Residual flux density in the single-phase transformer core can dramatically increase inrush current when the transformer is energized. To reduce inrush current, it is necessary to study residual flux density measurement. This paper proposes a new residual flux density measurement method based on time constant. Firstly, the generation principle of residual flux density is analyzed under different magnetization states, and it is found that the positive relative differential permeability is smaller than the negative at different residual flux density. To obtain the relative differential permeability, when an appropriate DC excitation is applied, the measurement circuit is equivalent to a first-order RL circuit. Then, combining magnetic circuit and transient circuit analysis, the relationship between time constant and relative differential permeability is obtained. It is conclusion that the positive time constant is less than the negative. Residual flux density direction is determined by comparing the positive and negative time constant, and the magnitude of residual flux density is calculated by the relationship between residual flux density and the difference of the positive and negative time constant. Finally, the empirical formula between residual flux density and time constant difference of the square core is obtained in finite element method, and then verified on the experimental platform. Compared with other measurement methods, the relative error of proposed empirical formula is within 4.58 %, and it has higher accuracy in this paper. The proposed method in this paper can provide a reference for selecting the demagnetization voltage, which improves the effectiveness of demagnetization. INDEX TERMS Residual flux density, relative differential permeability, time constant difference, empirical formula.

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Soft magnetic composite (SMC) is composed of ferromagnetic particles surrounded by an electrical insulation layer. The unique structure of SMC makes it difficult to calculate the eddy current loss of SMC. In addition, the temperature rise in the working condition affects the magnetic properties of the material, which makes it necessary to consider the temperature effect in the process of core loss prediction. In this paper, a magnetic property test platform that considers the influence of temperature is established. The core losses of the SMC ring sample were measured at 20-100 °C , and the experimental results show that core loss decreases as the temperature increases. Then, based on the method of using the SMC model to calculate the core loss, the effect of temperature increase on hysteresis loss and eddy current loss was analyzed, and the existing model was improved to consider the effect of temperature on the core loss. Finally, the measured core losses at different temperatures were compared with the calculated ones, and it is shown that the improved model considering the influence of temperature agreed well with the experimental results, which improves the accuracy and convenience of core loss calculation for the electromagnetic device with SMC cores.

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Due to the effect of higher harmonics on magnetic properties under actual complex operating conditions, the accurate calculation of core losses of soft magnetic composites (SMC) is complicated. First, this paper improves the existing SMC model by introducing a correction factor to correct the hysteresis loss coefficient so that the model can consider the local variation characteristics of the magnetic density waveform and then calculate the core loss under different harmonic excitation. Then, the influence of skin effect and inhomogeneous flux density within the ring sample model is analyzed. Finally, to validate the improved model, it is compared with other models in the reference based on experimental measurements, respectively. The results show that the core loss calculated by the improved model is closer to the experimental results under different harmonic excitations. In addition, the applicability of the improved SMC model under triangular and square wave excitations is also verified by the derivation of the equations.