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Why use Pure sinusoidal inverters for off-grid "mains" supply?


Advanced Member level 5
Jun 13, 2021
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The attached schem and LTspice sim, shows two 40W offline loads.
One represents a 40W SMPS with a 47uF HVDC Post rectifier Bus cap and the peak of its mains input current is
The other represents eg an 40W induction motor load with a sinusoidal
input current of just 240mA peak.

There is clearly a massive difference in their peak mains input current, even though they are both 40W Loads.

Now lets say that each of them is supplied the "mains" by a little portable, off-grid, 40W, 12V to 240VAC, pure sinusoidal inverter.

Such an inverter would have an overcurrent limit. At most, this would be about 240mA + 20%, ie, 288mA.

This is nowhere near enough peak current rating to be able to supply the 2.5A peak that the real mains can supply for the one with the
47uF cap.

As such, the 40W pure sinusoidal inverter will struggle to properly supply the 40W load with the 47uF bus cap.
Also, its current and voltage waveforms will get warped, and not be particularly sinusoidal at all.

It makes you wonder, why anyone would bother to ever use a Pure sinusoidal inverter for such a case?
Why would they? What is the reason for it?
And take this 500W pure sine inverter..
..It says it can supply motors, (so it must mean 50Hz induction motors) but you would have to hope that the particular motor
didnt need much high startup current (like most induction motors do), because this inverter would then just trip out and never start up.


  • Pure sine wave inverter 40W.png
    Pure sine wave inverter 40W.png
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  • Pure sine inverter
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Thanks, i am just amazed that anybody is still using pure sinusoidal inverters in portable inverters. Fans are the only load that would likely need a pure 50Hz sinusoid.
Even PFC'd loads would not need a pure sinusoid. 300VDC would be fine, even if it transitioned from -300VDC to +300VDC every 10ms, over apprx 2ms (for the transition) , that would be fine. Why are so many portable inverters using pure sinusoid output?
There are possibly a lot of poor designs out there that compromise quality for cost, to support your assumptions yet all high power GTI's are sinoid. I have seen harmonic crosstalk from lower quality emitters downstream that can ingress into better GTI's (Huawei) that have low impedance on harmonics and absorb in both directions which caused a 10% rise in fuse current,

I wonder if you have seen any anecdotal experience on seeing specs for load regulation errors on 100 to 300W inverters. That is the cause of all reactive load distortion.

Just imagine if you audio power amp reacted that way with about 50% of the impedance being reactive at some point.

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