SYNCHRONOUS IMPEDANCE 



57 



curves, care should be taken that consistent results are 

 obtained. 



The armature should first be short circuited; then, 

 with the machine running at normal speed and a weak 

 field current, the current in each phase should be read. 

 The field current should be increased gradually until 

 200 per cent normal armature current is reached, 

 readings being taken simultaneously of amperes arma- 

 ture and field, and volts field. 



Fig. 19. Synchronous Impedance Curve on a 500 Kw., 600 Volt, 360 R.P.M. 

 60 Cycle, 3-Phase, A-C. Generator 



Although the speed in this test should be held normal, 

 a small variation therefrom will not affect the curve, 

 because in the formula, current = 



the term R 2 is small compared 



impedance 



with LW 2 , and as E and W vary proportionally to the 



speed, the current remains practically constant. 



On some of the standard machines, a stationary 

 impedance is taken in addition to the synchronous 



