The paper develops an efficient computational method for establishing equivalent characteristics of magnetic joints of transformer cores, with special emphasis on step-lap design. By introducing an equivalent material, the method allows the real three-dimensional structure of the laminated thin sheets to be treated computationally as a two-dimensional problem and enables comparative analysis of designs. The characteristics of the equivalent material are established by minimizing the magnetic energy of the system. To verify the proposed approach, a series of experiments have been conducted. First, the anisotropic characteristics of the step-lap were established, and then space components of the flux density at specified positions measured. This enabled detailed analysis of the flux distribution in the step-lap region, in particular the way in which the flux travels between the laminations close to the air-gap steps. Encouraging correlation between the homogenized 2-D model and experiment has been observed.

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Purpose -The paper gives a new measurement method of the parameters characterising the magnetic laminations for broadband low-level signals defined at any operational point. Design/methodology/approach -High frequency phenomena machines fed by PWM inverters are related to low-level signals corresponding to minor hysteresis loops around the instantaneous working point, which moves on the main loop at the basic frequency. The minor loops are assimilated to ellipses, which are characterised by only two parameters: the incremental magnetic permeability (m) and the electric conductivity (s).Findings -For small signals high frequency field components, the laminated steel behaviour can be described by two local parameters (m, s) and skin effect. The values of m and s do not depend on frequency up to 1 MHz, but only on the operating point.Research limitations/implications -The proposed broadband characterisation should be associated with a Priesach model that defines the operating point for computer simulation of high frequency phenomena. Practical implications -The broadband characterisation of magnetic laminations is useful for studying the behaviour of the windings of the PWM-fed machines. Originality/value -Broadband measurements are now possible on small magnetic steel lamination samples.

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Abstruct-The paper presents a new method of taking into account overlapping zones in stacked transformer cores for magnetic field calculations. The method has been worked out with the use of homogenization technique under the assumption that the distribution of magnetic flux between the layers of overlaps results from the tendency to reach the minimum of the magnetic field energy. This enables replacement of a heterogeneous 3-D overlap structure with a simpler homogeneous 2-D one. The method has been used to calculate equivalent homogenized reluctivity of the whole structure and flux density vectors in its layers. The family of equivalent anisotropic magnetizing curves of an overlap has also been determined. To discuss the method, a classical overlap of a transformer core has been chosen. Calculation results and the results of measurements taken in two adjacent sheet layers of an overlap situated in a T-joint of a 3-limb 3-phase transformer core have been compared and are presented in the paper.

PurposeA new model able to describe the high frequency (HF) behaviour of the laminated cores of AC machines is proposed. The aim is to compute the external flux density of machine cores, corresponding to electromagnetic emissions in the HF range when the skin effect is predominant.Design/methodology/approachFor high frequencies, the skin depth is much lower than the thickness of a lamination and the external flux density is determined using a new analytical model. The validity of this model is confirmed by measurements performed on a magnetic core representing a small part of a large machine and a finite element 3D simulation.FindingsFor high frequencies, the external flux density is computed considering an equivalent current layer flowing on the laminated core external surface. Eddy currents in the laminated core have a large influence on the current density in this current layer.Research limitations/implicationsThe new model proposed is valid when the skin depth is lower than half the thickness of a lamination.Practical implicationsThe knowledge of the machine magnetic core behaviour in the frame of the HF electromagnetic emissions has practical applications for large AC machine maintenance such as the localization of partial discharges in the winding insulation. With this model, it is possible to analyse the information given by small magnetic sensors placed between the machine core and the external frame to solve all the insulation problems.Originality/valueThe new proposed model is able to establish a link between the electric HF phenomena in the windings of a working machine and the magnetic flux density outside the laminated core.

Purpose The purpose of this paper is to determine the physical design parameters that influence the total resistance of a twisted conductor (cable). One of the physical parameters characterizing this type of structures is the uneven distribution of resistivity due to hardening, which is the result of stress exerted on the wires during the manufacturing process. Design/methodology/approach The authors have developed a method to take into account the effect of localized hardening on the inhomogeneous distribution of electrical conductivity in the distorted structures of the conductor. To achieve this goal, the authors have implemented a mechanical-electrical simulation method. The resistance characteristics have been measured as a function of mechanical stress. Findings As demonstrated by the results of measurements conducted on various samples and with various cable design parameters, the resistance of a given material (copper or aluminum), expressed as a function of stress, does not depend on the type of force applied. Therefore, the same characteristics may be applied to various cable designs. Practical implications The method presented in this paper enables more detailed investigation of the influence of particular design parameters on the total resistance of a cable. It also provides the ability to determine optimal settings of design parameters. Originality/value The approach is distinct from similar studies because it takes into account the deformed geometry of the conductor and the uneven distribution of the resistivity within a filament. In the literature, it is sometimes stated that the distribution of resistivity in a compacted cable is uneven, but its measurement is deemed impossible. This paper provides a method for determining such a distribution.