1 76 ELEMENTS OF ELECTRICAL ENGINEERING. 



Sparking and noises are, of course, readily perceived. The 

 overheating of a dynamo may be most easily detected by apply- 

 ing the hand to the various parts of the machine, or by holding 

 the hand in the stream of air that is thrown off from the rotating 

 armature. In general a temperature that cannot be borne by the 

 hand is to be considered excessive, although carbon brushes and 

 a commutator in which there is no solder may be safely run at a 

 higher temperature than can be borne by the hand. Of course, 

 the hand can bear a much higher temperature in a non-conducting 

 substance like wood or varnish than it can in a conducting sub- 

 stance like metal, so that a little experience is necessary to enable 

 one to judge whether the temperature of any part of a dynamo 

 is excessive. An odor of burning varnish is indicative of serious 

 overheating, and a machine which shows this symptom should be 

 stopped at once. Never use water or ice to cool a dynamo. 



I. Sparking at the commutator. This may be due to the fol- 

 lowing causes : 



1. Excessive current on account of a simple overload on a 

 motor or generator, or to a short-circuit on the supply mains 

 leading out from a generator. 



2. Brushes not in proper position. (See Art. 78.) 



3. Commutator surface rough or uneven causing brushes to 

 jump. This defect may be remedied by using a fine file on the 

 commutator while the armature is in motion and the field unex- 

 cited, or by using fine sand-paper applied by a block of wood 

 which is hollowed out to fit the commutator ; care must be taken 

 to remove sand and copper dust when the job is finished. Never 

 use emery paper or emery cloth on a commutator. 



If the commutator surface is very uneven, the armature, if not 

 too large, must be removed, placed in a lathe, and the commu- 

 tator turned true and smooth, making one or more fine cuts with 

 a diamond-pointed tool. 



4. Very poor balance of the armature may also cause sufficient 

 vibration to make the brushes jump. 



5. Brush contact poor. This maybe due to insufficient brush 



