RE: [CAD_CAM_EDM_DRO] Mill Motor trips GFI outlet

I missed the original posting on this subject but from the ones I have read
it appears someone is attempting to operate a machine tool powered from a
GFCI outlet. Plainly stated, you don't do that! GFCI outlets are designed
to interrupt the power in case of shorting, say by water or some other
condition. They are also OK to use with small appliances such as toaster
ovens, mixers, food processors, etc. However, using them with larger
inductive loads, such as on a milling machine, will often result in
frustration due to them constantly tripping.



They don't have much delay built into them and any decent sized AC motor
will pull enormous amounts of current on startup and that increased current
draw can be present for a long enough period that the GFCI will trip,
virtually every time. Also, the more a GFCI trips, the weaker it gets and
will trip with less and less current draw until it simply will not reset or
will trip no matter how small the current through it. They are cheaply made
to start with, even the more expensive ones, so they are not manufactured
for repeated activation.



Unless there is some local code requiring the use of a GFCI in your location
and for your application, get rid of it and your problem should be solved.
IF the machine is properly grounded that is.

Randy Abernathy
CNC and Industrial Machinery
service, repair, installation and
design

4626 Old Stilesboro Rd NW
Acworth, GA 30101
Phone: 678-982-0235
Fax: 770-974-5295
E-mail: cnc002@...

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From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
Sent: Wednesday, November 07, 2012 9:07 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Mill Motor trips GFI outlet

On 11/07/2012 12:24 PM, Gary Click wrote:
> The GFI trip is most likely due to the inductive nature of the load, a
slight
> leak to ground or simply the misapplication of the GFI to the device.
>
Another cause of false GFCI trips is switching power supplies. The mill in
question, I believe, has one of those Chinese DC motor speed controls,
which is a switching power supply. It acts by turning current to the motor
on and off very fast. This produces large voltage swings at the motor
terminals with very fast rise times. For suppression of electromagnetic
interference from the motor, wiring, and conducted out the power
cord, they put capacitors to the safety ground. The GFCI sees this
as a fault current, and acts accordingly, if the magnitude exceeds
the trip threshold. So, many of these units will never coexist with
a GFCI. The curious thing is it used to work, and is now tripping
the protector. That probably indicates this EMI/capacitor thing
is not the cause.

Motor inductance really has no effect on a GFCI. It measures any
difference between current out the hot wire and current returning
through the neutral. If there is more than about one milli-ampere
difference, it shuts down. A high- or low-inductance motor will
all return the same current as it gets, not diverting any to
safety ground.

Jon



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