# vfd panel



## garfield (Jul 30, 2009)

I am building a vfd panel for a 75 hp pump for irrigation. The pump is only 25 feet away from the drive so I don't think I need load reactors. Do I need line reactors? If I understand correctly they can help with lightning when used with a lightning arrestor. For fuses is it important that I use the fast blowing fuses ahead of the drive? This will be operating on a pid loop that's programmed into the drive to maintain constant pressure when there's a different number of irrigators in use. He wants a price with and without a bypass contactor.


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## Kryptes (Aug 6, 2013)

Almost always use line reactors and time delay fuses.


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## garfield (Jul 30, 2009)

Kryptes said:


> Almost always use line reactors and time delay fuses.


Weg says they have a Built in DC link reactor so there's no need for an external line reactor? It seems like drive manufacturers like the fast blow fuses ahead of the drive. I don't know how much this decreases the risk of failure. Manufacturers are not paying for the fuses but no doubt do pay for some of the failures under warranty.


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## John Valdes (May 17, 2007)

Line reactors do only one thing. Correct impedance. The Weg drive has a built in line reactor? I am asking?

If it does, then you don't need one. If it does not have a line reactor built in, it is always a good idea to use one.
Some drive manufactures sell the reactor at 1/2 price with a drive and motor purchase. Weg does not, so bring it up and see what they offer you.

A line reactor is good insurance in regards to nuisance trips. Power induced nuisance trips.
Line and load reactors are actually 1:1 xfmr's. Some employ filtering along with isolation.
Contact TCI (Transcoil) about line reactors. Or ask Weg.

Weg will also provide the correct fuse type. It will be in the start up manual.
Follow their instructions.


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## ScooterMcGavin (Jan 24, 2011)

Line reactors are not transformers. The provide no electrical isolation whatsover. They are just an inductor in series with the line coming in or load going out. They help to limit the sudden changes in current going to the vfd and also filter out out the high frequency noise created by the drive as inductive reactance increases with frequency.


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## garfield (Jul 30, 2009)

On the fuses I think they are usually required in the start up but I rarely see them installed. Is a link reactor the same as a line reactor?


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## JRaef (Mar 23, 2009)

The #1 cause of VFD failures is excessive heat, the #2 cause is line transients. We all can see the heat issues and usually deal with it up front, but because people can't see line transients, they tend to ignore them.

Line reactors also add inductance to the input of the drive. The advantage to that is in what is called the "inductive time constant", which states that current in an inductive circuit can only change state at a rate commensurate with the inductance. So in other words, very fast transients on the line side will be slowed down by the reactor, that's likely what you heard with regard to lightning protection (although that's kind of a stretch). A much more common occurrence that a reactor helps protect against is utility grid switching transients.

As our infrastructure gets more and more stretched thin and more and more people say NIMBY to power lines, the utilities are forced to switch power around a LOT more than they used to. Every time a HV grid switch opens and closes, there is a transient spike of voltage and current. Every time a transient comes down the line to a VFD, it hits the diodes first. So even if you have a DC bus choke, which helps with harmonics and over voltage issues primarily, that does NOTHING to protect the diode bridge from line transients, because it is AFTER the diode bridge. In a study we did, we hooked up 2ea of our 25HP 480V drives (which by the way have DC bus chokes), one directly connected, one with a line reactor, to a 1000kVA transformer and observed the transients over a week. There were over 180 transients over 500A peak in that week, the worst were 805A spikes. Those were all seen by the drive connected raw to the line, but on the one with just a 3% reactor ahead of the drive, the highest peak seen by the drive was 55A. After the test the diodes on the raw connected drive were cut open and examined, there was already evidence of "burn through" in the silicon layers, meaning the diodes were on their way to becoming conductors instead of semiconductors, which is the failure mode of a diode. That's why I always tell people that a line reactor is "cheap insurance" even if you don't feel you need them for harmonic mitigation.

