40 



COMMERCIAL ELECTRICAL TESTING 



When compound wound generators are being tested 

 by this method the series field of the motor must be 

 included or the load will be unstable. 



Another method of "feeding back," often used, is 

 to feed the entire load back on the main supply circuit 

 from which the motor is run that drives the generator 

 under test. If the main supply circuit is likely to vary 

 in voltage, it may be necessary to insert resistances 

 between the generator and supply. It sometimes 

 happens that the no-load voltage of the generator- is 

 below that of the supply. As changing the line resist- 

 ances will have no effect at no-load, the generator 

 voltage must be increased until it is equal to that of 

 the main supply circuit. Having previously calculated 

 the full-load field current from the no-load current 

 and the ratio of compounding voltages, the machines 



Fig. 14. Connections for Induction Motor-Generator Set Pump Back 



are thrown together and full load put on the generator 

 by cutting out the variable resistance. 



Two similar motor-generator sets can be tested very 

 readily by the "feeding back" method. As an illustra- 

 tion, suppose each set consists of an induction motor 

 and a direct current generator. In this case connections 

 are made as in Fig. 14. The alternating current and 

 direct current ends of the sets are respectively connected 

 together, one set being run normally, and the other 

 inverted. The induction generator feeds back on the 

 induction motor, both taking their exciting current 



