On my mill the motor covers 20-200 Hz, and 2 speed ranges of ~ 40-450 and 400-4500, i.e. So on a smaller mill with a factory installed VFD system you often will see a 2-3Hp motor used with 2 or 3 mechanical ratios. This is one reason why VFD motors are often oversized and also driven above their base speed.
But you loose the 2:1 mechanical advantage if you maintained the motor's full rated speed and changed the mechanical ratio. So if you were to reduce the motor speed to say 30 Hz you loose 50% of the motor Hp, the torque is typically flat below the motor base speed.
As others as mentioned Hp is Hp be it single or 3 phase, but what is often missed is that below the motor's base speed (typically 50 or 60 Hz) you loose Hp in a linear fashion and you also loose the mechanical advantage.
