Showing papers by "Johann W. Kolar published in 2016"

Solid-State Transformers: On the Origins and Evolution of Key Concepts

Abstract: During the past two decades, solidstate transformers (SSTs) have evolved quickly and have been considered for replacing conventional low-frequency (LF) transformers in applications such as traction, where weight and volume savings and substantial efficiency improvements can be achieved, or in smart grids because of their controllability. As shown in this article, all main modern SST topologies realize the common key characteristics of these transformers-medium-frequency (MF) isolation stage, connection to medium voltage (MV), and controllability-by employing combinations of a very few key concepts, which have been described or patented as early as the 1960s. But still, key research challenges concerning protection, isolation, and reliability remain.

ZVS of Power MOSFETs Revisited

Abstract: Aiming for converters with high efficiency and high power density demands converter topologies with zero-voltage switching (ZVS) capabilities. This letter shows that in order to determine whether ZVS is provided at a given operating point, the stored charge within the mosfet s has to be considered and the condition $L I^2 \geq 2Q_\text{oss} V_\text{DC}$ has to be fulfilled. In the case of incomplete soft switching, nonzero losses occur which are analytically derived and experimentally verified in this letter. Furthermore, the issue of nonideal soft-switching behavior of Si superjunction mosfet s is addressed.

Multi-Objective Optimization of 50 kW/85 kHz IPT System for Public Transport

Abstract: Inductive power transfer (IPT) is an attractive solution for the automated battery charging of public transport electric vehicles (EVs) with low maintenance requirements. This paper presents the design of a 50 kW/85 kHz contactless EV charger, with a focus on the IPT transmitter and receiver coils. The multi-objective magnetics optimization reveals the Pareto tradeoffs and performance limitations in terms of high efficiency, high power density, and low stray field for high-power IPT systems without moving mechanical parts. A full-scale hardware prototype is realized and experimentally investigated. The dc-dc conversion efficiency, including all the power electronics stages, is measured as 95.8% at 50 kW power transfer across an air gap of 160 mm (coil dimensions $410\times 760\times 60$ mm3). With 150 mm coil misalignment in the worst case direction, the dc-dc efficiency drops to 92%. The measurements of the magnetic stray field show compliance with ICNIRP 2010 at 800 mm distance from the IPT coil center.

Comprehensive Evaluation of Rectangular and Double-D Coil Geometry for 50 kW/85 kHz IPT System

Abstract: In this paper, the influence of the inductive power transfer (IPT) coil geometry on the performance factors efficiency, power density, and stray field is studied for a public transport electric vehicle battery charging system. IPT coil geometries with rectangular winding and with double-D winding are compared based on the Pareto fronts obtained from a multi-objective optimization. In order to study the effect of the winding layout experimentally, two full-scale 50 kW/85 kHz hardware prototypes with the same outer coil dimensions ( $410\times 760\times 60$ mm3) and ferrite core structure are constructed. For both the prototypes, the measured dc–dc efficiency is approximately 95.5% at 50 kW with a 160 mm air gap and ideally positioned coils, which confirms the calculations. The positioning tolerance of the double-D prototype is inferior, because with coil misalignment the efficiency decays faster than for the rectangular winding prototype. Flux density measurements show that both the prototypes fulfill the ICNIRP 2010 standard at 800 mm lateral distance from the coil center. However, the measured magnetic stray field is a factor of two lower for the double-D prototype, which is a key advantage in high-power applications.

Inductive Power Transfer for Electric Vehicle Charging: Technical challenges and tradeoffs

Abstract: Increasing public awareness of the environmental impact of greenhouse gas emissions, together with the development of modern lithium-ion batteries, has triggered worldwide interest in electric mobility [1]. Together with environmentally sustainable energy production using renewable energy sources, electric vehicles (EVs) and plug-in hybrid EVs have a smaller CO2 footprint compared to traditional vehicles that rely exclusively on internal combustion engines. As an additional advantage, the total cost of ownership over the lifetime of an EV is lower than that of a traditional vehicle, despite the higher initial purchase price [2]. Hence, vehicle markets in the developed world have seen EV sales rapidly increasing over the past years.

