71 THE DESIGN OF STATIC TRANSFORMERS 



In contrast to the above transformer there are given in 

 Table 10 the efficiencies of a 20-kw transformer having a core 

 loss of 3GO watts and a copper loss at rated load of 190 watts. 

 The values in Table 10 are plotted in Fig. -1 



TAUI.F. K>. KFI iriF.xrv DATA OF 'JO-icw TKA.\SF.I;MFI;. 



With the employment of modern high-grade transformer 

 laminations the core can be worked at much higher flux 

 densities for the same core loss per kilogram than with the best 

 grades available three or four years ago. But when the core is 

 worked at high densities, the no-load current is considerably 

 increased. This is illustrated in Fig. 43, in which the upper 

 curve represents the percentage no-load current for a line of 

 transformers in which modern high-grade laminations worked 

 at high densities are employed, and the lower curve represents 

 this factor for a line of transformers built some few years ago 

 and employing the best material then available. The core 

 loss per kilogram is about the same in both cases for designs 

 for a given rated output, and the greater percentage no-load 

 current is due chiefly to the higher densities employed in 

 the modern designs. In fact, the densities employed run 

 up considerably higher than is desirable in the case of the 

 smaller rated outputs for the modern designs, since the low 

 power factors at light loads would constitute a distinctly 

 undesirable feature from the supply company's standpoint. 



