# VFD Ground Current



## jbonk (Nov 20, 2012)

AB PowerFlex 700 is putting about 15% of its current to ground. When I measure the wire from the drive to the cabinet with an amp clamp. Installed next to a similar drive with similar load and the other drive is showing <1 Amp. Each VFD is powering 50 1HP motors. Each motor has its own OL and fuse protection. I can control the motors individually via the starters that are after the VFD. There isn't one motor or group of motors that causes the problem. Thought the problem was the VFD but that has been replaced and still getting the same reading with an amp clamp. System operates fine as is but any time a motor winding fails and shorts to ground the whole system is brought down by a ground fault on the drive. In the past before the newer VFDs were installed, a ground fault on a single motor would trip its own OL/fuses without shutting down the whole system. Any ideas on how to track down the problem/correct it?

Josh


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## JRaef (Mar 23, 2009)

jbonk said:


> AB PowerFlex 700 is putting about 15% of its current to ground. When I measure the wire from the drive to the cabinet with an amp clamp. Installed next to a similar drive with similar load and the other drive is showing <1 Amp. Each VFD is powering 50 1HP motors. Each motor has its own OL and fuse protection. I can control the motors individually via the starters that are after the VFD. There isn't one motor or group of motors that causes the problem. Thought the problem was the VFD but that has been replaced and still getting the same reading with an amp clamp. System operates fine as is but any time a motor winding fails and shorts to ground the whole system is brought down by a ground fault on the drive. In the past before the newer VFDs were installed, a ground fault on a single motor would trip its own OL/fuses without shutting down the whole system. Any ideas on how to track down the problem/correct it?
> 
> 
> Josh


Your old system likely did not have GF sensing in the VFD, all new VFDs now do have it. But I'm not sure what you mean by saying 15% of the power is "going to ground". If that were true, the drive would trip. And what do you mean by "When I measure the wire from the drive to the cabinet with an amp clamp." Are you talking about the input conductors feeding the drive? What are you measuring with the ammeter, the ground conductor?


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## Peewee0413 (Oct 18, 2012)

what is the system? the motors are for what?


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## Jabberwoky (Sep 2, 2012)

I'm not sure what the 15% to ground means either.

If the drive is tripping when one of your motors fail I would say the ground fault protection on the drive is probably working as designed. If you want a system that will continue to run the other motors when one fails I would think you need to get a VFD for each motor and sync them. I can not recall if any new drives can have ground fault detection disabled. Another option would be to get a rebuilt version of the previous drive that was installed, if possible.


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## jbonk (Nov 20, 2012)

If I put an amp clamp on the ground wire going to the drive I am reading about 9-10amps. L1,L2,L3 current is about 45-50 Amps at 490V/60Hz. T1,T2,T3 current is about 60A at 450V/45Hz. All the motors are running fans in a pasta dryer(I am working at a Kraft Foods plant). The are two VFDs but each essentially does the same thing. The 1st VFD runs the fans in what they call the "pre-dryer". The 2nd VFD runs the fans in the "final dryer". I have read up on the ground fault and the manual says it is tripped when current to ground exceeds 25% of the output current. I think that if I can fix the continual 9-10A going to ground, a single motor could short and blow it's fuses/trip OL without faulting the drive.


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## wendon (Sep 27, 2010)

jbonk said:


> If I put an amp clamp on the ground wire going to the drive I am reading about 9-10amps. L1,L2,L3 current is about 45-50 Amps at 490V/60Hz. T1,T2,T3 current is about 60A at 450V/45Hz. All the motors are running fans in a pasta dryer(I am working at a Kraft Foods plant). The are two VFDs but each essentially does the same thing. The 1st VFD runs the fans in what they call the "pre-dryer". The 2nd VFD runs the fans in the "final dryer". I have read up on the ground fault and the manual says it is tripped when current to ground exceeds 25% of the output current. I think that if I can fix the continual 9-10A going to ground, a single motor could short and blow it's fuses/trip OL without faulting the drive.


Just curious, do you have shielded cable running between the drive and the motor? Is there a chance that you have a fault in the cable?


