EMI/RFI from VFDs in fiberglass enclosures
EMI/RFI from VFDs in fiberglass enclosures
(OP)
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)?





RE: EMI/RFI from VFDs in fiberglass enclosures
Mike Halloran
Pembroke Pines, FL, USA
RE: EMI/RFI from VFDs in fiberglass enclosures
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)
RE: EMI/RFI from VFDs in fiberglass enclosures
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?
RE: EMI/RFI from VFDs in fiberglass enclosures
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
RE: EMI/RFI from VFDs in fiberglass enclosures
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!
RE: EMI/RFI from VFDs in fiberglass enclosures
RE: EMI/RFI from VFDs in fiberglass enclosures
Thanks for that.
Mike Halloran
Pembroke Pines, FL, USA