Seeking help regarding instrument signal interference
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The problem is described as follows: A pressure transmitter (4-20mA) has been functioning properly for several years. About a month ago, when the pump started up, the data transmitted to the PLC and displayed on the touch screen was too high, approximately 1.5–2.0 times higher than normal. After the pump stopped, the data returned to normal. So it only rises when the pump is running; I suspect the pump is causing electromagnetic interference to it. I tried a simple grounding of the instrument, but it didn’t work; that’s why I’m seeking help from everyone. Thank you.
The shielding ground of the cable must be properly connected.
Check whether the shielding layer of the signal cable meets the standards and whether the grounding is proper
I trust the quality of the cables; I’ll check later to see if they’ve been crushed, as most of them were buried back then, and there’s a possibility that the shielding is defective.
My frequency converter is in the PLC control box, far away from the pressure transmitter; it shouldn’t be the frequency converter causing the interference.
What I most want to know is what usually causes such sudden disruptions
Based on what you’ve described, it’s the pump that causes the interference; check whether the shielding grounding is proper, and also verify that the shielding layer remains connected, from the gauge head to the DCS. To disable this transmitter, the shielding ground on the DCS side can be disconnected, the shielding layer can be short-circuited to either core of the transmitter cable, and then it can be checked on-site to see if there is a conductive path.
This post was last edited by zxb1990 on 2016-1-8 at 17:37. Based on what you’ve described, it’s possible that the instrument cable isn’t able to resist interference during transmission; the grounding of the instrument signals also needs to be checked (to see if the cables for these two signals share the same path or are too close to each other, and whether damage to the outer layer of the instrument cable has reduced its ability to resist interference).
I have encountered a similar problem: there are frequency converters in the workshop as well as pumps, and when the pumps start, the output of the voltage regulator fluctuates, while it remains normal when the pumps stop. The final solution involved connecting a 220V to 24V power conversion module, as well as an isolation barrier – or safety barrier, I think – and this successfully resolved the issue.
Measuring the pressure at a certain location doesn’t necessarily indicate electromagnetic interference; it could also be due to an obstruction.
We are facing a situation similar to yours as well: the PLC controls the frequency of the inverter, but the set frequency does not correspond to the 4–20 mA output signal; the output signal is higher than the set frequency. After stopping the pump, a value of 4 mA is measured, which is normal. It worked normally before