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## JRaef (Mar 23, 2009)

No. There is almost no relationship between the input and output of the drive. A drive converts the AC to DC, then recreates AC from the DC. The input AC is simply a resource, i.e. raw material that the drive uses to create a new AC source for the motor. When the drive refers to ground fault protection, that is strictly the output current, there is no line side protection for the drive unless you add it.

What you are reading at the input side is probably what is called "common mode" ground current, essentially noise created as a consequence of what the drive does. You can deal with it by adding a filter on the front end, or even at least just a reactor (or a shielded isolation transformer if you have a resistance grounded system). But that has zero to do with GF trips on the output side.

The GF protection on the output is done by what is called "residual current detection"' where the current sensors on each output phase are arranged such that if there is too much imbalance in the output of each phase, an amount of current flows in the pseudo neutral of the detectors, then presumably that imbalance represents current flowing to ground. However it can also be fooled by current flowing or not flowing elsewhere, because the drive's detection circuit cannot tell the difference. The problem with having a large number of small drives on the output of one drive is the increased risk that even relatively small current imbalance issues are multiplied which can trigger issues like this. For example you can get what are called "standing waves" in the output conductors that may normally not be harmful to one motor on a short run, but can be additive among the multiple motors and become problematic. If you do not have at least an output reactor ahead of where the drive output is split to go to each separate motor, this can be your first course of action. The output reactor slows down the rise time of the pulses going to the motors and decreases the standing wave generation. Under the right circumstances, standing waves can reach voltages of over 3x the RMS line voltage, and motors not designed for that can "leak" current through the insulation in the windings, eventually causing shorts in the weakest spots, such as the first turn in the windings.

So be advised, if you have been having this problem for a while, your motor windings may already be compromised, which is what I suspect. The ground fault problems may actually be motor winding shorts created by this and they are causing what the drive is _interpreting_ as ground faults.


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## denny3992 (Jul 12, 2010)

Very interesting setup, never heard of doing it that way!


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## Wirenuting (Sep 12, 2010)

JRaef said:


> No. There is almost no relationship between the input and output of the drive. A drive converts the AC to DC, then recreates AC from the DC. The input AC is simply a resource, i.e. raw material that the drive uses to create a new AC source for the motor. When the drive refers to ground fault protection, that is strictly the output current, there is no line side protection for the drive unless you add it.
> 
> What you are reading at the input side is probably what is called "common mode" ground current, essentially noise created as a consequence of what the drive does. You can deal with it by adding a filter on the front end, or even at least just a reactor (or a shielded isolation transformer if you have a resistance grounded system). But that has zero to do with GF trips on the output side.
> 
> ...


And isn't there still the problem of current flow thru the motor bearings? 
If I remember this issue can cause premature bearing failure.


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## Jabberwoky (Sep 2, 2012)

If you are getting OL trips because of bearing failure you have a mechanical issue. I would highly doubt the EMF from a 1HP motor could create any significant bearing current (microamps?) to cause any real reduced lifetime of the bearings. If you have current in the bearings from an insulation breakdown that is another issue. Like JRaef said the standing waves beat the hell out of the insulation in the windings.

I have only ever seen current flow evidence in bearings once. Little pits are visible if you have arcing across them. That situation was in fact a blower with bad insulation on the wires and the case not properly grounded.


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## Wirenuting (Sep 12, 2010)

Jabberwoky said:


> If you are getting OL trips because of bearing failure you have a mechanical issue. I would highly doubt the EMF from a 1HP motor could create any significant bearing current (microamps?) to cause any real reduced lifetime of the bearings. If you have current in the bearings from an insulation breakdown that is another issue. Like JRaef said the standing waves beat the hell out of the insulation in the windings.
> 
> I have only ever seen current flow evidence in bearings once. Little pits are visible if you have arcing across them. That situation was in fact a blower with bad insulation on the wires and the case not properly grounded.


Around here people think more grease is better. Bearing failures on air handlers are blamed on magic. They are never blamed on over heating due to over greasing. Few here even think to clean the old excess off. 
Motors fail for electrical reasons and not because the windings are filled with grease. 
Next 10+ HP motor I yank, I'm going to pop the bearings out and look for pitting. 
I'll post pics if I find some.


