I have recently installed a Click PLC with a 4-20ma card and (2) Thermocouple cards. I have 4 pump temps being read with K type thermocouples and then 4 channles ive twisted the K type wire on them for future use. The problem I am having is with the pumps. Friday all my temps were dead on and reading perfect. The 4 pumps were turned off and not running due to a plant shutdown. Today I come in and my temps are reading around 200f on one set of pumps (2&3) and the other set (7&8) pumps are reading around 400. pump 3 and pump 7 are running the others are backups. If i unplug the pumps that are not running and plug in a thermocouple into my pre-extisting wire run it works great. But if i touch the pump with the thermocouple ive plugged in externally it it goes crazy again. I have made sure the pump is grounded good and can not read any voltage on the pump frame (MV or AC). It is just pumping chilled water FYI. its like I'm getting noise but i cannot see it. The K-type wire is not shielded and the Click PLC is configured right for K-type. Keep in mind its only on the pumps that are not running and the 4 runs of K-type wire is all ran the same way into the same box.I've ran new wire outside the box and have the same issues. It's something with the pumps. They are driven by a GS2 automation direct drive.The power is off to the drives that feed the pumps that are not running as well. Any ideas would be appreciated.

 

On your PLC are the TC inputs designed for grounded or ungrounded thermocouple inputs? I ran myself into a similar problem once, matter of fact with pumps not that it matters much. I wanted some rough surface temperatures off my pumps so I just twisted some K wire and placed it under a washer in a few places. Problem was my junctions were grounded while my temp transmitters (4 - 20 mA) were designed for differential ungrounded TC inputs. In my case I just bought a few ungrounded thermocouples designed for doing surface temps. What you posted points to an issue with grounding your TC with the TC input cards you have. There is no problem if you just let your twisted thermocouples float in open air correct? The problems start when your TC tips are grounded correct? Check the specifications for your TC card and see if they are for grounded or ungrounded thermocouple tips.

Ron

 

hi,
I would agree with Ron, it sounds like a grounding problem.
Often after a plant shut down some of our process control equipment did not function as it did previously.
The plant electrical staff had made wiring changes in the mains distribution and grounding network, as part of the shut down service program.

Check to see what distribution changes may have been made during the shut down period.

E

 

Actually yesterday i managed to fix the issue about 2 hours after posting this. I was getting 150mv on the pump frames. I ran a ground connecting the 2 sets of pumps together. Each pump was already grounded under the peckerhead. I still have not been able to figure out what is inducing the mv though.

 

Ground loop?
It sounds as though you carried out equi-potential bonding for the pumps and eliminated the loop.
Max.