# VFD weirdness!



## Electrorecycler (Apr 3, 2013)

One more piece of info. The motor (5hp) is not running. If the motor is started before the fault occurs, the buss voltage drops to around 645. When the motor stops, the buss voltage climbs again. 


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## John Valdes (May 17, 2007)

Do you have a line reactor installed?
Over voltage fault in many cases can be a nuisance trip caused by the utility.

Think about power correction caps. They switch on and off as needed!


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## John Valdes (May 17, 2007)

Electrorecycler said:


> One more piece of info. The motor (5hp) is not running. If the motor is started before the fault occurs, the buss voltage drops to around 645. When the motor stops, the buss voltage climbs again.
> 
> 
> Sent from my iPhone using Tapatalk


Normal.


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## Electrorecycler (Apr 3, 2013)

No line reactor. I've been adding them throughout the facility, just haven't got around to this machine yet. I realize it's normal for the buss voltage to drop under load, but it's not normal for it to continuously climb until it faults out wen not under load. Over about 25 seconds the voltage goes from around 660 to over 800 before faulting. 


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## JRaef (Mar 23, 2009)

Electrorecycler said:


> No line reactor. I've been adding them throughout the facility, just haven't got around to this machine yet. I realize it's normal for the buss voltage to drop under load, but it's not normal for it to continuously climb until it faults out wen not under load. Over about 25 seconds the voltage goes from around 660 to over 800 before faulting.
> 
> 
> Sent from my iPhone using Tapatalk


As a gross general rule, this is usually caused by capacitive resonance; the DC bus caps inside of the VFD are resonating with caps somewhere else on your system. I would start by looking at anything that has been added to your system last week, even your next door neighbor. Most likely you will find that someone added a machine that has PFC caps on it and they are on-line all the time, not just when the motor is running. Happens a lot more than people realize. Another source is a Passive Harmonic Filter that is also connected incorrectly and the filter caps are connected all of the time, not just when the device they want to correct harmonics on is running. Better Passive Harmonic Filters come with a line contactor so they are off-line unless the device they are intended for is running, cheaper ones forgo that feature, hence they are cheaper.

If it's a neighbor and you can't do anything about it, try adding a 5% line reactor ahead of this drive.


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## Electrorecycler (Apr 3, 2013)

Thanks Jraef. We just gave that a try and no go. DC buss voltage is still climbing even with a 5% impedance line reactor in front. I'm going to send the VFD out for repair and insist that a line reactor is installed when it returns. I'm starting to wonder if the capacitors have degraded. 

On another note, at about the same time this VFD gave issues, another 15hp ABB VFD started giving an earth ground fault. This one does have a reactor and isn't even a year old. The fault can't be reset without removing the load, however the load meggers perfectly fine as do the cables going to it. 


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## JRaef (Mar 23, 2009)

If the capacitors in the drive are "degrading", they would be LOSING capacitance and you would get a fault as the DC bus ripple increases. The only thing that can make the DC bus rise when the load is off is resonance. 

Physically disconnect line power from the ABB drive and see if the problem with the PowerFlex drive goes away. You may have a shorted diode in the ABB drive.


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## triden (Jun 13, 2012)

Out of curiosity, what is the load driving? Is there any resistive braking on the system?


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## JRaef (Mar 23, 2009)

Wouldn't matter. The bus voltage goes up when it is NOT running the load!


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## triden (Jun 13, 2012)

I missed that information somehow. I'm trying to imagine how the DC bus voltage can climb with a reactor in front. If large transients from harmonics were being rectified, the reactor should have fixed it. It might be possible that other capacitance and inductors on the line form to make some sort of resonant tank circuit, but that seems like such a small possibility. I still think harmonics might be getting through the 5% reactor and are contributing to the bus voltage. Would be nice if someone could put a power monitor meter on the front end of the drive to look at power quality.


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## Electrorecycler (Apr 3, 2013)

JRaef said:


> If the capacitors in the drive are "degrading", they would be LOSING capacitance and you would get a fault as the DC bus ripple increases. The only thing that can make the DC bus rise when the load is off is resonance.
> 
> Physically disconnect line power from the ABB drive and see if the problem with the PowerFlex drive goes away. You may have a shorted diode in the ABB drive.




