adccommunitymod (AutomationDirect) asked a question.
Created Date: January 08,2015
Created By: Dean
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We have an air compressor unit with 3 scroll compressors in it. It has microprocessor control that sequences the 3, and should divide their run time equally. Of course depending on load, two or even all three may be running simultaneously. I maintain that the microprocessor handles this and that the run times are equalized over time, while my boss isn't convinced of this. He thinks that the Number 1 compressor runs more than the others. I think he's nuts. So I have decided to gather a little data. I have a basic CLICK laying around, (no RTC) that I wrote this little routine for. The motor starters do have NO Aux contacts, but they are being used by the unit, so I figured I would leave them alone, and use the NC contacts. This programs works, but I'm sure there is a more elegant way to do this with the hardware I have. What say you?
Created Date: January 08,2015
Created by: bcarlton
That looks fine to me. As the timers only accumulate to integers you need to accumulate their individual timings as you have done. If the compressors have relatively long run times the effect of loosing the partial minute of runtime will be negligible. If they cycle fairly often you may want a smaller duration timebase (possibly seconds) and store them.
Created Date: January 08,2015
Created by: bcarlton
As a side note let me pose a 'controls philosophy ' question.
If 'runtime ' is seen as a main contributor toward failure then an 'equalized runtime ' setup is trying to ensure that all 3 (in this case) fail at roughly the same time. Isn't that against what is desired?
Wouldn't it make more sense to run one as lead and the others as fallback until the lead failed (or reached a mandatory service hour point). The lead is taken out of service and repaired (or serviced) then placed back in as the last fallback.
Created Date: January 08,2015
Created by: Dean
Bernie,
Thanks for looking. Good point. I changed the time base to seconds, then in the math portion I convert it to hours, and changed the data type to DF to capture the the fraction. Each compressor runs about 3-4 minutes at a time, before the controller cycles to another.
Created Date: January 08,2015
Created by: Dean
Bernie,
This makes pretty good sense, so maybe my boss isn't as nuts as I think he is. Next week, I'm going to hook the PLC up to the unit, and monitor for a couple weeks. We shall see. Anecdotal observation indicates that the motors do sequence equally though, so my money is still on my position.
Created Date: January 23,2015
Created by: Dean
For anyone that is interested, I finally got a chance to hook up the PLC to the compressor. The first 24 hours show that each compressor runs approximately 12.5 hours out of 24. Our load is much lower on weekends, so I'll be curious to see how it does. Just anecdotally i know on weekends the unit will be completely idle for up to a couple minutes at a time. One interesting tidbit from this early data. We recently did a PM on one scroll. Part of this are new tip seals, the sealing surfaces between the orbiting and fixed side of the scrolls. This compressor in general has slightly shorter (it lags behind by about half an hour over 24 hours) cycle times than the other two that have older seals. I believe with the new seals less volume is lost, and pressure builds more quickly.
Created Date: January 23,2015
Created by: Cap
As a side note let me pose a 'controls philosophy ' question.
If 'runtime ' is seen as a main contributor toward failure then an 'equalized runtime ' setup is trying to ensure that all 3 (in this case) fail at roughly the same time. Isn't that against what is desired?
Wouldn't it make more sense to run one as lead and the others as fallback until the lead failed (or reached a mandatory service hour point). The lead is taken out of service and repaired (or serviced) then placed back in as the last fallback.
Good Point.. and this rates right up there with "Efficiency "..
There is no 'maximum Efficiency '.. there is only efficiency given the parameters you choose to base it on.. So the Discussion can proceed.
Back to your original point.. If you want to maximize the life of the machine, and run to failure, then replace.. Equal run time is desirable..
If your point is Utilize your Equipment to give maxium Protection of the Facility, Lead/Lag is a desirable option..
Point being.. there is no 'Best Way '.. it's just the way you chose to run your facility.. ( If you have an option.. )
Cap
Created Date: January 23,2015
Created by: Dean
The manufacturer has recommended PM frequencies, such as the tip seal replacement, that are I'm sure well short of the failure point. Of course we went over that limit because of the cost of parts. You would not believe what the manufacturer charges for a couple plastic spirals. Of course it only a fraction of what a new compressor costs. A point I finally impressed on the boss. In addition there are four grease points on the unit that call for a grease that costs $50 for 50 grams.
Created Date: January 08,2015
Created by: Dean
We have an air compressor unit with 3 scroll compressors in it. It has microprocessor control that sequences the 3, and should divide their run time equally. Of course depending on load, two or even all three may be running simultaneously. I maintain that the microprocessor handles this and that the run times are equalized over time, while my boss isn't convinced of this. He thinks that the Number 1 compressor runs more than the others. I think he's nuts. So I have decided to gather a little data. I have a basic CLICK laying around, (no RTC) that I wrote this little routine for. The motor starters do have NO Aux contacts, but they are being used by the unit, so I figured I would leave them alone, and use the NC contacts. This programs works, but I'm sure there is a more elegant way to do this with the hardware I have. What say you?