274 METHODS OF CALCULATION 



is negative. It is easy to mark these off on the characteristic. In- 

 versely, the drops due to the reactive current about the point C may be 

 deduced, and thus the coefficient K of direct reaction. It is sufficient, 

 starting from the point C, to trace two segments, CCi, CC 2 , representing 

 the two drops, and to trace the horizontal C^Ti, C^T^ thence the 

 abscissas O/i, O/2, which represent the virtually lost ampere-turns. 

 If the segments CD\, CD 2 are known, which represent the e.m f.'s. 

 of dispersion, and if the horizontal straight lines are drawn through 

 DI and >2, the corresponding abscissas t\ and 1% permit of calcu- 

 lating exactly the back ampere-turns, t\t\, t^', represented by the 

 armature, and which should have equal magnitudes. 



Method No. 2. Applicable to a Single Synchronous Machine 

 Operating upon an Actual Conducting System. When only one 

 alternator is available for the test, it is not possible to proceed so 

 conveniently as in the last case, and, in particular, the plan of testing 

 with variable angles of coupling must be given up. 



A similar test to that which we have indicated above can, how- 

 ever, be made by driving the alternator on open circuit as a syn- 

 chronous motor supplied from the conducting system on which it 

 is to be employed (supposing the factory to have other alternators 

 already installed) or by a current furnished from some other alter- 

 nator of equivalent power. The alternator, or alternators, serving 

 as the source, will then be excited in such a manner as always to 

 maintain the voltage constant at the terminals of the alternator under 

 test which operates as a synchronous motor; this voltage being the 



normal voltage of operation, the excita- 

 tion of the motor is to be varied, as if it 

 were desired to obtain the " V-curve " of 

 constant voltage. The latter gives by 

 its minimum ordinate AB (Fig. 23) the 

 value of the ohmic losses (at current 

 /o), and the indication of the condi- 

 tion of excitation OA corresponding to 

 a power-factor equal to unity (cos0= i) 

 FIG. 23. at least on the hypothesis that the 



effects of harmonics in the e.m.f. 



are inconsiderable. For any other excitation Oa, the strength of the 

 reactive current may be obtained by constructing upon ab a triangle 

 of which the angle at b is given by the wattmeter. The side bd=I w , 

 differs little from BA, that is to say, from the active current on 



