70 ELECTRICAL MACHINERY 



an outside turn would be in the neighborhood of 18 inches 

 (if the pole were of rectangular cross-section) so that the 

 average length of a turn would be 14 inches. 



Let E = ihe voltage of the field current supply; 



NI = the total ampere-turns required on the machine 

 NI per pole X number of poles; 

 Z = the average length of one turn, in feet; 

 y = the resistance per foot of the proper sized wire; 



this is to be determined; 

 7 = the current through the field circuit; 

 R = ihe resistance of the field circuit; 

 N = ihe total number of turns on all coils in series. 

 Then, 



R=Nlr,. (21) 



and 



, E E 



I = R = Wr' (22) 



multiplying both sides by N 



N I=-E 

 Nlr lr' 



or 



r= W7 (24) 



Now NI has been calculated, E is known, and I has been 



approximately determined from the assumed size of the 



HU coil. Therefore, r, the resistance per foot, of the 



oper sized wire to use, is determined. From the wire 



B the diameter of the wire and its cross-sectional area 



m circular mils can be obtained. 



Knowing the proper sized wire and the number of 

 required ampere-turns, the proper number of turns is now 



