102 



ALTERNATING CURRENTS 



short-circuit. The core loss is negligible, as 5 per cent, primary 

 voltage means only about 2J/ per cent, of the rated value of flux, 

 since half the impressed voltage on short-circuit is consumed in the 

 primary impedance drop. The core loss at 2 or 3 per cent, of the 

 rated flux is so small as to be negligible, for the core loss varies 

 nearly as the square of the flux. Therefore, the power at short- 

 circuit 



P = 



+ 



= /2/2o2 



where R Q i and R 02 are the transformer equivalent resistances 

 referred to the primary and secondary, respectively. 



#02 - Jj 



The value of equivalent resistance as found in this manner 

 may be checked with the value determined by measuring the 



resistance of each winding 

 with direct current. The 

 ratio of effective to ohmic 

 resistance is practically 

 unity in most transformers. 

 Figure 181 shows the 

 equivalent-circuit vector 

 diagram for the short-cir- 

 cuit test. This diagram 

 .7,-/ 8 is merely that of Fig. 176, 

 /J *"?" except that Vz now equals 



FIG. 181. Vector diagram for short-circuited zero, and all quantities are 



now referred to the pri- 

 mary side. It will be recognized in Fig. 181 that the entire 

 voltage Vi is consumed in the impedance drops of the two wind- 

 ings. From this it is obvious that if Z i be the quivalent 

 impedance of the transformer, referred to the primary side, 



(58) 



z " Gr 



(59) 



