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Author Topic: Inverter to run a Refrigerator  (Read 9447 times)
Fred Mc
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« on: July 07, 2007, 10:33:18 PM »

What would be the minimum size inverter to run one of the enegry efficient 110 refrigerators?

Thanks

Fred. Mc
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boogiethecat
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« Reply #1 on: July 07, 2007, 11:16:49 PM »

I have a VERY energy efficient fridge, a Summit that only draws 115 watts when running.  The problem is starting up... a trace 500 watt/1000 watt surge pure sine inverter wouldn't touch it.  I ended up with a 1000W pure sine that has 2000 watt surge capability and although it squeals a bit upon startup, it works fine.  Got it from Ebay for $265 (this is the same as the one I got, item 120139664807 ) it's a chinese special but it works very well.  Those little motors draw some hefty power when starting up!!!

I dedicated the inverter to my fridge... took the on/off switch out of the inverter and brought the wires out...and disconnected the temperature switch in the fridge and wired it to the inverter so the temperature switch now turns the inverter on and off, then hooked the compressor motor directly to the inverter output.  So basically I made it into a 12 volt fridge at a LOT less cost than factory 12 volt compresosr units (and bigger storage space too!) and when the fridge isn't running, the inverter is off so it's not drawing anything either.  It works great.

So the way it all works is, on shore my 65 amp intelli-power 12 volt converter keeps the house battery bank happy, and when the fridge kicks in, the converter power flows thru the battery connections to the inverter... when on the road the alternator does the same thing, and when in boonies my genset powers the intellipower...etc... or when it's all off and I'm sitting quietly in the desert, the batteries will power the fridge for 2 days or so.

The system is foolproof, no switches or relays, seamless and I'm VERY happy with it.  I had a 3-way propane fridge for years and since I changed over to this system, it's been super wonderful, not a care in the world about it anymore.  No more propane lighting, bus levelling, power switching, etc.
« Last Edit: July 07, 2007, 11:19:33 PM by boogiethecat » Logged

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jjrbus
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« Reply #2 on: July 08, 2007, 06:16:33 AM »

 You will need an inverter that has a surge capacity greater than the start up or Locked rotor amps of the unit you buy. The locked rotor amps are not something that consumers are interested in, so it is not easy to find. It should be in the owners manual. Try the manufacturers website. Ask the salesperson and watch the blank look.
 To figure the size you need the formular is amps X volts = watts.  1 amp X 110 volt = 110 watts
 There does not seem to be a rating agency for imported inverters so there watt rating is arbitrary. Also electrical items lose capacity as they warm up. Buy one that is at least 20% larger. I would buy one locally and talk with the manager, maybe even get it in writing that if it does not start the fridge you can return it for a diferent one.
« Last Edit: July 08, 2007, 06:25:12 AM by jjrbus » Logged

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« Reply #3 on: July 08, 2007, 12:04:39 PM »

I bought a Stabylex 1000 watt continuous inverter (24 volt) on eBay a year and a half ago.  I used it on our last trip with two refrigerators, fans, and a couple of lights.  I use my OTR air and didn't want to run the generator just for the 'fridges.  I felt the inverter from time to time and it was barely warm.  The 'fridges are fairly small - ones about 4 cu. ft. (undercounter) and the other is 2.7 cu. ft. behind the driver's seat - it's the refreshment (drink) fridge.  I wondered if the compressors would be happy with the inverters, but they were barely warm when I felt them.  I've tried the inverter with a TV and didn't notice any bad reception or anything from it.  I've not heard of the Stabylex brand before, but it seems to be very well made, is manufactured in the US, and was very inexpensive.  I should probably run it with no load to see how much DC power is consumed when the compressors are off.  In my popup camper, I've found that a single dorm-size 'fridge needs at least a 600 watt inverter to start the compressor.   If you look on eBay, there are a variety of 24 volt inverters (assuming you have a 24 volt bus system) available inexpensively.  I'd get one that's at least 1000 watts continuous.

David
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Sean
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« Reply #4 on: July 08, 2007, 05:28:34 PM »

I like Gary's solution (as I so often do).  But I'd also like to add two comments:

(1) If your goal is to run the fridge from battery power, I suggest you get a pure sine wave inverter (which is a good idea for a fridge, or other motors, anyway).  That's because there will be an efficiency loss with a modified sine wave.  Generally, the closer the steps in the waveform, the less efficiency loss you will see.  These losses, by the way, go directly into heating both the compressor motor and the inverter itself.  So for a given fridge run-time, you will need bigger batteries for MSW inverters than for SW inverters.  Also, you may shorten the life of the compressor.

And:

(2) Even if you get a high-efficiency fridge, I suggest you disconnect the frost-free mechanism, if it has one.  Some folks install a switch so that they can turn the frost-free feature back on when on shore power (or you can include a relay to do the same automatically).  Or just buy a manual-defrost fridge (harder and harder to find, unfortunately).  The auto-defrost, if left connected, will take a toll on your battery run-time.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com
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Nick Badame Refrig/ACC
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« Reply #5 on: July 08, 2007, 07:35:15 PM »

I like Gary's solution (as I so often do).  But I'd also like to add two comments:

(1) If your goal is to run the fridge from battery power, I suggest you get a pure sine wave inverter (which is a good idea for a fridge, or other motors, anyway).  That's because there will be an efficiency loss with a modified sine wave.  Generally, the closer the steps in the waveform, the less efficiency loss you will see.  These losses, by the way, go directly into heating both the compressor motor and the inverter itself.  So for a given fridge run-time, you will need bigger batteries for MSW inverters than for SW inverters.  Also, you may shorten the life of the compressor.

And:

(2) Even if you get a high-efficiency fridge, I suggest you disconnect the frost-free mechanism, if it has one.  Some folks install a switch so that they can turn the frost-free feature back on when on shore power (or you can include a relay to do the same automatically).  Or just buy a manual-defrost fridge (harder and harder to find, unfortunately).  The auto-defrost, if left connected, will take a toll on your battery run-time.

FWIW.

-Sean
http://http://OurOdyssey.BlogSpot.com



Just to add to Sean's post,
Also disable the door frame heaters that are around the freezer compartment. Theese heaters prevent the freezer gaskets
from freezing to the frame in high humidity conditions.
Good luck
Nick-
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« Reply #6 on: July 08, 2007, 09:04:22 PM »

FWIW
What Sean is saying is shown in this link:

http://nooutage.com/inverter1.htm#Output%20Voltage
Look for paragraph…”The image below shows a typical modified sine wave as compared with a true sine wave.”

In “Modified Sine Wave” image….shows both blue “ac” true sine wave and red modified sine wave. You will notice true sine wave “zero time” is zilch as compare to square wave with long zero voltage before ON pulse. Another word electric motor is energize about half the time “OFF” as well “ON”…mean less torque at a given rpm. Which it get hotter due from increase ampere to maintain normal rpm and at a given load. Any time it spins less than spec. rpm…it cause to ask for more ampere.

So if you want the least waste of battery power usages…stay away from modified sine wave inverter.

Sojourn for Christ, Jerry
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