ADDITIONS TO THE THEORY. SECOND APPLICATION 101 



fiih OA 2 a known angle, d. Since we assume the values of I d , 



2 



can calculate the values zl d and =. KN'I d . (As before z represents 



V2 



ie impedance of the armature only). 



Let us draw, on A 2 W, a segment A 2 A = zI d . The point A must 

 fall on a right line, A A 1} which passes through the point A and which 

 is parallel to A 2 D. Again, knowing 2, the excitation-curve N gives 

 ie excitation ampere-turns F. Subtracting \KN'I d , we have the 

 jsultant ampere-turns, and consequently, the effective E.M.F. 2. 

 et this be drawn on A 2 X, to A 2 O. The load-point A\ must fall on 

 circle described around O as a center with E\ as radius. It is 

 therefore determined by the intersection of that circle with the right 

 line AA\ previously drawn. 



We thus obtain the value of z! w =BD, and, consequently, the 

 irrent I w . From this we deduce the electric power applied to the 

 motor, 



L'I w Xl d - 



On the other hand, /= \/Id 2 +Iw 2 - We therefore obtain, by points, 

 ie relations between 7 and P^ both expressed as a function of 2 

 /hich serves as an intermediary variable. 



V-Curves. The diagram does not lend itself to the direct determina- 

 tion of the V-curves for constant power, owing to the complicated 

 expression, which is not explicit as a function of E% and I w , as it was in 

 the elementary theory. 



But these curves can be deduced from the curves of constant excita- 

 tion, when these are drawn by the process indicated in the preceding 

 paragraph. 



Influence of Field-Saturation on Stability. The stability is evidently 

 greater the higher the E.M.F. of the motor at the limit of lag. Now, for 

 the same reactive current, the loss of potential produced by the direct 

 reactance is all the less the higher the degree of excitation above the 

 knee of the characteristic. It would therefore be advantageous, from 

 the standpoint of stability alone, to have the fields saturated. But 

 lere is a limit, here, owing to the amount of energy required for excita- 

 tion, and the desirability of a certain margin to enable the voltage to 

 be raised. 



The cut-and-try process involved in finding the co-ordinates of 

 the V-curves may be obviated, without constructing any curves, by 



