OPERATION AND CARE OF ELECTRIC MACHINERY 431 



139. Spacing the Brushes. A machine may spark badly 

 at the brushes no matter how well the brushes may have 

 been ground and how smooth the commutator may be. 

 This may be due to the improper spacing of the brushes. 

 Of course many other causes may exist which produce 

 sparking; some of them have been taken up already and 

 they will be all grouped together in the tabulated list of 

 faults, which is given on a following page. 



If a two-pole machine is considered it is seen at once that 

 the brushes should be spaced 180 apart on the commutator; 

 on a four-pole machine they should be 90 apart, etc. On 

 the earlier types of machines, where the interpolar space 

 (space between the adjacent tips of consecutive poles) was 

 large, the necessity for setting the brushes exactly the 

 right distance apart did not exist. Around one side of the 

 commutator of a two-pole machine there might be 49 

 segments between brushes and around the other side per- 

 haps 51 segments and still there might not be excessive 

 sparking. 



Proper Spacing Important on Commutating Pole Ma- 

 chines. On the more modern machines, however, especially 

 on machines having commutating poles, the brushes must 

 be set exactly the right distance apart. On a high voltage 

 synchronous converter, if the spacing between one set of 

 brushes is greater than that between another set by half 

 the width of one commutator segment, bad sparking is 

 likely to result. In addition to the sparking trouble, the 

 armature of a machine on which the brushes are improp- 

 erly spaced is very likely to overheat, even though the 

 machine is not delivering full-load current to the outside 

 circuit. 



Uneven Spacing Produces Sparking. The sparking, 

 with unequally spaced brushes may be due to either of 

 two causes, or both. As we have shown before, the arma- 

 ture winding of any continuous current armature consists of 

 two or more circuits in parallel, the lap wound armature 



