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. 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. But you loose the 2:1 mechanical advantage if you maintained the motor's full rated speed and changed the mechanical ratio. This is one reason why VFD motors are often oversized and also driven above their base speed. 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. On my mill the motor covers 20-200 Hz, and 2 speed ranges of ~ 40-450 and 400-4500, i.e. direct belt drive with a back gear. On a lathe they are also typically have 2 mechanical speeds, on the lathe motor replacements I belt down the motor and run them to ~120 Hz, and recommend a usable speed range of 20-120 Hz.
VFD's are also able to provide higher torque levels over short periods of time (like 150-180% for up to a minute), one reason why the supply amperage/fusing needs to be higher than the motor specifications.
. It is always so informative. Thanks guys.
