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.

Minimum loss operation of high-frequency inductors

Abstract: This paper investigates the losses of a power inductor employed in a 2kW, 400V input DC-DC converter, in dependency of key operating parameters, i.e. switching frequency and current ripple. Based on detailed high-frequency winding and core loss models, including the implications of DC-bias, temperature and frequency on the core losses, and an analytic thermal model, a minimum loss inductor is designed for each combination of switching frequency, f, and current ripple, r. In the course of the optimization, the core (E55, N87) and the winding (litz wire, 100 μm) are considered given. Surprisingly, the evaluation of the losses calculated in the f-r domain reveals that nearly minimum inductor losses are obtained for a current ripple that is inversely proportional to the frequency, i.e., for a constant inductance. Further detailed investigations of the calculated inductor losses reveal a decrease of the losses for increasing frequencies up to a very high frequency of 500 kHz. In this regard, at f = 100kHz, minimum total losses of 4.0W result for r = 45%, which can be reduced to 1.8W at f = 500 kHz for r = 10%. Finally, the sensitivities of the losses with regard to different litz wires (71 μm, 200 μm) and a different core (E42, N87) are examined and a design guidance is extracted that summarizes the main findings of the detailed investigation. A calorimetric measurement set-up is used to measure the losses of a realized inductor at different operating points in order to confirm the theoretical considerations.

Dv/Dt-Control Methods for the SiC JFET/Si

Abstract: Switching devices based on SiC offer outstanding per- formance with respect to operating frequency, junction tempera- ture, and conduction losses enabling significant improvement of the performance of converter systems. There, the cascode consist- ing of a MOSFET and a JFET has additionally the advantage of being a normally off device and offering a simple control via the gate of the MOSFET. Without dv/dt-control, however, the tran- sients for hard commutation reach values of up to 45kV/μs, which could lead to electromagnetic interference problems. Especially in drive systems, problems could occur, which are related to earth currents (bearing currents) due to parasitic capacitances. There- fore, new dv/dt-control methods for the SiC JFET/Si MOSFET cascode as well as measurement results are presented in this pa- per. Based on this new concepts, the outstanding performance of the SiC devices can be fully utilized without impairing electromagnetic compatibility.

A Comparison of Separated and Combined Winding Concepts for Bearingless Centrifugal Pumps

Abstract: Bearingless centrifugal pump systems are employed in the semiconductor, pharmaceutical and medical industries due to their facility for pumping high purity fluids without particle contamination. Two types of forces have to be generated by the stator units, namely bearing forces for achieving magnetic levitation, and drive forces for producing the needed pump torque. The generation of these forces requires bearing and drive windings, which can be realized as separate bearing and drive coils or as identical, combined coils on the stator claws. In this paper, a detailed comparison between these two winding concepts is undertaken, whereby the copper losses, the power electronics losses, and the achievable pump output pressure are evaluated for both concepts. For each criterion a ratio of improvement is calculated analytically which allows evaluation of the performance of the two winding concepts for any given pump operating point and design. Finally, also practical features such as control complexity, cabling effort and manufacturability are discussed and measurements on prototype systems are carried out to validate the considerations.

Protection of MV/LV solid-state transformers in the distribution grid

Abstract: Solid-State Transformers (SSTs) are a promising technology since they combine a high efficiency with the integration of new functionalities and services in the grid. A SST establishes the interface between a MV AC three-phase grid and a LV AC or DC grid. For this reason a SST is subject to electrical stresses due to faults occurring in the grid. The MV and LV grids are in return exposed to failures of the SSTs. Therefore, a suitable design of the SST and its corresponding protection devices has to be found. This paper identifies the different stresses which are relevant for a SST, analyzes the protection mechanisms which are currently used for low-frequency transformers and proposes adapted protection schemes for SSTs. MV short circuits and overvoltages are identified as the most critical stresses and are analyzed in detail. Finally, some guidelines are extracted for designing a robust SST.

Multivariable State Feedback Control of a 500 000-r/min Self-Bearing Permanent-Magnet Motor

Abstract: The use of magnetic bearings in electrical drive systems enables very long lifetimes at highest speeds and the operation in high-purity or vacuum environments. The machine prototype presented in this paper overcomes several limitations of previously presented high-speed magnetically levitated electrical drive systems. The design features linear bearing characteristics, which enables applying linear state feedback control without linearization of bearing actuators. A multivariable rotor position control scheme is proposed and implemented on a signal processor. The implemented Kalman filter and the linear state feedback controller proved to perform well in practice, stabilizing the system over the design speed range of the machine with a single set of controller parameters. Closed-loop system transfer function measurements verify the presented system modeling and controller performance. Measurements of the machine spinning at 500 000 r/min verify the functionality of the overall system. To the authors’ knowledge, this is the highest speed achieved by magnetically levitated electrical drive systems so far.

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.