Ladder Programming

I am using RSView (HMI) and AB SLC5/05.

My scenario:
I have 2 HVAC systems, one is running and the other is used as a backup this is for our Server Room. Need to program this where only one unit can come on at a time. Also I have a wall sensor hooked up to the PLC to monitor temp. in this room. I would like to hook this up where if the temp. goes over 75 F the one unit shuts off and the other unit comes on. Any help on Ladder programming would be appreciated.
Thanks

You will get lots of help with any problem that you have if you show that you have put some effort into solving the problems first. From you post it looks like you know what you want to do, but it reads like you want someone else to do the program design.

How do you think it should work? Post your ideas, and ask for feedback.

need more information ...

Greetings 1stwave,



are the HVAC units already connected to the PLC? ... specifically, can the PLC turn each one ON or OFF individually? ... also, how is the room temperature sensor wired into the PLC? ... specifically, what type of analog input module are you using? ... and if it’s one of the configurable ones, please tell us how you have it scaled ...



the basic idea behind programming what you’ve mentioned so far is a couple of rungs with comparison instructions (something like LES or GRT, etc.) ... these could compare the room temperature with your desired 75 degree trip point and control the individual HVAC units ON or OFF ...



best idea: post your .RSS file ... once we have a look at that, then we can offer you more specific advice ...

1stwave,

Your scenario can be programmed, but for two units where one serves as a backup, the usual way is to ALTERNATE the units on a time schedule (say Number 1 runs 1st day, Number 2 runs 2nd day, Number 1 runs 3rd day, and so on), and if temperature goes above 75 degrees, BOTH units run until temperature drops back below 75.

I suppose that you have sized the units large enough that the temperature should never reach 75 and if it does, it means that the running unit has failed. This is okay except if the electric power goes off and your PLC is on a UPS, then it will switch units unnecessarily. Also, if the only time you switch to the second unit is when the temperature goes above 75, then Unit One will get a lot of wear and the Unit Two will hardly ever be run. This is usually not desirable.

Hopefully you dont mind.

I created a pdf of the ladder so those without RSLogix could look at it and assist. Lot of code there so will take time. One suggestion is to add more documentation (rung comments) to explain what is being done.

It was too large to upload so lets see how this works
http://www.patchn.com/Alarm.pdf

All I need help with is page 71 Rung 00 thru 09. Starting with HVAC Controls

Need to program this where only one unit can come on at a time. Also I have a wall sensor hooked up to the PLC to monitor temp. in this room. I would like to hook this up where if the temp. goes over 75 F the one unit shuts off and the other unit comes on

HVAC CONTROL, START OTHER UNIT WHEN > 75 DEGREES

1stWave,

Here is a revision, starting with your Page 71, Rung 4:

Thanks alot for your help. I'll try it and see how it works.

I can't start HVAC19 B3:7/4 or HVAC20 B3:7/6 Button to start unit.

Is Rung 4 Source B: suppose to be N51:2.

1stWave,

For the Start Buttons, I used your original memory locations. I assumed (wrongly?) that you had those set in your HMI program. If not, you will need to add some way to put the Air Conditioners in the Auto Run Mode from your HMI.

Rung 4, Source B should be your Thermostat Setpoint. I did not know if this is N51:2, or not. If it is, then you will need to use a different number in Rung 7, Source B. I suggest using a plain old "75" in Rung 7 for the temperature at which you switch units. Then in Rung 4 you can use N51:2 as an Input for the temperature setting that you want the air conditioners to maintain. It is important to have the Temperature Setpoint in the program, because this routine uses that to alternate the two air conditioners each On/Off cycle, and also to switch units when the temperature > 75.

I see that you are having trouble deciphering what I did and what I meant. That is because I did not have time to go back and add comments to each rung to explain exactly what it is supposed to do. I apologize for that, but I hope that you do not expect perfection for free. However do not make the same mistake in your final program. Document it well.

Sorry but still need help, program not working.

Had to add B3:7/1 Manual/Auto Switch to get it work on rung 9 & 10 or where ever you had the manual HVACM19(B3:7/0) or HVACM20(B3:7/2) switch it would run and not else work.

I only have a wall sensor monitoring temperature (N50:1)

I do not have a thermostat setpoint because in the room there is two independent thermostat's.

I only want to turn 1 HVAC system on at a time. But have the running HVAC unit turn off and switch to the other HVAC when over-temperature (B3:7/7 & B3:7/3) happens.

Right now I can turn both HVAC systems on.

Please help.

There is one concern which you should be aware of when controlling refrigeration compressors.

At the instant a compressor turns on a great deal of foam is created in the oil in the compressor, and this oil foam is blown out into the refrigerant lines with the refrigerant. This is normal and your refrigerant compressor is designed to accomodate for this. The oil is pushed through the lines with the refrigerant and retuened to the comperssor via the refrigerant suction line. But if you short cycle your compressor, that is turn it off and on several times over a short period then you will pump most of the compressor oil into the line without giving adequate time for oil to return down the suction line and seriously damage the compressor.

Compressor control logic should be designed to prevent a short cycle. When a compressor first turns on it should run for several minutes. Commercial anti-short cycle controllers usually operate the compressor for about 5-7 minutes minimum, provided the low pressure cut out or high pressure cut out are OK. You should check with the compressor manufacturer recommendation on short cycling the compressor. Anti-short cycle controllers also usually keeep a compressor off for several minutes before allowing it to come back on again. If the high pressure cut out trips the compressor should be shut off immediately and kept off until it can be serviced. The low pressure cut out response is different depending on the system. If the system is equipped with a liquid line solenoid and the liquid line solenoid has been closed then the compressor runs until the low pressure cut out activates. This is to be expected. If the system is not equipeed with a liquid line solenoid, or the liquid line solenoid is open, and the low pressure cut out switch activates, then the compressor should be kept off until it can be checked out. If you find it necessary to cycle the compressor off and on frequently to control capacity then you may need to install a hot gas bypass system rather than short cycle the compressors.

Unfortunately, the refrigeration industry has been slow to adapt to advances in control technology and innovations like variable speed compressors have not made their way into the field. Compressors with cylinder unloading valves are available but these compressors are quite a bit more expensive.

yes, thanks for the explaination. The service tech. told me the fan unit will start but the compressor won't start for 5 minutes. This is not a problem with cooling. We are using the 24v control on the system.

1stWave,

I am sorry, but I have misunderstood what you want. I still do not have a clear picture. Here is what I think you have:

Two room air conditioners, each with a temperature thermostat controlling its on/off cycle.

What you want to do is add a PLC routine with controlled output to start a unit if the other unit is running and the temperature goes above 75 degrees.

Do you realize that this addition will not add anything that the thermostats are not already providing? If you set both thermostats to the same temperature, say 72 degrees F, then if the temperatures goes above that, they will both come on and run until the temperature is back to 72 degrees. If you only want one to run, then set one to come on at 75 degrees. What can a PLC add to this? Please explain with more details. I must be missing something here.