PARTS OF A DYNAMO-ELECTRIC MACHINE 57 



may be 15 volts and at the same time two bars near one 

 set of brushes may have a difference in voltage of only 

 one or two volts. The above formula considers only 

 the average voltage between bars and so gives the min- 

 imum number of bars that should be used. 



Effect of Using too Few Bars. Between adjacent bars 

 of the commutator the mica insulation is generally about 

 0.02" thick and such a thickness of mica will stand a pres- 

 sure of perhaps 10,000 volts so that the limit of 10 volts 

 per bar is evidently not fixed by the dielectric strength 

 of the mica. The difficulty which arises if the voltage 

 per bar gets too high is caused by current leaking over 

 the surface of the mica insulation. The surface of the 

 mica becomes soaked with oil and if this oil carbonizes 

 it becomes a fair conductor. Even if there is no car- 

 bonized oil on the surface of the mica, there will always 

 be more or less dust rubbed into it, and also the whole 

 surface of the commutator may become coated with a 

 thin layer of oil and dust. If the voltage per bar is too 

 great under such conditions, current leaks over the sur- 

 face of the commutator from one brush to the next and 

 a " flash-over " may occur; i.e., a short circuit may occur 

 between a pair of brushes, the short circuit current fol- 

 lowing the surface of the commutator. 



16. Brushes and Brush Holders. As the commutator, 

 to which the armature coils are attached, revolves with 

 the armature, it is necessary to have some stationary 

 conductors for making a rubbing contact with the moving 

 surface of the commutator, the external circuit being 

 connected to these stationary conductors. Such con- 

 ductors are called brushes; they are generally made of 

 carbon blocks, but may sometimes be made of copper leaves, 

 copper gauze, etc. The choice between copper and carbon 

 depends entirely upon the voltage for which the machine 

 is designed. A low voltage machine, such as is used in 

 electroplating, must be equipped with copper brushes; carbon 



