I was getting a electrical tingle from my mill, so I took the motor off to check it. It had a "leak" of very high resistance so grounding it would stop the tingle. I put it back on and rewired with a ground. When I tried it, the motor ran strangely - what I would call a pulsing rotation.
My lathe also has a 3-ph motor and VFD so I tried using them to see if the mill's motor or VFD was causing the problem. I used the mill's VFD to drive the lathe motor and the lathe's VFD to drive the mill motor.
With the lathe VFD driving the mill motor, the motor had the same pulsing rotation. Ah-ha - the motor is the problem. But wait, when I drove the lathe motor with the mill VFD, it ran very slowly - maybe 100 rpm instead of 2000. Very confusing.
I checked the motor's windings for continuity & there were none open (it's a 2 voltage motor & has 6 windings).
Any idea of what might be wrong with the motor? (Never mind the strange slow-running with the other combination.)
Bob