15 TRANSFORMERS. [Exp. 



EXPERIMENT 5-B, Transformer Test by the Method of 

 Losses. 



i. Introductory. The losses in a transformer are the core 

 loss, which is dependent upon and varies with voltage, and the 

 copper (and load) losses, which are dependent upon and vary 

 with current. The most accurate* and the most convenient 

 method for testing a transformer is to measure these losses sepa- 

 rately, without loading the transformer, and compute the effi- 

 ciency and regulation. 



This requires two simple tests, each employing! a voltmeter, 

 ammeter and wattmeter: an open-circuit or no-load test for 

 determining the no-load or core loss and the exciting current 

 at various voltages, particularly at normal voltage; and a short- 

 circuit test at a low voltage (a few per cent, of normal) for 

 determining the copper and load losses and impedance drop for 

 various currents, particularly for normal full-load current. The 

 latter test gives, also, the equivalent resistance and leakage react- 

 ance of the transformer. 



Measurements are also made of primary and secondary re- 

 sistance. 



2. This method may be employed in testing any transformer, 

 whether it is intended for constant potential, constant current or 

 other service; the method will be described in detail with refer- 

 ence to its application to a constant potential transformer. 



* ( ia). This is most accurate for the reasons explained in ib, Exp. 

 2-B. It is not practicable to determine efficiencies accurately by loading 

 a transformer ( 16, Exp. 5-A) and measuring the input and output 

 .directly unless exceeding care be taken the two quantities measured 

 being so nearly equal. The indirect method of losses is, furthermore, 

 most convenient because no load is required and no high-potential meas- 

 urements are necessary. 



T In many cases the same instruments can be used in the two tests; com- 

 pare 44. Two similar* tests are made in testing alternators ; see 9, 

 Exp. 3-B. 



