60 THE DESIGN OF STATIC TRANSFORMERS 



which, as already explained, represent the total specific core 

 loss values for various densities and periodicities. 



In our transformer, the periodicity is 50 cycles per second 

 and the density is 11,5 kilolines per sq cm. Consequently the 

 specific core loss will he 2,6 watts per kilogram. The total 

 core loss will thus be 



2,6 X 73,0 = 190 watts, 



or 0,95 of 1 per cent, of the rated output. 



Before we can judge intelligently whether this is a satisfactory 

 value for the core loss, we must discuss a quantity to which the 

 term *' annual " (or " energy ") efficiency may he applied. If, as 

 will usually be the case for small transformers supplying lighting 

 loads, the transformer is always in circuit, i.e., 24 hours per day, 

 or 8750 hours per year, the core loss will amount to 



8750 X 190 



1000 



= 1660 kw hr per annum. 



The average lighting transformer is only loaded for a very 

 small percentage of the whole year, the precise percentage 

 depending upon the load factor. Let us assume that the trans- 

 former will be unloaded for 80 per cent, of the whole year, and 

 that it will operate at its rated load for the remaining 20 per 

 cent, of that time. In other words, let us assume the load 

 factor to be 20 per cent. Of course in reality these periods of 

 load and no load are the two extreme conditions, and the trans- 

 former will, as the hours go by, carry loads of widely vary- 

 ing amounts ; but it simplifies the calculation and introduces 

 no unreasonable inaccuracy to carry out our estimates on the 

 basis of these two definite loads, no load and full load, for 

 these two definite percentages of the entire time. At rated 

 load we have, in addition to the core loss, an I 2 B loss of 

 (0,018 X 20 000 =) 360 watts in the transformer windings. 

 Thus the 7 2 E loss amounts to 



i, 0>2 X 360 = 630 kw hr per annum. 

 1UUU 



