Micorlogix 1000 for load shed

We install a lot of stand by generators at residential sites. I have started using micrologix 1000 controlers for load shed of the air conditioner compressors. I am currently using analog input to monitor the load on the generator in amps and allowing or disallowing the compressors to run based on the load. so far the highest number of AC units that I've had to control is 4 but I have a job comming up that will require the control of 6 units. For 4 units I simply wrote one rung for each input / output combination with various combinations of XIC and XIO to allow or block an individual unit. This was okay with only 4 units as there are only 15 combinations possible. But with 6 units there will be 63 combinations and I would like to make a more generic program that will facilitate any number of units. I thought that I could use a compare block, or a move, but I don't seem to have the knowledge to write a program that will actually work. the gist of the program is that I need to monitor which thermostats are calling for cooling, and either allow or block the corrisponding AC compressors based on the generator load. Any help that would lead me in the right direction would be greatly appreciated.

Sorry, this is my first post. Here is a sample of how I am currently using the XIC and XIO to control the output.

R,

I would like to help....I understand your description of what you are trying to do.

I have a couple of questions:

I see no tag names, do you use them? could you include them in you diagram?
Specifically, what do your "B" contacts represent?

S

I do use tags but when I copied it to a picture the tags didn't show up. I= input from the thermostat, O = output to the AC compressor, I use some math, a SCL, and some GRT LEQ to determine how many units to allow to run. B = the number of units to allow to run. B3:0/1 = allow only 1 unit to run, B3:0/2 = allow 2 units to run, B3:0/3 = allow 3 units to run...etc. The flow of the rung is as follows: Tstat 2 is calling for cooling, AND the load will allow 2 units to run, AND 2 units are not already running, THEN energize output to unit 2.

Ok, first let me say that I am not familiar with AB micrlogix specific terminology (sorry), but I am well versed in general ladder logic terms. So far you don't have ANY replies so I will toss you some food for thought until the "gurus" can't stand it any more and jump in.

A lot of my programming is related to sequencing PUMPS on and off based on demand, status, time of day, etc. etc.

In the case of "load shedding" you might try switching your thinking around and create a relay (condition) that is true for "enough remaining capacity to start a compressor", call it "adcap" for "adequate capacity" . Then for each compressor you would have a rung to start it based on "tstat demand" AND "adcap" AFTER timedelayX. Latch that compressor on thru normally open contact of timer X and N-O contact of "tstat demand"so that compressor runs until its tstat drops open. Then you would repeat that same logic, rung after rung until you have provided for each compressor. This way you sequence thru rung by rung starting the next compressor if needed and and if there is remaining generator capacity and after a time delay and latch that compressor on until it is not needed. You will need to provide for time delays so that only one compressor is eligable to try to start at any one time. I may have to sketch something up to show you what I mean. This style of programming will allow you to write a similar logic rung per compressor regardless of HOW MANY compressors you have.

I'm getting ahead of myself....is there one tstat per compressor or one for the whole system? The "stop" logic will be different depending.

The "start" time delay preset would be determined by how long it may take each compressor to spin up, plus time for the generator to settle, plus time for your load monitoring sensor and math to aquire, before you let the logic consider starting another compressor.

ideas so far? questions?

S

I believe to put it more accurately this is selective loading not shedding. There would also have to be some programming to shed load if generator starts getting into overload.

How large are these compressors in terms of percent generator capacity.
For discussion let us assume each unit draws 10% generator capacity

Are they all equally important or are some more important than others.
IF so then for instance if 6 units then #1 most important and will start before any other and will be able to trip any other (if you choose). #2 more important than 3 etc etc. Also with this scheme you will need anti short cycle such that any one cannot start less than say 10 or 15 minute since its last shutdown. For example:

So the logic would be
1. I have more than 10% load available
2. Is # 1 called for YES-got to 3 NO go to # 7
3 Is it running no - go to 4 YES go to 7
4. Has 15 minute elapsed? YES if NO then wait
5 Start # 1.
6. 5 minute to let generator and loads stabilize
THEN
7 Go to # 2 and start same sequence.

Dan Bentler

I believe this wheel has been invented.

May be easier to see what the genset manufacturers offer.

You could try using an integer value instead of bits:

N7:0 allow this many units to run

N7:1 number of units running

N7:1 <= N7:0 then pump can run when a pump starts you add 1 to N7:1

Dont forget to subtract 1 from N7:1 when a pump stops

Hope that makes sense.

Worked with gensets for years but have no suggestions on the application.

What I can do is offer a way to show your ladder in detail. You need to get a program like http://www.pdf995.com/ This program will allow you to PRINT to a pdf file that you can attach here so all can view.