# Lets talk PF750 ground fault



## JRockey (Apr 9, 2018)

Prior to this summer we had only seen a few PF 750's come in with ground fault related problems. Currently we are dealing with about a half dozen and saw numerous others earlier this summer. They are mostly frame 7's but we have seen a couple frame 6 and 1 smaller. There also seems to be a relationship between these frame 7 drives that experience ground fault issues and then experience a catastrophic failure with the capacitors and capacitor board. Once this happen the drive is rendered unrepairable because Rockwell will not sell the capacitor board. 

From what our local distributor has told us, there are new (PINT2) gate driver boards that mitigate this issue by decreasing noise levels, but even this hasnt totally resolved the issue. I have read that cleaning the circuit board fingers and reseating can help. Odd, because we used to do this way back in the early 90's when I was a repair technician and worked on digital spread spectrum cordless phones. Id expect that to work on RF related issues but never expected noise issues on a VFD could be remediated in the same manner.

We have seen brand new units exhibit the ground fault as well as somewhat newer units and even older units. We can run a unit for hours on end and then send it out to a customer only to have them see it trip within 15 minutes. Makes us wonder if their electrical system grounding has something to do with it. Either way, the PF 750 seems way more sensitive to ground fault issues than any PF 70/700 we have seen. Are you guys seeing the same thing?


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## gpop (May 14, 2018)

I wonder what ground fault really means to the drive. (what has to happen for the program to produce this fault)

Im guessing that there are probably ct's and when one goes out of balance the program reads that as a ground fault. (current when somewhere). That way re-seating a card could correct the problem if the ct connection is loose or dirty.


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## JRockey (Apr 9, 2018)

gpop said:


> I wonder what ground fault really means to the drive. (what has to happen for the program to produce this fault)
> 
> Im guessing that there are probably ct's and when one goes out of balance the program reads that as a ground fault. (current when somewhere). That way re-seating a card could correct the problem if the ct connection is loose or dirty.



We do not believe this is a current sensor issue because from our experience when the CS is bad it causes a hard fault that is immediately tripped and cannot be reset. The issue I am describing is intermittent and can be reset. The ground fault for the PF750 is when the calculated current to earth ground exceeds 25% of the rated current for the drive. Exactly how it is calculated is probably info Rockwell keeps tight to the vest. Apparently excessive noise in the system can cause false positives in the calculation.


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## John Valdes (May 17, 2007)

I'm not sure what you got going on, but ground faults on VFD's are usually ground faults.
Are you connecting these controls to motors you know are in excellent health?


Have you removed the ground and tried to run these drives with a motor connected?
Since you are repairing theses drives I suspect you know what you are doing.


But I have had this issue with every drive manufacturer under the sun and its 99% of the time the motor.
Even motor shops cannot always find these hidden faults. But in most circumstances when the customer allowed us to send the drive and motor back to the manufacturer, it always seemed it was a motor problem. Not the drive but the load.


We were a EASA shop with Baker surge tester, high pot and 10 digit resolution resistance tester. In many instances we could not find the ground fault in the motor.
But the manufacturer did.


You say you work with a distributor? Have you gotten the manufacturer involved with this?
If your distributor is any good they would be looking to the manufacturer for answers.
And since you seem to be a repair facility are you a manufacturer repair center?


Manufacturer - Distributor - End user. Where do you fit into this?



But it looks like you are having grd fault issues that are control related. Maybe take a step back and actually address the fault.
You might be surprised.


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## JRaef (Mar 23, 2009)

There was an issue with Frame 6 drives for sure. Within a specific date code, they had a Honeywell current transducer inside that had a high failure rate, giving false signals. A-B issued a warning about it on their Knowledgebase system and will repair any drives that are within that date code but out of warranty (which would be all of them now) for free. I’ll look up those KB article numbers and post them for you. 

If you go back 6-7 years ago, there were more than just that frame size that used the Honeywell transducers and they did a service advisory on those, but those were all failing almost immediately so they were replaced under warranty. It was only the Frame 6 drives that failed later, requiring the special dispensation. Were these drives that you’ve seen as problems relatively new? I say that because in some cases, distributors sent out new drives to those customers under the advisory, who were supposed to return the bad ones. But some of those customers never did, choosing instead to try to make money on them by selling them on Fleabay or surplus sites, so they end up back in the field. If so, A-B will know because the serial numbers were recorded when the replacements were sent out to the iriginal users and they will not support those gray market drives, so be prepared for that. 

FYI, Ground Gault protection on the PowerFlex 750 series is triggered by 25% of the set current not returning. That seems high, but I have seen cable capacitive charging current cause that trip. Typically on that scenario, it happens immediately when the drive is first energized. But I’ve also seen it happen later as something else in the system changes and the capacitance of the conductor system changes. One of those changes is that someone cut the ground connection out at the motor end, ie copper thieves. Something to take a look at. 

If they were new from the factory, A-B needs to know about it. Have you contacted their Tech Support and started a service ticket?


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## MDShunk (Jan 7, 2007)

I don't tend to a whole lot of 750's. Maybe a couple dozen, but no fake ground faults that I'm aware of. One of the novel beauties of VFD's is that their alarms are believable. Sometimes they're early warnings to things that can be difficult to find with your own meters, but I tend to believe them. They've never let me down.


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## JRockey (Apr 9, 2018)

John and JRaef,

Let me try to clarify. We repair and rebuild drives but concentrate mostly on Powerflex VFD's. Most of the drives we supply for customers come from plant closings, mostly paper plants and food manufacturing plants. We are not affiliated with AB or Rockwell which is why we go thru a local distributor for parts etc.

