Simlpe 4-20mA control question.

Hi. I'm new to this group and hope this is the right forum to ask this in. I have an upcoming project that will require five VFD's to run
proportionally to a level sensor with a 4-20mA output. It will be in a single building treating mine runoff. I'm rather new to this type of work as my past life was one of shipboard communications. My question is, do I just need to series the devices in the loop without concern for how many there are? I understand the issues of interference and wire loss and such. Is a 4-20ma signal good for that many devices?? I appreciate any input this group may have. Thanks.
Joe
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On Tue, 06 Jul 2004 17:01:03 -0400, the renowned Joe

You need to look at the voltage drop across each VFD at 20mA, sum them up, plus the resistance of the wires * 0.02A and see if there is any headroom left with the output compliance of the sensor (and power supply, if it's a two-wire device).
All this information should be detailed in the specs of the devices that you are planning to specify.
Best regards, Spehro Pefhany
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Joe wrote:

That is the general idea, but details count and there are several exceptions.

Each load in the current loop has some resistance and that uses up some of the available voltage driving the loop. Many drives also have one side of their analog inputs grounded or otherwise commoned to some voltage. This causes problems when you try to use the same current on two or more of them. The iron clad way to share the same input would be to add an externally powered isolator for each drive (if the drive has a 24 volt DC output, this is a good power source).
These generally have very low load resistances (50 ohms is typical) so 3 of them in series use very little loop voltage. The isolation helps to keep the noise generated in one drive from interfering with the others, and if a drive fails, it can be replaced while the other two are in service on the loop. If you mount a low voltage zener diode across each of the isolator inputs, on a separate terminal strip (that have a zener voltage about twice the 20 ma voltage drop of the isolator) you can also pull and replace any one of the isolators while the other two keep working. Having two isolators between any two drives also limits the extent of damage during drive failure that involves any sort of flash over or other voltage surge on the signal wiring.
I don't know if any of that is important, but it is nice to know what the choices are.
Here is an example of the kind of isolator I am talking about (pages 6-8): http://www.mooreindustries.com/products/data_sheets/ect_din.pdf
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Depends on the drive, many (most?) will have ground as the return for a 4-20mA input so series connection is not an option.
A simple solution may be to use 0-10v inputs on the drives in parallel with a 500R load resistor to generate 2-10v from the 4-20mA. The drives will probably have input offset and scale calibrations which you can adjust to get 0-max speed from 4-20mA (if that is really what you want).
Even cheap drives have lots of fancy options. You may be able to feed 4-20mA into one drive and have it generate a 0-10v output from which you could slave the others.
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Joe,
There seems to be a misunderstanding here. While it is quite possible to series the transmitter, power supply and VFD, you are lacking a controller. The usual arrangement is to send the level transmitter to a controller and then the controller output to the final control element (VFD in this case). With this arrangement you can set the desired level setpoint and adjust the various tuning constants for optimal performance.
Without a controller your loop is the same as having a controller with a nonadjustable setpoint of zero, a nonadjustable proportional gain of one and no reset or derivative action at all. This MAY work in your case but is very limited in its capabilities. To change the setpoint you have to move the entire transmitter physically up and down. To change the effective gain, you have to change the proportional band of the transmitter. A bad side effect of this is that you cannot read any level outside the proportional band unless you have two transmitters (the very old fashioned method).
4-20 mA is THE stand signal for all process control.
Walter.

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I agree that you need a controller as well as the VFD and sensor. Here is an example of VFD level control:
http://www.inwusa.com/pdfs/level_control.pdf

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wrote:

Snip<
Thank you all very much for your input, it is all very helpful in steering me in the right direction and giving me the insight to at least ask the right questions. The simple questions often lead to complex answers. I do like the idea of being able to replace a failed VFD or isolator without shutting the entire system down for a small extra bit in the planning stage. I'm afraid my superiors think it is just a matter of running some wires. However, the signal running the raw water pumps will no doubt need to be adjusted proportionally to feed the pump for the polymer feed to the clarifiers and likewise for the sludge pumps.
This group is as great as the RCM group, but with a lot less noise ( : P
Again thanks fopr the input!
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Joe wrote:

