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There are several ways, use a normal audio amplifier driving a mains tranfromer, use a ferroresonant transformer driven from a normal h-bridge, or a class-d amplifier driving a high frequency transformer - explained below.
I would use a ferrite core transformer connected to a full h-bridge connected to a PWM working at about 40kHz and put a low pass filter on the output to only allow the 50Hz or 60Hz signal to pass.
A pure sinewave inverter isn't that complected, all it is is a class-D amplifier working at an extremely low frequency; it's a lot easier to build than a class-D audio amplifier.
Also not only will it give a good sinewave, it'll be more efficient and more compact than a modified sinewave inverter.
Google for class amplifier if you want more information.
The output emitter followers will draw high current if the impedance (inductive reactance at frequency) of the transformer primary is low.
You will need one hell of a lot of transistors and heat sinks and diodes and ballast resistors to get 500W out of that circuit, if it would work at all. Not to mention some huge batteries. In other words, it looks impractical to me.
Use the class D amplifier as Hero999 suggested.
That's a basic audio amplifier connected to a transformer, with a signal generator on the input. For such high power levels you're better off with a class-D amplifier, such a simple little audio amplifier design is only suitable for <10W loads as it's ver inefficient.
I'm feeling generous, I've modified someone else's block diagram of a class-D amp and illustrated how I would go about constructing a pure sine wave inverter.
**broken link removed**
Although the artical goes on about how hard it is to build a class-D audio amplifier, building a mains frequency aplifier is far easier as the lower frequency allows you to use a sensible switching frequency like 40kHz. Also not that my diagram doesn't have any negative feedback but the article talks about that.
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