The total or gross torque Ta developed in the armature of a motor is not available at the shaft because a part of it is lost in overcoming the iron and frictional losses in the motor. Therefore, shaft torque Tsh is somewhat less than the armature torque Ta. The difference Ta - Tsh is called lost torque.
Clearly,
As stated above, it is the shaft torque Tsh that produces the useful output. If the speed of the motor is N r.p.m., then,
