78 THE DESIGN OF STATIC TRANSFORMERS 



exceptionally low, then it might he admissible to use the same 

 design on a 25-cycle circuit. These proportions are not, 

 however, such as yield the best results at 50-cycles. 



The influence of the periodicity upon the core loss and 

 upon the no-load current of our 20-kw transformer is shown 

 in Fig. 45. By increasing the periodicity from 50 cycles to 

 100 cycles the core loss has been decreased from 190 watts to 

 125 watts, but. by decreasing the periodicity to 40 cycles the 

 core loss has been increased to 240 watts. The variations in 

 the no-load current, due to various periodicities, is still more 

 striking. Fig. 46 shows the influence of supplying different 

 pressures to the primary terminals, upon the core loss and the 

 no-load current. 



In Fig. 47 are given curves showing the relation between 

 the regulation and the output, corresponding to some of the 

 curves shown in Fig. 44. From these curves it is seen that in 

 three cases the regulation is closer the greater the rated out- 

 put, and in another case it is the same for all rated outputs. 

 Close regulation may be obtained either by working the copper 

 at a low current density in which case a fairly large number of 

 turns may be employed and a small flux, or it may be obtained 

 by employing a considerably higher current density, a small 

 number of turns, and a large flux. The latter plan generally 

 results in a greater core loss than the former plan. 



Comparing our 20-kw design, as regards core loss mid 

 regulation, with the data just given for various lines of trans- 

 formers, we find that the design is fairly in accord with good 

 practice as relates to these quantities. Of course, the 

 designer's object is to obtain a good result at a minimum total 

 works cost, and designs should also be compared from this 

 point of view. Broadly, however, it may be said that what 

 might appear an extravagant outlay when the cost of the 

 copper and core is alone considered, often constitutes but a 

 very inappreciable increase in the cost by the time the cost of 

 the oil, the insulation materials, the case, the bushings and the 



