TREATISE ON ALTERNATING CURRENTS. 



Then 



therefore 



(5) 



p is called the virtual conductance, <r the susceptance, and F the 

 admittance of the primary circuit. The power factor on open 

 secondary circuit is given by (see Fig. 37) 



cos d> = ~ 



(6) 



and should be as small as possible. 



When the transformer is on open secondary circuit, the primary 



current is so small that the loss of 

 energy due to ohmic resistance may 

 be neglected. As soon as load is put 



on the secondary, the primary current 



E increases, and the copper losses in 

 both circuits must be taken into 

 account. 



In designing a transformer, the 

 maximum induction B in the core is 

 usually fixed beforehand, and the permeability /* of the iron is 

 therefore known. The value of the magnetizing current i m may 

 then be determined as follows : 





FIG. 37. 





where N\ is the total number of primary turns and L the mean 

 length of the magnetic circuit ; hence 



(7) 



The hysteretic-energy current can also be determined in the 

 following manner : 



Let V be the volume of the iron core in cubic centimetres, 



