VH. Types of Transformers 123. As the transformer consists of a magnetic circuit inter- linked with two electric circuits, two constructive arrangements are possible : The electric circuits may be inside, and surrounded by the magnetic circuit as shell, shell-type transformer; or the magnetic circuit may be arranged inside, as core, and sur- rounded by the electric circuits, core-type transformer. In their simplest form, Fig. 163 shows diagrammatically the core-type transformer, with the iron Fe as inside circular core, built up of laminations or of iron wire, and the windings Cu outside; Fig. 164 shows diagrammatically the shell-type 296 ELEMENTS OF ELECTRICAL ENGINEERING transformer, with the copper windings inside, as Cu, and the iron shell Fe wound around it, of iron wire, etc. However, the circular form 163 is used to a limited extent only, in small trans- formers, autotransformers and reactances, and the form 164 practically never used, and in the constructive modification from these diagrammatic types, it is often difficult to decide to which type to assign the transformer. FIG. 163. — Diagram of core type transformer. FIG. 166. — Diagram of shell type transformer. The typical shell-type transformer of today is shown in section in Fig. 165, with the magnetic circuit Fe, and the high voltage windings P and low- voltage windings S intermixed with each other. Core-type transformers are shown in section in Figs. 166 and 167, the former with one, the latter with two cores, and with two different coil arrangements, the intermixed and the concentric. ALTERNATING-CURRENT TRANSFORMER 297 For the transformation of three-phase circuits, three separate single-phase transformers may be used, and their primaries and FIG. 165. — Shell type transformer. FIG. 166. — Single-coil core type transformer. FIG. 167. — Two coil core type transformer. FIG. 168. — Shell type three-phase FIG. 169. — Core type three-phase transformer diagram. transformer diagram. secondaries then connected in ring or delta connection or in star or Y connection, giving the four arrangements: AA, AF, FA, YY. Or two transformers may be used, arranged in T connection or in open A connection, as further discussed under three-phase systems. Or a three-phase transformer may be used. Diagram- 298 ELEMENTS OF ELECTRICAL ENGINEERING matically, the three-phase transformer can be represented by Fig. 168, shell type, and Fig. 169, core type. 124. While in its magnetic and electrical characteristics there is no essential difference between the single-phase shell- type and the single-phase core-type transformer, there is a material difference in the three-phase transformer. In the shell type, Fig. 168, a short circuit of one of the three phases does not affect the magnetic and thus the electric circuit of the other two phases, in the core type Fig. 169, however, a short circuit of one of the three phases short circuits the magnetic return of the other two phases, and so acts as a partial electrical short circuit of these two other phases. In shell-type transformers, Fig. 168, a triple harmonic of flux can exist, but not in the core type, Fig. 169. In the three- FIG. 170. — Shell type three-phase transformer. phase system, the three voltages, currents, etc., are displaced in phase from each other by 120°. Their third harmonics therefore are displaced in phase from each other by 3 X 120°, that is, by 360°, or in other words, are in phase with each other. In Fig. 169, such triple frequency fluxes in the three cores would have no magnetic return, except by leakage through the air, that is, cannot exist, except in negligible intensity, and there- fore the core type of three-phase transformer cannot give any serious triple frequency voltage. In the shell type Fig. 168, however, the three triple frequency fluxes, being in phase with each other, produce a triple frequency single-phase flux through a closed magnetic circuit. Where the circuit conditions and connections are such as to give a triple harmonic — as with YY connection — the shell-type three-phase transformer may produce triple frequency voltages, resulting from the triple frequency ALTERNATING-CURRENT TRANSFORMER 299 flux, and under unfavorable conditions, as when connecting to a system of high capacity — which intensifies these voltages — this may lead to destructive voltages, and YY connections with shell-type three-phase transformers thus lead to serious high voltage dangers. 125. The usual shell-type construction of three-phase trans- formers is shown in section in Fig. 170, the core type in Fig. 171. In Fig. 170 economy requires that the middle phase is con- nected in opposite direction to the outside phases, so that the iron between the successive phases, at 1, 2 and 2, 3, carries the sum of two of the three-phase fluxes, which, as the fluxes are 120 deg. apart, equals one of the fluxes. If the middle phase were not reversed, 1, 2 and 2, 3 would carry the difference of II II FIG. 171. — Core type three-phase transformer. two fluxes 120 deg. apart, and this difference is V3 times each flux, thus would give a much higher loss. In Fig. 171 usually the exciting current of the middle phase is somewhat less than that of the outside phase, since the magnetic reluctance of the middle phase is slightly lower.