Electrical noise from VFD affecting Temperature Probe 2

[veryconfused]

Firstly, I have no idea if I am posting this question in the correct forum, hopefully it's correct. Secondly, I am not an engineer in anyway. I have an issue with EMI from a 3 phase 480v VFD affecting a shielded temperature probe and was hoping somebody on the forum might be able to offer some advice on how to solve my issue.

If I leave the temp probe hanging in the air then the EMI has no affect on the temperature reading. However, as soon as I put the temp probe into the water I start getting temperature spikes. If I only needed to measure the air temp things would be fine, sadly this is not the case.

Does anybody have any ideas on how I might fix this issue? I have ordered some ferrite rings to install on the output side of the VFD. Also, I tried placing a small metal rod in the sump and grounding it with a wire but that hasn't helped.

Any help/advice would be greatly appreciated.

You can see the red temp probe in this image

Position of VFD relative to temp probe

Graph showing temp probe in and out of the water.

 

[RedSnake]

Itsmoke since you only addressed the guys, I guess I am excluded.

My suggestions with the glas jar/bottle are just for troubleshooting purposes, to figur out what is going on.

And as electricpete already stated I think we came to the conclusion early on that if it hade been a short circuit the motor CB or VDF drive would have tripped.

And as tug says and is shown in my post 17 Jul 21 06:18 protentional grounding is important vid VDF drives.

And protentional grounding should not be mixed up protection ground, to different things.
Here the protection ground "PE" cables are always yellow/green not sure what is used in US.
And for potential grounding we use black or transparent so we don't mix them up.
Protection ground is for personal safety and protentional ground for the function of the electronics and the machine.

BR A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[RedSnake]

If the bearings on the motor is not isolated to protect them from bearing currents it might end up looking like this.
And if the earth of the pump isn't there it might go through the salt water as suggested.

The solution might be to have a none electrically conductive shaft coupling between the motor and the pump.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[itsmoked]

Red; I didn't have a problem with your crazy bottle gambit, t'was interesting.

I'm very sensitive to water/electricity issues. I've done calls on boats that had problems. In one case a death from someone diving into the water next to a 'bad boat'. In another case someone was paralyzed but their inertia carried them out of the potential gradient, essentially back to life. So when someone talks about problems with electrical equipment associated with water works I get concerned. The very first thing done is to make absolutely sure there's no mains voltage during any part of the process modes in the solution (pump(s) on/off). Brine might as well be liquid metal as far as mains currents are concerned.

The OP's pump in the picture appears to be large and the volute housing appears to be metal. I'm unaware of any VFDs that would be able to detect one phase shorted to the case if the case isn't grounded correctly. Likely it is grounded which would cause an immediate VFD issue no doubt. Hard to tell as the only picture we have is of an unwired pump sitting on makeshift wood.

Did I see mentioned an additional sump pump in the tank too? (Whole nother can of worms if that's true.)

The temp sensor should not be tossed into the tank beyond its dipped part. Temperature excursions will work very hard to draw in brine thru the rinky-dink waterproofing. Rather, the sensor should be in a "life ring" of some sort so only the bottom 3/4ths is in the brine so it will rise and sink keeping the cable/dipped interface forever and always out of the brine. Never should the cable touch the solution. There should be no electrical interaction with that probe because there should be no galvanic connection to it at all.

There should be no high frequency potential in the brine if the VFD was correctly wired to the motor and the grounding rules detailed in the manual were followed. (Something I see ignored 90% of the time.) The high frequency energy between the motor and the VFD wants with-all-its-might to get back to the VFD. Directly back via a large surface area HF conductor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[veryconfused]

Thanks again everyone for the feedback and information. As soon as I'm able, I will be testing out and checking as many things you've all listed. I will certainly update as I go.

 

[RedSnake]

Likely it is grounded which would cause an immediate VFD issue no doubt. Hard to tell as the only picture we have is of an unwired pump sitting on makeshift wood.

I think that is a old picture from the installation.

I will add that the pump is properly grounded and protected. I don’t see anyway for it to energize the water without tripping something

Did I see mentioned an additional sump pump in the tank too?
I think it might have been the drum HEX filter that was turned by a small motor that turns the drum when needed.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[veryconfused]

Redsanke - There is an additional pump that pulls water from the sump and sends it through a plate heat exchanger, degassing tower and protein skimmer. However, this hasn't been running.

The Hex drum filter has a small motor that is used to turn the drum as needed.

 

[electricpete]

I'm very sensitive to water/electricity issues. I've done calls on boats that had problems. In one case a death from someone diving into the water next to a 'bad boat'. In another case someone was paralyzed but their inertia carried them out of the potential gradient, essentially back to life. So when someone talks about problems with electrical equipment associated with water works I get concerned. The very first thing done is to make absolutely sure there's no mains voltage during any part of the process modes in the solution (pump(s) on/off). Brine might as well be liquid metal as far as mains currents are concerned.

