Many VFD's on the same mains 3

[prex]

I'm building an application where some 50 asynchronous 3PH 400V 50Hz motors with powers between 2.2 and 1.1 kW are connected to the same mains. Each one of the motors is controlled by a VFD, as they have to run at different speeds in the range 1500 to 3000 rpm.
Not being a motor control expert, my question is: which one of the following alternatives is best (or which is the good choice, if not in the list)?
1) use VFD's each one with its EMC filter
2) use a single EMC filter on the mains (and how to dimension it?)
3) discard the EMC filters because the disturbances generated by the plurality of VFD's will cancel out.

prex

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[ozmosis]

prex
You appear to be in Europe. The product standard for Adjustable Speed Drives (EN61800-3) puts a level of responsibility onto the user/installer to clearly define the environment category.
Firstly, I would say no to your item #3. EMI is not cancelled out.
Secondly, #1 and #2 are a possibility but you need to understand the environment class: this is defined as C1,C2, C3 or C4.. and will determine whether or not it is Light Industrial/Residential/Domestic or Industrial. Simply put, are you sharing your PCC(point of common coupling) with others in the public domain (C1 & C2) or are you seperated electrically (C3,C4)? Then you can decide on the type of EMC filter class once you know. An important point to note is that "an EMC filter" is not a cover-all for all classes, meaning, one filter does not satisfy all requirements.
If going for one large EMC filter you have to understand the layout of the drives and the motors connected (cable lengths from VFD to motor) and determine with the help of the drive supplier if one large EMC filter, to the necessary class, will be suitable. The total length of motor cables needs to be considered if looking at one EMC filter. If the cables are long, then this will have an effect on the design of the filter and will determine the classification it will meet.
If using one EMC filter, ensure it is just the VFD's that are connected downstream otherwise any additional EMI will simply saturate the filter.
You will have to do all this in collaboration with the drive supplier/manufacturer otherwise your Technical Construction File will not meet the approvals of the relevant body who might come in and inspect your installation if there were to be a problem.

 

[DickDV]

A perfect job for a computer-based analysis program. Most VFD manufacturers will have this software and will often run the analysis for free. The tough part is getting the input data----lead lengths, loads, transformer impedances, etc.etc.etc. But, the good news is that the results can generally be trusted if the input data is reasonably accurate.

 

[ozmosis]

DickDV
are you referring to Harmonic analysis? My understanding of what prex is after is EMC issues within the high frequency range (EMI) rather than the lower frequency range (harmonics). I would have thought a computer sumulation on EMI would be quite difficult and involved. I could be wrong but for EMI related issues it would generally involve testing rather than simulation. I agree however, harmonic simulation is fairly straightforward given the correct network information.

 

[powerjunx]

prex,
1) use VFD's each one with its EMC filter:
This option is usually reccomended in fact filters comes with the VFD as an optional package and readily available.
2) use a single EMC filter on the mains (and how to dimension it?):
This is quite impractical, customized rating is rarely available. A non-simultanuiety factor is another issue here hence tuning is difficult to attain.
3) discard the EMC filters because the disturbances generated by the plurality of VFD's will cancel out: filters are highly recommended!

 

[Skogsgurra]

EMI don't cancel!

Where did you get that idea? (I guess from Danfoss and their article about harmonics cancellation when mixing the right proportions of single-phase and three-phase drives).

The standard practice is that EMI add RMS (square, sum sqrt) and that means that two VFDs output 1.4 times the single VFD and ten VFDs output 3 times the single VFD.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

Option 3 is definitely a non starter, that will not work.
Option 2 would be difficult to implement and gain a worthwhile gain. This option would be possible provided that all drives were in one steel enclosure and the filter was installed at the point where the supply entered that enclosure. If there was unenclosed cabling running between the filter and the drives, there would almost definitely be coupling of EMC into other conductors and equipment and the net result would be less effective.

By far the best way, is one filter per drive mounted as close to the drive as possible.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

sed2, I was referring to harmonic analysis, not EMI. My mistake. I am not aware of any software available to analyze EMI issues among multiple drives.

I'd go with the comments from the guys that have dealt with this before.

 

[ozmosis]

DickDV, no worries. You had me thinking "wow, how would that work?" but I was awaiting for Gunnar to pop up and say "ah yes, I have that..."