VOLTAGE REGULATION OF THE ALTERNATOR. 137 



(ft) For power factor equal to 0.85. From Fig. 1 20 the two 

 components of the total induced electromotive force, parallel to 

 and perpendicular to the current, are 1723.3 volts and 2207 

 volts respectively. The field currents required to produce total 

 induced voltages of 1723.3 and 2207 are found from Fig. 117 

 to be 68 amperes and 94 amperes respectively, and to give these 

 two quadrature electromotive forces simultaneously would require 

 116 amperes (= 1/68 2 + 94 2 ). The zero-load voltage corre- 

 sponding to 1 1 6 amperes of field current is found from Fig. 117 

 to be 2400 volts. Therefore the regulation of the alternator is 

 (2400 2000) / 2000 = 0.20 or 20 per cent. 



Comparison of the two methods for calculating voltage regula- 

 tion. The two methods for calculating voltage regulation are 

 based on assumptions that depart very widely indeed from the 

 conditions which actually exist in an alternator (compare Figs. 

 1130, 1140 and 115^, with Figs. 113^, 114^ and 115^) and a 

 theoretical comparison of the two methods would depend upon a 

 complete analysis of the actual conditions represented in Figs. 

 113^, 1 140 and 1150. The only satisfactory way to compare 

 the two methods is to compare the results of calculations 

 with actually observed values of voltage regulation. The elec- 

 tromotive force method generally gives a larger (poorer) regula- 

 tion than is found by actual test, and the magnetomotive force 

 method generally gives a smaller (better) regulation than is 

 found by actual test. The former method has therefore been 

 called the "pessimistic method" and the latter the "optimistic 

 method." * 



65. Armature demagnetizing action. The discussion of the 

 magnetizing action of the armature current, or currents, of an 

 alternator which is given in Arts. 63 and 64 covers the ground 

 as completely as it can be covered without entering into the theory 



* See " The Experimental Basis for the Theory of Regulation of Alternators," by 

 B. A. Behrend, Transactions of the American Institute of Electrical Engineers, 

 Vol. 21, pages 497-517, 1903; and "The Regulation of Alternators," by D. B. 

 Rushmore, Transactions of the International Electrical Congress, Vol. I., pages 

 729-761, St. Louis, 1904. 



