Erratic Chart Recorder Behavior

[JMPE]

Condition:
Five new shoulder to shoulder VFD's installed at plant approx 10-12' away from an existing yr 2000 vintage chart recorder. Chart recorder has two services to it, namely: 120V power (PH,Neut,gnd) and a shielded 4-20ma signal cable (no other cabling in conduit). 120V wiring fed from immediately adjacent panelboard, signal wiring fed from and instrument approx 150'away (instrument also fed 120V from a different panelboard we believe).
Motors are on average 125--250'' away, THWN wire, motors have well rated insulation (type F?), new VFD's have been fitted with dv/dt filters.

Issues:
Chart recorder goes wild when drives are operated, stopped or change speeds.

Fixes:
Grounding of upstream single phase transformer feeding the recorder panelboard checked ok. Chart recorder was fed with a double conversion UPS and behaved erratically when VFD's were operated.
We're trying caps on the signal wiring now.

Any situations similar to this one out there?

Thanks

 

[hurk27]

Is signal wire shield bonded at both ends?

 

[JMPE]

Good point, it was grounded at both ends, but we rectified that and now is grounded at the recorder end.

 

[Smart $]

Good point, it was grounded at both ends, but we rectified that and now is grounded at the recorder end.

...and the problem still exists... ???

Try grounding remote signal wire at instrument end and isolating recorder end.

If no improvement, try lifting remote signal wire and injecting local "dummy" signal to recorder... narrowing perhaps where interference is getting into the system.

Is the recorder inside a grounded ferrous enclosure or sitting open to surroundings?

 

[JMPE]

...and the problem still exists... ???

Try grounding remote signal wire at instrument end and isolating recorder end.

If no improvement, try lifting remote signal wire and injecting local "dummy" signal to recorder... narrowing perhaps where interference is getting into the system.

Is the recorder inside a grounded ferrous enclosure or sitting open to surroundings?

Yes, problem has existed with one grounding point at recorder end.
Have not tried grounding at instrument end.
How does one inject dummy signal into recorder?
Recorder I believe is inside ferrous enclosure.

Does it seem to you that the interference is radiated rather than conductive?

 

[Smart $]

Yes, problem has existed with one grounding point at recorder end.
Have not tried grounding at instrument end.
How does one inject dummy signal into recorder?
Recorder I believe is inside ferrous enclosure.

Does it seem to you that the interference is radiated rather than conductive?

Injecting a dummy signal is typically done with a calibrator (examples: http://www.altekcalibrators.com/ and http://www.fluke.com/Fluke/usen/Products/category.htm?category=CALPCL&parent=CAL(FlukeProducts)). Not sure what is being measured, but a crudely-fashioned signal simulator may serve the purpose. Depending on what is being measured and the input impedance, just connecting an ohmmeter may do... since you are not calibrating the recorder.

A connection across the recorder's terminals is to provide a path for any radiated interference generated within the enclosure. And now that I bring it up, what does the recorder do when the signal wire is totally disconnected, and just one or the other lead connected?

I'd hate to guess radiated or conductive at this point (and be wrong :roll.

 

[realolman]

How does one inject dummy signal into recorder?

I don't know what the internal resistance of the chart recorder would be, but assuming it was zero, could you use a 75 - 300 ohm 1/4 watt (or a little larger) variable resistor and a flashlight battery ?

That would eliminate whatever it is the recorder is getting it's signal from as being the path for whatever is making the recorder go wacko.

 

[Smart $]

I don't know what the internal resistance of the chart recorder would be, but assuming it was zero, could you use a 75 - 300 ohm 1/4 watt (or a little larger) variable resistor and a flashlight battery ?

...

That would be one example of "a crudely-fashioned signal simulator" that I mentioned in my previous post .

 

[realolman]

That would be one example of "a crudely-fashioned signal simulator" that I mentioned in my previous post .

Crude?!? hey ....I resemble that remark.

[FONT=Verdana,Arial,Helvetica]Model 134—2 PocketCal mA Calibrator =$510
[/FONT]

pot and flashlight battery = a little less [FONT=Verdana,Arial,Helvetica]
[/FONT]

 

[petersonra]

4-20 mA signals have quite a bit of inherent noise immunity. Are you sure it is not a voltage signal? Voltage signals can pick up a lot of noise along the way.

I would give the caps a shot and see what happens.

You might want to look at the power that feeds the instrument the signal is coming from.

You might also try grounding both ends of the shield and breaking it somewhere in the middle if there is a convenient place to do so.

pH is a funny thing to measure. It might be that the VFDs are somehow affecting the actual measurement.

I would hook the simulator up at the instrument end and see if the chart recorder still goes nuts. If not, that would indicate an issue with either the instrument or the measurement.

Is the pH cell installed in a metal pipe? Is it in a bypass line or inline?

 

[cadpoint]

...Chart recorder has two services to it, namely: 120V power (PH,Neut,gnd) and a shielded 4-20ma signal cable (no other cabling in conduit)....

I thought the rull of thumb was not to mix service where VFD's where involved!(period)

Is the insulation of the signal circuit the same value of power circuit? What size is the actual amperage rating of the signal circuit wire? How many conductors are in the signal wire + the shielding? Does the shielding have a true earth ground or is it terminated on the ground of a common/ground terminals?

What is the distances involved? It seems it was said but to me was misleading or nearly irrational as presented. Did you draw a line diagram of it, and truely add up all the travel distances?

I'm thinking that the signal wire is to small for the distance and is picking up noise,
(as said) or the machine reading is hick-uping due to the lack of signal that it's expecting to monitor.

 

[Smart $]

4-20 mA signals have quite a bit of inherent noise immunity. Are you sure it is not a voltage signal? Voltage signals can pick up a lot of noise along the way.

I would give the caps a shot and see what happens.

You might want to look at the power that feeds the instrument the signal is coming from.

You might also try grounding both ends of the shield and breaking it somewhere in the middle if there is a convenient place to do so.

pH is a funny thing to measure. It might be that the VFDs are somehow affecting the actual measurement.

I would hook the simulator up at the instrument end and see if the chart recorder still goes nuts. If not, that would indicate an issue with either the instrument or the measurement.

Is the pH cell installed in a metal pipe? Is it in a bypass line or inline?

I think his PH meant electrical "phase", as in Line, not chemical pH.

 

[gar]

110515-2158 EDT

JMPE:

Disconnect both signal wires at the recorder. Short the recorder input terminals together at the recorder, assuming the recorder is not the source of current for the current loop. Don't ground either input terminal. See if the problem is eliminated.

If the recorder is the source of the loop current, then determine if it is OK to short the terminals. Wherever the loop current is it should be designed to allow shorting its output.

If the recorder has problems with the signal input shorted and connected to nothing else, then the noise is from the AC input power to the recorded.

If no problem, then ground the shorted input terminals. Now is there a problem.

If no problem, then remove the short, and reconnect the cable. Do not ground the cable at either end. Disconnect the cable from the source end and short the wires together at the source end (instrument end). Is there a problem?

Report back.

.