Originally Posted by C5!
As mentioned previously, I don't know if that stator (no.3) died for the same reason as the other ones, but if the fault is electrical, it appears it comes from the connection between the end of the stator wiring and the external wires, and if it would be overheating I would assume that the wiring should be turning black all around the stator, not only one side !!
Because if you look at the 2 following pictures you see that the wiring is ok on the outside side, and starting to chip on the inside side. So it appears that the heat is coming from the flywheel. (and thus not from the exhaust either)
here is the picture from the outside side, the side facing the cover.
Interesting thread for sure.
I worked at a fractional hp electric motor mgr 38 years ago, so the idea of winding copper wire around steel annealed laminations is not new. In looking at the photo I could not tell if there was an insulator between the windings and the metal. In making motors the stator metal was coated with epoxy for isolation. Edge thickness of this isolation was important as the windings were tested to assure a hi- pot test of 1800 volts.
I was the reliability mgr for a while so we did testing of locking the rotor and life testing in it's application. Failure often occurred with a room of smoke with fried windings.
I would imagine the designers did testing to assure that the design would be good but full load in a hot ambient with rock protection may not have been in the testing plan.
In the case of some windings turning black seems a possible shorting to the laminations (gnd) should be investigated. That short would cause excessive current and heat IMO.
One other rule of thumb was that organic finishes on windings life decreases by 1/2 for every 10 degrees C over 150 degrees C.