I can't argue with huge respect for electrical safety. In my mind if we have this concern then the motor frame and pump casing should be viewed with the same level of concern as the water since the only possible path for line voltage in the water is from stator winding to stator frame to pump casing (line voltage can't get on the rotor)

Maybe some voltage measurements help along the way for checking both for safety and for understanding the problem.
Voltage measurements should have one lead of the meter connected to ground (not any circuit common). itsmoked gave a procedure including using the ground socket on a 3-prong extension cord plugged into a 3 wire outlet (his initial check validates you have a ground connection by checking for 120 volt).
You could check the water to ground as suggested. It would also be interesting to check the motor frame to ground and pump frame to ground and that metal flange a few feet from the pump to ground.
I know it seems like motor casing should be 0 voltage to ground but if the grounding is not effective as Keith mentioned you might still see a voltage from high frequency there.
Data is always good. But of course there is always a caveat. There is always the possibility to see a voltage due to the large loop area involving the test leads and the things they’re connected to and any path that in turn connects those two things.

It would be a easy test putting the probe in a glass bottle with freshwater just holding it in the tank to see if it makes any difference.

I see now that test would separate the effects of resistive conducted current (cannot flow through glass) from capacitive current (can flow through the glass). I did a calc (attached) based on the end-to-end resistance and capacitance in a pipe (assuming seawater conductivity 6 mho/m) and I now think the resistive conduction is far more important than the capacitive conduction in the water (by the way the selection of pipe length and diameter and number of pipes are all irrelevant to this particular conclusion since they cancel out in product R*C, but it was still interesting for me to put them in to see the individual R and C values in order to estimate what current might flow). So I suspect your test would cause the indication problem to go away. If the delay associated with temperature measurement in that configuration is considered acceptable (I don’t know, it might not be) it might possibly end up considered a permanent solution to the level indication problem. Would be better than a grounded can with holes.

(EDIT I rechecked my conductivity calc, and it was correct, but the comment "ohm/m" should have been mho/m... if you want to play with that file yourself it is available on smath cloud

https://en.smath.com/cloud/worksheet/ZeMETPxF).

To summarize the two potential paths for current to flow:
1 - stator winding capacitively coupled to rotor then conducted through coupling to pump impeller. This path could be solved by shaft grounding (which also improves bearing life) or by insulated coupling.
2 - stator winding capacitively coupled to stator frame then conducted to pump casing. this path might be solved by improving ground connections at the motor.

If we're looking for other quick-check solution I would try grounding that flange a few feet away from the pump, I don't see any harm in that myself.


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(2B)+(2B)' ?

 

[BrianE22]

Heading back a bit from where the discussion is going, try layering RTV sealant on the probe. It won't make it waterproof but it may give you a minute or so of dryness on the thermocouple. Also coat the wires and the entry of the wires into the probe head (that houses the electronics). Do your ON/OFF test with the VFD again (very soon after putting the probe in the water). See if the results look any different.

 

[veryconfused]

Electricpete - the flange is pvc not metal.

BrianE22 - I have some RTV sealant, so it’s something I can try

 

[RedSnake]

Since I have hade both fresh and saltwater aquariums during many years, it was my big hobby before.
I know that fish can tolerate quit large differences in temperatur as long as they are not rapid.
Of course for growth reasons I suspect that one wants to keep the optimal temperature for that, as konstant as possible.
A glas bottle with fresh water in it would get the same temperatur as the surrounding saltwater and how fast or slow it will even out the temperatur at a change I would guess depends of thickness of the glas.
I think it probable would be acceptable it would at least be possible to adjust the temperatur regulation taking this in to account.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[electricpete]

I see all sorts of equipment around the tank, any of it could be conducting to the brine.

I’d like to mention that the instrument malfunction was tied to the vfd both by observing it does not occur on other equipment AND that it went away when the vfd was deenergized. Those are two pretty good corroborating clues for purposes of troubleshooting. Add to that the exteremely low probability that a motor shorted to frame without tripping and the risk is pretty darned remote. But does it meet the high level of certainty expected for purposes of electrical safety? No it doesn’t, you're right. Standard safety practice is verify by test that it is deenergized before you put your hand on it if you are thinking about it as potentially-energized equipment.

I don’t in the least mind anyone interjecting safety concerns. If your advice to the op in this particular situation is not to touch the water or any electrical equipment without checking it, that’s fine it's a good idea.

I do feel that my comments in particular were singled out for some reason by you 19 Jul 21 21:20. I don’t see that anything I suggested makes the situation more hazardous other than potential to place the hand in the water which is not something you objected or warned about initially. And your objection seemed to be centered around my comment, citing ground rods and grounded fiberglass tank which I believe you misunderstood.

