# Voltage Fluctuation with Generator when connected to VFD



## Mfd (Aug 13, 2011)

My generator voltage is fluctuating 320-500V when i did connected to VFD (Altivar 61). The fluctuation is not there if i disconnected the VFD also all other starters are working good.

Please do advice on this to have a stable voltage with connected VFD


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## JRaef (Mar 23, 2009)

Probably an older genset or a cheap Asian one? 

VFDs create harmonics and the harmonics can fool the AVR (automatic voltage regulator) in the genset into trying to constantly correct a moving target of voltage. Genset mfrs figured this out years ago and added the proper filtering to the AVR sensing circuit, but units built prior to that, or cheap knockoffs, don't have it. There used to be aftermarket filter kits for some of the big names like Cat and Onan, don't know if they still offer them, they solved this problem over 10 years ago. Call some savvy generator service companies, they may know where to get something, but from what I remember it wasn't easy to just slap on some components, the filter had to be tuned to the AVR characteristics


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## John Valdes (May 17, 2007)

Generators and VFD's are not good companions. I would expect the VFD to be negatively effected, not the generator. Is that what you mean. The generator changes output voltage when the VFD is connected? Line voltage should be constant at the line input of the control (VFD). Even with a large load the VFD should bare the load and not reflect negatively the input voltage.
You most definitely need a line reactor between the generator and the VFD. The reactor should be installed as close as possible the VFD. This is a must.

http://www.transcoil.com/Products/Harmonics/KDR.htm


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## lefleuron (May 22, 2010)

Here are my thoughts, could be off track.

First I would make sure the VFD has no feedback from the gen set- or its going to hunt. Run it in V/Hz.

Your voltage out of VFD is full of massive swings in PP voltage because of switching- its not sinusoidal.

Then I would install a delta/star transformer for isolation between the VFD and gen set- this should smooth things out and take care of a lot of the harmonics. Oil filled would be ideal.

Not the cheapest solution, but I am thinking it would work. Correct me if I am wrong here guys.


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## micromind (Aug 11, 2007)

It's entirely possible that this gen will never be able to run the VFD. 

If possible, check the pitch of the stator. VFDs will work fine if the pitch is 2/3, but anything else will be trouble. 

2/3 pitch tends to cancel out the harmonics of the VFD, any other pitch tends to magnify them. 

Even a fairly small VFD will drive a big gen nuts.


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## fjl810 (Jul 20, 2011)

Good information to know.


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## Del (Feb 10, 2011)

*Gennys and VFDs*

A few years ago I did a project where we needed to temp a genny onto a vfd (300kw) while a panel was changed out, after speaking with genny suppliers, they all stated the generator had to be twice the rating of the VFD loading, to avoid issues, due to the harmonics.


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