Servo systems meter out power only as required - proportional to the load torque applied to the motor. Repeatability and torque production are a problem for steppers in this mode of operation. Again, servo systems overcome this problem by supplying power as indicated by the on-shaft encoder. The most common way to keep stepper motor systems from losing steps is to over design the system such that there is a very high probability that no steps will be lost due to high loads or friction. When current is fed into the windings of the stator it generates and electromagnetic field.
Induction motors are common because their construction is extremely rugged.
The motors are maintenance free and the manufacturing processes have been refined so that the costs are very low. There is another class of motors that address the efficiency and controllability issues. These motors, us permanent magnets in either the rotor or the stator. In sub-category called the brush type permanent magnet DC motors, the stator field is generated by permanent magnets. The rotor field is generated by the current flowing through the windings in a laminated steel core.