How to design a heat pump PID/inverter control system ?

[arne22]

Hello !

It is quite a usual thing today that heat pumps are designed using PID regulators (?!) and inverters (Frequency converters.)

I would like to do such a loaboratory (but full scale) heat pump, using
such PID regulators and inverter control for control of compressor (?) and fan (?).

My question is: How do I do that, using control principles and solutions that is as "normal" and commenly used for the industry as possible.

The first idea was to just replace the old ON/OFF thermostat room controller with a PID regulator, and to pull the compressor via a inverter (frequency converter).

Thinking this alternative over, I believe this will give a rater unstable operation. Here is a link (http://www.elektrofag.info/varmepumper/control-1)to a descriptin of this alternative.

I then tried to think out how to make a bether and more stable control system, and "someone" also told me that it was common (??!) to use the compressor suction pressure as the refference (feedback) for the compressor control loop.

Here is a link (http://www.elektrofag.info/varmepumper/control-2) to a suggestion for this "slightly more advanced design"

I would be thanful for any kind of information, links and etc, that can bring up some more information or experiences that can be used for this project.

Best reg Arne - Stavanger, Norway.

[arne22]

May be I ask or form the question in a too difficult way.

I would be interested in any "real world" information that could give some hints or information about how a heat pump with an inverter normally will work.

My theory is that if this should work well you can not only apply more or less power to the compressor alone, you will also have to do something with the capacity to the evaporator and the condenser as well.

If you increase the condenser fan speed, you also increase the condenser capacity, the same should also happen with the evaporator, etc.

To ask this way:

On an inverter heat pump, with a variable speed compressor, is it only the speed of the compressor that will vary, or does the speed of the evaporator and condenser fan also go up or down, to increase or decrease the capacity of these components ? - Any observations ?

And one other question if anyone should happen to know:

On a heat pump with a variable speed compressor, is it the compressor suction pressure, or is it the room temperature, itself, that is the feedback signal to the compressor controller ?

Any feedback or observations related to these questions, would help a lot to do such a control system.

[baycuclaudyu]

Firs, you will control the speed of pump on evaporator and condenser fan/pump too .
Second issue ,will control water temperature becouse is the main .
Pressure traductors is for inform and for allarm .
You can set the software to decrease the speed if the refrigerant pressure is above the limit, to not shut down the system .
This is my vision, probably is not the best idea .

[Brian_UK]

You should read one of the manufacturers service/design manuals to discover their various methods of control.

With inverter AC they control the fans, the compressor and the refrigerant flow so it is not a simple solution.

[desA]

You seem to have quite a bit going on in your system model. The room dynamics are probably reasonably straightforward. The RHVAC circuit dynamics may prove to be a little more of a challenge, however.

Things like evaporator & condenser time constants, for reacting to system changes, may prove to be very, very interesting. The TXV, or TEV response characteristics will be reasonably straightforward - in control terms, at least.

No idea of the realistic compressor response & piping hold-up responses.

Sounds like a huge amount of fun. Enjoy.

[arne22]

Thanks for information to both of you.

I certainly agree that it would be a good idea to look into such a service manual for some existing heatpump.

Do you have a suggestion on brand and model or a link to a pdf file ?

If there is no such pdf files on internet, I could call the manufactor or vendor and ask for such a service/design manual, if I knew brand and model.

[arne22]

desA -> I did not see your post as I was posting at the same time. Thanks for information !

Yes, you are right, it is many intersting things in the project, and what we will do is to build up a "double control" so everything in the system can be operated and controlled manually and then later on also automatically. (There will be two parallel and independent control system, one automatic and one manual that can override all automatic functions.)

Thinking it over a heat pump is really not only one dynamic process but a quite complex system of dynamic processes. Looking into this I am actually not sure that a PID regulator (or some of these) will be the right thing to use for the control, may be it should be something else. We are using a PLC with digital and analog in/outputs for the project, so we will try to find out.

To have the option of looking into a service or design manual for en existing commersial heat pump with inverter variable speed would definitly be of good help. (If we could find such a document.)

[desA]

Hi Arne. Tell us more about yourself, your project & where you hope to end up.

[arne22]

About me -> I am a teacher in industial control system, also trying to do something in the heat pumparea (that I actually do not know to much about.)

At or schoo in Stavanger, Norway, we try to set up a heat pump model, that will be used for heating our classrom. It has two purposes, to heat the classrom and to serve a a model to explain the heat pump theory, including also the control theory.

Our text book actually only cover on/off controls, and so does our exsisting labodels also. The idea was to make this model to work with an inverter control.

I do not have any backgrond from heatpumps, but rather form industrial automation, so I am teaching the elecrical system and automation system only.

Where we hope to end up -> Hopefully we will end up with a hot classrom, and except for or in addition to that, also laboratury heat pump model, that we can use for our studies.

We have a project page for this certain project here (http://www.elektrofag.info/varmepumper/stor-varmepumpe).

About the idea of finding a manual for an existing ssystem -> I googled a littel bit and found this one for the brand Daikin Link (http://www.daikinac.com/commercial/documents/VRV%20Heat%20Pump%20Service%20Manual%20-%20Daikin%20SiUS39-601.pdf)

In paragraph 2.2 it says something interesting about speed control of the compressor.