Noise on analog signals

lesmar96

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We worked in a sewage treatment plant and take care of their plc work. They have an issue with noise some of the analog signals in the plant. When the vfds on their blowers start, some of their tank levels and D.O. level feedback signals just go really hay-wire. Jump all around and trigger false alarms etc.

All the analog wiring is shielded and grounded. The wiring to the blower motors is all in EMT conduit that doesn't even go close to the EMT conduit where the signals come in from the tanks. The vfds and motors all have ground wires. It is only 5 analog signals that this happens on. All the rest are okay.

Incidentally, all 5 of these are on the same AI card, but the card is pretty new.

I am not a specialist in noise like this, but I checked for a few things I thought could be obvious, but I didn't find anything. Except whoever installed the blower vfds ran both the line and load wiring of the drives in the same conduit for about 10' to a junction box where they split out.
Could this cause such as issue??

Is there a possibility the ground wires are not connected to earth so it has no way to "drain" the system??

Where would you start for something like this??
 
Is the shielding terminated at a single end? Do the analog signals go through any j-boxes or terminal strips? Do the VFDs have line filters? What brand VFD are they?
 
A couple of things to check.

The shield in the cables for the analog signals must only be connected in one end to avoid ground loops.

How long are the blower motor cables ?
If very long, it may not be enough with shielding the motor cable. It may be necessary with a VFD output filter.
 
I've seen it happen when the noise is reflected back to the AC line, thus turning the entire power system into a big transmitting antenna.

Make sure there is an input line reactor to the VFD, and may need to add an EMI/RFI filter on the line input to the VFD. I't helped in numerous applications where I've had analog issues.
 
Is the shielding terminated at a single end? Do the analog signals go through any j-boxes or terminal strips? Do the VFDs have line filters? What brand VFD are they?

The analog signals' shields are only connected at the plc end. There are surge suppression terminal blocks at the edge of the control panel. And from there, I believe, it is directly to the sensors. The drives are Eaton HVX9000s.

A couple of things to check.

The shield in the cables for the analog signals must only be connected in one end to avoid ground loops.

How long are the blower motor cables ?
If very long, it may not be enough with shielding the motor cable. It may be necessary with a VFD output filter.

the motor wires are not shielded. its just THHN in EMT. I would estimate a maximum of 100' to the motors from the VFD. There is no output filtering at all.

I've seen it happen when the noise is reflected back to the AC line, thus turning the entire power system into a big transmitting antenna.

Make sure there is an input line reactor to the VFD, and may need to add an EMI/RFI filter on the line input to the VFD. I't helped in numerous applications where I've had analog issues.

There are line reactors on each vfd, but not output filtering. Do you suspect running the line and load wiring in the same conduit for several feet would transmit the noise back to line? I was suspicious of that, but thought it would be good to hear from someone that has more experience.
 
Is the "EMT conduit" some kind of shielding ?

The 100 feet could be borderline for what would require an output filter on the VFD.
But since this affects only some analog signals, I dont suspect it to be the cause. Neither that nor the parallel run of line and motor cables.

Do the cabling, placement etc. of the affected analog signals differ in any way from non-affected analog signals ?
 
Is the "EMT conduit" some kind of shielding ?

The 100 feet could be borderline for what would require an output filter on the VFD.
But since this affects only some analog signals, I dont suspect it to be the cause. Neither that nor the parallel run of line and motor cables.

Do the cabling, placement etc. of the affected analog signals differ in any way from non-affected analog signals ?


The conduit is metal conduit that is connected to the grounded control panels. So while it is not a shield, it might have some shielding ability.

I agree with you on the 100'. but agreed it also seems odd why all the analog signals are not affected.

3 of the affected signals go in their own conduit to one end of the plant. The other 2 go in another conduit to a tank. These signals are the only ones on an analog card. All other analogs are on other cards. But the wiring/shielding is basically the same to all sensors.
 
Are the affected signals voltage (0-10V) by chance? Are the unaffected signals current (4-20ma)? That could explain the difference in behavior. You said that all affected signals go to the same input card.


