# 3ma drop caused by vfd running



## wagnersydnor (Sep 23, 2008)

There are two seperate 4-20 loops with different power supplys. the only thing they have in commen is the water piping they are attached to and the bonding in the common grounding system. They have different brand pressure transducers powered, each are powered by their own power supply. 
when the new danfoss fc202 starts it causes a ma drop of 3 or 4 ma.

motor , vfd, grounding system is bonded together. I added a seperate wire and bonded them again and bonded the macc pressure transducer housing.
this reduced but did not eliminate the amp drop. 

two new ground rods were added and connected , no difference.

changing switching frequence made the difference worse when increased and better when reduced but did not eliminate drop.

I ran a seperate twisted shielded, and attached the drain at the supply end only

tomorow i will run pvc tubing and move the transducer up and away from the motor and piping , hoping this will stop the interference.

I beleive the vfd noise is causing the mili amp drop but how?


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## MDShunk (Jan 7, 2007)

Man, you got me on that one. That's why people use 4-20 transmitters. They aren't affected by much of anything. My best guess at this point might be some crappy power supplies, but that's just a guess. I really don't think grounding or bonding plays into what you're observing.


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## MDShunk (Jan 7, 2007)

Any reason why you're not powering the 4-20 loops from the 24vdc power supply in the drive? Are these transmitters associated with drive feedback, or do they go to some other analog input, and the drive running changing their values is something you just coincidentally noticed? I'd like to know a little bit more about the setup you have there.


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## wagnersydnor (Sep 23, 2008)

sorry I simplified to help to visualize the way I see it.

One transmitter is now powered from the vfd power supply thru the transducer back to the vfd.

the other is a power supply and equipment in a seperate vault, to a seperate and different brand transducer the only thing it has in common with the first is the same water piping and proximity to the motor that is driven by the vfd.

but both drop when the drive runs


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## MDShunk (Jan 7, 2007)

Weird. Someone will have an idea. Nothings coming to me. Is it possible this is genuine drop from a reduced pressure/temp(or whatever your sensors are) due to the effect of the running motor on the actual process? What's the process?


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## wagnersydnor (Sep 23, 2008)

that is the most logical cause, i have never had a signal drop unless the power was not sufficient, it has always spiked. 

we put two gauges with the piping and their is no water hammer, the pressure goes up a little and stays but never goes down, by the gauges, anyway.


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## MDShunk (Jan 7, 2007)

wagnersydnor said:


> that is the most logical cause, i have never had a signal drop unless the power was not sufficient, it has always spiked.
> 
> we put two gauges with the piping and their is no water hammer, the pressure goes up a little and stays but never goes down, by the gauges, anyway.


Gocha. If the process variable isn't actually changing, then it is an electrical problem. Color me puzzled.


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## wagnersydnor (Sep 23, 2008)

tomorrow I am extending the plumbing up to get one transducer away from the motor, I will post tomorow night with the results

all is bonded, all grounded, and the switching frequency makes it worse or better, it has to be the noise affecting the circuitry in the macc, and I think rosemount transducers

we will see tomorrow, thanks for noodling the problem


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## MDShunk (Jan 7, 2007)

I exclusively use Rosemount and Anderson transmitters in some pretty ugly environments, and have never had what you describe. I'm surprised.


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## Big John (May 23, 2010)

Just out of curiosity, have you thrown an ammeter or a scope on the pipe? The fact that changing the frequency affects the reading points pretty strongly to noise being induced in the pipe, especially if the recorded pressure doesn't change.

I wonder if dielectric fittings at the motor or at the transducers themselves wouldn't solve the problem?

-John


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## wagnersydnor (Sep 23, 2008)

I did not have a scope , and I agree about the dielectric fittings, I will try that if moving it away from the motor does not eliminate the affects


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## sparky970 (Mar 19, 2008)

Just a thought, would ferrite beads help with this issue?


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## oliquir (Jan 13, 2011)

had similar troubles with thermocouples signal running near 600v lines and the way i resolve them was putting capacitor between signal and ground. maybe you have a very noisy supply. is one of the power supply output bonded to ground?


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## wagnersydnor (Sep 23, 2008)

Update:

The next day after my post I went back. I took the transucer loose, used a gauge calibration pump to set the pressure at a constant psi. Used a twisted shielded cable 30' long and moved the transducer outside. The ma still dropped when the pump ran, not as much though.

