vfd cable grounding method 1

[nmark]

When installing drive cable with insulated green ground wire in addition to bare conducter and shield. What is the correct thing to do with bare conductor and shield?Prior experience with instrumentation says to isolate bare conductor at one end to eliminate ground loop.Thanks in advance for help.

 

[jraef]

Power conductors like this need to be grounded at both ends (if possible). You have a ground conductor already, the shield is for EMI/RFI attenuation.

"Venditori de oleum-vipera non vigere excordis populi"

 

[weh3]

The bare conductor is a shield drain, and the green is the equipment grounding conductor. The drain is connected at only one end; the other is open to atmosphere, so to speak.

Jraef, when you say the green wire needs to be grounded at both ends, I understand you to mean that the green wire needs to bond equipment at both ends. There should be only one grounding point, and that is at the service entrance or at the point of derivation for a separately derived system.

I saw advice on another thread suggesting driving more rods at the VFD. This is what creates ground loops.

Regards,
William

 

[dpc]

Actually, there can be multiple connections to ground in any electrical system. The green wire should be grounded at each end whenever possible. A typical motor feeder, for example has a green ground wire that is connected to the MCC ground bus, which is surely grounded and to the motor frame on the other end, which is also generally connected to ground.

I do agree that the neutral and ground are bonded together at only one location (in the US anyway).

I also agree that installing a separate ground rod that is not bonded to the building grounding system is not a good idea.

To get back to the original question, I agree with jraef - ground the green wire and shield of the power circuit at both ends.

 

[Skogsgurra]

The "ground everywhere" vs "ground only once" debate has been going on for as long as I can remember. And that is more than thirty years.

The reason seems to be that there are people who believe that the dreaded ground loop is the mother of all evil and other people that know that a low impedance ground - i.e. grounded everywhere - is tha best way to avoid undesired coupling of HF interference into equipment.

The "ground only once" team is using the old "ground loop creates hum" experience from audio installations where the signal level used to be microvolts and sub-mV. That may very well be so. But automation field signals are either digital/serial with noise margins in the 1 - 5 V range or analoge signals which can be filtered to avoid HF contamination. In other words; the "hum" is not an enemy in automation. But bad zero reference is. The best zero reference is a ground plane - the second best is the "ground everywhere" strategy.

The fear of ground loops is - in my opinion - very close to superstition.

Also, remeber that Americans mostly have TT systems while Europeans have TN or IT. Failure to understand the difference creates a lot of unnecessary hot air.

 

[jraef]

Actaully I was referring to the power conductor shielding being grounded at both ends. I had misread the question.

"Venditori de oleum-vipera non vigere excordis populi"

 

[weh3]

I say the ground question is a safety issue and not a noise issue. I used the term "ground loop" to mean a potential difference between points on the grounding conductor. However, I discovered these links

http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/The-Grounding-Truth~20040602.php

http://www.mikeholt.com/documents/mojofiles/electricalearthingworldwide.pdf

which I intend to study thoroughly before commenting further on the subject.

"Better to keep silent and be thought a fool than to speak up and remove all doubt." -Solomon

Regards,
William

 

[dpc]

weh3,

Thanks for link to the Earthing Worldwide reference. I've been wanting something like this.

dpc

 

[Skogsgurra]

Interesting links, weh3.

But I do not at all agree with the recommendation that TN-C shall be abandoned in favour of TT. I would say the opposite - for many good reasons. Perhaps there are very low-ohm soils in Puerto Rico? In that case, I can understand the recommendation.

Also, reference no 2 contains an error that makes the reasoning a bit confusing: The RCD protection level is 30 milliamps. Not 30 Amps as stated in the text.

 

[PowerfulStuff]

Hi,
I strongly disagree with the 'ground plane' concept throughout an industrial installation. I recently came across a situation where two large conductors cables had formed a loop inadvertently. With just the imbalance in cable construction there was a 15A current induced in the neutral by phase currents. This resulted (by poor design as much as anything) in a secondary current which repeatedly tripped the earth fault protection as the current is a circulating one rather than due to phase imbalance.

It is quite common for these currents in smaller installations to be a few 100 mA. With a longish run on communications cable this easily creates enough noise to swamp a signal - hence the invention of balanced digital systems! I've come across many analogue signals (4-20mA) where the disconnection of a second shield ground point immediately tidies up a horribly noisy signal (as seen on a CRO).

And a final point - where I work at the moment the ground is so dry and nonconductive that a grid of 15 or more earth stakes is required to get less than 2 ohms!

 

[Skogsgurra]

PS,

There is a *slight* difference in the meaning of "ground" in a TN and a TT system. The TN system has a ground wire (it is green/yellow) and "to ground" means connecting to this wire. The corresponding term in a TT system implies driving ground rods.

By using a TT system, you need very good ground rods or plates to avoid potential shifts and hence problems like those you describe. In a TN system where there is a grid of ground wires (close to a ground plane) you will not have those problems. The real ground resistance does not play an important role, but since all metallic parts in a building are tied together and connected to the ground system, it tends to be very low. The interconnections create a mesh with low resistance and (important when lightning hits) very low inductance.

