Why is sinusoidal mains inverter said to be difficult?

[cupoftea]

Hi,
Why is the sinusoidal mains inverter said to be so difficult to design?

The attached LTspice does it no problem..
There is no need even to adjust the amplitude of the reference sine wave.
If you change the load on the output, it automatically changes the sinusoidal current to suit it...and puts the same sine voltage across it.
All open loop!
Why is this said to be so difficult?
(Not Grid_tied here, of course)

Generating the ref sine signal....i'd say this is best done by a mini LLC converter, wouldnt you agree?....tap off the sine with a current transformer, and use that as the ref sine? Big L and C values for 50Hz sine, but low current. AYK, its a train of half-sines thats actually needed (rectified sine)

 

[Easy peasy]

How are you guaranteeing < 10mA DC in the output ? ( needed for driving std mains transformers )

 

[cupoftea]

Thanks
Generating the ref sine signal....i'd say this is best done by a mini LLC converter, wouldnt you agree?....tap off the sine with a current transformer, and use that as the ref sine? Big L and C values for 50Hz sine, but low current. AYK, its a train of half-sines thats actually needed (rectified sine)

 

[FvM]

I don't understand the application. You build a stand-alone (islanding) sine inverter, but the sine reference is taken from grid. Why? Why not from a stable sine oscillator?

 

[cupoftea]

Thanks, sorry, not using the mains, using a mini LLC converter switching say 5VDC...then tap off the sine from the current. So set Lres and Cres for 50Hz, then switch it at 50 Hz...and thence get the sine wave current going.....then tap it off with a bipolar rectified (FWB) current transformer.

 

[FvM]

I see. A rather awkward way to make a sine oscillator.

 

[cupoftea]

thanks, only other (analog) way to make a 50Hz sine i can see at the moment is to rc filter a 50Hz square wave, and then amplify it back up? Plus ideal diode rectification because we only want a train of positive haversines....of about 1V peak.

 

[betwixt]

Maybe I'm missing something here but if all you ask for is a 50Hz sine oscillator I could make one with a single transistor and a few resistors and capacitors. If you need crystal accuracy I could still do it in a few components.

Why start with a square wave then try to shape it? Much simpler to start with a sine oscillator.

Brian.

 

[cupoftea]

Thanks, as you know, when the load is purely a resistor of constant value, then the simple inverter principle above applies, but when there is rapid load changing , or non linear loads, then we need a control algorithm to be added.

Though the below appears to be the only chipset available to do this quickly. Do you know of any others?

https://www.ti.com/lit/ug/tiduay6e/tiduay6e.pdf?ts=1674208343185&ref_url=https%253A%252F%252Fwww.edaboard.com%252F