The PLC display panel shows fluctuations in the pressure of the ammonia tank, while the actual pressure remains stable
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On the PLC display panel of the Wu Leng ice machine, the ammonia tank pressure is shown as 3.1 Kpa, with a fluctuation range of plus or minus 0.4 Kpa; however, the actual pressure fluctuates very little, by at most around 0.02 Kpa. Since the ice machine is designed to start at 3.5 Kpa, fluctuations will cause it to start frequently. Detailed description: There are three separate pressure transmitters at the ammonia tank site, each of which sends signals to the ESD. The signals received by the ESD indicate that the pressure is stable. After undergoing triple averaging processing by the ESD, the results are transmitted to the on-site PLC via a two-core cable. The problem now is that the PLC shows fluctuations in the pressure of the ammonia tank, while the ESD indicates a stable pressure, and the transmitter readings also show stability. Currently, the voltage measured at the field wiring terminals of the PLC is 24.3 V. Whenever a wire is disconnected and the connections are remade, the voltage remains stable for about 20 minutes before starting to fluctuate again. What could be the reason for this?
Could it be interference? But there are more than just this signal entering the PLC, so it’s hard to say it’s interference. Could it be a problem with the PLC channels? Or is the signal cable damaged on the tray?
“The signal, after undergoing ESD triplexing processing, has its processed result transmitted to the field PLC via a two-core cable. What form of signal is sent? Is it 4~20mA or communication? Is the shielded wire connected?
How is the signal transmitted to the field PLC after undergoing ESD triplexing, with the resulting data sent over a two-core cable? Is it communication or an ESD AO outputting a 4-20mA signal? From what you say, it seems that an ESD AO is sending a 4-20mA signal; is this two-core cable shielded and grounded at the ESD end, while not grounded at the PLC end? If so, it is recommended to try disconnecting the ESD terminal from ground.
I initially thought the same way, but I connected the shielded wire on the PLC side, yet there were still fluctuations. I’m wondering now if there’s some kind of interference source somewhere
You can try using a single twisted shielded cable.
The tendency for interference is due to individual factors; one can use a multimeter to check whether there are such large fluctuations in the parameters, and then identify the source of the interference!