Emco Mentor Mill conversion

[Jonathan]

Cheers, I always thought that ac with vfd was more reliable but restrictive on high rpm also I had heard that torque at low speed can be an issue.

You can run a lot of standard induction motors at well over their rated frequency using a VFD and you will still get approximately rated power at the higher speed, however for that to happen clearly the torque drops proportionately. When you reduce the speed with a VFD, ideally the torque remains constant but in reality it drops a little. Even if the torque did remain constant, your power output will reduce with speed as power=torque*angular velocity.

The cheap VFDs from China seem to be good:
http://www.ebay.co.uk/itm/VARIABLE-F...item3cc89d5929

I use one of those to power the 2.2kW motor on my lathe and it works well.

the specs say 1.75hp continuous and 3hp peak. geared down to 3000 rpm for the mill as it has roller bearings then that would be about 3hp continuous. geared up 1:2 on another spindle it should still achieve 1hp at 12000rpm

Ideal gearing doesn't change the power output - if the input is 1.75hp, then whatever gearing you put in between the output will still be 1.75hp continuous, not 3hp. Clearly in reality you loose a few percent due to the inefficiency of the gearing.