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refrigerator on inverter

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Posted by Jim Wilkins on September 11, 2018, 11:05 am
 
My old Summit dorm-room-sized refrigerator ran without difficulty on  
an APC1400 900W/1400VA true sine UPS.during extended power outages. I  
recently bought a new 4.3 cubic foot Magic Chef which the manual says  
"is not designed to be used with an inverter." Their well informed  
service rep was noncomittal on the subject. I found that it starts and  
runs fine on my inverter, and uses less current than and so far 2/3  
the energy of the Summit.

What has your experience been with refrigerators on inverters?



Posted by danny burstein on September 11, 2018, 7:39 pm
 


Commodity chest freezer from one of the bigname places.
Pulls about 75 watts. There's no defrost cycle [a]

It worked fine for 15 hours (probably 25 percent duty cycle)
with a Sunforce 1,500 watt (nameplate, fwiw) true sine
wave inverter.

[ keep that auto defrost deal in mind. On my frost free refrig,
it pulls... 500 watts. yes, 500. that's enough to cause
lots of grief in a backup power situation ]

  
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Posted by Vaughn Simon on September 11, 2018, 10:08 pm
 On 9/11/2018 3:39 PM, danny burstein wrote:
In my experience, there are three possible problems with refrigerators  
on inverters:
1) Some refrigerators don't like square wave and modified square wave  
inverters.  You don't know until you try.

2) The inverter must be big enough to handle the compressor starting  
current, which may be several times its running current.

3) As already mentioned, the defrost cycle on a frost free refrigerator  
or freezer can cause a very heavy load because of the defrost heaters.  
I have seen 800 watts!

Really, you don't know for sure until you have tried a particular  
refrigerator with a particular inverter.

Vaughn

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Posted by Jim Wilkins on September 12, 2018, 3:04 am
 
1) The unsubstantiated rumor I heard was that modified square waves  
might overheat the motor and shorten its life.

2) The starting surge is proving hard to measure. I can take DVM peak  
hold or storage scope readings but they aren't consistent, and the  
compressor has to wait quite a while for the pressure to equalize  
between starts.  Maybe the useful test is to watch for a voltage dip  
on an analog meter or with a light bulb, since I don't really know how  
much starting surge current the inverter can tolerate either.

The Summit's starting current is roughly consistent with the cold  
resistance of the PTC thermistor in series with the start winding, as  
measured across the AC plug.  The Magic Chef's initial current appears  
to be lower than the resistance would suggest.

The Alpicool C20 DC-powered fridge/freezer soft-starts gradually. I'd  
be more impressed with it if it had better insulation and didn't fault  
occasionally.  Condensation forms on the outer walls if I add too much  
external insulation.

3) My three small fridges are all manual defrost. On older units the  
defrost timer motor could be temporarily disconnected when running on  
batteries, at the risk of clogging the evaporator and its fan with  
ice.  I don't know about modern ones. These manual defrost units put  
the evaporators inside the walls so frost buildup isn't as much of a  
problem.



Posted by ads on September 13, 2018, 3:17 am
 On Tue, 11 Sep 2018 23:04:03 -0400, "Jim Wilkins"


I have a 28 cu ft Samsung fridge/freezer that is kept cold during
power outages by a 2000 watt pure sine wave inverter (Reliable
Electric brand, about $20 on Ebay the last time I checked).  I had
the fridge on a Kill-A-Watt for 6 weeks and randomly checked the power
draw.  The highest I saw was 550 watts when both fridge and freezer
defrosters were active (dual evaporator model).  The average hourly
running watts over that time (KWH/hours) was 76 watts (summer, house
at 78F - winter average power is 59 watts hourly with the house at 68F
days/62F nights).

The inverter doesn't have a "peak watts" rating on its label but it's
never had a problem starting the Samsung fridge or the somewhat older
upright freezer (18-20 cu ft).  While I have a DSO, I haven't hooked
it up to check the starting current.  I don't hear any change in the
very quiet inverter when the fridge or freezer starts so I'd say their
starting current is well within its peak capacity.

The inverter is part of my "Wait until daylight" solar generator:
12 volt, 540AH AGM battery bank
1510 watt solar array
two 30 amp and one 40 amp MPPT charge controller

Depending on the season, the solar gen can provide 8 to 20 hours of
limited power without the sun and longer with sun.  That would be the
fridge, a few lights, internet, charging phones and tablets.   Maybe
the upright freezer a few hours a day.  The 470 watt blower on the
gas-fired central heat obviously eats up a lot of power as does the
427 watt, 5000BTU window A/C, neither of which will see much use as
limited cooling can be done with fans and limited heating with
battery-controlled gas logs and/or a kerosene heater.

Backup for that is a 1600/2000 watt inverter generator, an inexpensive
120 volt only 3000 watt gen and a 5500 watt 120/240 Generac (bought
used at a greatly reduced price of $0 when the selkler couldn't keep
it running long enough to prove that it worked).  The Generac is big,
heavy, loud and thirsty but it can power the 10" table saw.

In a long term outage, the fridge would be the counter height 4.4 cu
ft Kenmore that's currently filled with heirloom seeds.  It draws 55
watts and runs about 5 1/2 hours a day in summer.  That's 302WH and
less than some of the 12 volt compressor fridge/freezers that aren't
as big.  That fridge is no longer in stock online so perhaps it's been
discontinued, although there are some shown in the stores.  It's a
Kenmore 99783 fridge.

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