Minor Mystery

[Pat]

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

[Dean Hoffman]

A very quick look on Ebay showed different sized compressors. Some are

1/10 hp, others are 1/3 hp. Are the compressors the same within a single unit?

[Pat]

I hadn't thought of that possibility. I'll do some research. It's a model 642 by the way.

Thanks for the quick reply, Pat

[gfretwell]

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.

[Pat]

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

[Ralph Mowery]

I have had very few power outages, but did buy a dual fuel generator. By only using propane it can sit there for years and be ready to go for half an hour or days if needed, then shut off and not have to mess with it to the next time. Propane will not go bad and nothing left in the engine like it would if gas. Caught one on sale that is about 3.5 to 4 KW for only $ 350 including shipping.

It is batery or pull start, so I do check the charge of the battery every few months.

[trader_4]

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.

[Tekkie©]

On Tue, 30 Mar 2021 17:18:47 -0400, [e-mail] posted for all of us to digest...

O O O is that a listed raceway? ;-)

[gfretwell]

If you have access to a scope it might not be a bad idea to look at the wave form under load. It might be pretty ugly. My generator wasn't really that bad.

[gfretwell]

That is just a "duct", the wiring method is a cable and the boxes are listed. The assembly is not listed but it is made up from "recognized" parts. As AHJ here it is "approved" on a 90.4. "Come and get me copper"! ;-)

[Tekkie©]

On Wed, 31 Mar 2021 21:51:47 -0400, [e-mail] posted for all of us to digest...

I always wanted a scope but never got one. I was always interested in observing the A/C waveform but I was afraid if I plugged in the inputs I would blow the sucker up. A ham gave me one that weighed about 100lbs but had no leads. It sat in my living room until my wife said to get rid of it. I built a Heathkit automotive one which worked fine. Any recommendations?

[Tekkie©]

On Wed, 31 Mar 2021 21:57:53 -0400, [e-mail] posted for all of us to digest...

First you deny me shelter and now you deny me a fine. I think I should confiscate the scope and variac for my emotional health! Send it overnite.

[rbowman]

For a real low cost solution you can use the computer's audio card and free software:

20 years ago or so Radio Shack had an o-scope probe to use with a computer. The frequency range was limited by the processing power of the day but it was handy for some things.

[gfretwell]

I would look for an old tektronics like that 453. They were built to spec for IBM and real tanks. I don't think I ever saw a bad one and we beat the hell out of them. I gave away 3 after they closed our office. IBM must have dumped 10,000-15,000. Every 360/370 account had one and a lot of CEs also had one in their car. I fear a lot of them went straight to the dump. That is where the ones I had were going. I kept a 465. I would have saved my last 453 as a spare, the one in the picture, but I am just running out of room. A guy I barely know from another newsgroup asked me if I was giving it away somewhat facetiously and I said yes. He showed up at my door the next day.

[TimR]

See here:

[Ralph Mowery]

For around $ 200 to $ 300 Hantek makes some very good scopes. I have one of them I bought several years ago. It is a 200 MHz version. I was told at the time by several that the Hantek was the better one, but the Rigol is also very good for the money.

I don't know if it is just hype, but some sources say to get one that is sold in the US instead of ordering from China. Suppose to be a better quality for the same model.

Rigol is another China brand that has a good reputation for the same price range.

[Tekkie©]

On Thu, 1 Apr 2021 20:14:02 -0600, rbowman posted for all of us to digest...

What does the bandwidth do/affect? I can assume from the pricing that more is better and is there a good resource to learn how to use it? More questions to follow, I'm sure.

[Tekkie©]

On Thu, 01 Apr 2021 23:28:28 -0400, [e-mail] posted for all of us to digest...

Beat me to it. The district had a 360. I wonder where that one went? I wonder where a lot of interesting stuff went. Not to me. 8(

[Tekkie©]

On Fri, 2 Apr 2021 10:37:49 -0400, Ralph Mowery posted for all of us to digest...

rbowman sent a link which I looked at and that had Hantek. I need more information before I buy. Otherwise I just order the high price spread and never use it...

I hate buying any China stuff - just as a personal philosophy but it seems unavoidable.

Thanks

[Clare Snyder]

A Variac is a variable output autotransformer and can increase voltage as well as fecrease it and the output voltage is pretty much unrelated to the resistance of the load - unlike the "scariac" which is basically part of a voltage devider network, dropping part of the voltage across the load and dissipating the rest as heat in the "cell". The "scariac" will also work on DC - while a Variac is strictly AC. I have several rangng from 500ma to over 2a amps. I have one set up in a toolbox that can supply up to 160 volts - either AC or DC - from a 115 volt AC input and had another set up to supply adjustable 240 from 115 or adjustable 115 from 240.

I also have a "sstepiac" - a buck-boost transformer which has a 6 and 12 volt secondary connected in series with the primary - either forward or reversed - to boost or reduce the line voltage by 6, 12, or

18 volts depending on how it is configured - as well as an isolation transformer made of parts of 2 microwave oven transformers. I used to have an isolation trandformer made of 2 big fillament transformers connected back to back for working on "ac/dc" tube radios and TVs back when I was a kid.

A Variac is NOT an isolationtransformer - both sides of the line CAN be "hot" on the output!!!!!