EMI/RFI from VFDs in fiberglass enclosures 4

[bentov]

If VFDs are installed with EMI-RFI filters and shielded motor run cables, do they still need "Faraday" cage enclosure treatment? If an untreated fiberglass enclosure is used, does the (typically aluminum) interior mounting plate provide any useful directional shielding (i.e. if the plate is situated between the VFD and equipment known to be vulnerable to EMI or RFI)?

 

[MikeHalloran]

If you need shielding, you need mu-metal, or steel, which is much cheaper. Aluminum doesn't do it.





Mike Halloran
Pembroke Pines, FL, USA

 

[ozmosis]

It is not about a "wall" to shield the radiated interference, it is about a conductive path to route interference from the source and down, effectively, to earth.
Fibreglass is not too conductive as I recall.
Effectively means with the lowest impedance possible. High frequency interference follows different rules than 50/60Hz and therefore requires braided straps, flat metal straps etc to ensure the intereference travels away from all things susceptible (control wires, control equipment etc)

 

[bentov]

Thanks for your replies. I read about how the EMI/RFI filters work, and the importance of the motor run shielding/earthing. Mostly I see the problems described as switching transients gone astray, seeking return to the source (the VFD) via assorted parallel ground paths, and I can sort of follow how the output conductor management & input filters work to keep that noise off the upstream wiring.

Then I read something about Faraday cages (and conductive coatings for non-metallic enclosures, and "slot antennas" created by little gaps in conductive gaskets or scratches in the coating). Seems like trouble is everywhere!

We have a fiberglass enclosure already mounted on a dairy crowd gate (like a bridge crane, will have 1 VFD each for the travel & curtain lift gearmotors). The VFDs will live on the bridge (so very short motor runs), box faces away from the parlor pit where some RFID equipment reads tags on the cows (transmitting @ 92khz) as they arrive for milking (ushered in by the gate coaxing them forward). So the (filtered) VFDs will be facing away from the RF equipment - just wondered if the backplate provides any blocking effect.

If I'm diligent about cabling and filtering to EMC standards though, the main source of radiated noise as I understand it (the motor run conductors) should be effectively suppressed, yes? Or is there a whole other "airborne" radiation thing arising from the transistors themselves, requiring Faraday containment?

 

[itsmoked]

Your probably getting ahead of yourself. While dairies can be touchy newer VFDs are pretty good about not bringing everything down if they aren't ideally installed.

What hp are the motors?

Also you can change the VFD's modulation frequencies to shift interference a little ways around the spectrum. Have you run them? Are they actually causing a problem?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DickDV]

Since most smaller VFD's are wall mounted without the benefit of any enclosure, as far as the VFD is concerned, no further shielding is necessary.

Beyond that, it becomes a rather installation-specific thing. Since the motor leads are by far the largest noise generator, shielding and suppressing them is of first importance.

I would think that only in the most extremely sensitive cases would a Faraday shield around the drive itself by necessary. And, if you are going to do that, it may be that the motor will need one too. Now that can get really ugly!

 

[bentov]

Thanks for your replies. We used a single ceramic filter upstream of the twin (2hp 240V single phase input) VFDs plus shielded cable & terminations for the motor runs . . . reasonable cost, no reported problems to date so calling it a success.

 

[MikeHalloran]

Every story needs and ending.
Thanks for that.


Mike Halloran
Pembroke Pines, FL, USA