50 THE MAGNETIC CIRCUIT [ART. 20 



during the first few years of its operation used to be a serious 

 matter in the design and operation of transformers, because of the 

 subsequent overheating of the core and of the coils. Silicon steel 

 shows practically no increase in its hysteresis loss after several 

 years of operation. Moderate heating, which considerably in- 

 creases the hysteresis loss in ordinary steel, has no effect on silicon 

 steel. 



Impurities which are of such a nature as to produce a softer 

 iron or steel and a material of higher permeability, are as a rule 

 favorable to the reduction of the hysteresis loss, and vice versa. 

 Mechanical treatment and heating are also very important in their 

 effects on hysteresis loss. In particular, punching and hammering 

 increases hysteresis loss, while annealing reduces it. Therefore 

 laminations are always annealed carefully after being punched 

 into their final shape. 



The requirements for the steel used in permanent magnets are 

 entirely different from those for the cores of electrical machinery. 

 In permanent magnets a large and wide hysteresis loop is desired, 

 because it means a high percentage of residual magnetism (ratio 

 of CO to AP, Fig. 7) and a large coercive force, OF. Both are 

 favorable for obtaining strong permanent magnets of lasting 

 strength. Combined carbon is particularly important for obtain- 

 ing these qualities, as is also the proper heat treatment after mag- 

 netization. 



Prob. 16. In the 60-cycle transformer given in prob. 13, the core 

 weighs 89 kg. and is made of 26 gauge good carbon steel. The maxi- 

 mum flux density is 6.8 kl./sq. cm. What is the hysteresis loss assuming 

 T) to be equal to 0.0012? Ans. About 124 watt. 



Prob. 17. What is the constant in formula (21) in place of 10~ 7 , if, 

 with the same TJ, the density B is in kilo-maxwells per sq. cm., and the 

 volume is in cubic decimeters? Ans. 6.31. 



Prob. 18. Show how to determine the values of y and n in eq. (20), 

 knowing the values W^ and W 2 of the energy lost per cycle at two given 

 values of maximum flux density, B l and B 2 . 



Ans. n = (log TF 2 -log PFO/(log 2 log ,) 



Prob. 19. The following values of hysteresis loss per cu. decimeter 

 have been determined from a test at 25 cycles (after eliminating the eddy 

 current loss) : 



Flux density in kl/sq.cm., 5 = 5.0 

 Hysteresis loss in watts, Ph = 1 . 30 



6.5 

 2.00 



8.0 



2.8* 



10.0 

 4.11 



What are the values of 5? and n in formula (20)? Suggestion; 

 Use logarithmic paper to determine the most probable value of n, by 



