13 TRANSFORMERS. [Exp. 



The lost amperes are due to the fact that, even on no load, 

 a transformer takes a certain exciting current to maintain the 

 flux and to supply the core losses. For a more detailed discus- 

 sion, see Exps. 5~B and 5~C. 



3. Structurally, transformers are of two types, the core type 

 in which the core is on the inside and not the outside of the 

 coils; and the shell type in which the core is not only on the 

 inside of the coils but also encloses them, to a certain extent, on 

 the outside so as to form a divided return magnetic circuit. (See 

 hand-books and text-books.) Variations in structural arrange- 

 ments depend on commercial considerations, and do not affect 

 at all the principle of operation. 



Transformer losses eventually appear as heat and a trans- 

 former is so designed that this heat can be radiated or disposed 

 of without exceeding a limiting safe rise in temperature.* The 

 magnetic circuit is laminated to minimize the eddy current loss. 

 For all usual purposes, the magnetic circuit is closed. A trans- 

 former with an open magnetic circuit takes excessive magnetizing 

 current and while it might be used for some special purpose 

 it is never used for power and lighting. (The "Hedgehog" 

 transformer was of this type.) 



i *(3a). Heating of Transformers. This necessitates, on the part of 

 the designer, a consideration of radiating surface, etc., or the provision of 

 some special means of cooling. The radiating surface usually found nec- 

 essary varies between 2 and 4 sq. in. per watt. For the allowable rise of 

 temperature, see Standardization Rules which at present allow a rise of 

 50 C. above the air. Run at higher temperatures, transformer iron ages, 

 i. e., the core losses increase in the course of time. While this has been 

 true of the iron ordinarily used for years in transformer construction, it 

 is less true of the improved alloy steels which are being introduced. 

 Hence, aging ceases to be a factor and the allowable temperature rise 

 might be increased as much as the insulating material will stand. Good 

 insulation will stand continuously a temperature of 90 C. This will 

 increase the rating of a given size transformer, or will reduce the size 

 and cost of a transformer of a given rating. In rating new iron the 

 allowable magnetising current, and not temperature, may become the limit- 

 ing consideration. 



