What is the proper way to terminate the shielding in VFD motor cable? The manual for the Frenic Mini I'm using says to connect the shielded control cable to the low voltage ground pin, but doesn't mention the shielding for the output cable. I'm assuming I should connect it to the output ground on one end and the machine frame on the other end?
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Connecting VFD Cable Shielding
- Thread starter mesa
- Start date
- Replies 14
- Views 6,546
Jraef
Titanium
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- Aug 10, 2004
- Location
- San Francisco Bay Area
+1
In our manuals, we specifically tell installers to run the shield AND GROUND WIRE directly to the VFD ground (or "PE") terminal, then connect the VFD ground to any panel ground.
And just to be clear, on shielded instrument and control cables, it's best to ground the shield at only ONE end, but on shielded power cables, you want to ground the shield at BOTH ends.
rons
Diamond
- Joined
- Mar 5, 2009
- Location
- California, USA
To avoid the ground loop.
WHHJR
Stainless
- Joined
- Jun 5, 2009
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- Near:Louisville, KY
In the case of the Power conductors the cable or conduit (flex or rigid) ground is there and connected at each end for code reasons as much as anything. ie Safety.
2¢
2¢
I'm gonna go against the grain a little.....
OFTEN it is best to ground the control cable at one end.... NOT every case is a good idea to handle that way.
Reasons NOT to include the possibility of inducing large currents in a small wire, cases in which that could be induced into signal wires, and so forth. Also ases in which there is what the audio people call a "pin 1 problem", usually meaning the grounding. is interconnected with the signal wiring so that they can cross-couple signals.
When there is a lot of interference, it may be best to ground it at each end, and maybe in between as well. It just depends on the situation. Electromagnetic interference is actually predictable, but it can look like a black art. Another case where it is best to know WHY you are doing what you do.
OFTEN it is best to ground the control cable at one end.... NOT every case is a good idea to handle that way.
Reasons NOT to include the possibility of inducing large currents in a small wire, cases in which that could be induced into signal wires, and so forth. Also ases in which there is what the audio people call a "pin 1 problem", usually meaning the grounding. is interconnected with the signal wiring so that they can cross-couple signals.
When there is a lot of interference, it may be best to ground it at each end, and maybe in between as well. It just depends on the situation. Electromagnetic interference is actually predictable, but it can look like a black art. Another case where it is best to know WHY you are doing what you do.
Chip Chester
Diamond
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- Dec 13, 2007
- Location
- Central Ohio USA
I'm an "audio people", and it's treated much like the control side of the VFD equation. Usually grounded at one end, although there are some situations where both ends are grounded... or occasionally one end is grounded via a capacitor, and the other is grounded directly. We'll save the 'star ground' discussion for another day.
To my understanding, the power cable shield is to contain interference, and the control cable shield is to protect from interference.
Chip
To my understanding, the power cable shield is to contain interference, and the control cable shield is to protect from interference.
Chip
rons
Diamond
- Joined
- Mar 5, 2009
- Location
- California, USA
Probably not understood, clarification:
Occurs when earthed points on two ends of a path are NOT at the SAME potential. Signals must have a life too.
Spent 28+ years in the audio biz designing musical and PA equipment, as well as using it. Some installations get so crazy that no ground scheme will work right, but I usually found that without a "pin 1 problem", the both end grounded way was much more predictable.
Sometimes the one end only was better but then when some seemingly unrelated change was made, all hell would break loose.
of course it makes a huge difference it you are talking about balanced or single-ended cabling.... And lots of folks have no clue that bad grounding can mess up digital signals, like RS-485, horribly.
mike_kilroy
Stainless
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- Sep 2, 2010
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- Farmersville, oh (Dayton/Cinti area)
To jst point too, some (most?) drives mfgrs now demand signal shields tied both ends - due to the faster and faster igbt switching times.
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jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
For the motor power leads, the green wire and sheild connection are best tied to the same terminal at both the motor and
VFD ends.
Unless you have a proper connector where the shield can be fanned out into a proper sheilded connector termination, where
it is brought to chassis directly at the connector, at both ends. Probably not your case.
VFD ends.