ηρ-Pareto optimization and comparative evaluation of inverter concepts considered for the GOOGLE Little Box Challenge

Abstract: In recent years, driven by worldwide growing environmental awareness the research in power electronics was focusing on the development of highly efficient but mostly bulky converter systems e.g. for interfacing renewable energy to the grid. The GOOGLE Little Box Challenge was impulse to give the power density again more attention by motivating engineers worldwide to design a single-phase solar inverter system at the cutting edge of what is technically possible. In this paper a comparative evaluation of inverter concepts considered by a team of ETH Zurich, FH-IZM and Fraza company for the GOOGLE Little Box Challenge is given. Based on the lessons learned from the participation in the competition, for the considered inverter concepts the achievable efficiency, power density and the optimal modulation scheme are identified with a multi-objective ηρ-Pareto optimization. This provides a sound basis for the redesign of the existing system pushing the forefront of power density even further.

Comprehensive Conceptualization, Design, and Experimental Verification of a Weight-Optimized All-SiC 2 kV/700 V DAB for an Airborne Wind Turbine

Abstract: This paper details the design, implementation, and experimental verification of a minimum weight input series output parallel structured dual active bridge (DAB) converter for an airborne wind turbine system. The main power components of the DAB converter, particularly the bridge circuits, the actively cooled high-frequency transformer and inductor, and the cooling system, which largely contribute to the total system weight, are designed and realized based on multiobjective considerations, i.e., with respect to weight and efficiency. Furthermore, the design includes all necessary considerations to realize a fully functional prototype, i.e., it also considers the auxiliary supply, the control for a stable operation of the system, which also comprises an input filter, over the specified operating range, and the start-up and shut down procedures. These considerations show the complex interactions of the various system parts and reveal that a comprehensive conceptualization is necessary to arrive at a reliable minimum weight design. Experimental results validate the proposed design procedure for a realized lightweight DAB hardware prototype with a rated power of $ \mathrm {6.25~\,kW}$ . The prototype weighs $ \mathrm {1.46~\,kg}$ , i.e., features a power-to-weight ratio of $ \mathrm {4.28~\,kW{/}\,kg}$ ( $ \mathrm {1.94~\,kW{/}\,lb}$ ), and achieves a maximum full-load efficiency of 97.5%.

Ultra-compact Power Pulsation Buffer for single-phase DC/AC converter systems

Abstract: In single-phase power conversion systems, typically bulky electrolytic capacitors are installed in order to cope with the intrinsic double-line frequency power pulsation. However, since the voltage ripple at the dc bus is typ. limited to just a few percent of the nominal voltage, only a small fraction of the actually stored energy in the capacitors is used for the power decoupling. In this paper an auxiliary buffer converter is employed, shifting the double-line frequency power pulsation away from dc bus to a buffer capacitor. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed across the buffer capacitor, significantly reducing the capacitance requirement. In this paper an ultra compact Power Pulsation Buffer (PPB) is designed for a 2kW PV-inverter application by means of a comprehensive Pareto optimization. Besides compensating the power pulsation, the PPB must be able to quickly stabilize the dc bus in case of abrupt load variations and maintain an average buffer capacitor voltage. In this paper, a novel cascaded control structure is presented, meeting all aforementioned control objectives. A constructed prototype of the optimized PPB is presented in the paper and experimental measurements verify the outstanding performance of the proposed control system.

Comprehensive evaluation of GaN GIT in low- and high-frequency bridge leg applications

Abstract: In power electronics applications with power ratings around several kilowatts, wide band gap semiconductors are more and more replacing state-of-the-art Si MOSFET. SiC MOSFETs with blocking voltage rating up to 1200V and low-voltage GaN devices are already commercially available on the market since a couple of years. Now also 600V GaN devices are entering the market, which are a cost-effective solution in many 400V key applications in order to increase the system performance in terms of achievable efficiencies or power density. Besides the employed semiconductor devices also the design of the appropriate gate drive circuit is important. In this paper a simple and reliable gate drive circuit for driving GaN switches is presented. In addition, the proposed gate drive is used to evaluate the switching performance of a GaN Gate Injection Transistor (GIT) under soft- and hard-switching condition, which provides a basis for further optimization of totem-pole converter systems.

