The purpose of the modifications, is to enable PLC automated control based on air demand to start and stop the compressors as needed, while enabling the compressors to also be switched over into a manual mode, to allow independant operation if necessary.
The initial plc I chose, was a Crouzet model CD20, because of it's economical cost, and I/O compatibility, although any plc would work fine, with enough inputs, outputs, and memory. I have since, upgraded to an XD26, and utilized the CD20 elsewhere.
I have included three of the most useful ladder programs for tandem operation. The main distinguishing feature of each, is whether it is based on two, three, or four demand ranges. The determining values for demand (load time, and idle time), can be modified to suit any particular load and supply system.
The three range program was initially intended to facilitate two compressors, with one being a bit less capable of heavier loads, although the program has been modified to switch demands between three compressors with varying load capabilities. I initially designed it for two compressor operation, with one compressor dedicated for light loads, another for medium loads, and both for heavy loads. Now I toggle between two close to being comparable screw compressors, unless both are needed, based on known typical demands during specific time periods, and downshift to a Cambell Hausfeld upright tank reciprocating compressor during low demand periods. Ample time is incorporated into run times, to avoid frequent start/stop activity.
The two range program is ideal for just two compressors, that are about equal in capability. Only one compressor runs during a light load, and the other is toggled to run the next light load, after any heavy demand requiring both to run.
The four range program, is built upon the three range, and modified, and improved, to suit three basic compressors, similarly as the three range, although the low range is intended only for the reciprocating compressor, the second range for the weaker of the two screws, the third for the stronger, and the fourth range, for both screws, under the heaviest loads.
The compressors share a common pressure switch (main), that is plumbed into the main air line that they feed. That switch gets the compressors running, and ensures both screw compressors should run if the line pressure drops low enough. In addition to this, the settings of the pressure switches that cycle each individual compressor, should be matched as closely as possible to each other (similar make and break pressures), or functional problems could occur. The break setting (high) of the main pressure switch, should be about 5 to 10 psi less than the make setting (low) of the individual compressor pressure cycle switches. The main pressure switch is not necessary, although I have incorporated it as sort of a fail safe, and it is convenient for certain aspects of the program to rely upon.
Perhaps either program could be simplified a bit, and still be as functional, although each one suffices well for it's intended purpose. Some additional programming has been added, to switch some ambient air cooling fans.
As I make improvements and modifications, sometimes I have to rearrange the rung order a bit, to fit everything into limited rung space, until such a time as when I may extend into other modules, to gain more memory, and or, capabilities.
I had to modify the reciprocating compressor (which is referred to, as "blue", which describes it's painted color), to provide some inputs for motor overheat, and power faults.
I will include the additional wiring documentation, and updates to what is presented, as I take the time, to do so.
I have also incorporated utilizing a Keystone USB server (model US-4A) http://www.google.com/search?hl=en&q=keyspan+usb+server&btnG=Google+Search (not to be confused with a model PS-4A, which I do not think is compatible with the plc), to be able to monitor, upload, and download programs, using either a Crouzet USB programming cable, or a Crouzet serial programming cable, with a USB to serial adapter ( http://www.keyspan.com/products/usa19hs/ ), remotely, over the ethernet. I can even monitor, or control the plc from home, by connecting to my office pc, using free LogMeIn software ( https://secure.logmein.com/home.asp?lang=en ). Sure, I could setup my own vpn, but LogMeIn is so much easier, and reliable, than dealing with the cumbersome Microsoft networking utility. Bill Gates, has lately become too preoccupied with attempting to resolve problems with the world at large, rather than focusing his attention on trying to eliminate the flaws in the empire he has already established.
I have also, added a Ranco temperature controller ( http://www.rancoetc.com/-c-36.html?osCsid=963dbbd0b7d718dfa14eea534c5d8f97 ), which outputs a 0-10VDC temperature reference signal (note that not all of them do), which I can monitor remotely, using a Dataq DI-194RS data acquisition kit ( http://www.dataq.com/194.htm ), and another USB to serial adapter, plugged into the USB server. Furthermore, I am using the same temperature reference signal, through a plc relay output contact, to input to the acquisition device, to display, record, and or save, the resulting pulse train of the compressor on-off load times. Furthermore, 120 volt outputs from the other plc relay contacts, can be input into cheap cube style plug-in power supplies, to provide any less than 10 volt DC outputs, to input as additional data acquisition channel information. The acquisition device, has software, that makes oscilliscope mode waveforms, which can also be imported into Excel, (or free open office software: http://www.openoffice.org/ ) spreadsheets. In other words, it works as a data logger, and I can save, and print out chronological temperature, and load cycle graphs, and data.
Way, too cool!
The plc programs available on this page, require Crouzet programming software to be able to utilize and edit.
Adobe Reader is necessary to view the following PDF files, and can be downloaded from this website:
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| Quincy Wiring PDF | Sullair Wiring PDF | Recip Wiring PDF | Compressor IO List PDF |
Click here to download the two-range plc program
Click here to download the three-range plc program
Click here to download the four-range plc program