144 



PRINCIPLES OF ELECTRICAL DESIGN 



the flux distribution over the armature surface, including the 

 interpolar space, was outlined in Chap. VII (refer to Art. 41, 

 42 and 43), and the flux-distribution curves of Fig. 54 might have 

 been obtained in the same manner as the flux curve C referred to 

 in Art. 43. The coil of Fig. 54 is supposed to be moving from 

 left to right, and measurements on the horizontal axis XX may 

 represent either distance travelled or lapse of time, since the arma- 

 ture is revolving at a uniform speed. The case considered is 

 that of a dynamo without commutating poles, with brushes 

 moved forward from the geometric neutral or no-load com- 

 mutation position until a neutral commutating zone is again 



FIG. 54. Diagram showing ideal armature coil in commutating zone. 



found. The flux curves as drawn are the result of the combined 

 m.m.fs. of field coils and armature windings. During the time 

 of commutation, t c) which, if we neglect the effect of mica thick- 

 ness, is the time taken by a point on the commutator to pass 

 under the brush of width. W, the conductors on the right-hand 

 side of the short-circuited coil have been moved through the 

 neutral zone from a weak field of positive polarity into a weak 

 field of negative polarity, while the conductors on the left-hand 

 side of the coil have moved from a weak field of negative polarity 

 into a weak field of positive polarity. Owing to the symmetry 

 of the fields under the poles of opposite kind (i.e., the similarity 

 in shape and equality in magnitude of the shaded flux curves), 

 and the fact that the small portions of the flux curves near the 