The ideal switch is not enough

Abstract: There is an ongoing demand for increased power density and efficiency along with lower costs of converter systems and shorter development time for specific applications in the field of power electronics. In order to expedite the technology development Google and IEEE initiated the Google Little Box Challenge (GLBC) including $1 million prize money. Aim of the GLBC was to build the worldwide smallest 2 kVA/400…450 VDC/230 VAC single-phase converter with η > 95% efficiency and an air-cooled case temperature of less than 60 °C by using latest semiconductor technology and innovative topological concepts. Out of 2000+ applications 18 finalists have been selected, whose converter systems exhibited power densities mostly in the range of 120…220 W/in3. With this, a clear performance increase compared to the state of the art (ρ < 50 W/in3) was achieved, but in the end it represented only a limited performance improvement. In this work, a high power density DC/AC converter system, developed by a team of the ETH Zurich, the Fraunhofer Institute for Reliability and Microintegration (IZM) and the Fraza company and presented at the GLBC finale in Golden, Colorado, will be described and further optimized. Given the converter system, it will be clarified which components and technologies are finally limiting an increase in performance. In a first step, the optimum solution will be identified by means of a ηρ-Pareto front obtained from a multi-objective optimization. The analysis will be based on detailed loss and volume models of the utilized GaN GIT power switches, inductors and capacitors as well as on component stresses, resulting for advanced modulation and control techniques. Thereafter, the models of the power semiconductors will be gradually idealized by means of reducing the switching and conduction losses. The resulting shift of the Pareto front reveals the sensitivity of the system performance with respect to the semiconductor technology and ultimately leads to an ‘absolute’ performance limit imposed by the passive components and the cooling system. It is shown that for fully idealized semiconductors a maximum possible performance increase of 50 % regarding power density or losses is feasible, whereby the switching frequencies are limited to & 1 MHz due to the losses in the magnetic components. Thus, for the realization of highly compact systems, high frequency core materials and winding concepts of the magnetic components, new heat management concepts and 3D-packaging will gain further importance in future along with the ongoing improvement of semiconductor technologies.

Comprehensive large-signal performance analysis of ceramic capacitors for power pulsation buffers

Abstract: In high power density single-phase PV inverter systems, active auxiliary circuits are installed, shifting the double line-frequency power pulsation away from the dc link to a dedicated buffer capacitor. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed at the buffer capacitor, significantly reducing the capacitance requirements and consequently the overall volume of the converter system. Since the capacitance density of electrolytic capacitors must be derated because of imposed current limitations, ceramic capacitors become the preferred choice in power pulsation buffer applications. In particular, TDK's class II X6S MLCC with BaTiO 3 ceramic and the recently launched CeraLink with PLZT ceramic are promising candidates due to their high energy density. The actual prevailing capacitance of these two types of ceramics, strongly depends on the operating point, that is, a dc bias voltage and a superimposed large-signal amplitude ac voltage. Unfortunately, the large-signal behavior and performance of these ceramic capacitors is not specified by the manufacturer. In this paper, a comprehensive characterization of the X6S MLCC and the CeraLink large-signal performance is carried out by means of experimental measurements. The acquired data enables an accurate dimensioning of the power pulsation buffer capacitor and an estimate of the capacitor losses during operation. Since the device temperature has a strong impact on the ceramic properties, results for 30 °C, 60 °C and 90 °C are presented.

Computation and analysis of dielectric losses in MV power electronic converter insulation

Abstract: The newly available Medium Voltage (MV) Silicon-Carbide (SiC) devices enable a great extension of the design space of MV inverters. This includes the utilization of unprecedented blocking voltages, higher switching frequencies, higher commutation speeds, and high temperature operation. However, all these factors considerably increase the insulation stress. This paper details the computation of dielectric losses, which are directly related to the insulation stress and can be used for the insulation design and diagnostic. After a review of the method used to compute dielectric losses, scalable analytical expressions are derived for the losses produced by PWM waveforms of DC-DC, DC-AC, and multilevel DC-AC inverters. Finally, a Medium-Frequency (MF) transformer is analyzed and the impacts of the insulation material and the operating temperature on the dielectric losses are discussed. It is found that the insulation losses can represent a significant share (17%) of the total transformer losses.

