98 



ELECTRICAL MACHINERY 



and in the same direction with respect to the external circuit. 

 The line e.m.f., obtained by adding the coil e.m.fs., is shown 

 by the broken line of Fig. 59 (6). It is seen that although 

 the line e.m.f. is not yet regular, it never goes to zero value 

 as with the one coil winding (Fig. 55) and that the fluctua- 

 tion is only 50% of the maximum value. 



27T 



37T 



37T 



47T 



ITT 



FIG. 59. Wave Forms for Four-coil Armature. 



E.M.F. Variation in an Ordinary Generator. By con- 

 sidering in the same way an 8-coil armature with an eight- 

 part commutator it will be found that, as the number of 

 coils is increased, the pulsation in the line e.m.f. continually 

 decreases, and, on an ordinary commercial machine having 

 perhaps 20 coils between brushes, the variation is scarcely 

 perceptible and can only be detected by some sensitive 

 instrument like the telephone. Besides getting smaller 

 in magnitude as the number of coils is increased the fluctua- 



