Transformer : Cores

Cores

Any material inside a coil, used to serve as a form to support it, is called a core. Cores are made of different materials with permeability ranging from 1 to over 10000. The higher permeability aid in providing low reluctance path of the ?ux and the ?ux lines mostly con?ne themselves to the core. The permeability of air is 1 whereas the permeability of common “ferro-magnetic” materials is about 300 for ordinary steel, about 5,000 for 4% silicon transformer steel, and up to about 100,000 for some nickel-iron-molybdenum alloys. Because such materials concentrate magnetic flux, they greatly increase the inductance of a coil. Coil inductance is directly proportional to the square of the number of turns and also, direct proportional to the permeability of the core. Silicon steel, hot rolled grain oriented steel, Cold Rolled Grain Oriented (CRGO), etc. are some of the material used in the form of thin laminations for the core; the laminations (in the form of E & I, C & I or O) are coated with a layer of insulating varnish, oxide or phosphate.

Ferrite cores are best suited for high frequency applications and steel laminations are best suited for low frequency applications. For lower frequencies, core material selection is governed by core saturation considerations. Eddy current losses are low so steel laminations can be considered. For higher frequencies, core material selection is governed by core loss considerations. Eddy currents can be significant. In such applications, ferrites are commonly used.

Numeric Codes representing the power handling ability have been assigned to the cores by the manufacturers; the assigned number is the product of its window area and the core cross-section area. The codes are available for laminations, C cores, pot cores, powder cores, and Toroidal tape-wound cores.