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Shielded Cable Terminating

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Splat

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#1
So finally, maybe, getting my VFD install started this weekend. I won't even think I could finish it this weekend, too. That's just foolish thinking there. Ha! So I've got shielded cables for the motor power and control wires. One thing I'm curious about is how you guys are terminating the shielded cables' grounds. I'm thinking backshells would be overkill, and expensive. I've heard pigtails are a big no-no because of the round form that will increase impedance. I was thinking of getting some copper braid off some coax I have and wrapping that around the end of the cable and bring it out to a ground screw. How you guys doing it?
 
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markba633csi

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#2
I didn't think they even needed to be shielded, just that they don't run together for any appreciable distance
Not sure
Mark
 

Splat

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#3
The VFD output wires to the motor act like antennas so they're outputting major RF to everything around them. True, you would worry more about long lengths of motor power wires, but even short wires can influence control wiring.
 
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whitmore

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#4
The VFD output wires to the motor act like antennas so they're outputting major RF...
The motor metal won't pass real RF, but there's some switch noise from the VFD all right. Why not run (from
the VFD to the motor) flexible metal condiut (greenfield, it used to be called)? It's under a buck a foot... and
there's vinyl-coated variants if you have coolant issues.
 

mksj

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I usually separate the braid out, put some heat shrink tubing over it and connect it to ground at the VFD only. Where the braid is exposed on both ends, I put heat shrink tubing over it, but you could use black electrical tape. The other alternative is to connect the shield braid to the ground wire (yellow/green wire below) at the VFD end, I strip a small section on the ground wire, tightly wind the shield around it and then solder. Then place heat shrink over that section and around the cable. This forms a pigtail. The other option is there are strain reliefs that ground the shield to the metal enclosure. Recently made up a 10AWG motor VFD cable which had a separate shield drain wire in addition to the foil and braid, so made grounding the shield much easier. I always cover any exposed shield to prevent the possibility of shorting against a HV terminal. Typically I use ring spades for all the grounds/shield and the motor terminals, I use locking spades for the VFD motor connections because they are easier to attach.

Without a cable shield I have had a number of individuals have issue with RF noise on the TV and radio, sometimes I will add a EMI/RFI filter on the power line just before the VFD.

1440GT VFD system, the motor cable shield ground is connected at the VFD end. The control cables and speed pot cable all have drain wires from the shielding, these are covered with shrink tubing and grounded at the VFD end.

20180302_102659.jpg
 
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Splat

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#6
Found a nice little video from Belden on this topic that I never found before.
 

Bi11Hudson

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#7
To quote:
I usually separate the braid out, put some heat shrink tubing over it and connect it to ground at the VFD only. Where the braid is exposed on both ends, I put heat shrink tubing over it, but you could use black electrical tape. The other alternative is to connect the shield braid to the ground wire (yellow/green wire below) at the VFD end, I strip a small section on the ground wire, tightly wind the shield around it and then solder.
Unquote

From instrumentation: The shield wire should be grounded at the source (VFD) only. It should be insulated otherwise for its' entire length. This is implied in the above quote but not stated specifically. A layer of tape is sufficient unless it rubs somewhere. A lug, fork tongue, is highly recommended.

Hats off to "Chief" Reinhard.

Bill Hudson​
 

mksj

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#8
As shown in the picture, and specific to VFD cable, there is always an outer shield insulating layer along the full length of all VFD cables. The particular cable that I used in the build shown has an inner insulated core cable for the three power cables and the ground, then the braid shield, and then an outer insulating cover. What I was outlining and per the VFD instructions of various manufactures is that where the shield is exposed that it is insulated to prevent shorting. I typically use a star ground terminal or ground bus bar connected to a metal back plane to minimize ground loops. I only use eye crimp terminals for ground connections and for the motor terminals, forks can shift with tightening in particular with multiple terminals

. At the VFD motor terminals and power to the VFD I recommend locking spades, as they are a bit easier to get onto the terminals. I use a commercial style crimper and also 3M crimp terminals with adhesive shrink insulation material. If one was to use a metal enclosure, then there are specific cable terminations that ground the shield to the enclosure or one can use a cable shield clamp kit to attach to the back plate.

VFD cable.jpg

20160731_162016.jpg
20160731_163302.jpg
 

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Blackjackjacques

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#9
Found a nice little video from Belden on this topic that I never found before.
Your Belden Video shows the proper way to thwart EMI as is commonly understood and applied, at least by radio amateurs.

KV4WA
 
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