Sorry, forgot to mention that the ABB drive is on another machine. Both machines are fed from the same transformer, but the machine containing the ABB drive with the fault was turned off while we were dealing with the pf70. That's today's mystery. [emoji6]


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## Electrorecycler (Apr 3, 2013)

triden said:


> I missed that information somehow. I'm trying to imagine how the DC bus voltage can climb with a reactor in front. If large transients from harmonics were being rectified, the reactor should have fixed it. It might be possible that other capacitance and inductors on the line form to make some sort of resonant tank circuit, but that seems like such a small possibility. I still think harmonics might be getting through the 5% reactor and are contributing to the bus voltage. Would be nice if someone could put a power monitor meter on the front end of the drive to look at power quality.




If only I had the toys and the time to look at power quality. I can tell you this, our power quality is BAD! I've been telling that to the powers that be from the first week I started. They'd rather replace drives and motors than invest in the company's future apparently. 

I've sent the original problematic pf70 out for repair. I'll know today if it's a hardware issue or not. 

I should also mention this: not only did we try the pf70 with a 5% impedance line reactor, but we did it in our shop, which is fed from a completely separate transformer. 


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## John Valdes (May 17, 2007)

Electrorecycler said:


> I've sent the original problematic pf70 out for repair. I'll know today if it's a hardware issue or not.


I bet you will not find out anything other than the repair facility telling you they cannot find anything wrong. Hope they are an honest shop.
Who does you repair work?
Are they an AB distributor?


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## mitch65 (Mar 26, 2015)

You say you have been installing reactors in the plant? Have you installed one recently on another piece of equipment that could be causing this?


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## Electrorecycler (Apr 3, 2013)

John Valdes said:


> I bet you will not find out anything other than the repair facility telling you they cannot find anything wrong. Hope they are an honest shop.
> Who does you repair work?
> Are they an AB distributor?




They're an honest shop. I'm given a report upon completion describing what was repaired/replaced and why. I use a company called Radwell. They've always done a fantastic job and can rush a repair in 24-48 hours. Not an AB distributor, but why pay a premium for repairs, especially when our local distributor sends all their repairs to the US. I'm sure the repairs are done right, but the delivery time is a little lengthy. 




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## Electrorecycler (Apr 3, 2013)

mitch65 said:


> You say you have been installing reactors in the plant? Have you installed one recently on another piece of equipment that could be causing this?




Nothing at all. 


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## Electrorecycler (Apr 3, 2013)

tannerberendt1 said:


> I'm curious as to what kind of jobs you think are most rewarding? i.e. What jobs do you enjoy doing and get paid well for. Thanks.


Although we can answer your questions here, I would recommend that you start your own thread using the questions you posed. You'll get far more opinions that way. Plus you won't be throwing in a question that's right off the subject matter. :blink:


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## mitch65 (Mar 26, 2015)

Transformer issue? Seems odd that 3 drives running off the same transformer are having issues while nothing else in the plant seems affected. Any other equipment using a PF70 that you could run with one of the questionable drives to see if the issue is the drive?


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## John Valdes (May 17, 2007)

Electrorecycler said:


> They're an honest shop. I'm given a report upon completion describing what was repaired/replaced and why. I use a company called Radwell. They've always done a fantastic job and can rush a repair in 24-48 hours. Not an AB distributor, but why pay a premium for repairs, especially when our local distributor sends all their repairs to the US. I'm sure the repairs are done right, but the delivery time is a little lengthy.


That's good. Electronic repair shops are becoming a thing of the past.
We/they replace more today than they repair.
That goes for motors and gearing as well.

But since I worked in a repair facility myself, I have a good idea of how things are approached.
There were many times we could find nothing wrong with the equipment and only solved the issue by a site visit.

Keep us posted as I am interested in how things work out. :thumbsup:


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## Electrorecycler (Apr 3, 2013)

Findings: The PF70 returned to us from the repair company cleaned and tested. With no findings of any major issues, as John Valdes warned. So we started looking harder. We found a ground fault on Phase A of the delta transformer. After further tracing, it turned out to be caused by a screw driven through a conduit by our roofers, on the load side of a 480-600V transformer. When we opened the breaker to the transformer, the ground fault cleared. We powered up the PF70, and it worked perfectly. 

