114 PRINCIPLES OF ELECTRICAL DESIGN 



been considered in some detail in Chaps. II and III. It might 

 appear, therefore, that little more need be said in connection with 

 the design of a continuous-current generator, but it must be 

 remembered that certain assumptions were made in order that 

 the broad questions of design might not be obscured by too much 

 detail, and in order also that the leading dimensions of the 

 machine might be decided upon. 



It was assumed that the flux in the air gap was uniformly dis- 

 tributed under the pole face; but is it so distributed, and if not, 

 how does this affect the tooth saturation and the ampere-turns 

 required to overcome the reluctance of the teeth and gap? 

 What is the influence of the air-gap flux distribution on arma- 

 ture reaction and voltage regulation, and how can we calculate 

 the field excitation required at different loads in order that the 

 proper terminal voltage may be obtained? These and similar 

 questions cannot be answered without a more thorough study 

 of the magnetic field cut by the conductors, at full load as well 

 as on open circuit. 



Again, with a non-uniform field under the poles, the flux 

 density in the teeth may be much higher than would be indicated 

 by calculations based on a uniform field, and this might lead to 

 excessive heating. 



Perhaps the most important problem in the design of direct- 

 current machines is that of commutation which, so far, has barely 

 been touched upon. It is proposed to devote a whole chapter 

 to the study of commutation phenomena. 



These various matters will be taken up in the following 

 order : First, a study of the flux distribution over the armature 

 surface, and what follows therefrom in relation to tooth densities, 

 regulation, and the excitation required at various loads; next, 

 commutation and the design of commutating poles; and finally, 

 some notes on the design of the field system, with a brief 

 reference to the factors that must be taken into account when 

 calculating the efficiency of a continuous-current generator. 



