A Practical Solution to Mitigate Vibrations in Industrial PM Motors Having Concentric Windings

Industry applications involving torque brushless dc motors require high efficiency over a large speed range. In order to achieve such specifications, flux weakening is necessary, which could lead to relative high electromagnetically induced vibrations and consequently to audible noises. Furthermore, in order to decrease cogging torque and to satisfy motor manufacturing constraints, concentric windings are usually considered. Such motor configurations lead to harmonics of the electromagnetic forces being the sources of mechanical vibrations. This paper focuses on the electromagnetic forces created in surface-mounted permanent magnet and internal permanent magnet motors for two concentric windings: one and two coil sides per slot. These forces are determined using both lumped magnetic scheme and two-dimensional finite-element (FE) simulations. Then, three-dimensional FE simulations enable to predict the mechanical modes and the acoustic radiated power. Finally, the authors propose a simple method to mitigate the magnetic forces and their variations. This method deals with skewing the stator. The skewing angle is optimized differently as in the usual case of decreasing the cogging torque. This method is really interesting for industry applications, because does not require any important change in the motor construction. The acoustic measurements confirm the analysis and the proposed improvement technique (stator skewing).

Power line communications

Emmanuel Marthe

With the deregulation of the electricity and telecommunications markets, a great deal of new operators providing some innovative services have appeared. In the telecommunications' field, the promise of broadband Internet access with easy installation procedure at low cost may help to win new customers. The Power Line Communications (PLC) technology is a way to fulfil this function. PLC uses the low voltage (LV) and the medium voltage (MV) networks as communication support. It consists in an additional signal carrying data information and superimposed to the 50 Hz power wave. As PLC is easy to install, it can be used to extend the Internet coverage to areas that are still badly covered by the other broadband technologies, or to provide high speed Internet access to every power socket of a building. This thesis, done as a part of research project called Digital Power Line Access, is devoted to the study of electromagnetic compatibility (EMC) aspects involved with the PLC. There are still a certain number of problems due to the fact that PLC, which is a quite recent technology, has not been regulated yet. Among these problems, all the aspects associated with the radiation created by the transmission of PLC signals over unshielded conductors and their potential disturbances are very significant. The aim of this thesis is the analysis of PLC data transmission over the low voltage network and the related EMC problems, mainly the radiation due to the transmission of PLC signals. The measurement and the analysis of these radiations lead us to develop a method to reduce these radiations. The mitigation of radiation created by the use of PLC within buildings is still an important EMC challenge, because there are no possibilities for network conditioning. The first two chapters of this thesis present a state of the art of PLC, its regulation and the related EMC problems. The second chapter describes the working of PLC and emphasizes the hostile nature of its environment. The problems (filtering, attenuation, generation of noise,...) caused by the use of a signal in the [1 – 30 MHz] band over a channel designed for a 50 Hz power wave are presented. On the other hand, the various technologies used to allow the PLC to work (suitable modulation scheme, use of repeater,...) are illustrated. An analysis of the different PLC concepts and of the successful and unfruitful commercial deployment ends this chapter. In the third chapter, we present the technical criterion (use of the PLC as a multi-purpose port: as a main port and as a telecommunication port) that we have to consider and the evolution of the conducted and radiated limits that we have to respect. The pollution of the radio amateur band, which could be the result of a massive deployment of PLC, is the subject of tremendous discussions and proves that if according to some persons the current limits are too permissive, for the PLC manufacturer they are already too restrictive. The coexistence of the PLC with other broadband technologies (xDSL, CA-TV, Wireless,...) is also addressed and we show that only the coexistence with an alternative of the DSL – VDSL – could be problematic. The main original contributions of this thesis are presented in chapters 4 to 6. We concentrated on the problems related to the electromagnetic radiations associated with PLC signals, especially in indoor environment. The reason for this is that the emission of electromagnetic noise, which can interfere with services such as public and amateur radio, remains an obstacle to the development of PLC. Furthermore, to the best of our knowledge, the mitigation of indoor radiation has not been addressed in the literature. In chapter 4, we present a complete characterization of the problems resulting from the use of PLC in an indoor environment. Two approaches are presented for the measurement of electromagnetic fields. The first one consists in the injection of a reference signal in the network and the measurement of the resulting radiations. The second one deals with the measurements of interferences due to the work of real PLC equipments. These two approaches enlight several aspects. The main aspect is the strong dispersion of the measurements, resulting from the influence of the position of the observation point and of the power network load. Measurements on real PLC networks show excessive radiations, which confirm that the limits are, at this time, too restrictive. We also present and discuss PLC radiated emissions in typical indoor environments. In particular, we analyze the field levels resulting from PLC signals with magnitudes given by EN55022/CISPR22 limits and compare them with various proposals for PLC radiated field limits. Thus, we show that the resulting fields exceed, for a wide range of frequencies, some of the current emission limits such as the German NB30, which is considered as a very strict limit. In this same chapter, we present a technique for the analysis of power line communication (PLC) signals, based on the formalism of the scattering parameters. This technique allows an efficient and fast characterization of any transmission medium of a power network. The analysis of the variations of the magnitude and phase of the scattering parameters gives useful insights on the increasing of the losses with frequency and confirm the need of a technique to reduce impedance mismatch at both injection and reception ports. The chapter 5 presents the modelling of the PLC environment and the computation of its radiations using a commercial code, the Numerical Electromagnetics Code (NEC), which solves the electric field integral equation in the frequency domain using the method of moments. We present some comparisons between the NEC simulations and the experimental data for the total magnetic field presented in the previous chapter. The simulations of simple models give results in good agreement with the experimental data and therefore allow the validation of certain principles. Unfortunately, the simulations on a model of increasing complexity give less satisfactory results. Because of the difficulty to respect the topology of the network and to take into account all of the components of the network coupled with the constraints of NEC, it is extremely difficult to obtain accurate results on the complete PLC band. The strong electromagnetic radiations appeared as the major problem of this technology. Therefore, the development of a method of mitigation was the logical continuation of this work, especially because there is still no solution to this problem. We developed a technique, which is presented in chapter 6, aiming at reducing the electromagnetic field radiated by indoor PLC signals. This technique is the major original contribution of this thesis and led us to file a patent. The central idea is to take advantage of the additional ground conductor and inject a signal similar to the PLC signal into the ground-neutral line, but with a reversed phase. As a result, the overall antenna mode current is significantly reduced, as well as the radiated electromagnetic fields. The method is specifically developed for indoor applications and applies to uni-directional and bi-directional data transmission systems with an implementation using switches. Experimental data obtained on a test network have shown that the use of the proposed technique implies a significant reduction (up to 20 dB) of the PLC electromagnetic radiation in the considered frequency range. It is also shown that the performance of the proposed technique increases when the ground-neutral line is terminated by an impedance equal to the one of the PLC modem. Other tests performed on 230 V network confirm this trend but show that further work is needed in this direction. This work could consist in the development of a system allowing the synchronisation of the real PLC signal and the auxiliary signal according to the behavior of the PLC channel, in order to optimise the performances for any frequency used by the PLC.