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## jbonk (Nov 20, 2012)

Thanks for the response JRaef! I will work on getting the reactor ordered and installed and then report back on my findings. Do you have a particular one you recommend or are they all pretty much the same?


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## JRaef (Mar 23, 2009)

:whistling2:


Wirenuting said:


> And isn't there still the problem of current flow thru the motor bearings?
> If I remember this issue can cause premature bearing failure.





Jabberwoky said:


> If you are getting OL trips because of bearing failure you have a mechanical issue. I would highly doubt the EMF from a 1HP motor could create any significant bearing current (microamps?) to cause any real reduced lifetime of the bearings. If you have current in the bearings from an insulation breakdown that is another issue. Like JRaef said the standing waves beat the hell out of the insulation in the windings.
> 
> I have only ever seen current flow evidence in bearings once. Little pits are visible if you have arcing across them. That situation was in fact a blower with bad insulation on the wires and the case not properly grounded.





Wirenuting said:


> Around here people think more grease is better. Bearing failures on air handlers are blamed on magic. They are never blamed on over heating due to over greasing. Few here even think to clean the old excess off.
> Motors fail for electrical reasons and not because the windings are filled with grease.
> Next 10+ HP motor I yank, I'm going to pop the bearings out and look for pitting.
> I'll post pics if I find some.


There is always a risk of bearing failure, that's true, but that is not directly germaine to this situation. The OP never said anythng about bearing failures, he only mentioned winding failures. Bearing failures are usually not as big a deal on small motors either, the amount of EDM current flowing across the bearing races is somewhat proportional to the motor current, so on small motors there is not as much going on there. That's one of those things that I wouldn't waste a lot of money on until someone shows me a failed bearing. It might be happening, but if the bearings last 15 years instead of 25 years, is anyone really going to notice the difference?

And yes, over greasing is more often a problem for bearings than EDM pitting. I once did a project at a chocolate plant where the Millwrights came along while I had the system shut down and pumped every motor with a zerk fitting so full of grease that it oozed out of the motor flanges. When I suggested that it might be too much, the response was "We have a lot of bearing failures around here, so we make sure to pump 'em full like this at least once a month." They just didn't get it.


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## JRaef (Mar 23, 2009)

jbonk said:


> Thanks for the response JRaef! I will work on getting the reactor ordered and installed and then report back on my findings. Do you have a particular one you recommend or are they all pretty much the same?


Nah, six of one, half dozen of the other, it's pretty difficult to screw up winding some wire around an iron core. TCI or MTE, most of the others sold by VFD people are brand labels of one or the other of those two. I tend to use TCI because I have a friend that works there and he buys me lunch a couple times a year when he comes to town. I'm a cheap date....


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## Wirenuting (Sep 12, 2010)

JRaef said:


> :whistling2:
> 
> There is always a risk of bearing failure, that's true, but that is not directly germaine to this situation. The OP never said anythng about bearing failures, he only mentioned winding failures. Bearing failures are usually not as big a deal on small motors either, the amount of EDM current flowing across the bearing races is somewhat proportional to the motor current, so on small motors there is not as much going on there. That's one of those things that I wouldn't waste a lot of money on until someone shows me a failed bearing. It might be happening, but if the bearings last 15 years instead of 25 years, is anyone really going to notice the difference?
> 
> And yes, over greasing is more often a problem for bearings than EDM pitting. I once did a project at a chocolate plant where the Millwrights came along while I had the system shut down and pumped every motor with a zerk fitting so full of grease that it oozed out of the motor flanges. When I suggested that it might be too much, the response was "We have a lot of bearing failures around here, so we make sure to pump 'em full like this at least once a month." They just didn't get it.


I saw you addressed the OP's question really well as always. 
I just tossed in that statement to see you feelings on it. 
I feel your pain about over greasing. We had that done at the hospital here years ago. Them old plug in/55 gal drum grease guns did wonders. 
The nice thing was back then we rewound our own motors. Them old guys did great jobs. But it became a lost skill here after they retired. Now we dispose of everything less then a 25.


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