I5 8 TRANSFORMERS. [Exp. 



12. Fig. 5 also shows a core-loss curve corrected for 60 

 cycles, although no measurements were made at that frequency. 

 Such a derived curve can be drawn for any desired frequency. 



13. It is seen that core loss becomes less as frequency is 

 increased. A transformer designed for one frequency can, 

 therefore, be operated more efficiently at a higher frequency, the 

 voltage remaining unchanged. Operated at a lower frequency, 

 however, the transformer will have larger core loss and will 

 therefore, heat up more unless operated at a lower voltage 

 and reduced output. 



The transformer to which Fig. 5 refers is seen to have the 

 same core loss (41.6 watts), and so would have the same tem- 

 perature rise, when run at 81 volts, 29 cycles; 98 volts, 60 cycles; 

 100 volts, 66.2 cycles; 118 volts, 140 cycles. The volt-ampere 

 capacity of any transformer is, accordingly, less at lower fre- 

 quencies. For the same capacity, a larger and more expensive 

 transformer is required. 



If transformers were the only consideration, the frequencies 

 of 125 and 133 cycles in early use would not have been abandoned 

 for lower ones. (See 3, Exp. 3~A.) 



14. Flux Densities. Compare the flux densities at different 

 frequencies, for normal voltage. The values of flux density 

 can be computed, as in 33-35, Exp. 5~A, if the number of 

 turns and iron cross-section are known. Without calculating 

 the actual values and without knowing the construction data of 

 the transformer, the relative values can be found by the rela- 

 tion B oc E-t-n. Thus, if at 60 cycles B is taken as i.o, the 

 flux density at 30 cycles is 2.0, at 120 cycles 0.5, etc. 



