Controlling two PF525 drives with a PID Loop

Hello all,

I have been tasked to control two PF525 VFDs with at least one PID loop in a duplexing pump scenario. The project pumps water from a lower tank into a tank on top of the lower one, which can then be drained back into the lower one via a hand operated valve. No matter how fast water pumps back into the lower one, the level has to be maintained in the lower tank. The level on the lower tank is monitored with a banner ultrasonic sensor.

I have one working PID loop which calculates how far off the tank level on the lower tank is from my setpoint. I have it tuned to operate how I would like it to but I am struggling incorporating my two VFDs into the project.

How would I be able to control two VFDs (or just one depending on rate of flow) with one (or more if needed) PID loops?

My first question is : what about tank capacities, are they the same ?.

widelto said:

My first question is : what about tank capacities, are they the same ?.

Click to expand...

They are both 35 gallon tanks.

Welcome to the forum!

So if I understand correctly, you are not alternating pumps, but rather using 1 pump for low flow demand and two pumps for high flow demand. Is this correct?

If so, there was a similar scenario presented in a post several months back. A search of the forum posts would be helpful. I'm up to my armpits in alligators presently, so I cannot do much good other than to say the solution involved a bit of an overlap between the two pumps, if I remember correctly. Only one PID loop was used in that solution.

My guess is that with only one pid you an control lower tank level, be careful with valve on upper tank, I would use open and close limit switches on that valve just for security, and in some cases to stop pumping from bottom tank.

I guess the other question is where is your pid in a PLC or the drives themselves

This thread is the one that came to my mind at first. Here is another one that might help.

There are possibly others on this site as well. Using the "search forum" tool can dig up a ton of good stuff.

Hydraulically, are the pumps in series or parallel? In other words are there two pumps because you have a high static head and need the boost (series) or do you potentially need higher flow rates (parallel). Depending on that answer, this might be very simple.