Foil vs braid screening for VSD application 3

[DrDrreeeaaa]

G'Day All

We are currently in a cost and benefit exercise regarding the specification of VSD cable.

Our project is a new manufacturing plant requiring the installation of ~120 VSDs (VVVF drives). Here is some key data for this discussion:

1) Drive sizes - 0.37kW to 90kW
2) Drives are AB Powerflex 525 and 753
3) Distance from drives to motors is around 100m
4) The drive cables will be installed on ladder probably touching other, non shielded control cables.
4) Drive cables are required to be steel wire armored (SWA)

The installing contractor has a significantly cheaper offer for the supply of VSD cable having only one of a braid screen or a foil screen, but not both.

What do you think the best option is, foil screen (laminate tape) or braided screen?

From my understanding the tape provides a lower transfer impedance for higher frequency noise, while the braid is more suited for lower frequency noise.

In addition, the benefits of SWA for shielding have been debated considerably and, in the past, I have not considered SWA to provide any shielding benefit. However, practically, it probably will provide some shielding for low frequencies - hence with respect to having to choose between foil or braid, perhaps the combination of SWA and the foil is the best option?

Any thoughts are much appreciated

Regards, Michael.

 

[jraef]

How I consider it is that foil is better, but if the machine moves, you need the braid. Most braided shielding will come in at 80%, whereas foil comes in at 100%, or should.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[DrDrreeeaaa]

Thanks Jraef.

Curious that the AB reference DRIVES-IN001 talks about providing a braid screen as a minimum, then a tape screen as additional protection.

 

[GroovyGuy]

For what it's worth; I have never used any special (shielded) drive cable for LV PWM 6-pulse type ASDs. I have yet to hear of any issues. Being in Canada, we always use metal-clad (TECK type) cable for all motor feeders. TECK cable has an interlocked aluminum armour, which I suppose can knock out some of the higher level harmonics. We always provide a dv/dT filter on the ASD output as well; this also helps in knocking down the higher level harmonics.
Mind you we seldom install control cables in the same tray as drive cables. Control cables are generally located in their own tray system, even though this is not a Code requirement with armoured cables.

BTW: Most of the plants that I design are very drive rich with all of the larger motors being controlled by PWM type ASDs. These Plants are also located in very remote locations, which is maybe why there have never been any complaints.

The biggest issue that we have with 6-pulse drives is cleaning up the harmonics mess produces by the drive inputs. Generally we use LV active harmonic filters. MV ASDs are generally (minimum) 24-pulse type, so harmonics are not an issue with these ASDs.
GG

ps I would appreciate feedback form other experienced large-scale ASD users as to their experience with noise issues.



"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[jraef]

GroveyGuy,
I believe you, but the danger of anecdotal stories like yours is that people who are looking for a way to justify NOT doing things the right way will use these anecdotes as "proof" that it's unnecessary. There are several pertinent aspects to your story that seem irrelevant, but are not. The other issue is that shielding of the output cables has nothing to do with harmonics.

Shielding is a MAGNETIC issue, in that the PWM output will be creating a lot of EM/RF interference that will be radiating away from those conductors into the surrounding area; they are essentially FM radio broadcast antennae. If you have nothing in the surrounding area that can be affected by this, then you are causing a problem that has no effect on anything; a tree falling in a forest with nobody there to hear it. Using Teck cable also helps because, like the shielded VFD cable, the 3 power conductors will have a consistent geometry in their relation to each other, which helps reduce standing wave formation. The fact that you have DV/DT filters on the output of every drive also helps GREATLY in preventing motor failure from reflected waves. If you have proper separation of cables in tray, the inductive coupling problems again have little effect as well. But few people go to that much effort and simply connect to the output of a VFD and lay cables in trays randomly using THHN. So yes, your facility seems to have some very good installation practices that override the need for further measures to be taken, but this can't be used as a generalization to justify never using shielded cable.

The other thing I'll add is that I hear the story that "we've never had any issues", only to find out later that they "lost a few motors, but that's normal". Chances are they lost motors and didn't bother to discover that it was related to the reflected wave phenomenon in their unprotected output circuits.

Again, I'm not meaning to chastise you, I just wanted to warn future readers that what you are describing is an exception, not the rule, and other important aspects of your standard practices are the likely reason you are having success without having to use VFD shielded cables.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[GroovyGuy]

Hey jraef,
My only reason for my earlier reply to this thread was to let others know of my experiences with this issue, and hopefully entice responses from other large-scale ASD users.

