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Pfc Using Vfds


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#1 m_in_oregon

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Posted 10 November 2006 - 06:41 AM

Hello All,

Thanks to everyone for their posts- there is much to think about. For the slightly less technical of us the answers sometimes raise more questions.

I have been looking at using VFDs (for a variety of reasons) as a method of phase shift pf correction, but having read several posts I am now moderately concerned about the much quoted distortion pf. Is this related to the current spiking caused by the VFD? I had planned to try and suppress this using line reactors. I had the impression that Rockwell thought this a less brilliant idea than did Yaskawa after reading their information sheets. Would the distortion pf greatly undermine the benefits of improving the phase shift?

I have been looking at our plant supply which is a good deal smaller than I was led to believe (3000A instead of 4000A @ 460V) and our pf is about .57. The plant has maybe as many as 200 motors usually about 5 or 10HP but ranging up to 500HP started DOL (ouch!) With several projects in the pipeline I need to encourage more of my electrons be involved with the KW thing and a little less with KVAR.

At the end of the day we are running out of power. The utility company has said that if we improve our pf we can get by without installing a new/ additional power supply.

Does anyone think I am about to make a monumental mistake? I enjoy my job and like where I live and would hate to have to move away......

Kind Regards,
Mat.



#2 marke

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Posted 10 November 2006 - 08:14 AM

Hello Mat

I would not use VFDs as a means of pf correction because they shift the problem from displacement power factor to distortion power factor. - By all means use VFDs for other reasons where variable speed is required.

If yo are looking for displacement power factor correction, this can be achieved by the use of power factor correction capacitors. If there is a high level of harmonic voltage, detuning reactors should be used.

To reduce the current distortion on the input of the VFD, a line reactor will help, but only to a limit due to the conduction angles of the current.
You can correct the distortion power factor by the use of active front end VFDs, Passive filters or active filters. - An expensive option.
Unfortunately, there is no easy answer with distortion pf.

Best regards,

#3 m_in_oregon

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Posted 13 November 2006 - 05:06 PM

Thanks Mark.

I guess thats what I thought you would say but since I need to add a minimum of soft starts in many places, I thought maybe I could solve two problems with one basic solution. Thanks again.

#4 jraef

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Posted 14 November 2006 - 01:28 AM

I don't think Marke meant to totally discount the benefits of using VFDs, just not primarily as a PF correction method. Certainly if you also have trouble with starting power in certain locations, the VFD method could prove beneficial. But they would also likely cost more than using PFC caps too.

Another thing to consider for a large facility is to look at any large motors that run continuously as an integral part of your process, such as air or refrigeration compressors etc. What can be done is to replace the motors with synchronous motors and run them in leading PF mode, acting as synchronous condensers for the entire facility. Some people even go so far as to install them without having a load connected just to save them from PF penalties.
"He's not dead, he's just pinin' for the fjords!"

#5 m_in_oregon

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Posted 15 November 2006 - 01:13 AM

QUOTE(jraef @ Nov 13 2006, 05:28 PM) View Post

I don't think Marke meant to totally discount the benefits of using VFDs, just not primarily as a PF correction method. Certainly if you also have trouble with starting power in certain locations, the VFD method could prove beneficial. But they would also likely cost more than using PFC caps too.

Another thing to consider for a large facility is to look at any large motors that run continuously as an integral part of your process, such as air or refrigeration compressors etc. What can be done is to replace the motors with synchronous motors and run them in leading PF mode, acting as synchronous condensers for the entire facility. Some people even go so far as to install them without having a load connected just to save them from PF penalties.



#6 m_in_oregon

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Posted 15 November 2006 - 01:21 AM

Thanks jraef- that sounds quite promising. Do you have any links to further information on the syncronous motor being used in this way? Do they make soft starts for these motors? Sounds like they would be similar to a dc drive.

Regards,
Mat

#7 jraef

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Posted 16 November 2006 - 05:09 AM

If you do a Google search on the term "synchronous condenser" you will see a lot of pertinent information. They will discuss that specifically, but of course a synchronous condenser is a motor with no load hooked up. What you would want to consider is using one or more of your big motors as condensers. This article discusses it in passing, but makes reference to an older article from 1990 that specifically discusses using motors with loads connected in this way. Unfortunately that older article predates the Internet expansion so it is apparently not available on-line, but you can go to Electrical Apparatus magazine and buy a copy.

http://www.findartic...202/ai_n9052999


If you're not familiar with synchronous motors in general, this is an excellent resource.

http://www.kilowattc.../SyncMotors.pdf
"He's not dead, he's just pinin' for the fjords!"




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