422 ELEMENTS OF ELECTRICAL ENGINEERING. 



her of commutator bars for a simple ring winding may be equal 

 to the total number of inductors Z, or the number of bars may 

 be Z\m y where m is any integer which is a factor of Z. 



The necessary relation between the number of inductors and 

 the number of commutator bars in the case of a drum armature 

 may be shown as follows : The drum armature may be looked 

 upon as a ring armature modified by transferring each inside re- 

 turn wire, a, Fig. 5 1 , to the opposite side * of the armature at b, 

 the wire c being extended across one end of the armature, and 

 the wire d across the other end. Now, if both b and the adjacent 

 inductor are commutator bars, or are connected to adjacent com- 

 mutator bars, it will be seen from Fig. 5 1 that both wires c and 

 d will connect across directly from the positive brush to the nega- 

 tive brush and the dynamo armature will be short-circuited. 

 Therefore a drum armature cannot have adjacent inductors con- 

 nected to commutator segments. Every alternate inductor of a 

 drum winding corresponds to an inside return conductor of a ring 

 winding, and, at most, every alternate inductor of a drum winding 

 can be a commutator bar, or be connected to a commutator bar. 

 That is to say, the greatest possible number of commutator bars 

 in case of a drum winding is Z\2, where Z is the number of 

 inductors. The number of commutator bars may of course be 

 less than this. In fact the number of commutator bars in case 

 of a drum winding may be Z\2m where m is any integer, and 

 2m is a factor of Z. 



44, Lap and wave windings. There are two different schemes 

 of connecting the ends of the inductors of a drum armature to 

 form a symmetrical closed coil winding. One of these schemes 

 is called the lap winding and the other is called the wave winding. 

 There is no essential difference between these two schemes in 

 case of a bipolar dynamo ; but in case of a multipolar dynamo 

 the (simplex) lap winding always gives as many paths between 

 the brushes as there are field poles, while the (simplex) wave 



* For a two pole drum armature. 



