M. Lobue

3-D Formal Resolution of Maxwell Equations for the Computation of the No-Load Flux in an Axial Flux Permanent-Magnet Synchronous Machine

This paper presents a 3-D analytical model of an axial flux permanent-magnet synchronous machine, based on formal resolution of Maxwell equations. This method requires much less computation time than conventional 3-D finite elements, and is therefore suitable for optimization purposes. In a first part, the mathematical procedure used to compute the machine no-load flux is described in detail. This method is 3-D, and then takes into account the radial edge effects of the machine, as well as the curvature effects by a resolution in cylindrical coordinates. Moreover, the originality of this method lies in the fact that it is totally analytical. The obtained results are verified using 3-D finite elements, and compared with simpler analytical models of axial flux machines, taken from the literature. This work puts in evidence the advantages of the proposed model. In particular, it is shown that the radial edge effects are important for a correct estimation of the no-load flux. On the contrary, the curvature effects are a second-order phenomenon.

Classical eddy current losses in Soft Magnetic Composites

This paper deals with the problem of loss evaluation in Soft Magnetic Composites (SMCs), focusing on the classical loss component. It is known that eddy currents can flow in these granular materials at two different scales, that of the single particle (microscopic eddy currents) and that of the specimen cross-section (macroscopic eddy currents), the latter ensuing from imperfect insulation between particles. It is often argued that this macroscopic loss component can be calculated considering an equivalent homogeneous material of same bulk resistivity. This assumption has not found so far clear and general experimental validation. In this paper, we discuss energy loss experiments in two different SMC materials, obtained using different binder types, and we verify that a classical macroscopic loss component, the sole size-dependent term, can be separately identified. It is also put in evidence that, depending on the material, the measured sample resistivity and the equivalent resistivity entering the calculation of the macroscopic eddy currents may not be the same. A corrective coefficient is, therefore, introduced and experimentally identified. This coefficient appears to depend on the material type only. An efficient way to calculate the macroscopic classical loss in these materials is thus provided.

Extended frequency analysis of magnetic losses under rotating induction in soft magnetic composites

We present novel results on magnetic losses in soft magnetic composites (SMCs) excited with rotating field. Soft composites are very promising in electrical engineering applications, where new topologies of electrical machines with two- and three-dimensional induction loci are increasingly found. An experimental characterization of industrial SMC products has, therefore, been carried out, up to the kilohertz range, under alternating and circular flux loci, making use of a specifically designed and optimized loss measuring setup. The obtained results have been analyzed for all kinds of excitation, according to the loss separation concept, with the emphasis being placed on the relationship between the rotational and the alternating loss components. In particular, it is found that the ratio between the rotational and the alternating losses is, for any given peak induction, independent of frequency.

Direct calorimetric measurements of isothermal entropy change on single crystal W-type hexaferrites at the spin reorientation transition

We report on the magnetic field induced isothermal entropy change, Δs(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are performed by direct calorimetry. Single crystals of the composition BaCo0.62Zn1.38Fe16O27, prepared by the flux method, are measured at different fixed temperatures under an applied field perpendicular and parallel to the c axis. At 296u2009K one deduces a value of K1u2009=u20098.7u2009×u2009104u2009 J m−3 for the first anisotropy constant, which is in good agreement with the literature. The spin reorientation transition temperature is estimated to take place between 200 and 220u2009K.

Skin effect in steel sheets under rotating induction

By means of a newly developed broadband measuring setup we have overcome the usual upper limit for the test frequency, around a few hundred Hz, which is encountered in the two-dimensional characterization of magnetic steel sheets at technical inductions and we have measured the rotational losses in low-carbon steels up to 1 kHz and peak induction 1.7 T. An important piece of information is thus retrieved upon a frequency range useful to predict the performance of high-speed electrical machines. Our experiments, performed on thick (0.640 mm) laminations, have brought to light the emergence of the skin effect under rotational fields. This is revealed by an abrupt deviation of the excess loss component, calculated under the conventional loss separation procedure, from its well-known linear dependence on the square root of the frequency. A simple magnetic constitutive law under rotating induction is proposed and introduced into the electromagnetic diffusion equation, which is solved by finite elements coupled to a non-linear algorithm. The classical rotational eddy current loss, largely prevalent with respect to the hysteresis and excess loss components on approaching the kHz frequencies in low-carbon steels, is then calculated in the presence of skin effect, permitting one to achieve full analysis of the rotational losses and good predicting capability upon a broad range of frequencies and peak inductions.

