Applications. The uses to which motor-dynamos are put are very various. They are extensively used in the larger sizes as "Boosters," for giving the necessary extra force on long electric supply circuits to carry the current to the end with the same pressure as that which reaches the ends of the shorter circuits from the station.
Motor-dynamos have the advantage over dynamotors, described later, of having the secondary voltage easily and economically varied over wide ranges by means of a regulator in the dynamo field.
137. Dynamotors. Fig. 153. In Dynamotors the motor and dynamo armatures are combined in one, thus requiring a single field only. The primary armature winding, which operates as a motor to drive the machine, and the secondary or dynamo winding, which operates as a generator to produce a new current, are upon the same armature core, so that the armature reaction of one winding neutralizes that of the other. They therefore have no tendency to spark, and do not require shifting of the brushes with varying load. Having but one field and two bearings, they are also more efficient than motor-dynamos.
Applications. They have largely displaced batteries for telegraph work. The size shown, occupying a space of about 8-inch cube, and having an output of 40 watts, will displace about 800 gravity cells, occupying a space of about 10 feet cube. The cost of maintenance of such a battery per year, exclusive of rent, is about $800, whereas the 1-6 dynamotor can be operated at an annual expense of $150.
Dynamotors are largely used by telephone companies for charging storage batteries, and for transforming from direct to alternating current, for ringing telephone bells. Electro-cautery, electroplating, and electric heating also give use to dynamotors.
CHAPTER XIX.
HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.
Fig. 154.