CHAP. IX] ARMATURE REACTION IN D.C. MACHINES 167 



the necessary induced voltage. In a generator the induced voltage 

 is equal to the specified terminal voltage plus the internal ir drop 

 in the machine. In a motor the induced voltage is less than the 

 line voltage by the amount of the internal voltage drop. The 

 actual ampere-turns to be provided on the field poles are larger 

 than the net excitation by the amount necessary for the compensa- 

 tion of the armature reaction. 



The direct reaction is compensated for by adding to each field 

 coil the ampere-turns given by eq. (94). For instance, in 

 prob. 3 above, 3470 ampere-turns per pole must be added to the 

 required net excitation, in order to compensate for the effect of the 

 direct armature reaction. The necessary shift of the brushes is 

 only roughly estimated from one's experience with previously 

 built machines, though it could be determined more accurately 

 from the distribution of flux density in the pole-tip fringe. The 

 poles usually cover not over 70 per cent of the armature periphery, 

 so that the distance between the geometric neutral and the pole- 

 tip is about 15 per cent of the pole pitch. In preliminary esti- 

 mates, the brush shift may be taken to be about 10 per cent of the 

 pole pitch; this brings the brushes not quite to the pole-tips 

 though well within their fringe. In actual operation a smaller 

 shift may be expected. In machines provided with commutating 

 poles, and in motors intended for rotation in both directions, the 

 brushes are usually in the geometric neutral, so that the demag- 

 netizing action is zero. 



In a machine with a low saturation in the teeth and in the pole- 

 tips, the cross-magnetizing m.m.f. of the armature does not affect 

 the magnitude of the total flux per pole, because the flux is 

 increased on one-half of the pole as much as it is reduced on the 

 other half. It is shown in Art. 31 that the induced e.in.f. of a 

 direct-current machine is independent of the flux distribution, 

 provided that the total flux is the same, so that no extra field 

 ampere-turns are necessary in such a machine to compensate for 

 the distortion of the flux. 



However, the teeth and the pole-tips are usually saturated to 

 a considerable extent, so that the flux is increased on one side of 

 the pole less than it is reduced on the other side. Thus, the useful 

 flux is not only distorted by the transverse armature reaction. I nit 

 is also weakened. This latter effect has to be counterbalanced by 

 additional ampere-turns on the field poles. In most cases these 