High-frequency rotational losses in different soft magnetic composites

The isotropic properties of Soft Magnetic Composites (SMC) favor the design of new machine topologies and their granular structure can induce a potential decrease of the dynamic loss component. This paper is devoted to the characterization of the broadband magnetic losses of different SMC types under alternating and circular induction. The investigated materials differ by their grain size, heat treatment, compaction rate, and binder type. It is shown that, up to peak polarization J p = 1.25 T, the ratios between the rotational and the alternating loss components (classical, hysteresis, and excess) are quite independent of the material structural details, quite analogous to the known behavior of nonoriented steel laminations. On the contrary, at higher inductions, it is observed that the J p value at which the rotational hysteresis loss attains its maximum, related to the progressive disappearance of the domain walls under increasing rotational fields, decreases with the material susceptibility.

X-ray diffraction analysis of the magnetoelastic phase transition in the Mn-Fe-P-Si magnetocaloric alloy

Structural characterization of the Mn1.3Fe0.65P0.5Si0.5 powder is reported. The rare-earth-free magnetocaloric material was prepared by ball milling and solid-state synthesis. X-ray diffraction data were collected in a wide temperature range across the magnetoelastic phase transition. The lattice parameters and volume fractions of the paramagnetic and ferromagnetic phases were determined as functions of temperature using Rietveld fitting. The virgin effect (a delay of the phase transition on first cooling) and associated variation of lattice parameters are analyzed on the assumption of elastic constraints imposed on the paramagnetic phase by the defect structure. A simple Landau model with magnetoelastic coupling illustrates the observed first-order behavior.

Computation of the Losses in a Laminated Ferromagnetic Material Under Bidirectional Induction Excitation

In this paper, an analytical method for the calculation of power losses in FeSi laminations, taking into account skin effect and the ferromagnetic material nonlinearity, is used to compute iron losses. The electromagnetic field is assumed to be bidimensional, as it is the case in many applications in electrical engineering. The results of the analytical method used to solve the nonlinear diffusion equation are used as a starting point for the losses computation in the ferromagnetic material, using a losses separation model. The computation assumptions are detailed, and the results are discussed.

First- Versus Second-Order Magnetocaloric Material for Thermomagnetic Energy Conversion

We estimate the power and efficiency of a thermal energy harvesting thermodynamic Brayton cycle using the first- and second-order magnetocaloric materials as active substance. The thermodynamic cycle was computed using a simple thermal exchange model and an equation of the state deduced from a phenomenological Landau model. For the first- and second-order materials, narrow- and high-frequency cycles are optimum and give similar performances. Considering technological issues hindering the increase of frequency, we introduced a more detailed approach, where we take into account the time needed to switch the material between two heat reservoirs. We show that the first-order material equation of the state leads thermodynamic cycle shape keeping it closer to the optimum cycle. Conditions to improve the performance of the second-order materials are discussed. In addition, we infer key remarks for prototype design regarding the power density and efficiency reachable in different configurations.

Enhanced magnetoelectric voltage in ferrite/PZT/ferrite composite for AC current sensor application

In this paper, we report the fabrication of a rare-earth free current sensor based on a PZT/NiCoZn-ferrite magnetoelectric (ME) trilayer composite disk. To improve the sensitivity of the sensor, the structure uses an in-plane series connection, which increases the ME voltage by two for a fixed volume. Then, we propose a full characterization of the sensor: electrical modeling, low and high frequency limits, sensitivity, linearity, distortion and resolution. The device is also evaluated under sinus, square, and triangle waveform currents, which are excitation signals commonly used in the field of electrical engineering. The current sensor shows high current sensitivity (90xa0mV/A), good linearity from 0.001 to 30xa0A and low added impedance (0.01xa0Ω) in a frequency range of 10xa0Hz–30xa0kHz for a very compact structure (4xa0cm3). It also exhibits a high resolution of 1xa0mA (for a 1xa0A peak signal) and good stability.