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2010
Valtchev, Stanimir, and Jorge Pamies Teixeira. "The Charging of Hybrid and Electrical Vehicles? Batteries: Contactless Energy Transfer as the key to the future." 2010 International Symposium on Electric Vehicle and 2-nd Annual Conference of Polish Society for Environment Friendly Vehicles. 2010. Abstract
Would it be necessary to charge rapidly the electric vehicle (EV) and hybrid electric vehicle (HEV)? It is not easy to answer without a thorough look at the capability of the modern propulsion battery and the power necessities of the EV and HEV. The different possible solutions are compared and maximum speed of charging is analysed. The wired connection for charging is compared to the wireless energy transfer and because of the limitations imposed by the modern and future batteries, the contactless energy transfer is chosen as the future charging method.
Valtchev, Stanimir, and Svilen Valtchev. "Improved strategy for an instantaneous super-resonant converter regulation." 12th International Conference on Optimization of Electrical and Electronic Equipment (OPTIM). 2010. 631-638. Abstract
The proposed regulation strategy consists in obtaining at the right time the correct information about the energy contained in the resonant tank. This information allows a stable operation of the switches and a higher efficiency of any Series Loaded Series Resonant (SLSR) power converter, especially when contactless energy transfer is concerned. The strategy is based on guaranteeing the correct portion of energy transported by the resonant tank to the load. The portion has to vary corresponding to the error signal taken from the output voltage and remains unchanged if the error signal is at its minimum. In a certain way this method is similar to the Current Mode Control of the classical power converters. The viability of the idea is demonstrated by simulation of a realistic analogue circuit (on preparation for a digital implementation in the near future).
Teixeira, Jorge Pamies, and Stanimir Valtchev. "An Insight to Potential Actions Facing Future Needs of Electric and Hybrid Electric Vehicles." 2010 International Symposium on Electric Vehicle and 2-nd Annual Conference of Polish Society for Environment Friendly Vehicles. 2010. Abstract
Recent issues of global heating and energy shortages are imposing a need to change our paradigm around transportation. Somehow, electric vehicles are progressively standing as a strong and necessary alternative for the society. Technically and technologically the acceptance of the EV is easier now than ever but the psychology of the consumers and the running business of internal combustion vehicles, the whole existing infrastructure are too much conservative to be changed easily. The changes in technology require changes in the engineering society and its human resources. The objective of this paper is to give a contribution to the discussion and reflection of potential future scenarios where EV?HEV?s are spread across the society. It gives an overview of the range of knowledge and competences necessary for a sustainable and streamlined development of those. In fact, it is expected that a new kind of professional profiles need to be created or developed to supply the work market with the right human resources. The paper provides some discussion on the creation of new profiles or adaption of existing ones. Among different possible scenarios the creation of post-graduation courses for students holding undergraduate profiles in the fields referred to earlier would be an interesting and viable solution for fast response. The post graduation would be focused in specific key areas of the EV?HEV. Several factors are pointed out to endorse this scenario
Valtchev, Stanimir. "Resonant power conversion in contactless battery charging for Electric Vehicle ? Hybrid Electric Vehicle." 2010 International Symposium on Electric Vehicle and 2-nd Annual Conference of Polish Society for Environment Friendly Vehicles. 2010. Abstract
The electromagnetic resonance became irreplaceable tool for wirelessly transfer energy and information. Since long this effect is widely used in communications but recently it is deeply studied and applied at the contactless transfer of energy. The electric?hybrid car battery charging is an urgent need and the knowledge about the resonant contactless transfer became very important. The study of the Series Loaded Series Resonant converter shows it as well suitable for the contactless energy transfer. The idealized Series Resonant Power Converter is used as a base for defining the best (most efficient) modes of operation. Based on the magnetic parameters of the loosely coupled transformer (magnetic link), the characteristics of the contactless power converter are described in approximated form, thus permitting an easier and faster calculation of the converter variables. This is used for instantaneous control of the converter, free from previously known defects.