The "ground everywhere" strategy is universally adopted in process industry in Europe and there are guides as to how connect all grounds (TN grounds, not TT) together with equipotential connections. It works very well and is being used in paper mills, mines, steel works phamaceutics and lots of other industries.

The other grid configuration (IT, where the transformer neutral is floating and the apparatuses are grounded) is used in high-power drives at 500 and 690 V levels. But that does not mean that TN is absent. It is used in parallel for all control, communication, instrumentation etc purposes.

 

[mc5w]

There are only 3 instances where a local ground rod at a machine will help you:

1. The machine manufacturer wants a heftier grounding electrode conductor than the 3/0 maximum that U.S. National Electrical Code requires or a redundant conductor if the first one breaks. In this kind of instance a 4/0 equipment ground that is inside of the conduit from the service ground bus to the local equipment disconnect ground bus is typical. Conductors that are sized for 50 degree Celsius operation are also typical i.e. the manufacturer wants a deliberate overkill.

2. You have a high frequency machine such as an induction furnace where you need a local radio requency ground to both a ground rod and the reinforcing mesh in the floor. An equipment grounding conductor that is for electrical safety is no good at radio frequencies either because it is too long OR because it is in metal conduit.

3. You need a local path for lightning current such as with solid state kilowatthour meters.

 

[DickDV]

Returning to the subject of grounds in VFD systems, it is basic good practice to install a continuous copper ground conductor between the motor frame and the drive frame. This is not a safety issue but rather a way of preventing false drive faults (especially ground faults) due to different ground reference voltages. This ground conductor will be referenced to earth ground at the drive end but does not need to be referenced to earth ground at the motor end. In practice, the motor almost always is grounded too which produces a potential ground loop. As skogsgurra mentions, ground loops are overrated in power systems and, at least in the case of VFD's and motor leads, can be ignored.

If you have a ground shield or screen around your motor leads, in my view, the best practice is to follow signal and communication screening practice and ground the shield at the source end only--that would be the VFD end. The issue with the screen is entirely about suppressing EMI/RFI noise and ground loop currents are better avoided.

Be warned, however, that screening motor leads can lead to grievous common mode noise problems if your power supply system to the drive is floating. The only reliable solution I have found in those cases is the installation of a drive isolation transformer with a grounded center wye secondary. Not a pleasant solution if your customer was not expecting the extra cost and space requirements.

I hear that some reactor manufacturers are developing simpler, cheaper common mode noise solutions but I haven't used them and can't comment on their effectiveness. If anyone has experience with these newer devices, please post on their effectiveness.

 

[lgh1]

I have 18 two horsepower motors running on a 65 Kw ABB ACS 800 VFD. The motors are connected delta and a ground wire is connected to the drive and the motor frame. The conductors along with the ground wire between the motors and drive run in a cable tray or conduit. The conduit tray is grounded to the motor ground at both ends. All grounds in the system are connected, that is, the input to the drive, the output from the drive, motors, equipment cabinet etc. are at the same ground potential. I am running the system off a 137 KVA generator at 400 V and 50 Hz which is also grounded to earth. I also have an input line filter between the generator and drive and a dv/dt filter on the output of the drive. The filters are properly sized and recommended by ABB.

My problem is in meeting European standards for conducted emissions. At the moment I meet the standard but with little margin. The most significant energy peaks at 150 KHZ and 1 MHZ and appears to be largly on the ground wires. The signal is cleanest at or near full speed and degrades significantly at 150 KHZ when I turn the speed down. I am considering installing shielded three conductor motor wire which is common in Europe but wonder if I am overlooking something in my present set up, or perhaps have created ground loops through excessive grounding. Any comments are most appreciated.

 

[dpc]

Best solution (but not the least cost) is probably to use metal-clad cable, such as Okonite C-L-X for the motor feeders. This will provide much better shielding than the tray. And you can still use the tray.

 

[DickDV]

Observing that the noise problem is worse at light loads almost would indicate input supply harmonics. However you indicate that you have the input leads filters.

I would have to rely on more testing or the advice of better experts than me on this one.

 

[Marke]

Hello nmark

Treat the frame of the motor, the screen on the cable and the case of the drive as making up a complete enclosed Faraday cage.

You need to properly gland the screen of the cable into the motor such that there is a continuous enclosure formed by the cable screen, gland and motor frame. It is not good enough to pull the screen out to make a pigtail and terminate that in a connector.

At the drive end, you should do the same. Use a proper screen terminating gland at the entry of the cable into the enclosure so that the enclosure formed by the screen flows into the drive enclosure.
RF is like water. If water could get out, RF will also!!

You must terminate the screen at both ends using a proper sheilding type gland so that the screen is terminated in the gland. Additional earthing using an earth conductor within the cable will help to improve things.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Quite so, Mark! A well shielded cable. With shields connected at both ends with EMC glands is what helps.

But there's one little thing that I have had problems with; the gland has to have a very good contact with the flange in the motor terminal box. And the flange has to have a very good contact with the terminal box. It is also necessary that the terminal box in itself has a low-impedance connection to the motor frame. Something that is not always true. A metal gasket and no paint is usually needed.

This is actually why most of the seemingly good installations fail an EMC test.