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.

Comparison of winding concepts for bearingless pumps

Abstract: Bearingless slice motors as employed in semiconductor, pharmaceutical and medical industry perform magnetic levitation by radial bearing forces and rotation by tangential forces. This requires bearing and drive windings, which can be realized as separate bearing and drive coils or as identical, concentrated coils on the stator claws. In this paper, a detailed comparison between these two winding concepts is undertaken, where the coil losses, the coil volume, the power electronics requirements and the achievable rotation speed are evaluated. Furthermore, practical features such as control complexity, cabling effort and manufacturability are taken into account. Finally, the trade-off between losses, volume and realization effort will be discussed in order to give a guideline for the selection of the appropriate winding concept for a specific application of a bearingless pump.

EMI filter design for high switching frequency three-phase/level PWM rectifier systems

Abstract: The actual attenuation characteristic of EMI filters in practice often differs from theoretical predictions and minor changes could result in a significant performance improvement. Whereas the performance of the differential-mode (DM) filter stage usually can be well predicted, the common-mode (CM) behavior is more difficult to handle. This is especially true for three-phase PWM rectifier systems, which show a large high-frequency CM voltage at the rectifier output. In this work the possible CM noise current paths of a three-phase/level PWM rectifier are analyzed where parasitic capacitances to the heat sink and to earth are considered. Additionally, a concept to significantly reduce CM emissions is discussed in detail. Based on the proposed models an EMI filter design for a system with 1 MHz switching frequency is shown. Experimental verification of the designed EMI filter is presented by impedance and conducted emission (CE) measurements taken from a 10 kW hardware prototype. Several practical aspects of filter realization like component arrangement, shielding layers, magnetic coupling etc. are discussed and verified by measurements.

Wide input voltage range high power density high efficiency 10 kW three-phase three-level unity power factor PWM rectifier

Abstract: In this paper the current stresses on the power components of a direct three-phase boost-type unity power factor rectifier are analysed in order to provide a basis for a system design under restriction of the height to 2-U. The conduction losses of the power semiconductors are calculated using analytical approximations of the average and RMS values of the component currents. The switching losses are taken from experimental investigations where a novel turn-on snubber has been employed. Based on this data an overview of the estimated power losses is given for a rectifier system of 10.5 kW/800 VDC output for 320 V/400 V/480 V/530 V (RMS, line-to-line) mains voltage. Corresponding efficiency figures are calculated and the improvement achieved by the turn-on snubber as compared to hard switching is determined. The snubber topology and operating principle is discussed in detail. Finally, the theoretical results are verified by experimental investigation of a system prototype.

Medium-frequency transformer scaling laws: Derivation, verification, and critical analysis

Abstract: Medium-frequency transformers (MFTs) are one of the fundamental building blocks of modern power electronic converters. The usage of increased frequencies leads to improved characteristics, i.e., efficiency and power density (volumetric and gravimetric) but also to design challenges and constraints. This paper reviews the analytical modeling of MFTs. More particularly, the mapping between the design space and the performance space is analyzed. It is found that wide regions of the design space are mapped to a narrow region in the performance space, i.e., the optimum is flat and designs with very different parameters features similar performances (design space diversity). Scaling laws are derived for optimal MFTs operated at different power ratings and power densities, which provide a comprehensive and general insight on the achievable performances. In a next step, the results obtained with the analytical model are compared to numerical simulations. It is concluded that the derived scaling laws capture qualitatively and quantitatively the behavior of MFTs, but should be used with caution for accurate design processes.

Virtual-Flux Direct Power Control for Mains Connected Three-Level NPC Inverter Systems

Abstract: This paper proposes a control strategy which extends the virtual-flux direct power control traditionally employed for the conventional two-level VSI to a three-level NPC inverter. This topology generates a higher number of output voltage levels, increasing the flexibility for selecting an appropriate voltage vector. The mid-point potential is controlled according to the direction of the mid-point current and the sign of the mid-point voltage deviation. The method is adapted to be used with an LCL output filter, where some undesirable characteristics, such as filter resonance, have to be compensated. Further investigation concerning the dependency of the mid-point voltage with the current phase-shift is given. Theoretical analysis is presented and the performance of the proposed method is verified by simulation.

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.