Figure 1.6(a) shows the complete magnetic circuit of a core-type transformer in the shape of a hollow rectangle having two limbs. It has a single magnetic circuit. In Figure 1.6, I0 is the no-load current and Φis the flux produced by it. Number of turns of the primary and secondary are N1 and N2, respectively.
Figure 1.4 Schematic and Sectional View of a Core-type Transformer
Figure 1.5 Schematic View of a Shell-type Transformer
The windings surround the core. The coils used are wound and are of cylindrical type having the general form circular, oval or rectangular.
Core-type transformer has a longer mean length of core and a shorter mean length of coil turn. Core has a small cross section of iron; more number of turns is required because the high flux may not reach the core. Core type is used for high-voltage service, since it has sufficient room for insulation.
Figure 1.6 Core-type Transformer
Figure 1.7 Different Cross Sections of Core-type Transformer
Figure 1.6(b) shows the actual view of a core-type transformer. The different cross sections used in core-type transformer are shown in Figure 1.7.
In small core-type transformers, rectangular cores with rectangular cylindrical coils are used as shown in Figure 1.7(a), whereas circular cylindrical coils are used for large transformers; hence, square cores are preferred as shown in Figure 1.7(b). If rectangular cores are used for large transformers, it becomes wasteful. Figure 1.7(c) shows the cruciform core, which is an improvement of square core. Figure 1.7(d) shows further core stepping (three-stepped cores) for large transformers resulting in reduced length of mean turn and copper (Cu) loss.