RATINGS AND GUARANTEES. 151 



76. Sparking. One effect of the sparking of a dynamo is to 

 roughen the commutator and cause the brushes to chatter and 

 to wear the commutator rapidly ; another effect is to cause ex- 

 cessive heating of the commutator and brushes. It is not pos- 

 sible to specify precisely the degree of sparking that is permis- 

 sible, nor is there any satisfactory quantitative sparking test 

 to which a finished machine may be subjected. It is for this rea- 

 son that ratings and guarantees are based specifically upon rise 

 of temperature. It may be stated in a general way that a dynamo 

 should not show any visible sparking when its output is changed 

 from zero load to full load, or from full load to zero load, the 

 brushes being left in a fixed position. 



A dynamo will bear a maximum output rating, with a mini- 

 mum cost of materials, when it reaches its heating and sparking 

 limits simultaneously. 



The cause of sparking, and the means adopted by the designer 

 to obviate sparking are discussed in the following articles on 

 armature reaction and commutation. 



77. Armature reaction. The magnetizing action of the current 

 in a dynamo armature is called armature reaction. Armature 

 reaction has a two-fold effect, (a) It causes the magnetic flux 

 which enters the armature from the field magnet poles to be 

 crowded toward the trailing pole tips of a generator, or towards 

 the leading pole tips of a motor, as shown in Figs. 97, 98 and 99. 

 (b) It opposes (generally) the passage of the flux through the 

 armature and thus reduces the amount of flux. 



The part (a) of armature reaction is called cross -magnetizing 

 action, and the part (&) is called demagnetizing action. The cross- 

 magnetizing action has an important influence upon the sparking 

 of a dynamo, and therefore it has to do more or less with the 

 rating of a given machine. The demagnetizing action, inas- 

 much as it reduces the flux through the armature, affects the 

 voltage regulation of a generator and the speed regulation of a 

 motor. 



