4-20mA Noise??

shoelesscraig

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Apr 2009
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Long story short, I am building an electrical panel at work that involves roughly 20 Allen-Bradley PowerFlex 4 VFDs. All VFDs run on 480 volts. The speed reference for all of them are coming from analog output cards on a SLC, 4-20mA. Because of the VFD design, the power wires coming in and the motor wires going out are going to be passing side by side with the 4-20mA signal wires. And, because of the size and layout of the panel, 15 of these VFD's will be side by side. So, the 4-20mA wires are going to be packed into a Panduit tray that has tons and tons of high voltage wires in it. I know that it is good practice to pass power and signal wires at 90 degree angle to each other, but this time it will be impossible for me to do. The wires will have to run parallel with each other for several feet. The drives are 1 HP drives, so current will be at a minimum. And, the VFDs are for conveyor speed control, so I'm not dealing with anything incredibly demanding such as motion control.

Assuming everything is grounded correctly and the speed wires are shielded and grounded correctly, should I have any problems with noise?? I have done many an installation, but never one with this many VFDs in such a tight space.
 
Noise is a voltage phenomenon and I have never experienced noise on a current signal. This should normally be no problem. In fact, avoiding noise is the main reason why current signals are used.

Kind regards,
 
With that many drives you should be considering a bus communications. just the analog cards alone will be very expensive.
Just saying,that's all
Roy
 
If you keep your wires as pairs any pickup of noise in one wire is cancelled out by the pickup in the return wire.
If the resultant current flow in each pair is zero then the noise pick up will also be zero.
IN THEORY
 
I have seen a panel with ten Powerflex VSD's installed & I believed that they were causing some noise issues.

I am sure that there was an improvement when each drive was fitted with a filter (filters were not originally installed).
 
With that many drives you should be considering a bus communications. just the analog cards alone will be very expensive.
Just saying,that's all
Roy

I couldn't agree more. I wanted to do this but the boss didn't. We had all of the cards in stock already and he's kinda old school. I wanted to do a DeviceNet setup. Is that what you would've choosen?? And, does anybody know where I can do some reading about how to set up that kind of DeviceNet system?? Thanks guys!
 
Make sure to use appropriately insulated wire for the 4-20mA signals. It will be running in a high voltage area and will need to be rated for that use.

Brian
 
Unfortunately, the communication options on the PF4 are limited. You can control up to 5 PF4s over DSI with an 22-XCOMM-DCBASE. The PF40 is a better choice, you can directly mount a COMM Card in them with a different cover.

I agree with Roy, I just about exclusively use communications to control VFDs these days. It is much more flexible, because you have access to all the status and control bits. Plus you have access to all the parameters via the communications link.

In your case, when you look at the cost of two Input card, an Output Card, and two Analog Output Cards. Also, if it is field I/O, you are also looking at a Communications Adapter, and rack or Flex I/O Bases. There is additional cost. I find the communications method more cost effective and flexible.


Stu.....
 
You didn't mention it but you might also want to consider using VFD approved cable from the motor to the VFD to minimize the noise.
See this link for one type:
http://www.houwire.com/products/belden.asp
Edit:
The link above in danw's post also covers this issue.
 
Last edited:
Can you run the signal wires in a separate tray or bundle outside of it? Even a small separation will significantly reduce pick-up. Also, run the signals in twisted pairs, preferably shielded, with the shield grounded at the VFD only.

As others have mentioned, current signals are pretty robust. If you can adjust the VFD input time constant you'll be pretty safe even if there is some pick-up.
 

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