Figs. 362 and 363.—Side and end view of built up armature core. The sheet metal ring sections containing the teeth are fastened into dovetail notches in the spider as shown. The layers of ring sections are placed so as to break joints and are held by end clamps and through bolts B. Distance pieces are inserted at intervals to provide ventilating spaces D, D, D.
Ques. What is the approved method of core construction in large armatures?
Ans. The core should be of the built up construction to avoid waste of material in the stampings.
Ques. Describe the construction of a built up core.
Ans. Ring sections stamped, from sheet metal are fastened to a central support or spider, which consists of an iron hub with radiating spokes and a rim with provision for fastening the rings. The rim of the spider is provided with dovetail notches into which fit similarly shaped internal projections on the core segments. These features are shown in figs. 362 to 364. Each layer of core sections is placed on the spider so as to break joints and the core thus formed is firmly held in place by end clamps as shown. The manner of fastening the rings to the spider is an important point, for it must be done without reducing the effective cross section of the core in order not to choke the magnetic flux.
Fig. 364.—Built up core with four spoke spider, each spoke carrying two dovetail notches In this construction a little more air space is obtained for ventilation than where a separate spoke is provided for each notch.
In order to secure a better fit and reduce the machine work, the spider hub in large machines is sometimes cored with enlarged section between the outer bearing surfaces, and it is not unusual to find these surfaces turned to two different sizes as in [fig. 365], to admit of easier erecting.
To avoid any trouble that may arise by unequal expansion, the rim of the spider is not made continuous, but in several sections as shown in [fig. 364]. The rim here consists of four sections each of which has two dovetail notches. By thus dividing the rim into sections, its weight is somewhat reduced and the ventilating spaces between the sections increased.
Ventilation.—In the operation of a dynamo more or less heat is generated, depending on the load; hence it is desirable that provision be made to carry off some of this heat to prevent excessive rise of temperature.
