436 ELECTRICAL MACHINERY 



the armature core than the others, resulting in excessive 

 pressure on the bearing and thus tending to produce a hot 

 bearing. 



Development of Unequal Air Gap by Wear of Bearings. 

 This inequality in air gap may not exist when the machine 

 is sent out from the factory but may develop if the bearings 

 are allowed to wear down to an appreciable extent. This 

 allows the armature to drop slightly, shortening the air 

 gap for the magnetic circuits on the lower side of the arma- 

 ture and lengthening it for those above. 



This same difficulty occurs if one field coil becomes 

 short circuited from any cause. The inductors under the 

 pole with the short-circuited coil will generate practically 

 no voltage. 



142. Faults Occurring in Electrical Machinery.* The 

 difficulties which may be encountered in the operation 

 of electrical machinery are tabulated below and an explana- 

 tion as to why they occur and how they may be remedied, 

 according to table on p. 437. 



1. A rough or dirty commutator always produces spark- 

 ing at the brushes. The causes for the roughening of a 

 commutator have been given before. When the roughening 

 exists to a slight extent only, it may be remedied by polish- 

 ing the surface of the commutator with sandpaper. If the 

 machine is a motor, all brushes but one pair should be 

 lifted out of the holders; the one pair left in contact with 

 the commutator will carry enough current to run the motor 

 with no load. 



* In making up the following table the author had in mind prin- 

 cipally continuous current machinery, because faults are so much 

 more likely to develop on c-c. than on a-c. machinery. There is 

 generally no commutator on an a-c. machine and it is much more 

 rugged than a c-c. machine. For this reason trouble is seldom 

 experienced unless a machine is actually burned out or damaged 

 very seriously. In this case the machine would of course be sent 

 back to the factory. 



