DIRECT-CURRENT MACHINERY 



175 



equal parts I c which follow the two paths through the winding 

 and unite again at the brush B\. Any coil c while moving from 

 B 2 to BI carries a current 7 C . After passing BI it carries an equal 

 current I c but in the opposite direction and, therefore, while pass- 

 ing under the brush BI the current changes from I c to 7 C , that 

 is, it is commutated or reversed. 



The factors which affect commutation are, I c the intensity of 

 the current to be commutated, T the time of commutation, r c the 

 resistance of the contact of the brush with the commutator, r the 

 resistance of the armature coil short circuited, L the self-induct- 

 ance of the coil, and finally the direction and intensity of flux cut 

 by the coil during commutation. 



deb 



nnmnn 



FIG. 142. 



The current to be commutated is that carried by each con- 

 ductor of the armature winding. If / is the load current of the 

 machine and pi is the number of paths in parallel through the 

 winding, the current per conductor is 



and increases directly as the load current. 



The time of commutation is the time during which two adjacent 

 commutator bars are short circuited by the brush. In Fig. 142 

 commutation of the current in coil c begins as soon as the brush 



