PRACTICAL OPERATION OF DYNAMOS. 



pressure, or to the fact that the ends of the brushes are not worn 

 to the exact shape of the commutator so that the brush makes 

 contact at a few points only. When the commutator becomes 

 foul with grease and dust the ends of the brushes become dirty 

 and the contact between the brushes and the commutator is poor. 



6. A short-circuited coil, or an open-circuited (broken) coil in 

 the armature may cause sparking. This sparking is always 

 located at one point on the armature, namely at the commutator 

 segments in which the defective armature coil terminates. A 

 short-circuited coil is distinguished from an open-circuited coil 

 partly by less violent sparking, and, partly by excessive local 

 heating in the former case. Sometimes a short-circuit or break 

 occurs on account of the displacement of the armature conductors 

 by centrifugal force while the machine is in motion. Such a fault 

 *s often very perplexing inasmuch as it cannot be detected by 

 test on the machine when it is standing still. 



7. " Grounds " in armature winding, that is, accidental elec- 

 trical contact between two or more points of the armature con- 

 ductors and the armature core, will produce sparking. 



8. Weak field excitation. This cause of sparking may be 

 identified in any one of three ways, namely : (a) by the low 

 voltage which is associated with it in a generator, or the high 

 speed in a motor ; (b) by the weakness of the magnet poles as 

 indicated by their attraction for a bit of iron held in the hand 

 (care being taken that the bit of iron is not pulled out of the hand 

 and thrown against the rotating armature) ; (c) by the abnormally 

 large angle of lead of the brushes required to give sparkless run- 

 ning. This defect, weak field excitation, is usually due, in a 

 shunt or compound dynamo, to an excessively high resistance in 

 the shunt field circuit due to a faulty contact either in the field 

 winding itself or in the field rheostat 



9. Chatter of brushes. When the brushes are improperly held 

 in the brush-holders the vibration may cause them to chatter. 



II. Heating of commutator and brushes. This may be due to 

 the following causes : 



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