wire size is determined by amp draw, 10ga wire is suitable for 30amps.
30 amps of 220 3-phase (low voltage) is about a 10hp motor.
3-phase motors will run just fine without a ground but it is a very good idea to install one for safetys sake. also if there are any single phase controlls on your equipment (most have) they will make use of the ground wire to complete their circuit.
i do not use a convertor so can`t offer advice on building one.
if you are using 10ga wire leaving the convertor you should install a breaker no larger than 30amp between the convertor and the 10ga wire.
breakers are designed to protect wire not motors.
if you want to add motor protection it is best to fuse each line at the controlls.
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