ARMATURE REACTION IN ALTERNATORS. 135 



the field which thread through the armature coil is then a 

 minimum, and the rate of cutting lines of force is a maximum. 



If the current is in phase with the E.M.F., it is evident that 

 the average armature reaction is zero, because the current then 

 attains its maximum value when the coil is midway between two 

 consecutive field-coils, and (the rotation of the armature being 

 clockwise) whatever effect an armature pole, A (see Fig. 45), has 

 on a field pole, S, as it recedes from it, there will be an equal but 

 opposite effect on the consecutive pole, N t on its approach. The 

 same would be true if the difference of phase between E.M.F. and 

 current were 180, as might be the case if the machine were 

 running as a motor. Thus, in the case of a generator, the average 

 armature reaction is zero if the E.M.F. and armature current are 

 in phase, and in the case of a motor, if the counter E.M.F. and 

 current are in exact opposition. 



Now suppose the machine running as a generator and that the 

 current and E.M.F. are not in phase, but that the former leads over 

 the latter by some angle (less than 180). Consider the same poles 

 in Fig. 45. The current in the coil A will now attain its maximum 

 value before it reaches the position midway between S and JN, so 

 that the effect of the current in A on the pole S will be greater 

 in magnitude than its (opposite) effect on N. But the lines of 

 force due to the field current pass through the armature in the 

 direction N to S, as shown by the dotted lines in the figure, and 

 the lines of force due to the armature current are in the direction 

 from armature to field, from A to $; thus the effect of the current 

 in A, as A passes from to N t is to strengthen the pole S, and 

 weaken the pole N t but to a less extent; therefore the average 

 effect of the leading current is to strengthen the field. By similar 

 reasoning we may show that a lagging current weakens the field 

 when the machine is running as a generator. 



If the machine were running as a motor, the armature current 

 at any instant would flow in opposition to the counter E.M.F., so 

 that the tendency of the current in A would be to weaken the 

 pole S and strengthen the pole N, and the average effect of a lead- 

 ing armature current is to weaken the field of a motor. Similarly 

 the average effect of a lagging armature current is to strengthen 

 the field of a motor. 



We have now proved that the field of a generator is 

 Strengthened when the current leads before its E.M.F., 