Unless you have a proper connector where the shield can be fanned out into a proper sheilded connector termination, where
it is brought to chassis directly at the connector, at both ends. Probably not your case.
mike_kilroy
Stainless
- Joined
- Sep 2, 2010
- Location
- Farmersville, oh (Dayton/Cinti area)
Just to add to this Jim, newest technology shows that just tying that shield to the green ground wire does little today to help reduce noise. Remember, the harmonics of the .01usec (10mhz) igbt switching times are up in the 100mhz and ghz range... means that jumper stub from shield to the ground lug of 6" is a 1/4 wavelength at around 2ghz so effectively a simple open circuit - might as well not bother jumpering it over to the green wire. As time goes on, igbt's will switch faster and faster until EVERYTHING better be tied at both ends as many are beginning to suggest or require, AND by stripping the pvc off the outer cable and physically laying that shield against a decent ground and use a plastic wire tie to hold it. Remember to that most of the noise current is at these high freqs so actually is only on the outer molecular atoms of the shield itself; so unless the shield is physically layed against a ground surface, most of the high freq noise will not even attempt to travel to ground. Very interesting subject!
jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
Not 2 GHz - the inductance of that six inch wire is large enough that it presents a pretty big impedance
around 10 MHz.
But unless you have something better, it's the thing to do. In my most recent VFD install, the motor got a
metal strain relief, with a rubber grommet to grab the cable jacked. I stripped back the jacked and
brought the sheild up between the aluminum strain relief body and the rubber. When the jacked is
clamped down, the sheild is brought into contact with the aluminum body all around.
At the other end the Hitachi drive didn't have a good way to do this - so the braid got a lug and was clamped
with the green wire.
Bottling up the RFI genie in a bottle is really hard to do. Most efforts accidentally leave a hole big enough to
drive a truck through.
around 10 MHz.
But unless you have something better, it's the thing to do. In my most recent VFD install, the motor got a
metal strain relief, with a rubber grommet to grab the cable jacked. I stripped back the jacked and
brought the sheild up between the aluminum strain relief body and the rubber. When the jacked is
clamped down, the sheild is brought into contact with the aluminum body all around.
At the other end the Hitachi drive didn't have a good way to do this - so the braid got a lug and was clamped
with the green wire.
Bottling up the RFI genie in a bottle is really hard to do. Most efforts accidentally leave a hole big enough to
drive a truck through.
Depends on your goal.
If you are sealing up for EMI, so that your VFD powered military whatzit is not detectable via emissions, or not zapping your comm link, that's one thing.
If you need to meet agency emissions requirements, that's another.
If you just want to avoid getting complaints, yet another. That's going to be the usual goal. The agencies do not go looking for you, they rely on complaints.
Each need has its bandwidth. Most agency requirements go to 1 GHz. While an IGBT may have harmonics there, they are usually small. The real payload is going to be in the sub 100 or so MHz area, maybe 200. That is 2 or 3 m wavelengths, and does not require the same sort of attention as higher frequencies, not to mention that the most usual problem bands for complaints are in that area, likely the lower portion.
Any time you are worried about the radio impedance of a short length of wire, make it bigger, make it multiple, or best, make it wider. A wide thin strip is far better than a piece of 10 ga wire with more actual cross-section,
If you are sealing up for EMI, so that your VFD powered military whatzit is not detectable via emissions, or not zapping your comm link, that's one thing.
If you need to meet agency emissions requirements, that's another.
If you just want to avoid getting complaints, yet another. That's going to be the usual goal. The agencies do not go looking for you, they rely on complaints.
Each need has its bandwidth. Most agency requirements go to 1 GHz. While an IGBT may have harmonics there, they are usually small. The real payload is going to be in the sub 100 or so MHz area, maybe 200. That is 2 or 3 m wavelengths, and does not require the same sort of attention as higher frequencies, not to mention that the most usual problem bands for complaints are in that area, likely the lower portion.
Any time you are worried about the radio impedance of a short length of wire, make it bigger, make it multiple, or best, make it wider. A wide thin strip is far better than a piece of 10 ga wire with more actual cross-section,
jim rozen
Diamond
- Joined
- Feb 26, 2004
- Location
- peekskill, NY
