Ques. What is the construction of the core end plates, and why?
Ans. The rims are beveled quite thin to avoid eddy currents.
Ques. How is the core connected to the shaft?
Ans. Since the core has the full torque exerted upon it by the drag of the inductors, it must be firmly connected to the shaft by means of a key, as shown, so that it may be positively driven.
Core discs are stamped in one piece up to about 30 inches in diameter, and for larger sizes they are built up from sections as later described.
Figs. 353 and 354 show two forms of disc stamped in one piece. The first illustrates a solid disc, and the second a ventilated disc in which more or less of the metal is cut away near the center, thus providing passages for the circulation of air which carries away some of the heat generated in the armature.
Figs. 353 and 354.—Solid and ventilated core discs. In fig. 353, the metal cut away near the center reduces the weight and provides passages for air circulation. In some instances a forced circulation is secured by means of a fan attached to the armature, as shown in [fig. 366].
Insulation of Core Discs.—When the discs are stamped from very thin metal, the mere existence of a film of oxide is sufficient insulation. It is usual, however, to apply a quick drying varnish that will give a hard tough coat and not soften with heat or become brittle and crumble under vibration. The varnish may be applied either by dipping or with a japanning machine; it must be very thin, and the solvent employed should be a very volatile spirit.
Forms of Armature Teeth.—The teeth stamped in the core discs are made in various shapes, depending largely on the method of securing the inductors in the slots against electromagnetic drag and centrifugal force. The teeth may be cut with their sides:
1. Inclined;
2. Projecting;
3. Notched.