REVOLVING FIELD ALTERNATOR 83 



an<l which is generally divided into two sections, an upper and a 

 lower one, bolted together. The lower section is provided with two 

 feet, by means of which it rests on a foundation-plate. In very large 

 machines, however, the yoke-ring is made in four sections. This 

 IUIL; is provided with numerous ventilating apertures, only a few of 

 which are shown in Fig. 61. The revolving field consists of a fly- 

 wheel or spider, supporting a ring to which are bolted the field poles. 

 In large machines the ring is a separate structure, bolted to the 

 spider arms ; or the boss of the spider is split, and after having been 

 bored out to fit the shaft, is strengthened by two steel rings, which 

 are shrunk on. Either of these expedients ensures freedom from any 

 dangerous strains which may be set up in the arms of a large wheel 

 during cooling when cast in one piece. 



In order to render the armature ring more rigid, the laminations 

 are arranged to overlap or break joint, as shown in Fig. 62, where the 



FIG. 62. Method of building up Armature Core. 



full radial lines indicate the laminations of one layer, and the dotted 

 lines those of the next. The thickness of the core-plates or lamina- 

 tions is from 0'014 inch to 0*018 inch. 



FIG. 63. Method of supporting Armature Laminations. 



There are various methods of supporting the laminations from 

 the yoke-ring. One of these is illustrated in Fig. 63. As will be 



