PID loop with multiple powerflex 40 vfds

Hi Guys

I have been asked to set up 3 evaporative condenser fan motors on a single
pressure transducer.
I want to stage the drives on and off according to what the pressure will
be and maintain an Ideal pressure level.
I think I can do this with a 4-20 MA retransmitter?
Or, can the 3 drives share a PID loop having 1 transducer?
Or, should I use 3 transducers on a common header?
I have never done the PID thing before and I would like to set it up and
run it on the bench to make sure it works.

Thanks in advance

Do you have a PLC or are you programming the PID into the drives?

I am programming the PID into the drives.

Thanks

Do the VFDs have analog outputs also? My first thought is to run the PT to one drive, and have that drive retransmit its speed to the other drives (using a splitter) and have the other drives follow it. That would be the easiest to configure, but not knowing the process I can't say for sure that will provide acceptable control.

Giving each drive its own PT and its own PID loop is not likely to work well as the drive speeds will float around as they "fight" each other.

pid control of multiple powerflex 40s

The process involves maintaining a set point pressure for an ammonia
evaporative condenser, were condensing hot gas into a liquid.
I need for the vfds to start and ramp as the pressure level changes.
Lets take 100 PSI as the pressure minimum with no drives running.
At 110 PSI drive 1 starts, but the MA it sees certainly wont be 4 MA.
any way at 140 PSI drive 2 starts and at 160 PSI all drives run.
There's going to be some scaling issues to work around.
Yes, the drives have analog outputs.
My minimum frequency will be 20 HZ.

Happy
Happy

I need a raise

That would be an interesting project
i would use a small PLC like the AB Micro850 to control the drives
or a better choice would be to use a compact logics usig ethernet to communicate to the VFD's over network
Trying to cascade from VFD to VFD would be prone to failures
how are you going to deal with a VFD failure the system needs to detect that and move on to the next VFD
you would also want to stagger the run times on the VFD's and motors otherwise they may all need to be shut down at the same time for servicing
a single pressure transducer would be questionable I would use at least 2 so can keep things running if one fails
Just my thoughts

A lot of drives support a multi-drive mode where one VFD is the master and it will call the others for "help" to control a load. I don't know if the Powerflex 40 VFDs have something like this but I have seen it done with Yaskawa IQPump drives and some others.

The biggest drawback to this is that the master VFD needs to be fully functional at all times, but with no separate controller (PLC) this is the sort of scheme you will need to work with in order to have them communicate in order to stage in and out like you describe.

This has not been very well thought out by the higher ups.
I think we are going to wait for a compact logix system to be installed or
I just plain don't know

Another note... why the PF40? I know it’s still listed as active but I would guess it will go to active mature before to long. The are other capable and current technology options from Rockwell.

mgodby said:

The process involves maintaining a set point pressure for an ammonia
evaporative condenser, were condensing hot gas into a liquid.
I need for the vfds to start and ramp as the pressure level changes.
Lets take 100 PSI as the pressure minimum with no drives running.
At 110 PSI drive 1 starts, but the MA it sees certainly wont be 4 MA.
any way at 140 PSI drive 2 starts and at 160 PSI all drives run.
There's going to be some scaling issues to work around.
Yes, the drives have analog outputs.
My minimum frequency will be 20 HZ.

Happy
Happy

I need a raise

Click to expand...

This was interesting right up until I read "ammonia".
Ammonia is a great refrigerant, but also quite deadly. Please be careful.

Bubba.