220 



PRINCIPLES OF ELECTRICAL DESIGN 



flux curve, the value of B g at all points on the armature periphery 

 can be determined. 



The ampere-turns between pole and armature, due to field 

 excitation on open circuit, have at every point on the armature 



B a 



surface the value SI = 



QAw X permeance per square centimeter 



M.M.F. Curves 



Ampere Turns per Pole 



Field M.M.F. (Open Circuit) 



\FieM M.M.F. (Fill) Load) 



\ 



Brush 



Pos tioD 



Ita 



Directio 



UFull Load) 



a of Tra 



do 



\\ 



Coiductors\ '\ 



kj ihg f e d c b a b c d e f ghijk I m 



Reference Points 



FIG. 83. Curves showing distribution of m.m.f. between pole and 



armature. 



The actual figures are given in the following table. 



From these figures the dotted m.rn.f. curve of Fig. 83 has been 

 plotted. Observe now that the m.m.f. represented by 4,860 

 ampere-turns per pole is not sufficient to overcome the reluctance 

 of the teeth, and in order to obtain the required total flux on 



