172 



PRINCIPLES OF ELECTRICAL DESIGN 



In regard to the slot flux, at the beginning or end of commuta- 

 tion (depending upon the position of the coil in the slot) the con- 

 ductors in the slot affected are not all carrying the full armature 

 current. This will be seen by reference to Fig. 65, where coil 

 A (consisting of two turns) is just about to be short-circuited by 

 the brush. At this instant the current in the coil-sides B and D 



is i = I c X y-~H = 0.428/c as indicated by the diagram sketched 

 on the brush of width W covering three and one-half bars, the 



/-4- 

 i 1 



i r 



I ! 



^=.-=, ^ 



K - w M 



FIG. 65. Illustrating example of interpole design. 



straight-line law of current variation being assumed. At the 

 end of commutation, when the position of the affected slot rela- 

 tively to brush and interpole is as indicated by the dotted lines, 

 the current in all four coil-sides is 7 C . We may, therefore, 

 assume the slot flux to be produced by a current of which the 



3 -f- 428 

 average value is not I c =- 76 amp., but I c X -^- - = 65 amp. 



The slot flux $>e, in formula (85), which is the "equivalent" value 



