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cFP-RTD-122 Noise Issues with VFD

Hi,, we are running a Field Point application that has 2 RTD-122 cards (as well as other AI/AO/DI/DO) that monitor various temperatures of our test stand.  Next to the Field Point cabinet we have a 10HP and 40HP VFD that drive each respective water pump.  The temperatures (3-wire 100 ohm platinum) read perfectly fine with the pumps off, but as soon as we start either pump, the RTD signals go crazy.  If fact, they get worse when switch from the 10HP pump to the 40 HP pump.  I have placed a UPS on the 110V feed to the power supply that knocks it down to 24V to feed the Field Point module.  I have also grounded the shield wire in the field point cabinet to earth ground.  Any idea what is going on here and how I can fix it? 
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Message 1 of 6



Sounds like you are encountering conducted emissions ( noise ) from the drives.


A couple of things to look at.


1 - Make sure you have a good dedicated ground from the drives to the motors.

2 - Make sure that your RTD wiring is nowhere near the power feed to the drive or the wiring to the motors. 

3 - Consider installing line filters on the power inputs to the drives and outputs to the motors.  It is always best to go after noise like this at the source.

4 - Try installing a large ferrite on the RTD wiring.

Message 2 of 6

Centerbolt, thanks for the reply, here is some more info:


1.) Yes the drives to have a good ground

2.) RTD wiring is in separate conduit from 480V wiring to the pump, but just to make sure I wired from the RTD-122 card directly to the RTD which I removed from the piping and kept it away from the pump wiring and the problem still continued.

3.) The VFD's have line reactors on them(input to drive, but not on output to motor)

4.) Some sort of magnet?  I will see what I can come up with?


I also have the shielded wire on every channel going to COM as NI suggests, but this had no effect.  Like I said, everything is well with no pumps on, and with the 10HP pump noise is present but minimal(temps change about 5 degrees), but with the 40HP pump on everything goes wacky(temps read 20F to 200F).  I also have analog input channels reading voltages and they are not affected by the running pumps.  I am at a dead end here, any more help is appreciated!

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Message 3 of 6
What are the RTD's mounted to? Are they contacting anything electrically conductive? I'm thinking conducted or capacitively coupled noise or ground loops here.
"It’s the questions that drive us.”
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I still have not figured out this issue.  I tried a thermocouple card in place of the RTD card and the same problem occurs.  The RTD's we have hooked up are 100ohm platnium, and the thermocouples I tried were J-Type.  These are the same exact probes we use throughout our entie plant with no issues, however, this is our first Field Point application.  Any more thoughts?

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I'm still convinced that this is EMI from the drives.  I would start with a line filter on the input power to the smaller drive.  I think the ones we use are from Schaffner


I have also fixed a couple of problems similar to this by placing a snap-on ferrite on the incoming cabling from sensors right at the cFP.  I usually choose one with an apeture large enough to put several turns of the sensor cable through it.


One more thought.  Have you tried connecting just one temp sensor to see if severity of problem changes?  The theory being that if the noise is getting into the cFP via the sensor cabling, then the more sensors connected, the more severe the problem should be.

Message Edited by Wayne.C on 05-15-2009 11:45 AM
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