THE TRANSFORMER 



293 



load, each of them being from 1 to 3% of the full-load 

 capacity of the transformer. The larger percentage applies 

 to the smaller sizes. 



Core Loss. The core loss in a transformer is due to 

 hysteresis, and also to the eddy currents, which are set up 

 in the laminations of which the core is built. 



As explained in Chapter I, it requires energy (work) 

 to carry iron through cycles of magnetization, a measure 

 of this energy being the area of 

 the hysteresis loop. As it is de- 

 sired to keep this loss in the 

 transformer as low as possible, 

 the iron for transformer cores 

 is carefully selected; only that 

 grade of iron which has a nar- 

 row hysteresis loop is used. For 

 a given transformer the hyster- 

 esis loss varies directly with the 

 frequency and with the 1.6 power 

 of the maximum flux density. 



The eddy currents in the core 

 circulate in the laminations as 

 shown in Fig. 184, in which the 

 laminations are represented as 

 much thicker than they really 



are. These currents heat the iron and so represent loss in 

 the transformer. This loss is kept low by using thin lami- 

 nations as explained in Chapter II. Fig. 184 gives the 

 cross-section of one leg of a core type transformer. 



For a given transformer the eddy current loss varies with 

 the square of the frequency and with the square of the maxi- 

 mum flux density. 



Core Loss Independent of Load. The flux density in a 

 transformer is nearly independent of the load on the secondary 

 and, as the frequency of the supply current does not vary 

 with the load, we may conclude that both hysteresis and 



FIG. 184. Sketch Showing 

 the Nature of the Paths 

 Taken by Eddy Currents 

 in a Transformer Core. 

 Here the laminations are 

 shown much thicker than 

 they really are used in 

 transformers. 