Chalk that one up to a learning experience! Now all we have to do is clean up the mess...


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## mitch65 (Mar 26, 2015)

Gotta like the 6" screws on a 4" thick roof


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## JRaef (Mar 23, 2009)

I'm trying to wrap my head around how that would result in an over voltage fault in the drive...


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## micromind (Aug 11, 2007)

Is the transformer an ungrounded ∆?


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## Electrorecycler (Apr 3, 2013)

The transformer is an ungrounded Delta, which was inadvertently grounded by a roofing screw. He did a good job at it too, through the middle of the pipe and the middle of the A Phase conductor.

This is what made us chase after a ground fault. I was looking for possible reasons why the drive would go into overvoltage on startup. I came across an AB support page online. It said " If the line voltage is known to be within specifications, then find the jumper used to ground the common mode capacitors". Of coarse, it didn't give reasons why. So the I kept looking.

I then came across a publication (Publication 20A-IN010B-EN-P ) which explained reasons why you would want to remove this grounding jumper. One reason was a "B phase ground". I immediately thought about our one and only Delta transformer in the facility and sent my co-worker over to check the idiot lights in the substation. Sure enough, A Phase was grounded.


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## Cow (Jan 16, 2008)

Ungrounded systems are required to have a ground monitor per 250.21(B). 

If you had a ground monitor, you would have known A phase had grounded and saved yourself a lot of trouble.


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## ponyboy (Nov 18, 2012)

Cow said:


> Ungrounded systems are required to have a ground monitor per 250.21(B).
> 
> If you had a ground monitor, you would have known A phase had grounded and saved yourself a lot of trouble.




Spend some time in heavy industrial and you'll see things are never that easy. Most likely that wasn't his call and he couldn't change it if he wanted to nor even be aware of the ungrounded secondary in the first place. 


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## Electrorecycler (Apr 3, 2013)

Cow said:


> Ungrounded systems are required to have a ground monitor per 250.21(B).
> 
> If you had a ground monitor, you would have known A phase had grounded and saved yourself a lot of trouble.


That's what I meant by "idiot lights". Unfortunately we have 5 substations and only two slightly overworked electricians. Only one substation has a Delta transformer. If we had a newer monitor capable of sending a remote alarm it would make my life so much easier, but I work for a company that doesn't see the value in such things. They won't even make the obvious changes to save money, let alone make changes to save time...which is money...this company is so confusing. So, we continue with our visual inspections of the substations when time permits, once o twice a week.


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## JRaef (Mar 23, 2009)

OK, here's my conjecture:

The screw into the phase became an arcing ground fault and because of it being an ungrounded delta system, the system stayed energized, which is the purpose behind using it; the first Ground Fault essentially just makes it a corner grounded delta, life (actually equipment) goes on. But in this case because it was arcing, the arc itself acts like a capacitor, which will increase the voltage in that phase. It's the same thing that happens when you open a set of contacts; the arc that forms looks like a capacitor at first and the voltage across the contacts increases until the contacts separate far enough to where the dielectric of the air is enough to stop it. But in an arcing ground fault, there is no real separation mechanism, other than the eventual melting of the arcing components, and inside of a conduit, that keeps everything tight and close and the arc doesn't self extinguish, yet the amount of current that could flow was relatively low because it was a high resistance GF, so no circuit breaker or fuse cleared either.With nothing shutting down that delta circuit, the arcing likely continued until the higher voltage on the two phases fed by that conductor caused the VFD DC bus to charge up.


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## JRaef (Mar 23, 2009)

Side note: on the PF70, there are MOVs on the line side of the diode bridge to protect it. If you have an ungrounded delta system, the instruction manual told the installer to remove a ground reference jumper in that MOV circuit, because there are 4 MOVs in a Wye configuration referenced to ground. In a GF on that delta system, the ground referenced MOV will attempt to become the Wye point for your entire delta circuit, for a few milliseconds until it vaporizes. When it does, it can take out other things around it. Removing the ground reference jumper prevents that. If whomever installed that did not read the manual or understand this section and left that jumper in place, you may have a damaged drive now, or at best, you will need to replace that MOV module.

Manual page showing jumper information.


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