Using a PLC to monitor current draw on multiple circuits

I am very new to the PLC world so please keep that in mind.

We have several heat trace elements keeping a pipe from freezing over the winter. This system is setup using individual thermostats(20 total). We would like to monitor and track the current draw of each of these circuit lines so when one fails we can just look at the log and say, "Ok circuit 15 has not pulled current in over 48 hours we know this unit has failed" and then replace it. I have no idea where to start with this and I dont know what kind of unit I should be searching for as an input to the PLC.

Thanks for any information you provide.

You could add a relay coil in series with each heater element and wire a contasct from each relay as a PLC input.

Do you actually want to measure the current or just know whether or not current is flowing at all?

If you want to measure the current you will need some sort of signal conditioner that generates an analog signal to take into your PLC.

If you just want to know if there is current you could use something like this and wire it into a digital input in your PLC.

http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR4395

The later would be much simpler/cheaper

What is the current draw on a working heater ? How many volts are applied to the heater ?

I believe you should monitor what you are trying to prevent ie freezing SO monitor temperature.
The heat trace current could be correct but if it has fallen off the pipe then it is not doing anything.
Using PLC with either theromocouple or RTDs to control heaters via relay would be fairly easy. You could have PLC monitor temp and set off alarm when temp gets too low. YOu can get small current transformers to measue current with a 4-20 mA or 0-10V output for the PLC. Again you could trip alarm if current falls and temp is below a desired setpoing.

Dan Bentler

http://www.ohiosemitronics.com has current transducers that will provide a 0-10V or 4-20mA signal that is proportional to the current flowing in your heat tapes. You pass one leg of the heat trace through a doughnut shaped sensor and it measures the current flow, just like a handheld current meter, except it sends the value to a PLC analog input.

Hello and welcome to the forum kroxic !

Look here for an inexpensive little gizmo that does exactly what you ask for:
http://www.plctalk.com/qanda/showpost.php?p=56411&postcount=9

Omron also has an "Heater element burnout detector" K2CU similar to the Carlo Gavazzi unit.

kroxic said:

I am very new to the PLC world so please keep that in mind.

We have several heat trace elements keeping a pipe from freezing over the winter. This system is setup using individual thermostats(20 total). We would like to monitor and track the current draw of each of these circuit lines so when one fails we can just look at the log and say, "Ok circuit 15 has not pulled current in over 48 hours we know this unit has failed" and then replace it. I have no idea where to start with this and I dont know what kind of unit I should be searching for as an input to the PLC.

Thanks for any information you provide.

Click to expand...

First, check with your heat trace vendor. They may have a better way of monitoring. Is the heat trace self-limiting? The current draw may be variable and you just need to be aware of that if you use a switch type.
Second, it is possible that a small graphic recorder may be a more cost effective way of monitoring. You mention viewing a log. Where, in what way. A PC running some MMI software? Cost of programming time need to be considered along with software cost.

A trend chart (recorder or MMI) is a quick way of checking instead of a log. There should be some kind of a pattern showing on/off cycles. If no pattern change then it is either stuck on/off or background temp is high enough to not be needed.
Keep it simple.

There is a flow meter on the pipe so the we will know right away if it has frozen. due to the heat trace falling off.

I am still looking into if the unit pulls variable amounts of current. I dont think it does the simplest solution seems to be to put a coil into each heat trace circuit(about 20 cricuits) and then have the contact hooked into a 24 volt power supply going into the PLC. I hear that its better practice to use a 24 volt DC PLC rather than a 120VAC PLC when possible. From there I would just be sending all those signals back to the shop where a computer would pull this info off the network and log it. Am I on the right track of thinking here?

My boss insists that in this day and age all things need to be put on monitoring systems that are tracked in the computer in a database or something along those lines. So I dont think a simple graph logged solution will apply. We also checked with the vendor and their solution will cost around $15 grand and lock us into proprietary software

....the simplest solution seems to be to put a coil into each heat trace circuit(about 20 cricuits) and then have the contact hooked into a 24 volt power supply going into the PLC.

Click to expand...

Be careful with the coil idea. Because current seeks the easiest path, a relay coil in parallel with a resistor (the heating element) may cause most of the current to flow through the relay coil, bypassing the heater. On the other hand if you put it in series with the heat tape, the resistance may be so high that the coil does not energize. For a series coil, careful selection of the coil voltage would be necessary.

Because you do not need CONTINUOUS current inputs in this case, but simply need to know when one is not working in some time period, then you would not need an analog input for each sensor. You can "multi-plex" them so that only one analog input is needed. For me the easier solution would be to add a doughnut current sensor around each heat tape. (It will be necessary to split the tape at that point.) Run these sensor wire outputs back to a PLC through DIGITAL switching outputs then to ONE analog input. The PLC steps through a sequence every T seconds, reading each current sensor in turn using only 1 analog input. It should then be easy to pick out the ones that are not working.

The main additional cost would be for the current sensors and wiring, so this may be the cheapest method, if a PLC is used anyway.

The PLC program is also fairly simple, consisting of a Timer and a Counter. Every T seconds, the timer advances the Counter. When Count = 1, energize Output 1, wait X seconds, then read Sensor 1. If Sensor 1 < Some Current Value, then Circuit 1 Not Working. Now advance to Sensor 2, and repeat for each heat trace circuit.

PS: Normally a digital PLC output has a "Common" terminal that may be internally connected to other outputs and then externally to some voltage supply. In your case, you would use the external Common terminal as your "output" to feed the analog input. In this way, PLC outputs can be used to multiplex (feed many signal inputs into) one analog input module.

www.nktechnologies.com has some current switches that you can bring back to discrete inputs on your PLC - similar to devices that were mentioned above.

Take a look at these:

http://www.veris.com/product.asp?idMainCategory=45&idCategory=106&idProduct=98

Thanks for all of the suggestions everyone. I am definately going to get a better solution for this problem using the information in this thread.

I decided that before doing this project full scale(even though it is a small project compared to many PLC projects), I want to get a PLC for my own personal use at home. I want to do some lighting automation and maybe some things with servos or something like that. I think I will feel much more comfortable with this project with that experience. I have been looking at ebay. I have also heard that many companies or distributors have test models or something along those lines that they will give companies to see what their product is like free of charge. Has anyone here heard of such an offer and if so which do you recommend. Thanks again everyone.