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Open Roads Forum  >  Tech Issues

 > Replacing Dometic refer with home type; what size inverter?

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VDOCAD

Arcadia, CA

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Posted: 06/28/09 09:37pm Link  |  Print  |  Notify Moderator

Like some of you, I got tired of dealing with my old Dometic refer and its bad cooling unit. I refuse to replace it with another propane refer so I read quite a bit about people who were successfull in using a small home type refer like the Magic Chefs at Lowes, HD... Mine is Avanti brand, and the label on the back says:

Power Input 90W
Current 1.45A

I own two small inverters, a 400/800W Coleman and a Harbor Freight unit of the same size. I tried the fridge with both. With the Coleman I get nothing and with the HF I hear a small buzz made by the starter I guess, but the compressor never kicks in. With AC obviously it works great.
So it seems I need an inverter with more "umph" or maybe a start capacitor? I would like to purchase the minimum size inverter that would run it if possible since $$ is at a premium.

Your input as always will be greatly appreciated.

damac

usa

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Posted: 06/28/09 11:06pm Link  |  Print  |  Notify Moderator

click here for one that works.

this is a popular unit that has been talked about on this forum for over a year, and works great thus far from what we can see.

Basically for us it was either the inverter and a household fridge/freezer or repair the cooling unit on our older smaller dometic 3 way.

It was a no brainer for us.

That specific inverter will run the unit, which has similar stats as yours, and there is actually allot of headroom for using other devices. Just make sure you watch what you plug in though, I would not try a microwave

MrWizard

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Posted: 06/29/09 03:17pm Link  |  Print  |  Notify Moderator

HOW did you wire the inverter, from a cig plug? or to the 12volt wires behind the fridge, or directly to the battery with a log 120v cord from the inverter to the fridge.

the last way is the best way.

the inverter must be hardwired and must be close to the batteries

* This post was edited 06/30/09 08:01pm by MrWizard *


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VDOCAD

Arcadia, CA

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Posted: 06/29/09 01:35pm Link  |  Print  |  Notify Moderator

damac wrote:

click here for one that works.

this is a popular unit that has been talked about on this forum for over a year, and works great thus far from what we can see.

Basically for us it was either the inverter and a household fridge/freezer or repair the cooling unit on our older smaller dometic 3 way.

It was a no brainer for us.

That specific inverter will run the unit, which has similar stats as yours, and there is actually allot of headroom for using other devices. Just make sure you watch what you plug in though, I would not try a microwave


I am sure this particular one will work just fine and have headroom for more stuff. I just went and got a 700/1400W from HF and will test it later on.

Thanks for the info.

Gdetrailer

PA

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Posted: 06/29/09 03:58pm Link  |  Print  |  Notify Moderator

VDOCAD writes "Mine is Avanti brand, and the label on the back says:

Power Input 90W
Current 1.45A...

...I am sure this particular one will work just fine and have headroom for more stuff. I just went and got a 700/1400W from HF and will test it later on"


The startup surge current will be about TEN times the run current listed on the name plate. So in your case that would be 14.5A @ 120V AC or 1740W, the startup surge is more than your inverter will be able to provide.

damac is correct in the fact you need that extra head room, without it the inverter output will sag to the point that the compressor motor will fail to run and then it will hum until it or the inverter overheats.

Additionally you do need to understand that not all MSW inverters will be able to cope with large inductive loads. To get over this you need to consider an inverter that has a CONTINUOUS rating that is nearly the same as the surge current of the inductive load. This will allow the inverter more than enough capacity without voltage sag.

I in fact have actively promoted the inverter that damac linked, it is a very robust inverter that has no problem starting a inductive load.

I myself am running a 10 cu ft apartment size fridge in my TT from that inverter, two years now without any problems. The inverter and fridge conversion have already paided for its self.

Also should be noted, with working with such large inductive loads which have a large surge current, your 12V wiring MUST account for the additional current drawn. Good rule of thumb since the inverter must boost the voltage by TEN times, the current on the 12V side will be at least TEN times more than the 120V AC load.

So if you have 120V startup surge of 14.5A then your 12V current surge will be at least 145A! Knowing this you must plan to use some very heavy ga wire on the 12V side, in this case 2 Ga might work if kept under 10 ft total (5ft for Pos and 5 ft for Neg), I used 1/0 and the run is less than 8 ft total.

So there is much more to this than just slapping any old inverter into use.

I would highly recommend that you take a look at MY RV HINTS AND TIPS

This document has all the details about my fridge/inverter selection along with my notes on the entire conversion. Has lots of other RV related tidbits also, all free of charge and for your use! No strings attached... honest.

67Airstreamer

Louisiana

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Posted: 06/29/09 05:07am Link  |  Print  |  Notify Moderator

Your inverter may be a bit undersized.

I used a modified sine wave inverter with a capacity of 600w (1200w peak) to power a 3.7 cf refrigerator with a 1.01 amp draw. It worked very well for 10 years, and was still working when I sold the trailer.

Also, the battery must be in good shape to adequately power these units. I only used mine on battery when towing between campgrounds, so the battery was always well-charged while on the road. If I boondocked, I would have had to have more than one trailer battery to keep from running the generator too much.

