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.

Stator Tooth Design Study for Bearingless Exterior Rotor PMSM

Abstract: This paper describes important design considerations for a bearingless brushless motor (permanent magnet synchronous motor, PMSM) in exterior rotor construction. In order to come up with a compact energy-dense setup which can provide both bearing forces and high torque, several parameters have to be accounted for while considering mutual dependences. Moreover, the magnetic bearing and the drive are interlinked for this disk-shaped bearingless motor (with combined concentrated windings). A detailed analysis about the design of the stator teeth has been undertaken, and the influence on torque and active and passive radial forces has been investigated.

Comparative Evaluation of a Full- and Partial-Power Processing Active Power Buffer for Ultracompact Single-Phase DC/AC Converter Systems

Abstract: One of the key technical challenges of the Google and IEEE Little Box competition, an international contest to build the world’s smallest 2-kW single-phase inverter in 2015, was to shrink the volume of the energy storage required to cope with the twice mains-frequency (120 Hz) pulsating power at the ac side and meet the stringent 2.5% input voltage ripple at the dc side. In this article, first, a full-power processing buck-type converter active buffer approach, selected by the first prize winner of the Little Box Challenge (LBC), is analyzed in detail. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed across the buffer capacitor significantly reducing the capacitance requirement. Second, a partial-power active buffer approach, selected by the second prize winner of the LBC, where conventional passive capacitive buffering of the dc-link is combined with a series-connected auxiliary converter, used to compensate for the remaining 120-Hz voltage ripple across the dc-link capacitance, is studied in detail. In this article, both the selected concepts are comparatively evaluated in terms of achievable efficiency, power density, and ripple compensation performance under both stationary and transient conditions. Novel control schemes and optimally designed hardware prototypes for both considered buffer concepts are presented and accompanied by experimental measurements to support the claimed efficiency and power density and assess the performance of the implemented control systems. Finally, by means of comparison with conventional passive dc-link buffering using only electrolytic capacitors, it is determined at what voltage ripple requirement it actually becomes beneficial in terms of volume to employ the considered active buffer concepts.

Full PEEC Modeling of EMI Filter Inductors in the Frequency Domain

Abstract: In this paper, the performance of a new method based on the coupling of the partial element equivalent circuit method and boundary integral method (the PEEC-BIM method) for 3D modeling of toroidal inductors, which are typically used in electromagnetic interference (EMI) filter applications, is presented. The presence of magnetic materials is modeled by replacing the surface of magnetic regions with an equivalent distribution of fictitious current loops. It is shown that the influence of the magnetic core on the impedance and the stray field of EMI filter inductors can be modeled and explained in detail by PEEC-BIM simulation results. The developed PEEC-BIM approach is verified by both 3D finite-element method (FEM) simulations and near-field measurements for different winding configurations and magnetic cores. Regarding computational complexity, the developed PEEC-BIM method applied to toroidal inductors performs extremely well. The PEEC-BIM simulation is at least twice faster than the corresponding FEM-based analysis. The PEEC-BIM method has been implemented in a PEEC-based simulation tool, which facilitates the simulation of entire EMI filter structures.

Three-phase buck-boost Y-inverter with wide DC input voltage range

Abstract: Driven by the needs of the continuously growing fuel-cell industry, a promising three-phase inverter topology, the Y-inverter, is proposed, which comprises three identical buck-boost DC/DC converter modules connected to a common star point. Each module constitutes a phase-leg and can be operated in similar fashion to conventional DC/DC converters, independent of the remaining two phases. Therefore, a straightforward and simple operation is possible. In addition, the Y-inverter allows for continuous output AC voltage waveforms, eliminating the need of additional AC-side filtering. Due to the buck-boost nature of each phase leg, the AC voltages can be higher or lower than the DC input voltage. This is an essential feature for fuel-cell applications, which suffer from a wide DC input voltage range. This paper details the operating principle of the Y-inverter, outlines the control system design and verifies its functionality by means of simulation results. The Y-inverter performance in terms of efficiency η and power density ρ is briefly analyzed by means of a multi-objective optimization and a converter design is selected which is compared to a benchmark system realized with a conventional inverter solution.

Multi-cell power conversion method with failure detection and multi-cell power converter

Abstract: A power converter includes a plurality of first converter cells, a plurality of second converter cells, and a plurality of DC link capacitors. Each DC link capacitor links one of the plurality of first converter cells and one of the plurality of second converter cells. A failure management unit is configured to detect a faulty converter cell, and to deactivate the faulty converter cell while maintaining a power conversion operation of the power converter.

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.