WOW. I'm sure getting a lot of great advice here. I don't think the client will like the transformer idea, and has already stated that 600V is not available. It needs to be a VFD, they say. I TOTALLY forgot about the critical speed issue. That could be the achiles heel. Thanks for the reminder. It has no gearbox, and will likely run at 1750 rpm. The pump curves are running up from about 450 HP to 600+ HP depending on flowrate. I was hoping to limit the HP to 500, but I can see now that won't be possible. I will need to talk this over with the pump supplier concerning the torque -- constant or not? What am I after besides more pump head and flow? Only the process engineer can tell me.
To help clarify my inquiring mind, just what features of how an induction motor is designed allows it to go overspeed, while a synchronous motor cannot?
Again, thanks to all of you for this phenomenal thread.