DIRECT-CURRENT MACHINERY 147 



mutated is not generating any e.m.f., the e.m.f . ; s generated in all 

 conductors under one pole will act in the same direction and will 

 combine to give the terminal e.m.f. of the generator. The e.m.f. 's 

 generated under the other pole will be equal in magnitude but will 

 act in the opposite direction. Thus, there is no e.m.f. tending to 

 cause current to circulate through the winding at no load and 

 there are two paths in multiple for the current flowing through 

 the armature. 



The connection from one conductor to the next is run through 

 inside the armature, where it cannot cut magnetic flux, and Con- 

 sequently one half of the winding is not effective in generating 

 e.m.f. This extra wire increases the resistance of the armature 

 and adds to the weight and cost of the machine. The ring wind- 

 ing has the further disadvantage, that it is very difficult to replace 

 injured coils. On the other hand, the voltage between adjacent 

 coils is so low that very little insulation is required between them. 

 This type of winding is now obsolete. 



Fig. Ill shows a six-pole ring winding with 36 coils connected 

 to a commutator with 36 bars. This winding must be tapped at 



Six-pole ring winding. 



six equidistant points by brushes; there are six paths in parallel 

 through the armature from positive to negative terminals and the 

 voltage of the machine is that generated in one sixth of the winding. 



94. Drum Winding. In drum-wound machines the whole of 

 the armature winding is carried in slots on the outside of the arma- 

 ture core and both sides of any coil are effective in generating 

 electromotive force. The single coils are of the shapes shown in 

 Fig. 112 and may consist of one or more turns. 



The conductors forming the two sides of a coil must be situated 



