Transformers of this type have a large number of turns in the primary winding and a small number in the secondary, in ratio depending on the amount of pressure reduction required.
Core Transformers.—This type of transformer may be defined as one having an iron core, upon which the wire is wound in such a manner that the iron is enveloped within the coils, the outer surface of the coils being exposed to the air as shown in figs. 1,926 and 1,927.
Shell Transformers.—In the shell type of transformer, as shown in fig. 1,928, the core is in the form of a shell, being built around and through the coils. A shell transformer has, as a rule, fewer turns and a higher voltage per turn than the core type.
Ques. What is the comparison between core and shell transformers?
Ans. The relative advantages of the two types has been the subject of considerable discussion among manufacturers; the companies who formerly built only shell type transformers, now build core types, while with other builders the opposite practice obtains.
Fig. 1,928.—Shell type transformer. In construction the laminated core is built around and through the coils as shown. For large ratings this type has some advantages with respect to insulation, while for small ratings the core type is to be preferred in this respect. The shell arrangement of the core gives better cooling; with this arrangement minimum magnetic leakage is easily obtained.
Ques. Upon what does the choice between the two types chiefly depend?
Ans. Upon manufacturing convenience rather than operating characteristics.