Some effects of grain boundaries on the field distribution on the surface of grain oriented electrical steels

Abstract

A non-destructive system using a Hall effect I.C. sensor has been used to measure the distribution of the vertical component of stray magnetic field on the opposite surfaces of a magnetised specimen of high permeability, grain-oriented electrical steel. An optical microscope was used to establish positions of grain boundaries in order to explain differences in field profiles outside the systematic accuracy of measurements. It was found that unlike previously assumed, grain boundaries may not necessarily be perpendicular to the sheet surface so different field distribution may occur. This should be considered when evaluating local power loss from surface sensor movements. Future studies are also necessary to quantify the effect of this phenomenon in the interpretation of surface or bulk properties of electrical steels using surface field sensors. Electrical steels are used in many transformer and motor core applications where a.c. magnetic permeability or losses are important. They are usually used in sheet form typically 0.23 mm to 0.65 mm thick. Grain oriented 3.25% Silicon Steel has a strong (001)(110) texture with grains of up to 10 mm in diameter. Its surfaces are usually coated with complex non-magnetic layers to impose beneficial tensile stress in the steel as well as providing the necessary interlaminar or interturn insulation when built into cores. The energy loss under a.c. magnetisation is proportional to the integral of the product of the instantaneous tangential component of surface field and the instantaneous spacial averaged rate of change of flux density. In grain oriented electrical steel the bulk of the material in the demagnetised state comprises anti- parallel bar domains with 180 ◦ walls. The domain, structure which depends on the grain size and orientation as well as several other factors is changed by application of an external field and during the process magnetic losses occur. The surface magnetic field in the magnetised or demagnetised state can give an indication of material structure which in turn has a large influence on the losses and permeability. The surface magnetic field can be measured in several ways. Hall sensors have been used for the measurement of the tangential component of surface field of the coated material at scanning steps of around 0.3 mm, sensor length being typically 3 mm (1,2). More recently smaller magneto resistive sensors are used with greater flexibility (3). Close relationships are found between grain structure orientation and tangential field (4).