158 ELEMENTS OF ELECTRICAL ENGINEERING. 



D- a ZI ~ 



(35) 



in which D is the demagnetizing ampere-turns (per magnetic cir- 

 cuit) of a dynamo armature, a is the angle of forward lead of the 

 brushes, Z is the total number of armature conductors, / is the 

 total armature current, and /' is the number of paths into which 

 the current I a divides in flowing through the armature. Equation 

 (35) is true whatever the number of field magnet poles may be. 



The cross-magnetizing action of the 

 armature is represented in the above dis- 

 cussion as being due to all the armature 

 conductors which lie at a greater distance 

 than a from the lines AB. It is 

 evident, however, that the conductors 

 which are beyond the pole tips cannot 

 contribute to the magnetic flux shown 

 under the pole tips in Fig. 103. In fact 

 the cross-magnetizing action of the arma- 

 ture depends only upon the number of armature conductors 

 which lie under a pole face as shown in Fig. 107. 



The intensity cV, of the magnetic field under the pole tips in 

 Fig. 103 due to the armature alone, may be quite easily calcu- 

 lated as follows : Consider the magnetic circuit CC, Fig. 107, 

 which encircles all of the conductors under a pole face. The 

 number of these conductors is Z x ^360, and the current in each 

 conductor is /,//', so that the magnetomotive force around CC 

 is Zx /3/$6o x I a lp'* ampere-turns, or 47r/io x Zx /3/36o x 

 IJp' c.g.s. units, f Now the iron of the pole face and of the arma- 

 ture core may be considered to have zero magnetic reluctance as 

 compared with the air in the gap space, so that approximately 



* This gives the cross-magnetizing ampere-turns (per magnet pole) of a dynamo 

 armature. 



f See Appendix A for a full discussion of magnetomotive force and of the magneti- 

 zation of iron. 



Fig. 107. 



