refrigerator control

[junhonguk]

Dear experts,

It's my first post.
Hope someone could help me to clear the cloud.

As far as I know, there are two control modules in refrigeration system.
One is the thermostat controller for cabinet temperature.
Another one is the controller for the compressor.

Does anyone can tell me how this two control modules are coupled or provide some relevant information? Is there any control logic behind?

Many thanks.

[Brian_UK]

The thermostat control monitors the actual temperature within the refrigerated space and switches the plant on and off to suit.

By compressor controller I assume that you are referring to a pressure switch which monitors the temperature of the refrigerant returning from the cooled space. The pressure switch will also switch the plant on and off.

[junhonguk]

Thanks, Brian_UK.
You mean that there are two separate input variables to determine the compressor on/off.

Or

the temperature of the refrigerated space changes the pressure or temperature of the refrigerant, and then make the compressor on/off.

[Brian_UK]

Sorry, I may have misread your question.

A compressor may have some form of capacity control built in which can 'unload' the compressor thereby reducing the refrigerating effect. It's a bit like running a V8 engine at full blast when you need maximum cooling and then removing 4 sparkplugs when you are near to the desired temperature.

[junhonguk]

hi Brain_UK
Could you identify the main input control variables for the operation of the refrigerator, if we think the refrigerator as a black box?


thanks.

[Brian_UK]

I think that you are now asking very basic questions that you should be able to research for yourself.

[junhonguk]

Brain_UK,Thank you for your answer, although I didn't get the answer what I expected.

To make my question clear, I explain again. Hope someone can understand.

I would like to know what exact the control logic for the operation of refrigerator is.

If I am right, there are two thermostat modules available. One (located inside cabinet) is responsible for regulating the temperature within the refrigerated place. And the other one (located inside the refrigeration system) is to control the compressor on/off depending on the suction pressure or temperature of the refrigerant.

I'm sure that these control modules are coupled together.

My question:
Assume the temperature within cabinet is increasing, does the thermostat module inside cabinet generate the control signal and directly switch off the compressor?

or

The increasing cabinet temperature affects the suction temperature/pressure of the refrigerant within the refrigeration system, which makes the refrigerant temperature increase. and then switch on the compressor.

which one is the correct one? if neither one is correct, then what is correct then?

Thanks for your attention.

[Brian_UK]

OK, with a basic domestic refrigerator there is normally one control - the thermostat which senses the temperature of the box or the evaporator coil.

The thermostat contacts are strong enough to switch the compressor on/off directly.

Some of the larger/modern/technical fridges use electronics instead of a mechanical thermostat but achieve the same basic control.

In commercial cold stores/freezer stores then a pressure switch may be used instead of a thermostat. But whatever the control system the controls will switch the compressor via motor contactors as the control switches are not strong enough to take the electrical current drawn during start up.

[cold beer]

Box, compressor, controls.
Needed temperature needs to be maintained within parametric +_ few degrees.
Thermostat seeks and controls T on rise closes, when satisfied open.
Over time in evaporator section accumulates ice and needs to get rid of it.
Then comes defrost cycle with many different ways to achieve that.
Timer usually come in cycle 4 times per 24 hr period or sensor senses the ice and trigger heaters for short period of time, melts the ice and at the same time cuts off the compressor.
Depend what kind of fridge you have 1,2, 3 compartments. Hi, middle, low T.
Some bigger systems above size of domestic refrigerators are controlled with pressure switch
and solenoid valve on liquid side of the system. Compressor runs until suction pressure reach set point and pressure switch opens respectively opens the solenoid valve electric coil and spring forces piston on the valve closes /chokes/ liquid refrigerant line.
When T rises in the box high enough pressure on the suction side rises and triggers pressure switch to close and cycle goes on again. Temperature ranges require different type of defrost,
Some time hot gas defrosts system i.e. achieving flow of hot gas refrigerant trough evaporator.
Defrost systems are most complex, some time they can give some trouble to diagnose properly the problems and have call backs.