Comparative evaluation of isolated front end and isolated back end multi-cell SSTs

Abstract: Solid-state transformers (SSTs) are power electronic interfaces between medium voltage (MV) and low voltage (LV) systems that provide galvanic isolation by means of medium frequency (MF) transformers, making them suitable for MVAC to LVDC conversion in environments where weight and volume constraints apply. This paper discusses an isolated front end (IFE) SST concept that allows to reduce the complexity and physical size of the MV side converter assemblies compared to the well-known isolated back end (IBE) SST topologies. The IFE approach performs the entire grid current and output voltage control on the LV side using standard non-isolated |AC|-DC boost converter stages. A generic comparison of the IFE and the IBE concepts reveals that the lower complexity of the IFE, e. g., a lower total MV blocking voltage requirement (number of cascaded cells), comes along with higher device RMS currents and hence slightly higher chip area requirements. On the other hand, a case study considering a 25kW, 6.6 kV AC to 400V DC SST shows advantages of the IFE in part-load operation due to lower switching and transformer core losses. This makes the IFE approach interesting for applications where MF isolation instead of low frequency isolation is required because of space and weight constraints (e. g., traction, subsea or aircraft environments), and where low system complexity is desirable.

Accurate transient calorimetric measurement of soft-switching losses of 10kV SiC MOSFETs

Abstract: The characterization of soft-switching losses of modern high-voltage SiC MOSFETs is a difficult but necessary task in order to accurately model converter systems such as medium-voltage-connected Solid-State Transformers (SSTs), where soft-switching techniques are employed for an increased efficiency. Usually, switching losses in general are measured with the well-known double pulse method. However, in case of soft-switching loss measurements, this method is very sensitive to several effects such as probe skew and limited measurement accuracy, among others, and thus unsuitable for the characterization of fast switching high-voltage MOSFETs. This paper presents an accurate and reliable calorimetric method for the measurement of soft-switching losses using the example of 10 kV SiC MOSFETs. Finally, measured soft-switching loss curves of these 10 kV SiC MOSFETs are presented for different DC-link voltages, currents and gate resistors.

Full-ZVS modulation for all-SiC ISOP-type isolated front end (IFE) solid-state transformer

Abstract: Thanks to their comparatively low system complexity, SSTs based on an isolated front end (IFE) approach are suitable for space and weight-constrained medium voltage (MV) AC to low voltage (LV) DC power supply applications, e.g., in future traction, naval, subsea or aerospace systems. The IFE approach connects series resonant isolation stages operating in the half-cycle discontinuous-conduction-mode (HC-DCM) directly to the MV AC grid in an input-series, output-parallel (ISOP) configuration, but the entire control, i.e., the shaping of the grid current for unity power factor and output voltage regulation, is carried out by a second, non-isolated conversion stage on the LV side. However, since the isolation stages do not operate with a DC but with an AC or |AC| input voltage, the transformer magnetizing current available for ZVS as well as the voltage to be switched vary over the grid period. Taking into account also component tolerances among the cascaded converter cells, this paper provides an in-depth analysis of the ZVS behavior under these conditions, and of the associated losses and EMI considerations, presenting a loss-optimal choice of the magnetizing inductance value and of the dead time (interlock time) of the isolation stages' bridge legs. A time-dependent variation of the latter to achieve ZVS over the entire grid period without an increase of the isolation stage losses is proposed. The considerations are verified at the example of the Swiss SST (S3T), an all-SiC 25 kW, 6.6 kV MVAC to 400 V LVDC converter system, using a detailed simulation model, including non-linear MOSFET capacitances.

Comparative evaluation of a Triangular Current Mode (TCM) and Clamp-Switch TCM DC-DC boost converter

