REGULATION OF ALTERNATORS 



307 



signer to predetermine with but little error the regulation on zero 

 power factor provided he can correctly calculate the reactances 

 required for the vector quantities of Fig. 126. The complete 

 load characteristic O'R is quickly obtained by sliding the triangle 

 MNR along the corrected no-load saturation curve. 1 The dif- 

 ference of pressure, SR, corresponding to any particular value 

 OR' of field excitation (Fig. 125) is called the synchronous react- 

 ance drop because, although it is made up partly of real reactance 

 drop and partly of armature reaction, it may conveniently be 

 treated as if it were due to an equivalent or fictitious reactance 

 capable of producing the same total loss of pressure if the magneto- 

 motive force of the armature had no demagnetizing or distortional 

 effect. Thus, by producing the line PE' g to E in Fig. 126, so 

 that PE is equal to RS of Fig. 125, the vector diagram shows the 

 difference between the open-circuit pressure OE and the terminal 



FIG. 126. Vector diagram for zero power factor. 



pressure OE t under load conditions at zero power factor when the 

 field excitation is maintained constant. The additional (ficti- 

 tious) reactance drop E E' g is correctly drawn at right angles to 

 the current vector because on zero power factor the effect of the 

 armature magnetomotive force is wholly demagnetizing; in other 

 words, it tends to set up a magnetic field displaced exactly 90 

 degrees (electrical space) behind the current producing it; hence 

 when the load is thrown off, the balancing m.m.f. component on 

 the field poles will generate the additional voltage in the phase 

 OE . It should be realized -that the fictitious reactance drop, 

 E E' of Fig. 126, cannot be predetermined until the whole of the 



f S. H. MORTENSEN. "Regulation of Definite Pole Alternators." Trans, 

 A. I. E. E., vol. 32, p. 789, 1913. Also B. A. BEHREND. "The Experimental 

 Basis for the Theory of the Regulation of Alternators." Trans. A. I. E. E., 

 vol. 21, p. 497, 1903; and B. T. McCoRMiCK in discussion on "A Con- 

 tribution to the Theory of the Regulation of Alternators" (H. M. HOBART 

 and F. PUNQA). Ibid^ vol. 23, p. 330, 1904- 



