I have a remote facility powered by a generator that operates a water pumping system. The pressure output of this system is monitored at a well with a battery powered pressure transmitter which sends the transmitter information wirelessly back to the main facility PLC. THe PLC is powered with a 24VDC battery bank and has a charger system connected to keep the battery bank charged. The transmitter signal coming in is 1-5V and then converted to 4-20mA for the input to the main PLC (Momentum). The reciever board for this radio to the transmitter conversion is powered by 12VDC. This 12VDC is created bt a 24 - 12 converter. The problem found is (was) that the readings on the PLC output are 6 pounds higher than the actual readings from the transmitter. When looking at the 1-5 voltages, they convert to the actual readings so we had determined that the radio and pressure transmitter were good. What was found was that the ground on the analog input card had 23 mV on it. The battery charger is a MDS model and has different alarms and faults, one of the faults that it shows is a Neg Ground. This has been on the system since startup over 2 1/2 years ago, but it appears it may have come back to haunt us. I remember at startup we had removed all the remote devices from the buss and the fault never cleared, they were in a rush to get running and all systems were working correctly so it was swpt under the rug. The charger powers a 24 VDC battery bank as I mentioned. There are two circuits that then feed 24 power to the PLC. These two circuits are bussed out on terminal strips (~30 per circuit) in the PLC cabinet for the many different devices in the facility and to the multiple I/O cards on the PLC. The negative busses are next to each otherand we found one circuit when measured to earth ground was reading -26 mV and the other was +15- 18mV. I don't recall which circuit the analog input card was on initially but we did switch the card ground and the reading changed. Figuring we were on to something we set about getting the best ground situation we could. THe terminals on the grounds are Weidmuellr terminals with the screw in jumper bars in the center. We jumpered a #12 wire across the two sets of terminals which brought the ground potential to -13mV and the difference on the pressure transmitter to the PLC reading to just under 2 pounds. This fix has worked well for the past four months but they are now seeing more flucuations. I'm getting ready to go back out to the site and wanted to get some input on what to look at more closely. I have considered grounding the negative busses to the earth ground but I was told by the engineer that had done the design that the 24 VDC is to remain isloated at all times for these control situations. Thanks
How To Get The Correct Ground?
Re: How To Get The Correct Ground?
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