MAGNETIC SHIELDING OF TRANSFORMERS 417 



impairs other important characteristics of the transformer and is 

 therefore undesirable. 



Another method is by shielding the transformer from the disturbing 

 magnetic field. It is the object of this paper to consider such shielding 

 and to present some data in this connection that may be of general 

 interest. 



Theory 



When a transformer is placed in an a.c. magnetic field, there will, 

 in general, be a voltage induced in the windings. This voltage is 

 proportional to the intensity of the magnetic field. Therefore, if the 

 intensity of the magnetic field in the space occupied by the transformer 

 is reduced, the induced voltage will be correspondingly reduced. 

 This can be accomplished by enclosing the transformer in a case made 

 of material which shields against magnetic flux. Let Hi be the 

 intensity of the field inside the case and He the intensity of the field 

 when the case is removed. The ratio He/Hi will then indicate the 

 shielding efficiency of the case. Expressed in decibels: 



Shielding efficiency — 20 \ogio He/Hi. (1) 



The shielding efficiency of the case depends primarily upon the 

 permeability and conductivity of the material, and the mechanical 

 construction of the case. 



A high permeability material provides a magnetic path in the walls 

 of the case of much less reluctance than the air space inside the case. 

 The greater part of the flux will, therefore, follow the low reluctance 

 path, and only a small part will enter the space inside the case. The 

 higher the permeability is, the less the flux that will enter the space 

 inside the case.* With a steady magnetic field all the shielding is 

 due to this cause. 



An alternating magnetic flux induces eddy currents in the material 

 of the case as shown in Fig. 1. These eddy currents are a function 

 of the conductivity and permeability of the material. They may 

 increase or decrease the shielding efficiency of the case. That is, 

 the eddy currents iei (Fig. 1), which are due to the component of the 

 magnetic field perpendicular to a wall of the case, will set up a counter 

 mmf. which will oppose flux entering the case. In a copper case, 

 the shielding is primarily due to such eddy currents. On the other 

 hand, the eddy currents ie2 (Fig. 1), which are due to the component 

 of the field parallel to a wall of the case will set up a counter mmf. 



* It is assumed here, of course, that the source of the magnetic flux is at some 

 distance so that the amount of flux leaving the source is not appreciably affected 

 by the case. 