EPFL2005

Magnetic states and spin-wave modes in single ferromagnetic nanotubes

Daniel Rüffer

In this thesis the electrical properties, magnetic states and spin wave resonances of individual magnetically hollow ferromagnetic nanotubes have been studied. They were prepared from the different materials Nickel (Ni), Permalloy (Py) and Cobalt-Iron-Boron (CoFeB), deposited as shells onto non-magnetic Gallium-Arsenide (GaAs) semiconductor nanowires via Atomic Layer Deposition (ALD), thermal evaporation and magnetron sputtering, respectively. The resulting nanotubes had lengths between 10 to 20 μm, diameters of 150 to 400 nm and tube walls (shells) which were 20 to 40nm thick. Structural analysis of the tubes by Transmission Electron Microscopy revealed a poly(nano)crystalline (Ni, Py) and amorphous (CoFeB) structure. Electrical transport experiments as a function of temperature revealed different transport mechanisms for each of the materials. Electron-phonon scattering dominated the temperature dependence of the resistivity in Ni, while a clear evidence for electron magnon scattering was observed in Py. Electron-electron interaction in granular and amorphous media was identified as the major contribution to the temperature dependence in CoFeB. The Anisotropic Magnetoresistance (AMR) ratios have been determined for all tubes and different temperatures. Ni nanotubes exhibited a large relative AMR effect of 1.4% at room temperature. The AMR measurements provided information about the magnetic configurations as well as the magnetization reversal mechanism. Indications for the formation of vortex segments in Ni tubes were found for the magnetization reversal when the magnetic field was perpendicular to the nanotube axis. In cooperation with the Poggio group in Basel, cantilever magnetometry has been used for the further characterization of the nanotube magnetization. The magnetization curves were compared to the AMR measurements and finite element method (FEM) micromagnetic simulations. The comparison between the experimental results and the simulations suggested that the roughness of Ni tubes gave rise to segmented magnetic switching. An almost perfect axial alignment of the remanent magnetization has been observed in Py and CoFeB nanotubes. The influence of the inhomogeneous internal field in transverse magnetic fields was investigated by simulation. The segment-wise alignment of spins with the field direction is argued to provoke characteristic kinks in the hysteresis curve and measured AMR effect. Magnetothermal spatial mapping experiments using the anomalous Nernst effect (ANE) complemented the magnetotransport experiments in cooperation with the group of Prof. Grundler in Munich. Here, first evidence of end-vortices entering the nanotube before reversal could be found. Electrically detected spin wave resonance experiments have been performed in cooperation with the group of Prof. Grundler on individual nanotubes. The detected voltage, generated by the spin rectification effect, revealed multiple resonances in the GHz frequency. The experimentally observed resonances were compared to calculated ones extracted from dynamic simulations. With this comparison, the signatures could be attributed to azimuthally confined spin-wave modes. The deduced dispersion relation suggested the quantization of exchange dominated spin waves in that resonance frequencies follow roughly a quadratic dependence on the wave vector.

