Construction

Most electronic transformers are small, and for small transformers the shell-type core is usually most suitable because only one coil is required. Figure 8 shows shell-type transformer assemblies.

Fig. 8. Transformers with shell-type core.

The magnetic path is divided, half the flux enclosing one side of the coil and half the other. The coil opening is called the window. Between the windows is the core tongue, which is twice as wide as the iron around the rest of the window. The core is built up of thin laminations to reduce eddy-current losses; typical shapes are shown in Fig. 9.

Fig. 9. Shell-type laminations.

Alternate stacking of the lamination pairs may be used to reduce magnetic reluctance and keep magnetizing current small. To reduce assembly cost, this alternate stacking is sometimes done in groups of two or more laminations, with some increase in magnetizing current. A wide range of sizes of shell-type laminations is available. At 60 cycles, common thicknesses are 0.014 in., 0.019 in., and 0.025 in.

Shell-type laminations are made with proportions to suit the transformer. In the E-I shape a scrapless lamination is widely used. Two E's facing each other are first punched, and the punched-out strips are of the right dimensions to form two I's. Then the E's are cut apart. This economy of material is not justified in transformers in which turns per layer, and hence window width, must be reduced relative to window height.

Fig. 10. Core-type transformer.

For some applications, the core-type transformer is preferable. In these there is only one magnetic path, but there are two coils, one on each leg of the core. A core-type transformer is shown in Fig. 10, and some core-type laminations in Fig. 11.

Fig. 11. Core-type laminations.

Cores wound from continuous steel strip are widely used. One common shape is illustrated in Fig. 12; it is known as the type C core.

Fig. 12. Type C cores.

Steel strip is first wound to the proper build-up on a mandrel. The wound core is then annealed, impregnated with a bond, and cut in two to permit assembly with the coil. After assembly with the coil, the core is held together with a steel band as in Fig. 10. Several advantages accrue from this construction, which will be discussed in Section 15.

Fig. 13. Partly assembled transformer.

Fig. 14. Assembled type C cores and coil.

Typical assemblies using two type C cores are shown in Figs. 13 and 14; they correspond to shell-type laminations. Because it is simpler to assemble a single-core loop, a single core is often used, especially in small sizes. See Fig. 15.

Fig. 15. Single-coil, single-core assemblies.

In 60-cycle service the laminations are usually stacked alternately to produce an overlapping joint. This is approximated in the type C cores with ground gap surfaces which fit closely together. Either type of core can be used with core gaps; laminations are stacked butting, with no overlap. The desired amount of gap material, such as fish-paper, is inserted between the gap surfaces.