Well, here's that broken piece of the magnet.
I had a chance to stop by the forklift rebuilding place today, and I spoke with one of the technicians there. He said that a smaller chunk of magnet breaking off really isn't that big of a deal. It shouldn't make that much of a difference in the overall magnet field. The bigger issue is that the rest of the magnet is still nicely glued to the shell of the motor, which it appears to be.
He said that sometimes, motors can get hot enough for the epoxy to soften, and the magnet actually rotates a little bit inside the shell, and messes up the timing.
I cleaned up the motor as best I could.
It has new bearings, and I green scrubbied the commutator to remove any surface corrosion there.
Cleaned up commutator and new bearing.
Drive end and bearing.
Drive motor armature.
I put the entire motor back together and re-installed the motor, pulley, and belt. I filled the rear tires with air (even though it won't hold more than a few hours) and did some test mowing.
The drive motor does get hot. Not nearly as bad as it did the other night, but the fact that it gets warm at all is a problem. I was able to mow most of my yard, but I didn't want to abuse the motor too much, and my batteries were running low anyways.
I hooked up a couple chargers to the mower and will finish mowing tomorrow. Seems like the next EV Build Day can't come soon enough. I really need to have the guys take a look at this thing and see what we can figure out.
It would also be great to put a clamp-on ammeter on the mower to see how many amps it's pulling.