DIRECT-CURRENT MACHINERY 171 



face if the air gap is uniform but falls off rapidly beyond the pole 

 tips due to the increased reluctance of the air path and to the 

 decrease in the m.m,f. acting, Midway between the poles it is 

 zero. It is represented by B and its values are plotted in curve 

 2. The total flux entering the armature is represented by the 

 area under curve 2 and this is the value of 3> which appears in the 

 eoin.f. equation. 



urve (2), B . 



Curve (1), M/ 

 OOOOOOOOOOOOOOOe&OOOOOOOOOOOOOOO 



r" N \\ 01 



(a) 

 FIG. 136. Distribution of flux and m.m.f. at no load. 



When, however, the armature is carrying current it exerts a 

 magnetomotive force, called armature reaction, which combines 

 with the magnetomotive force of the field winding and changes 

 both the distribution and the total value of the flux entering the 

 armature. 



Figo 137 (a) shows the distribution of flux produced by the 

 armature m.m.f. acting alone, and the values of the m.m.f. of the 



Curve (1), Ma 



Curve (2), B 



(a) 



FIG. 137. Armature m.m.f. and flux. 



armature at all points around the circumference are plotted in 

 curve 1, Fig, 137 (6) The brushes are placed on the no-load 

 neutral points. The armature m.niof. M a is a maximum in line 

 with the brushes and falls off as a linear function to zero under 

 the centre of the poles. The distribution of the flux produced by 

 the armature m.m.f. is shown in curve 2, Fig. 137 (6). 



Fig. 138 (a) represents the conditions when the m.m.f.'s of field 

 and armature are acting together and with the brushes still on the 



