THE DYNAMO. 63 



Fig. 28. In dynamos of earlier design, the shunt and series 

 windings were wound one over the other on a single spool. 

 Present practice favors the separation of these windings for the 

 sake of more perfect ventilation, and on account of greater acces- 

 sibility for repairs. 



34. Details of Commutator. The commutator, the function of 

 which is explained in Art. 28, is always made in the form of a 

 rigid self-contained structure. Forged copper bars, bb, Fig. 53, 

 slightly thinner at one 



edge than at the other, are 

 built up in the form of a 

 hollow cylinder with ac- 

 curately gauged mica 

 plates between the bars. 

 This built up structure is 

 clamped in a massive ring 

 RR, placed in a lathe, and 

 the conical seats ss are 

 turned out. A steel hub is 

 provided with conical lips 

 which fit into the grooves 

 ss, Fig. 53, and the ends 

 of the hub are drawn to- 

 gether by bolts, as shown 

 in Fig. 54. A thin conical 

 washer of moulded mica 



Fig. 55. 



separates the steel hub 



from the copper bars, as shown in Fig. 54. The radial lugs //, 

 one on each commutator bar, or segment, serve for the attachment 

 of the wire leads which are tapped into the armature windings. 

 Fig- 5 5 ' ls a general view of a completed commutator manufactured 

 by the General Electric Company. 



35. Brushes and Brush Holders. Metal brushes were formerly 

 almost exclusively used for dynamos. Such brushes were made 