Bonifácio, Paulo Seguidor fotovoltaico: uma variação do P&O - Simulação e Prototipagem. Eds. Stanimir Valtchev. FCT-UNL, 2010. Abstract
This dissertation tries to track the route of power production from photovoltaic sources. A worldwide look into the photovoltaic solar power production is given. The theory behind the PV cell and its application in PV power modules is presented. The DC ? DC power converters usually associated with solar PV panels are showed. The main maximum power point tracking (MPPT) algorithms are also showed. A solar PV system was dimensioned using the theoretical models and the solar PV modules (BS ? 40) available at the Departamento de Engenharia Electrotécnica (DEE). This system was used as a reference for designing a simulator implemented in MatLab?Simulink. This simulator includes all the components needed to test the different MPP tracking algorithms. Those components are: A PV solar module, which can be associated with others to form a solar panel; A Flyback DC ? DC power converter and a classic perturb and observe (P&O) tracking algorithm. A solar power meter that gives values in W?m2 was built in order to validate simulation values of the solar PV modules. Measuring the solar module characteristics (tension ? current) and knowing the solar irradiation at that given time it is possible to check if the simulated values and the measured ones agree. With the data collected in the simulation a new MPP tracking algorithm was presented. This is based in the classic P&O algorithm, but using modules that try to overcome the local maxima problem. It also tries to minimize the control oscillations in the converter?inverter, this is done in order to minimize losses and to maximize power production.
Valtchev, Stanimir Resonant Effects Applied in Power Conversion for Contactless Energy Transfer. Seminar on Resonant Processes and Contactless Energy., 2010.
Valtchev, Stanimir. "Resonant Effects Applied in Power Conversion for Contactless Energy Transfer and Energy Harvesting." Invited Seminar (Lecture) of S. Valtchev. 2010.
2009
Valtchev, Stanimir Battery chargers and battery management for the future electric vehicles. Cooperation work between S. Valtchev and TU Delft., 2009. Abstract

This was a cooperation with Electric Power Processing laboratory of TU Delft. During this stay organized and financed by TU Delft, S.Valtchev supported by teaching some students, PhD students and post-doc associates (in workshops?seminars).

Valtchev, Stanimir, Rosen Miletiev, Rumen Arnaoudov, and Svilen Valtchev. "Control strategy for efficient operation of super-resonant SLSR (contactless) converters." XLIV INTERNATIONAL SCIENTIFIC CONFERENCE ON INFORMATION, COMMUNICATION AND ENERGY SYSTEMS AND TECHNOLOGIES (before 1990 was: "Day of the Radio Conference"). 2009. 527-530.
Sousa, Pedro Accionamento de um Motor Passo a Passo com Velocidade e Direcção Controladas e com Binário de Arranque Aumentado. Eds. Stanimir Valtchev, and Mário Neves. FCT-UNL, 2009. Abstract
Os motores passo a passo podem ser vistos como motores eléctricos sem comutação electromecânica (escovas e anéis). Tipicamente, todos os enrolamentos no motor são parte do estator, e o rotor ou é um magneto permanente ou, no caso dos motores de relutância variável, um bloco dentado de algum material ferro-magnético macio. Toda a comutação deve ser lidada externamente pelo controlador do motor, e normalmente, os motores e os controladores são dimensionados de modo que o motor possa ser posicionado numa posição fixa, assim como rodar em ambos os sentidos. Os motores passo a passo podem ser usados em sistemas de controlo em malha aberta; estes são geralmente adequados para sistemas que operam em baixas acelerações com cargas fixas, mas o controlo em malha fechada pode ser essencial para acelerações elevadas, particularmente se envolverem cargas variáveis. Se um motor de passo num sistema de controlo em malha aberta sofre um binário demasiado grande, todo o conhecimento da posição do rotor será perdido e o sistema deverá ser reiniciado. Nesta dissertação, é proposto um novo conversor elétrico em malha aberta, especialmente dimensionado para motores de pequenas dimensões e de alto binário, alimentados com correntes unidirecionais. Este conversor é baseado na mudança do declive de subida da corrente em cada enrolamento, através da descarga de um condensador adicional carregado.
