234 ELECTRICAL MACHINERY 



and form a three-pnase winding. Probably ninety-five per 

 cent of the alternators in use to-day are three phase 

 machines. The two-phase winding sometimes employs four 

 slip-rings and sometimes three; the three-phase winding 

 always has three slip-rings. 



Development of the Different Types of Winding. The 

 development of these three styles of winding and the mean- 

 ing of the names will now be taken up. Suppose an ele- 

 mentary generator, having only one coil and two slip-rings, 

 as shown in Fig. 138. The e.m.f. wave which such an 

 armature would generate is shown in Fig. 138. It is a 

 simple sine wave. 



/ \18027( 



FIG. 138. E.M.F. Wave Form of Single-phase Generator. 



Now suppose another coil is wound on the same arma- 

 ture, insulated from the first coil and connected to two 

 more slip-rings. This armature is shown in Fig. 139; the 

 four rings are shown one outside the other, whereas really 

 they would be all of the same size placed by the side of 

 each other on the armature shaft. 



The e.m.f. generated by coil 2-2' will evidently be of the 

 same shape and magnitude as that generated by coil 1-1', 

 but its maximum value will occur 90 electrical degrees later 

 in time than the maximum value of that generated in coil 1-1'. 

 The e.m.f. waves of the two coils are shown in Fig. 

 that of coil 1-1' being given by the full-line curve and that 

 of coil 2-2' by the dotted curve. 



This armature would, therefore, feed power to two 

 separate circuits, one connected to the brushes a-a' and 

 the other to the brushes b-b'. The e.m.fs. on these two 



