4-20 Analog signal bouncing

hayesfaze

Member
Join Date
Feb 2011
Location
Traverse City, MI
Posts
1
Hello,

I have micro 1400 with a 1762-if4 card installed. Currently I am only using one channel (4-20ma) hooked to a 0-15 psi transducer. It's a dual pump station with 10hp vfd's driving 2.5 hp motors. I have seperate conduits & plenty of spacing between my cables, but when the drives start my analog signal bounces causing my level to jump by about 3 tenths. Normally this wouldn't be a problem except we are trying to maintain a level in a collection trench & 3 tenths really messes with this. I was advised to try & install a capacitor from my signal return to common. This has worked in the past for me, but I would like further input. Thanks a bunch!
 
these two wires carring 4-20 ma should be separated from any other ac wires especially the ones for the vfd motor power ( 99% these the ones causing the mutual inductance you facing)
 
Also make sure the Micrologix PLC is well grounded...the noise could be getting to the backplane, and a good ground should help drain it away from the controller.
 
Once you have tried all the above you may want to try a filter. Using the cards
configuration or a filter in your logic. See pdf and text below.
 

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