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Jim Yanik
 
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w_tom wrote in :

Modified sine wave means what?


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.

Nothing useful because it is
not nor describes a number. One critical number is THD. Does
that inverter even provide a THD number?


Doubtful.


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.


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.


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:
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?


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

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.


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


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.



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
at
kua.net