EDDY-CURRENT LOSS 



107 



(1) above, the constancy of B. Thus the core losses, which depend 

 on B, will remain nearly constant at all loads. As a matter of fact, 

 there will be a very slight decrease with increasing load, owing to the 

 slight decrease in the counter e.m.f. brought about by the increase in 

 the resistance drop. 



According to Steinmetz's law, the hysteretic loss, in ergs per c.c. 

 of the material per cycle, is given by rjB 1 ' 6 , where TJ has a value, for 

 good transformer sheets, ranging from O'OOl to 0'0012. If we 

 multiply the loss per c.c. per cycle by the frequency and the total 

 volume of iron in the core we obtain the hysteresis core loss. 



The hysteresis loss is found to increase considerably if the core- 

 plates are subjected to great mechanical pressure. It is therefore 

 advisable, in building up the core, to apply as little pressure as 

 possible. 



The core-sheets employed in some of the earlier transformers were 

 found gradually to deteriorate, the hysteresis loss steadily increasing 

 as time went on. This effect was found to be due to the maintenance 

 of the core at a fairly high temperature by the losses occurring in 

 it, and is known as the ageing of transformer iron. Lately, however, 

 manufacturers of transformer sheets have succeeded in producing 

 material which does not suffer from the " ageing " defect to any 

 serious extent. 



54. Calculation of Eddy-Current Loss in Core. 

 Discrepancy between calculated and Actual 

 Core Losses 



The eddy-current loss which occurs in sheets of a given thickness 

 may be approximately calculated. Let in Fig. 87 AB represent part 

 of the cross-section of a sheet, the 

 magnetic flux being normal to the 

 plane of the paper. The alterna- 

 tions in the flux give rise to eddy 

 currents, and these will flow parallel 

 to the boundaries of the sheet 

 if we except the extreme edges of the 

 sheet, where the currents flowing 

 along the upper half cross over to 

 join those flowing in the lower half 

 of the sheet. There will obviously 

 be no flow along the central line 

 OO'. Above this line the currents 

 will, at a given instant, flow in one 

 direction, below it, in the other, as 

 indicated by the arrows. Consider a double strip, CDEF, 1 cm. long 



A 



-0' 



B 



FIG. 87. To illustrate Eddy-current 

 LOBB in Core. 



