Wednesday, July 2, 2014

Pure Sine Wave Inverters Are Astonishingly Expensive Compared to Modified Sine Wave Inverters

I am guessing that there is something about the electronics required to produce a pure sine wave that explains the astonishing price difference.  As I understand it, if you are running a computer or other digital device, a pure sine wave AC output really matters.  What about other devices, such as garage door openers, electric motors, drill presses?  Is there any advantage to a pure sine wave inverter over a modified sine wave inverter?


  1. Desktop computers shouldn't care much about the difference between pure sine wave and modified sine wave power. It'll be a little tougher on the PSU, but any mainstream PSU can handle much worse.

    You'll want to avoid using modified sine AC power to charge batteries, for lights (especially LED/florescent lights), for high-speed motors such as in hand tools or fans. Clock circuits sometimes don't like it, especially cheaper clocks. Some older microwaves would have issues, but pretty much anything on the market today will do fine.

  2. Depends on your load as to whether a modified sine wave or a pure sine wave is needed (they are ALL modified sine wave, just the steps are a lot smaller on the "pure" sine wave inverter.)

    Most motors and other inductive loads don't really care. Some battery chargers (like for cordless drills) do, because of the construction of the power supply.

    Resistive loads don't care at all. incandescent light bulbs, or folded flourescent bulbs with internal power supplies generally don't care either.

    Wanna make modified sine wave units work with nearly anything? Find a 1:1 isolation transformer rated for the appropriate wattage (I like to size 'em 20 bigger than the load) and put it between the inverter and the load. Generally this will smooth out the steps in a modified sine wave unit so that it looks like a real sine wave because of the inductance of the transformer.


  3. Clayton,

    In my experience, many modern electronic components no longer use AC. They either use a wall-wart AC/DC transformer, or an internal transformer. It is my understanding that the waveform is not an issue in such utilization since the transformer is also rectifying.

  4. I wish I could, but can't answer your specific question.

    But... a few comments.

    There is no such thing as a pure sign wave - not in the real world - although power line power comes close.

    A quick Googling says that "modified sine wave" is done with a three-state (on high, off, on low) waveform, which is a rough approximation. It's also quite cheap to do.

    A more pure sine wave is going to have more steps - it still isn't going to be perfectly smooth unless they add a bunch of filtering (or resonance), which is probably not very useful.

    Basically, a "pure" sine wave is going to have lower distortion than a modified sine wave. You care a lot about distortion in audio, less so in power.

    It is likely to be quite a bit more expensive to generate a "pure" sine wave.

    Over the years, I've been involved in a few power supply designs for computers. They were not particularly sensitive to the waveform. Switching supplies pretty much shouldn't care (YMMV) - the ones I have seen turn the AC into DC before doing anything else with it. Motors are going to be more sensitive than those switchers. Lights and other gadgets... who knows (see link).

    But... all this is theory. I don't know where you can find an unbiased expert opinion on real world devices - after all, "pure" is great for marketing.

    The following link has some nice background, and recommends "pure" sine wave.

  5. There are serious differences between the two. The pure sine wave inverter is really an all analog circuit that puts out a pretty competent copy of a sine wave. The "modified sine wave" is really a pair of digital pulses: a T/4 pulse up, T/4 zero, T/4 pulse down, T/4 pulse 0.

    A modified sine wave waveform will sort of work for most consumer things, but those square transitions have a lot more harmonics in them (remember Fourier transforms from college?) and can often cause interference in things like stereos, radios, etc. And for motors you will often have much poorer power because the motor won't respond correctly to the fields generated with square waves since they won't have the right phase relations.

    In general, for crappy line regulation and very light use a modified sine wave is tolerable, but it's not a good solution if you want to run a motor or have high quality devices to run.

  6. The humidifier unit for my CPAP machine dies (permanently) if fed square waves.

    (Killed several when I was driving a truck finding that out.)

    Why expensive? Filters.

    Mathematically a square wave is the summation of the fundamental and all the even harmonics (really hairy formula), but if you want a nice sine wave at the fundamental you have to filter off (read discard the energy in) the harmonics.

    Discarded energy is waste heat--so you have to make more to have some to throw away, AND you have to deal with heat.


  8. I wouldn't be so sure about the computers, since they all use switching power supplies with huge input ranges (often being automatically capable of switching for 110 to 240 volts and 50 or 60 hz.)

  9. Depends what kind of computer you are running.

    Laptops, printers, monitors? No problem at all. Server farms? Usually have full time pure sine wave inverters.

    But I run Outback (tm) inverters on my house, boat, RV and run everything on them.

  10. From a PDF for a pure sine wave converter (so watch for marketing hype):

    Examples of devices that may not work properly with modified
    sine wave and may also get damaged
    • Laser printers, photocopiers, magneto-optical hard drives.
    • Built-in clocks in devices such as clock radios, alarm clocks, coffee makers, bread-makers, VCR, microwave ovens etc may not keep time correctly.
    • Output voltage control devices like dimmers, ceiling fan / motor speed control may not work properly (dimming / speed control may not function).
    • Sewing machines with speed / microprocessor control.
    • Transformer-less capacitive input powered devices like (i) Razors, flashlights, night-lights, smoke detectors etc (ii) Rechargers for battery packs used in hand power tools. These may get damaged. Please check with the manufacturer of these types of devices for suitability.
    • Devices that use radio frequency signals carried by the AC distribution wiring.
    • Some new furnaces with microprocessor control / Oil burner primary controls.
    • High intensity discharge (HID) lamps like Metal Halide lamps. These may get damaged. Please check with the manufacturer of these types of devices for suitability.
    • Some fluorescent lamps / light fixtures that have power factor correction capacitors. The inverter may shut down indicating overload.

    I've used a small modified sine wave inverter running off a deep cycle 12 VDC battery to power a netbook during power outages several times, with no problems.

    For large loads where this might matter, I would use a gas generator fed through a line conditioner. Like this one: