# Vsd motor wiring



## Frank Mc (Nov 7, 2010)

Hi Guys

We are having a debate over in the Oz sparkie Forum regarding cable used for wiring motors connected to vsd,s...

Firstly do you guys use vsd cable (has outer shield)...?

Secondly if you use vsd cable do you ground the shield ?if so at one end only or both....Any motors i have wired to vsd,s we use vsd cable and ground at both ends...

Thanks for any feedback
Frank


----------



## Jlarson (Jun 28, 2009)

Sometimes I use shielded drive cable, sometimes shielded tray cable, with the shield drain connected at each end. 

With a drive output power cable the idea of the shield is to prevent radiating all the EMI crap drive output conductors can throw off and conduct it back to the drive.


----------



## BBQ (Nov 16, 2010)

I have not done a ton but have used standard building wire or VSD cable depending on the specifications.

As far as the shield it is my understanding it is no longer a 'shield' if grounded at both ends so that it can carry current. At least that is how it works on the LANs I have worked with.


----------



## Jlarson (Jun 28, 2009)

BBQ said:


> As far as the shield it is my understanding it is no longer a 'shield' if grounded at both ends so that it can carry current. At least that is how it works on the LANs I have worked with.


With signal cable the idea is to keep EMI out and prevent ground loops that could induce noise to the signal, so the shield is only bonded at one end. 

If you run drive outputs in non shielded cable there will be stray capacitance to ground and noise current will be coupled to ground, with the shielded cable bonded at both ends the capacitance is now coupled to the shield and the noise current will flow back to the drive.


----------



## BBQ (Nov 16, 2010)

Jlarson said:


> With signal cable the idea is to keep EMI out and prevent ground loops that could induce noise to the signal, so the shield is only bonded at one end.


Understood.



> If you run drive outputs in non shielded cable there will be stray capacitance to ground and noise current will be coupled to ground, with the shielded cable bonded at both ends the capacitance is now coupled to the shield and the noise current will flow back to the drive.


Either way, keeping noise in vs keeping nose out I am not sure it is necessary, or even desirable to bond the shield at both ends.

I would be happy to learn the truth of the matter.


----------



## Jlarson (Jun 28, 2009)

Just about every drive manufacture recommended bonding the shield at both ends if you read enough of their tech info it's in there somewhere.

The idea is to make sure you have a continuous shield from the metal motor case and j box, through the cable all the way back to the drive and provide a return path for the noise current, and prevent creating an antenna.


----------



## Jlarson (Jun 28, 2009)

Hey Frank got a link so I can read the discussion over at the other forum?


----------



## oliquir (Jan 13, 2011)

no i use regular teck cable unless it is a very long distance. you may follow drive manual for best cable to use, but vfd cable cost $$$ and may not be required depending on application


----------



## Frank Mc (Nov 7, 2010)

Jlarson said:


> Hey Frank got a link so I can read the discussion over at the other forum?


http://www.phased.com.au/forumdisplay.php?f=29

Frank


----------



## Jlarson (Jun 28, 2009)

oliquir said:


> no i use regular teck cable unless it is a very long distance. you may follow drive manual for best cable to use, but vfd cable cost $$$ and may not be required depending on application


I think we will be seeing Drive cable become the norm as time goes on, there are many benefits to good quality cable. 



Frank Mc said:


> http://www.phased.com.au/forumdisplay.php?f=29


Harry found it and PM'ed it to me this morning. :laughing:


----------



## steelersman (Mar 15, 2009)

I believe (and nobody will change my mind) that you should only ground one end. Preferably the end at the VFD.


----------



## Frank Mc (Nov 7, 2010)

steelersman said:


> I believe (and nobody will change my mind) that you should only ground one end. Preferably the end at the VFD.


Cmon Steelers gotta keep that mind open ........To be honest when i was first told to ground both ends i wasnt cool about doing so...But having read a little about the subject and having no issues i ground both ends without thinking....As jLarson stated grounding only one end the shield acts like an antenna........

BTW Are the pittsburgh steelers colours black + yellow (gold)...???

Take it easy

Frank


----------



## JRaef (Mar 23, 2009)

steelersman said:


> I believe (and nobody will change my mind) that you should only ground one end. Preferably the end at the VFD.


You will believe it and be wrong.

This is absolutely proper for SIGNAL wiring, but the need for shielding in VFD OUTPUT _POWER _conductors is for a totally different reasons and needs a totally different approach. BOTH ends of the shield must be grounded for it to be of any use. If you ground only one end, you might make the problem worse.

Think of it this way: when you are running signal conductors, you have to worry about small subtle changes in the electrical properties, such as noise and induced voltages, that result from the wires acting like an antenna and picking up stray EMI and RFI from all around it. The SHIELD acts like a conductor for those stray effects and by grounding one end, you give them a nicer cleared path to ground.

But in a set of VFD output POWER cables, you are not trying to protect those cables from picking up stray EMI and RFI, it's exactly the opposite! The PWM output of the VFD is essentially a very powerful local FM radio transmitter. Those cables are emitting EMI and RFI into everything around them, sometimes for great distances (see story below). By grounding BOTH ENDS of the shield, you are essentially creating a Faraday cage around those transmitters and preventing the EMI/RFI from bleeding all over everything around them. If you ground only one end, the "cage" essentially becomes another conductor and adds to the problem rather than solving it.

*VFD EMI/RFI story:
I once worked on a machine designed to pour concrete into the test holes at the underground nuke test site in Nevada. We had to use flexible cables to the motors because everything in the conveyors was moving: extending and retracting while rotating. One conveyor motor was on a VFD. In those days we didn't have shielded flexible cable but the run was under 10 feet and the VFD was only 20HP so we used regular SO cord. When we did the test run on site, scientists from *25 miles in all directions* could "see" us in their instrumentation and had security jump all over us wanting to know what we were up to. We found a way to shield the cables, problem went away.


----------



## Jlarson (Jun 28, 2009)

The other thing is good shield termination, especially with newer large drives. Just twisting a shield braid together and putting it under a screw ain't gonna cut it at HF. 

Get used to seeing corrugated copper shields with clamps or special fittings to bond the shield. Even with braided shields get used to using clamps to clamp the shield to a plate. 


And If you are using metallic pipe and building wire try to isolate the pipe from anything else metal and pull a copper wire EGC.


----------



## steelersman (Mar 15, 2009)

Frank Mc said:


> BTW Are the pittsburgh steelers colours black + yellow (gold)...???
> 
> Take it easy
> 
> Frank


Yes they are....and now that enough of you have chimed in and told me otherwise I will now believe it......:thumbsup:


----------



## John Valdes (May 17, 2007)

THHN and a load reactor located as close to the drive as possible has always been sufficient for most every application.
I know the VFD cables are getting to be standard SOP, so I will just say it's personal preference as to connecting a motor to a VFD. I would use either method depending on the customer or facility requirements. Cost would be my argument for THHN and a reactor.


----------

