AC noise on DC power
2

Question

AC noise on DC power

shelyle (Electrical)

(OP)

14 Sep 00 11:24

I am working on an Industrial control system, which uses a Foxboro DCS, and a Triconex shutdown system. We have been experienceing problems with Rosemount transmitters locking up, and to communicate with them we have to cycle the DC power to them. We have found some AC component on our DC power. Does anyone have any similar experience??

jbartos (Electrical) · Answer 1 · 2000-12-23 09:10:03

Due to more intense EMI emissions in some industrial environments, either the source of the strong emssion(s) shall be treated (mitigated) or substantial shielding of sensitive control and instrumentation circuits shall be implemented.

don01 (Electrical) · Answer 2 · 2001-01-04 06:06:10

shelyle,
   possibility 1: rosemount ( and most other transmitters ) "smart transmitters" are supply voltage sensitive. Are these IS circuits if they are talk to your barrier manufacturer. They must be specifically chosen for smart txs. Apparently the tx will draw some hefty currents when they recal. or or carry out some internal self checks. Of course the barrier pulls down the supply rail and the processor chucks a wobbly and dont wanna work.
   possibility 2: both end of your shields are grounded. One end only should be to instrument ground.
   possibility 3: you  have a long tray run next to a VF motor feed cable or large feeder. If your instrument cable is over all shield and the drain has a good connection then this should be minimal.

Comment: Both Fox and rosemount gear is usually quite forgiving I have seen this  combo many times and no problems.

Ps which DCS? and which txs, how big is the ac and what Hz
Sorry if this is telling you what you already know

Let us know how it goes

Regards Don

shelyle (Electrical) · Answer 3 · 2001-01-04 08:20:22

don01, Thanks for the response! Since I had posted the question, we dug further into our system, and found that the grounding was a definate problem. One interesting point, is that we interface control signals with another plant that supplies us with steam, oxygen and nitrogen. We have both fibre optic and electronic signals going back and forth. Of course the fibre signals are no problem, but the wiring for the electronic signals passes through a junction box at the fence between the plants. What we found was the shields for these signals were tied together in this JB. Essentially what we had was a big ground loop between the two plants. Although this wasn't the only ground problem we found, it is a point to remember when interfacing between DCS systems.

We are running Fox IA, with Rosemount 3051 series transmitters. We were seeing up to 150mVp-p of AC on the 24VDC.

Thanks again!

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AC noise on DC power
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