Inácio, D., J. A. Inácio, J. Pina, S. Valtchev, M. Neves, J. Martins, and A. Rodrigues. "Conventional and HTS Disc motor with pole variation control." 2nd International Conference on Power Engineering, Energy and Electrical Drives (POWERENG'2009). 2009. 513-518. Abstract
In this paper, a poly-phase disc motor innovative feeding and control strategy, based on a variable poles approach, and its application to a high temperature superconductor (HTS) disc motor, are presented. The stator windings may be electronically commutated to implement a 2, 4, 6 or 8 poles winding, thus changing the motor's torque?speed characteristics. The motor may be a conventional induction motor with a conductive disc rotor, or a new HTS disc motor, with conventional copper windings at its two iron semi-stators and a HTS disc as a rotor. The conventional induction motor's operation principle is related with the induced electromotive forces in the conductive rotor. Its behaviour, characteristics and modelling through Steinmetz and others theories are well known. The operation principle of the motor with HTS rotor, however, is rather different and is related with vortices' dynamics and pinning characteristics; this is a much more complex process than induction, and its modelling is quite complicated. In this paper, the operation was simulated through finite-elements commercial software (FLUX2D), whereas superconductivity was simulated by the E-J power law. The electromechanical performance of both motor's computed are compared. Considerations about the systems overall efficiency, including cryogenics, are also discussed.
Sousa, P., Stanimir Valtchev, Mário Neves, and A. Rodrigues. "A New Open-Loop Control Method for Stepping Motor Driving." 2nd International Conference on Power Engineering, Energy and Electrical Drives (POWERENG'2009). 2009. 605-610.
Valtchev, Stanimir, Beatriz Borges, Kostadin Brandisky, and Ben J. Klaassens. "Resonant Contactless Energy Transfer With Improved Efficiency." IEEE Transactions on Power Electronics. 24 (2009): 685-699. AbstractWebsite
This paper describes the theoretical and experimental results achieved in optimizing the application of the series loaded series resonant converter for contactless energy transfer. The main goal of this work is to define the power stage operation mode that guarantees the highest possible efficiency. The results suggest a method to select the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that efficiency improvement. The research clarifies also the effects of the physical separation between both halves of the ferromagnetic core on the characteristics of the transformer. It is shown that for practical values of the separation distance, the leakage inductance, being part of the resonant inductor, remains almost unchanged. Nevertheless, the current distribution between the primary and the secondary windings changes significantly due to the large variation of the magnetizing inductance. An approximation in the circuit analysis permits to obtain more rapidly the changing values of the converter parameters. The analysis results in a set of equations which solutions are presented graphically. The graphics show a shift of the best efficiency operation zone, compared to the converter with an ideally coupled transformer. Experimental results are presented confirming that expected tendency.
2008
Inácio, Steve Projecto de uma caixa de velocidades eléctrica por meio da variação do número de pólos para motores em disco de rotor em alumínio e em materiais supercondutores de alta temperatura (SAT). Eds. Mário Neves, João Pina, and Stanimir Valtchev. FCT-UNL, 2008. Abstract
Nesta dissertação apresenta-se um motor em disco polifásico inovador bem como uma estratégia de controlo com base no método de variação de velocidade por comutação do número de pares de pólos. A configuração das bobinas aliada à escolha das correntes e tensões que se injectam nas bobinas dos estatores, permite comutar electronicamente o número de pólos do motor entre 2, 4 6 e 8 pólos, conseguindo-se controlar a característica binário?velocidade do motor. O motor em disco possui a bobinagem feita em cobre com dois semi-estatores, em que quando utiliza o rotor em alumínio (com condutividade diferente de zero) comporta-se como um motor de indução convencional. Quando se substitui o rotor em alumínio por um constituído por um supercondutor de alta temperatura (SAT), o dispositivo comporta-se como um motor de histerese. O princípio de funcionamento do motor em disco convencional é baseado na indução de força electromotrizes no rotor e, consequentemente, uma vez que o alumínio é bom condutor eléctrico, correntes eléctricas induzidas, originadas por haver um campo magnético variável que é criado pelos semi-estatores. O comportamento deste tipo de motores, no que diz respeito a principais características (como o binário?velocidade para os diferentes números de pares de pólos), circuito equivalente de Steinmetz, entre outras teorias associadas é já conhecido há bastante tempo. O princípio de funcionamento do motor SAT é diferente do apresentado anteriormente, funciona com base na dinâmica de vórtices e devido ao facto de aparecer o fenómeno de ancoragem de fluxo (flux pinning) nos supercondutores de alta temperatura. Como o campo magnético varia, então o disco roda. Este motor tem um princípio de funcionamento muito mais complexo que o motor de indução sendo a obtenção do modelo do motor SAT complicada. A obtenção do modelo do motor SAT não é abordado nesta dissertação. Os comportamentos e modos de operação do motor com disco de alumínio e em materiais SAT são simulados através de um programa comercial de elementos finitos, nesta dissertação, sendo a supercondutividade simulada com base na relação entre o campo eléctrico e a densidade de corrente pela lei da potenciação (E-J power law). Com as simulações pretende-se comparar o rendimento electromecânico de ambos os motores.
