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Old 07-01-2014, 10:32 PM   #1
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Power Inverter For Residential Refrigerator

Can anyone explain to me the set up for a power inverter to run a residential refrigerator?

By way of background, I will want to run the refrigerator off the 12v through the inverter while traveling but be able to switch to standard 110v when I get to my destination. Would I have to unplug the refrigerator from the inverter and plug it into the standard 110v outlet or is there an inverter that will stop "inverting" once the system is connected to the 30amp service at the campground?

Relative newbie here, so use small words and lots of them. Pictures help too since I am visually minded.

Thanks.
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Old 07-01-2014, 11:09 PM   #2
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I will try.

First, you will need an inverter which has enough capacity for the refrigerator load. There are two parts to the load, the starting load and the running load. Then there are two kinds of inverters, pure sine wave and modified sine wave.

A refrigerator may run just fine on a modified sine wave inverter (MSW), but it probably will not be as efficient, and it is hard to tell if the compressor will be harmed by the somewhat strange waveform that they produce. MSW inverters are considerably less expensive than pure sine wave inverters (PSW), although the difference is decreasing. Today, in general, I recommend you use a PSW type of inverter, but you may find the price difference pushes you to the MSW type.

A small residential type refrigerator will probably have an actual running wattage of 150 to 200 watts but the starting wattage will be considerably higher, in the range of 800 to 1000 watts. (I am generalizing here but probably will not be too far off).

So, I would size the inverter to the starting wattage of the refrigerator, that is in the range of 1000 watts.

The inverter should be very close to the batteries you intend for the system, as the wire for a 1000 watt inverter will be large, and properly fused, as it will have to cary in the range of 100 amps at 12 volts for the starting current of the refrigerator compressor. Smaller wire can be used from the inverter to the refrigerator itself.

The running wattage of the refrigerator (assuming a 200 watt compressor draw at 120 volts) fed through the inverter at 12 volts will probably mean the 12 volt DC system will need to supply between 16 and 20 amps at 12 volts. That is more than most tow vehicle to trailer charging systems can deliver, so your batteries in the trailer will become discharged as you drive, although by just how much is difficult to say because the refrigerator does not run full time.

As to the switching from inverter to grid power, there would be several options, depending on how much money you want to spend on this proposed system. Simple would be switch the refrigerator plug from a grid outlet to an inverter outlet, complex would involve a transfer switch of some kind.

You may find that the somewhat expensive sine wave inverter, wiring, and switching make the residential refrigerator on an inverter a more costly and involved system than you imagine. And if you are not electrically well versed, and are paying someone else to do the job for you, could easily approach the cost of a propane/electric refrigerator. You may have to beef up your batteries in the trailer, as well as the converter/charger to recharge them when you get to your campsite.

This is not a cheap inverter plugged into a cigarette lighter job.

i have outlined my feelings and information on the issue at any rate. Hope I didn't use too many big words....lol.
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Old 07-01-2014, 11:27 PM   #3
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JB,

I do this type of installation on a regular basis. First, I would suggest looking at a marine fridge that uses a Danfoss compressor, as it runs equally well on either 120VAC or 12VDC,thus eliminating the need for an inverter completely. Isotherm, Tundra, Vitrifrigo are just a few makers of these units.

You will also need a rather robust battery bank to power this fridge, as your umbilical will not do the job and you will need sufficient DC reserves to run the unit while traveling. Solar is one distinct possibility.

If you are dead set on a residential unit, just be aware that they are NOT built for the rigors of the road, where a marine unit is.

Let me know if you have any specific questions.




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Old 07-01-2014, 11:33 PM   #4
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Questions:
What size is the refer?
How many amps does it draw?
If you are referring to one of the small (4 cubic foot) dorm or apartment refers. They typically will draw 1.5 to 2.0 amps at 120 volts.
This means you will need an inverter that can handle the startup current of the refer. Which could be in the 10 amp range. If the inverter has internal protection (most do) and it is not capable of handling the start up current. It will simply crowbar and shut down. And the refer will not run.
10 amps at 120 volts is 1200 watts. Since the inverter is not 100% efficient. You are looking at a minimum of 1,600 watts. Which means the inverter will require a 12 volt DC supply which is capable, at least during the start up, of handling the current. Without a significant voltage drop. This means a capacity close to 150 amps at 12 volts which is 1,800 watts.
If you are trying to avoid buying an RV refer at close to $1K.
An alternative may be to use a generator while traveling. A 2,000 watt generator will cost between $500 and $1,000 depending on the brand.
You will be approaching or exceeding the generator cost by the time you purchase an inverter and enough battery capacity to run the refer for an extended period of time.
You can plug the generator into the coach. With no modifications to the coach. Other than mounting the generator and figuring out how to wire it for safe road travel.
A 2,000 watt generator would be more than enough for your refer needs.
Batteries are heavy and will require replacement eventually. A generator, properly maintained, will last for years and is multi purpose.
Good luck with your plan.
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Old 07-02-2014, 04:37 AM   #5
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Just a note...my Allegro Bus had a residential refrigerator and six house batteries in addition to the two chassis batteries. This would allow overnight running with no generator or shore power. But after 8 to 10 hours, the 10,000 watt generator would kick in to recharge batteries.
My opinion would be a residential refrige is not a practical configuration for a relatively light weight trailer. My bus weighed in at 21 tons.


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Old 07-02-2014, 12:52 PM   #6
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Thanks to everyone for the information. It sounds about as involved as I figured it would be and probably more than I want to spend. I'll file that one away in the "maybe later" file.

I am going to stick with the residential refrigerator. I've got a family with 4 young kids and the only camping we do is in locations with full hookups. We will just have to purchase groceries at the destination if we go more than a few hours away for a long weekend or week long trip.
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