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"modified" sine wave inverters ?

Discussion in 'Electronic Design' started by [email protected], Jun 30, 2005.

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  1. Guest

    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
  2. mike

    mike Guest

    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.

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  3. Joel Kolstad

    Joel Kolstad Guest

    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
  4. Andy

    Andy Guest

    Andy writes:

    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

    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...

  5. Pooh Bear

    Pooh Bear Guest

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

  6. Guest

    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.
  7. Joel Kolstad

    Joel Kolstad Guest

    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.
  8. Guest

    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

    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

    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".


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

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

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

  10. kell

    kell Guest

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

    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email:

    Please visit my GURU's LAIR web site at
  12. Rich Grise

    Rich Grise Guest

    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

  13. Guest

    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.

  14. wrote...
    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.
  15. Guest

    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

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