120 



ELECTRICAL MACHINERY 



removed to permit a clear view of the field construction. 

 The commutating poles are made very narrow; generally 

 they reach over only two teeth (and the included slot) of 

 the armature core. 



22. Armature Reaction. When the armature of a 

 dynamo electric machine is carrying current, the armature 

 acts like an electro-magnet and tends to distort the mag- 

 netic field produced by the field coils. This action of the 

 armature windings is called armature reaction. This 

 action may be a distorting one only, in which case the mag- 

 netic field of the machine is not directly strengthened or 

 weakened by the armature reaction, but is merely twisted 



FIG. 74. Magnetic Field Produced by Field Coils Alone. 



out of its normal path. In other cases the armature m.m.f. 

 may not only distort the main field but may either magnetize 

 (strengthen) it or demagnetize (weaken) it. 



Armature Reaction Components. The distorting action 

 is called the cross magnetizing action of the armature and 

 that component of the armature's m.m.f. which directly 

 strengthens or weakens the main field is called its magnet- 

 izing or demagnetizing action, as the case may be. This 

 effect of the armature windings is shown by figures 74, 

 75 and 76. Fig. 74 shows the field produced by the main 

 field coils, called the main field; Fig. 75 shows the field 

 produced by the armature windings when they are carry- 



