It may have been mentioned before or not, but as it is late I haven't had time to read the entire thread properly. Having said that I have just recently built myself a largish wood lathe with a 4HP induction 3 phase motor and 2 step pulley and a VFD to controll the RPM in both ratios, using the the VFD to drop the speed also results in significant drop in torque as well to the point that if I drop the speed down too slow ie; 60 RPM you can actually stop the motor by hand. Also the more mass your lathe has, the less it will want to "walk" due to imbalance. I think that mine is about 400-450Kg so it takes a lot to vibrate, but nothing is immune, give it a big enough, heavy, out of balance log at too high a speed, and mine will do the same. I just don't want to be near it if that happens.

Torque is a function of a motor running at its designed RPM, the more you electronically/electrically drop the RPM's the less torque you have. The only way I know to actually increase the torque and at the same time drop the RPM's so your out of balance wood doesn't shake it self to bits or damage the lathe, is to use lots of step pulleys, as in like the metal lathes do or your cars transmission.

As the previous poster mentioned you might be able to mount a router on some sort of frame and when the wood revolves underneath it, it will take out the high spots and you won't have a problem with low torque as the router will be doing all the work and the lathe just turning slowly, (once you work out how to slow it down), and which is what I am planning to do for mine in the future. . VFD's don't work on all type of motors from what I have been told. There are some video's of the router setups on youtube. Just my 2 cents worth.

Cheers

Ed.