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.
Some typical numbers have been inserted in Figure 2 to illustrate an example. The
important aspects are the constant torque region, which extends from zero speed to 1,500 rpm
(base speed), and the constant horsepower region which starts at base speed and continues to
5,250 rpm, which is 3.5 times the base speed.