184 ALTERNATL\<; CURRENTS 



error is introduced by using the secondary voltage as a basis for 

 calculating the primary voltage. 



82. Equivalent Resistance and Reactance. The preceding 

 discussion refers specifically to transformers having one-to-one 

 ratios. There is little difference, however, when other than 

 one-to-one transformers are considered. For example, in Fig. 

 174, EI and EI are considered as being equal. If there were 

 Ni primary turns and N 2 secondary turns, the true primary 

 voltage would be 



F Nl F 



El= W, El 



Likewise the primary current would be. 



7 Ni 7 

 7l = N[ h 



In testing transformers and in computing their performance, 

 it is more convenient to work with one side of the transformer 

 only. The method of treating such a problem is as follows: 



First, consider the resistance of the primary and of the second- 

 ary. The total copper loss in the transformer 



PC = ISRi + /2 2 fl 2 (I) 



where I\ and R \ are the primary current and resistance respec- 

 tively and 7 2 and R% are the secondary current and resistance 

 respectively. 



If the exciting current is neglected, 



h _ Ni T T Ni 



h ~ N, lz = ll N z 

 Substituting in (I) 



p c = / 1 2 /e 1 + / 



This means that the total copper loss can be found by multi- 

 plying the primary current squared into the expression 



This expression is equal to the primary resistance added to the 

 secondary resistance when multiplied by the ratio of primary to 

 secondary turns squared. This quantity is called the equivalent 



