On Fri, 22 Dec 2017 17:23:53 -0500, Bob Engelhardt
wrote:

Thanks, Martin & Iggy.

The lathe VFD runs the lathe motor and a spare 3-ph motor just fine, so
it isn't the problem. The mill is floating and has no connection to
anything but the VFD.

The problem-motor winding resistances are all good. Their resistances
to the case are all in excess of the DMM's range (megaohms). As are the
winding-winding resistances.

As Iggy pointed out, the VFD output is PWM to simulate a sine wave. The
base voltage is 300vdc+- (240vac peak). I agree that the leak is
probably a matter of this 300v breaching the winding insulation, to the
case.

I suspect that the erratic rotation is a matter of the breaching being a
winding-winding short, because the motor was not grounded in the
alternate-VFD test.

I'm going to _carefully_ try putting 300v between windings & see if I
get any shorting. Stay tuned.

Bob

(BTW - I'm leaving the funny behavior of the mill VFD with a good motor
for later - it's a separate issue.)
Greetings Bob,
With a VFD you can get much higher voltage spikes than the base
voltage. The insulation in your motor should be good for at least 600
volts. So if there is any shorting at 300 volts you know the motor is
hosed but if there isn't that still doesn't mean the insulation is
good.
Eric