1 66 



ALTERNATING CURRENTS 



(G \ 2 

 ^ J r 2 . Or, if we prefer to refer every- 

 thing to the secondary circuit,* the transformer behaves as if 



magnetic leakage and coil resistance were absent, and as if in series 



g \ ay 



with the secondary there were connected an impeda i ( e ~ ) ^> 



bi ' li 



/ g \ 2 



whose resistance component is r 2 + ( ^ 2 ) ?'i- 



When the transformer is short-circuited, the induction in its core 

 is very small, and hence the magnetic leakage is not quite the same 

 as that occurring at full load. In order to overcome this objection 

 to the method of measuring leakage reactance just explained, Berry 

 recommends f measuring the impedance which corresponds to 

 normal working conditions, the arrangement of connections being as 

 shown in Fig. 120. The method can only be used if two similar 



FIG. 120. Berry's Method of determining the Equivalent Impedance of a 

 Transformer. 



transformers are available. The secondaries Si and S 2 are connected 

 in series with each other, the connections being so arranged that the 

 e.m.f.'s induced in them oppose each other. One of the primaries, P 2 , 

 is connected directly across mains between which the normal work- 

 ing primary p.d. is maintained. If the primary PI of the other 

 transformer were similarly connected, the secondary e.m.f.'s would 

 exactly balance each other, there would be no secondary current, and 

 each primary would take only its small magnetizing current; the 

 transformers would, in fact, behave as if their secondaries were open- 

 circuited. In order to cause the full-load currents to flow in the 

 transformer windings, a small auxiliary transformer is used, whose 

 primary P is connected in series with an adjustable resistance E, 

 while its secondary S is connected in series with PI, the e.m.f. 



* As was done in 81. 



f "Modern Electric Practice," vol. ii. p. 86. 