PS regarding fuses.
I know that VFD mfrs tend to recommend high speed fuses, or what are called 'semiconductor" fuses. But to be honest, in 30+ years of being in the VFD industry I have NEVER seen a semiconductor fuse clear a fault BEFORE there is damage to a diode bridge, the damage always happens first, THEN the expensive fuse clears. I just don't see the point in spending the extra money. All of our drives now are listed to be used behind CBs or standard RK-5 (LPS / LPN) fuses. The problem is, when people get UL listing on their drives, if they send them to UL for testing with fuses connected, UL will list them ONLY with those specific fuses installed. If you want them listed behind different fuses or CBs, you have to have UL test them with those other combinations as well, and that costs money. So once they get listed with only one type of fuse, that becomes the _*only *_approved fuse. This issue happens a LOT with European and Asian VFD mfrs, because they hate having to get UL listing so they spend as little as possible to get that bug, even though it might cost more for the end user in the long run.


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## ScooterMcGavin (Jan 24, 2011)

Here's a good write up on Line reactors for VFD's from our local utility. It explains in more detail what Jraef is talking about.

http://www.pge.com/includes/docs/pd...e/energystatus/powerquality/line_reactors.pdf


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## garfield (Jul 30, 2009)

Another question I have is about bypass. Can I open the line and load simultaneously before switching to across the line start?


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## John Valdes (May 17, 2007)

garfield said:


> Another question I have is about bypass. Can I open the line and load simultaneously before switching to across the line start?


Best practice is to stop the drive before switching the load.
Another good question for Weg.


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## garfield (Jul 30, 2009)

John Valdes said:


> Best practice is to stop the drive before switching the load.
> Another good question for Weg.


The support person I talked to said it would be fine but if I end up having a problem it will likely be my problem. I guess it seems to me like drives are similar enough in technology that if most manufacturers have an opinion different than the Weg support person I happened to get I should go with it.


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## pudge565 (Dec 8, 2007)

JRaef said:


> The #1 cause of VFD failures is excessive heat, the #2 cause is line transients. We all can see the heat issues and usually deal with it up front, but because people can't see line transients, they tend to ignore them.
> 
> Line reactors also add inductance to the input of the drive. The advantage to that is in what is called the "inductive time constant", which states that current in an inductive circuit can only change state at a rate commensurate with the inductance. So in other words, very fast transients on the line side will be slowed down by the reactor, that's likely what you heard with regard to lightning protection (although that's kind of a stretch). A much more common occurrence that a reactor helps protect against is utility grid switching transients.
> 
> ...


I really enjoy reading your very informative posts.


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## Safari (Jul 9, 2013)

JRaef said:


> T
> 
> In a study we did, we hooked up 2ea of our 25HP 480V drives (which by the way have DC bus chokes), one directly connected, one with a line reactor, to a 1000kVA transformer and observed the transients over a week. There were over 180 transients over 500A peak in that week, the worst were 805A spikes. Those were all seen by the drive connected raw to the line, but on the one with just a 3% reactor ahead of the drive, the highest peak seen by the drive was 55A. After the test the diodes on the raw connected drive were cut open and examined, there was already evidence of "burn through" in the silicon layers, meaning the diodes were on their way to becoming conductors instead of semiconductors, which is the failure mode of a diode. That's why I always tell people that a line reactor is "cheap insurance" even if you don't feel you need them for harmonic mitigation.
> .


 this is awesome jraef,this study puts the facts straight.


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## Jlarson (Jun 28, 2009)

JRaef said:


> already evidence of "burn through" in the silicon layers, meaning the diodes were on their way to becoming conductors instead of semiconductors, which is the failure mode of a diode.


Try explaining that concept to a well install monkey. :no: :laughing:


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## John Valdes (May 17, 2007)

garfield said:


> The support person I talked to said it would be fine but if I end up having a problem it will likely be my problem. I guess it seems to me like drives are similar enough in technology that if most manufacturers have an opinion different than the Weg support person I happened to get I should go with it.


If you are not real sure about the the guy you spoke to, call the Atlanta office.
This is where you get a real drive guy. 

If anyone can answer your questions regarding a bypass on a Weg control it is the guys in Atlanta.


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