OPERATION AND CARE OF ELECTRIC MACHINERY 429 



can be provided. Even though an electric machine is very 

 well balanced before leaving the factory it is sure to vibrate 

 more or less when running, unless it is -fastened to a solid 

 bed of some sort. If a machine is allowed to vibrate when 

 in operation, there is likely to be sparking at the brush 

 contacts, owing to the fact that the brushes make poor 

 contact with the vibrating commutator; the vibration may 

 also make the bearings heat excessively. 



Effect of Dust and Dirt. Any continuous-current machine 

 is almost sure to develop commutator trouble if it is 

 operated in such a location that dust or dirt can fall on 

 the commutator. The dirt will work into the bearing 

 surface of the brush, producing a rough surface and spoiling 

 the commutator surface by scratching and undue wear. 

 Also it is likely to work into the insulation between the 

 commutator bars and produce a partial short-circuit between 

 adjacent bars. When a partial short-circuit occurs, it soon 

 develops into a complete short-circuit and the coil which 

 is attached to the two segments likely to burn out. 



Faults Tend to get Worse. A point for the operator to 

 keep constantly in mind is that nearly all of the fauks 

 developed by electrical machinery tend to aggravate themselves 

 if not immediately remedied. Thus a slight roughening of 

 the commutator will produce but imperceptible sparking; 

 unless, however, the rough spot is removed (the cause also 

 removed) the commutator will soon be unserviceable. 



If it is absolutely necessary to run a machine in a room 

 where dust accumulates, as, for example, in a cement 

 mill or flour mill, an enclosed type of machine should be 

 installed if possible. This type of machine is completely 

 closed by end pieces being fitted to the sides of the yoke; 

 the bearings are fitted into these end pieces. The armature, 

 commutator, etc. are thus completely shut in so that dirt 

 and dust can collect only on the outside of the machine, 

 where it can do but little harm. (All railway motors, 

 e.g., are of this enclosed type, so that the working parts 