Valtchev, Stanimir Contactless Power Transfer. Seminar of S.Valtchev on Contactless Power Transfer., 2008. Abstract
This seminar was a presentation of S.Valtchev on the methods of wireless energy transfer. The seminar was invited by the colleagues of the Power Processing Laboratory of TU Delft, the Netherlands.
Valtchev, Stanimir Series Resonant Power Converter for Contactless Energy Transfer with Improved Efficiency. Eds. Beatriz Borges, and Ben J. Klaassens. UTL-IST (Portugal), TU Delft (the Netherlands), 2008. Abstract
The development of more efficient power converters is the most important and challenging task for Power Electronics specialists. In the same time, many currently existing or yet to appear future applications require full mechanical independence between the transmitter and receiver of the electrical energy. This contactless form of energy transfer is the concern of the presented work. The work is based on the study of the Series Loaded Series Resonant converter which prove to be the best suitable for the contactless energy transfer. The work investigates the idealized Series Resonant Power Converter with the objective to find the best efficiency zones of operation. Generalized expressions obtained are original and useful. Based on the magnetic parameters of the loosely coupled transformer (magnetic link), the characteristics of the contactless power converter are described in approximated form. The approximation permits easier and faster calculation of the converter variables, thus predicting a shift of the maximum efficiency zone compared to the ideal converter case. The approximated form of the equations permitted to present a new instantaneous form of regulation which combines the frequency and pulse width modes which is free from the previously known defects. The method is based on calculating the energy portions supplied to the load during each half period. Measurements performed on industrial converters and on the laboratory experimental converter, confirm the predicted theoretically behaviour of the converter.
Inácio, S., D. Inácio, J. M. Pina, Stanimir Valtchev, M. V. Neves, and A. L. Rodrigues. "An electrical gearbox by means of pole variation for induction and superconducting disc motor." Journal of Physics: Conference Series. 97 (2008): 012221. AbstractWebsite
In this paper, a poly-phase disc motor innovative feeding and control strategy, based on a variable poles approach, and its application to a HTS disc motor, are presented. The stator windings may be electronically commutated to implement a 2, 4, 6 or 8 poles winding, thus changing the motor's torque?speed characteristics. The motor may be a conventional induction motor with a conductive disc rotor, or a new HTS disc motor, with conventional copper windings at its two iron semi-stators, and a HTS disc as a rotor. The conventional induction motor's operation principle is related with the induced electromotive forces in the conductive rotor. Its behaviour, characteristics (namely their torque?speed characteristics for different number of pole pairs) and modelling through Steinmetz and others theories are well known. The operation principle of the motor with HTS rotor, however, is rather different and is related with vortices' dynamics and pinning characteristics; this is a much more complex process than induction, and its modelling is quite complicated. In this paper, the operation was simulated through finite-elements commercial software, whereas superconductivity was simulated by the E-J power law. The Electromechanical performances of both motors where computed and are presented and compared. Considerations about the systems overall efficiency, including cryogenics, are also discussed.
