The transformer core provides a path to the magnetic field that is required to induce the voltage between the two windings.
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The core of a transformer in a given frequency range is usually selected by keeping efficiency in mind. The efficiency is determined by loss of iron, loss of copper and also by magnetising currents in small devices.
The core is usually made of laminated steel sheet which is assembled to provide a continuous magnetic path with a minimal air gap. The steel used is heat-treated with high silicon content to produce high permeability and low hysteresis loss at normal operating flux densities.
Laminating the core minimises the eddy current failure. Laminations are used by a lightweight coating of the core plate or oxide on the surface. The lamination thickness ranges from 0.35 mm for 50Hz to 0.5 mm for 25Hz frequency.
Presently, the Cold Rolled Grain Oriented Silicon Steel (CRGO) is the major material used to manufacture the laminated core. The material is used for the core to reduce the losses in the core. The core losses are mostly hysteresis and eddy current losses. CRGO is the best suitable solution to minimise both of these losses in the core and increase its resistivity.
Mostly the material chosen for the transformer core has high permeability. The CRGO can give high permeability to the flow of magnetic flux and low conductivity to break the strength of eddy current losses. If silicon steel crystals are placed parallel to the magnetisation direction; the material has a much greater relative permeability and can be used at much higher flow densities with moderate current flow. The cold-rolling of silicon steel sheet can achieve this crystal structure.