A rotary phase converter would be less efficient than a VFD (though possibly cheaper if you're buying it new), while a static phase converter is, for the most part, a VFD.
3 phase is pretty much standard in a commercial environment. He'll almost certainly have to have an electrician add a 3 phase circuit to use a 3 phase compressor, but a single phase compressor can just plug in if there's already an outlet with sufficient current capacity.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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