PARTS OF A DYNAMO-ELECTRIC MACHINE 59 



If the machine is delivering current to the external 

 circuit when this happens, sparking will occur at the com- 

 mutator surface and if this occurs for any length of time 

 the surface of the commutator becomes so roughened 

 as to be unserviceable. In so far as flexibility is concerned, 

 therefore, copper is preferable to carbon. The non-flexi- 

 bility of the carbon brush is partly overcome by using 

 instead of one big brush, several small brushes, each cap- 

 able of movement separately. Then, as one of these small 

 brushes is lifted from the commutator accidentally, no 

 open circuit is produced because the rest of the small 



FIG 30. Five Small Brushes, instead of One Large One, are Here 

 Used to Obtain Flexibility in the Brush as a Whole. Allis- 

 Chalmers Co. 



brushes are still making contact. Fig. 30 illustrates this con- 

 struction; on the one brush holder stud there are mounted 

 five separate brushes, each of which is free to move by itself. 

 Brush Holders. As the brush (whether copper or car- 

 bon) wears away with use, some arrangement must be 

 made to feed continually the brush toward the commutator 

 so that the proper pressure between the brush and the 

 commutator is always maintained. The device by which 

 this is accomplished is called the brush holder. It is a 

 sort of clamp, through which the brush is capable of motion, 

 and is mounted on the brush holder stud. This motion 



