Measurement and modelling of magnetic properties of materials with rotating fluxes
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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- With magnetic materials being widely used, the demand for understanding the characteristics of magnetic materials, such as the relation between magnetic flux density B and magnetic field strength H or B-H curve and electromagnetic power losses or core losses with various kinds of magnetic field excitations, is inevitable. These characteristics are used in the study of magnetisation mechanisms, and performance simulation and assessment in design of electromagnetic devices. This dissertation is focused on two main areas: the numerical vector modelling of magnetic properties and the measuring methods and techniques for vector magnetic properties. The comparative study of three major macroscopic vector hysteresis models shows that the Stoner-Wohlfarth (S-W) model appears to be most suitable for describing the vector characteristics of magnetic materials, because it is inherently a vector magnetisation model due to its vector elemental operator, and it can instantaneously take both reversible and irreversible magnetisation into account. A new S-W elemental operator that can describe the vector magnetic characteristics of a single domain particle in three dimensions was derived. A method to determine the position of the magnetic moment ms of an S-W particle in three dimensional (3D) space corresponding to an excitation magnetic field was proposed. In addition, an algorithm that incorporates the mean interaction field into the S-W theory was implemented to model the 3D behaviour of multiple single domain particles under alternating and rotating excitation fields. The simulation results are presented. For the measurement of vector characteristics of magnetic materials under alternating and rotating magnetic excitation fields, a single sheet tester (SST) developed at UTS was modified. The precision of two dimensional (2D) field strength measurement at the surface of specimen is improved by a 2D Rogowski-Chattock H sensing coil. A soft magnetic composite material, Somaloy 500™ recently developed for rotating electrical machines by Höganäs AB, Sweden, was measured using the SST with various alternating and 2D rotating excitation fields at different frequencies. These measurements provided much useful information for both understanding of the mechanisms and modelling of magnetisation. An induction field, which is related to the magnetization in the magnetic material under test, is introduced to explain the discrepancy between the core losses measured with 2D rotating magnetic fluxes in opposite rotating directions. To eliminate its effects to obtain the correct measurement results, the author suggests that both the rotating transform and the averaging methods should be employed. A totally new experimental instrument, 3D magnetic tester, for studying the magnetic properties with 3D magnetic excitations was designed, constructed, and calibrated. ANSYS, a finite element package was used for field analysis in the design. A formulation to calculate the power losses in the sample under 3D magnetic excitations from the measured flux density vector B in the sample and magnetic field strength vector H on the sample surface was derived from the Poynting's theorem. A novel 3D surface H sensing coil was designed and elaborately manufactured. A method to correct the misalignment angle of the 3D H sensor was also proposed. Finally, a sample of grain oriented SiFe sheets was tested by using the newly developed 3D magnetic property tester and the experimental results with alternating and 2D rotating fields are in substantial agreement with those obtained by conventional 1D and 2D measurements.
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