294 



ELECTRICAL MACHINERY 



eddy current losses are independent of the load; hence the 

 core loss (which is the sum of these two) is independent of 

 load. 



Copper Loss. The PR loss in the windings evidently 

 increases with the square of the load current. This is 

 true for both transformer coils because we have previously 

 shown that the primary current increases proportionately 

 with any increase in the secondary current. The total 

 copper loss may be calculated from the equation: 



Copper Loss =h 2 Ri +I 2 2 R 2 = Ii 2 (Ri+a 2 R 2 ), (75) 



20 30 40 

 Load current 



50 



FIG. 185. Curves of Losses in a Transformer, Showing Their Variation 



with Load. 



where the term (Ri-\-a 2 R2) is called the effective resistance 

 of the transformer (of both coils) in terms of the primary 

 resistance. 



Loss Curves. Fig. 185 shows the two loss curves of a 

 4-kv-a. transformer plotted against the secondary current; 

 the total loss curve is obtained by adding the two. 



From the total loss curve the efficiency can easily be 

 calculated. We know that 



efficiency = output /input = output/output + losses. 



Now assume any output, as for example, 10 amperes. 

 As this transformer is rated 2200 volts-110 volts, the 



