Modified Square Wave to Pure Sine Wave Converter

Siegfried Meier

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Hey guys,

Bit of a different request here.  I have a remote home that runs off wind and solar power.  The power is collected into deep cell batteries where it is fed to Power Inverters that convert the power from 24vdc to 120vac.  It then feeds the home with approximately 5000 watts of power.  It's fantastic for lights and pretty much everything else, including even the water pump and small electric heaters at times.  The problem is that the output of these specific inverters is a modified square wave signal.  For sensitive electronics such as washing machines, refrigerators and computers, this modified square wave can damage components, and has in the past.  My question is, does anything exist commercially that I can purchase that I can plug into the wall, and that will convert this modified square wave to a pure sine wave signal (I do not want to purchase new inverters, but there are commercially available pure sine wave inverters out there, this I know).  Or perhaps if nothing is available, is there something I can build with bridge rectifiers that will convert this signal correctly?

Any help would be much appreciated.

Thanks!
Sig
 

Mark Burnley

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Well,

All you need to do is filter off the higher harmonics of the mains supply.

In industry, some of the inverter motor drives use L-C filters to achieve this-

like this

These are expensive, as an inductor has to be used that has sufficient reactance to allow the blocking of the HF component, but has enough current carrying capacity (i.e. copper + core capacity) to pass the full load current without too much series R.

I should imagine a series of L-C (with L as input, C as shunt) filters would suffice here- you wouldn't use rectifiers, as these would only convert your AC back to DC (Diodes are used sometimes to convert other waveforms into approximated sine waves- I've got an old Feedback Instruments function generator that does exactly this with a diode ladder....check internal schem page 4 of this datasheet...anyway that's OT)

You'd want to experiment with values to get your 60Hz of reasonable purity, bearing in mind your expected load current, and the fact that the waveshape may get more messy the more I you pull (depending on your inverter!)

Sounds like an interesting project!

Mark
 

Siegfried Meier

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OK, you certainly seem to know what you're talking about haha.

So, there's nothing commercially available out there like this, is there?

Thanks,
Sig
 

PRR

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> So, there's nothing commercially available out there like this, is there?

The link Mark gave IS a commercial filter intended to turn inverter waveforms into stuff that electronic equipment will enjoy.
 

Mark Burnley

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Also,

Watch out if you're getting an ex-industrial unit- they're usually three-phase (i.e. 3-lives) for delta connected motor drives (no neutral)

You'd need a single-phase unit, or cobble a single-phase unit from a 3-phase unit (can get confusing with wiring- make sure you get a datasheet!)

Mark
 

Speedskater

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Aug 4, 2009
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Cleveland Ohio
What about a Sola ferro-resonate transformer?  They should be available on the surplus market. But one thing about Sola ferro-resonate transformer is that they are fussy about the load.  They have a sweet spot between about 1/3 to 3/4 rate power. Outside that area they don't do so well.  On a test stand at work, we had to add a dummy load to keep it happy.
 

SSLtech

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Florida (Formerly UK)
We installed a ferro-resonant with our Neve Capricorn...

Wow! -Never again!

Enormous thing, ran hot, made noise, fussy, ugly, caused more trouble than I appreciated...

I had to KEEP explaining that it was doing some good with the full-online UPS which I'd specified... the sheer enervating effort of explaining -again- why that hot, noisy eyesore was doing as part of a high-tech installation just got to be too much in the end.

Keith
 

Siegfried Meier

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Ontario, Canada
Ok, well this is sounding much more complicated than what I initially expected.  I was simply hoping for a commercially made device that I could plug into the existing North American 120v outlet, and get a sine wave outlet at the same voltage. 

Sounds like perhaps getting a dedicated Pure Sine Wave inverter might be the actual answer.  Yes?

Thanks guys,
Sig
 
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