I have a problem with cascade controlling..

We have three 11kW Grundfos pumps and three Danfoss VLT800 VFDs at our plant to supply water to our hotel and we maintain a constant pressure of 2.5bars.

VFDs are programmed in standard cascade control mode and all the three pumps have cascade control cards.

Pump one starts first and if it cannot deliver 2.5bars then the second pump starts and so on. Now the problem is VFD one doesn't run at full speed when it delivers 2.5bars and hence VFD two and three never starts.

Now I want to remove one VFD from the plant and I want to run two remaining pumps alternating at 24hr intervals as only one pump can easily deliver the requirement.

I want to know can I do this by changing parameters or do I have to change the wiring?

best regards,
Kanchana

 

Looks like you have a split-range control logic where the pressure controller output (0-100%) is split in three ranges:

0-33% becomes 0-100% for drive 1
33-66% becomes 0-100% for drive 2
66-100% becomes 0-100% for drive 3

You need to know where the split-range is being done. If the same 4-20 mA signal is used for all drives than you will be able to change the program in the drive to get drive 2 as your priority drive. This is not great though because it means you need to reprogram your drive everytime you want to switch pumps.

Hopefully, you have three seperate 4-20 mA signals going to each drive. If this is the case, than your PLC can be programmed with a switch to change the split-range online. This is the best way and the cleanest way to do it. The hard part might be if you want to do a bumpless pump split-changeover (but this is usually not the case for this kind of application)

Good luck.
Ben Janvier, bjanvier [at] EneroSolutions.com