442 ELECTRICAL MACHINERY 



happens, it takes a skilful mechanic to get it off; it should 

 not be attempted by the machine operator as the shaft 

 is likely to be spoiled by scratching and scoring in removing 

 the box. 



13. If a commutator is allowed to become too hot, all 

 of the coil connections are likely to be melted loose from 

 the commutator bars. The over-heating of a commutator 

 may be due to excessive brush pressure, sparking at brushes, 

 too high a current density in the brush contact surface, 

 or having improper lubrication. 



If too high a current density is used in the brushes, 

 either larger brushes must be employed or the rating of 

 the machine must be decreased. A commutator often 

 gets hot because of insufficient lubrication at the brush 

 contact surface; most brushes are made self-lubricating 

 but a little machine oil applied to the surface of the com- 

 mutator with a piece of rag often helps to keep the 

 commutator in good condition and stops heating and 

 sparking. All superfluous oil must be carefully removed 

 from the commutator surface by a clean rag or it will 

 soon result in a dirty commutator and, therefore, in 

 sparking. 



14. The armature may become too hot either because 

 of excessive core loss or excessive I 2 R loss. If the first 

 cause exists, the core has been improperly constructed 

 or else the machine is operating at a higher voltage than 

 that for which it was designed. If the armature heats 

 because of a high PR loss, there may be a short circuited 

 coil or else the machine is carrying too much load. In 

 either case the remedy is evident. 



15. Excessive heating of the field coils is likely to occur 

 only if they are forced to carry more current than that 

 for which they were designed. One field coil may become 

 short circuited and thus cut out of the circuit; the rest of 

 the coils will then carry more than the normal current. 

 This fault very seldom occurs. 



