50 THE DESIGN OF STATIC TRANSFORMERS 



to bring about this equality in the losses) than is employed in 

 the secondary. Of course, even for the same current density, 

 the section of the primary coil will be the greater because of 

 the greater amount of space devoted to insulation between the 

 many turns and many layers. In other words, the cross- 

 section through the primary coil is, for two reasons, much 

 greater than that through the secondary coil. The first reason 

 is that in this particular example the plan has been followed 

 of having the full-load loss in the primary coil equal to the full- 

 load loss in the secondary coil. The second reason relates to 

 the inherently and necessarily lower utilisation of the space in 

 the primary coil as compared with the utilisation of the space 

 in the secondary coil. The second reason is fundamental, but 

 as to the first reason, the transformer might equally well have 

 been proportioned for a greater loss in the primary coil at full 

 load than in the secondary coil. Had this latter plan been 

 followed, the depth of the secondary winding would have more 

 nearly approached the depth of the primary winding. 



The winding used (see Fig. 29) is that known as the simple 

 concentric type, which is the commonest form for this type of 

 transformer. As applied to a core-type transformer, it is 

 shown in plan and elevation in Fig. 30. Its advantages are 

 that it gives a high space factor, and at the same time presents 

 a large cooling surface to the action of the cooling medium, 

 thus enabling the copper to be worked at comparatively high 

 densities. To secure close regulation on inductive load, it is 

 sometimes necessary to adopt the triple concentric winding 

 which reduces the leakage of lines to well down toward half 

 the value obtained with the simple concentric type. 1 A triple 

 concentric winding is shown diagrammatically in Fig. 31. 



An arrangement is shown in Fig. 32 which may be termed a 

 sandwich winding, and it might at first sight be supposed that 

 it would lead to obtaining still better regulation. This will be 



1 For a further discussion of this subject of magnetic leakage the reader 

 should consult Chapter VII. on the regulation of transformers. 



