COMMUTATION 



165 



The flux actually entering the armature teeth in the com- 

 mutation zone should therefore be equal to the sum of the end 

 flux $ e and the equivalent slot flux S^. It is because this con- 

 clusion is not obvious that it has been deduced from the fore- 

 going arguments. 



Having determined the value of the flux <i> c which must enter 

 the teeth comprised in the commutating zone of width W a , 

 it is evident that the average air-gap density in this zone, to 

 produce perfect commutation, must be 



B c 



W a X la 



(83) 



By referring to the final flux distribution curve, C, obtained by 

 the method outlined in Art. 43, Chap. VII, it may easily be 

 seen whether or not the desired field can be obtained in the 

 fringe of the leading pole tip. If the required field is greater than 

 that obtainable in the interpolar space, commutating poles must 

 be provided, or the machine must be re-designed. If the flux- 

 distribution curves have not been drawn, the calculated density 

 B c required in the commutating zone, as expressed by formula 

 (83), may be compared with the average air-gap density under 

 the main poles. If the required density does not exceed one-half 

 of the average density of the main flux taken over the pole pitch, 

 it will usually be possible to obtain satisfactory commutation in 

 the fringe of the leading pole tip, provided carbon brushes are 

 used. In the event of B c being in excess of this value, interpoles 

 will probably be necessary. 



50. Commutating Interpoles. Assuming the same number of 

 interpoles as there are main poles, and an axial length of interpole 

 equal to that of the main pole, the flux from each interpole 

 which enters the armature teeth included in the commutating 

 zone of width W a is, as before, $ w + $e> 



If, as is usually the case, the interpole face does not cover 

 the whole length of the armature core, then some flux due to the 

 total m.m.f. of the armature windings will enter or leave the 

 armature core by the teeth included in the commutating zone, 

 and this flux will be cut by that portion of the slot conductors 

 which is not covered by the interpole. With the brushes on 

 the geometric neutral line, this armature flux is unaffected by 

 the excitation of the main poles ; its value depends only upon the 

 armature ampere-turns and the reluctance of the air paths 



