DIRECT-CURRENT MACHINERY 



173 



than the part which is added to it and therefore the resultant 

 m.m.f. is reduced and the flux is both distorted and decreased. 



Referring to Fig. 139 (a) the armature conductors may be sep- 

 arated into two groups; namely those between a and d included in 

 the double angle of advance a with their return conductors from c 

 to b and those under the pole between b and a with their return con- 

 ductors between d and c. The first group acts in direct opposition 



3urve CO, M= M/+ Ma 

 -Curve (.2), B 



FIG. 139. Distribution of flux and m.m.f. under load, with the brushes under 



the pole tips. 



to the field m.m.f. and decreases the flux crossing the air gap. 

 They are therefore called the demagnetizing ampere turns of the 

 armature. This demagnetizing m.m.f. increases as the shift of the 

 brushes is increased and it also increases directly with the armature 

 current. 



The second group exerts a m.m.f. at right angles to the field 

 m.m.f. and distorts the flux as in Fig. 137 (a) but only causes a 

 slight decrease due to saturation. They are called the cross-mag- 

 netizing ampere turns of the armature. 



For sparkless commutation without the use of interpoles the 

 brushes must be moved ahead of the neutral points in order that 

 the coils short circuited by them may be cutting the fringe of flux 

 at the pole tips. E.m.f.'s are thus generated in the coils opposing 

 the back e.m.f.'s due to inductance, and they aid in reversing the 

 current. As the armature current is increased a point is finally 

 reached where the armature m.m.f. is so strong that it over- 

 balances the field m.m.f. at the pole tips and therefore no reversing 

 field is left and commutation is not possible without interpoles. 

 It is of no use to move the brushes further ahead because that only 

 increases the demagnetizing component of armature m.m.f. and 

 decreases the flux more. In direct-current machines without 

 interpoles the armature ampere turns per pair of poles at full load 



