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Further processing hints
Grain oriented electrical steel sheet is used for the building of magnetic cores. It is important to point out that the best magnetic properties exist only in the rolling direction. When the magnetising is done outside the rolling direction, the core loss values increase substantially; e.g. at 90° to the rolling direction more than triple and at 60° more than quadruple. Therefore, it is absolutely necessary to constrain the magnetising in the whole magnetic circuit as close as possible to the rolling direction for optimal performance.
Mechanical stresses
Mechanical stresses exert a significant effect on the magnetic properties of grain oriented electrical steel.
Such stresses can be introduced into the strip in many ways:
- by external forces (external stresses)
- by plastic deformation (internal stresses)
External stresses may arise if the laminations in a magnetic core are pressed into a wavy or curved shape by a too strong or uneven compression. Internal stresses are generated along the cut edges during each slitting operation and as a result of bending the sheet or subjecting it to tension beyond the yield point. Through stress relief annealing it is possible to reduce these unavoidable stresses nearly completely. Such annealing can be done in a continuous furnace under air (short annealing cycle) or in a batch type furnace under nitrogen (long annealing cycle). Whether stress relief annealing is necessary or depends on the conditions prelavent at each custumer's site.
Short annealing cycle
For the annealing of laminations the short annealing cycle in a roller hearth furnace is advatageous. This cycle lasts only few minutes, the soaking time being 1-2 minutes at the soaking temperature of 860°C maximum. Since annealing is done in a atmospheric air, the edges are subject to oxidation which produces an insulating coating. Grease and oil from previous processes are burnt off. Therefore, they generally are not harmful, provided that they do not occur in excessive amounts. Short-time annealing does not cause the flatness of the laminations to deteriorate and the electrical resistance of the insulating coating is not reduced at all by this process.
Batch annealing cycle
The batch annealing cycle is recommended for wound cores. It is performed in a batch type furnace or a continuous type furnace. The maximum temperature is 820°C. The heating and cooling times largely depend on the furnace size and the amount of material to be annealed. Heating and cooling have to be long enough in order to avoid warping or distortion. The soaking time must be as long as it takes for the material to reach the soaking temperature throughout the cores. Nitrogen is recommended as a protective atmosphere and traces of oxygen are harmless. An addition of hydrogen is not useful. The material to be annealed must be free from grease, oil and other organic substances to prevent any recarburisation. By stress relief annealing of laser irradiated PowerCore high permeable grades, the improvement of the cores loss due to laser treatment will be reversed. Such
PowerCore grades maintain their excellent adhesion and the high resistance value of their insulating due to the special design of the laser beam. Hence, laser irradiated PowerCore high permeable grades exhibit the same low noise levels in a transformer as high permeable grades without laser irradiation.
Domain refined material
If laser irradiated PowerCore H-material is stressed relief annealed, the improvement of the core loss due to laser treatment will be reversed. As far as our PowerCore grades are concerned the excellent adhesion and the high resistance value of the insulation remain constant due the special design of the laser beam. Hence laser irradiated PowerCore H-grades have the same beneficial influence on the noise level of a transformer as PowerCore H-grades without laser irradiation. |
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