112 



COMMERCIAL ELECTRICAL TESTING 



ratio should next be taken, after which the heat runs 

 may be made. 



A line shunt must be used in each side of the direct 

 current circuit, otherwise one line will have more resist- 

 ance than the other and the currents flowing through 

 them will have unequal values, the unbalanced current 



Fig. 33. Connections for Pumping Back Rotary Converters 

 with Regulator 



returning through the alternating current ends of the 

 machines. The currents in these lines can be balanced 

 by decreasing the resistance in the low reading line. 

 The direct currents should be balanced before attempt- 

 ing to balance the alternating current. 



In running a pump back test there will be a slight 

 difference in the direct current voltages of the two 

 machines, equal to the CR drop of the set. The field 

 of the inverted machine will be less than that required 

 for minimum input and will carry the additional current 

 necessary for supplying the core losses. 



