MC, Fred stated his lathe has a gearbox. Agreed there are allot of unknowns with this problem due to lack of information. Fred seems like a nice guy BUT doesn't have the technical abilities to understand what he's trying to accomplish. He's missing the entire cause of the problem and focusing on why he can't take the same depth of cut and expects someone on the forum to give a "magic" answer.
First, a noisy gearbox is a sign of mechanical problems that should be addressed first. The 750W, 220V, (no given rpm), single phase motor, when geared down to 900 rpms (chuck speed?) had enough torque to make a heavy cut. Changed to a 1.1kw, 1400 rpm, 3 phase motor and VFD and can't get enough torque at 1500 rpm chuck speed to take the same cut. It's very simple... the torque ratio and rpm between the new motor and chuck is backward.