Jean-Philippe Lecointe

Equivalent Permeability of Step-Lap Joints of Transformer Cores: Computational and Experimental Considerations

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.

Analysis of the Magnetic Flux Distribution in a New Shifted Non-Segmented Grain Oriented AC Motor Magnetic Circuit

This paper deals with a new AC electric motor magnetic circuit structure made of shifted non-segmented grain oriented steel laminations. The aim of the paper is to understand with a finite element approach the local distribution of the magnetic flux density inside shifted laminations as well as in the air-gaps between them. It is shown that the air-gap flux distribution is not trivial at all and very difficult to appreciate.

Use of the External Magnetic Field for Induction Machine Leakage Inductance Distinction

This paper is about the exploration and the analysis of the AC rotating machine external magnetic field. An experimental procedure is presented: it makes it possible to distinguish the stator and rotor leakage inductances which occur in the induction machine single phase equivalent circuit.

Using the FEM Meshes Adaption and Genetic Algorithms for Identification of Permeability in Normal Direction of Anisotropic Sheets

This paper proposes the identification of the B-H curve of anisotropic magnetic laminations in the direction normal to the sheet surface. An automated procedure for linking an identification algorithm implemented in a Matlab environment with a commercial finite-element code Opera for 3-D magnetic field analysis has been used. In particular, an optimization procedure is proposed, where the computation time is reduced by adjusting the refinement of the FEM mesh. The use of a multiprocessor computer allowed to perform parallel computations and to realize the calculation in a reasonable time. The finite element mesh density is changed during operation of the procedure depending on the level of magnetic flux density difference between calculation and measurements.

Simplified models including eddy currents for laminated structures

Purpose – The purpose of this paper is to present a comparative analysis concerning the influence of eddy currents on the distribution of the magnetic flux density in the laminated anisotropic structures.Design/methodology/approach – The influence of the magnetic flux normal to the lamination surface is particularly analysed. Several models containing internal air gaps and overlapping are tested. For every structure, the eddy currents are first taken into account and then, they are neglected. At last, the 3D simulation of the anisotropic conductivity permits to analyse separately the longitudinal and normal flux in the structure and the eddy currents induced by those fluxes.Findings – The study leads to a more realistic numerical model with conducting laminations. The results show that the normal flux does not turn at once on lamination. The normal and longitudinal fluxes induce eddy currents which modify the flux distribution in the laminated structure.Practical implications – The results of the presente...

A new approach to the critical induction in transformer cores

The most critical areas in power transformers are their corner joints where the magnetic flux distribution presents a very high heterogeneity. This heterogeneity results from several parameters such as the geometry of the magnetic core, the anisotropy level of the electrical steel laminations and a critical induction. This paper is focused on the magnetic steel grades influence on the critical induction, adding a new parameter to those classically considered, such as the number of steps in a multiple step lap stack. Due to the difficulty to perform local measurements inside a real transformer corner, the study is performed both experimentally and by Finite Element analyses on a simplified magnetic core structure. The paper highlights that, in a corner of a core, it exists a critical induction level which depends on the magnetic steel grades and that, in a lamination, the magnetic flux density value along the Transverse Direction can be of the same order that it has along the Rolling Direction.

On the use of carrier phase jumps to reduce some PWM switching effects

This paper is based on a method recently developed by the authors to characterize analytically, without switching angle determination, the voltage harmonic three-phase systems generated by the switching of PWM inverters. On this basis, it is possible to define control strategies that modify the spectral content of the output signals. The presented developments are focused on the carrier phase jump which aims to act on two harmonics simultaneously by controlling the effects on the other spectral components.

An Improved 2-D Subdomain Model of Squirrel-Cage Induction Machine Including Winding and Slotting Harmonics at Steady State

This paper proposes an improved subdomain model of squirrel-cage induction machines (SCIMs) with imposed stator currents. This new model enables to accurately compute electromagnetic quantities such as air-gap flux density, instantaneous torque and forces, and electromotive force including all harmonic contents. The first improvement is to explicitly account for rotor motion with time-stepping technique. The second improvement consists in modeling the skin effect in rotor bars by considering each space harmonic of the stator magnetomotive force separately. Eddy currents in rotor bars are therefore “skin limited” and not “resistance limited.” The results are then validated with linear transient finite-element analysis for both no-load and load cases, taking a topology of squirrel-cage machine already used in the previous references. The time and spatial harmonic content of all electromagnetic quantities is also validated by comparison with analytical expressions. The computation time is a hundred times lower than finite element as it does not require achieving a numerical transient first, and the resolution for a time step is shorter. Thanks to its computational efficiency and intrinsic mesh insensitivity, this method is particularly suited to magnetic forces computation and vibroacoustic analysis of SCIMs.

Non-invasive detection of winding short-circuit faults in salient-pole synchronous machine

This study presents an analytical and experimental method for detecting internal turn-to-turn short circuits both in the rotor or in the stator in Salient-pole Synchronous Generator Machines (SSGM) connected to local power grid. Two main measurements are proposed to confirm or discard these electric faults: External Magnetic Flux Density (EMFD) and External Housing Vibrations (EHV). In the event of a short-circuit winding fault, a correlation between EMFD and EHV is confirmed. Furthermore, the electric power injection dependency is introduced in order to establish the validity range of this method. In this study, only non-invasive and on-line equipment are required to diagnose the SSGM. A diagnostic tool has been developed to show the viability of an operative wireless monitoring system in an industrial environment.

Using ARDUINO development platform in the diagnosis of AC electrical machines

Low cost electronic platforms based on Atmel microcontroller, as Arduino platform, are used in many applications and in several fields. Indeed, their characteristics in terms of inputs, outputs, CPU and RAM make possible to develop rapid smart-systems for measurement avoiding additional components. CPU and RAM performance allows analyzing complex numerical signals. The aim of this paper is to show some boarddevelopments (ARDUINO UNO, M0 and DUE) coupled with specific sensors for electrical rotating machine applications in industrial environment.