3-B] PREDETERMINATION. 93 



turns (or amperes) give us the value of the magnetomotive force 

 MT', for, in this case, A/z O. 



On short circuit, the field ampere-turns (or amperes) give us the 

 value of Mz; for, in this case, Mx = O. That is, on short circuit 

 the field and armature ampere-turns are (practically) equal and oppo- 

 site (compare 21). 



In Fig. I it is seen that, on short circuit, full-load current (43.4 

 amp.) is given by a magnetomotive force Mz = OG = i2i ampere- 

 turns (2.6 amperes) ; the corresponding impedance voltage, as used 

 in the electromotive force method, is Ez = GF = 234. 



38. Procedure; Any Power Factor. The value of Mz is known, 

 as in the preceding paragraph; also the power factor, cos 0, of the 

 load. 



Given ET to find EQ. Construct the triangle OBA, Fig. 8, from 

 the known values of Mz and cos 0, and the value of MT corresponding 

 to ET; the value of Mo and the corresponding value of EQ is thus 

 determined. 



Given EQ, the converse procedure is followed to obtain ET. 



The most important cases are for unity and zero power factors. 



39. Unity Power Factor. For this case, cos 0= i, and OBA (Fig. 

 8) becomes a right triangle. The same procedure is followed as in 

 the preceding paragraph. 



40. The following procedure, known as the Institute* Method 

 (proposed by a committee but not adopted) differs from the fore- 

 going by taking special account of the armature RI drop. Armature 

 RI drop is significant at unity power factor; it becomes less so as 

 the power factor decreases and becomes negligible at zero power 

 factor. The Institute Rule is : 



"When in synchronous machines the regulation is computed from the 

 terminal voltage and impedance voltage, the exciting ampere-turns corre- 

 sponding to terminal voltage plus armature resistance-drop, and the 

 ampere-turns at short-circuit corresponding to the armature impedance- 

 drop, should be combined vectorially to obtain the resultant ampere-turns, 

 and the corresponding internal e.m.f. should be taken from the saturation 

 curve." 



By the reverse procedure ET is determined when EQ is known. 

 * Rule 71, p. 1087, Vol. XIX. 



