Noise on analog signals

These are some really good answers. I have a few noise troubleshooting stories over the last couple of years that are similar. Most of the solutions are covered by the above replies. ...This is a good post to save.

I still have one issue that I never resolved that may be related, so I want to share it. I was doing VFD PMs at a water plant that had intermittent 4-20 mA signal issues. Because I mainly work with VFDs, I use a Fluke 87V meter, which has a low pass filter for measuring output voltage from the drive to the motor. I suppose I take readings on about 1,000 - 1,200 drives a year. I do check phase to ground on all the drives, for both inputs and outputs (don't ask why... long story). While checking L1 to ground at the water plant, I noticed the voltage was much higher than the usual 280v. Just out of curiosity, I hit the low pass filter, which usually doesn't have any affect until looking at the outputs... it fell to the normal readings. I actually took a video of it to show my senior tech: https://youtu.be/uCPMNWSo2oM

We discussed the building's grounding, but I didn't have enough information from the customer to take it further. It's still something I wonder about, since I never found the cause.

As far as analog signal issues, I experienced a new one (for me) about two months ago. The power for the flow controller was on a different ground plane than the 480v for the well pump drive. The potential difference was traveling through the analog speed reference wire from the controller to the VFD. I found it by checking for voltage between the ground terminals for the controller and the VFD. Once I bridged the grounds with a wire, the issue with the analog voltage went away.

Another situation that stands out that might be related: A local university had issues with all of their values for their environmental controls SCADA screen intermittently X-ing out. It had happened for over 8 years and no one could figure out the cause. All the analog wires were properly grounded at one end, and the building's ground was eliminated as a possibility. They mentioned it to me while I was working on one of their large drives and I took a quick look around. First thing I noticed was a small conduit running parallel, 4" away from the large drive's motor lead conduit for at least 50 ft. I explained that that was a likely cause, but the electrician dismissed it saying the conduits were all properly grounded. Long story short... they moved the small conduit and the problem went away.

I'll be interested in learning what you find as the cause.
 
Do the drives have any analog inputs and outputs connected to the PLC? I've had Danfoss drives that caused noise on the analog inputs when both the drive current output and the speed reference input were connected to the same PLC. The solution was to add a loop isolator on the speed reference.

If the analog input card doesn't have isolated inputs then its possible one input could be influencing all the others. I'd try disconnecting all analogs and reconnecting individually to see if one is the cause. If it's possible to do that without causing too many problems.

Yes, the plc is giving the vfds their speed reference via analog output. The original system had speed feedback from the drives, but that has been disconnected previously. I like the idea of disconnecting them, but I will see if that is possible since this is a WWTP online all the time.

Do the motors use the emt conduit as a ground, or do they have a ground wire?

They have a physical ground wire.
 

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