On Wednesday, March 31, 2021 at 6:37:51 AM UTC-4, Pat wrote:
On Tue, 30 Mar 2021 17:18:47 -0400, wrote:

On Tue, 30 Mar 2021 14:08:46 -0400, Pat
wrote:

We had a power outage here last Friday that lasted about 19 hours. (It
was caused by a wind storm blowing a large tree into power lines a few
blocks away). I happen to have four 100 Ah LiFePO4 12v batteries near
my circuit breaker box. (Their normal use is to power my boat during
the summer, but they were just being stored in the basement this time
of year). I also have a 1500 watt 12v to 120 VAC inverter. You can
probably see where this is going... Can I power my
refridgerator/freezer during the power outage using the inverter and
one of the 12v batteries? I thought, why not give it a try?

The refridgerator/freezer is a Sub-Zero brand built-in side-by-side
that is about 15 years old. If you aren't familiar with these, they
have separate systems for the refridgerator and freezer (ie, two
separate compressors). It has a dedicated 15 amp circuit, so it was
easy to pull the breaker and separate the wires from the breaker box.
I did that and then put a regular three prong plug on the end of the
wires and plugged the plug into the inverter socket and gave it a try.
The Sub-zero powered up normally and appeared to run. It drew about
800 watts for short periods and about 100 watts for longer periods.
(For example, 800 watts for 20 seconds followed by 100 watts for 2
minutes ... then repeat). So far, so good. After an hour or so, the
refridgerator section was back down to its normal 38 degrees F, but
the freezer temperature was slowly rising rather than dropping as
expected. Near the time the power was restored, the refridgerator
temp was still at 38F, but the freezer had risen from 0F to 22F. In
other words, the freezer wasn't really working at all. When the power
came back on, I restored the circuit to normal. Within a short time,
the freezer was back down to 0 and everything was back to normal.

So, my mystery is why did the refridgerator run normally off my
inverter but the freezer did not? The inverter is of the sine-wave
variety. I don't have a scope to look at the wave form, but it kept
the voltage up at 114 VAC even under the heaviest load (measured with
a Fluke 87). It was 120 VAC with no load. I would like to understand
this so I can use this setup during any future outages.

Thanks,
Pat


It is possible it just doesn't like 114v. That might be a bit lower
than that by the time it gets to the fridge. My generator puts out 115
under a fairly full load and my newer Whirlpool side by side didn't
like it much. The actual voltage at the plug was 109-110.
I ended up putting this together and it solved the problem.
When I ran it up to around 120 all was well.
http://gfretwell.com/electrical/Henc%20Variac.jpg
After reading Dean Hoffman's suggestion, I did a little more research.
I didn't find much other than lots of warnings not to use generators.
Sub-zero didn't really say why, though. They did say that starting
both compressors could peak at 24 amps. But, since they recommend a
15 amp breaker, that peak can't last very long. Who knows what the
voltage would drop to at the compressors during that peak. Other than
duplicating your variac setup, the only thing I can think to try next
time is moving the battery/inverter to the kitchen to avoid some of
the voltage drop. The inverter claims 1500 watts continuous with 3000
watts peak. Sub-zero's stated 24 amps peak would very near that 3000
watts max, so what I measured at 114vac during a 800 watt load might
actually be much lower during that peak demand. We've lived here for
almost 20 years and that was the first power outage that lasted longer
than a few hours. Other than quick glitches during lightning storms,
we have only had 2 or 3 outages in total over those 20 years. I'll
keep my eye out for a higher power inverter and plan to use it in the
kitchen rather than at the breaker box for "next time".

Thanks for your comments,
Pat

I would suspect that the freezer compressor takes more current to start.
So the fridge one wins out, it starts and then it reduces the available
current. IDK if they have electronic smarts that figure that out or if
it's just a thermal breaker for the compressor. If it's the latter you'd think
you'd hear a click when the freezer one stops trying.

Another consideration is that you're fiddling with Sub Zero that cost
thousands and repair would not be cheap either. I'd make sure I had
an inverter plenty big. Inverters are cheap compared to a repair bill
if the compressor gets hosed.