EPFL2014

Numerical modelling of high temperature superconducting tapes and cables

Francesco Grilli

This Ph.D. thesis is focused on the numerical modelling of high-Tc superconductors (HTS) at the operating temperature of 77 K (liquid nitrogen). The purpose of numerical modelling is to precisely calculate the current and field distributions inside HTS devices (tapes, cables) and the corresponding AC losses, which are one of the most important limiting factors for a large-scale application of such materials. From the electrical point of view, superconductors are characterized by a strongly non-linear voltage-current relation, which defines the transition from superconducting to normal state. In the case of HTS, the steepness of this transition is smoother than for low-Tc superconductors (LTS), so that the commonly used critical state model (CSM) gives a too simplified representation of their electromagnetic behaviour and can be used for a qualitative description only. In this thesis the finite element method (FEM) has been used for precisely computing the current and field distributions as well as for evaluating the AC losses in HTS devices. The superconducting transition is modelled with a power-law relation, E(J) = Ec(J/Jc)n, which is derived from the fit of transport measurements. Firstly, the results obtained with the software package FLUX3D on multi-filamentary tapes have been validated by means of a comparison with the ones obtained by another software package (FLUX2D). The results from FLUX2D, having already been successfully compared with experimental measurements within the framework of two previous Ph.D. theses at LANOS, have been used as reference. Secondly, two power-law models, which take into account the spatial variation of the critical current density inside HTS tapes and its strongly anisotropic dependence on the magnetic field, have been implemented in FLUX3D. In most cases, this latter dependence is extremely important, since the transport capacity of the superconductor is considerably reduced (and its power loss sensibly increased) by the presence of a magnetic field. Afterwards, FEM modelling of HTS tapes has been extended to cables. HTS cables are in general composed by different layers of several tapes and have a quite complex three-dimensional structure: in fact, the layers are wound around a central cylindrical support with different pitch lengths and relative winding orientations, in order to obtain a uniform repartition of the transport current among the layers, which minimizes the total AC losses. For overcoming the difficulties of a direct 3D FEM simulation, a simple electrical model, which allows to find the optimal pitch lengths and whose results are the input data for a 2D FEM evaluation of the AC losses, has been developed. FEM computations have also been used to investigate the influence of the non-uniformity of the tape properties (contact resistance, critical current, power index) on the global loss performance of a single-layer HTS cable. As an alternative to FEM computations, an equivalent circuit model of HTS cables has been utilized. It describes the cable from the macroscopic point of view and allows to compute the current repartition among the layers and the corresponding AC losses, without the necessity of having detailed information about individual tapes. In the framework of the European project BIG-POWA, I have collaborated to the assembling process of a HTS power-link at the Pirelli Labs, in the Milan region, Italy. Finally, 3D simulations have been performed to extensively study the coupling effect between two superconducting filaments via the resistive matrix, which is a typical case where analytical solutions exist for very peculiar geometries and physical conditions only. FEM simulations have been utilized to study the dependence of the filament coupling on the physical and geometrical parameters of the conductors.

EPFL2004