1 62 ALTERNATING CURRENTS 



LV (corresponding to PP' in Fig. 117), and we have E = -j , 



MV 

 A r = -.* 



87. Potier's Method of Predetermining the 

 Regulation of an Alternator 



Making use of the above results, Potier f determines the p.d. corre- 

 sponding to given excitation, armature current, and external power 

 factor cos as follows. Eothert's method (Fig. 116) is first used to 

 determine the effective excitation, and from the open-circuit curve the 

 e.m.f. corresponding to this excitation is found. From this e.m.f. is 

 next subtracted vectorially the combined drop corresponding to 

 armature resistance and leakage self-inductance, as in Behn-Eschen- 

 burg's method.} This gives a rough value for the p.d., and also for 

 the angle of phase difference 8 between the e.m.f. and the current. 

 If differs appreciably from 0, the entire construction is repeated, 

 using in place of 0, and a more accurate value of the p.d. is obtained. 



Potier's method has been found to give much better results than 

 either Behn-Eschen burg's or Eothert's. This is due to the fact that 

 the variation in the reluctance of the magnetic circuit is taken into 

 account. 



Where accuracy is required, it is advisable to calculate the drop, 

 making use of the diagram only for the purpose of deducing the 

 necessary simple formulae. 



* In order to secure accuracy, the point A must (in either method) be chosen well 

 above the knee of the open-circuit curve, otherwise the intersection (T in Fig. 118, 

 and L in Fig. 119) will take place at a very acute angle, and accuracy will be impossible. 



t Elektrotechnische Zeitschrift, vol. xxii. p. 1061 (1901). 



j Using, however, the leakage self-inductance drop instead of the total self-inductance 

 drop. 



Behn-Eschenburg has recently (Tlie Electrical Engineer, vol. xxxiii. p. 979 (1904)) 

 published a more exact and elaborate method than that originally proposed by him, and 

 an almost identical method was published, practically at the same time, by Torda- 

 Heymann (The Electrician, vol. liii. p. 6 (1904)). This method involves the application 

 of Kirchhoffa well-known network laws to the magnetic circuits of the alternator. As, 

 however, the theory of the method is much more complicated than that of Potier's 

 elegant construction, and as the accuracy of the results does not appear to be any 

 greater, we do not propose to consider this method here in detail. 



A very elaborate and exhaustive investigation of the voltage drop in alternators has 

 recently been published by Messrs. Henderson and Nicholson, Journal of the Institution 

 of Electrical Engineers, vol. xxxiv. p. 465 (1905). 



