Analog Output Question

[gjenkins]

I have a system where with a pressure loop that's running three pumps on separate VFD's. I'm limited by the number of analog outputs that I have available on my PLC. Is there a product available that will allow me to output one signal and duplicate it on 3 channels?

Thanks,

Jenkins

 

[dpc]

Yes!

http://www.actionio.com/products/slimpak2/wv408_spec.html

Three loop isolators can be placed in series in your PLC output (assuming 4-20 mA PLC output - if voltage output, you'll need to configure as voltage-to-current).

You just need to make sure that the burden of the three isolators is within the capability of the PLC output.

There are many options for signal conditioners/isolators that can do what you want to do. You might even get away with putting all three VFDs analog inputs in series on the same loop.

 

[gjenkins]

Great thanks. I thought about putting the drives in series but I was concerned about the PLC output being able to drive that many circuits.

 

[dpc]

The specs for the PLC output should provide a maximum impedance that can be driven. The VFD specs should provide info on the input impedance presented by the input. It can probably work if the VFD inputs are properly isolated. But with loop isolators, the VFDs are more independent of each other.

 

[danw2]

As others mentioned, you might run a couple of the drives in series if the output is 4-20mA and the drive inputs are isolated and of a low enough low resistance to not load the output.

If you need one, Phoenix Contact has a DIN rail mount signal multiplier:
one 4-20mA input, two 4-20mA outputs which follow the input. The input, both outputs and the power supply (24Vdc) are isolated from each other, so it is an isolator, as well. model MCR-FL-C-UI-2UI-DCI 2814867

Since the input resistance of the module is only 50 ohms, you could probably run one drive and this module in series, and use one of the two outputs for each of the remaining drives.

Dan

 

[HEC]

A fundamental question, why do you need to run all three pumps on VSD? What is the pump configuration, parallel or series? What are the process constraints? You may be able to get better response and control by running only one pump on a VSD and the other two as on/off to meet demand. I prefer simple solutions.


Mark Hutton