RS5000 Programming Question

[grider75]

Hello all, I have a small programming issue for you. I have 6 250hp water pumps with 2 of them on VFD's. Engineers are wanting to automate these pumps. What they want to do is run the 2 pumps on VFD's all the time with a setpoint based on header pressure. If the VFD's cannot handle the load then start one of the other pumps, and after a certain time if the pressure does not come up then start another pump and so on. I have the programming done for one and I am testing. My issue is that I need to be able to rotate the starting of these pumps so that we are not accumulating excessive hours on one pump. Any one ever done this before? Do you have any sample code I could work with? Thanks,

 

[Brijm]

There are a few examples here
http://www.plctalk.net/qanda/showthread.php?t=51640

 

[cbuysse]

To begin with, I have never understood the desire to keep the runtime among a collection of redundant pumps the same. To me, that just implies that you're more likely to have them all fail at roughly the same time. 250HP pumps are not cheap and having to rebuild/replace all of them in close succession is just not an easy thing for a department to budget. It has always made more sense to keep one of them as the primary pump until it fails, then you have a group of (relatively) unused pumps to take over. I do recommend to the operators to periodically switch to the other pumps to make sure all of the pumps get at least some runtime to make sure they remain fully functional, but not to gain any significant hours.

I don't have any code on hand to share, but when I've been forced to do what you're asking about, I've set up a counter (or just a DINT with ADD and SUB instructions) to determine the number of pumps to run. Assign each pump a number to determine order of operation. PumpA = 1, PumpB = 2, PumpC = 3, PumpD = 4. These pump numbers change when you change the lead. So PumpA = 4, PumpB = 1, PumpC = 2, PumpD = 3.
When the number of pumps required is GEQ than the assigned number, run that pump.

I'm sure you'll get a handful of other options, but that one has worked out for me pretty well.

 

[redbarron55]

I assume that the pressure remains constant and the flow changes a great deal.
I have 4 350 hp pumps all on VFDs. These each are setup to control pressure PID with the onboard 25 VDC powering a small pressure transmitted dedicated to that drive.
In this case the setpoints are slightly staggered and the pump with the highest setpoint is the lead. When the flow starts to exceed the output on that pump the second starts pumping. There is no control in the PLC for these pumps and as long as there is power the pumps will supply the plant with industrial water.
If the base load = 1+ pumps then the answer is to run one fixed speed pump and trim with the VFDs.
If the base load is less than one pump then I would program the VFD to maintain pressure and when it reaches 100% load start the second pump. If you want to run both VFD's the stagger them as above and when you reach 150% start the third (or more) pumps.
If the pressure control is critical then the surges from starting and stopping these pumps may cause a problem for operations.
Are these pumps started across t he line or do you have soft starts? Also are these low or medium voltage motors? While the normal cutoff for Medium/low voltage is often 200 HP the low voltage drives are much less expensive, but the copper costs are higher.