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Old 03-14-2011, 09:02 PM   #1
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120V refrigerator with a power inverter?- Can be done?

We are sure that this is a subject that has been discussed before but we need some help from the RV Gurus.
Our 3 way refrigerator is dead and we were wondering if that can be replaced by a compact 120W bedroom size refrigerator.
These are the Facts:
The refrigerator is a Magic chef 120W 4.3 amps
We have 2 Deep Cycle Marine battery class 24 in the camper plus the 2 heavy duty batteries in the truck(diesel truck with a 110amps alternator).
Power inverter is Vector 750W continuos power and 1500W peak.
Since the camper is a Slide in Truck Camper it is connected to the truck so the alternator can provide constant charging to the batteries in the camper.
--------
We wonder if this can be done since we used that same power inverter to power the big refrigerator from the house and our fish tank air and filter needs more than once when the power went out with storms.
I called the RV dealer and a Tech Support member told me over the phone: that this small refrigerator will draw up to 50 amps when trying to cycle the compressor in and that we will the burn the alternator & batteries out not considering that the power inverter has to be at least 2000W running power.
If all this is true how in this world we did run the big refrigerator and the fish tank air pump and filter before?
Does the fact that the refrigerator was already cold and just need to keep the temp means that it doesn't need to draw that many amps?
Please help.
thanks to all of you.
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Old 03-14-2011, 09:44 PM   #2
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Is your inverter a Pure Sine Wave or Modified Sine Wave? Some refrigerators will not run or can be damaged by a Modified Sine Wave.
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Old 03-14-2011, 10:13 PM   #3
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12 volt swing compressor refrigerator - Google Search=

Check out some of these...good luck
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Old 03-14-2011, 10:54 PM   #4
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Some sketchy math will help:

Power (watts) is the product of current (i) (in amps) times voltage (e). I never forget this because I like "Pie" (P=IE).

So the wattage consumed by your running refer is 4.3 times 120, or 516 watts....if we use these values literally. But, of course, your inverter is creating the "120 volts" from a 12 volt source....which means that 43 amps are needed from the 12VDC source. If we assume that the surge current needed at refer start-up is 5 times that of 4.3 amps, then at the 120v level, we need 21.5 amps (ever so briefly) to get the motor/compressor started from a dead stop). To get 21.5 amps at 120 volts is to need 215 amps from the 12 volt source. (again: ever so briefly).

I don't see any red flags here. Just be aware and a continuous draw from batteries of 43 amps is quite a large draw. So running your motor will be necessary.
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Old 03-14-2011, 11:38 PM   #5
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A modern Energy Star rated refrigerator is a solution for a number of people in their RV. The cost is such that you can buy the fridge, a couple of extra batteries, and a solar system for about the cost of a typical RV fridge.

There are some good discussions about using a modern Energy Star refrigerator in an RV around. It has proven to be a practical solution for a number of folks from what I can tell.

My house fridge runs at a bit under 1 KwH/day and it wasn't selected for efficiency. That rate of energy use could easily be handled by a 4 RV battery bank and perhaps 300 watts solar.

The 500 watt calculation and the 50 times surge claim sound rather off to me. You need to check the actual data on your fridge and get the units right to know what is going on.
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Old 03-15-2011, 06:36 AM   #6
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Some residential fridges will work with a modified sine wave inverter - many will not.

You may also need to add batteries to be able to power the fridge for any length of time.
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Old 03-15-2011, 06:55 AM   #7
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Quote:
Originally Posted by Dadeaux View Post
Is your inverter a Pure Sine Wave or Modified Sine Wave? Some refrigerators will not run or can be damaged by a Modified Sine Wave.
Good morning Del & Charlotte.
Until last night We didn't know that there is such a thing as Modified or pure sine.
We have 2 power inverter on our trucks that we got from a Truck Stop to run laptops/internet/phone chargers and that kind of device.
We did notice the "pure sine signal" on some generators when we were looking around but since we are new on this I think that the ignorance factor is not a good allied.
According to our research this is a Modified Sine Wave.
Would you mind to describe the difference?
Thanks
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Old 03-15-2011, 07:16 AM   #8
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Quote:
Originally Posted by Robi.1014 View Post
Some sketchy math will help:

Power (watts) is the product of current (i) (in amps) times voltage (e). I never forget this because I like "Pie" (P=IE).

