CHAPTER VI 



II I K INFLUENCE OF THE FREQUENCY 



IT has already been stated on p. 78 that transformers 

 designed for a given periodicity may be and are often used on 

 circuits having periodicities above that for which they have 

 been designed. For circuits having periodicities below the 

 designed figure, the large no-load current consequent upon the 

 very high densities in the core, stands in the way of their 

 successful use, the power factor of the circuit being seriously 

 lowered. Moreover, the core loss at the lower periodicity is 

 higher. The lower periodicity in itself would occasion a 

 decrease in the core loss, but the higher flux density produces 

 an increase in the core loss which more than suffices to offset 

 the decrease, and the result is a considerably increased core 

 loss. It is, therefore, often necessary and usually preferable to 

 prepare separate designs for circuits of lower periodicities. If 

 calculations according to the method given in the previous 

 chapters are carried through for two other 20-kva oil-cooled 

 single-phase transformers, differing as regards rating from the 

 one we have considered, only in respect to their periodicities, 

 which are respectively 15 cycles per second and 25 cycles per 

 second, then the specifications given in columns A and B of 

 Table 11 will be obtained. Column C of the same table con- 

 tains the data of the 50-cycle transformer which we have been 

 designing. It will be seen that with the lower periodicities 

 the core loss has increased considerably above that of the 

 design for 50 cycles per second, notwithstanding the much 

 greater amount of material employed. 



