# A refrigerator on a modified sine wave inverter?

Harbor Freight has an 800w inverter on sale for \$45. It claims to have surge capacity of 1800w. My refrigerator draws 13 amps for a second when starting, and then less than 2 amps. So, will the inverter be able to start my refrigerator? Is it safe to run a refrigerator on modified sine wave? When they installed my furnace last year they told me the msw inverters will fry the furnace, but refrigerators seem less demanding.
I don't plan on using it, but for \$45, back up to my generator would be nice; but not if it will trash a \$1,000 refrigerator.
And since I am on the subject, what would happen if I tried to start it with inadequate current? Would it burn out, or is there a safety feature? I tried to get an answer from the manufacturer, but they wouldn't tell me.
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Modified sine wave means what? Nothing useful because it is not nor describes a number. One critical number is THD. Does that inverter even provide a THD number?
Many computer UPSes output modified sine waves. That means they are 'computer grade'. Another term used to confuse. 'Computer grade' may mean it is only good for computers because inverter may damage small electric motors. Myths forget to mention that computers can be more robust than other appliances. A modified sine wave is not destructive to a computer but could be destructive to something less robust such as a furnace.
But again, what is the THD number? It's not which is more demanding. It's about which is more robust. But then I have told you little that is useful since I provided a subjective word (robust) and did not provide numbers.
Toller wrote:

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It means they use a stepped waveform to simulate a sine wave(instead of a pure square wave),so it's still full of harmonics,but at lower amplitudes. So there is less energy at the higher freqs to be dissipated as heat. But still more than a pure sine wave.

Doubtful.
Computer power supplies these days are all switchers,so the input AC is immeidately rectified and filtered to DC,so the harmonics would not bother them.They have EMI filters on the inputs anyways,to keep internally generated harmonics and noise from going out the input lines!
It seems that furnace controls operate on 24VAC from a 60Hz transformer and the motor runs on 220VAC/60Hz.(I believe).Thats a lot of current for an inverter to supply,though.

No. Don't count on any "surge capacity".

Some modern fridges have electronic controls.MSW inverter might harm those.

electric motors do not like undervoltage for any long time period. "brownouts" burn out motors. Your inverter output V would drop then either recover,or decide it can't maintain and shut down all the way.Depends on how sophisticated the circuitry is;for a inexpensive one,it probably will not have the better circuitry.
--
Jim Yanik
jyanik
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I've got a couple inverters which do mod sine. First, a common volt meter reads 90 volts, not 115. If your meter reads 90, it's just account of the waveform.
The book I had with mine said that charging things like flash lights or shavers (which don't use a transformer) will fry the charger. But, the implication is that charge systems with a transformer are OK.
I don't know the answer about furnace boards. (I'd like to know cause I do have a mod sine inverter, and also a furnace with a board).
Does your refrig have a circuit board, or is it all analog?
--

Christopher A. Young
Do good work.
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Pay attention to whatever Stormin Mormin says, he knows lots and lots about tricity. Especially pacacitators and circuit breakers.
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It would be better to ask this on alt.energy.homepower. I know that Trace, Xanax and Outback inverters are used to run refrigerators. I also know that there have been reports that MSW inverters won't start up furnace motors.
Bottom line: There is a good chance it will work. But, get better advice on alt.energy.homepower.

--
Just my \$0.02 worth. Hope it helps
Gordon Reeder
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Thank you; I will try there. I presume it is like everything else at HF; cheap as possible, but just barely doing the job.
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Without that THD number, then no one can responsibly answer your question here or there.
Toller wrote:

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I am guessing that THD means total harmonic distortion, so something like that, and also that HF doesn't provide it.
But, in case they do, what sort of THD would I be looking for?

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2% THD would be sufficient for anything. A higher number on the inverter should be less than what specs permit for that appliance (ie furnace). 20% THD is probably what that inverter is outputting meaning that it causes stress to refrigerator and furnace - but is more than sufficient for electronics.
Notice a shortage of numbers. This because too many just somehow know and don't need the numbers. Well, to answer your question, numbers are required. There is no way to answer your question without those numbers. Less than 1% of us understand those numbers ... which is why the other 99% should be demanding numbers. So that the 1% cannot blow the whistle, many manufacturers intentionally don't provide numbers. Then people in your position only get answers from those who don't really know such as, "I did it and it did not explode. Therefore you can also so it."
Toller wrote:

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I run 2 fridges on a square wave inverter (1700W) and they perform perfectly. No odd noises. I just have to start them one at a time. One fridge is about 6 years old and the other is less than 6 months.
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