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I am making a modified sine wave inverter. The PWM wave is generated, switched using transistors, and then passed through a transformer. What is the PWM frequency required to generate a modified sine wave of 50Hz?

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Here are a few pictures: -

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They are all using (from what I can see) different PWM frequencies to get the same result. Clearly the higher the PWM frequency, the less grainier will be the final sine wave but, on the other hand there could be more losses of you decide to use (say) a 1MHz pwm frequency as opposed to 1 kHz.

The best frequency to use is totally dependent on your input specification and the availability of hardware in your area plus your cost/build requirements. I don't have that information - do you?

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You have to use a PWM with a base frequency that is several times higher than the sine wave frequency you like to generate if you want the sine wave to be generated without too much distortion. E.g. solar inverters generally work with PWM frequencies from 10 kHz to several 10th of kHz. The amount of distortion largly depends of the power you want to put through the inverter in comparison to the filtering done with output capacitors, coils and perhaps an output transformer.

A quite detailled insight into this matter with practical aspects gives this Major Qualifying Project about PWM Techniques: A Pure Sine Wave Inverter.

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