CT Used to Control CLogix 5000

Hey everyone. I found a suitable CT/Transducer with a 4-20 mA output for a fan motor wire. The motor is 3 phase with 4 FLA and I am going to monitor just one phase. I plan on using an AB Analog Flex module for the input and then programming in CLogix for proper shutdown sequencing.

My question is how will this data be read into CLogix and what should I do to make it into Engineering Units (aka User-Friendly) so that, for example, I would like to trigger an alarm if the amperage falls below 3 Amps (where the user can read 3 Amps in CLogix or whatever software it may be on)?

Thanks!

There are better ways.
Take a look at www.loadcontrols.com. They have a simple easy to understand article there about why single phase current monitoring does not give good results.

This comes up a lot.

For any unit conversion on any PLC you can use four function math with the following, which asumes integer math:

DataOffset = Data in input register at 4 mA (or zero analog signal of whatever range - for example 0 VDC on a 0-10 VDC transmitter)

DataSPan = Data at 20 mA - Data @ 4 mA

FACTOR = arbitrary factor (multiple of 10) needed to get proper resolution and accuracy

EU = Engineering Units x FACTOR

EUOffset = Engineering Units @ 4 mA x FACTOR
EUSpan = (Engineering Units @ 20 mA - Engineering Units @ 4 mA) x FACTOR

DATA = Actual data reading in input register

EU = ((EUSpan x (DATA - DataOffset))/DataSpan) + EUOffset
ProcessValue = EU / FACTOR
For Example, 0-60 psig from 4-20 mA with resolution of 0.1 psig:
FACTOR = 10
Data @ 4 mA = 6240
Data @ 20 mA = 31208
DataOffset = 6240
DataSpan = 31208 - 6240 = 24968
EUOffset = 0 x 10 = 0
EUSpan = (60 - 0) x 10 = 600
DATA = 18975
EU = ((600 x (18975 - 6240)) / 24968) + 0 = 306

Process Value = 306 / 10 = 30.6 psig

Hi:
What I have done is those cases is to install a soft starter or a overload relay thru devicenet and then i can read all the parameters.

william

Being that ControlLogix is being utilized, use a CT (or set of 3) to determine current. Use a transducer to read CT and output mA. Wire this into 1756 Analog Input module. Then scale directly into Float. Gives all the resolution one may desire.

There may be some area of improvement, but this is how I have done high voltage / high current applications.
By the way, CT in this case is a current transformer.

Actually, with ControlLogix, you define the engineering units in the tag definition of an analog tag. That's what I do with my CT's... Then I don't have to deal with Tom's math.

Thank you for all of your responses! I think I will try out OZEE's method to avoid all those calculations. Oakley's method may also work for my situation. Right now I'm trying to get a price quote of the Transducers from the vendor and then I have to wait for the lead time.

OZEE's Method is without a doubt the easiest.

Your first post said you were going to be using a FLEX analog module. The onboard scaling is only on the 1756 type of analog cards. You are still going to have to do the math.

Note in OZEE's post that the screen shot is from a 1756-IF6I.

It really is not that big of deal. You really need to know how to scale a variable anyway. There are a lot of other uses for it too.


RSL

Oops -- I missed the flex module... Sorry if that's an issue. I haven't used flex yet.

Yes it's important to know how to manually scale values, but that's only 6th grade math (remember y=mx+b??)

Unfortunately, in Logix5k the SCL function is available only in Function Block. (It really needs to get moved over to ladder!) But if you have FB functionality with your Logix5k, it's easy to use, too.