Converting a 240v fridge to 12v?

[Sea Change]

Hypothetically, would it be practical/possible to open up the compressor unit on a domestic type fridge, and swap out the AC motor for a 12v DC one? Would any other parts need alerted?
Given the huge price difference between marine and domestic appliances I can't be the first person to have thought of this.

 

[lustyd]

You wouldn't open it up but you could very easily swap the compressor. The problem is that those compressors are not cheap so you'd have to really want the domestic fridge!

Secop (formerly Danfoss) are the people that make most of them DC Compressors for Battery-Driven Refrigeration | Secop

If you did do a swap, you'd need to be aware of the differences in gases as new compressors use a different gas than older ones which use a different gas to even older ones. You also need to be aware that you need (in most countries, coming soon to the US but currently EU and UK) an F-gas certificate to buy these gases now so you'll need a pro to do the install. It's also a legal requirement to capture the gas from the old system before removal.

 

[PCUK]

Why not simply use a small capacity inverter? No dismantling of fridge required!

 

[PaulRainbow]

Opening up the compressor or swapping it for a 12v one isn't practical or financially viable. The only viable solution is to fit an inverter, just big enough for the fridge to keep inefficiencies and losses to a minimum.

 

[Roberto]

From some old notes, danfoss BD35 etc compressor motors all run on 24v triphase current, which is provided by the electronic unit (in turn fed by 230AC, 12, 24v etc). A number of units provide rather 18-19-20v triphase currents with rough sinusoids, the compressor motor seems to have a wide range of acceptance I do not know if a simple control unit swap is feasible or not.

 

[Sea Change]

Thanks all. I was thinking that it might be possible to separate the motor from the actual compressor, surely it's just a direct drive on a spindle? And therefore a case of matching the rpm/torque of the motor.

It's just a musing at this stage

 

[rogerthebodger]

Thanks all. I was thinking that it might be possible to separate the motor from the actual compressor, surely it's just a direct drive on a spindle? And therefore a case of matching the rpm/torque of the motor.

It's just a musing at this stage

Most domestic fridge compressors are totally sealed so you cannot dismantle the compressor from the motor as they are both inside a sealed case

Have a look here

https://www.bing.com/images/search?q=domestic+fridge+compressor&form=HDRSC3&first=1

 

[AntarcticPilot]

If you look at the diagrams here (fridge compressor diagram - Google Suche) YOu'll see a) that the motor and compressor are highly integrated and b) that it's located within the pressure vessel.

 

[Sea Change]

A non starter then!
Inverter it is.
It's not actually my own boat, it's someone else who is rubbing this via a 1000w inverter already and we were discussing ways to improve efficiency.
More insulation is probably the best option tbh.

 

[rogerthebodger]

More insulation is probably the best option tbh.

That is the way I improved efficiency when I built my second boat by increasing the fridge insulation from 50 mm PU foam to 75 mm PU foam

In summer cut the duty cycle of the fridge from on all the time to less than 50 % duty cycle

 

[vas]

over the last few years I had most of my boating colleagues swapping 12/24V Danfoss BD35 powered fridges with domestic ones (fitting a suitable - or not!) inverter.
ALL are v.happy with the performance and energy consumption.

V.

 

[geem]

Why not simply use a small capacity inverter? No dismantling of fridge required!

That makes it very inefficient. The inverter needs to big enough to deal with the compressor starting load. Having the inverter on permanently can be a significant power draw.

 

[AntarcticPilot]

That makes it very inefficient. The inverter needs to big enough to deal with the compressor starting load. Having the inverter on permanently can be a significant power draw.

Could you arrange for the thermostat to operate a relay on the 12v supply so the inverter was powered down EXCEPT when required? It'd have to be a fairly meaty relay, because the current would be significant.

 

[rogerthebodger]

Could you arrange for the thermostat to operate a relay on the 12v supply so the inverter was powered down EXCEPT when required? It'd have to be a fairly meaty relay, because the current would be significant.

I have a 100 Amp relay on my inverter input operated by a toggle switch as my inverter has a constant 12 Vdc draw even when the inverter is not operating

 

[AntarcticPilot]

I have a 100 Amp relay on my inverter input operated by a toggle switch as my inverter has a constant 12 Vdc draw even when the inverter is not operating

I guessed it would be around that, but don't have the wattage of the fridge so couldn't be sure.

 

[rogerthebodger]

I guessed it would be around that, but don't have the wattage of the fridge so couldn't nr sure

I use my inverter for all kinds of mains driven appliances, so my relay is sized for the total DCV input

 

[William_H]

The 12v type Danfoss fridge units use effectively an inverter in the control box to produce 3 phase AC to the motor. I think domestic fridges use a capacitor to provide a phase shift to drive an induction motor in the compressor unit. It follows then that a 240v fridge on an inverter would not be less effiecient than the Danfoss. (except for the fact that the inverter could end up running full time and the fridge load only part time. (must be pure sine wave inverter)
However it seems to me that domestic 240v fridgesd are not very well insulated. I bought a new fridge (Lg) a while back. Expewcting to see condenser tubes on the back in relatively free air. Not so it seems the condenser tubes are embedded inside the side walls. (Which get warm) must be pretty good insulation or perhaps not. Plus of course if he has a 240v fridge no problem on shore power. ol'will

 

[PCUK]

That makes it very inefficient. The inverter needs to big enough to deal with the compressor starting load. Having the inverter on permanently can be a significant power draw.

Works for me and has done for years. I don't really care if it is a little inefficient as I've got a big alternator and solar panels. It is simply an easy and reliable solution.

 

[Alex_Blackwood]

Why not simply use a small capacity inverter? No dismantling of fridge required!

Quite agree but why not give some figures as to size of fridge and inverter. The OP will then have some ball park to kick off from.

 

[Sea Change]

As I said, the boat (which isn't mine) is currently set up using a 1kw inverter to run the fridge.