Johann W. Kolar

Bio: Johann W. Kolar is an academic researcher from ETH Zurich. The author has contributed to research in topics: Rectifier & Three-phase. The author has an hindex of 97, co-authored 965 publications receiving 36902 citations. Previous affiliations of Johann W. Kolar include Alstom & Infineon Technologies.

Novel Sinusoidal Input Current Single-to-Three-Phase Z-Source Buck+Boost AC/AC Converter

Abstract: This paper introduces a novel unidirectional unity power factor single-to-three-phase Z-source Buck+Boost AC/AC Converter (123ZBBC) topology to supply three-phase AC machines with widely varying rated voltage directly from the single-phase mains. Due to the integration of the boost circuit into the inverter stage, the proposed circuit benefits from a reduced realization effort and an increased robustness. Furthermore, the insertion of a front-end buck-stage allows to select an intermediate voltage which is lower than the peak mains voltage and on the other hand enables to achieve a sinusoidal input current within the entire mains period. The paper gives a detailed analysis of the proposed converter including the different conduction states, the modulation schemes in order to implement the power factor correction and the inverter functionality, as well as the corresponding closed-loop control enabling sinusoidal input current and output voltages. Furthermore, the converter operation is verified by circuit simulations and the stresses on the main components are analyzed and compared to a conventional single-to-three-phase Z-source based AC/AC converter system.

Multi-cell power converter with improved start-up routine

Abstract: A power converter circuit includes a plurality of first converter cells, a plurality of second converter cells, and a plurality of DC link capacitors. Each of the plurality of first converter cells is coupled to a corresponding one of the plurality of DC link capacitors. Each of the plurality of second converter cells is coupled to a corresponding one of the plurality of DC link capacitors. At least one of the plurality of second converter cells is configured to, during start-up of the power converter, internally dissipate power received from the corresponding DC link capacitor while a cell output power of the at least one of the plurality of second converter cells is substantially zero.

Switching device with a JFET series arrangement

Abstract: A switching device for switching a current between a first connection and a second connection including a series circuit of at least two JFETs (J1-Jn), with further JFETs (J2-Jn), which are connected in series to a lowest JFET (J1), and wherein a wiring network for stabilizing the gate voltages of the JFETs (J1-Jn) is connected between the second connection and the first termination. One additional circuit is connected between each gate connection (GJ2, GJ3 . . . GjN) of the further JFETs (J2-Jn) and associated cathode connections of diodes (DAV) of the wiring network. During switch-on and in the switched-on state, said additional circuit keeps the potential of the respective gate connection higher than the potential of the associated source connection.

Kilohertz-Frequency Rotation of Acoustically Levitated Particles

Abstract: The achievable rotational frequency of acoustically levitated particles is limited by the suspension stability and the achievable driving torque. In this work, a spherical ring arrangement of piezoelectric transducers and an improved excitation concept are presented to increase the rotational speed of an acoustically levitated particle by more than a factor of 10 compared to previously published results. A maximum rotational frequency of 3.6 kHz using asymmetric expanded polystyrene (EPS) particles is demonstrated. At such rotational speeds, high-frequency resonances of the transducers cause disturbances of the acoustic field which present a previously unexplored limit to the achievable manipulation rate of the particle. This limit is investigated in this work by means of calculations based on an analytical model and high precision measurements of the transducer characteristics beyond the conventional frequency range.

Exploring the Physical Limits of Axial Magnetic Bearings Featuring Extremely Large Vertical Levitation Distances

Abstract: This work provides an analytical method, based on the Amperian model of permanent magnets (PM), for a fast calculation of the magnetic flux density in three-dimensional space applying Biot–Savart law, and the calculation of the forces using Lorentz's law. The applied approach enables the characterization regarding forces, torques, and stiffnesses of the levitating PM for any arbitrary position in space. Furthermore, it permits the extension of the investigation to any shape and configuration of ironless magnetic bearings (MB). In order to demonstrate the simple use of the analytical model, in this article, the dimensions of an ironless axial MB employing PMs are optimized with a multiobjective Pareto analysis, which reveals the physical limits concerning maximum achievable levitation height with respect to given constraints on, e.g., the required force and tilting stiffnesses, and the MB robustness defined by the maximum allowable payload on the levitating magnet. Moreover, the optimized axial MB and a corresponding test bench are realized to validate the proposed model with experimental results. For the sake of completeness, it should be mentioned that in a later stage, the optimized MB can also be scaled with simple scaling laws if the demanded specifications, e.g., concerning desired maximum levitation height or payload capability, would have changed.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Multilevel inverters: a survey of topologies, controls, and applications

Abstract: Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

Recent Advances and Industrial Applications of Multilevel Converters

Abstract: Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.