While dynamos vary in the manner of winding the fields and armatures as described above, the most important difference between the different types is in the kind of current generated. This classification divides dynamos into the two types of direct and alternating. An alternating current dynamo is similar in its action to a direct current dynamo, except that in the former the two ends of the various armature coils are connected in a ring. As the armature travels past the poles of the field magnet, the armature coils cut through the magnetic field in opposite directions. This produces a flow of current in the coils which reverses as the particular wire passes each pole. The current is collected by means of the rings, and is transmitted through the circuit as a series of rapidly oscillating pulsations. It is necessary to have or maintain the magnetic field of an alternating current dynamo in a constant condition; that is, the lines of force must always travel between the poles in a constant direction. To attain this result, the field must be excited (receive its power) from a dynamo generating a continuous current.
Fig. 87. - Compound-Wound Dynamo.