NOISE ON 4-20ma

[KEN_KACEL]

The Phase 2 of our project uses deviceNet IF4I analog input cards. The devices hooked to the input cards are Pressure transducers. When the device(PT) is grounded to the building ground the inputs goes to 11ma when disconnected it will go to the normal 6ma. This value is affected by the 100HP vfd in the cabinet. We have searched high and low for wiring problems and also added 1746I8n to the main rack and still get the rereferenced signal. The potential between the PT device and ground is 20-40mv.
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At our wits end !!!!!!
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[Electrically_Bound]

This is an interesting problem. I think a rough diagram showing the layout/locations of the devices mentioned would be very helpful in helping others to diagnose the problem.

 

[danw]

It isn't clear why you would ground the pressure transmitter.

What you are seeing is a classic ground loop.

Please show us the wiring, including the power supply; and include model & brand of the pressure transmitter.

 

[Electrically_Bound]

I'm with danw, I don't know what you mean by grounding the pressure transmitter. We need more info.

 

[shooter]

The sensor should be grounded as it can be touched, however the signal ground should only be grounded at the cabinet.
in many sensors the - of the supply is grounded. so this way you will need a separator in between.
what is voltage between machine and cabinet ground.
Is the ground cable from motor big enough?
is cable shielded?