CHAP. IX] ARMATURE REACTION IN D.C. MACHINES 165 



winding the currents within each D belt flow partly in the opposite 

 directions and neutralize each other's action. 



Let the specific electric loading of the machine, as defined 

 above, be (AC). Then, with a full-pitch winding, the demagnetiz- 

 ing ampere-turns per pole are 1 



(94) 



The belts T e T e constitute together a coil the center of which is 

 along the axis O e 9 '\ the adjacent belts T W T W form a coil with its 

 axis along W W '. The m.m.f. distribution of these coils is indi- 

 cated by the broken line ABC, which shows that the T belts pro- 

 duce a transverse armature reaction. The line ABC is also the 

 curve of the flux density distribution which would be produced by 

 the transversal reaction alone, if the active layer of the machine 

 were the same throughout (non-salient poles). On account of a 

 much higher reluctance of the paths in the interpolar regions the 

 flux density there is much lower, and is shown by the dotted lines. 

 The actual distribution of the field in a loaded machine is obtained 

 considering from point to point the field and armature m.m.fs. 

 acting upon the individual magnetic paths. 



The transverse reaction opposes the field m.m.f. under one-half 

 of each pole and assists it under the other half, so that the main 

 field is distorted. In a generator the field is shifted in the direc- 

 tion of rotation, in a motor it is crowded against the direction of 

 rotation of the armature. This is the same as what happens in 

 synchronous machines, when the armature is revolving and the 

 poles stationary (see Fig. 36). 



The brushes must be shifted in the sanio direction in which the 

 flux is shifted, because the magnetic neutral is displaced with 

 respect to the geometric neutral. Usually, the brushes are shifted 

 ad t he magnet ic IK ut ral, in order to obtain a proper flux den- 

 si ty for commutation. Namely, to assist the reversal of the cur- 

 rent in the conductors which are short-circuited by the brushes, 

 these conductors must be brought into the fringe of a field of such 

 a direction as assists the comnmtaiinn. In the case of a gener- 

 ator this means the field under the influence of which the conduc- 



1 The effect of the coils short-circuited under the brushes is not considered 

 separately, for the sake of simplicity. For an analysis of the reaction of the 

 short-circuited coils upon the field see E. Arnold, DM GMckitnmma9cki* t 

 1 (1006), Chap. 23. 



