Regarding the issue of fluctuations in 4-20mA signal transmission over long distances
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As the title suggests, I have a valve position transmitter that outputs a 4-20MA signal with power supply to the DCS (passive) for opening degree feedback. It has been fluctuating between 4-6MA. The DCS occasionally reports a short-circuit fault. After removing the 4-20MA output wire of the valve position transmitter at the site, the measured current remained stable, and both the zero and full-scale values were appropriate. Moreover, when signals are sent from the field using a signal generator to the DCS, the performance remains stable. However, fluctuations occur as soon as the connection is made, and similar fluctuations are seen when measuring current at the DCS end after disconnecting the cable. Dear experts, please help find a solution.
Is the valve position output equipped with power distribution? What model? It seems that by default, power distribution is not provided; here we have three brands – Siemens, Fox, and Fisher – and none of them come with power distribution
A domestic brand. It is powered by 220VAC, with an MA output of around 18VDC. There is also a pointer-type opening gauge on site; it is possible to use just this gauge as well. Fluctuations occur as soon as the DCS is connected!
Are there any photos? Photos of the inside of the junction box
It’s probably the power supply module that’s having problems; the output power is low and unstable, and its capacity to handle loads is severely insufficient
Check whether the cable is damaged in any way and whether its insulation from ground is proper.
You can just use a different locator, or even a locator from another brand. It’s like when two people who are divorced think it’s fine to be together, but it’s not okay when they are together
Connect the power supply wiring from a nearby valve to the faulty valve, to see if the fluctuations persist; or replace the power cables at both ends of the DCS internal card
It is also recommended to check the grounding; it could be interference as well.
Floors 9 and 10 are the solution to this problem. I personally lean towards strong electrical interference or insufficient power supply capacity... Is there a Zener diode safety barrier?