I'll plead guilty of not thinking through electrical safety before giving my advice. But I'd like to point out that most of the others in this thread did the same. We talked about lifting probe in and out there was no mention of safety. We talked about possible glass, there was no mention of safety. Your own previous comment telling how to measure with an extension cord could have easily been construed by op as a troubleshooting recommendation rather than a safety precaution (that's the way I interpreted it at the time although I can see in retrospect that may not be what you intended). And the water could have been very easily contacted during that process. You did post later talking about lethal but that could've been too late. As for myself I didn't notice your later post using the word lethal in the long string of posts.

So if you're judging this thread for perfect electrical safety, then I screwed up a little. But I'm not the only one (that doesn't mean I screwed up any less but it makes me feel better).

Perhaps a reminder for me to keep electrical safety forefront when giving advice to non-electrical people.

PS - If it's not obvious you touched a nerve with your post 19 Jul 21 21:20. It arouses my defensiveness although you were more concerned about safety than anything else. I felt a need to respond.


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(2B)+(2B)' ?

 

[sferrari]

The suspense is killing me. I need to know what's happening.

I have three additions to confuse this even further.

First, you can't even assume that it's the VFD. There could be other aspects of this process that coincide with the VFD running.

Second, when I have problems with instrumentation, it usually comes back to improper grounding. Make sure everything is grounded PROPERLY.

Third, if there's a question of whether this probe is suitable for use in this submerged brine application can't it get put in a thermowell? Drop it in a copper pipe so it stays dry. You might lose a bit of definition but it's a huge mass of water. The temperature doesn't change quickly.

I'm talking out of my ass so take it all with a grain of salt.

 

[RedSnake]

Sea salt then ;-)

First, you can't even assume that it's the VFD. There could be other aspects of this process that coincide with the VFD running

At least to my understanding veryconfused have stated that nothing else has been running while he has conducted his own tests and this is the only VFD in the plant.
So what else could it be?

Third, if there's a question of whether this probe is suitable for use in this submerged brine application can't it get put in a thermowell? Drop it in a copper pipe so it stays dry. You might lose a bit of definition but it's a huge mass of water.

Not sure a copper pipe would be good for the saltwater fishes.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[BrianE22]

Veryconfused - the 2 points per second is low sampling rate. Your readout electronics probably has an antialiasing filter that sets up at the proper frequency for the chosen sample rate. You might want to check though. You'd need a very low pass analog filter for 2 pts/sec.

 

[LionelHutz]

sferrari - I was thinking that too, use a grounded metal thermal well.

 

[electricpete]

My thoughts on thermowell suggestion:
[ul]
[li]The existing shielded digital temperature probe (with varying diameters) is an oddball shape to be used in a thermowell. It would have to be a large diameter thermowell. Which part of the probe needs to be in contact with the well I have no idea. Maybe it's ok if you fill it up with thermal compound or water inside. That might be an option to consider for permanent solution if you have something like stainless steel.[/li]
[li]After Keith's comments I was thinking about effect of grounded component in the tank. Assuming we are seeing high frequency capacitively coupled voltage, it would create a small voltage gradient although very low. I assume it wouldn't have any long effect on the fishies, but I don't know for sure.[/li]
[li]I vote let's try the fresh water glass test first.[/li]
[/ul]
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(2B)+(2B)' ?

 

[itsmoked]

Hi Pete; No worries, I wasn't targeting you. I ran aquariums for nearly 50 years and have seen so many electrical connection to the water over that time it was ridiculous. Almost every "Waterproof" thing wasn't at some point. I'm always leery of water with electrical apparatus associated with it. I was talking instrumentation while thinking safety at the same time.

There is no good reason for the brine to have enough VFD related electrical noise in it to screw up the data logging unless it's galvanically connected to the power somehow. Furthermore the probe shouldn't be caring unless it's also galvanically connected, i.e. wet to it's insides. Otherwise the problem is probably the cable routing OR power filtering OR radiated noise. getting to the cable on the way to the soup or the data logger/controller directly.

Confused: I will repeat that you do not in anyway ground the brine for the sake of the fish. Grounding it is harmless unless something in the future energizes or even small leakage currents get imposed from somewhere. Then the aquaculture will go to hell quickly.

Pay attention to my probe life-ring suggestion above. The cable - insulation goo interface is to be very suspect. It will absolutely fail at some point if not already.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RedSnake]

If I do not misremember veryconfused sade they use this probe in all other of there already running tanks without a problem.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[itsmoked]

Red; I hope you're not inferring that if a couple of earlier probes haven't leaked yet that means a couple of later ones won't?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[veryconfused]

Briane22 - I don’t think I can change the sampling rate. I will have to read about antialiasing filters.

Itsmoked - I will not ground the brine. I will add that there aren’t any fish in the white tank. And I will try out the probe life ring after I’ve tried the probe in a glass of freshwater.

Redsnake - you are correct, I use these probes in other systems without this issue.

I’m going to install a new probe to completely rule out any issue with the probe.