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- Magnetic flux density distribution in the air gap of a ferromagnetic core with superconducting blocks: three-dimensional analysis and experimental NMR resultsPublication . Roque, António; Sousa, Duarte M.; Margato, Elmano; Machado, Vítor Malo; Sebastião, Pedro J.; Marques, G. D.The design of magnetic cores can be carried out by taking into account the optimization of different parameters in accordance with the application requirements. Considering the specifications of the fast field cycling nuclear magnetic resonance (FFC-NMR) technique, the magnetic flux density distribution, at the sample insertion volume, is one of the core parameters that needs to be evaluated. Recently, it has been shown that the FFC-NMR magnets can be built on the basis of solenoid coils with ferromagnetic cores. Since this type of apparatus requires magnets with high magnetic flux density uniformity, a new type of magnet using a ferromagnetic core, copper coils, and superconducting blocks was designed with improved magnetic flux density distribution. In this paper, the designing aspects of the magnet are described and discussed with emphasis on the improvement of the magnetic flux density homogeneity (Delta B/B-0) in the air gap. The magnetic flux density distribution is analyzed based on 3-D simulations and NMR experimental results.
- On the Problem of Balancing the DC Capacitor Voltage Divider in Back-to-Back Multilevel ConvertersPublication . Chaves, Miguel; Margato, Elmano; Silva, J. Fernando; Pinto, Sónia F.This paper presents a new generalized solution for DC bus capacitors voltage balancing in back-to-back m level diode-clamped multilevel converters connecting AC networks. The solution is based on the DC bus average power flow and exploits the switching configuration redundancies. The proposed balancing solution is particularized for the back-to-back multilevel structure with m=5 levels. This back-to-back converter is studied working with bidirectional power flow, connecting an induction machine to the power grid.
- Fast optimum-predictive control and capacitor voltage balancing strategy for bipolar back-to-back NPC converters in high-voltage direct current transmission systemsPublication . Chaves, Miguel; Margato, Elmano; Silva, J. F.; Pinto, S. F.; Santana, J.Multilevel power converters have been introduced as the solution for high-power high-voltage switching applications where they have well-known advantages. Recently, full back-to-back connected multilevel neutral point diode clamped converters (NPC converter) have been used inhigh-voltage direct current (HVDC) transmission systems. Bipolar-connected back-to-back NPC converters have advantages in long-distance HVDCtransmission systems over the full back-to-back connection, but greater difficulty to balance the dc capacitor voltage divider on both sending and receiving end NPC converters. This study shows that power flow control and dc capacitor voltage balancing are feasible using fast optimum-predictive-based controllers in HVDC systems using bipolar back-to-back-connected five-level NPC multilevel converters. For both converter sides, the control strategytakes in account active and reactive power, which establishes ac grid currents in both ends, and guarantees the balancing of dc bus capacitor voltages inboth NPC converters. Additionally, the semiconductor switching frequency is minimised to reduce switching losses. The performance and robustness of the new fast predictive control strategy, and its capability to solve the DC capacitor voltage balancing problem of bipolar-connected back-to-back NPCconverters are evaluated.
- Comparing two power supplies for fast-field cycling nuclear magnetic resonance relaxometers power losses and performancePublication . Lima, Marco; Pereira, Bruno; Sousa, Duarte M.; Roque, António; Margato, ElmanoThe main power supply of a Fast Field-Cycling Nuclear Magnetic Resonance (FFC-NMR) is the key element comparing the performance of different solutions. The power supply is a current source that supplies a magnet being the current controlled in order to perform adjustable and repetitive current cycles. This power supply can be based on different topologies, operating principles and controlled using distinct techniques. If for the final users of this experimental technique the current cycles of the equipment is the core feature, for the developers also the power losses distribution needs to be analyzed in order to develop efficient solutions. In this paper, the power losses and the dynamic behavior of two solutions for the FFC-NMR power supply are compared and discussed.
