9 6 SYNCHRONOUS ALTERNATORS. [Exp. 



nearly so, on account of high armature reactance). The armature 

 and field ampere-turns on short circuit are, therefore, practically equal 

 and opposite. If they were exactly equal and opposite, there would 

 be no electromotive force generated; as a matter of fact, there is a 

 very small electromotive force equal to the armature RI drop. 



That the armature ampere-turns due to a current lagging 90 

 opposes or weakens (and does not aid or strengthen) the field is 

 verified by this short-circuit test, and its resultant small electromotive 

 force. 



48. A leading current, on the other hand, directly aids and 

 strengthens the field. 



49. In the foregoing discussion of Figs. 9 and 10, the reaction 

 of the armature has been considered for the particular moment and 

 position when the armature current is a maximum. In reality, the 

 armature assumes successively all positions and the current takes all 

 values; in intermediate positions, demagnetization and cross-magneti- 

 zation are both present in varying amounts dependent upon the posi- 

 tion of the armature and the armature current at any instant. The 

 general nature of the reaction, however, may be considered as defined 

 by its character when the current is a maximum. The real effect is 

 a summation of the effects at each instant through a cycle. A more 

 complete discussion would involve some knowledge or assumption as 

 to flux distribution in the pole pieces, and other design factors. 



As a matter of fact, a sinusoidal flux distribution has been assumed 

 in order to make it possible to treat Mo as a vector in Fig. 8; the 

 assumption tacitly made is that the field flux passing through an arma- 

 ture coil varies as a sine function of time, so that the generated elec- 

 tromotive force (<? = d(f>-+-dt) is also a sine function differing in 

 phase by 90. This assumption justifies the treatment of Mo and EQ 

 as vectors at 90. 



But distortion, by its very nature, disturbs the flux distribution and 

 makes the assumption necessarily an impossible one. No diagram 

 using plane vectors can exactly represent all the quantities. The 

 justification of the magnetomotive force method is, therefore, partly 

 empirical. It is found to give fairly good result on many modern 

 alternators in which armature reaction is large as compared with 

 armature reactance and in which too high saturation is not reached; 

 it is least accurate in alternators with high saturation and relatively 

 large armature reactance. 



