THE DC MOTOR

Magnetic fields can produce considerable mechanical forces. These forces can be harnessed to do work. The device that converts dc energy into rotating mechanical energy is a dc motor. In this sense, a dc motor is a form of transducer. Motors can be microscopic in size or as big as a house. Some tiny motors are being considered for use in medical devices that actually can circulate in the bloodstream or be installed in body organs. Others can pull a train at freeway speeds.
In a dc motor, the source of electricity is connected to a set of coils producing magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is switched in such a way that a constant torque, or rotational force, results. The greater the current that flows in the coils, the stronger is the torque, and the more electrical energy is needed. One set of coils, called the armature coil, goes around with the motor shaft. The other set of coils, called the field coil, is stationary (Fig. 1). In some motors, the field coils are replaced by a pair of permanent magnets. The current direction in the armature coil is reversed every halfrotation by the commutator. This keeps the force going in the same angular direction. The shaft is carried along by its own angular momentum so that it doesn’t come to a stop during those instants when the current is being switched in polarity.

Fig. 1. Simplified drawing of a dc electric motor. Straight lines represent wires. Intersecting lines indicate connections only when there is a dot at the point where the lines cross.