# Limiting Inrush on Capacitors



## MDShunk (Jan 7, 2007)

How often do you place them online?


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## Handasee (Dec 18, 2009)

Capacitor is turned on a the beginning of the day and off at night.
Technically the capacitor should be able to handle the inrush without a problem, but this is a replacement for one that burned out after one year.
Doing a power quality study now, but the majority of the machines on the branch circuit are just induction motors.


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## JRaef (Mar 23, 2009)

"Serial air coils" sounds as though someone didnt know what an Air Core Reactor (Inductor) is. Adding a reactor adds in an inductive time constant to the change in current, so yes, it can act as an inrush current limiter. Air Core Reactors are typically used in high voltage, but can be used in low voltage as well. 

However in the case where you had seen it, it's more likely they didn't use the correct term because that implies a little more thought and engineering. Anyone can make one just by coiling wires in a slightly organized way. Think of wrapping wire around a toilet paper tube, then removing the tube (only because it looks funny). I suspect that the PFC system mfr just used that to explain why they had a hank of extra wire coiled up like a spring in the box, so that the installer wouldn't think it was surplus and remove it. The number of coils is going to depend on the current and voltage involved, and spacing will be a factor as well. So you can't necessarily use a formula derived for one system in another one unless it is the same. If you are willing to study and research it for hours, you can find the information on building a tuned Air Core Reactor for your system, or you can just go buy an iron core reactor off the shelf from someone like TCI or MTE and get it over with.

http://www.physicsforums.com/showthread.php?t=330539


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## MDShunk (Jan 7, 2007)

I like MTE, mostly. IIRC, they make A-B's reactors.


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## Handasee (Dec 18, 2009)

To JRaef


JRaef said:


> "Serial air coils" sounds as though someone didnt know what an Air Core Reactor (Inductor) is.


Terminology may be different all over the world. In this case, I decided to use the terminology used by a small company called EPCOS (they supply the capacitors to Eaton.) So I consider the use of "serial air coils" as correct.
You want it in writing, here is the article: http://www.electromagazine.com.uy/downloads/EPCOS 4.pdf
And yes, I am willing to take the time to study. Thats what makes this trade interesting to me.


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## JRaef (Mar 23, 2009)

Handasee said:


> To JRaef
> 
> Terminology may be different all over the world. In this case, I decided to use the terminology used by a small company called EPCOS (they supply the capacitors to Eaton.) So I consider the use of "serial air coils" as correct.
> You want it in writing, here is the article: http://www.electromagazine.com.uy/downloads/EPCOS%204.pdf
> And yes, I am willing to take the time to study. Thats what makes this trade interesting to me.


Sorry, I was not implying that about YOU, I was implying that about whomever wrote what you had read.

Reading that, it appears this is just a translation issue for them, it was written by Germans, who tend to insist on using their own terms for things (I used to work for Siemens) and translate literally if they don't know there is another commonly used term.

So what's your question? They give you the information you need in that brochure. Are you looking for corroboration? 

By the way, to some extent this "problem" is being over blown in that brochure. The risk of damage to the caps themselves is not what they claim, most capacitor mfrs KNOW this will happen and design for it. There is a risk to the switching device however, hence capacitor switching ratings on contactors are different from conventional current ratings. So you can either try to limit the current, as they explain here, or oversize your contacts to handle it. Here in North America where we like to use NEMA rated contactors, that's what we are doing. In IEC world where they will spend 6 hours engineering a smaller contactor, they chose to deal with it externally rather than just use a larger contactor. But to be honest, if you want to go that way, you can buy a "capacitor switching contactor" from the likes of Siemens that has that inductor already built-in and pre-wired to the late-break aux contacts that are needed for it. That's what they are showing in that brochure where you see the coils of wires installed.


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## Handasee (Dec 18, 2009)

As I said, I have only found one article, (the link that I posted).
I have found reference to air core coils/reactors in most major mfg of capacitor banks, but only one line references. I wanted to see if there is more detailed info from other sources.


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