So the wattage consumed by your running refer is 4.3 times 120, or 516 watts....if we use these values literally. But, of course, your inverter is creating the "120 volts" from a 12 volt source....which means that 43 amps are needed from the 12VDC source. If we assume that the surge current needed at refer start-up is 5 times that of 4.3 amps, then at the 120v level, we need 21.5 amps (ever so briefly) to get the motor/compressor started from a dead stop). To get 21.5 amps at 120 volts is to need 215 amps from the 12 volt source. (again: ever so briefly).

I don't see any red flags here. Just be aware and a continuous draw from batteries of 43 amps is quite a large draw. So running your motor will be necessary.
====
Good morning Robi.
Thanks for the math calculations. We did the math estimates based on the same criteria as you did and according to that it seems to it should work.
now 2 questions:
1) Where the criteria of what some people say that the compressor needs 5 to 10 times more for the start-up comes from?
2) if we need 215 amps to start the compressor that is huge! I don't think that any regular alternator on a 1ton truck has that power capacity. I also got the info about the power inverter :
Features:
750 Watt Continuous Power / 1500 Watt Peak
Powers up to 6.8 Amps
Two GFCI 110/120 Volt AC receptacle
Maxx SST (Soft Start Technology) - gradually & safely ramps up output voltage
Automatic reset if inverter shuts down
Noise-free technology
Turbo cooling
Specifications:
Maximum Continuous Power: 750 Watts
Surge Capacity: 1500 Watts
No Load Current Draw: 0.4 Amps
Maximum Efficiency: Approx. 90%
Input Voltage Range: 10.5 - 15.5 VDC
Output Voltage Range: 110VAC 60HZ
Under Voltage Shutdown: 10.0 Volts
Thermal Shutdown: Auto Reset
Wave Form: Filtered Modified Sine Wave
Internal Fuse: 35 Amp
Dimensions: 10.7"L x 6.25 "W x 1.75 "H
====
So if it only power up to 6.8 amps I can't do it. But then is the question of: Why we were able to run the big RR with that power inverter?
Does it make a big difference that the RR was running already and that the Compact Refrigerator is completely dead?(it has not been running for the last couple of weeks) and the very first start-up takes a lot more than when it is cold already?
As we said before we are new on this and we need help.
Thanks for the help
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Old 03-15-2011, 07:25 AM   #9
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Quote:
Originally Posted by BryanL View Post
A modern Energy Star rated refrigerator is a solution for a number of people in their RV. The cost is such that you can buy the fridge, a couple of extra batteries, and a solar system for about the cost of a typical RV fridge.

There are some good discussions about using a modern Energy Star refrigerator in an RV around. It has proven to be a practical solution for a number of folks from what I can tell.

My house fridge runs at a bit under 1 KwH/day and it wasn't selected for efficiency. That rate of energy use could easily be handled by a 4 RV battery bank and perhaps 300 watts solar.

The 500 watt calculation and the 50 times surge claim sound rather off to me. You need to check the actual data on your fridge and get the units right to know what is going on.
===
Hello BryanL
Thanks for the input.
According to the Ref manufacture it is a energy star. So we hope that makes a difference. But we will get one of the energy consumption meters from home depot to check the actual consumption on watts and amps while running/starting etc. I guess that a good reading chart will help our math estimates.
Wow. This is getting a quite of a big project. I was under the impression that I just need to do the 4.3 amps x 115V and since we have a big enough power inverter in Watts we were just fine!
The whole idea is to be able to run the RR while driving. Since the prices of an RV Ref are so high and we have the RR sitting at home we said why not?
We are not planning to keep the RR running all night or for a couple of days without running the motor but solar sounds an exciting option!
Please let us know what do you think.
Thanks
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Old 03-15-2011, 07:27 AM   #10
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Quote:
Originally Posted by paz View Post
Some residential fridges will work with a modified sine wave inverter - many will not.

