How good is cable armour as an EMF shield ?

[maxime.levesque]

I've read that the "armour" on armoured cable are primarily meant for mechanical protection, but can they make a substitute for "real" EMF shielded wires ?


Would it for example, be a good choice for the wire to connect a 3 phase AC motor to a VFD ?

 

[Reddinr]

Not an easy question but It should be pretty good up to certain noise frequencies and certainly better than nothing. There are probably gaps here and there between the spirals. It would be hard to tell how "sprung" the coils are or will become so it might be hard to predict. Maybe aluminum tape over the top could make it better even.

I have a motor that is fed by the stuff (about 12 feet long) and the very nearby CNC controls seem to behave around it. It is well bonded on both ends. Might be better to bond on just one end but I didn't bother.

To give an idea of how hard it might be to have an accurate answer, here is a paper that talks about what frequencies will be passed / blocked depending on what the gap widths / gap lengths there are between the spirals. The same theory applies but the widths/lengths are hard to estimate.

Applying The Waveguide Below Cut-off Principle To Shielded Enclosure Design. ​

https://ieeexplore.ieee.org/document/626096

 

[MrWhoopee]

Way back in the mid 80s, Fred bought and moved into a new shop. It was a 125' long metal building with offices added onto the front end. We got a new "fast" computer for the office (25 mhz 80286 IIRC). We soon began experiencing problems with data corruption. After some head scratching, we figured out that it happened whenever the weldor was running aluminum with hi-freq. The welding department was at the far end of the building, but everything was grounded through the metal building. We covered the torch lead with braided shielding, disconnected the grounds from the building and attached everything (including the shielding braid) to a ground stake thru the concrete. Problem solved.

 

[matthewsx]

Are you trying to solve a problem, or just prevent one?

If you have the material I'd say give it a try, I've just run THHN wires from my VFD to motor and haven't experienced any issues. Do you have particularly sensitive electronics in the area?

John

 

[homebrewed]

It probably will work for the proposed application but you will need to pay careful attention to the grounding configuration. Also see below!

If the armor is stretched enough to force ground currents to flow around the armor in a spiral fashion, it will become a pretty good inductor, which could pretty much negate any advantage, maybe even turn the armor into an antenna and make problems worse.

In my experience, trying to use some product for something it wasn't specifically designed for is a darned good recipe for future problems, perhaps ones that are hard to track down: and, once the stuff is installed, difficult and expensive to replace with the "right" stuff.

Another possibility: the situation changes and the cables now are being used for some other application, which may be even MORE sensitive to EMI. Plan for the future.

At my old company us "office plankton" used to say that management was willing to send a dollar to save a dime. You don't need to prove that axiom to be true. Save yourself some potential headaches down the road.

 

[ChazzC]

In an industrial environment we had problems with VFD’s that were traced to cable lengths from the VFD to the motor. My advice is to follow whatever guidance you can get from the VFD manufacturer with regard to conductor size, routing, grounding, conduit, etc.: it will save headaches.

 

[markba633csi]

Braided is probably a better shield, if you were to do spectrum analysis

 

[maxime.levesque]

I don't have any problem yet, I just connected a VFD to a lathe motor with left over "construction" 14/3 wire that I already had, and I'm trying to do things "correctly".

I have about 5 feet of wire from the motor to the VFD enclosure, I've looked around for "VFD wires, and they get kind of expensive at 14 gauge and above.

I will do a bit more reading, but it seems something like this could work: https://www.amazon.ca/Tinned-Copper-Braided-Sleeving-Diameter/dp/B01BIBQ9TA/ref=sr_1_56?keywords=cable+shield+sleeve&qid=1696990680&sr=8-56&th=1

I'm all for "listening to manufacturers", but I like to try to understand principles and see if there is room for using what I have, and the principle here is the Faraday cage, ... heck, I even have dozens of feet of scrap 3/4 inch copper pipe that aren't doing anything !

Are you trying to solve a problem, or just prevent one?

If you have the material I'd say give it a try, I've just run THHN wires from my VFD to motor and haven't experienced any issues. Do you have particularly sensitive electronics in the area?

John

 

[matthewsx]

Well, I spent most of a lifetime working with sound, video and networking equipment.

I doubt you’ll run into trouble with that 14/3

Just start making chips and worry about problems as they arise, I doubt this’ll be one of them.

John