174 



ELECTRICAL ENGINEERING 



should not exceed 80 per cent of the field ampere turns per pair of 

 poles. 



Fig. 140 shows the effect of moving the brushes to the centre 

 of the poles. The whole armature m.m.f. is demagnetizing and 



Curve (1), M= M/t- Ma 

 'Curve (2 ), B 



FIG. 140. Distribution of flux and m.m.f. with the brushes under the centres 



of the poles. 



the flux is reduced to a small value. There is no difference of 

 potential between the brushes since the sum of the e.m.f.'s gen- 

 erated in one half of the conductors in series between the brushes 

 is exactly equal and opposite to that generated in the other half. 



FIG. 141. Commutation. 



110. Commutation. Commutation is the most important 

 problem in direct-current machinery. 



Fig. 141 represents the armature winding of a bipolar gen- 

 erator. The current entering by the brush B 2 divides into two 



