38 



COMMERCIAL ELECTRICAL TESTING 



loss and reduce the cost of testing, the "feeding back" 

 method is used when possible, especially in the case 

 of large direct current machines and motor-generator 

 sets. In this method the total machine losses are 

 supplied either mechanically or electrically from an 

 external source. In the mechanical loss supply method, 

 two machines of the same size and voltage are belted 

 or direct connected together and driven by a third 

 machine large enough to carry the losses of the set. 

 Connections are made as shown in Fig. 12. If the 

 machines have series fields, these should be connected 

 to boost one another. Both machines should then be 

 started up as generators and thrown together by closing 

 the switch between them when the voltage across this 

 switch is zero. The field of the machine that is to act 



Fig. 12. Connections for Mechanical Loss Supply Pump Back 



as motor should then be weakened, which operation 

 throws load on both machines. The speed is held 

 constant by the loss supply motor. After running at 

 the proper load for the specified time, temperatures 

 should be taken and tests finished according to stand- 

 ard requirements. 



If the machines are motors, the same connections 

 should be made and the machines thrown together as 

 before. The voltage of the system must be held by 

 the machine running as generator. The only correct 

 way of obtaining load is by changing the speed of the 

 set, the brushes having previously been set in the 

 running position. Usually the speed will have to be 

 decreased, and the difference between full load and no 

 load speed will be the normal drop in speed for the 



