54 THE DESIGN OF STATIC TRANSFORMERS 



164 000 cm. The section of the primary winding to give the 

 above resistance at 60 C. must therefore equal 



164 000 X 0,0000020 



10>9 = 0,080 sq cm 



and since the total length of the secondary conductor is 

 (124 X 40 =) 4960 cm, the section of the secondary conductor 

 must equal 



4960 x 0,0000020 



-pi8~ '- ' 545 sq cm ' 



The current density at rated load equals 



4,12 



= 137 amperes per sq cm 



0,030 

 in the primary copper, and 



100 



n KAK = 184 amperes per sq cm 



U,o4o 



in the secondary copper. The difference in these two densities 

 is due to the plan adopted in this design, namely, that the 

 percentage pressure drop shall be the same in the primary and 

 secondary windings. 



It will be observed that in our example we have adopted a 

 method by which the current density has been deduced. There 

 are innumerable alternative methods of carrying out a set of 

 designing calculations, and, as a matter of fact, the experienced 

 designer employs sometimes one and sometimes another, 

 according to the way in which each case as it arises appeals to 

 him, or according to his mood. Thus it is useful to have at 

 hand rough representative values of current densities which, in 

 this type of design, are found by experience to lead to good 

 results. 



As a matter of fact, however, widely different current 

 densities may be employed in equally good designs, since, as 

 we shall see later, the heating of an oil transformer is mainly a 