VDOCAD

Arcadia, CA

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Posted: 06/30/09 02:48pm Link  |  Print  |  Notify Moderator

MrWizard wrote:

or directly to the battery with a log 120v cord from the inverter to the fridge.

the last way is the best way.

the inverter must be hardwired and must be close to the batteries


It is wired with 6 gage wires, less than 12" long to the battery.

VDOCAD

Arcadia, CA

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Posted: 06/30/09 03:02pm Link  |  Print  |  Notify Moderator

Gdetrailer wrote:

VDOCAD writes "Mine is Avanti brand, and the label on the back says:

Power Input 90W
Current 1.45A...

...I am sure this particular one will work just fine and have headroom for more stuff. I just went and got a 700/1400W from HF and will test it later on"


The startup surge current will be about TEN times the run current listed on the name plate. So in your case that would be 14.5A @ 120V AC or 1740W, the startup surge is more than your inverter will be able to provide.

damac is correct in the fact you need that extra head room, without it the inverter output will sag to the point that the compressor motor will fail to run and then it will hum until it or the inverter overheats.

Additionally you do need to understand that not all MSW inverters will be able to cope with large inductive loads. To get over this you need to consider an inverter that has a CONTINUOUS rating that is nearly the same as the surge current of the inductive load. This will allow the inverter more than enough capacity without voltage sag.

I in fact have actively promoted the inverter that damac linked, it is a very robust inverter that has no problem starting a inductive load.

I myself am running a 10 cu ft apartment size fridge in my TT from that inverter, two years now without any problems. The inverter and fridge conversion have already paided for its self.

Also should be noted, with working with such large inductive loads which have a large surge current, your 12V wiring MUST account for the additional current drawn. Good rule of thumb since the inverter must boost the voltage by TEN times, the current on the 12V side will be at least TEN times more than the 120V AC load.

So if you have 120V startup surge of 14.5A then your 12V current surge will be at least 145A! Knowing this you must plan to use some very heavy ga wire on the 12V side, in this case 2 Ga might work if kept under 10 ft total (5ft for Pos and 5 ft for Neg), I used 1/0 and the run is less than 8 ft total.

So there is much more to this than just slapping any old inverter into use.

I would highly recommend that you take a look at MY RV HINTS AND TIPS

This document has all the details about my fridge/inverter selection along with my notes on the entire conversion. Has lots of other RV related tidbits also, all free of charge and for your use! No strings attached... honest.


My new 700/1400W inverter ran the fridge just fine for a couple of hours. So from what you are saying, either the fridge or the inverter or both will die soon because it is borderline? Since this inverter's only task will be to power the fridge, I do not need extra headroom on it(I have two smaller inverters for the TV and laptop, etc... attached to other batteries).

It was actually your document MY RV HINTS AND TIPS that got me on this track. Since the fridge was free, I am willing to take the risk of using it this way and see what happens...

VDOCAD

Arcadia, CA

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Posted: 07/01/09 05:49pm Link  |  Print  |  Notify Moderator

I did the suggested temperature comparison running the fridge from shore power for the same amount of hours(only 3 hrs). The compressor temp was about the same, hot to the touch, but this could be because it just ran non stop for at least 2 hrs to get the fridge cold... I guess a better test would 24 hrs. I may try that.

Gdetrailer

PA

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Posted: 06/30/09 04:10pm Link  |  Print  |  Notify Moderator

VDOCAD writes "My new 700/1400W inverter ran the fridge just fine for a couple of hours. So from what you are saying, either the fridge or the inverter or both will die soon because it is borderline? Since this inverter's only task will be to power the fridge, I do not need extra headroom on it"

I am glad it is working for you. What I am saying is that often times the low cost inverters are way over rated and may not be able to support starting heavy inductive loads.

The "extra" head room is a fudge factor just in case my calculations (and actual observations) miss the mark.

Think of it as if you are trying to run it from a small generator, too small of a generator will either stall it or blow the breaker of said gen.

A gen that is sized slightly larger than the load will start the load but you will hear it struggle and strain against the load.

A gen sized twice the size of the load will not only start the load but just barely even struggle, most likely not even notice the load.

In which of the above illustrations above have the longest and most reliable life of both gen and load?

Just change the gen to an inverter in the illustrations above and you will have your answer.

Number one enemy of stuff as preached on every thread in this forum is LOW voltage kills stuff. While I don't personally say that is fully true, I can say that having the inverter voltage sag at startup due to an inverter being sized too close to the load may over time cause one or the other (both?) to break down.

MSW is known to cause some heating of inductive loads (motors, transformers) above what a PSW inverter will do. Add in a motor that is under load and under voltage and you have a good mix to over heat the motor.

For these reasons I chose to intentionally oversize the inverter. I actually do not have anything else connected to my inverter, it is only for the fridge.

In your case perhaps monitor the temp of the compressor for a while, could even do a baseline temp using shore power. Compare the two, if the motor temps are considerably higher on the inverter then you should reconsider your choice of inverter size.

Free or not, damaging the fridge will result in your need to replace it down the road. I would hate to see you have that happen while out on the road.

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