48 THE DESIGN OF STATIC TRANSFORMERS 



to adopt the cruciform or stepped cruciform core, in order to 

 secure maximum cross-section of iron with good provision for 

 cooling and with short length of mean turn. The polygonal 

 core (Fig. 23) is undesirable on account of the many different 

 widths of the stampings required and of the consequently 

 increased cost attending cutting and assembling. 



For our present design, let us adopt the oblong shape 

 (Fig. 21), and without entering upon a consideration of the 

 most economical dimensions of the rectangle (as this is purely 

 a matter of experience and judgment), let us employ cores 

 measuring 7,0 cm wide by 10,0 cm deep. 



FIG. 28. Section of 20-kw 5000/200-volt transformer core (dimensions 

 in centimeters). 



In transformers of this type, it is generally undesirable to 

 employ great depths of winding, and, bearing this consideration 

 in mind, there have been plotted in the curves in Fig. 27 

 values for the gross width of the winding space, i.e., the width 

 of the window, for various outputs and periodicities. These 

 curves may be used for preliminary trial designs. It is by no 

 means intended that these values should be strictly adhered to 

 in the final design. On the contrary, it is often necessary to 

 appreciably alter the width of the winding window when we 

 reach the point of laying out the shapes of the coils them- 

 selves. From Fig. 27 we find that some 8,5 cm constitutes a 

 reasonable trial value for the width of the winding window of 

 our 20-kw transformer. 



