Stanimir Valtchev

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Important requirements for the modern electric drives are the high overload capacity and a wide range of speed control. A two-phase adjustable low-power drive has these properties, but its implementation in small-scale mechanics is hindered by the need a frequency converter that provides a three-phase power grid into a two-phase network, which is important when the power of the mechanisms increases. Previous studies have already shown the possibility of using a typical frequency converter based on a three-phase full-bridge voltage inverter applying space-vector PWM method. The switching frequency of the inverter remains, unfortunately, relatively high. It is not possible to reduce this frequency without degrading the harmonic composition. The goal of this work is to develop an algorithm for controlling the two-phase electric drive system, while reducing the numberof commutations of the switching devices of the three-phase inverter, and at the same time keeping the deviations of the instantaneous values of the phase currents close enough to the reference.

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Problem of the reliability increasing of the vehicles electric drives is discussed in context of the reduction of the electric motor heating during variable character of movement. Opportunities of the induction motor stator winding heating reduction by means of the control of the vehicles electric drives in transient and steady state modes of movement are shown. Analytical expressions for dynamic torque, optimal on minimal of the winding temperature rise during acceleration, as well as expressions for dynamic torque and value of the motor torque limitation, optimal on energy consumption during acceleration are presented. It is shown that desire to reduce the stator winding temperature rise by means of dynamic torque optimal value selection or motor torque limitation can lead to the rise of energy consumption. Results of the modeling are presented.

This paper presents a wireless energy transfer system operating at the frequency values of kHz order: modeling, simulation, and comparison with prototype measurement results. Wireless energy transfer system model using finite element method was carried out to simulate the electric field and the magnetic flux density for different air gap sizes between the transmitter and the receiver coils. Results are presented and compared with the electromagnetic emission measurements radiated by the wireless energy transfer system prototype. The electric field comparison between the simulated and the prototype measurement values shows an error of roughly 8.7{%}. In the recent years, the interest in the wireless energy transfer technology, especially for electric vehicles batteries charging, is rapidly increasing. As a result of the increasing application of this technology in the industrial and consumer electronic products, more concerns are raised about the electromagnetic compatibility, since the wireless energy transfer systems produce electromagnetic emissions in the surrounding environment.

Hardware-in-The-loop (HiL) systems nowadays become popular. During the HiL creation process it is important to select a proper numerical method because the accuracy of the process simulation, in case of the HiL, depends on the correct numerical approach selection. That is why it is important to evaluate the most popular numerical approaches. The {"}Sequential{"} Euler (solves equations step-by-step), the Forward Euler (the most popular method for HiL creators), and the Second-order Adams-Bashforth approach are under consideration. A mathematical model based on the DC-motor with independent excitation is selected as a basis for the experimental numerical calculation. In consequence, recommendations for the numerical method selection are given, based on the obtained results.

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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.

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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.

This paper describes some theoretical and experimental results obtained in an effort to optimize the Series Resonant Converter (SRC) when used with a loosely coupled transformer for Inductive Coupling Power Transfer (ICPT). The main goal of this work is to define precisely which mode of operation of the power stage is the most efficient. The results also suggest a way to choose the design criteria for the physical parameters (operation frequency, characteristic impedance, transformer ratio, etc.) to achieve that mode of operation. The analysis involves also the investigation of the separated in two halves pot core ferrite transformer, especially the way it changes its magnetizing and leakage fluxes and hence, inductances. It is shown that for the practical values of the separation distance, the leakage inductance remains almost unchanged. Nevertheless the current distribution between the primary and the secondary windings changes drastically due to the large variation of the magnetizing inductance. The analysis has lead to a set of equations with solutions that show graphically the way to an optimized operation of the converter, i.e. higher primary currents and higher transformer ratios to fit in the desired mode.

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Series connection of power devices has evolved into a mature technique and is widely applied in HV dc systems. Static and dynamic voltage balance is ensured by shunting individual devices with dissipative snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages, but they require a zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. A high-voltage dc-dc power converter with multiple series-connected devices is proposed. It allows the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and lowers turn-off losses. Simulation results obtained with a circuit simulator are validated in an experimental converter operating with two series-connected devices. The behavior of the series connection is examined for MOSFET's and insulated gate bipolar transistors (IGBT's) by both experimental work with a 2-kW prototype and computer simulation. Applications can be found in traction and heavy industry, where the soft-switching converter is directly powered from a high-voltage source.

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VEEM93A13

The DC analysis of a series-resonant converter operating above resonant frequency is presented. The results are used to analyze the current form factor and its effect on the efficiency. The selection of the switching frequency to maximize the efficiency is considered. The derived expressions are generalized and can be applied to calculations in any of the switching modes for a series-resonant circuit. For switching frequencies higher than the resonant frequency, an area of more efficient operation is indicated which will aid in the design of this class of converters and power supplies. It is pointed out that (especially for power MOSFETs where ohmic losses dominate) it is more attractive to select switching frequencies that are higher than the resonant frequency because of the possibility of nondissipative snubbers. Slowing down the rise of the gate voltage and, hence, the slow decrease of ON resistance during turn-on is also not a drawback to high-frequency switching. Because of this safer operation, the standard intrinsic diode of the power MOSFET could be used at high frequencies instead of the more expensive FREDFET

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