PARTS OF Dvxxmo-MActusgs.—The principal organs of all dynamos are the armature in which the currents are generated, and which, as a, rule, forms the moving or driven part of the machine; and the field magnets, which create the magnetic field through which the arma ture-conductors pass. To these principal organs we may add the roma/ardor, or collector, into which the currents generated in the armature are led, and the brushes which bear upon the commutator, and are connected with the external circuit.
ARMATURES.—Various constructors have adopted different forms of armatures, which, however, may be grouped under four general heads, as follows: 1. Cylindrical or Drum.-Armatures, in which the coils are wound longitudinally over the surface of a drum or cylinder. This type of armature is shown diagrammatically in Fig. I, which illustrates a 4-part drum-armature with closed coil. In prac tice, of course, the coils thus wound may reach several hundreds in number, with a corresponding number of commutator-bars. Arma tures of this type are employed in the machines of Edison, Weston, Siemens (Alteneek). Stanley (alternating), and a large aninher of others. A modified forin of the drum-armature is employed in the Thomson-Houston arc-light dynamo (see below) Which has a spheri cal shape.
Drum-armatures, a typical form of which (Weston) is shown in Figs. 54 and 55. are usually built up of disks of the softest charcoal iron, insulated from each other by layers of t issue-paper, and screwed together to form a solid cylinder, which is keyed to the shaft. The corn thus formed is covered with canvas soaked in shellac, and upon it (he insulated wires are wound. The object of building up the core with thin disks is to avoid the formation of Foneault or "eddy" currents, which absorb power, and which would quickly heat the armature and destroy the insulation of the wires. In the early types of thrsearmat tiros feet la were generally employed on the periphery, but were later on abandoned ; however, at present tends strongly to their re-employment, as they serve to decrease the resistance of I he magnetic circuit and to aid largely in holding the wires firmly in place.
2. liing-Arinaliirm—In these the coils are wound around an iron ring, usually mounted on a spider of brass or gun metal keyed to the shaft. This type is shown diagrammatically in Fig. 2. This illustrates the usual type of Gramme armature, and, as will be seen, the coils form one continuous winding, which is tapped at the proper intervals, and connected by short wires to the commutator-segments, against which the brushes bear, fling-armatures have been adopted by a large number of constructors, among them Paci notti (who was the first to use a ring-armature with teeth), Gramme, Brush, Schuckert, Fein, etc. This type of armature is coining more and more into general use, on account of its simple construction ; repairs are made easy, owing to the in dependence of each coil, which can be removed without interfering with the others.
Various methods have been etni‘loyed in the construction of the iron core in ring-armatures. In order to avoid the generation of Foucault currents, Gramme employed a ring built up of iron wire covered with a Japan compound, so as to insulate the convolutions from each other. Later constructors have used hoop or band iron wound as a continuous spiral, the layers of which are insulated by paper. The most recent machines of approved type have cores built up of ring-slumped soft iron disks, insulated from each other, and pressed together to form a hollow cylinder. The wires are, as a rule, wound on the surface of the core, but in some recent machines, such as those of Brown and Wenstrom, the wires are led through holes close to the periphery of the armature, being thus entirely imbedded in the iron. This avoids the use of the band wires usually employed to holm( the Wires in place, and allows the iron of the armature to be brought close to that of the pole-pieces. thus reducing the magnetic resistance.