54 



ELECTRICAL MACHINERY 



sized commutator is shown in Fig. 28. The end view, 

 in (6), Fig. 28, shows the trapezoidal form of the bar; the 

 angle between the two sides depends upon how many bars 

 are to be used in the commutator. If there were to be 

 360 bars in the complete commutator this angle would 

 be about 1; if there were to be 720 bars it would be about 

 one-half of a degree, etc. 



Insulation. As the bars are assembled, sheets of insu- 

 lation must be placed between every bar and its neighbor; 

 also on the ends of the bar insulation must be used to 



Wires 



Wires 



Lug for making 

 Connection 



Bearing Surface for Brush 



Lug 



9 



(a) (6) 



FIG. 28. Sketch of a Commutator Bar. 



keep it from contact with the spider, which clamps the 

 bars together. For the insulation between bars a special 

 grade of mica is always used; a grade of mica must be 

 employed which has about the same wearing qualities as the 

 copper bar itself. If the mica should be too tough and 

 wear away more slowly than the copper, the mica insu- 

 lation would soon project above the copper bars and would 

 cause sparking at the commutator. 



The ability of mica to stand high temperature without 

 deterioration makes it preferable to any such insulation 

 as fiber, oiled cambric, etc. These substances are good 

 insulators when kept cool and dry but commutators fre- 

 quently become very hot when the machine is operating 

 and also sparking is likely to occur where the brushes bear 



