230 THE MAGNETIC CIRCUIT URT. 67 



Uni-coil windings in open slots .... 3 to 6 perms per cm. 

 Thoroughly distributed windings in 



open slots 1.5 to 3 " 



Uni-coil windings in completely 



closed slots 7 to 14 " " 



Thoroughly distributed windings in 



completely closed slots 3 to 6 " " 



The much larger values of <p/ for closed slots, as compared 

 to those with open slots, were to be expected because the bridge 

 which closes the slot offers a path of high permeance. The 

 lower values for windings distributed in several slots per pole 

 per phase, as compared to uni-slot windings, are due to the 

 fact that the partial linkages become more and more pronounced 

 as the winding is distributed into a larger number of separate 

 coils, and also because the length of the paths is greater. This 

 is somewhat analogous to splitting a transmission line into two 

 or more lines; see prob. 21 in Art. 61. The greatest reduction 

 in the value of the inductance results when the number of slots 

 is increased from one to two; a further subdivision is of much 

 less importance. For instance, if the permeance (Pi with a 

 uni-slot coil is 7, then dividing the same coil into two slots 

 reduces the permeance to less than 5. On the other hand, a 

 change from four to five slots per phase per pole would hardly 

 reduce the equivalent permeance more than from say 3.5 to 3.4. 

 The data in the table above give rather a wide range from which 

 to select a value of (Pj for a particular machine, and the designer 

 must exercise his judgment as to whether his machine will have 

 a permeance nearer the upper or lower limit. This judgment 

 comes with experience, by comparing the predicted performance 

 of machines with that actually observed. 



The values of <P a ' and (P e f depend upon the number of coils 

 per group, in other words, upon the number of slots per pole 

 per phase. Until more accurate and detailed data are available, 



he considers separately the equivalent permeance (P 8 of each slot, instead 

 of the group of slots per pole per phase. Thus, his formula for the leakag 6 

 inductance per pole, with our notation, is L pp = Sp P C 8 2 (Pg, where S pp is 

 the number of slots per pole per phase, and C 8 is the number of armature 

 conductors per slot. The values of unit permeance, which he gives and denotes 

 by A, refer to this formula. 



