearing installed in the motor. Next, I made another endcap for the other side of the motor.

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It began looking like a real motor at this point! Next, I had to make a rather complicated shaft to hold the stator and bearings.

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Many aluminum shavings were produced that day. After machining the other side, I drilled holes and used bolts to keep the stator from spinning on the shaft.

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The semi-finished shaft inserted into the motor:

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Next, I had to mill flat-spots into the shaft, since the axle on hub motors is torque-bearing.

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Finally, I had to do one of the most tedious part of the project: wind the stator. I didn’t try to absolutely maximize wire density this time, since I knew I would be re-winding this motor after feeling how much torque/speed this winding delivered.

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As you can see, all the wires have to exit through the bearing’s inner hole, since the rest of the motor spins. I packed them against the shaft and slipped the bearing over it.

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At this point, it is a complete hub motor! I’m still working on getting it to spin (either my tiny airplane speed controller can’t handle the massive inductance, or I soldered one of the wires backwards).

Predictions for the future: HUB MOTOR ELECTRIC SCOOTER!

 

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Posted in Hub Motor

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