RATINGS AND GUARANTEES. 



161 



ordinarily warranted on account of its cost. It is, however, an 

 essential feature of the ordinary commutator motor when driven 





Fig. 109. 



by alternating current, as exemplified in the recent development 

 of the single phase alternating current motor for railways. 



78, The phenomena of commutation. Non-sparking condition. 



The cause of sparking, and a clear idea of the means available for 

 reducing sparking, may be obtained by a careful consideration of 

 what takes place in a section of the armature winding and at the 

 two commutator bars to which it is connected, when the two bars 

 pass under a brush. 



Fig. 1 10 shows the positive brush of a generator, a portion of 

 the commutator with its segments or bars c, d, e, f and g t a por- 

 tion of the ring winding of the armature with its sections T, /, V, 

 W and X, and the leading tip of one of the pole pieces. The line 

 nn' represents the neutral axis (see Art. 51) 



Before a given section of the armature winding reaches the 

 positive brush, current is flowing through it towards the brush, as 

 ii 



