One potential feature of a lathe that tends to be ignored when making buy decisions is the availability of multiple speed ranges, usually obtained by shifting a drive belt between pairs of pulleys in the spindle drive. This is a very valuable feature, particularly when selecting a lathe to run on 120 V AC, where one is limited to about 1.5 HP or less. A close parallel is an auto that has only direct drive (no gear selection). It is fine at highway speeds, but is severely challenged when climbing steep hills.
A VFD motor will deliver its maximum torque when operating at an RPS (revolutions per second) that is a little less than the frequency of the AC power generated by the VFD. This maximum torque will be nearly the same for all settings of the VFD. Motor horsepower is proportional to the product of torque and rotating speed. Therefore, the maximum horsepower will occur at the highest speed setting and will fall off proportionally as the speed is reduced. In other words, a 1 HP motor operating in a VFD system with a maximum speed of say 3000 RPM will deliver only 0.1 HP at a speed of 300 RPM. Compare this to a system that with a belt change can gear down the maximum spindle speed by say 4:1. In this case, the HP available at 300 RPM would be 0.4 HP and the maximum torque available would be four times larger than in the first case. Ideally there would be several speed ranges so that one can maximize the available torque at the desired speed for a particular project.
A VFD motor will deliver its maximum torque when operating at an RPS (revolutions per second) that is a little less than the frequency of the AC power generated by the VFD. This maximum torque will be nearly the same for all settings of the VFD. Motor horsepower is proportional to the product of torque and rotating speed. Therefore, the maximum horsepower will occur at the highest speed setting and will fall off proportionally as the speed is reduced. In other words, a 1 HP motor operating in a VFD system with a maximum speed of say 3000 RPM will deliver only 0.1 HP at a speed of 300 RPM. Compare this to a system that with a belt change can gear down the maximum spindle speed by say 4:1. In this case, the HP available at 300 RPM would be 0.4 HP and the maximum torque available would be four times larger than in the first case. Ideally there would be several speed ranges so that one can maximize the available torque at the desired speed for a particular project.