432 ELEMENTS OF ELECTRICAL ENGINEERING. 



The use of many turns of wire in a winding element increases 

 the tendency to spark at the brushes. 



The conductor of an armature winding is frequently composed 

 of a number of small wires in parallel, instead of one large solid 

 wire, in order that the conductor may be flexible and easily 

 wound. Such a conductor is said to be a stranded conductor. 

 A stranded conductor is electrically a single inductor and is to be 

 counted as such in applying the formula for calculating the elec- 

 tromotive force of a machine. 



50. Number of brushes required. In any closed coil winding 

 whatever, there are p neutral points around the commutator, and 

 p brush sets may be used in any case. 



1. Lap Winding. In the lap winding, simplex or multiplex, 

 / brush sets must be used to permit the delivery of the greatest 

 current output with the least PR loss in the armature windings. 



2. Wave Winding. In the wave winding, simplex or multi- 

 plex, two brush sets, a positive set and a negative set, are neces- 

 sary and sufficient, but any number of sets up to and including 

 p sets may be used. 



The fact that two brush sets are sufficient for the wave wind- 

 ing is shown in Figs. 54 and 55, and is evident from the follow- 

 ing considerations : Suppose first that there are four brush sets, 

 one at each neutral point. The commutator segments I and 6, 

 under the two positive brushes are electrically connected through 

 the two inductors Nos. I and 6, and their end connections. 

 The figure shows that these two inductors are situated be- 

 tween the pole pieces and have no electromotive forces in- 

 duced in them, hence they may be considered simply as cross 

 connections between the two positive neutral points. The nega- 

 tive neutral points are similarly connected by the idle inductors 

 Nos. 5 and 10. In consequence of these cross connections the 

 only function of the second pair of brush sets is to aid the first 

 set in collecting current from and delivering current to the com- 

 mutator segments by the increased contact surfaces of brushes 

 with commutator ; but the distribution of current in the windings 



