282 METHODS OF CALCULATION 



ture at rest, alternating currents derived from any available external 

 source, and measuring the apparent self-induction by the method 

 of Joubert for the two characteristic positions of the field-magnet 

 poles. The simplest method applicable to a star-wound alternator 

 consists of sending an alternating current 7 through one of the phases 

 which divides between the two other phases, starting at the neutral 

 point, and which is received at its emergence by a connection applied 

 to the two terminals A 2 A^. Fig. 29 represents the connections: a 

 is an auxiliary alternator of which the excitation is adjusted at will 

 in order to vary the current 7. It is easy to see that an alternator 

 thus traversed by a current is placed in the same conditions as if it 

 were fed by three-phase currents at the moment when one of these 

 currents passed through its maximum. This connection being made, 

 the field-magnet is arranged so that the 

 armature poles have their axes directed in 

 a line with those of the field-magnet poles, 

 and then in such a manner that these axes 

 are directed midway between the field- 

 magnet poles. In each case the self-induc- 

 tion developed by the current 7 is measured 

 according to the method of Joubert; the 

 p ratio of the two measures is that of the 



direct self-induction to the transverse self- 



induction, or of the coefficients K and K t applicable to these two 

 reactions. If the difference of potential u is measured between A 



11 



and the center of the star O, the ratio y represents the impedance 



of one phase. From this the inductance may be determined, since 

 the resistance of one phase is known. This measure made in the 

 second position indicated above, gives, therefore, the precise and 

 approximate value of the transverse inductance sought. It must be 

 remembered that this inductance also comprises the inductance of 

 dispersion; therefore has the value 



calling a>\ the transverse reaction properly so called, not including 

 the dispersion a>s. 



Repeating the experiment for various values of the current, the 

 constancy of the coefficient L may be determined. 



