Interference issues in the instrument system cabinet and electrical cabinet
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There is a Siemens PLC about one meter away from the electrical cabinet, and the electrical cabinet contains an inverter. Then several 380V cables run under the instrument cabinet. : The instruments in the system cabinet are not grounded; all grounding is connected to the safety ground. As soon as the frequency converter is started, there are sudden changes in the pressure and liquid level in my system. For example, the pressure read by the transmitter on site is 6 mA. When connected in series with the ammeter, a current of 14 mA is shown. The problem disappears once the frequency converter is turned off. If it’s interference, where does it come from – the cables or the system hardware? How to handle it?
The hardware and cables are subject to interference; the solution is to replace the electrical cables in that pathway with shielded cables, cover the surface of these cables with tin foil for protection, and add reactors at the output of the frequency converter； The instrument cable shielding has been properly connected, and an isolation safety barrier has been added
Use shielded cable and ground it; if that doesn’t work, then move to another location
You can check whether the grounding of the frequency converter and the cabinet in which it is installed is reliable, as well as whether the cabinet is made of metal. All signal wires must be shielded and properly grounded; if that doesn’t work, you can try adding an iron plate between the system cabinet and the frequency converter cabinet
I’ve encountered the same problem as well; I’ve tried grounding the cabinet and grounding the sensor shielding, but it didn’t solve the issue. Later, capacitors and isolation modules were added to the instruments; the data from some instruments became stable, while those of others still did not. For instruments that have not been properly treated, they must be sent back to the factory. The factory’s technical staff need to be informed about the power level of the inverter on site as well as the layout of the wiring, so that adjustments can be made inside the instrument; this approach yields better results.
This interference originates from electrical issues; generally, facilities rarely install and arrange things in this way. To resolve such problems, one can try the following: 1. Change the route of the electrical cables； 2. Install the electrical cables and instrument cables separately, placing each in its own conduit box, which should be covered with a metal shield for isolation and grounding ； 3. Add an isolation shielding element between the instruments and the electrical cabinet, and ground it ； 4. If possible, move the instrument panel away from the electrical cabinet. Instrument cables should be shielded, while electrical cables should at least be armored with steel strips. Compared to the power of the instruments, the power of the motors used in electrical systems is quite high; coupled with frequency conversion interference, this should result in significant issues.
Inverter, a dirty device. The farther away from him, the better.