18 COMMERCIAL ELECTRICAL TESTING 



saturation curve, depending upon the armature wind- 

 ings, leakage coefficient, galvanometer constant and 

 resistance of the galvanometer circuit. Before taking 

 galvanometer readings at a given field current value, 

 the field current should be reversed one or more times 

 in order to obtain the same magnetic conditions as were 

 obtained in the running saturation curve. Unless this 

 precaution is observed, the saturation curve will not 

 agree closely with the running saturation test of the 

 same machine. 



The running saturation for any machine may be 

 calculated from the machine constants and this test, 

 without further information. Such a calculation is, 

 however, somewhat involved, and uncertain. It is 

 best to take the ratio of the galvanometer deflection 

 for a given field current, to the armature voltage gener- 

 ated when the machine is running at normal speed 

 with the same field current for excitation. This ratio 

 will hold with sufficient accuracy for all practical pur- 

 poses throughout the ballistic test, and hence allows 

 of a ready determination of a running saturation curve 

 being obtained on any machine which is a duplicate 

 as regards armature winding and core, and field struc- 

 ture. This method may be used to great advantage to 

 check up individual standard machines and insure 

 that the magnetic circuit and windings are correct. 

 Similar machines should be tested with the same gal- 

 vanometer and resistance values. 



To obtain the ballistic curve, the armature need 

 only be wedged central with the field bore by wooden 

 or composition wedges to obtain a uniform air gap. 

 Fig. 5 shows the results of a ballistic test and running 

 saturation test, plotted on the same sheet. 