You may also need to add batteries to be able to power the fridge for any length of time.
Hello Paz.
Thanks for the input but how can we know if ours will or will not?
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Old 03-15-2011, 07:33 AM   #11
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Quote:
Originally Posted by Twomed View Post
12 volt swing compressor refrigerator - Google Search=

Check out some of these...good luck
==
Hello Twomed
Thanks for the link!
That is a good option but a little pricy for our budget.
Our idea it was to replace out 3 way dometic (dead) for the compatc RR that we have sitting at home.
We are not planning to run it all the time or leave it running overnight. We just want to be able to run it while driving (10 maybe 14 hours) and when we get to our destination will be either generator or a hook up connection.
Please let us know what do you think.
Thanks
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Old 03-15-2011, 08:36 AM   #12
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Back when I towed a trailer, the trailer did not have a fridge, it had an ice box.. So I bought ice till the campground where I was most likely to park started charging for electric hook up.. Then I bought a compact "office" type fridge from Sears.. This unit, per the tag, needed 100 watts to run.. Please not this.

I tried it on my 250 watt inverter (MSW) no joy

I tried it on a 450 watt MSW, Still no joy

The 1000 watt would run it.

The moral, you need a lot more watts of inverter than what the tag suggests (i'd guess a 750 would run it too)

Page 2: Others have spoken about differences between the "may not work" list for TSW and MSW inverters so I'll not re-post that,... Just reference it.. I recommend True Sine Inverter of at least 5 times the watts or volt-amps (Whichever is greater) rating of the fridge.

If you wish, I can explain the difference between V-A and Watts.
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Old 03-15-2011, 11:29 AM   #13
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Quote:
Originally Posted by wa8yxm View Post
Back when I towed a trailer, the trailer did not have a fridge, it had an ice box.. So I bought ice till the campground where I was most likely to park started charging for electric hook up.. Then I bought a compact "office" type fridge from Sears.. This unit, per the tag, needed 100 watts to run.. Please not this.

I tried it on my 250 watt inverter (MSW) no joy

I tried it on a 450 watt MSW, Still no joy

The 1000 watt would run it.

The moral, you need a lot more watts of inverter than what the tag suggests (i'd guess a 750 would run it too)

Page 2: Others have spoken about differences between the "may not work" list for TSW and MSW inverters so I'll not re-post that,... Just reference it.. I recommend True Sine Inverter of at least 5 times the watts or volt-amps (Whichever is greater) rating of the fridge.

If you wish, I can explain the difference between V-A and Watts.
Thanks for the reply.
Well the first thing that got our attention was as you said that there is a bigger need of watts than the 115 that the tag suggest.
Now the whole amps/v/w calculations that we have no idea about starting to play a big part.
a) if we have a 750 continuous wand 1500max peak the theory say: YES WE CAN RUN IT.
b) The max amps for the inverter is 6.8amps and other people are saying that it takes about 25amps to start-up the refrigerator.
So what are the options?
I would like to learn about the whole Watts-amps-Volts so if you don't mind to share with us we will love it.
the other thing is:
Some people say that since very first start of the compressor is the one that takes the most amps. Once is running and cold every time that starts the compressor to keep the temp it will not take much. Is it true?
If that is the case I can just Plug it in the pre-cooled a day before we leave and will that solve or help anything?
Thanks for the imput
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Old 03-15-2011, 11:46 AM   #14
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Quote:
Originally Posted by Quest View Post
Hello Paz.
Thanks for the input but how can we know if ours will or will not?
I don't know if there is a way to tell other than experience - unless you can get into the engineering specs of the control boards and motors.

Manufacturers of some diesel pushers offer a residential fridge as an option. These would be the refrigerator models I would stick with, and I would stick only with those specific models. Most people who add a residential fridge also add a new pure sine wave inverter and a pair of 6V batteries specifically for the fridge.
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