ReplySet all of the parameters to the same as the one that is working ok. Check the sensor that's being used for the controlled process variable, to see if it's swinging up and down erratically.Originally Posted by Igotworms
Professional Member
I have a building with several air handlers with an NAE and UNTs controlling the equipment. Two compressors are short cycling in the AHUs. These two AHUs are identical to two others that are working fine. The same program and everything else with the exception of some parameters. Im just wondering if the PID in the UNT needs to be reset. That's the only thing I can think of to fix the problem. Any other thoughts or suggestions? TIA
Professional Member
Set all of the parameters to the same as the one that is working ok. Check the sensor that's being used for the controlled process variable, to see if it's swinging up and down erratically.
Regular Guest
Couple of things... 1st Look in the BO object in the UNT itself to see if you have the min on/off times and interstage delays set up this will help with cycling. Is your cooling command going up and down at a fast rate in Hpro? If so check the prop and intergration times. you don't mention what stragety you are using. Is it possible that your controlling sensor is in a poor location as opposed to the other ones? IE: discharge control and the sensor is right up against the cooling coil!
Professional Member
This cooling stage is being controlled by a one stage sequencer in Hvacpro. The cooling prop band and intrgral times seem ok, the cooling command ramps up pretty slow. The min on/off times are the same as the ones that are working properly but im probably going to change them. As far as zone sensor location and DA-T go ill have to check, that's really the only thing left to look at since I have only been looking at trends and programming.
Professional Member
also, this is programmed as a 100% OA single path AHU with room control cooling/room reset
Professional Member
You check the HvacPro program. You say you have two stages of cooling but only one output defined in a sequencer. Check the Q/A in the cooling section and see if only one cooling stage is defined. If you have two stages change it to two. If you are doing room control of the cooling you may need to confirm the room setpoint, propband, deadband to determine if they are not too tight. Also make sure you don't have air blowing on the room sensor.
Hope this helps.
Professional Member
The way I typed it is confusing. There are two AHUs with the problem. Both AHUs have only one stage of cooling. I think the zone sensor may need to be relocated, since the PID parameters seem to be ok
Professional Member
Every manufacturer writes PID controls, but they don't cross over well.
I find a UNT will want to see a I value of 600 to 1000. Or none at all for a single compressor.
D.
Regular Guest
Remember that the IT in ASCs are in seconds so you should have a big number for room control, like +/-600.
Professional Member*
Igotworms,
Upload the controller, set your HVAC-Pro to generate *.prn and *.ddl files, use long form print format and email all 4 file to me. I think I know what your problem is but it will take too much typing to explain but 2 minutes to find/fix.
BTW, Integration (IT) is in "Ticks" not seconds. 1 tick is aprox. 1.5 seconds. Found that out the hard way because of a very anal (EPITA) engineer during commissioning.
If sense were so common everyone would have it !You cannot protect the Stupid from themselves !"Experience is the ability to recognize a mistake Before you make it again!" (Stolen Quote)
Professional Member
If two units are working fine with virtually the same program, I can't help but think something other than controls. What is the delta T across the evaporator, airflow maybe?. I have seen many UNT'S setup this way resetting discharge and without the absolute correct setup they will short cycle like crazy and usually a compressor death!!.Remember that evaporator needs a good load on it which will provide longer run cycles, if you have say 65 degrees air on the evaporator and you start cooling cycle, and your DAT set point is say 55 degrees it will take no time to reach set point, IE short cycle. Now with minimum on time you can sometime stop that a bit, but that usually drives the DAT really low which in turn drives the suction pressure down also. I personally don't care for DAT reset with DX, if you read one of my other post I explain in detail why, that instance I was explaining was a unt with DX also, that system ate three compressor before I was asked to look at it. I found a short cycling mess with that one also!!. So good luck .
Regular Guest
But rather a refrigeration system problem ???
Is there Hot Gas Bypass ??
Are the systems charged correctly ??
Are the TXV and other refrigeration components set-up correctly ??
Professional Member
I dont think it is a mechanical problem because I can see the trends at the NAE. The controller is calling it on and off.
Norriski Tech - The cooling is not being controlled off of DA-T. That is only for the heat, the cooling is looking at the zoom sensor.
After troubleshooting, I think the zone sensor is faulty. Its located in a good spot but for some reason its reading fluctuates too much causing the PID to go up and down too fast. Thanks for all the input.
Professional Member
Bad cabling can cause this behavior too. Often we have changed the sensor, The problem seems to go away for a while then comes back. ( I think wiggling the wires may make the cable work for a while.) PIA but after the sensor is replaced we usually change the wire next. Controllers are too expensive to change when that usually does not cure a problem like this.
Bill
Professional Member*
Igotworms,
Check your wiring connections and...
You've got mail !
If sense were so common everyone would have it !You cannot protect the Stupid from themselves !"Experience is the ability to recognize a mistake Before you make it again!" (Stolen Quote)
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