control 10 compressors

I am looking for suggestions to control the following:

10 ammonia compressors
anywhere from 0 to 10 compressors can run at the same time depending on demand
when starting I would like any one of 10 to start first but not the same one every time

The current program starts #1 then #2 then #3 etc

The goal is to get equal run time on the 10 compressors rather than #1 having the most runime then #2 etc.

TIA for any suggestions.

e

I have a similar application with 8 hyd pumps. and do something similar with water well pumps.
What type of processor?

Does it really matter the type controller? What is being asked is a theory question.


If you know your run time, then base rules on that. To me, you have a basic set of requirements in your statement - just put that into logic.

the processor type only matters for example code.
Many large plant cooling systems use opto 22 systems , the theory is the same but the code is much different

It is controlled by a AB PLC5

e

A first approximation might be :

Setup a 10-word file (N12:0)

Create a counter (C5:0, PRE=10) - the counter .ACC value will be used as an index into N12:0

Populate N12:0 with the values 1-10 by some means

When a cycle starts, C5:0 is sitting at zero.

N12:[C5:0.ACC] is compared with '1' in an EQU instruction which starts line #1. There is also an EQU comparison to '2' and so on for each of the other lines.

After the line just started is at speed (or whatever criteria you use), C5:0 is incremented to point to the next N12 location and the process repeats.

When C5:0 is DONE, rotate the N12 file thusly: copy N12:0 to a scratchpad location; COP N12:1 to N12:0 for LEN=9; put the saved value of N12:0 back at N12:9. This will move everything up (down?) so next time line 2 will start first.

Reset C5:0 to be ready for the next cycle.

YMMV

If you are using an enhanced PLC5, 5/20, 5/40, etc, then you can build on Doug's suggestion by using the FAL command instead of moving data around.

Enter FAL R6:0 10 0 ALL #N12:0 (#N12:0 MOD 10) + 1

This will perform integer modulo division by 10 and then add 1. Thus 1 becomes 2, 2 becomes 3... and 10 becomes 1 without the need for any intermediate storage.

CCE,

Here is one for a 5-compressor rotation for an SLC 5/02. There are no special commands used, just indexed addressing, comparisons, one-shots, and relay latches. It should be easy to translate to your PLC5 and add 5 more motors.

It has the Auto/Man switch feature, so that if any compressor is switched out of Automatic for maintenance, that unit is removed from the rotation "queue" until it's Auto switch is again turned on.

Be warned that if all compressors run the same amount of time, then they may also all wear out and go down at about the same time. This may or may not be desirable.

Thanks to everyone for your replys and ideas. I have to work through it now that I have some good suggestions.


Thanks again

Have a coke and a smile!!

cce