Jraef, thank you very much for the insight on the frame 6 issue, that is most helpful. However the slew of drives we have seen recently are all frame 7. I may be confusing two issues but they really seem to be related.

1) Customers complaining of a resettable ground fault that becomes more and more problematic. We have verified the issue in house with these drives and they trip seemingly randomly between 15 minutes to 1 hour. The ones that trip at customer sites also trip here. It is not a test motor or cable issue here. There is an EASA motor shop int he same building we are in and everything has been verified. We have been told a revised gate driver board helps alleviate (but not totally resolves) these drives to the ground fault issue.

2) At least two of these customers have also experienced catastrophic capacitor board failure on a frame 7 drive. Ill see if I can attach pictures. We have seen many more frame 7 cap boards blown up recently but were unable to verify with the original owners if they also experienced ground fault issues.

Are these two issue related? I find it hard to beleive that with the number of failures we have seen that more people here havent seen them. Then again, frame 7 is a pretty decent sized drive and not as prevalent as the 15-50 hp drives out there. AB is aware of the blown cap boards and will not sell them simply stating that there is more wrong than just the caps/board and it needs to be sent in for repair.

Are there certain grounding systems out there that the PF750 frame 7 really needs to be setup right with the jumpers in order to prevent this from happening? We had one customer that recently had ground fault issues with his frame 7 and instead of sending it in for evaluation he just ordered a replacement drive from us. He put it in service and within 5 seconds the drive blew up (pictures show cap board blown up). we are waiting for this customer to return the drive to us for an eval. Also waiting for info on type of electrical system and if he is using a line reactor.


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## MDShunk (Jan 7, 2007)

Sure sounds suspicious for a drive installed on a non-solidly grounded system without the MOV jumper removed. To my memory, the 750's have two such jumpers that need disconnected.


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## JRockey (Apr 9, 2018)

MDShunk said:


> Sure sounds suspicious for a drive installed on a non-solidly grounded system without the MOV jumper removed. To my memory, the 750's have two such jumpers that need disconnected.


Correct, there are two jumpers. Jumper PE-A(MOV Input Filter) and Jumper PE-B (DC Bus Common Mode Caps), From what we were told, If a customer does not have a solid ground and they connect the jumpers then the VFD could very well ground itself and blow up.


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## MDShunk (Jan 7, 2007)

Yup. Plenty of stories about drives catching fire, although I've not seen that myself. Usually they just go boom in my experience or die some other less eventful way. I think there's a lot of people that don't even know for sure if they have an ungrounded or resistance grounded system. 

The jumpers are connected out of the box on most drives, but something sticks in my head that the 750's might be the one that has them unconnected and landed in a parking spot. I forget.


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## MDShunk (Jan 7, 2007)

These customers local? It might be worth a drive to a customer site or two to see what's going on first hand to protect your reputation. Particularly since one of them just bought an 18 or 20 thousand dollar drive from you and smoked it.


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## JRockey (Apr 9, 2018)

I was just about ready to ask how I can verify his system since he is on the east coast and we are in Ohio. lol If he had blown anything else up on this drive it wouldve been no problem, We have lots of spare parts but the cap board is an absolute deal breaker. I cant find any of these even in China and Europe


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## MDShunk (Jan 7, 2007)

Just curious, since I don't fix drives... what's so special about the cap board? It it just a board the caps are bolted to that is more or less a printed circuit jumper?


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## JRockey (Apr 9, 2018)

You would understand if I was allowed to post a picture. Unfortunately I dont have enough posts for privilege. It is a multi layered board with hi voltage laminations between the conductive layers. Its a critical piece that we never even acknowledged on most drives because they would rarely fail. Frame 7 PF750 is a game changer.


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## JRockey (Apr 9, 2018)

Google ECI-37018-1. Then click on images


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## JRaef (Mar 23, 2009)

Knowledgebase articles:
900207 and 60178


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## gpop (May 14, 2018)

MDShunk said:


> These customers local? It might be worth a drive to a customer site or two to see what's going on first hand to protect your reputation. Particularly since one of them just bought an 18 or 20 thousand dollar drive from you and smoked it.



I have had many AB drives blow during there first power up over the years. I have also had drives that i have refused to install as you can hear parts rattling around inside the drive. (rebuilds were the worst for that reason)

I was pleasantly surprised when the last drive we bought seemed to come in a box that was designed to survive being delivered to the swamps. 

Just because it worked before postage doesn't mean it survived the trip. Another thing we learnt was to allow the drive to acclimatized if it has been allow to get cold before being installed. (common problem for us when we are installing in a refrigerated room as the drive gets cold before being installed in a warm cabinet).


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## JRockey (Apr 9, 2018)

Shipping has never been an issue with anything we ship. We invested in a Sealed Air -Speedy Pack (Instapak) machine and it does an amazing job filling all voids. All smaller frames get single or double wall boxes and the larger frames get crated up with a cushion layer between the drive and crate base.

We did receive the drive back and discovered the customer removed the leg rails for mounting in his cabinet. Upon doing so, a fastener was dropped down into the drive and fell into the space between the cap board and the reactor plate. The rest is history and from what we understand made a very interesting show at the extruder plant.


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## gpop (May 14, 2018)

JRockey said:


> Shipping has never been an issue with anything we ship. We invested in a Sealed Air -Speedy Pack (Instapak) machine and it does an amazing job filling all voids. All smaller frames get single or double wall boxes and the larger frames get crated up with a cushion layer between the drive and crate base.
> 
> We did receive the drive back and discovered the customer removed the leg rails for mounting in his cabinet. Upon doing so, a fastener was dropped down into the drive and fell into the space between the cap board and the reactor plate. The rest is history and from what we understand made a very interesting show at the extruder plant.



Always nice to find the why.


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