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.

High-temperature (250 °C / 500 °F) 19′000 rpm BLDC fan for forced air-cooling of advanced automotive power electronics

Abstract: In an increasing number of application areas and industry sectors, such as the automotive, aerospace, military or oil and gas industry, a trend towards higher ambient temperature rating from 120 °C upward for electrical machines and power electronic converters can be observed. Forced air-cooling of power electronic converters offers reduced complexity of the cooling circuit compared to water-cooling. For high ambient temperature rated converters fans are required that withstand these temperatures and still feature performance comparable to standard conditions in order to still enable a high converter power density and efficiency. Commercially available fans for power electronics cooling are typically rated up to 75 °C, very rarely fans are specified up to 105 °C. In this paper, the electrical and mechanical design of a 40mm × 40mm × 28mm fan is presented in detail that offers an operational temperature range up to 250 °C at the rated speed of 19′000 rpm and similar fluid dynamical performance in terms of static pressure and volume flow at 120 °C as commercial high performance fans at 20 °C. The 3-phase BLDC machine driving the fan is integrated into its hub and has got an input power of 15W. The fan can be driven using a 3-phase inverter supplied from 12V DC voltage with an inverter switching frequency of less than 1.3 kHz.

Analysis and design of an ultra-high-speed slotless self-bearing permanent-magnet motor

Abstract: Active magnetic bearings (AMB) enable contactless operation and can therefore be used for supporting rotors spinning at high speeds. However, the rotational speed in conventional reluctance-force-based AMB topologies is limited which is mainly due to high rotor losses and achievable force control bandwidths. In this paper, a prototype of a self-bearing motor designed for rotational speeds of up to 500 000 revolutions per minute (rpm) is presented. Due to the employed AMB, the motor can be operated in high-purity or vacuum environments. An analytical mechanical and electrical bearing model is introduced and verified by measurements. Furthermore, a bearing inverter system is designed and its controller performance is shown. Closed-loop system measurements of a spinning levitated rotor at 400 000 rpm verify the functionality of the overall system. To the authors knowledge, this is the world record speed for magnetically-levitated electrical drive systems.

Optimized Modulation of a Four-Port Isolated DC–DC Converter Formed by Integration of Three Dual Active Bridge Converter Stages

Abstract: Multi-port converters have gained more and more interest in research during recent years, due to the increasing field of possible applications, e.g., DC micro grids, energy distribution in electric vehicles and more electric aircraft, and power supplies for cascaded multi-cell converters. This paper presents an optimized modulation strategy for a bidirectional multi-port DC–DC converter, which consists of the Integration of Three (3) conventional Dual-Active Bridge (I3DAB) converters into a structure that combines the primary-side full bridges into a common three-phase two-level inverter. The resulting structure features one input port and three isolated output ports. By utilizing so far unused degrees of freedom for the control of the power converter, it is shown that a reduction of the power dissipation can be achieved by adapting the primary-side duty cycles to the output power levels. According to the outcome of a comparative evaluation of conventional and optimized modulation strategies for an example system with input and output port voltages of 700 V and 3 × 100 V, respectively, and with a total nominal power of 3 × 4 kW, reductions of the conduction losses of up to 23% and reduced additional hardware efforts to achieve ZVS operation (with regard to reduced transformers’ magnetizing inductances) within wide power ranges are achievable and are also expected for deviating port voltages. Thus, in combination with the presented optimized modulation strategy, the I3DAB converter is found to be well suited to multi-port applications that require bidirectional conversion capabilities, galvanic isolation, and are subject to unequal load conditions with substantially different power levels provided by the output ports.

Novel Converter Concept for Bearingless Slice Motor Systems

Abstract: The general trend towards smaller and more economical designs has also reached bearingless slice motor (BSM) systems. Up to now, full-bridge converters have been used in order to independently control the phases of the active magnetic bearing and the drive system. In this paper, a simplified topology based on three half-bridges is proposed to replace two conventional full-bridges. With this, the number of required power semi-conductors is reduced by 25%. The three half-bridges can be advantageously realized by one integrated IGBT module, thus achieving a reduction of the total converter volume by 43% as compared to a discrete full-bridge solution. The principle of operation is explained and analytical expressions for the calculation of the current stresses on the power components and dc-link capacitors are provided as a basis for a design of the system. Finally, the performance of the system is shown for a 600W prototype of the topology.

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.