The original Bantam motor is a 2 speed motor, 1500 nom rpm in low speed, 3,000 nom rpm in high speed, if your original motor would only take the lathe up to 800rpm, it was probably a 1500 rpm motor, unless the pulleys were changed as well.

Chris Gunn

3hp 3000rpm sounds pretty sensible. You should still be able to develop similar torque at low speeds to the original in low speed mode as well as being able to develop decent power at high speed. You can make more use of the variable speed and less of the gearbox if you have more power reserve like this. It's handy for varying the speed as you face off a workpiece with power feed, allowing you to keep the surface speed more constant.

With an inverter you can always overspeed it a bit as well, eg for small diameters. A bit of an overspeed isn't going to bother the bearings unduly, as the forces will be fairly light.

3-phase motors are smaller than the equivalent single-phase motors and there's plenty of space at the back of the machine anyway. My 3-phase motor is only something like 1.5 – 2hp and I've managed to bog it down a couple of times, so wish I'd been able to get hold of a larger example when I changed it. I have a 5.5kW inverter, although obviously I have set the limits to suit the actual motor nameplate ratings.

Murray

Alan, that sounds like a plan, my only comment would be to still use the gear box a little, so that you keep the smaller motor running at a good speed, a) to keep the torque up, and b) to keep the motor cool. If one is working at a lower spindle speed. select a lowish gear and fine tune the speed with the inverter.

Chris Gunn