26 THE DESIGN OF STATIC TRANSFORMERS 



Few transformers of the natural air-cooled type are, however, 

 built nowadays for so large an output as that in question, 

 muni'ly, 20 kva. From Fig. 7 we find that the full-load 

 efficiency will he of the order of 97,1 per cent. In the final 

 design it may he found that the calculated efficiency does not 

 agree absolutely with that assumed from Fig. 7. No radical 

 change will have to be introduced in the calculations on this 

 account. It will only be necessary to make a slight alteration 

 in the current so that it shall correspond with the true efficiency 

 and also to alter the factors dependent upon the current. 

 The primary current at rated load is 



20 000 



5000 X 0,971 ' 

 and the corresponding secondary current is 



20 000 

 2QQ = 100 amperes. 1 



At the next step a factor is introduced and considered wherein 

 large differences may exist, even in equally good designs. 

 This factor is the " volts per turn." In general, the pressure* 

 between each turn of the winding may be varied over a very 

 wide range, depending partly upon the object which the designer 

 has in view, but more especially upon the instinct, preferences 

 and experience of the individual designer. In designs for 

 fairly large outputs, say, up to 300 or 400 kva, and for 

 high pressures, the determining factor for this quantity 

 (the " volts per turn ") is the permissible pressure, in volts, 

 which can, with a sufficiently large factor of safety, and with due 

 regard to economy in the space factor, be allowed between 

 each turn of the windings. This, of course, applies to both 

 primary and secondary windings, since the pressure JUT him 



1 Strictly speaking, the secondary current at rated load would !>< loo amperes 

 only if the transformer were wound to give a terminal pressure of 200 volts nt 

 v'/.v i-iitril load. Usually, the secondary pressure ascribed to a transformer is 

 that at no load. 