Abstract: For the power management of a wireless power transfer system for implantable mechanical heart pumps, an additional boost DC-DC converter stage is needed in order to control the power delivered to the implant. Particularly, battery powered and implantable medical devices pose special demands on the efficiency and/or power density of the employed converters. Accordingly, soft-switching and/or high switching frequencies must be targeted. Modulation schemes that allow for Zero-Voltage-Switching (ZVS) such as Triangular Current Mode (TCM) offer a highly efficient operation, but suffer from a large operating frequency variation, which is mainly limited by the digital control. Therefore the Clamp-Switch TCM (CL-TCM) converter can be employed which allows also for the control of the switching frequency variation. In this paper, the CL-TCM and the TCM converter are compared regarding the power conversion efficiency and the power density of the converter. Since the CL-TCM converter is not well known in the literature, the converter is analysed in detail and a modulation scheme is explained that allows for ZVS for all switches in the entire range of operation. In addition, the requirements for ZVS and a control scheme (i.e. timing calculations) are provided for the converter in order to limit the maximum switching frequency. The modulation and control scheme are verified with a hardware prototype. Finally, the performance of the CL-TCM converter is measured and compared to the performance of the converter operated in TCM mode. The measurements show that the CL-TCM converter offers similar performance compared to the TCM operation at lower inductor power density, but has the advantage of a significantly reduced switching frequency variation. In applications, where a very high power density is needed, the TCM converter outperforms the CL-TCM converter in terms of efficiency.

99% efficient three-phase buck-type SiC MOSFET PFC rectifier minimizing life cycle cost in DC data centers

Abstract: Due to the increasing power consumption of data centers, efficient dc power distribution systems have become an important topic in research and industry over the last years. Furthermore the power consumed by data centers is an economic factor, which implies that all parts of the distribution system should be designed to minimize the life cycle cost, i.e. the sum of first cost and the cost of dissipation. This paper analyzes a three-phase buck-type PFC rectifier with integrated active 3rd harmonic current injection for dc distribution systems. Switching frequency, chip area and magnetic components are selected based on a life cycle cost optimization, showing that a peak efficiency of 99% is technically and economically feasible with state-of-the-art SiC MOSFETs and magnetic components. Measurements taken on an 8 kW, 4 kW dm−-3 hardware prototype demonstrate the validity and feasibility of the design.

New optimum modulation of three-phase ZVS triangular current mode GaN inverter ensuring limited switching frequency variation

Abstract: In three-phase pulse width modulation inverters with an LC output filter, switching losses can be reduced by increasing the current ripple in the inductor until a current zero crossing occurs between the two switching instances of a bridge leg. A disadvantage of this triangular current mode (TCM) operation is the varying switching frequency. This paper presents new modulation methods where the current ripple amplitude is adjusted and a common-mode voltage is introduced in order to limit the switching frequency variation. Moreover, the possibility of increasing the efficiency and minimizing the filter volume are investigated. Numerical optimization results are presented for a 2.5 kW inverter with 400 V DC link voltage as a case study.

Weight and efficiency analysis of switched circuit topologies for modular power electronics in MEA

Abstract: This paper details weight and efficiency analysis of a bidirectional ±270V dc to 3-phase 115V ac power electronic interface as part of an integrated modular power electronics cabinet, which supplies various loads, such as motors and ac buses from a HVDC bus in more electric aircraft. Through a case study the analysis shows that a significant reduction of filter component weight is achieved with a 3-level instead of a 2-level topology. The interactions and trade offs between the power electronic switched circuits and the EMI filter are analyzed and quantified. The potential benefits through the use of SiC-MOSFET's instead of Si-IGBTs and the implications for the circuit topologies are detailed. Recommendations for the choice of the optimal switched circuit topology, dependent on the importances of efficiency, weight, and reliability, are proposed. Furthermore, recommendations for the focus of future research enabling the competitiveness of the MEA concept is given.

Analysis of capacitive power transfer GaN ISOP multi-cell DC/DC converter systems for single-phase telecom power supply modules

Abstract: The Input Series Output Parallel (ISOP) multi-cell converter approach allows breaking the performance barriers of conventional single-cell telecom rectifier systems by leveraging the advantages of using multiple interleaved low-voltage and/or low-current converter cells. The ISOP interconnection in the DC/DC converter part of the cells, however, requires the employment of some kind of isolation in each cell, which is typically provided by transformers. An analysis of the losses and of the volume of the entire multi-cell system reveals that these transformers contribute a major part to the system losses and are responsible for a significant share of the total volume. However, as the transformers are mainly required for providing galvanic isolation in the ISOP structure and not for voltage conversion, series capacitors represent an alternative to decouple the series connected input terminals of the cells from the parallel connected output terminals. Compared to conventional solutions with transformers, the resulting capacitive power transfer ISOP multi-cell DC/DC converter (CPT-ISOP-MCC) system features lower losses and a smaller volume. In this paper, the benefits as well as the limitations in the design and operation of CPT-ISOP-MCC systems are analyzed in detail. In order to comprehensively evaluate the CPT against the magnetically isolated concept, i.e. inductive power transfer (IPT) converter topology, a multi-objective optimization is performed with respect to the achievable efficiency and power density for both types of converters. Based on the optimization result, a prototype of a CPT GaN DC/DC converter is realized and compared to its IPT GaN DC/DC converter counterpart, along with measurement results. Furthermore, a complete single-phase 3.3kWAC/DC converter system with a high power density of ρ = 3.92kW/dm3 and an efficiency of η = 97% is presented, incorporating the CPT-DC/DC converter stages in ISOP topology.