Pereira, P., S. Valtchev, J. Pina, A. Gonçalves, Ventim M. Neves, and A. L. Rodrigues. "Power electronics performance in cryogenic environment: evaluation for use in HTS power devices." Journal of Physics: Conference Series. 97 (2008): 012219. AbstractWebsite
Power electronics (PE) plays a major role in electrical devices and systems, namely in electromechanical drives, in motor and generator controllers, and in power grids, including high-voltage DC (HVDC) power transmission. PE is also used in devices for the protection against grid disturbances, like voltage sags or power breakdowns. To cope with these disturbances, back-up energy storage devices are used, like uninterruptible power supplies (UPS) and flywheels. Some of these devices may use superconductivity. Commercial PE semiconductor devices (power diodes, power MOSFETs, IGBTs, power Darlington transistors and others) are rarely (or never) experimented for cryogenic temperatures, even when designed for military applications. This means that its integration with HTS power devices is usually done in the hot environment, raising several implementation restrictions. These reasons led to the natural desire of characterising PE under extreme conditions, e. g. at liquid nitrogen temperatures, for use in HTS devices. Some researchers expect that cryogenic temperatures may increase power electronics' performance when compared with room-temperature operation, namely reducing conduction losses and switching time. Also the overall system efficiency may increase due to improved properties of semiconductor materials at low temperatures, reduced losses, and removal of dissipation elements. In this work, steady state operation of commercial PE semiconductors and devices were investigated at liquid nitrogen and room temperatures. Performances in cryogenic and room temperatures are compared. Results help to decide which environment is to be used for different power HTS applications.
Pina, J., P. Pereira, S. Valtchev, A. Gonçalves, Ventim M. Neves, A. Alvarez, and L. Rodrigues. "A test rig for thrust force measurements of an all HTS linear synchronous motor." Journal of Physics: Conference Series. 97 (2008): 012220. AbstractWebsite
This paper presents the design of a test rig for an all HTS linear synchronous motor. Although this motor showed to have several unattractive characteristics, its design raised a number of problems which must be considered in future HTS machines design. HTS electromagnetic properties led to the development of new paradigms in electrical machines and power systems, as e. g. in some cases iron removal and consequent assembly of lighter devices. This is due to superconductor's ability to carry high currents with minimum losses and consequent generation in the surrounding air of flux densities much higher than the allowed by ferromagnetic saturation. However, severe restrictions in HTS power devices design that goes further beyond cryogenic considerations must be accounted in. This is usually the case when BSCCO tapes are used as conductors. Its bending limitations and the presence of flux components perpendicular to tape surface, due to the absence of iron, have to be considered for it may turn some possible applications not so attractive or even practically unfeasible. An all HTS linear synchronous motor built by BSCCO tapes as armature conductors and two trapped-flux YBCO bulks in the mover was constructed and thrust force measurements are starting to be performed. Although the device presents severe restrictions due to the exposed and other reasons, it allowed systematising its design. A pulsed-field magnetiser to generate opposite fluxes for both YBCO bulks is also detailed. Thrust force numerical predictions were already derived and presented.
