MAGNETIC SHIELDING OF TRANSFORMERS 



429 



1/8" clearance between the core of the coil and the case. The shielding 

 efficiency is given for a coil having a permalloy core and also for a 

 coil having a non-magnetic core. These curves illustrate the effect of 

 the magnetic core upon the shielding efficiency. At low frequencies 

 the shielding is mainly due to the magnetic properties of the shield 

 material. As the frequency increases, however, the effect due to the 

 eddy currents, ie\ (Fig. 1), increases and the shielding efficiency 

 increases. At approximately 300 cycles a maximum is reached and 

 from here on up to 4000 cycles the shielding efficiency decreases. 

 This is due to the fact that at these frequencies the eddy currents, ie2 

 (see Fig. 1), decrease the effective permeability of the material at a 

 greater rate than the shielding efficiency is increased due to the eddy 

 currents iei. The slope of the curves at 50 cycles is not zero. This 

 shows that even at 50 cycles there is a considerable shielding effect 

 due to the beneficial eddy currents. 



200 500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 8 — Observed shielding efficiency of cases made of 1/32" thick permalloy 

 and siHcon steel sheet. 



The curves of Fig. 8 show the shielding efficiency vs. frequency of 

 a cylindrical permalloy case, the thickness of the walls of which is 

 1/32". The approximate dimensions of this case are 3 1/4" high 

 X 2 5/8" diameter and the relative dimensions of the case and core 

 are such that there is approximately 1/8" clearance between the core 

 and the case. A comparison between the two curves for permalloy 

 shows the effect of the cover upon the shielding efficiency. 



