40 THE DESIGN OF STATIC TRANSFORM]-: i:s 



TABLE 3. CORE DENSITIES FOB USE A.S I'IM-.UMINAUY ASSUMPTIONS 



IN i in: I >I>K. N <>r XAH-KAL AJB-OOOLED AND <r An;-lli.\sr 

 TRANSFORMERS, 



For air-blast transformers or transformers of large capacities, 

 the values set forth in Table 8, will be found more appropriate. 

 These densities may be departed from to a considerable extent 

 without necessarily sacrificing the quality of the designs ; and 

 this matter of the selection of densities affords another instance 

 where individual discretion must be applied. Some designers 

 have adopted the practice, when calculating a line of standard 

 transformers, of arranging that the density for a given 

 periodicity shall be the same for a large range of capacities ; 

 but such a procedure either entails high no-load currents in 

 the small sizes or uneconomical use of the steel in the larger 

 sizes. It is for this reason that the values of the core density 

 given in Table 2 have been adjusted to increase with increased 

 output. The lower values are necessary with transformers of 

 small sizes in order to obtain suitably-low values for the no- 

 load current, while in large transformers the no-load current 

 will not exceed permissible values even when somewhat higher 

 magnetic densities are employed. These higher values neces- 

 sarily involve more iron loss per unit of weight, but the 

 increased losses may be taken care of by the cooling medium, 



1 These densities can only be used where ample ventilating ducts an 

 employed in the core. 



