96 THE DESIGN OF STATIC TRANSFORMERS 



leakage flux. But even in such a case as that of Fig. 36, it 

 would not suffice to have the primary windings occupy one of 

 the vertical legs and the secondary windings the other. Such 

 an arrangement would still afford far too great a cross-section 

 for the leakage flux between the primary and secondary wind- 

 ings. But by winding the secondary the full length of each 

 vertical leg and by arranging the primary coils immediately over 

 the secondary coils, as shown in Figs. 49 and 50, on pp. 82 and 

 83, only leaving enough room between them to ensure effective 

 insulation, the opportunity for leakage flux to pass between 

 primary and secondary is much reduced. Moreover by having 

 the windings shallow we keep down the magnetomotive force 

 per centimeter of length of winding window, and since the 

 leakage flux is proportional to this specific mmf, the use of these 

 shallow windings is in the interests of low inductance. For 

 such a case as our 20-kva transformer, the simple concentric 

 arrangement shown in Fig. 30, on p. 51, and in Figs. 53, 54 

 and 55, on p. 92, with the secondary wound inside and the 

 primary outside on each vertical leg, leads to a sufficiently low 

 inductance, but cases often arise where a triple-concentric 

 arrangement, such as that indicated in Fig. 31, on p. 51, must 

 be employed in order to obtain the required low inductance and 

 close regulation. The sandwich winding of Fig. 32, p. 52, may 

 also lead to low inductance and close regulation if the sub- 

 division is carried far enough, but when this arrangement is 

 employed with a long and narrow winding space, it must be kept 

 in mind that the length of the leakage path from one side of the 

 narrow winding space to the other, is very low, and this in itself 

 tends to low magnetic reluctance of the leakage path between 

 the primary and secondary windings. 



Other tilings being equal, the inductance will obviously be 

 less the higher the magnetic density employed in the core, for 

 this will tend toward minimising the mean length of both 

 primary and secondary turns and consequently will reduce the 

 cross-section of the leakage path. 



