178 ELEMENTS OF ELECTRICAL ENGINEERING. 



1. Conduction of heat from another and hotter part of the 

 dynamo. 



2. Sparking. Sometimes a degree of sparking that is scarcely 

 visible may cause overheating of the brushes and commutator. 



3. Arcing or short-circuit between the commutator segments 

 across the mica insulation. This may be due to the collection of 

 metallic particles. The remedy is to clean the commutator thor- 

 oughly. 



4. Imperfect electrical connections between the brushes and the 

 brush holder. This frequently happens with carbon brushes. In 

 any case, carbon brushes heat more than metallic brushes ; this 

 is especially the case if the ends of the carbon brushes do not fit 

 the commutator accurately, thus giving a surface of contact which 

 is far from uniform. Carbon brushes should not be depended 

 upon to carry much more than 40 amperes per square inch of 

 contact surface. 



III. Heating of the armature. This may be due to the fol- 

 lowing causes : 



1. Conduction of heat from another and hotter part of the 

 dynamo. 



2. Overload as explained under cause (i) for sparking. This 

 always causes an overheating of the entire armature. 



3. Poor insulation of armature windings due to moisture. This 

 may cause local heating if the moisture is localized. It is indi- 

 cated by the steaming of the armature. The wet armature should 

 be baked for several hours at about 240 F. in an oven, near a 

 fire, or by passing full-load current through it. In the latter case 

 the armature may be left in the machine and kept in motion, but 

 the field should not be excited. 



4. Short-circuited coils. This produces local heating. See 

 discussion of cause (5) of sparking. 



IV. Heating of field coils. This may be due to : 



1. Conduction of heat to the field coils from another hotter 

 part of the dynamo. 



2. Excessive current in the field coils caused either by an exces- 