You may not agree with my observations, but that is all that they are - observations.

My question to you is " Why would anyone respond, and risk being 'chastised' by Others ? ".

Regards,
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[Marke]

There is an ongoing debate in this area with the concensous being that it is about keeping the monkey in the cage, however there are two mechanisms in place:
1) radiated high frequency energy which can be coupled into other nearby conductors,
2) conducted high frequency currents which can pass through other equipment on it's way home.

Here in New Zealand, we operate under the MEN system where the three phase supply is fed from a star secondary transformer and the star point (neutral) is connected to earth.
There is considerable switching current injected into the frame of the motor due to the switching voltage waveform applied to the stator windings. This is capacitively coupled coupled to the frame of the motor and this current has to find it's way home to the source which is the DC bus supplying the IGBTs on the drive output. One path home is via the the earth cable and or screen connected between the motor frame and the VFD frame provided that there are small capacitors connected between the DC bus and the VFD frame. (Common in European VFDs that have to comply with EMC regulations). Another path is via the rectifier back to the DC bus. This requires the current to flow from the system earth back to the three phase inputs via every piece of equipment that has an EMC or similar capacitor between phase and earth. The VFD will often also have capacitors between the input phases and earth for EMC reduction.
If the issue is radiated, then foil with the greatest coverage is best. If the issue is conducted, then you need maximum surface area AND at least 1mm conductor thickness. The high frequency currents penetrate into the conductor by less than 0.5mm at the higher frequencies.

Most of the EMC problems that we experience here in NZ, are conducted rather than radiated and the big answer is to provide a very low impedance earth return path between the motor frame and the VFD frame and to do this, you need surface area the whole way between the motor and VFD. Any constriction (pigtail etc) will lift the path impedance and increase the "stray currents".
Provided a good path is provided, we find that it is not necessary to screen the cable, but the cheapest way to get a good return path, is to use a properly terminated screened cable with a continuous screen.
In many cases, we have retrofitted an alternative return path using a wide range of conductors, water pipe, aluminium strip, copper braid etc. The target is about 1mm thickness or greater, and a surface area of around 50 times the surface area of the phase conductor.

Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[DrDrreeeaaa]

Thanks very much for your reply, Mark.

We have spoken before on the LMP Forum - we also have EMX3 soft starters on this job

Care to weigh in on the foil vs braid debate?

Cheers mate

Michael

 

[Marke]

Hello Michael

As described above, there are two issues, providing the lowest impedance return path back to the VFD at around 200KHz, and this is best achieved with braid as it is thicker, and the other is the screen to keep noise in and this is best achieved with foil as it has a higher coverage.
If your control wiring is in very close proximity, then screening is important, otherwise reducing "stray currents" may be more important.
Generally, pure radiated noise will occur at the higher frequencies, in the MHz, but conducted noise will occur in the 100KHz's.

My experience has been primarily fixing problems of conducted noise and for this I would choose braid, and it does not have to be around the conductors, but can follow a different path if required.

So I wold recommend foil for close control cabling etc and MHz radiated issues, or braid for interference with other equipment connected on the same supply.

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[controlsdude]

Some installs use conduit where all vfd cables are run in conduit with a proper gauge ground cable. Typical install would be like below.

Dry installation.
Conduit is bonded by connectors from panel where vfds mounted to motor.
THHN wires in conduit.
Line reactor before the vfd drive
3 phase and ground to vfd drive
3 phase and ground from vfd drive to motor.
Depending on drive distance to motor as specified by the vfd manufacture there is either some output load reactor or some filter.

Now as you can see all vfd wire was contain in conduit with a bonded ground and ground wire to motor frame.

If you ran any wire that was not 3 phase with the other vfd wires i think you would have the results that jref is talking about with radiated energy interference.

I have seen this type of interference where its created in the panel due to improper install where 120vac wiring had problems with the VFD interference.

Separation is also critical even with 120vac control wiring and higher voltage vfd wiring.

 

[dogleg43]

Allen-Bradley should be specifying the cable type for their drives and for this application. Show them the cable type the contractor is proposing and get them AB's opinion on it. If you don't when there are problems they (AB) won't support their product properly. Get the cable manufacturer and AB together.

Now don't let AB overwhelm you with double-talk or try and weasel their way out of whether they approve of the cable or not. This sounds like a large project and manufacturers should be willing to work together to help you.