VFD Pumps Flow Control Using One PID


Thread Starter


I have a set of 4 pumps i.e. two VFD pumps and two AC fixed speed pumps. The two VFD speed are controlled via one flow PID controller. The controller output feeds both VFD pumps to control the speed at the same time (whichever VFD is running).

the problem is whenever operation is switching between the two VFD pumps or putting a new pump online, the flow spikes up before the PID brings it back down to the set point. The flow overshoot violates some water downstream filters capacity and creates water quality violations.

The PID is tuned to act very slow since the setpoint doesn't changed often.

I am using Schneider Electric PIDP function block (PID in the parallel form).

What is your thoughts of the control scheme and how can I prevent this flow overshoot when switching between the VFD pumps or putting a new pump online. Is it a tuning issue, which I don't think so, or is it a control scheme issue.
I would suggest placing the PID in manual before switching the pumps during a normal switchover. After the new VFD has reached its commanded speed, put the loop back in Auto.

My guess is that there will be an upset regardless but this may minimize it as the PID loop won't be trying to increase it's output when the pump is not at normal speed.

You need a better control scheme. Your pump loop should have:

-an auto/man station for each VFD

-a bias station to switch between pumps or have them share the load. A bias of B=0-100% applied to the first AM station as -B and the 2nd AM station as -(100-B)

-a low output limit on the AM stations of 0 to -3%

-optional logic for auto loading and backup pump that would switch pumps if there is a pump trip.
I agree that special logic may be needed. a switch function to set point tracking may help during the switch over. as an interim measure I would use lambda tuning for the loop since lambda tuning will decrease the amplitude of the pressure spikes during switch over.