[Fig. 224] exhibits the armature for the above alternator; it is familiarly called a “squirrel cage armature” on account of its resemblance to the wheel in a squirrel’s cage.
Its peculiar construction enables it to run without producing any sparks; this feature renders it safe to run where there are explosive gases which might be ignited by an electric spark. In the machine the bearings are cast solid with the end shields, thus assuring perfect alignment when properly turned. Another feature is the automatic self adjusting bearings which are lubricated mechanically by rings resting upon the shaft. These rings were formerly a failure, but by the use of mineral oils are now a success.
This machine is one of the simplest designs of alternating motors, the example, [Fig. 223], is one developing one hundred horse-power.
Fig. 225.
[Fig. 225] shows a revolving field with “spider.” In this construction of generators or motors the field revolves in place of the armatures, the first object of this design is to reduce the high rotative speed; it is also claimed to have a better electrical efficiency.
The field spider consists of an extra heavy cast iron pulley which is keyed to the shaft; the low speed at which it runs permits the employment of bolts to secure the field coils and laminated pole pieces to the rim of the spider, as shown in the engraving. With this construction each individual pole piece can be removed and replaced independent of the others.
The laminated pole piece, one of which is shown in detail in [Figs. 226-229], takes its name from the fact that it is built up of a large number of layers of soft sheet iron, which it has been demonstrated give a better electrical efficiency than a solid iron. Soft iron is the most magnetic of all metals and is better suited for pole pieces than steel.
It should be understood that each individual pole piece is insulated from the others as well as from the spider. The pieces of sheet iron are stamped out—like washers and are cut apart and the ends united so as to form a continuous coil, like a coil of wire and each coil is isolated; mica is used between the layers.