Okay... as far as the 3 phase direct current motor that I have and was thinking that potentially by long shot, this answer searching individual might also have...
Probably not, but hey, I have what was considered a 3 phase/speed/split motor that hooked up to AC as long as the common was common 100% of the way (thus, making it DC operated).
If the common works it's way to a ground when the ground is also a ground, it shorts out a system.. maybe not completely, but nonetheless, it creates a short.
So here's the deal... maybe the answer to the question is that IF this motor is a newer motor was at one time set up in a wiring situation as spelled out above, the older motor (or whatever potential crossed common to ground) dragged the newer AC motor down and shorted the field. Just a thought... But since I've never been union, I look at reworking the old hack jobs without complaint and the extra break in between.
Kind of like that old wall switch that gives you the shock when you flip in Aunt Emma's Chicago basement apartment... Or the grounded to common water pipes in Iraq done up with mixed wiring schematics from across the globe where common and ground sometimes interchange.
