[OP]
Contributor
I am moving into my new shop and am slowly moving equipment. When I set up my Oneway 1224 on a 120V circuit with a gfci, it would trip every time I started the lathe. I tried the lathe on 4 different circuits. I even changed out the breakers, still the lathe tripped the gfci. I finally had to remove the gfci from the circuit and the lathe ran fine. I would guess that there is something in the variable speed wiring that would cause this. Anyone have any thoughts on this?
Contributor
Does the lathe have a VFD? If so, it will trip the GFCI unit, hardwire it add a disconnect switch on a non-GFCI protected circuit & you should be good to go.
If using GFCI receptacles pick up power ahead of the GFCI instead of the load side, hardwiring is a code compliant way to avoid a GFCI as long as there is a means of disconnecting power.
Contributor
I've heard what Rollie says - that is, using a VFD on a GFCI circuit will cause the GFCI to trip.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Moderator
Well, GFCIs measure the current on the hot wire and the current on the return wire (neutral) and compares the difference. If a detection of about 4-6 mA (milliamps) is found, the GFCI will trip. In this situation, the GFCI assumes the current is flowing elsewhere...like in the green/bare wire and open circuits to stop the flow. Note that a short between black/red and white will not cause a GFCI to trip since the current is the same. Let's hope in this situation that the circuit breaker trips...unless this is an inrush/starting current.
Now I know very little about how a VFD fires up but I think it must have an imbalance in the currents the GFCI is monitoring in the hot and neutral wire. I understand that a VFD can create a lot of electrical noise like crosstalk and common-mode currents so that could be it...the GFCI is sensitive to such situations.
Last edited by Chris Padilla; 06-09-2009 at 11:26 AM.
Member
I have a Jet 1642 lathe and it exhibits the same behavior. I called Jet and it is typical with VFD powered drives. I still have a few non GFCI circuits available so it is not really a problem for me
Member
There are a couple of ways to look at the reasons a GFCI would trip with a VFD or for that matter even a large size motor. The simplest explanation and I believe the most likely requires a basic understanding of how a VFD works. For those easily bored by really technical stuff skip to the last couple of sentances for the easy explanation. When you see battery, you have the basic explanation.
For those still awake, In order to change a speed on a motor, the frequency of the Sine wave to the motor is altered. The VFD alters the 60 HZ output of input power by first converting the input voltage to a large DC value. It then cycles that DC voltage on and off at the desired frequency which produces a square wave output. Since a square wave at some X frequency is really a Sine wave of frequency X plus another sine wave at frequncy 2X and another at multiples of the base frequency X we simply have to remove all the other higher multiples and then you have a sine wave output of your desired frequency. The VFD removes these mulitple by filtering the output using large capacitors.
Simple Explanation... VFD's have Large capacitors...
Large Capacitors have the ability to store large amounts of electric energy and release it like a battery that lasts a very short time. It's that ability to store and release power that really messes with GFCI's as instead of returning current at times, the capicators are storing that energy and at other times the Capacitors are releasing energy that isn't coming into the VFD. The GFCI is looking for that return current to be the the same as the hot leg, doesn't see it and trips. There are other reasons as well, but by far that's the simplest, and since I've been out of school many years, its the one I'm sticking with.
Ok, one more thought, VFD's do nasty things to eachother. It's so bad, that in most industrial setting on lower grade VFD's that can't handle the swings in Input power, we install devices to help filter the incoming power to a VFD when multiple VFD's are on the same circuit to prevent them from causing problems with eachother. The GFCI doesn't stand a chance against that VFD.
Moderator
Thomas,
A square wave is made up of harmonics of a sine wave where f0 is called the fundamental and fn is the nth harmonic where n is an integer multiple of the fundamental frequency. Fourier analysis is a good way to look at the various frequency components that make up a square wave.
Member
By removing the odd harmonics from a square wave you have a resulting sine wave. I worked on UPSs (uninterruptable power supply) many years ago and the output transformer removed the 1st 3rd and 7th harmonic and caps were used to remove the 5th 9th 11th and 13th. The company that made the UPS said that any high order harmonic had little effect on the wave.
However, AC drives use PWM (pulse width modulation) to recreate the current waveform. If you put a scope on the output it has a varying series of square pulses. AC drives don’t use caps on the output to filter the wave. The motor windings do that for you. On the other hand the pulse train is running at about 20,000 cycles. This causes the current being pulled off the line supply to be in a nonlinear draw. It creates some very nasty harmonics on the line side and that does trip the newer GFI breakers.
For years I ran my Delta with VFD in the garage on an older GFI with no problems but after building a new shop a couple of years ago with new GFIs I had the same problem.
Member
Chris,
I was actually trying to explain that concept in the post by explaining the x 2x etc. Joe's actually correct though, most lower end VFD's will not do any filtering on the output, and simply use the winding on the motor as a low pass filter.
Member
So I know what GFCI stands for and I know what it does. I now know what a VFD does, but I don't know what it stands for... anyone?
Contributor
Variable Frequency DriveOriginally Posted by Kevin Rector
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Member
You can still leave the plug on your lathe and plug it into a non GFI circuit, but it will need to be a dedicated circuit with only one plug.
Contributor
If GFCI protection is required in a area (such as residential garages & outbuildings/ accessory structures) that is in error unless there is a specific exception in the code allowing a dedicated circuit, which means if it's cord & plug connected & 120 volts GFCI protection must be used.Hard wiring & a disconnect is the only code complaint way around it, unless it can be reconnected to 240V.
The exceptions are slowly being eliminated in the NEC.
Member
Thanks Mike, I like learning new stuff.
Member
If a circuit is dedicated to an appliance such as a freezer or a cump pump it does not need to be GFCI protected. If the lathe is the only thing that can be plugged into this circuit, then you don't have a lamp cord or TV cord that can get wet.
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