58 



COMMERCIAL ELECTRICAL TESTING 



impedance. First block the armature or field, in the 

 case of a revolving field machine, then connect the 

 armature leads to an alternator giving the same fre- 

 quency as that of the machine being tested. Starting 

 with about 50 per cent normal current, the current 

 in the armature of the machine tested is increased by 

 steps to about 150 per cent normal, readings of volts 

 and amperes armature being recorded. 



This method should be followed in taking stationary 

 impedance on induction motors, except that it is only 

 necessary to take one reading at normal current. A 

 special stationary impedance test is sometimes taken 

 on induction motors; this is treated under the heading 

 of induction motors. 



In the calculation of synchronous impedance all read- 

 ings should be correct for the constants of instruments 

 and ratios, and a curve plotted on the same sheet as the 

 saturation curve, amperes or ampere turns field being 

 plotted as abscissae and amperes armature as ordinates. 

 See Table VII and Fig. 19. 



TABLE VII 



Synchronous Impedance on a 500 Kw., 600 Volt, 20-Pole, 

 60 Cycle, 3-Phase Generator 



