Figs. 1,474 to 1,477.—Various types of armature; fig. 1,474 ring armature; fig. 1,475 disc armature; fig. 1,476 drum armature. The latter type is now almost universally used, the others being practically obsolete. A Gramme ring wound and connected to collector rings as in fig. 1,474, will yield an alternating current. In a multipolar field, the ring will need multipolar connections alternated at points corresponding to the pitch of the poles. Fig. 1,475 illustrates the so-called "Siemens" disc armature. The armature coils are arranged around the periphery of a thin disc. The field magnets consist of two crowns of fixed coils, with iron cores arranged so that their free poles are opposite one another. This type was created in 1878 by Herr von Hefner, engineer to Messrs. Siemens and Halske. Fig, 1,476 shows a modern drum armature of a three phase machine. It is similar in appearance to a direct current armature except for the absence of the commutator and its connections. The drum armature is the prevailing type.
2. With respect to the core, as
- a. Ring;
- b. Disc;
- c. Drum.
Ring and disc armatures are practically obsolete and need not be further considered. A ring armature has the inherent defect that the copper inside the ring is inactive.
Disc armatures were employed by Pacinotti in 1878, and afterwards adopted by Brush in his arc lighting dynamos.
The design failed for mechanical reasons, but electrically it is, in a sense, an improvement upon the Gramme ring, in that inductors on both sides of the ring are active, these being connected together by circumferential connectors from pole to pole, thus, corresponding to the end connectors on modern drum armatures.
3. With respect to the core surface, as
- a. Smooth core;
- b. Slotted core.
In early dynamos the armature windings were placed upon an iron core with a smooth surface. A chief disadvantage of this arrangement is that the magnetic drag comes upon the inductors and tends to displace them around the armature. To prevent