For Electrical Engineers Only?
The system is about a motor driving a pump
2 of them are located in the panel in which the analog module is installed ,those have no problem
As for the others which are installed outside the panel ,give good reading as long as the motor is off
But just when the motor starts for,zero pressure readings the readings goes like this:
Motor Off Motor On
4mA(Zero pressure) 13mA
4mA(Zero pressure) 13mA
4mA(Zero Pressure) 3mA
4mA(Zero Pressure) 3mA
Note that this has nothing to do with pressure because it is zero wether motor running or no
And the first 2 Transmitters (with reading 13mA) are of the same type and position
The second 2 Transmitters (with reading 13mA) are of the same type and position
and i made sure that there are no ground loops
Answer:
so check cabling, did you use shielded cable? did you isolate control signals from power signals in conduits/raceways? did you terminate shields on one end? have you looked at the analog signal with an oscope to determine noise levels?
99.9% chance your pickup is magnetic field dominant. The answer here is to use twisted pair cables for your sensors. Shielding is good for electric field dominated problems or plane wave RF problems, but the word "motor" says magnetic. The general rule is to make an estimate of the frequency of the interference. Calculate its wavelength. If you are within 1/6 of the wavelength, the dominant mode of coupling is magnetic. For this situation, the 1/6 wavelength is very likely in the next town so it is a very safe bet to deal with it as a magentic pickup problem. Twisting works because each twist reverses the polarity of the induced interference and it tends to add to zero over reasonable distances.
If your problem is just at motor start, then there is a big surge in motor current at start time.
Have you used a single point ground approach for your sensors and the system. If not, you will see off-sets from the sensors as a function of different currents or loads on the system.
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