TRANSFORMERS 379 



Then: 



1. For unity power-factor, we have approximately: 



2 



Per cent regulation = 9,.+. 



2. For inductive loads, where the power-factor (cos <f>) equals 

 m and the reactive factor (sin </>) equals n. Per cent regulation 



2QQ 



Core and Shell Type. Transformers are of two fundamental 

 designs, namely: The shell and the core type, and occasionally a 

 combination of these two is also used (Fig. 225). In the shell type 

 the iron circuit surrounds the transformer coils, while in the core 

 type the copper windings surround the iron core. While the shell- 

 type transformers have been most extensively used in the past, 

 core-type transformers are now built for the largest sizes, and are 

 rapidly superseding the former type. With the core type design, 

 the arrangements of cores and the circular coils present a con- 

 struction which offers a maximum resistance to the mechanical 

 distorting forces. This mechanical strength, combined with the 

 inherent reactance of this type of transformer, produces a unit 

 which is exceptionally able to withstand severe service. On the 

 other hand, the circular coils can readily be insulated for the very 

 highest voltages in use. 



Method of Cooling. Transformers may be divided into four 

 classes, depending upon the method of cooling, viz., natural draft, 

 air blast, oil immersed self-cooled and oil immersed water-cooled. 

 Natural-draft transformers have the core and coils exposed directly 

 to the air, and depend entirely upon the natural circulation of 

 the air for their cooling. They are built only for very low voltages 

 and small sizes. Air-blast transformers depend upon a forced cir- 

 culation of air over the surface of the core and coils to carry away 



