A Magnetic Circuits or core of a transformer is designed to provide a path for the magnetic field, which is necessary for induction of voltages between windings. A path of low reluctance (i.e., resistance to magnetic lines of force), consisting of thin silicon, sheet steel laminations, is used for this purpose. In addition to providing a low reluctance path for the magnetic field, the core is designed to prevent circulating electric currents within the steel itself. Circulating currents, called eddy currents, cause heating and energy loss. They are due to voltages induced in the steel of the core, which is constantly subject to alternating magnetic fields. Steel itself is a conductor, and changing lines of magnetic flux also induce a voltage and current in this conductor. By using very thin sheets of steel with insulating material between sheets, eddy currents (losses) are greatly reduced. The two common arrangements of the magnetic path and the windings are shown in figure 6 and 7. In the core-type (core form) transformer, the windings surround the core.
A section of both primary and secondary windings are wound on each leg of the core, the low voltage winding is wound next to the core, and the high voltage winding is wound over the low voltage.
Figure 6 – Magnetic Circuits
Figure 7 – Three-Phase Core Form and Three-Phase Shell
Form Transformer Units
In a shell-type (shell form) transformer, the steel magnetic circuit (core) forms a shell surrounding the windings. In a core form, the windings are on the outside; in a shell form, the windings are on the inside. In power transformers, the electrical windings are arranged so that practically all of the magnetic lines of force go through both the primary and secondary windings. A small percentage of the magnetic lines of force goes outside the core, and this is called leakage flux. Larger transformers, such as Reclamation GSU transformers, are almost always shell type.
Note that, in the shell form transformers, (see figure 7) the magnetic flux, external to the coils on both left and right extremes, has complete magnetic paths for stray and zero sequence flux to return to the coils. In the core form, it can easily be seen from the figure that, on both left and right extremes, there are no return paths. This means that the flux must use external tank walls and the insulating medium for return paths. This increases core losses and decreases overall efficiency and shows why most large transformers are built as shell form units.