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

Does the VFD drive like before if not - maybe you blew a phase with a
bad Mill motor and then it the VFD can't drive like before.

I would make sure the lathe and mill are not connected. Is the mill
motor messing with the lathe as it is checked ? Maybe disconnect the
mill and try the lathe with the VFD and see it it is fast. If not,
maybe you blew a phase and need to detect waveforms / voltages on each
phase of the output of the VFD.

The mill motor likely has a ground now somewhere in the winding and
causing this issue. Take off the ground and see if the motor runs ok.
The winding is causing odd voltages to appear on the 3 phase.

The motor likely needs re-winding or replacing.

Martin

On 12/20/2017 10:03 PM, Bob Engelhardt wrote:
I was getting a electrical tingle from my mill, so I took the motor off
to check it.Â* It had a "leak" of very high resistance so grounding it
would stop the tingle.Â* 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.Â* 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.Â* Ah-ha - the motor is the problem.Â* But wait, when I
drove the lathe motor with the mill VFD, it ran very slowly - maybe 100
rpm instead of 2000.Â* 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?Â* (Never mind the strange
slow-running with the other combination.)

Bob

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

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.)

"Bob Engelhardt" wrote in message
news
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.)

This megger applies up to 1000VDC to detect high voltage breakdown.
https://www.amazon.com/Insulation-Me.../dp/B00975X2FY

The meter movement has opposing voltage and current windings which
make the resistance reading (V/I ratio) valid at less than full
centrifugal-clutch-limited cranking speed, and voltage.

You can roughly measure breakdown voltage with it by connecting a DVM
in parallel and watching for the megohm reading to dip below 10 as you
increase the cranking speed. How rough the voltage reading is depends
on how constant you can hold the cranking speed. The reading is fairly
stable when measuring the breakdown voltage of a Zener diode or a
rectifier.

Don't use a cheap HF meter to read voltage, their input is only 1
megohm. They are OK to monitor the current, which is tens to hundreds
of microamps, up to 1 mA into a short. A 90 megohm divide-by-10
resistor in series with the voltmeter will let the generated voltage
go higher.

The crank pivot can benefit from a drop of light oil, which also lubes
the concentric speed-up gear shaft.

It's hand-powered so you will stop if you get shocked.
http://www.biddlemegger.com/biddle-ug/Megtypes_UG.pdf

-jsw

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