Fig. 365.—Hub and shaft design on large machines to reduce the machine work and facilitate erecting.
Ques. Why do armature cores heat?
Ans. They heat from these causes: eddy currents, hysteresis, and heat generated in the inductors.
Ques. How is adequate ventilation secured?
Ans. The spider is constructed with as much open space as possible through which air currents may circulate. The core is divided into several sections with intervening air spaces D as shown in [fig. 363], the discs being kept apart at these points by distance pieces. These openings between the discs are called ventilating ducts; they are usually spaced from 2 to 4 inches apart.
Fig. 366.—Western Electric barrel wound armature, having a fan attached at one end to induce a circulation of air for ventilation.
Ques. What other provision is sometimes made to secure ventilation?
Ans. In some machines a forced circulation of air is secured by means of a fan attached to one end of the armature as shown in [fig. 366].
Insulation of Core.—Before the winding is assembled on the core, the latter should be thoroughly insulated. Japan or enamel insulation is not sufficient because it is liable to have bubbles or minute holes in it, or be pierced by particles of metal or by the rough edges of the core discs. Two or more layers of strong paper, fibre, canvas or mica, should be applied to the core before placing the inductors in position. The ends of the core should be insulated with thicker material, since the strain upon it is greater, especially at the edges.