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ELECTRICAL ENGINEERING 



no-load neutral points. Curve 1, Fig. 138 (6), shows the resultant 

 m.m.f. acting at each point. Its ordinates are represented by M 

 and they are the sum of the corresponding ordinates M f and M a . 

 The m.m.f. across the pole face is no longer constant but is de- 

 creased over one half and increased over the other half by the 

 same amount. The flux density B (curve 2) at each point is still 

 proportional to the m.m.f. and inversely proportional to the 



-Curved), M=M/+Ma 

 -Curve (2). B 



FIG. 138. Distribution of flux and m.m.f. under load, with the brushes on 

 the no-load neutral points. 



reluctance of the path, but, since part of the path is made up of 

 a magnetic material, due to the effect of saturation, the increase 

 of flux in one half of the pole is less than the decrease in the other 

 half and consequently the total flux is decreased. 



If it were not for the effect of saturation the ordinates of curve 

 2, Fig. 138 (b), could be obtained by adding the corresponding or- 

 dinates of Fig. 136 (b) and Fig. 137 (6). 



The neutral points are no longer midway between the poles but 

 have been shifted in the counter-clockwise direction in Fig 138 (a) 

 and to the right in Fig. 138 (6). To prevent sparking the brushes 

 must be moved up to or a little beyond the load neutral points. 

 In a generator the brushes must be moved forward in the direction 

 of rotation and in a motor must be moved backward against the 

 direction of rotation as indicated in Fig. 138. 



With the brushes midway between the poles the direction of 

 the armature m.m.f. is at right angles to the field m.m.fo anfl it 

 therefore does not weaken it but only causes a distortion of the 

 flux and a slight decrease due to saturation. In this case the 

 m.m.f. of the armature is cross magnetizing. 



When, however, the brushes are moved into the fringe of lines 

 at the pole tips, as in Fig. 139 (a), the two m.m.f o*s are no longer at 

 right angles and as seen in Fig. 139 (b) the part of the armature 

 m.m.f. which is subtracted from the field m.m.f. is much greater 



