42 



THE MAGNETIC CIRCUIT 



[ART. 18 



expense and poor space factor. Iron-wire cores are used at present 

 in only high-frequency apparatus, in which eddy currents must 

 1)0 carefully guarded against; for instance in the induction coils 

 (transformers) employed in telephone circuits. 



It will be seen by an inspection of Fig. 9 that eddy currents 

 a ro much smaller in the laminated core because the resistance of 

 each lamination is increased while the flux per lamination and con- 

 sequently the induced e.m.f . is considerably reduced. It is proved 

 in Art. 21 below that the power lost in eddy currents per kilogram 

 of laminations is proportional to the square of the thickness of 



FIG. 9. Eddy currents in a solid and in a laminated core. 



the laminations, the square of the frequency, and the square of 

 the flux density. 



Prob. 8. Show that the armature cores of revolving machinery must 

 be laminated in planes perpendicular to the axis of rotation. 



Prob. 9. Show that assuming the temperature-resistance coefficient of 

 iron laminations to be 0.0046 per degree Centigrade the eddy current loss of 

 a core at 70 C. is only about 82.5 per cent of that at 20 C. 



Prob. 10. Explain the reason for which the hysteresis loss in a given 

 core and at a given frequency depends only on the amplitude of the excit- 

 ing current, while the eddy-current loss depends also upon the wave-form 

 of the current. 



18. The Significance of Iron Loss in Electrical Machinery. 

 The power lost in an iron core on account of hysteresis and eddy 

 currents, taken together, is called iron loss or core loss. It is of 

 importance to understand the effect of this loss in the iron cores 



