# Analog input interfering with analog output



## wagnersydnor (Sep 23, 2008)

Installed Danfoss VFD. Interfacing with customers SCADA. When the vfd's analog output for speed indication is connected to the scada analog input the Scada's analog output spikes to a full 21ma and stays untill our input is removed. We installed an analog option card to isolate our vfd input from our vfd output but that did not help. Something is connecting in the Scada that shouldnt , anyone have any ideas where?


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## Charlie K (Aug 14, 2008)

Sounds like the 4-2o feedback (output) is being powered from the drive.
Check if you need an active output or passive input. ABB drives can be changed with dip switchs, not sure with Danfoss.

Charlie


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## wagnersydnor (Sep 23, 2008)

You are correct the feedback signal is generated by the drive. I do not think that the danfoss has that option but maybe the scada does.

thanks


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## captkirk (Nov 21, 2007)

What kind of facility is this..?


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## Jlarson (Jun 28, 2009)

wagnersydnor said:


> Installed Danfoss VFD. Interfacing with customers SCADA. When the vfd's analog output for speed indication is connected to the scada analog input the Scada's analog output spikes to a full 21ma and stays untill our input is removed. We installed an analog option card to isolate our vfd input from our vfd output but that did not help. Something is connecting in the Scada that shouldnt , anyone have any ideas where?


I have dealt with Danfoss drives before they can be a PITA. What model is it? I probably have the manuals and info. Also what type of SCADA, control micro systems, AB, Modicon, or some strange obscure one?


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## wagnersydnor (Sep 23, 2008)

This is a remote industrial size well pump station, turbine pump. the main water plant controls speed and needs to see actual speed


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## Jlarson (Jun 28, 2009)

wagnersydnor said:


> This is a remote industrial size well pump station, turbine pump. the main water plant controls speed and needs to see actual speed


Is it by any chance a Danfoss VLT8000 drive?


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## wagnersydnor (Sep 23, 2008)

The Drive is a danfoss fc202 aqua drive, it has a standard programmable 4-20 . it also has a 0-10 volt option card wich is galvanically isolated from the main control card which we used to isolate on our vfd side.

the scada is an obscure model about 6 or 7 years old. did not have time on the last visit to get the model but I am going back tomorow and will post what i found


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## Jlarson (Jun 28, 2009)

All the Danfosses I have seen have had a 4-20 in and and 4-20 out. They also had an internal 24VDC supply with an option for an external one too. The internal and external commons were isolated and I think you needed to jumper them together.


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## miller_elex (Jan 25, 2008)

Jlarson said:


> I have dealt with Danfoss drives before they can be a PITA.


HELL NO, Danfoss is a Cadillac!

Get rid of that lousy 4-20 crap, and run that sh1t 1-5Volts.
250ohm resistor will take care of that. LATE!


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## MDShunk (Jan 7, 2007)

When unexpected results happen on a 4-20 loop, I generally first measure what's going on with a Fluke 707. Often, you'll find the cure by using a loop power isolation relay.


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## Jlarson (Jun 28, 2009)

miller_elex said:


> HELL NO, Danfoss is a Cadillac!


I will try to remember that the next time I'm out in the desert troubleshooting one that's faulting out for no reason or constantly blowing internal fuses for no reason(PS a tech looked at it too and didn't find a reason ether but it finally stopped at least for now). 



MDShunk said:


> When unexpected results happen on a 4-20 loop, I generally first measure what's going on with a Fluke 707. Often, you'll find the cure by using a loop power isolation relay.


Yeah the more information the better. Even if I got it working without an isolator I might be tempted to isolate the drive and the RTU it might just save the SCADA system one day.


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## MDShunk (Jan 7, 2007)

For many of the same reasons that I feel every PLC output should be fused, I think that every 4-20 input should come through an isolator. Seldom is in the budget, but I happen to think it's a good practice for bulletproof installations. Some believe that I am just adding another potential failure point. I'd rather say that I'm installing a cheap, sacrificial part to save a more expensive input or output card. In the case of a SCADA system, Lord only knows what they're using, and how much it costs... or, if the parts are even still made. I sure don't want to be the guy that blows some card on a 1970's SCADA system that hasn't been made in the last 20 years.

Having said that, input isolators solve the lion's share of weird problems on 4-20 loops, assuming you have no wiring errors causing the problem in the first place. In this case, I'm guessing that he's got loop power coming from two places, and the isolator will solve that too. Just an educated guess at this point.


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## Jlarson (Jun 28, 2009)

Every PLC panel I do is isolated (its a really easy up sell with most utilities here with all the lightning and power problems most of them have), the only thing the PLC outputs drive directly are LED pilot lights, everything else goes through a SS IO module, an ice cube, or some type of isolator or surge protector.


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## sparky970 (Mar 19, 2008)

wagnersydnor said:


> Installed Danfoss VFD. Interfacing with customers SCADA. When the vfd's analog output for speed indication is connected to the scada analog input the Scada's analog output spikes to a full 21ma and stays untill our input is removed. We installed an analog option card to isolate our vfd input from our vfd output but that did not help. Something is connecting in the Scada that shouldnt , anyone have any ideas where?


It sounds like both devices are providing loop power.


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## wagnersydnor (Sep 23, 2008)

We installed an analog option card to isolate our vfd input from our vfd output but that did not help. 



When I installed the drive I did not have the General Purpose MCB-01 option card. On the second visit someone else installed the card. In testing the customer (at the remote site) did not see a responce on the motor speed indication. When I arrived yesterday (third visit) I checked everything out with a meter and did not see any interference with the output signal. I discovered that the only other problem was that the feedback signal was not properly scaled.this fixed the sluggish or non excistent indication. To sum up, the input and output commons are linked on the standard control card this is where my vfd voltage was being added to the scada voltage causing the scada speed reference to stay at 20ma. Us filter installed the SCADA the actual components were Autocon


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## Hunter1151 (Nov 4, 2010)

wagnersydnor said:


> The Drive is a danfoss fc202 aqua drive, it has a standard programmable 4-20 . it also has a 0-10 volt option card wich is galvanically isolated from the main control card which we used to isolate on our vfd side.
> 
> the scada is an obscure model about 6 or 7 years old. did not have time on the last visit to get the model but I am going back tomorow and will post what i found


The fc202 had dip switched under keypad to determine the output of the analog, is one set to voltage? We use these elusively in the water industry, you should not need option card. You should have a 4/20 out as a feedback to scad a and 4/20 input to give ref to drive, the danfoss will also do an auto tune for pid


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## John Valdes (May 17, 2007)

Hunter1151 said:


> The fc202 had dip switched under keypad to determine the output of the analog, is one set to voltage? We use these elusively in the water industry, you should not need option card. You should have a 4/20 out as a feedback to scad a and 4/20 input to give ref to drive, the danfoss will also do an auto tune for pid


See what I mean Hunter. This post is as old as Moses. Take it easy. You can prove how smart you are in good time. :whistling2:


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