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Tuesday, 6 August 2019

Grain Oriented Steel Sheet

The ferro magnetic materials are crystalline in structure. Each of these crystals have high permeability in a particular direction. When they are magnetised in that direction they can be magnetised very easily and quickly. This is called "easy or soft direction". It becomes hard and difficult to magnetise a material along any axis other than this. For easy magnetisation the crystal directions should be oriented so that their axes lie in parallel with the axis of magnetisation. This can be obtained by rolling the sheets carefully and annealing them after rolling. Fig (a) indicates irregular directions of axes and in Fig (b) the arrangement when the crystals axes are in one direction is indicated. Sheets like this are called grain oriented and they require less magnetisation which cause less hysteresis loss. The process in which the grain orientation is done is called cold rolling and the sheets obtained by this method are called "Cold Rolled Grain Oriented" or CRGO sheets. These are used in transformer core plates. The magnetising force required is very small because the grains are already oriented and they are in soft directions. But in case of non-oriented grain sheets the magnetising force and thus the losses are more.
Grain Orientation in Sheet Steel
When CRGO steel is used to manufacture the transformer cores, the core should be manufactured in such a way that the crystal direction is parallel to the flux path otherwise the core will offer low permeability rather than high permeability. The benefit of grain orientation cannot be generally utilised in the manufacture of the magnetic core for electrical rotating machines due to their circular nature of the core. If a circular core is shaped of a CRGO sheet steel, in the majority part of the iron path the grains be not oriented in the direction of magnetic flux.

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