TABLE OF CONTENTS Because an automobile has a good supply of dc from the storage battery, there are many dc motors for such tasks as raising and lowering the windows, and many motor-like actuators for things like automatically regulating the air intake. Much larger dc motors are also used in industrial machines, elevators, and other activities involving a lot of starting and stopping, and changing of speed.
A simplified dc electric motor is diagrammed in Fig. 20.1. A coil "stator" is stationary, and a steel magnet "rotor" can rotate around the axle signified by a black dot. The half-circle is a copper metal "commutator," so the electromagnet can be turned on in order to attract the "north" pole of the iron permanent magnet, which then swings up to the position shown by the dashed lines. In order to prevent this pole from tending to stop when it gets to that position, the conductive graphite "brushes" signified by the small white circles will lose contact with the commutator. The coil then loses its magnetism, and the rotor can continue spinning by inertia, until it comes around again to a position where it will be attracted when the commutator regains contact to the brushes.
The reader probably knows already that real motors have more coils and poles, and more complex commutators. There is more than one pulse of attraction, and sometimes the current is reversed to also cause repulsion. The...
