80 PRINCIPLES OF ELECTRICAL DESIGN 



they were actually in contact with the conductors on the " geo- 

 metric neutral" line. The armature m.m.f. will always be a 

 maximum at the point where the brushes are placed, because the 

 direction of the current in the conductors changes at this point, 

 producing between the brushes belts of ampere-conductors of 

 opposite magnetizing effect, as indicated in Fig. 25. The broken 

 straight line indicates the distribution of the armature m.m.f. 

 over the surface. Its maximum positive value occurs at A 

 and its maximum negative value at B. These maximum ordi- 

 nates are of the same height, and equal to one-half the ampere- 

 conductors per pole pitch as will be readily understood by inspect- 

 ing the diagram. Thus, whether the machine is bipolar or 

 multipolar, the armature ampere-turns per pole are 



(SI) a = ~ ampere-conductors per pole pitch 



= f 



15 000 

 In practice, a safe limit for the pole pitch is r = - 



If q = 750, the maximum allowable pole pitch is r = 20 in., 

 which dimension is rarely exceeded in ordinary types of dynamos. 

 Number of Poles. The formula (45) gives the output in terms 

 of the peripheral velocity. In its complete form it would be 

 written 



kw. output = l a Dv X B g qr X 4 X lO" 11 (49) 



By eliminating l a and D from the equation, it is possible to 

 arrive at an expression for the number of poles in terms of the 

 peripheral velocity and other quantities for which values can 

 be assumed. Thus, by (46) 



rirD 

 pk 



In order to eliminate D, let (SI) a be the armature ampere- 

 turns per pole. Then 



P q 



whence ^ 2p(SI) a 



-^r 



By substituting these values for l a and D in equation (49) we 

 get 



i -^ vk V x 

 = kw ' X 



