Help using analog Input for monitor

[monta]

Hello, I need some help adding a project to an existing PLC and HMI. We have a SLC 5/03 controlling a filtration systems. I will like to add a current transducer (4- 20 ma CT) to each of the three phase legs of two different pumps for monitor the Amps for PM purpose. This is the deal, I don’t want to use six analog input. I want to use a relay for switch one pump at the time and that way I use just three inputs.
Now. I want to record the maximum amps reading in the HMI for each pump and in every PM reset the Amps reading in the HMI.
The questions is, What instruction do you recommend and how? I got the HMI part.

 

[GaryS]

I don’t see where you need more than 1 CT per pump.
In the US if you have a large current imbalance phase to phase you have some big problems and they need to be addressed first.
In fact with some creative thinking you could do it with just 1 CT
If you only run 1 pump at a time run 1 motor lead from each motor through the same CT than in the PLC move the input data to a separate address depending on the motor that is running.
If you are using this for PM then you should also track the run time on each pump.

 

[ASF]

Welcome to the forum!

One quick thing to watch for - many relays are not suitable for switching the low voltage, low current analog signals that you're talking about. The *best* way to do what you're describing (aside from biting the bullet and using 6 analog inputs) is a multiplexer. But, if you're going to use a relay, then at least make sure you get a suitable model.

As for your programming, the first thing you'll need is the SCP instruction. That will scale your analog input 4-20mA into something useful, like 0-100% or 0-50A. Or whatever.

Then, it's a simple matter of a GRT instruction. Let's say your "realtime" amp reading is in N7:0, and your maximum amp reading is in N7:1. If ever N7:0 is greater than N7:1, move N7:0 to N7:1. Easy!

The only thing you'll have to think about is how you reset that maximum value - is it reset each time the motor starts, each time the motor stops, every day, only when the operator presses a button, etc, etc.