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.

Rotor losses in an ultra-high speed spinning ball motor

Abstract: The ongoing miniaturization trend of electric machines demands for higher rotational speeds to provide a required power level at decreased size. The goal of this project is to push the limits of miniaturization by researching new concepts for bearingless machines with ultra-high rotational speeds exceeding 25 million rotations per minute (Mrpm). The employed sub-millimeter size spherical solid steel rotors are accelerated by the principle of a solid rotor induction machine. This study presents an analysis of the resulting eddy current losses and a thermal model of the rotor. The temperature of the rotor material is of interest, as it influences the achievable rotational speed at which failure of the rotor due to centrifugal loading occurs.

Active Reset Circuit with Energy Recovery for Solid State Modulators

Abstract: Summary form only given. In order to reduce pulse losses two active reset circuits are proposed in this paper. The first one is based on the passive reset circuit and offers the possibility' to feed the excessive energy stored in the transformer/inductor partly back to the source instead of dissipating it. This leads to a reduction of the losses by 20-30% of the passive circuit and a significant improve of the efficiency. The second reset circuit is based on the topology presented, where the excessive energy is stored in an capacitor which is in series to the freewheeling diode. This energy is used to premagnetize the transformer core. In the paper a charging circuit which compensates for the losses is added to this topology. Furthermore, control strategies are presented which allow to keep the premagnetizing current constant for a certain period of time and which could be applied for using the remanence of the core for a pulse premagnetization. In this case the core is premagnetized with a current which is turned off after the premagnitization. In the paper detailed information on the waveforms and the design of the proposed topologies is given and the losses of the different topologies are compared.

Comparative Evaluation of Three-Phase AC-AC Voltage/Current-Source Converter Systems Employing Latest GaN Power Transistor Technology

Abstract: The emergence of monolithic bidirectional GaN power transistors sparks renewed interest in AC-AC current-source converters (CSCs) as an alternative to AC-AC voltage-source converters (VSCs) for motor drive applications. This paper compares the two approaches with the CSC utilizing novel 600 V, 140 $\mathrm{m}\Omega$ monolithic bidirectional GaN transistors. Aiming for a comparison of next-generation plug-and-play motor drives, both systems must fulfill the same EMI limits that cover an extended frequency range from 9 kHz to 30 MHz at the grid and at the motor interface. Designing both systems with roughly the same total chip area for the same AC-AC efficiency of 97 % at a nominal power of 1.4 kW (200 V nominal line-to-line RMS voltage, 4 A nominal RMS current), we identify a suitable switching frequency of 72 kHz, and we discuss efficiency characteristics over the motor current (torque) and voltage (speed) ranges. A comparison of the total magnetic component volumes finds advantages for the VSC system, which disappear if motor-integration (voiding the need for a motor-side differential-mode EMI filter) is considered.

"Looking Through Walls" – Actuator Position Measurement Through a Conductive Wall

Abstract: This paper investigates the design of an Eddy Current Sensor (ECS) for position measurement of a moving conductive target located behind a fixed conductive shielding surface. Such a sensor can e.g. be used in completely sealed actuators with magnetically levitated rotor or mover for high purity applications. A detailed analysis of the sensor is conducted, starting from the conventional ECS and its equivalent circuit model, which is then further extended to an ECS looking through a conductive wall. With the aid of Finite Element Methods (FEM) simulations, a deeper understanding of the sensor operating principle is provided, together with guidelines to select its optimal excitation frequency, based on the properties of the selected materials for the target and the shielding surface. A sensitivity analysis is then conducted and the results are finally verified with measurements on a hardware sensor prototype, showing that the sensor can be used to capture the mover position in an active magnetic bearing control structure.

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.