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senthilkodisundaram
Associate

Join Date: Jun 2010
Location: India
Posts: 25

Field Bus Interference

06/29/2010 1:26 AM

I am using Profibus PA for my process, through Profibus PA I have connected my Transmitters. Nearby my transmitters we have HT motors, due to that EMF I can not get good reading. I can not re-locate HT motors.

Anyone can help me how can I get good result from my transmitters.
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Tornado
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Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21017
Good Answers: 795
#1

Re: Field bus Interference

06/29/2010 1:39 AM

What sort of shielding do your instrument cables use? I am not knowledgeable to advise, but it seems wise to look at this aspect.

Some related strategies include 1) Install instrumentation cable in separate conduit from either control power or main power, and 2) Run instrument cable/conduit perpendicular, rather than parallel, to higher voltage wiring.

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senthilkodisundaram
Associate

Join Date: Jun 2010
Location: India
Posts: 25
#2
In reply to #1

Re: Field bus Interference

06/29/2010 2:06 AM

Thanks for your advice, we are done electronics shielding for our instruments.
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stevem
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Join Date: Sep 2007
Location: Defreestville, NY
Posts: 1072
Good Answers: 87
#4
In reply to #1

Re: Field bus Interference

06/29/2010 3:20 AM

Good advice.

In extremely high EMI environments you may have to upgrade to fiber optic cables to get your transmitter signals reliably to your controller.

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PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#3

Re: Field bus Interference

06/29/2010 3:18 AM

Remove the Profibus unit from the area. Change to 4-20mA analog signals between the transmitters and the Profibus unit.

"Simples!ξ"

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stevem
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Join Date: Sep 2007
Location: Defreestville, NY
Posts: 1072
Good Answers: 87
#5
In reply to #3

Re: Field bus Interference

06/29/2010 3:24 AM

Also good advice. Use shielded twisted pair cables with the shield grounded at the transmitter end only.

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Carl_E
Power-User

Join Date: Nov 2009
Posts: 335
Good Answers: 63
#6

Re: Field bus Interference

06/30/2010 12:30 AM

Were the installers instructed in cable and connector construction?

Here's the Profibus cable installation guide, entirely illustrated (54 web pages):
http://www.profibus.com/fileadmin/media/wbt/WBT_Assembly_V10_Dec06/Start.html

key points for different cable styles when connecting shields:


D sub shield connection:

hybrid shield connection:


I seem to recall that DP shielding needs to be grounded at all node points, whereas PA shielding was to be grounded only at the power supply, but I can't find the reference at the moment. Each uses a different physical layer electronics.
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Bud
Power-User

Join Date: Jul 2010
Posts: 136
Good Answers: 19
#7

Re: Field Bus Interference

07/10/2010 3:06 PM

Profibus has engineered cabling, connectors and terminators, which is more than one can say for generic RS-485 used by Modbus RTU.

Good installation procedures are more critical for Profibus DP because of its inherently higher speeds, but even PA installation warrants attention to detail, particularly grounding the shields as the graphics shown by Carl_E point out.

That Profibus wiring guide is exceptionally well illustrated. I give it 5 stars *****.
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Bud (1); Carl_E (1); PWSlack (1); senthilkodisundaram (1); stevem (2); Tornado (1)

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