Bench milling machine tripping GFCI

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peter d

peter d

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My dad purchased a small bench sized milling machine for making small parts. It's a Micro-Mark, not sure of the model. It's a variable speed model with an electronic power supply.

Anyway, I assemble all the controls and fire it up. The unit runs fine until about 10% speed on the controller, then it trips the Square D QO GFCI breaker. :( I'm thinking that the power supply is creating a lot of harmonics and other dirty power that is causing the GFCI to trip. Your thoughts?
 
peter d said:
I'm thinking that the power supply is creating a lot of harmonics and other dirty power that is causing the GFCI to trip. Your thoughts?
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Harmonics and dirty power are very unlikely to trip a quality GFCI.

More likely your power supply is 'leaking' current to ground through some of its filtering circuits.
 
jim dungar said:
Harmonics and dirty power are very unlikely to trip a quality GFCI.

More likely your power supply is 'leaking' current to ground through some of its filtering circuits.
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Thanks, that makes sense. So I'm assuming there is no fix for this other than putting the machine on a non-GFCI circuit?
 
GFCIs are a stupid waste of money, remove the GFCI.

As a mater of fact just slip the branch circuit hot under the panel feeder lug. :p
 
Maybe some portion of the power supply is going to completely fail sometime soon, or maybe you do have a ground fault on the load side of the power supply and doesn't reach enough current to trip GFCI until certain speed is reached?
 
kwired said:
Maybe some portion of the power supply is going to completely fail sometime soon, or maybe you do have a ground fault on the load side of the power supply and doesn't reach enough current to trip GFCI until certain speed is reached?
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Could be, I didn't have my ammeter with me to verify that, but current is certainly making its way to the EGC somehow.
 
peter d said:
Could be, I didn't have my ammeter with me to verify that, but current is certainly making its way to the EGC somehow.
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Remember that current making its way to the EGC is important only in that it allows for an imbalance between the hot line current and the neutral/grounded conductor current.
If the current comes from somewhere other than the protected phase or neutral wires and gets to the EGC it will not trip the GFCI.
 
GoldDigger said:
Remember that current making its way to the EGC is important only in that it allows for an imbalance between the hot line current and the neutral/grounded conductor current.
If the current comes from somewhere other than the protected phase or neutral wires and gets to the EGC it will not trip the GFCI.
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Yes, I'm aware of that. :)
 
peter d said:
Yes, I'm aware of that. :)
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That could even make a difference if the suspected problem is on the load side of an electronic power supply/convertor. I just hate to see the workings of a GFCI described casually in a potentially misleading way. :)
Not that I am not guilty of similar things myself.
 
GoldDigger said:
That could even make a difference if the suspected problem is on the load side of an electronic power supply/convertor. I just hate to see the workings of a GFCI described casually in a potentially misleading way. :)
Not that I am not guilty of similar things myself.
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Mostly will depend on if the load side of any power conversion equipment is separately derived from the supply side. Basic rectification of the supply does not result in a separately derived output, a transformer that does not tie any secondary conductors to any primary conductors (but can have a point tied to equipment grounding conductor) is a separately derived system, and any ground faults on that secondary will not effect a GFCI protecting the primary conductors.
 
Don't rule out the neutral being connected to ground. That will trip the GFCI every time. Easy enough to check with your ohm-meter.
 
sgunsel said:
Don't rule out the neutral being connected to ground. That will trip the GFCI every time. Easy enough to check with your ohm-meter.
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If the machine will run up to a certain speed before it trips it is not a solid low impedance neutral to ground connection as that would trip it immediately every time.

If there is a DC drive in this machine then there could be a fault in the DC circuit but we may need to reach a certain speed (and voltage) before enough current leaks to ground to trip the GFCI. Could be something as simple as needing to clean the carbon deposits off the area surrounding brushes of a brush type motor.
 
Update: So my dad talked to MicroMark today and apparently they are aware of the issue. They told him not to plug it into a GFCI.

Okkkkaaayyy then. :blink:
 
It might be worth trying a regular circuit breaker, and a GFCI outlet.

How far is the breaker from the mill? I think longer wires between a GFCI device and the load increase the chances of tripping.

GFCI's can be finiky things.
 
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