Hello Joe,
If these VFD's are driving pumps (via motors) you have to consider the paraters of the fluids being pump(how much does the properties of the fluids vary if any) and the characteristics of the chosen pumps. Often VFD(motor)pump combinations will not actually move fluids until some minimum speed is reached. As you look at your pump curves (go learn about pump curves if this is a new concept to you) may have various flow rates that cannot be used (such as the case when your equipment reaches harmonic speeds and the equipment shakes excessively.
Also, you may want to ask how many pumps you will run at a give flow rate( as 3 or 4 pumps might work for a given flow rate) depending if you want to minimize your electricity costs, or runtime on the equipment etc. Also, depending on your piping, valves and headers, and automationed valves you might want alarms for such things as high pressure, low level, low line voltage, bearing temperatures, vibration, just to name a few. Last, as you run equipment, you will have failures and routine maintenance. How will you automatically manage that and provide for equipment lockout (mechanical, electrical and software)?
Best thing is to use a programmable system, like what is used elsewhere in your facilities (SCADA, PLC, DCS, etc).
PS, keep the code (software you develop) very well documented, because quite often, you have to go back and work on things, after some elapsed time. The older you get, the more things and time lapses on you..... Remembering/Relearning things when a plant/facility is under duress, should be avoided.
James
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Well, all of this makes very good sense and leaves me to believe that the 'AUTO' position of the 3-pos switches we have envisioned may seldom, if ever, get used. The system consists of a sump that collects the raw water and is monitored by an ultrasonic level detector. The amount of raw water entering the sump from the old mine runoff will vary with rainfall. We plan on limiting the system to 500 gallons/hour; excess will bypass the system. The raw water from the sump is fed via two VFD run 20 hp motor/pumps to a clarifier. The clarifier takes the raw water and mixes it with a polymer (via 1 hp VFD/motor/pump) to coagulate stuff in the raw water, mostly iron. The coagulated sludge is pumped (via two 5 hp vfd/motor/xmsn/pump) to a holding tank, and from there, floats in that tank will control feed to the sludge press. The amount of polymer pumped to the clarifier and the sludge pumps out of the clarifier should be in sync with the amount of raw water being fed to the system. I guess I can't just run some wire and hope everything works harmoniously, eh? I will need set points for each and be able to proportion the activity of all the VFD's so the end result is improved watershed to the Mississippi from here in PA. There is a similar system up the road a bit, but it is all manual and the money folks want a bit more automation. This all sure is a change from installing and fixing satcom gear that someone else has already designed. I'm not an engineer, just a technician. Perhaps I have learned enough already that I can see some hired professional help is in order...anybody out there interested???
Joe
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Joe,
OK, I'll bite. Before changing jobs last year, I had almost 10 years as an EE with a company that provides control system integration primarily for water and wastewater treatment. The outfit I work for now is more diversified and always looking for new work. Plus, Syracuse, NY isn't too far from Pennsylvania.
From your description, it seems like the engineering group designing this project has covered the civil, mechanical and electrical up to the motor control, but needs help on the details of the control system. I have several questions:
Did you mean 500 gallons/hour or 500 gallons/minute? The latter makes more sense with 20 HP pumps.
The polymer coagulant typically isn't just flow paced, since the amount of stuff to be coagulated in the raw water varies. Instead, we used an instrument called a streaming current detector to measure the surface charge potential of the particles in the raw water downstream from the coagulant injection point.

Often
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Perhaps I am not even learning to ask the right questions yet. As I said I am new to this field and I do not have an engineers's grasp of the math involved in a project like this. Yes 500 gallons per minute. This isn't a large project, but a local solution to water quality improvement for the sins of our forefathers. I haven't heard anyone I work with use the term 'streaming current detector' and as a technician, I am as close to an EE as there is in this outfit. They are mostly chemists and mechanical types and it is a small company. Your statement makes sense as rainfall would certainly dilute the incoming raw water and less polymer would be needed. As an aside, my outfit also sells chemicals so the principle of economy may be a bit, er, conflicted? I don't really know how they think as I am a newbie there. Are you offering service on a consultant basis? I could set up a temp mailbox to exchange contact info if you are. I know I am a bit over my head and may be able to contract professional input into this project. Again thank you all for the input you have given.
Joe
On Sun, 11 Jul 2004 05:13:53 GMT, "Mike Lamond"

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Joe,
If by consultant you're referring to providing services to specify the instrumentation and control requirements for the process, that's not what we do. That would typically be done by the process engineer, and you may or may not have a requirement for the design to be stamped by a licensed professional engineer. We do have resources that we can call on if needed.
What we do is system integration, where we take the engineer's spec and provide detailed design, fabrication, programming and field commissioning services for the control system. Installation would be done by a local electrical contractor.
If you're interested, contact me at my work email - lamond @ deadlinesolutions.com
Mike

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