THE TEANSFOEMEE. 



175 



(2) After reading the no-load current and losses on A 

 and W l} close the switch S r This will either add to or sub- 

 tract from the primary voltage of transformer T lt the voltage 

 of the secondary of the auxiliary transformer. In either 

 case, the equality of the voltages in the closed circuit formed 

 by the two secondary windings of the test transformers will 

 be destroyed and a current will flow. By adjustment of 

 the resistance R in series with the primary of the auxiliary 

 transformer, vary the current passing through the ammeter A 

 so that about six readings can be taken corresponding to 

 loads varying by nearly equal amounts from light load to 

 slightly above full load. 



300 



250 100 



90 



200 . 80 



L. 70 



150'S" 60 



50 



100 40 



50 



30 



20 



10 





/ 



m 



IRON LOSSES 



I 2 4 6 8 10 12 14 16 

 Load current-amperes. 



FIG. 82. SUMPNER'S TEST OF TRANSFORMER. 



18 20 



The load on the transformer T 2 is the product of the 

 reading of the ammeter A and voltmeter V. The losses in 

 the two transformers are the sum of the watts read on the 

 wattmeters W l and W r 



The wattmeter W^ carries the additional current supplied 

 from the mains in order to supply the transformers with the 

 magnetising and iron loss current. The readings of W 1 are 

 thus the watts spent in overcoming the iron losses. 



