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