I use the VFD as a single to three phase converter. This is because the original motor are not really meant to give good torque with varying frequency. I do sometimes drop the speed using the vfd but it mostly stays on 10/10. As you are doing the tool changes manually anyway, you can set the speed manually too right? Now there is the possibility of running the motor to 60hz as opposed to 50 and that will give give you a few more rpm.