TRANSFORMERS 



309 



The eddy current loss, therefore, varies as the square of the 

 frequency, the square of the maximum induction density and the 

 square of the thickness of the plates and it also depends on 

 the electric conductivity of the iron. 



To reduce the eddy current loss the core is built up of sheets 

 0.014 in. thick and an iron with a high electric resistance is used. 



Since iron has a positive temperature coefficient for resistance, 

 the eddy current loss will decrease slightly as the transformer 

 heats up. 



189. Efficiency. The efficiency of transformers is very high. 

 For small sizes it varies from 93 per cent at J load to 97 per cent 

 at full load and for large sizes from 97 per cent at J load to 99 per 

 cent at full load. 



Fig. 282 shows the loss and efficiency curves for a large trans- 

 former. 



100 



90 



80 



| 70 



3 60 



33 



i 



2 



50 100 



Percent Load 



150 



FIG. 282. Efficiency and loss curves of a 550-kw., 10,500- 

 volt, 60-cycle, air-blast transformer. 



190. Types of Transformers. Transformers may be divided 

 into two general types according to the arrangement of the core 

 and windings, the core type and the shell type. These two types 

 are illustrated in Fig. 283 and Fig. 284. 



Three-phase transformers are built of both the core and shell 

 types, Fig. 285 and Fig. 286. In the great majority of polyphase 

 systems, however, groups of single-phase transformers instead of 

 three-phase units are used. This gives a more flexible system and 

 there is less danger of shut down, since an injury to a single-phase 



