Initially, all motors used direct c

Initially, all motors used direct current (dc). This current creates magnetic fields in the stator and rotor, then mechanically energizing and de-energizing stator coils cause a moving field that draws the rotor around. With alternating current (ac) motors, the current itself switches in polarity from positive to negative.As with spindle motors, positioning motors were initially dc brush type. Then ac induction motors became popular, followed by hydraulic motors .Position control is the key feature in feed motor operation. It is important to know that accurate positioning depends on feedback, or a closed-loop system.With feedback or closed loop, the motor obeys the command, but has a senor that sends back the signal, "Here is where I am." If it is not the right place, the control warns the operator or sends a corrective signal. The "conversation" between the motor and control takes place almost instantaneously.