ABB as pump control: attn Dickdv

[milldrone]

Dick,

Does anything special need to be done if you were using a positive displacement pump VS a centrifugal pump as far as the pid goes? It seems that the PID in a positve displacement situation would need the variable pump near zero speed as a DOL pump came on line. But if this were a centrifugal situation the variable pump would need to be near 50% when the DOL pump came on line. Assuming the pumps are all the same size.

 

[DickDV]

In general, I won't use PID control on positive displacement pumps. The pressure/speed curve is just too steep.

Centrifugal pumps and fans have rather soft speed responses and are easy to set up on PID control.

 

[DickDV]

I probably should modify my last post a little. Regarding positive displacement pumps, I haven't tried this but I suspect that the addition of an accumulator tank would "soften" the pump response enough to use PID control. Has anybody tried this?

Also, I have run into a similar problem with turbine vane pumps, especially the submersible type. They also have a very steep speed/pressure characteristic and tuning them can be really tricky. Adding an accumulator tank always helps with these and it sure makes the sleep function work better!

 

[DickDV]

Continuing on, notice that simple Lead/Lag requires only a mag starter for each auxiliary motor with the variable speed motor connected directly to the VFD.

As we move on to Lead/Lag with Rotation, notice that, in that configuration, each motor needs a mag starter and each motor also needs a contactor for connection to the VFD. This adds significant cost and complexity to the system but keeps operating hours about equal among the several motors.

Parameter 8118 sets the number of hours the variable speed motor accumulates before the drive moves the variable speed operation to the next motor. I usually set this around 600 hours.

Parameter 8119 sets the output level at which the automatic rotation is blocked. This is not usually used but is handy if, at high outputs, the disruption in output pressure would cause problems.

Parameter 8120 tells the drive how many motors are out there. Each one has an interlock signal coming back to the drive which tells the drive if it is online or not. A motor offline simply falls out of the sequence and rotation with the drive skipping over the offline motor or motors.

The rest of Group 81 parameters deal with obscure detail issues which I am going to skip.

Hope this clarifies what the newer drives can do with multiple fans and pumps. If you've got questions, please ask.

Otherwise, that's the short course on Lead/Lag pump control.