Many machine tool spindle drives require maintaining a constant tool force and a
constant surface speed at the cutting tool. This requirement defines constant power by the drive,
because as the radius decreases the torque decreases (for a constant cutting force) and the
speed should increase to maintain a constant cutting velocity.
Thus, to be economical, most spindle motors should be able to operate in a constant
horsepower range, Figure 2. Without this capability, the motor would have to be significantly oversized.