A DC motor needs a commutator, but the commutator is a source of problems as arcing (caused when a coil is "disconnected" as the brush rotates to a different commutator segment) increases wear and produces electromagnetic interference which can adversely affect other electronic equipment nearby. Arcing increases with altitude.
The slip rings in an AC motor, although a source of wear, are a vast improvement over commutators.
Only the simplest motors utilize a permanent magnet for the field.
Motors are classified as shunt (field coils in parallel with armature coils), series (field coils in series with armature coils) or compound (combination of shunt and series)
One class of AC motor is the induction motor, this one does not require armature coils or slip rings but functions on the principle of eddy currents, these induced currents create the armature's magnetic field, induction motors have limited torque but are quite adequate for situations where the load is light or constant and applications include fans, blowers and flywheels. These motors are well suited to inertial loads and increased torque is produced at the expense of rotational speed but once the motor attains operating speed the load is constant, this is a feature of inertial loads.
two kinds of motor that can hold a position are the servo and the stepper, in the servo motor, external positional feedback from the shaft to the control electronics causes the motor to precisely hold a stationary position, but in the stepper motor, positional control is achieved by "programming" the motor to turn a given number of angular steps... stepper motors are designed to turn in small steps, the 360 degrees of the circle are equally divided into 8 of more poles, and complete rotation will not occur unless the poles are pulsed sequentially in the correct order.
speed, torque, braking and positioning are motor characteristics that may be electronically controlled and a whole range of Digital Signal Processors (DSP) integrated circuits (IC's) are available to do the job for almost any type of motor.
© copyright 2002 Stephen E. Mendes, Barbados.