Inverter AC Voltage

alkitalkit Registered Users Posts: 6

Just a quicky:

Busy testing some 500W MSW inverters.
I find that some of them start losing AC voltage as the battery starts draining.
They start at 220V AC output but go down to around 180-190V.

Is it safe to power my electronics at that low voltage?
Is it normal to go down to that range or does it indicate a fault with the inverter?

Thanks in advance!


  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    It is even a bit more complex a question than that with MSW inverters.

    Yes, many MSW inverters have variable output voltage vs DC input voltage--Presumably "better" MSW inverters will vary less... Here is a person that took scope pictures of an MSW inverter with variable output voltage:

    Another issue is the MSW (modified square wave) vs TSW (true or pure sine wave) wave form... Attachment not found.

    The above picture is a of a "square wave" vs modified square wave (which spends a bit more time at zero volts)... But the overall picture is still useful.

    An AC sine wave has a higher "peak voltage" and a MSW inverter has a lower peak voltage but longer "on time" to get the same/equivalent RMS voltage.

    RMS is Root Mean Square--It is a way to measure "average voltage" based on Power (not voltage)... One equation for power is P=V2R -- So the math is a bit different than an "average".

    But notice that the "peak voltage" of a MSW is lower than a TSW peak. This can cause issues for many "simple" electronic power supplies. The basic power supply uses a diode to charge a high voltage capacitor on the input of the Power Supply An MSW inverter already looks like "brown out" to a typical AC to DC power supply. In itself, this does not (usually) cause damage--But can cause an AC supply to have DC output failure due to "low" input voltage.

    Another "issue" with digital volt meters--Many measure "peak voltage" and use a conversion factor (divide by the square root of 2 -- The proper peak to RMS voltage conversion for a sine wave) to indicate voltage. MSW output needs a different conversion from peak to RMS voltage.

    More expenisve Digital Meters actually measure the whole wave form and "do the RMS Math" to indicate "true RMS" voltage... So between a "cheap" meter and a True RMS meter, you will get a different set of readings on MSW.

    What MSW issues are is that they are not Sine Wave output... Many electrical loads make assumptions that they are connected to utility power with TSW output. This can cause other issues.

    For example, that diode/high voltage capacitor input power supply? A sine wave is a "soft" edge that rises slowly to the peak voltage. This keeps the average current (and RMS) lower through the diode/capacitor. With an MSW input, the voltage goes from zero to peak with a very "hard" or vertical wave form. This causes high peak current (and higher RMS) current. Another power equation is Power=I2R -- Notice if the current doubles, the power goes up by 4x ... So these high "peak currents" generate a lot more heat and are very "hard" on electronics (heat is the enemy of life--for electronics, motors, etc.).

    With induction motors and AC transformers, the MSW wave form is "not a sine wave" -- The extra "harmonics" in the voltage (and current) wave forms for MSW is "useless" energy in many cases--This extra energy is usually wasted as heat (and a bit of noise). Again, the extra heat can cause shorter life. The "noise" can be a buzzing in motors/transformers/sheet metal/wiring. It can also be "electrical noise" in audio/visual devices.

    Here is an article about MSW vs TSW inverters: Choosing an Inverter - Home Power Magazine

    Sorry for the long answer--The short answer is that ~80% of "stuff" will work OK on MSW inverters. And about 10% will fail sooner because of MSW.

    Heaters/Filament Lamps and other "resistive loads" -- MSW vs TSW -- They don't care.

    Small wall plug transformers, many small "cord bump" power supplies will run hot/over heat. And refrigerator motor tend to run hot too (early life failure). Many newer/higher quality Electronics will work OK on MSW (technically, usually supplies with Power Factor Correction--They do not use a simple diode+capacitor on their input--Another long discussion).

    The big issue is, many times, it is not really easy to tell which will run OK and which others will have problems.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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