250 PRINCIPLES OF ELECTRICAL DESIGN 



sible value of the pole pitch; but this limit must sometimes 

 be exceeded, as in the case of steam-turbine-driven machines, 

 in which a pole pitch of 3 to 4 ft. is by no means uncommon. 



The pole arc is a smaller portion of the total pitch than in 

 continuous-current machines; the value of r (i.e., the ratio 



T~fcO I f* 1 T*C* \ 



r) rarely exceeds 0.65. A very common value is 0.6, 

 pole pitch/ 



while it may frequently be as low as 0.55. The reason for 

 the smaller circumferential space occupied by the pole face is 

 partly to avoid excessive magnetic leakage, but mainly to pro- 

 vide a proper distribution of flux over the pole pitch. An at- 

 tempt is usually made to obtain sinusoidal distribution; but the 

 means of obtaining this will be explained later. 



75. Specific Loading. As in the case of D.C. machines, the 

 specific loading, g, is defined as the number of ampere-conductors 



nTJ a TJ a 



per inch of armature periphery; q - = r Ine con- 

 ductors of all phases are counted, and the current considered is 

 the virtual, or r.m.s., value of the armature current. The mag- 

 netizing effect of the armature as a whole will, at any moment, 

 depend upon the instantaneous value of the currents in the indi- 

 vidual conductors, but this matter will be taken up later. 



The following are average values of q, as found in commercial 

 machines : 



Output of A.C. gener- Average value 



ator (k.v.a.) of q 



50 400 



100 430 



200 470 



500 520 



1,000 570 



5,000 625 



10,000 670 



The proper value of q to be used in a given design will depend 

 on several factors. Apart from the fact that its value increases 

 with the size of the machine, it will depend somewhat upon the 

 following factors: 



(a) Number of poles. 



(6) Frequency. 



(c) Voltage. 



(a) Machines with a small number of poles usually ha,ve a 

 small armature diameter and a large pole pitch; calling for a 

 small value of q. In modern steam-turbine-driven generators 



iORY, 



JCE. 





