RMS readings

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adas
adas Solar Expert Posts: 136 ✭✭✭✭✭
in Advanced Solar Electric Technical Forum #1
Aloha, I get 124.6V with my RMS voltmeter and 119.8 with the el cheapo multimeter on my modified sine inverter. Is it true that the cheapo multimeter will read closer to the RMS voltage if the Mod Sine inverter is a better wave form.

ie can I judge the inverter quality of sine wave by the difference between these two multimeters?

And is the quality of the sine wave the indication of the performance of the inverters?

thanks
Frank
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: RMS readings

    A true RMS reading meter integrates the area under the curve to derive the "total work" available under the curve (remembering it is Root Mean Square--So the the higher voltage is squared and averaged--So RMS will always be equal to, or larger than the arithmetic average).

    Also, you have to be careful measuring RMS... Most meters measure just the AC component, other measure the AC+DC component (which is actually the definition of RMS). Also, as the frequency components go up (sharper edges), the DVM meters are less able to calculate an accurate RMS (they cannot measure the higher frequencies--they don't sample the wave form fast enough).

    And the average DVM just measures the peak of the AC coupled voltage and multiplies by 0.707 (1/sq root of 2)...

    When you put any other periodic wave form into the DVM, just multiplying the voltage peak by 0.707 is not going to give you any meaningful information.

    Regarding the quality of the inverter... You can have very good quality inverters that output very nice square waves--and you can have very cheaply built inverters that output very nice looking sine waves.

    And this get back your question of performance... Performance doing what? Powering heaters/lights that don't care about square waves. Or powering motors and electronics that will overheat when subjected to square waves...

    If you are going to use square wave inverters and need transformers (up/down voltage/isolation/etc.)... Then you are going to need to look for transformers designed to run with high "K factors".... Basically designed not to overheat/waste energy when presented with non-sinusoidal input voltage/current.

    In order to measure the harmonics properly, at the very least you will probably need something like a Fluke 43B meter. But that is something near $3,000... Perhaps you can find a used spectrum analyzer...

    In the end, you have been running your shop successfully (?) on Mod Square Wave inverters and have been pretty happy?

    Are you looking to see how to predict the performance of a different unit in your shop? Or what...

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