Bob, I am not really very conversant on this, but as I understand,
VFDs produce "chopped" sine waves, and due to inductance and
capacitance of wires the VFD produces barely noticeable (for the
motor's mechanical behavior) voltage spikes due to fast changes of
square wave voltage dv/dt, that break through the insulation, what is
what you feel. That is damaging to the motor insulation as it degrades
it, and is unsafe in many ways.

You can read this to begin with.

http://www.transcoil.com/Public/Docu...orFailures.pdf

On 2017-12-21, Bob Engelhardt wrote:
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