There are three seperate power supplys doing this.

What size capacitor would you use for 24vdc/20ma?

What are and how do you use Ferrite beads?

Danfoss has a new device , not a load reactor , that they may try on the output , I will post more as this issue is resolved.

This installation is cupped into the side of a rock hill, with a large metal storage tank on the other side, it is almost like a satilite dish. the istruments that are affected are in three seperate underground vaults.


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## erics37 (May 7, 2009)

Weird.

The inputs aren't set on 0-20 mA or something are they? I doubt that'd do it but who knows.

And what's the fluid being pumped?


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## frenchelectrican (Mar 15, 2007)

That is pretty odd one.

Was any chance did you use the isolated transfomer then use the control transfomer?

The reason why I ask if you did use the isolated transfomer* that may reduce the " noise " in the system and did you do the FOP test as well somecase may have "substandard " connection may show up.


Merci,
Marc 

* hook up the isolated transfomer on line side of VSD.


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## drsparky (Nov 13, 2008)

Is the supply voltage for the power supplies changing when the VFD/motor runs?


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## wagnersydnor (Sep 23, 2008)

The voltage remains the same, Danfoss wants me to go back with an ocillosclope which I will do today, will post results

also their are three seperatepower supllys being affected


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## MDShunk (Jan 7, 2007)

The beauty of 4-20 controls is that the voltage can vary, and the current remains rock steady. The voltage range of most 4-20 transmitters is between 11 and 40 volts, and the current will remain rock-solid, no matter how much the voltage varies within that range. I'm curious what you might find with the scope. Keep us posted.


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## JD_ (Jul 8, 2011)

I'd probably try putting isolators on each loop.


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## oliquir (Jan 13, 2011)

check if any of the power supply output are grounded (some sensors or vfd analog outputs are grounded) , if they are you may need isolators or group all the negative from the 3 power supply together
can you post a schematic of those loops?


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## wagnersydnor (Sep 23, 2008)

I went back the day before last and put an ocilloscope:


No Voltage drop on power supply stopped/running
changed vfd input to voltage by switching dip switch and adding a 500ohm resistor
with ocillisope on dc, only saw the voltage drop, on the analog loop when the vfd ran the motor
switched to ac and started in auto time and volt division, and saw ac voltage on the analog loop when the motor ran
set the trigger to manual and moved range to the lower end of the 50 mv division, 10ms , no activity with the motor stopped, a lot of noise when running, I saw up to 42mv.





Yesterday, the customer re-installed a linereactor , but on the load of the VFD, *This stopped the signal drop on all analog signals.*

We are going to furnish him the proper Danfoss sine wave filter for the load sides of both vfds.

Just for your info , other suggestions from danfoss :

Make sure load wires in metal conduit , and shielded
oversize the bond between motor and vfd


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## Big John (May 23, 2010)

wagnersydnor said:


> ...Yesterday, the customer re-installed a linereactor , but on the load of the VFD, *This stopped the signal drop on all analog signals....*


 Yeah, the line reactor filters out high frequency noise, which is what you were picking up in your process loop.

I'm still a little unsure why moving the transducer 30' away didn't get rid of that noise. Was the shield still grounded at the drive?

-John


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## wagnersydnor (Sep 23, 2008)

I went back last week and changed the 4-20ma to 0-10v by installing a 500ohm resistor and switching the dip switch.

I connected the ocilloscope to the power supply first and set it on dc,
there was no changed in the voltage, whether motor was on or off.

I then connected to the anolog input, remained on dc, I saw a voltage drop in proportion to the ma drop I saw when the analog was wired for ma

I then changed to ocilloscope to ac and set the time division and the volt division to auto. I picked up the noise when the motor ran
I then changed the divions to manual , using the time and volt it had switched to. I then moved the trigger to a position where it would not register anything while the motor rested. when I started the motor the noise showed up clearly. How it is affecting the signal I guess is by inductance but I really dont know, does anyone have a guess how 42+mv/ac induced on the wire could made the dc voltage drop?

you can watch a edited video that does not show the vfd on you tube
http://www.youtube.com/watch?v=VIJboh4Mc94


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## wagnersydnor (Sep 23, 2008)

When we moved the transducer outstide we used a twisted shielded, 18gauge cable with the drain hooked to the power supply side , there is a lot of metal around, and it is cupped in the side of a hill, maybe this amplifies or contains the noise?


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