ARM AT URE WINDINGS AND SLOT INS ULA TION 89 



commutation then occurs, each brush set collecting current in 

 proportion to the conductance of the brush contact. This leads 

 to sparking troubles unless ample brush surface is provided. 



The fact that there may be only two sets of brushes on a multi- 

 polar dynamo does not necessarily indicate a wave-wound 

 armature. The commutators of lap-wound machines are some- 

 times provided with an internal system of cross-connections 

 whereby all commutator bars of the same potential are joined 



(a) (6) 



FIG. 30. Appearance of lap and wave windings. 



together. This allows of only two sets of brushes being used; 

 but the length of the commutator must, of course, be increased 

 to provide the brush-contact surface necessary for the proper 

 collection of the current. The external appearance of 

 parallel and series windings respectively is indicated by sketc 

 (a) and (6) of Fig. 30. The observer is supposed to be looking 

 down on the cylindrical surface of the finished armature. 



If there are two coil sides in each slot, the number of commu- 

 tator bars will be the same as the number of armature slots, 

 whether the coils are connected to form a multiple or a serie 

 winding. It is, however, by no means necessary to limit the 



