PI problem

Hi,

I have a strange problem on a site running a VFD deep water pump with a pressure sensor. The pump is feeding water to watering machines that is watering agricultural fields. So the system can both be started onto a closed system or an open system.

To stop the pump we have a condition that if the VFD is running on to low speed (indicating that it can maintain the pressure easy and thus the system is closed and the watering machines have closed their valve) the pump is stopped and must be restarted next time they want to water.

It is running fine aside from the stop condition, when the system is closed the pump is still running at 70% at 8bar. When we stop the pump manually the pressure is maintained so there is no leaks. Why isn't the PI regulator regulating down the pump? I know that the pump must run somewhere in the upper 70s% to build pressure.

We have other machines with the same setup but where we control the VFD with a Siemens LOGO, the PI regulator is in the LOGO. Then there is no problem.

/Tim

What are you using for process variable and control variables?

rQx said:

It is running fine aside from the stop condition, when the system is closed the pump is still running at 70% at 8bar. When we stop the pump manually the pressure is maintained so there is no leaks. Why isn't the PI regulator regulating down the pump? I know that the pump must run somewhere in the upper 70s% to build pressure.

We have other machines with the same setup but where we control the VFD with a Siemens LOGO, the PI regulator is in the LOGO. Then there is no problem.

/Tim

Click to expand...

It is because your PI-control is happy with 70%, pressure neither drops or goes bigger, and PI-control starts to maintain speed with 70%. There is nothing strange
It maybe can avoided if changing PI-parameters, but only maybe. Then your VFD deaccelerate to 0% or minimum frequence, and pump isn't still stopped.

Is PI on VFD or PLC now?

VFD should have PID sleeping and awaking functions, but if you control pressure on PLC, then you need to program them to PLC.
Basic idea is that if pump frequency is below certain point (Pump isn't making any flow) and pressure is good, then stop VFD (Sleep)
And if pressure drops below certain point, then start VFD again (awake).