METHODS OF TESTING TRANSFORMERS 121 



The ammeter and voltmeter in the secondary circuit might, 

 of course, be replaced by a wattmeter if desired ; and any system 

 of instruments which would give the true input of the transformer 

 might replace the wattmeter in the primary circuit. 



74. Second Method. In the first method it is assumed 

 that an alternator is at hand which is capable of supplying power 

 to the transformer when working on full-load. If the transformer 

 is a large one, and if the alternators at disposal are of insufficient 

 capacity to supply the transformer at full-load, other methods of 

 determining the efficiency must be adopted. 



The output of the transformer at any load is the difference 

 between the input and the losses. These losses consist of (1) 

 hysteresis loss, (2) eddy-current loss, and (3) copper (iV) losses in 

 both primary and secondary circuits. The iron losses are practi- 

 cally the input of the transformer on open secondary circuit ; and 

 the copper losses can be calculated for any assumed load, and 

 hence the efficiency corresponding to that load may be determined. 



Let W be the iron losses. 



?'i the primary resistance. 



r>2 the secondary internal resistance. 



i\ the primary current corresponding to the assumed 



output. 



^2 the corresponding secondary output. 

 w the assumed output in watts. 

 r\ the efficiency. 



Then 



__ output 

 ~ output + losses 



The copper losses on open secondary circuit are so small that 

 they may be neglected. We may, therefore, determine W by 

 placing a wattmeter in the primary circuit when the secondary 

 circuit is open. 



The next step is to measure, by means of a Wheatstone's bridge 

 (or otherwise), the resistances r\ and r 2 of the two coils, correcting 

 for the temperature corresponding to the load. 



By giving the assumed values to w, the efficiency at any out- 

 put may be calculated. 



