CHAP. VIII] REACTION IN SYNCHRONOUS MACHINES 141 



the same as if the armature winding created no leakage fluxes 

 around it, but a separate reactance coil were connected in series 

 with each armature lead. The calculation of the armature react- 

 ance, or of the local fluxes, is treated in Art. 67, the subject of this 

 chapter being armature reaction only, that is, the effect of the load 

 upon the main magnetic circuit. In the numerical problems of t hi.s 

 chapter, for the solution of which it is necessary to know the value 

 of the armature reactance, this value is given. It is not quite 

 correct, strictly speaking, to separate the local distortion of the 

 main flux as a phenomenon by itself; moreover, the separation is 

 somewhat indefinite and arbitrary. However, the flux so separated 

 is comparatively small, and the treatment of the armature reaction 

 proper is thereby greatly simplified. 



The distribution shown in Fig. 36 varies from instant to instant 

 because the relative position of the armature changes with refer- 

 ence to the poles, as well as the value of the armature current. 

 Besides, there are usually two or three armature phases, and sev- 

 eral slots per pole per phase. It would be out of the question to 

 calculate the actual fluxes for each instant and to take into account 

 their true influence upon the e.m.f. induced in the armature. In 

 practice, certain approximate average values of armature reaction 

 and of armature reactance are employed, which permit one to 

 predict the actual performance of a machine with a sufficient 

 accuracy. 



In the case of a synchronous generator (alternator) the problem 

 usually presents itself in the following form : It is required to pre- 

 determine the field ampere-turns necessary for a prescribed ter- 

 minal voltage at a given load. Knowing the resistance and the 

 leakage reactance of the armature, the voltage drop in the arma- 

 ture is added geometrically to the terminal voltage ; this gives the 

 induced voltage in the machine. Knowing from the no-load satura- 

 tion curve the required net excitation at this voltage, and correct- 

 ing it for the effect of the armature reaction, the necessary field 

 ampere-turns are obtained. The results of such calculations for 

 different values of the armature current and for various power 

 factors, plotted as curves, are called the load characteristics of the 

 alternator. 



In the case of a synchronous motor the terminal voltage is usu- 

 ally given, and it is required to determine the field -\< -nation sm-h 

 that, at a given mechanical output, the input to tin- armature be at 



