46 THE MAGNETIC CIRCUIT [ART. 19 



The thickness of lamination to be used in each particular 

 case is a matter of judgment based on previous experience, and 

 no general rule can br laid down, except what is said in Art. 17 

 above, in regard' to the factors upon which the eddy -current loss 

 depends. The gauges 26 to 29 are representative of the usual 

 practice. If it should be necessary to estimate the core loss for a 

 different thickness and at another frequency than those given in 

 Fig. 10, the method explained in Art. 22 below may be used. 



The core loss in the armatures and teeth of revolving machinery 

 is found from tests to be considerably above that calculated from 

 the curves of loss on the same material when tested in stationary 

 strips. This is probably due in part to the fact that the conditions 

 of magnetization are different in the two cases. In the one case 

 the cycles of magnetization are due to a pulsating m.m.f., which 

 simply changes its magnitude; in the other case to a gliding m.m.f., 

 with which the magnetic intensity at a point changes its direction 

 as well. Besides, the distribution of flux densities in teeth and in 

 armature cores is very far from being uniform. Therefore, 

 when using the curves given in Fig. 10, for the calculation of iron 

 loss in generators and motors, it is necessary to multiply the results 

 by certain empirical coefficients obtained from the results of tests 

 made on similar machines. Mr. I. E. Hanssen recommends add- 

 ing 30, 35, and 40 per cent to the loss calculated from the curves 

 obtained on stationary samples when estimating the iron loss in 

 an armature back of its teeth, at 25, 40, and 60 cycles respectively. 

 For teeth he recommends adding 30, 60, and 80 per cent, at the 

 same frequencies. 1 These values are quoted here merely to give 

 a general idea of the magnitude of the excess of core loss in revolv- 

 ing machinery; a responsible designer should compile the values 

 of such coefficients from actual tests made on the particular class 

 of machines which he is designing. 



Some engineers do not use for revolving machinery values of 

 core loss obtained on stationary samples, but plot the curves of core 

 . loss obtained directly from tests on machines of a particular kind, 

 for various frequencies and flux densities. This is a reliable and 

 convenient method provided that sufficient data are available to 

 separate the core loss in the teeth from that in the core itself. Mr. 

 H. M. Hobart advocates this method, and curves of core loss 



1 Hanssen, "Calculation of Iron Losses in Dynamo-electric Machinery," 

 Trans. Amer. Inst. Elec. Eng., Vol. 28 (1909), Part II, p. 993. 



