5-B] TEST BY LOSSES. *75 



(eddy currents). Hysteresis is the chief loss and has most weight in 

 determining a and b. Hysteresis loss*, and hence the values of a 

 and b, are affected by wave form. It will be understood that the 

 hysteresis exponent 1.6 is not a constant but represents a fair average 

 value for moderate ranges of. flux densities; at high densities the 

 hysteretic exponent may have a value as high as 2 or more. 



52. One-Voltage and One-Frequency Method. If a and b are 

 known, it is possible, having determined the core loss at one voltage 

 for a particular frequency, to compute the core loss for any other 

 voltage and frequency. It then becomes unnecessary to test a trans- 

 former at the exact rated voltage and frequency which is indeed 

 difficult to do. 



Taking as average values a 1.666 and b = -4474, correction 

 factorsf for variation of core loss with frequency and voltage are 

 given in the following tables. 



CORRECTION TABLES. 



VARIATION OF CORE Loss WITH VOLTAGE. 



Volts (per cent, normal) oo 95 96 97 98 99 100 



Core loss 83.7 91.7 93.3 95 96.7 98.4 100 



Volts (per cent, normal) 101 102 103 104 105 no 



Core loss 101.6 103.3 105 106.6 108.3 116.6 



VARIATION OF CORE Loss WITH FREQUENCY. 



Cycles 55 56 57 58 59 60 6 1 62 63 64 65 

 Loss 103.9 103.12 102.3 101.6 100.8 100 99.3 98.5 97.8 97 06.4 



53. Separation of Hysteresis and Eddy Currents. To determine 

 the eddy current loss in watts, the core loss is to be measured at two 

 frequencies and at the same flux density. Let W be total core loss at 

 normal voltage E' and frequency n'. At a lower frequency n" and a 



* (5 x a). When the wave of electromotive force is peaked, the maxi- 

 mum flux density and the core loss are less. 



t (52a). These are taken from a series of tests made in 1809 by W. F. 

 Kelley and H. Spoehrer (see thesis, Cornell University Library). The 

 transformers were small (1-15 K. W.) and were designed for 60 cycles 

 and over. The writer has no data on most recent transformer iron. 

 These tests also showed that each per cent, variation in voltage caused 

 about .7 per cent. (.6945) variation in exciting current. 



