44 THE MAGNETIC CIRCUIT [ART. 19 



the hysteresis loop. With alternating current the effect of hystere- 

 sis is automatically eliminated by the reversals of the current 

 which passes through the instrument. 



From these examples the reader can judge as to the effect of 

 hysteresis in other types of electrical apparatus not considered 

 above. 



Prob. 11. Show that in an 8-pole direct-current motor running at a 

 speed of 525 r.p.m. the armature core and teeth undergo 35 complete 

 hysteresis cycles per second. 



Prob. 12. Show that for two points in an armature stamping, taken 

 on the same radius, one in a tooth, the other near the inner periphery of 

 the armature, the hysteresis loops are displaced in time by one-quarter 

 of a cycle. 



19. The Total Core Loss. In practical calculations on electrical 

 machinery the total core loss is of interest, rather than the hystere- 

 sis and the eddy current losses separately. For such computations 

 empirical curves are used, obtained from tests on steel of the same 

 quality and thickness. The curves of total core loss given in 

 Fig. 10 have been compiled from various sources, and give a 

 fak idea of the order of magnitude of core loss in various grades 

 of commercial steel laminations. The specimens were tested in 

 the Epstein apparatus, which is a miniature transformer (see 

 the author's Experimental Electrical Engineering, Vol. 1, p. 197), 

 and the values given can be used for estimating the core loss in 

 transformers and in other stationary apparatus with a simple 

 magnetic circuit. 



In using the curves one should note that the ordinates are watts 

 per cubic decimeter of laminations, hence the gross volume and 

 not the volume of the iron itself is represented. On the other hand, 

 the abscissae are the true flux densities in the iron. In choosing a 

 material the following points are worthy of note: (1) Silicon steel 

 is now used for 60-cycle transformers, almost to the exclusion of 

 any other, on account of its lower core loss; it is sometimes used 

 for 25-cycle transformers also. (2) The material called " Good 

 carbon steel " is that which is used for induction motor stators, 

 and in general for the armatures of alternating and direct- 

 current machinery ; also, sometimes for the cores oMow frequency 

 transformers. (3) The material called " Ordinary carbon steel " 

 should be used only in those cases for which the core loss is of 

 small importance. 



