"modified" sine wave inverters ?

Anyone have any idea just "how modified" the waveform is on these inexpensive inverters. Certainly if you look for True Sine Wave Inverters, there is a huge price difference. Are MSW just square wave, maybe with a big capacitor to round off the edges? Regards, Don

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blueflash
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Think of a square wave with a flat spot (zero volts) at each transition. The peak voltage is scaled to be approximately peak of sine wave. Pulse width is adjsted to make the RMS value approximately the RMS of a sine wave. This often works. I have some equipment, TEK scope, that uses a series capacitor to limit the input based on a sine wave. If I try to run it from a MSW inverter, it blows the fulse. mike

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Reply to
mike

No, modified square wave is 120 degrees +V, 60 degrees 0V, 120 degrees -V, 60 degrees 0V, and repeat. The magnitude of the voltage is something like 177V, I believe.

The benefit of this method over a a regular square wave is that it eliminates (or, in practice, noticeably attenuates) the third harmonic relative to a simple square wave. This produces less losses (and hence heating) in transformer operated equipment.

---Joel Kolstad

Reply to
Joel Kolstad

Another advantage is that a diode peak rectifier will output the same DC peak as a sine wave, which is an advantage for simple power supplies that are lightly loaded.

Also the DC heating value is the same as a sine wave, into a resistive load, which is good for hot filaments and things....

So the modified sine wave has the same peak and the same DC heating value as a true sine wave. I think, a very good idea.

The old style square wave 50/50 inverters can not provide these simulataneously, and the power supplies of some equipments did not work the same as with a true sine wave...

Andy

Reply to
Andy

If you did the Fourier transform, you'd find that the three-level waveform had no third harmonic content, and a much lower fifth harmonic content than a simple square wave. Higher harmonics are reduced progressively less with rising harmonic number, but the amplitude of the highner harmonics in a square wave already goes down in proportion to the harmonic number.

It is a neat trick. "Magic sine" pulse width modulation, as popularised by Don Lancaster, has to work a lot harder but can get rid of more of the lower harmonics.

--------- Bill Sloman, Nijmegen

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bill.sloman

Not in my country it wouldn't be, nor most of the world actually !

Graham

Reply to
Pooh Bear

60
177V,

Well, you know what they say... at least here in the US, we can find where the cables are routed just by plugging in a space heater and feeling the walls. :-)

Reply to
Joel Kolstad

I have the "modified" inverter and a sine wave inverter, too.

"Modified sine wave" is a marketing term. The more accurate term, used less frequently is "modified square wave". "Effective sine wave", while not used, has meaning, and the "modified" type inverters have the same peak to peak voltage and RMS voltage as the sine wave they are used to replace. What they do *not* have is the same *spectrum*. They have harmonics, while pure sine power has only the one spike in the spectrum.

It helps to have both types and test all loads with each type to see if the "modified" type induces unacceptable heating, or perhaps unsteady operation in the connected device. Even a sine wave inverter will not drive the most sensitive loads. But they will drive almost any load.

It makes sense to drive small loads, when possible with a modified type inverter, as they are substantially more efficient with small loads. Larger loads that can use either inverter are best driven with the least expennsive inverter. My inverters are rated in idle current and full load current. The pure sine wave inverter draws about four times the idle current that the modified type draws. At the same larger load, their consumption is almost the same, but you already mentioned the price....

A true sine wave inverter would be like a class C amplifier, or even class A. Very ineffienct, but high quality power. It's said this has been done with high power car audio amps. I cannot confirm it.

I am experimenting with self-excited induction generators, a heavy, medium efficiency source of extremely pure sine wave power, when adapted to a particular load. Search "SEIG" for these mechanical-to-electrical "converters".

Yours,

Doug Goncz Replikon Research Falls Church, VA 22044-0394 DGoncz at aol dot com email

Reply to
DGoncz

Sorry. SEIG is an acronym and also the word associated with the Nazi "Heil".

Search "self-excited induction generator OR generators" in Google, Google Groups, Google Scholar, or your favorite search engine to get the best results.

Doug

Reply to
DGoncz

The common German word you're thinking of means "victory" and is spelled Sieg, not Seig.

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kell

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Don Lancaster

It's a "rectangular wave". Something like this:

--- --- --- | | | | | | | | | | | | -- -- -- -- -- -- -- etc. | | | | | | | | | | | | | | --- --- ---

I once had occasion to work with a line of brute-force inverters - these guys were awesome, for the day - 2400VA in a box about the size of "a breadbox" - now that I think about it, about the volume of a "desktop" computer. (don't most people put them on the floor these days?) A little thicker and squattier than the box that computers come in these days - and one whole side of the box was extruded anodized aluminum heatsink. The idiot that designed it had TWO - count 'em, TWO - mongo transformers with their secondaries _IN SERIES_ so that during the "dead spots" (the segments at zero volts in the diagram) the two transformers were actually bucking each other (yes, I know the joke potential there, for jokes, please prepend "OT:" thanks) and the output was regulated by changing the relative phase of, essentially, two whole inverters. They were awesome. There's something to be said about the visceral feeling you get while watching ~100 amps coming off a bank of 24V lead-acid batteries running several desk lamps, a bench grinder, and a hand drill simultaneously, and no fires. ;-)

Of course, the product died an unceremonious death. Nobody could afford the damn things, and they weighed close to 75 lbs. (~35 kg) apiece.

But they _were_ fun to work on! They had about a dozen TO-3 transistors, (per each!) and in the scrap pile I saw a few boards that had failed catastrophically - all of those TO-3 transistors (in the example "oops!" boards) had little craters in the case, where the die had vaporized. =:-O

Cheers! Rich

Reply to
Rich Grise

WTF? Sure, incredibly precise sequences of carefully chosen bits can emulate a sinewave Class D, but if even one solder joint isn't right, the whole thing won't work.

I'm talking about a *reliable* generator that won't fail when it takes a bullet.

Doug

Reply to
DGoncz

Solder joints? Reliability? Doug, Doug. We're talking about some lookup tables in a microprocessor, not solder joints. Once those are designed and typed in, they never change. No solder joints, or unreliable connections would affect a magic sinewave, or other PWM technique, preferentially over more simple approaches, once they're embedded inside the microprocessor code. Bullets? Bring them on.

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    - Win
Reply to
Winfield Hill

No way. uP code depends on the existence of a uP, and it's hard to harden such. If you pry a bullet out of a motor/generator, there's a reasonable chance you can improvise a repair. If you pry a bullet of a uP case, there's almost no chance of getting it to run again using a Swiss Army knife, tape, and a bit of wire.

Sure, you can harden computers. Motor/generators are inherently hard to begin with. It's a power source, fer heck's sake! It's got to be the

*last* thing to fail. Everything depends on the source. Live are at stake.

Doug

Reply to
DGoncz

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