Evaluation of One- and Two-Pole-Pair Slotless Bearingless Motors With Toroidal Windings

Abstract: In this paper, winding topologies for one- and two-pole-pair rotors are analyzed and compared for a slotless bearingless disk drive with toroidal windings. The basis of the studies is a six-phase motor with a diametrically magnetized one-pole-pair rotor. Due to the absence of mechanical bearings and the significantly large air-gap capability, the motor is suitable for applications with high purity and special chemical demands. Its slotless design results in low losses even at high rotational speeds. To improve the operational behavior of the rotor in different applications, the influence of higher pole pair numbers on the passive bearing stiffness is examined. A possible winding configuration for these rotors is proposed and evaluated for their bearing and motor performance. Based on the results, a further prototype was built and is presented in this paper.

Eddy-current-based contactless speed sensing of conductive surfaces

Abstract: Contactless speed sensors are used in a broad area of applications in various industries such as machining, assembly lines and transportation. Commonly used technologies are based on optics (e.g. cameras, encoders), or electromagnetic effects (e.g. variable reluctance sensors, Hall sensors). However, these sensors require a non-uniform property of the moving target that can be detected. For example, variable reluctance sensors rely on the variation of the air gap, and Hall sensors require a magnetic field whose spatial distribution is dependent on the position of the mover. A clear disadvantage of all these systems is the fact that they require modifications on the target's geometry and/or magnetic properties, since they cannot measure the speed of a smooth body/surface. Moreover, some of them are sensitive to environmental conditions; e.g. dirt in case of optical encoders and high temperature in case of permanent magnets can render these systems ineffective. Therefore, an eddy-current-based contactless speed sensor is developed in this work for measuring the speed of smooth, electrically conductive surfaces in harsh operating conditions. An injection coil is used to induce eddy currents in the mover whose speed is to be detected, and two differentially wound pick-up coils are used to detect the speed-dependent deformation of the eddy-current field. Two-dimensional finite-element method (2-D FEM) is used for modeling the system and optimizing the sensor geometry as well as the injection frequency. Measurements taken from a prototype verify the validity of the design procedure and the analyzed speed-sensing concept.

Optimization and Comparative Evaluation of Multiloop Control Schemes for Controllable AC Sources With Two-Stage $LC$ Output Filters

Abstract: This study investigates the control system design and its small-signal properties for the output stage of a high-bandwidth four-quadrant three-phase switch-mode controllable AC voltage source (CVS) with an output power of $10\;\mathrm{kW}$ , a switching frequency of $48\;\mathrm{kHz}$ , and a two-stage $LC$ output filter. Each output phase of the CVS is operated individually, i.e., the phase voltages are generated with reference to the DC input-voltage midpoint, to allow maximum flexibility in the generation of the output-voltage waveforms to supply a wide range of different load types, such as DC, single-phase, and general three-phase loads including constant-power loads leading to negative small-signal load-resistance values. Three suitable multiloop control structures with inner-current- and outer-voltage-control loops are motivated, modeled, and are optimized with respect to different control performance indicators, e.g., small-signal control bandwidth, and for common boundary conditions, e.g., maximum overshoot of the output voltage in case of a reference voltage step. All structures employ conventional P and PI controllers, due to their simplicity and widespread use. Among the three structures, the capacitor–current feedback-control structure, which controls the two filter capacitor currents and the output voltage, is identified to be most competitive. The small-signal bandwidth determined for this structure is between $7.1\;\mathrm{kHz}$ and $15.5\;\mathrm{kHz}$ , depending on the value of the load resistance. This result, in combination with an excellent matching of calculated and measured step responses of the output voltage of a $10\;\mathrm{kW}$ hardware prototype, point out the effectiveness of the selected control structure and the usability of control structures that are composed of conventional P and PI controllers.

