audio-talk

Once you have to voltage amplify may aswell forget the behringer and build 3 class B push pull valve amps with a choke load on the output delivering direct to the three phases of the motor at 400v per phase.

Whichever way you look at it chokes / autoformers are involved.

Might be better to buy a rotary inverter.

Even better just replace motor with single phase one, but I was just pointing out that 3kW for £500 is hard to match with any valve solution, even before we look at the efficiency question.

Actually, it's better to keep the three phase motor and use either a static converter or a rotary converter. For best finish, you need smooth rotation, and single-phase motors produce far more vibration than three phase. I chose a static converter over the (cheaper) VFD solution not just because I didn't want the RF hash but also because it enables you to plug a choice of three phase machines into one box, whereas a VFD has to be matched to its motor, so you need one per motor.