Wiring up a Fridge

[Norman_E]

I posted this thread about the problems of my inverter driven fridge. http://www.ybw.com/forums/showthread.php?400459-Boat-Fridge-Expert-Needed
I have now concluded that the only option left is to run the fridge without the inverter, which contained the circuitry that controlled the fridge. What I need to know is how to wire up the fridge without the inverter. My own view is that this should work:

Using the existing mains supply and its switch connect the compressor via the normally closed over pressure switch and the contacts of a relay pulled in by a 12 volt coil which will be energised from a 12 volt source only when the fridge thermostat is demanding cooling. A second 12 volt circuit wired in parallel with the relay coil could drive the cooling water pump but I suspect that this would cause too much current to flow through the thermostat contacts, so a second all 12 volt relay would appear to be needed.

Can anyone tell me if I am right? To recap from my original post its a 230 volt compressor with an overpressure cut out switch which was wired to the inverter separately from the compressor, and an adjustable thermostat which I am pretty sure is only intended for 12 volt DC. It also has a 24 volt pump, running quietly on a 12 volt supply and providing a flow of cooling water.

Finally can anyone point me towards suitable relays, 12 volt coil and contacts capable of withstanding mains voltage or 12 volts with about 10 amp switching capability. There is a bewildering variety of relays on sale, and as yet I have failed to see anything that appears suitable. Plug in ones are fine, provided I can get the bases as well.

 

[RobWheatley]

http://www.maplin.co.uk/p/30a-spdt-power-relay-12dc-n23aw
240v 30amp contacts & 12v coil so more that enough rating.
Have you checked the rating of the pressure switch to make sure its 240v and high enough for the compressor?

Also if you use the thermostat on 12v to control the relay, add a second relay rated for 24v contacts to turn the water pump on and off with the compressor!

 

[Norman_E]

Thank you for that. That relay looks man enough for the job. When I go back to the boat I will check the pressure switch voltage, but I think it is mains because it is built into the unit consisting of a baseplate carrying the 230 volt compressor and water cooling coil as well as the filter. I suspect that if I fed the pressure switch from the 12 volt feed in series with the thermostat it would probably work just as well because both pressure switch and the thermostat demand for cooling switch would have to be closed for anything to run.
Although the pump is a 24 volt one it runs on 12 volts and it will be run off a second relay.

If I can get the fridge to work on this setup, I will then consider adding a suitable inverter to provide the mains voltage when not connected to shore power, and fitting a break before make changeover switch on the mains side to select the power source. The boat has a 110 amp alternator designed to cope with the old fridge inverter load when motoring.

 

[superheat6k]

If the pressure switch is directly wired to the compressor I would leave it this way. If it operates you really want the compressor to stop. The compressor will place quite an inductive load on the relay and it could cause contact pitting and could stick, if this happened nothing would stop the compressor if it over pressurised the system

 

[Norman_E]

Superheat6K, The pressure switch and the compressor were both wired into the inverter. Without an internal circuit diagram for the latter I cannot tell how they were connected. (I would think that they were in series if the switch was at mains voltage) All I know for certain however was that the compressor received mains voltage, but it is entirely possible that the pressure switch was on 12 volts. The only way that I can tell is that when I return to the boat in September I will examine the failed inverter more closely and try and figure out whether the switch received high or low voltage. My best guess is that it was on mains voltage because the flexible cable to it was the same mains type as the cable to the compressor, whereas the wires that I know carry 12 volts are all automotive type single wires.