Core flux losses decrease with thinner silicon transformer steel and were originally graded by a number that represented the loss in W/kg. Then as they were able to produce very thin materials of cold rolled grain oriented steel or CRGOS the numbers did not always correlate to precise losses, yet were able to reduce losses from 0.9 W/kg down to 0.3 and even less.
Iron and hot rolled steel have far greater losses, while ferrite has far lower permeability.
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The losses are a function of eddy currents in the thickness of the material. Each layer is coated with a very thin silicate coating for an electrical insulation which gives each layer high capacitance with a tiny gap yet very high permeability to increase saturation levels of layers and greater linearity. The LC equivalent circuit of the core thus makes it suitable only for low frequencies.
MOT units are also made this way but welded across the seams to eliminate hum but that shorts all of the insulated laminations and makes them far less efficient as a power transformer. For some high voltage cores it is important to remember if you want higher performance to keep the edges clean of magnetic conductive dust particles, which is another topic of insulation breakdown from contaminated partial discharge.
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