2007
Pereira, Pedro, S. Valtchev, João Pina, Anabela Gonçalves, Mário Neves, and Amadeu Rodrigues. "Power Electronics Performance in Cryogenic Environment: Evaluation for Use in HTS Power Devices." 8th European Conference on Applied Superconductivity (EUCAS). 2007. Abstract
Power electronics (PE) plays a major role in electrical devices and systems, namely in electromechanical drives, in motor and generator controllers, and in power grids, including high-voltage DC (HVDC) power transmission. PE is also used in devices for the protection against grid disturbances, like voltage sags or power breakdowns. To cope with these disturbances, back-up energy storage devices are used, like uninterruptible power supplies (UPS) and flywheels. Some of these devices may use superconductivity. Commercial PE semiconductor devices (power diodes, power MOSFETs, IGBTs, power Darlington transistors and others) are rarely (or never) experimented for cryogenic temperatures, even when designed for military applications. This means that its integration with HTS power devices is usually done in the hot environment, raising several implementation restrictions. These reasons led to the natural desire of characterising PE under extreme conditions, e. g. at liquid nitrogen temperatures, for use in HTS devices. Some researchers expect that cryogenic temperatures may increase power electronics' performance when compared with room-temperature operation, namely reducing conduction losses and switching time. Also the overall system efficiency may increase due to improved properties of semiconductor materials at low temperatures, reduced losses, and removal of dissipation elements. In this work, steady state operation of commercial PE semiconductors and devices were investigated at liquid nitrogen and room temperatures. Performances in cryogenic and room temperatures are compared. Results help to decide which environment is to be used for different power HTS applications
Inácio, S., D. Inácio, J. Pina, S. Valtchev, Ventim M. Neves, and A. Rodrigues. "An Electrical Gearbox by means of pole variation for induction and superconducting disc motor." 8th European Conference on Applied Superconductivity (EUCAS). 2007. Abstract
In this paper, a poly-phase disc motor innovative feeding and control strategy, based on a variable poles approach, and its application to a HTS disc motor, are presented. The stator windings may be electronically commutated to implement a 2, 4, 6 or 8 poles winding, thus changing the motor's torque?speed characteristics. The motor may be a conventional induction motor with a conductive disc rotor, or a new HTS disc motor, with conventional copper windings at its two iron semi-stators, and a HTS disc as a rotor. The conventional induction motor's operation principle is related with the induced electromotive forces in the conductive rotor. Its behaviour, characteristics (namely their torque?speed characteristics for different number of pole pairs) and modelling through Steinmetz and others theories are well known. The operation principle of the motor with HTS rotor, however, is rather different and is related with vortices' dynamics and pinning characteristics; this is a much more complex process than induction, and its modelling is quite complicated. In this paper, the operation was simulated through finite-elements commercial software, whereas superconductivity was simulated by the E-J power law. The Electromechanical performances of both motors where computed and are presented and compared. Considerations about the systems overall efficiency, including cryogenics, are also discussed.
Pina, João, C. Caracaleanu, A. Gonçalves, Pedro Pereira, S. Valtchev, Mário Neves, and A. Rodrigues. "High Performance, Environment Friendly, Modular and Fault Tolerant Renewable Energy Microgrid." 12th International Energy Conference & Exhibition (ENERGEX2007). 2007.
Pina, João, Pedro Pereira, S. Valtchev, A. Gonçalves, Mário Neves, and A. Rodrigues. "A test rig for thrust force measurements f an all HTS linear synchronous motor." 8th European Conference on Applied Superconductivity (EUCAS). 2007. Abstract
This paper presents the design of a test rig for an all HTS linear synchronous motor. Although this motor showed to have several unattractive characteristics, its design raised a number of problems which must be considered in future HTS machines design. HTS electromagnetic properties led to the development of new paradigms in electrical machines and power systems, as e. g. in some cases iron removal and consequent assembly of lighter devices. This is due to superconductor's ability to carry high currents with minimum losses and consequent generation in the surrounding air of flux densities much higher than the allowed by ferromagnetic saturation. However, severe restrictions in HTS power devices design that goes further beyond cryogenic considerations must be accounted in. This is usually the case when BSCCO tapes are used as conductors. Its bending limitations and the presence of flux components perpendicular to tape surface, due to the absence of iron, have to be considered for it may turn some possible applications not so attractive or even practically unfeasible. An all HTS linear synchronous motor built by BSCCO tapes as armature conductors and two trapped-flux YBCO bulks in the mover was constructed and thrust force measurements are starting to be performed. Although the device presents severe restrictions due to the exposed and other reasons, it allowed systematising its design. A pulsed-field magnetiser to generate opposite fluxes for both YBCO bulks is also detailed. Thrust force numerical predictions were already derived and presented.
Steve, Inácio, João Pina, Stanimir Valtchev, Mário Neves, and Amadeu Rodrigues. "Topology of an Electrical Gearbox with Variable Poles for Induction and Superconducting Disc Motors." X Portuguese-Spanish Congress in Electrical Engineering. 2007.