I had a problem with the (now old) replacement motor of a Sta-Rite Dura-Glas II or Max E Glas II (What's the difference?).
It happened only once so far.
When the motor was powered, it hummed / buzzed for a couple seconds then tripped the breaker.
I tested the start capacitor and run capacitor (microfarads). The start capacitor is in its operating range. The run capacitor is only 2% lower than its operating range.
The shaft spins freely.
What could it have been?