fridge question

Submitted: Thursday, Oct 30, 2003 at 22:29
ThreadID: 8208 Views:1826 Replies:4 FollowUps:4
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i was chatting to a friend today , that puirchased a inverter and runs a home type half bar fridge in his car , it's around 5o litres and has a small freezer compartment . I had a look at these and noticed that they are real cheap if you hunt around , I can get one for $200 to $300 , inverter will cost around $500 for a 1500 watt . Sounds like a cheap alternative to a $1600 12 vold car fridge , has anyone else tried this set up ?? any sucsess ??Venus Bay
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Reply By: joc45 - Friday, Oct 31, 2003 at 00:42

Friday, Oct 31, 2003 at 00:42
Eric,
On the surface, they sound a bargain. But...
The efficiency of the bar fridges is way below that of purpose-built 12v compressor fridges. Check the spec plate on the back of the fridge for power consumption, and you'll see what I mean - you'll find that it translates to about 10-15A from your 12v system when using an inverter.
The insulation is also thinner - the bar fridges are the worst for efficiency of the domestic fridges (compare the efficiency sticker on the front with that of bigger fridges).
The start-up current for 240v compressors is also higher than that for 12v compressor fridges, meaning you need a bigger inverter to turn it over.
And the bar fridges cannot take the vibration that portable fridges are designed to take.
Having said that, one or more Aust co's do use a 240v compressor with an inverter in a portable fridge, but I understand that they also use heaps of power.
If you really want an upright fridge,
- Engel make an upright (bar) fridge, but it uses the Engel swing motor; ie is reasonably efficient.
- 12v Shop (Perth) also carries a range of upright fridges using the Danfoss 12v compressor.
Gerry
AnswerID: 35768

Follow Up By: Member - Timothy - Friday, Oct 31, 2003 at 08:02

Friday, Oct 31, 2003 at 08:02
Also all the cold air
and more importantly beer
falls out when you open the door
of an upright
Defender Extreme
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FollowupID: 25832

Reply By: Member - Oskar(Bris) - Friday, Oct 31, 2003 at 10:18

Friday, Oct 31, 2003 at 10:18
Eric
I've thought about that as well, but the figures don't really stack up.
An inverter is usually fairly inefficient (they can get quite warm as they are losing power to heat) thus the power draw can be quite high so battery charge life is limited.
You can get a 12 volt fridge (Waeco, Evakool etc) about 50 ltr for $900 - $1000 (check the archives) which is not that much more than $800 for the other system.
As already stated uprights lose their cold air as soon as you open them up and then you have to go through another cooling cycle and use more battery.
If you are intending 4WD touring I doubt that a cheap upright domestic unit will last very long.
For camping only, the Chescold 3-way type is very good and cheaper.
I understand where you are coming from in this and I agree that there are heaps of other alternatives to achieve similar results in many areas of off-roading and camping etc without spending heaps of "hard earned".
I would, however, have serious doubts about the economy of the fridge idea.
Having said that, you can always "suck it and see", it might be the go for you.
It will be interesting to see responses from others who have had success with that system.
Cheers
OskarThe real oskar
AnswerID: 35783

Reply By: Terry - Friday, Oct 31, 2003 at 10:22

Friday, Oct 31, 2003 at 10:22
Eric

I would be surprised if you could buy a decent inverter for those prices. They would be square wave or modified wave inverters and produce dirty power which can cause all kinds of problems to your electrical equipment.

There is a reason why car fridges cost as much as they do, and thats because most of them are designed for an enviroment where they will be knocked about.

The other point to make is that all fridge compressors aren't true DC units. Induction motors need an AC current to make them rotate, so regardless of what people are told there are no DC compressors out there yet.

We could talk about you wanting to use a domestic fridge for your application,but I'm sure your not driving a sedan or similar when you go off road so your logic is almost right.
AnswerID: 35785

Follow Up By: joc45 - Friday, Oct 31, 2003 at 20:26

Friday, Oct 31, 2003 at 20:26
Terry,
The Danfoss does not use any inverter - the DC is fed directly to the motor. As far as I can glean from the design info, the Danfoss 12/24v compressor does use a DC motor of sorts; ie, it is electronically commutated, that is, there are no brushes, just electronics which take the part of the old commutator and it uses permanent magnets instead of the old field winding. As such, it has high efficiency and excellent starting torque, and can re-start when the fridge gas is up to pressure. You can't do this on an AC compressor, which has a shaded pole motor, and has much poorer starting torque, hence the need for a hefty inverter. The efficiency of the AC shaded pole motor is lower than that of a modern DC motor.
The Engels are another design story, particularly in simplicity. These use an oscillating piston (no rotation) driven by a coil inside a permanent magnet (much like a loud speaker). They are driven by a square wave inverter. The inverter and compressor are both quite efficient.
Gerry
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FollowupID: 25879

Follow Up By: Terry - Friday, Nov 07, 2003 at 21:14

Friday, Nov 07, 2003 at 21:14
Joc

I dont think your right about the Danfoss DC motor, I cut a BD35f open to see what was in side and its almost identical to the AC unit. I was interested in how they were able to get the unit to function. The little black box on the side is where the smarts are and its not DC when it hits the motor, its not true AC but the wave is similar. The units are very good but there COP leave a bit to be desired when compared to other options. The rotary compressor in the Reefer has a COP (ASHRAE) (-23.33C, 3500rpm) of 1.19, the Danfoss at the same speed is 1.09 The Engel has an efficent compressor, but struggles in high temps and low humidity and I've seen them in a heat chamber, they plateau out at 3C at 45C ambient, I have no idea yet what there COP levels are but I suspect they're not as good as the Danfoss.
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FollowupID: 26389

Follow Up By: joc45 - Wednesday, Nov 12, 2003 at 16:09

Wednesday, Nov 12, 2003 at 16:09
Hi Terry, I guess I stand corrected; I'm going by my recollections from reading a service manual some years ago.
Would be interested in what you found out;
gerron@bigpond.net.au
rgds
Gerry
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FollowupID: 26762

Reply By: Member - Eskimo - Friday, Oct 31, 2003 at 19:38

Friday, Oct 31, 2003 at 19:38
Eric,
A standard 1500 watt inverter is not adequate for a 240 compressor or any thing else that has a top or bottom dead centre! Ask the experts like Jaycar before buying!!!

Wow! am I cute
If yer ain't fishing, Yer ain't livin
Richard
AnswerID: 35837

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