Strange problems with DCS systems and low-voltage electrical motors
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We have a 380VAC low-pressure pump on site; this pump can operate when the DCS system is shut down, but not when it is started； The start and stop functions of this pump can be implemented on-site. This pump sends a 4-20mA current signal through the motor protector in the electrical drawer cabinet to the DCS system; this signal corresponds to a current range of 0-160A for the pump, with the current usually being around 70A when the pump is operating normally. In recent months, this pump has frequently experienced sudden drops in current during operation – the current would drop from around 70A to 0A before returning to its normal level. This drop in current and subsequent return to normal occur within just 1 second (this time interval is accurately recorded by the DCS system’s alarm functions). This phenomenon occurs every few days while the pump is running. The electrical and instrumentation teams have been unable to identify the cause, as by the time they go to check, the pump has already returned to normal and all signals appear to be fine. I would appreciate it if experts could help analyze the reasons. Thank you! !
It is caused by eighty or ninety types of interference. Check whether the shielding layer of the signal line is properly grounded, and whether there is an analog signal isolator on the DCS side of the circuit.
As for DO control, I have encountered situations where high induced voltage in the wires connected to the electrical system caused the intermediate relay on the electrical side to malfunction.
Try adding an isolation barrier to the circuit.
Check if there are any sources of interference; I think it’s caused by interference
It is estimated that there is AC voltage interference in the circuit; first, install an isolator, and then check whether there is any feedback voltage in the circuit.
Check the current transmitter or issues with line shielding or grounding.
The 4-20mA signal sent from the motor is isolated on the DCS system side
The isolator connected to the 4-20mA signal from the electrical system is also uncharged. Dear experts, please offer more guidance and share your opinions. Thank you! ! ! !
If there was no problem originally, try replacing the current transmitter, safety barrier, and shielding one by one; otherwise it will be really difficult to fix.
There is shielding and isolation; also, are the wiring terminals for analog signals on-site located in the electrical distribution cabinet, where there is a lot of interference?