162 TRANSFORMERS. [Exp. 



Although these losses may be considerable in large transformer^, 

 in small well built transformers they are usually insignificant. 



19. Impedance Voltage. In an ideal transformer on short 

 circuit, with zero secondary resistance and no magnetic leakage, 

 the only voltage necessary to cause a given current to flow would 

 be RJi, to overcome the resistance of the primary. The effect 

 of secondary resistance is to apparently increase the resistance 

 of the primary to 



and the resistance drop is, accordingly, RI lt 



On account of the magnetic leakage, the transformer appar- 

 ently has a reactance X, called leakage reactance;* this causes 

 (in terms of the primary) a reactance drop XI lf in addition to 

 the resistance drop, RI t . For a given frequency, this reactance 

 due to leakage is a constant of the transformer, the same as 

 resistance. It is the same on open circuit or short circuit, and 

 is the same at no load or full load. 



The total impedance, which limits the flow of current in the 

 short circuit test, is a combination of the equivalent resistance 

 and leakage reactance, being 



The voltmeter reading gives the total impedance voltage, 



necessary to overcome both resistance and leakage reactance. 



20. Data. At rated frequency, take, say, five readings of the 

 impedance voltage (z) and the copper loss (We) for various 

 currents from about J to ij full-load current. 



21. Readings at various currents are chiefly for illustration 

 and are not essential. When facilities for varying the current 

 are lacking, one accurate reading (or better the mean of five 



* Discussed more fully in Exp. S-C. 



