OPERATION AND CAKE OF ELECTRIC MACHINERY 435 



put on the machine, excessive heating will develop at this 

 point. Fig. 284 shows how the sandpaper should be held 

 and also how the brush when so ground fits the commutator 

 over its whole cross-section. While the brush is being 

 ground the sandpaper should be drawn under the brush 

 only in the same direction as the machine is designed to 

 run. 



Use of Emery Cloth. If emery cloth is used instead 

 of sandpaper, two faults are likely to develop. The abrasive 

 material used in making the rough surface of the emery 

 cloth is a partial conductor and small particles of it are 

 likely to become imbedded in the insulation separating the 

 copper bars thus producing a partial short circuit of the 

 commutator. Or small particles of it may become imbedded 

 in the contact surface of the brush and so, when the machine 

 is in operation, may produce severe cutting (roughening) 

 of the commutator surface because the emery grains are 

 extremely hard. 



141. Effect of Unequal Air Gaps. The inequality of 

 voltage in the different paths will also occur even when 

 the brushes are properly spaced, if the air gap under some 

 poles is less than under others. As there is the same mag- 

 netizing force on all field poles (same number of turns 

 and same current) that magnetic circuit having the smaller 

 air gap will have the smaller reluctance and therefore 

 the greater flux. Therefore the inductors lying under 

 those poles having the smaller air gaps will generate a 

 greater e.m.f. than the inductors lying under the poles with 

 a larger air gap. Hence those paths in the armature lying 

 under the poles with the smallest air gap length will generate 

 a greater e.m.f. than the other paths and so current will 

 flow from one path to the other in such an armature, even 

 though no load is being supplied by the machine. 



The inequality in air-gaps is also likely to produce 

 heating in the bearings. That pole under which the mag- 

 netic field is most dense will exert a greater pull on 



