60 ELECTKICAL MACHINERY 



of the brush is produced by a spring fastened tightly OD 

 one end to the brush clamp; the other end of the spring 

 presses directly on the brush itself. There may be several 

 brush holders on the same stud. On large machines as 

 many as twelve or more may be used; there are seldom 

 less than two because one carbon brush has no flexibility 

 and sparking is likely to result if only one is used. 



Number of Sets of Brushes. There must be at least 

 two brush holder studs, one at which the current leaves 



FIG. 31. A Common Type of Brush Holder, Showing Brush in Place 

 General Electric Co. 



the machine and another at which it enters. With the 

 type of armature winding commonly employed, as many 

 sets of brushes (i.e., groups of brushes on the same 

 stud) are required as there are field poles. A 12-pole 

 generator would have 12 brush holder studs, mounted rigidly 

 on the brush yoke; the studs would be equally spaced on 

 the yoke and insulated from it; every other stud would 

 be connected together and the two sets of studs so formed 

 could be connected to the two terminals of the generator. 

 Jn Figs. 31 and 32 are shown two types of brush holders, 



