310 



ELECTRICAL ENGINEERING 



transformer in a group does not necessarily shut down the line 

 fed by that group, but operation can be carried on more or less 

 satisfactorily with two transformers in open delta. An injury to 

 one coil of a three-phase transformer puts the whole transformer 

 out of commission. Thus for the same reliability of operation the 

 cost of spare apparatus is increased with three-phase units. 



Coils 



FIG. 283. Core-type transformer. FIG. 284. Shell-type transformer. 



Core 



FIG. 285. Three-phase core-type transformers. 



191. Methods of Cooling. Very small transformers do not 

 require any special method of cooling but are so designed that the 

 exposed surface is large enough to radiate the heat generated by 

 the power losses in the windings and core without a temperature 

 rise exceeding the limits consistent with the life of the insula- 

 tion. Transformers must be designed to operate for 24 hours at 



