DIRECT-CURRENT MACHINERY 



179 



almost twice as large as in the case of fractional pitch windings. 

 This is partly a mutual inductance effect but may all be included 

 under self -inductance. (See Fig. 146.) 



lc 



Time of Commutation^ 



Curve (1) 

 Curve (2) 

 Curve (3) 



T f 



Time 



^l 



-I 



( 1 ) Commutation by contact resistance only 

 (.2) Effect of the resistance of the cofl 

 (3) Effect of the inductance of the cofl 



FIG. 145. 



When the current in a coil of self -inductance L henrys is changing 

 at the rate -=- amperes per second, an e.m.f. L -r- volts is gen- 

 erated in a direction opposing the change of current. 



1,1,1 



UB 2 - 

 FIG. 146. Inductive flux in a full-pitch drum winding. 



In Fig. 144 the drop of potential from bar 2 to the brush is the 

 same by the two paths, one through the resistance r 2 and the other 

 through the coil in series with the resistance ri; and thus 



