178 



ELECTRICAL ENGINEERING 



If the resistance of the coil is taken into account, the drop 

 of potential across r 2 (Fig. 144) must be greater than the drop 



u, 



nn 



nti 



2 



6 



rn 



r l 



Bi + 



FIG. 144. 



across n by the amount required to maintain the current i through 

 the resistance r; therefore, 



T T 



r T^i + ir = ^ Ic ~ ^ Tc T 



and 



r r c (T 2 - 2Tt) 

 c r(Tt-t z )+r c T z ' 



. . . . (205) 



T . 



When t = ;j-, i = 0, and when t = T, i = I c . 



The current therefore passes through zero at the same instant 

 as before and is completely reversed in the same time, but the 

 variation does not follow a straight line law but a curve as shown 

 in curve (2), Fig. 145. The effect of the coil resistance is very- 

 small and may be neglected. 



The effect of the self -inductance of the armature coil must next 

 be considered. Armature coils are partially surrounded by iron 

 and therefore have a large self-inductance, which is proportional 

 to the square of the number of turns in the coil. With full-pitch 

 drum windings both the coils in one slot will be short circuited at 

 one time and the inductive flux linking with each of them will be 



