I had an ESC shutdown on me mid flight. I had to land and reboot then it ran fine for a few minutes then did it again.
Later I discovered that the motor controlled by that ESC was harder to turn than the other motors. I took the motor apart and found one of the bearings was gummed up and not spinning free so I ordered some replacement bearings. When I reassembled it with the new bearings it still had the same problem.
While turning the motor by hand it wanted to stop about every 1/2 inch of rotation (the distance between the magnets). It didn’t feel like a physical obstruction but like a magnetic field was stopping it. The other three motors don’t feel like that. Turning it by hand felt like a motor with fewer stators which wants to stop as the magnets pass each stator. These motors have so many magnets and stators that they don’t normally feel like that.
Under power the motor had a loud whine that I didn’t notice while flying. After a minute of power at low throttle without a load (no prop) the motor got quite warm and the lead wires got hot.
I noticed a black wire inside the motor. There was a short circuit across a phase where a wire heading to a lead made contact with a coil. Note, the wires inside of motors are not bare wire. They are coated with an epoxy coating. Not sure what caused the epoxy insulation to come off the wire to allow the copper to make contact and create a short (maybe vibration or a piece of debris). The short circuit caused just the one wire to get hot and blacken the epoxy coating:
I pulled that wire away from the coils and suddenly my motor spun free again while turning it by hand and no longer wanted to hesitate as the magnets passed the stators.
At this point I decided fix the epoxy coating by brushing any suspect area with silicone modified conformal coating. That stuff is made for waterproofing circuit boards. I generously brushed two coats over any suspect area. It glows purple in UV light so when I took it outside to test it I could see where the coating was applied:
I was only able to repair the coating because the fried wire was loose and I was able to pull it away from the other wires. If a wire wrapped tight around a stator got fried you wouldn’t be able to get conformal coating between the wraps.
I ran it up to full throttle and it runs smooth and didn’t get hot. I might take it for a flight later today if the weather is good.