Powercore

Granular electrical steel is used for the manufacture of magnetic conductors. It should be noted that the best magnetic properties are found only in the rolling direction. If the magnetization is outside the direction of rolling, the losses in the core will increase significantly, for example, at an angle of 90° to the direction of rolling, the losses increase more than three times, and at 60° they increase more than four times. It is therefore important that the steel is magnetized as accurately as possible along the rolling direction throughout the magnetic circuit.

Mechanical tension

Mechanical stress has a very negative effect on the magnetic properties of granular electrical steel. Strips can be subjected to such stress for various reasons:

• external forces (external stresses)
• plastic deformation (internal stresses)

External stress is caused by excessive or uneven compression, which causes the magnetic core plates to become wavy or curved. Internal stress is generated along the cut edges during each cutting operation or as a result of the sheet being bent or stretched beyond the yield point.

This sometimes unavoidable stress can be almost completely eliminated by stress relief annealing. The material can be annealed in a continuous air annealing line (short-time annealing) or in a chamber annealing line in a nitrogen atmosphere (long-term annealing). Whether the material is annealed for stress relief depends on the conditions at the customer’s installation site.

Annealing by the customer

Short-term annealing

Plates are usually subjected to short-term annealing in a roller furnace. This process takes several minutes and requires a soaking time of 1 to 2 minutes at a maximum temperature of 860 °C. As the layers are annealed in an air atmosphere, the cut edges are oxidized, thus forming an insulating coating. Any grease or oil from previous processing steps will burn off and is generally harmless in small amounts.

Long annealing

Coiled cores and stack transformers are annealed for a long time in a box-type furnace. Prolonged annealing must be carried out under the following conditions:

• Soaking temperature: Min. 820 °C,
  max. 840 °C to 850 °C
• Soaking time: 2 hours (the coldest part of the material must be at least 800 °C)
• Cooling: preferably in an oven to about 200–300 °C
• Protective atmosphere: Preferably 100% nitrogen. Adding hydrogen is not recommended.

The heating, soaking and cooling times are largely determined by the type and size of the furnace and the amount of material to be annealed. The annealing cycle must be adapted to the above parameters. As a rule, too fast heating of the material can lead to local overheating, especially in the outer cores. This risk can be reduced by controlling the temperature with a thermocouple near the heating conductors. The holding time must be long enough to ensure that the annealing material reaches the soaking temperature (minimum 800 °C) throughout.

If the material cools too quickly, the cores can warp or deform. In addition, it is recommended that the soak temperature be controlled by thermocouples located at the hottest and coldest points of the annealing material. The cores should be allowed to cool in the furnace to a temperature of 200 °C to 300 °C to avoid the hardening effect during unloading. The annealing material must be free of grease, oil and other organic matter to prevent carbonization.

Refined domain material

Laser-irradiated powercore ^® H stress relief annealing reverses the reduction in core loss caused by laser treatment. The special design of our laser beam ensures that excellent adhesion properties and high insulation resistance values ​​are maintained in our laser-irradiated powercore H grades. As a result, laser-irradiated powercore^® H grades demonstrate the same favorable noise behavior in finished transformers as powercore^® Hgrades  that are not laser treated.