# VFD Ground Current



## wendon (Sep 27, 2010)

I'm just curious if anyone has any experience in dealing with the current a VFD puts on the grounding conductor?


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## JRaef (Mar 23, 2009)

Yes, it's called "common mode noise" and is largely unavoidable. You can take steps to reduce it though. Better drives include mitigation techniques, cheaper ones don't. A method used by many on smaller drives is to put the conductors through a "common mode choke", a ring of ferrite, a non-conductive magnetic material. I know people who make their own common mode chokes by adding what are called "ferrite beads" to the output conductors, someting a friend of mine swears works to reduce the negative effects this has on the dairy industry. Common mode high frequency noise has been demonstrated to cause milk cows to stop producing because they feel it when hooked up to the milking machines. He buys a ferrite core and passes the output conductors through it a few times. The problem with that is, it's guesswork. The common mode noise is at high frequencies and the choke must be selected to be "tuned" to those frequencies. If you don't know what they are, you are shooting in the dark.

Another method used in larger drives is a DC Bus Choke (inductor), something that many Asian drives don't come with but US and EU drives often have. We do it because all capacitors (a very expensive part of VFDs) come from the same 4 or 5 Asian suppliers, who also make VFDs. So to help avoid having their pricing controlled by competitors, US and EU drive mfrs used chokes on the DC bus to reduce the amount of capacitance needed. Turned out that had other benefits, but it's expensive for Asian suppliers to ship iron and copper in every box. So most Asian drive suppliers just give you terminals to hook up your own external DC Bus choke if you want one. Nobody reads the manuals though and they don't do it, because usually if you are buying one of those, you are usually buying on price, and if the price is the most important issue you don't go looking to add cost.


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## chicken steve (Mar 22, 2011)

They _(the guys with lots of letters after their names)_ had us install VFD cable from drive to motor . The best explanation , at least in terms i could understand, was farady cage.....~CS~


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## wendon (Sep 27, 2010)

JRaef said:


> Yes, it's called "common mode noise" and is largely unavoidable. You can take steps to reduce it though. Better drives include mitigation techniques, cheaper ones don't. A method used by many on smaller drives is to put the conductors through a "common mode choke", a ring of ferrite, a non-conductive magnetic material. I know people who make their own common mode chokes by adding what are called "ferrite beads" to the output conductors, someting a friend of mine swears works to reduce the negative effects this has on the dairy industry. Common mode high frequency noise has been demonstrated to cause milk cows to stop producing because they feel it when hooked up to the milking machines. He buys a ferrite core and passes the output conductors through it a few times. The problem with that is, it's guesswork. The common mode noise is at high frequencies and the choke must be selected to be "tuned" to those frequencies. If you don't know what they are, you are shooting in the dark.
> 
> Another method used in larger drives is a DC Bus Choke (inductor), something that many Asian drives don't come with but US and EU drives often have. We do it because all capacitors (a very expensive part of VFDs) come from the same 4 or 5 Asian suppliers, who also make VFDs. So to help avoid having their pricing controlled by competitors, US and EU drive mfrs used chokes on the DC bus to reduce the amount of capacitance needed. Turned out that had other benefits, but it's expensive for Asian suppliers to ship iron and copper in every box. So most Asian drive suppliers just give you terminals to hook up your own external DC Bus choke if you want one. Nobody reads the manuals though and they don't do it, because usually if you are buying one of those, you are usually buying on price, and if the price is the most important issue you don't go looking to add cost.


We just installed 9 freq drives in a cattle facility. The drives are located next to the fans and each drive is fed from a transformer (240volt PRI - 240/120 volt SEC) mounted next to it. I checked one of the smaller drives and I had an 9 MA reading on the ground. If you add up that amount x 9 drives, it amounts to a considerable amount of current flow on the ground. I'm just curious why the drive companies can't design a drive that will eliminate this problem.
When we finished, I had 0 ma on the grounding conductor.


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## JRaef (Mar 23, 2009)

wendon said:


> ... I'm just curious why the drive companies can't design a drive that will eliminate this problem.


We can... but not enough people would buy them because they'd be too expensive.

I used to use drives for certain industries like dairies and mining (where grounding is a big issue too) that were made by a company in New Zealand called PDL. They were fantastic drives; simple to use, very capable, very little common mode noise, low harmonics, virtually bullet proof and every one was made in a NEMA 12 enclosure with no air exchanged around the electronics so they never got dirty inside. But they went belly up and were bought by Schneider, who put a bullet in their head. Why did they fail? Too expensive, they couldn't compete with cheaper drives.


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## wendon (Sep 27, 2010)

JRaef said:


> We can... but not enough people would buy them because they'd be too expensive.
> 
> I used to use drives for certain industries like dairies and mining (where grounding is a big issue too) that were made by a company in New Zealand called PDL. They were fantastic drives; simple to use, very capable, very little common mode noise, low harmonics, virtually bullet proof and every one was made in a NEMA 12 enclosure with no air exchanged around the electronics so they never got dirty inside. But they went belly up and were bought by Schneider, who put a bullet in their head. Why did they fail? Too expensive, they couldn't compete with cheaper drives.


These were Lenze drives. They have a dust proof enclosure. I've never tested one with a ferrite filter installed.


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