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 three-phase Y-rectifier cyclic 2 out of 3 DC output voltage balancing

Abstract: A three-phase Y-rectifier is formed by the star connection of single-phase unity power factor rectifier systems and represents a highly interesting concept for the realisation of the input stage of high-power telecommunications power supply modules using established single-phase technology. However, for stable operation, control and balancing of the independent DC output voltages of the phase rectifier systems is required. A novel, easy to implement DC output voltage control concept is proposed in this paper. There, the mean value of all three DC output voltages is controlled and, in addition, always 2 out of the 3 DC voltages are compared and balanced. The basic operating principle of the control is described and the theoretical limit for the admissible asymmetric loading of the DC voltages is calculated. Finally, the theoretical considerations are verified by measurements on a 3times1 kW Y-rectifier prototype.

A Bearingless Synchronous Reluctance Slice Motor with Rotor Flux Barriers

Abstract: This paper presents a bearingless synchronous reluctance slice motor, which contains no permanent magnets. The rotor with four iron poles and flux barriers is levitated and rotated through a stator winding system with six coils wired as two three-phase systems. After applying a constant rotor oriented magnetization current, the system can be controlled just like a bearingless permanent magnet synchronous slice motor, including the passive stabilization of axial and tilting movements. In a first step, the motor geometry is being optimized and the performance characteristics of the designed motor are examined. The motor is then compared to two other designs, which contain permanent magnets either in the rotor or the stator. The comparison includes torque generation, radial force generation, passive axial and tilting stiffnesses and wide air gap suitability. The introduced topology outperforms the others for ultra high process or ambient temperatures and rotor disposable applications with a short exchange interval.

Hybrid rectifier and method for operating such hybrid rectifier

Abstract: A hybrid rectifier (14a) comprises a line-commuted rectifier (11) and a self-commuted rectifier (15) working in parallel between a 3-phase AC input (17) and a DC output (18). The efficiency of the rectifier is improved by choosing said self-commuted rectifier (15) to be a 2-level unidirectional delta-switch rectifier.

High bandwidth non-resonant high voltage generator for X-ray systems

Abstract: High-voltage supplies for medical X-ray systems require up to 150 kV DC at up to 100 kW to supply the tube. The challenge for the power supply however is not only to deal with the high voltage and high power demand, but also to provide short voltage rise- and fall times in the range of 100μs. A coupled interleaved circuit variant of a single active bridge concept, the coupled interleaved single active bridge converter, is demonstrated in this work providing very fast output voltage control due to its non-resonant structure. The conduction modes of the circuit are analysed in detail and compared to the uncoupled interleaved single active bridge approach, demonstrating the advantages of low inverter RMS current, zero voltage switching (or at least zero current switching) over the full operating range with constant switching frequency. Analytic expressions for the output current-duty cycle relationship are provided. Experiments on a 60 kW (low voltage equivalent) prototype show that rise times as low as five switching cycles are possible, allowing to reach the target of 100μs rise time using a switching frequency of only 50 kHz.

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.