COMMUTATION 



167 



$d = portion of interpole flux per centimeter length, which 

 enters armature core through root of teeth. 



$ = armature flux per centimeter length, which leaves 

 teeth over the commutating zone of width W a and length 

 l a - l p (Fig. 63). 



The equivalent flux to be cut by conductors under the interpole 

 must equal the total of all the flux components that have to be 

 neutralized. This leads to the equation 



*Ap + &eslp = $e + $a(la ~ lp) + 3>e,(l a ~ lp) 



from which a value for $d can be calculated. The total flux 

 leaving interpole is 



Inserting for <<* in this last equation the value derived from the 

 previous equation, we get 



$c = $,lp + $e+ $a(la ~ lp) + $e,(la ~ lp) $'elp (84) 



This equation may be simplified by expressing to total slot flux 

 $ 8 and the equivalent slot flux &' ea in terms of the equivalent slot 

 flux <i> c ,. The relation between these quantities is obtained by 

 comparing the previously developed equations (79), (80) and 



(77). Thus, 

 and 



Inserting these values in equation (84) we get 



(85) 



wherein the symbols $>, and $0 stand for flux components per unit 

 length of armature core, as previously mentioned. 



Knowing the amount of the flux to be provided by each inter" 

 pole, its cross-section can be decided upon and the necessary 

 exciting ampere-turns calculated, bearing in mind the following 

 requirements : 



(a) The average air-gap density should be low (about 4,000 

 gausses corresponding to full-load current), to allow of increase 

 on overloads. 



(b) The leakage factor should be as small as possible. This 

 involves keeping the width and axial length of interpole small, 