- Predictive Optimal Matrix Converter Control for a Dynamic Voltage Restorer with Flywheel Energy StoragePublication . Gamboa, Paulo; Silva, J. Fernando; Pinto, S. Ferreira; Margato, ElmanoThis paper presents a predictive optimal matrix converter controller for a flywheel energy storage system used as Dynamic Voltage Restorer (DVR). The flywheel energy storage device is based on a steel seamless tube mounted as a vertical axis flywheel to store kinetic energy. The motor/generator is a Permanent Magnet Synchronous Machine driven by the AC-AC Matrix Converter. The matrix control method uses a discrete-time model of the converter system to predict the expected values of the input and output currents for all the 27 possible vectors generated by the matrix converter. An optimal controller minimizes control errors using a weighted cost functional. The flywheel and control process was tested as a DVR to mitigate voltage sags and swells. Simulation results show that the DVR is able to compensate the critical load voltage without delays, voltage undershoots or overshoots, overcoming the input/output coupling of matrix converters.
- Control of a power supply with cycling current using different controllersPublication . Roque, António; Maia, José; Margato, Elmano; Sousa, Duarte de Mesquita e; Marques, GilFast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometers require controlled current sources in order to get accurate flux density with respect to its magnet. The main elements of the proposed solution are a power semiconductor, a DC voltage source and the magnet. The power semiconductor is commanded in order to get a linear control of the flux density. To implement the flux density control, a Hall Effect sensor is used. Furthermore, the dynamic behavior of the current source is analyzed and compared when using a PI controller and a PD2I controller.
- Temperature distribution of a Fast-Field Cycling Nuclear Magnetic Resonance relaxometer's electromagnet with reduced volumePublication . Videira, P.; Sebastião, P.; Roque, A.; Sousa, D. M.; Fernandes, A.; Margato, ElmanoThe temperature distribution of a Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) electromagnet plays an important role in the operation of this type of apparatus. The designed electromagnet presents a reduced volume and is iron and copper based, fulfilling the technical requirements for the magnetic field. With this solution, it is possible to increase the overall performance in comparison with former similar FFC relaxometers. Electromagnet's simulation results evaluating the temperature distribution, heating effects and cooling requirements are presented.
- Fast-Field Cycling Nuclear Magnetic Resonance relaxometer's electromagnet with optimized homogeneity and reduced volumePublication . Videira, P.; Sebastião, P.; Roque, A.; Sousa, D. M.; Margato, ElmanoIn this article a Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) electromagnet with low power consumption (less than 200 W), high field homogeneity and reduced volume is projected and described. The electromagnet is iron and copper based, possessing a high permeability and allowing for good magnetic field homogeneity in the operating range of 0 to 0.33 T. With this solution, it is possible to increase 65% the maximum magnetic field keeping the magnetic field homogeneity in comparison with former similar FFC relaxometers. Electromagnet's experimental and simulation results evaluating the generated magnetic field, field homogeneity, heating effects and cooling requirements are also presented. In addition, some technical aspects of the required coupled systems such as the cooling, sample heating are assessed.
- Discurso do presidente do Instituto Politécnico de Lisboa na sessão de aberturaPublication . Margato, ElmanoA Escola Superior de Educação de Lisboa comemora hoje os 100 anos do início da construção do seu edifício sede. Este belo edifício, do qual ainda hoje desfrutamos, da autoria do Arquiteto Arnaldo Adães Bermudes, foi mandado construir pelo Governo da Primeira República para o funcionamento da Escola Normal Primária de Lisboa, posteriormente, Escola do Magistério Primário de Lisboa e, atualmente, Escola Superior de Educação de Lisboa. Coube ao então Presidente da República, Bernardino Machado, a sua inauguração e cabe-nos hoje a nós e à Sr.ª Prof.ª Isabel Alçada, em representação do atual Presidente da República, Prof. Marcelo Rebelo de Sousa, o assinalar, com enorme regozijo, do trabalho que tem vindo a ser desenvolvido por esta Escola em prol de uma melhor educação para todos.
- Risk Assessment of Chronic Exposure to Magnetic Fields near Electrical ApparatusPublication . Nunes, Fernando; Margato, ElmanoThe paper presents a numerical model to calculate ELF magnetic fields for typical conductor arrangements of electrical power lines. After a presentation of the several sources of the magnetic field and an introduction to the problems arising from the continued exposure of human beings to low frequency magnetic fields, a description of the numerical model for the magnetic field evaluation is presented, discussing its application. The attention is then devoted to the analysis of possible solutions for magnetic field reduction.
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