DESIGN OF ALTERNATORS 249 



the armature m.m.f. overpowering the field excitation; but this 

 leads to increased cost. 



The pole pitch, r, is a function of the peripheral speed and 

 the frequency, thus: 



P 



where p = number of poles, and D = diameter of armature 

 (inches) . But 



irDN 

 V= 12" 

 and 



120 



where v = peripheral velocity of armature in feet per minute. 

 It follows that 



r = (95) 



which explains why the pole pitch is always large in steam- 

 turbine-driven alternators. 



In 60-cycle machines running at moderate speeds, the pole 

 pitch usually lies between 6 and 12 in., a pitch of 8 to 10 in. 

 being very common. In 25-cycle generators, the pole pitch might 

 measure from 10 to 20 in. The speed of the machine which is 

 usually a factor in determining the peripheral velocity has 

 an appreciable influence upon the choice of pole pitch; pole cores 

 of approximately circular or square section are not always feasible 

 or economical, and the designer must make some sort of a com- 

 promise to get the best proportions. The output formula, as 

 used for determining the proportions of dynamos, is not so 

 readily applicable to the design of alternators, because the arma- 

 ture diameter in A.C. machines will be settled largely by con- 

 siderations of peripheral velocity and pole pitch, the propor- 

 tions of the pole face being a secondary matter. The value of 



k, or ratio -rr, is therefore determined largely by 



armature length' 



the limits of peripheral velocity, which may lead to a smaller 

 diameter and greater axial length than would be strictly eco- 

 nomical if the weight of copper in the field coils were the only 

 consideration. 



It is usual, when possible, to limit the armature ampere-turns 

 per pole to 10,000, which will determine the maximum permis- 



