Magnetic shielding using electrical steel panels at extremely low frequencies
A new test method and finite element modelling were used to investigate how material properties of electrical steel panels affect their shielding factors. Both experiment and modelling showed an improved DC shielding factor with increasing thickness for the shields of similar magnetic properties. En...
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Cardiff University
2008
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| Online Access: | http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.584325 |
| Summary: | A new test method and finite element modelling were used to investigate how material properties of electrical steel panels affect their shielding factors. Both experiment and modelling showed an improved DC shielding factor with increasing thickness for the shields of similar magnetic properties. Enhanced shielding by the eddy currents was demonstrated by testing the same steel panel under AC and DC field conditions. Comparing to non-oriented steel panels, a decreasing shielding factor with the frequency from 50 Hz to 400 Hz was found for grain-oriented steels. This was also investigated by measuring magnetizations along rolling and transverse directions within the panels. It was found that measured shielding factors of double-layer shields with two grain-oriented steel panels could be improved significantly with orthogonally arranged rolling directions. Different shielding factors were found by placing different panel closer to the field source in the test of double-layer shields formed by one grain-oriented and one non-oriented steel panels. Although little shielding effect of aluminium panels are found at 50 Hz, adding the same aluminium panel with single electrical steel shield was dramatically improved the shielding. The magnetization of shielding sample at AC conditions has been modelled and measured. The magnetization was found very low due to the demagnetizing effect. Therefore, the permeability at very low magnetization range has a large effect on the magnetic shielding factors of the steel panels. Drilled hole as a defect in the panel and overlap of the panels have been tested with the new test method. Small hole in the panel would not cause the degradation of the overall shielding factor of the shield rooms. Overlap was proved to be an effective way to reduce the flux leakage at the joints between the panels The difference between the computed and measured shielding factors is addressed by analysing the capability of the solver used in the finite element modelling and the uncertainty of the measured B-H characteristic of the material as the input to the model. |
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