High-speed magnetically levitated reaction wheels for small satellites

Abstract: Mechanical wear and stiction can be eliminated by using magnetic bearings instead of ball bearings in spacecraft reaction wheels. Accordingly, the lifetime of attitude control systems can be increased and a smooth torque can be achieved also at speed zero crossings. Moreover, the microvibration emission of the reaction wheels can be minimized and actively controlled. A further advantage of magnetically levitated reaction wheels is the ability to rotate at speeds higher than the state of the art. High-speed reaction wheels can be made smaller in size; however, they require higher power levels for the same reaction torque. Therefore, this paper analyzes the circulation of energy within different reaction wheels of an attitude control system in order to limit the external power requirement. Moreover, integrated position sensors are used for limiting the axial length of the rotors, thereby further increasing the speed limits that may be resulting from rotor dynamics. A functional demonstrator is built and equipped with two different reaction wheel rotors for initial tests. The electrical machine design and the operation of the magnetically levitated reaction wheel systems are verified experimentally.

Novel contactless axial-flux permanent-magnet electromechanical energy harvester

Abstract: This paper proposes a novel type of watt-range permanent-magnet energy harvester, which harvests energy from a moving conductive body or surface without mechanical contact, as its operation is purely based on eddy-current coupling. The harvester's main advantage over existing solutions is that it allows energy transfer over atypically large (2 … 15 mm) air gaps, which are unavoidable in certain industrial applications. The paper provides a detailed description of the system's operating principle, and elaborates its modeling using 3-D Finite-Element Method (FEM) analysis. Two prototypes are built and tested for verifying the models. A power of 2.42W is harvested from an aluminum surface moving with 10 m/s, over 12mm air gap using a prototype with ≈ 14 cm3 magnet volume. Moreover, the effects of the harvester's placement as well as the speed of the moving conductive surface on the maximum harvested power and the system's efficiency are analyzed, both with FEM simulations and measurements.

Characterization of Electromagnetic Rotor Material Properties and Their Impact on an Ultra-High Speed Spinning Ball Motor

Abstract: The ongoing miniaturization trend of electric machines increases the demand for higher rotational speeds to provide a required power level at decreased size. In this paper, new concepts for bearingless machines with ultra-high rotational speeds exceeding 25 million rotations per minute are researched. While the mechanical properties of the employed sub-millimeter-sized spherical steel rotors are documented, insufficient information is available on the electromagnetic characteristics that are crucial for magnetic levitation and acceleration. This paper outlines the relations between the relative permeability and conductivity of the rotor material and the achievable active magnetic bearing force and angular acceleration. Measured results for complete hysteresis curves of different rotor steels are presented.

Voltage/current measurement performance and power supply rejection in all-digital Class-D power amplifiers

Abstract: Low-noise and low-distortion voltage and current acquisition circuits, both analog and digital, are required for the control systems of precision, high-power switch-mode (Class-D) amplifiers used in nanometer-accuracy mechatronic systems. In this paper, the achievable linearity and noise of analog signal processing elements, such as sensors or active filters, is investigated and their performance in a closed-loop control system is demonstrated. Resistive voltage dividers are sufficiently linear but are subject to resistor noise. Nonetheless, a signal-to-noise ratio (SNR) of 130dB is achievable. Shunt resistors can achieve a total harmonic distortion (THD), limited by self-heating, of 110dB while operating at switching frequencies up to 500kHz.

4D-Interleaving of Isolated ISOP Multi-Cell Converter Systems for Single Phase AC/DC Conversion

Abstract: The multi-cell converter approach allows to break the performance barriers of conventional systems by leveraging the advantages of using multiple interleaved low voltage and/or low current converter cells. In this digest, a fourth dimension of interleaving is proposed which considers the time dependent degrees of freedom in the control of the entire multi-cell converter system. This new control concept is based on the possibility to decouple the operation of the series connected input stages from the parallel connected output stages by using the energy storage capability of the DC-link capacitors. This digest shows how the 4D-interleaving concept improves the system performance such as the efficiency which will be demonstrated with measurement results on a hardware prototype system in the final paper.