116 



ELECTRICAL MACHINERY 



Condition for Sparkless Commutation. If the current 

 in B was completely reversed during the short circuit 

 interval (as in the full line of Fig. 69) there would be no 

 change of current in B, as segment g left the brush and 

 hence no counter e.m.f. of self-induction to overcome. 

 In this case there would be no tendency of the current 

 from C to arc over the commutator, as segment g left the 

 brush and there would be no sparking. In fact, com- 

 mutation is always sparkless when the current in the coil 



FIG. 71. Use of Pole Fringe for e.m.f. Commutation. 



undergoing commutation is completely reversed during the 

 time the coil is short circuited by the brush. 



E.M.F. Commutation. E.m.f. commutation is used on 

 nearly all modern machines. With high commutator 

 speeds the resistance variation is never sufficient to reverse 

 completely the current in the coil short circuited, so an 

 e.m.f. is induced in the coil while it is short circuited in 

 such a direction that it assists the current to reverse. This 

 e.m.f. may be induced by the pole fringe or by separate 