-rpoet
 
If the analog card that has the issues is another model then all the rest, there is a chance that the other cards have higher damping on the input signal.
Try hooking up a signal converter/isolator on one of the problematic signals if you have on laying around.
If this helps I would simply change the analog card to whatever model the rest are.

Nothing else that differs with these 5 signals? Like these being passive 4-20mA while the rest are active?
 
I have experienced similar issues. In addition to shield grounding and other items covered above, I have seen improper grounding of the drive itself and improper grounding of the drive control card cause this.

Another common problem is the use of wire nuts on the analog signal wiring. Check every junction box - some %$@#%#$%# electricians LOVE their wire nuts. Replacing them with terminal blocks has saved my rump many times.
 
I would try a signal conditioner (4-20mA In, 4-20mA Out) on one of the signals that gets erratic.

If that makes a difference then consider them for all the effected channels.
 
We worked in a sewage treatment plant and take care of their plc work. They have an issue with noise some of the analog signals in the plant. When the vfds on their blowers start, some of their tank levels and D.O. level feedback signals just go really hay-wire. Jump all around and trigger false alarms etc.

Curious: do all of the analog signals go up and down in unison?

All the analog wiring is shielded and grounded. The wiring to the blower motors is all in EMT conduit that doesn't even go close to the EMT conduit where the signals come in from the tanks. The vfds and motors all have ground wires. It is only 5 analog signals that this happens on. All the rest are okay.

Incidentally, all 5 of these are on the same AI card, but the card is pretty new.

I am not a specialist in noise like this, but I checked for a few things I thought could be obvious, but I didn't find anything. Except whoever installed the blower vfds ran both the line and load wiring of the drives in the same conduit for about 10' to a junction box where they split out.
Could this cause such as issue??

Is there a possibility the ground wires are not connected to earth so it has no way to "drain" the system??

You have not mentioned what type of 480? power is supplied to the Vfd's. I hope it's not a floating delta. If you have a solidly grounded wye, did the VFD have it's common mode MOV's removed/disconnected at some time? You mentioned ground wires on the motors, do they go from the motors to the VFD then to the plant ground? When powering VFD's with a floating delta the common mode MOV's need to be disconnected to prevent them from "spraying their guts" all over the rest of the electronics during a ground fault event. But they are also a path for noise back to the transformer when the grounds are connected properly. If they are removed then there are no low impedance routes for the noise to get back to the transformer.

Where would you start for something like this??

The best place to start is doing just what you are doing, research of the system. Then attack the low lying fruit or low cost "fixes" and proceeding to the more expensive as warranted.

3 of the affected signals go in their own conduit to one end of the plant. The other 2 go in another conduit to a tank. These signals are the only ones on an analog card. All other analogs are on other cards. But the wiring/shielding is basically the same to all sensors.

You may have uncovered an important clue. If possible, try an independent cable not in the same conduit but going to the same sensor. If you get the same results try a 4 to 20 signal generator (battery powered) in place of one of the errant sensors.

You did mention that other sensors were not affected and that the conduits were "doesn't even go close to the EMT conduit where the signals come in from the tanks". My question is: do they go to the same "corner" of the plant. If so then the practice of using THHN for VFD motor leads may be the problem. The corona discharge from micro bubbles in the insulation adds another leakage path for noise especially at the end of a long wire run.

Nothing else that differs with these 5 signals? Like these being passive 4-20mA while the rest are active?

Important to check.

I would try a signal conditioner (4-20mA In, 4-20mA Out) on one of the signals that gets erratic.

If that makes a difference then consider them for all the effected channels.

More good advice

Edit: I forgot to mention Tom Jenkin's suggestions, he has worked around water treatment for many years.
 
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Do the drives have any analog inputs and outputs connected to the PLC? I've had Danfoss drives that caused noise on the analog inputs when both the drive current output and the speed reference input were connected to the same PLC. The solution was to add a loop isolator on the speed reference.

If the analog input card doesn't have isolated inputs then its possible one input could be influencing all the others. I'd try disconnecting all analogs and reconnecting individually to see if one is the cause. If it's possible to do that without causing too many problems.
 

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