i6o 



ALTERNATING CURRENTS 



86. Experimental Determination of Two 

 Components of Armature Reactance 



The method just explained of finding E g and A r involves the com- 

 plete determination of the " load curve " LP'. Blondel * and Fischer- 

 Hinnen f have, however, indicated methods by means of which E 8 and 

 A r may be found from a single point on LP', involving the taking of 

 a single reading instead of the determination of the entire curve. 



Let in Fig. 118 PBA be the open-circuit curve, P'A giving the 

 value of the e.m.f. which corresponds to the excitation OP', and let us 

 provisionally assume that we know the position of P, where P is that 

 point on the open-circuit curve which corresponds to P' on the load 



' R C E S P' 



FIG. 118. Blondel's Determination of Two Component8 of Armature Reactance. 



curve for a wattless armature current equal to the armature short- 

 circuit current at excitation OP'. If the position of P were actually 

 known, E a and A r could be found at once.J Let I, = short-circuit 

 armature current at excitation OP' ; this may be found from the short- 

 circuit curve. Further, let V = the experimentally determined p.d. 

 corresponding to excitation OP' and any convenient wattless armature 

 current I (this is the single additional reading required for determining 

 E and A r , besides the open and short-circuit curves). Corresponding 

 to a wattless armature current I, the reduction in the ampere-turns 



is P'S = Y . P'R, and the leakage self-inductance voltage drop is 



* EleMrotechnische Zeitsehrift, vol. xxii. p. 474 (1901). 

 t Ibid., vol. xxii. p. 1061 (1901). 



PR HP' 



J E, = =p , and A,. = -~- , by equations (1) and (2) of 85 ; I, being the short- 



1 



circuit current at excitation OP'. 



