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.

Comparative evaluation of linear-rotary actuator topologies for highly dynamic applications

Abstract: A Linear-Rotary Actuator (LiRA) is an electrical machine whose mover can move linearly and rotate as a direct drive (without mechanical transmissions/gearboxes). Such actuators are successfully used in various applications such as pick-and-place machines that mount components on Printed Circuit Boards (PCBs), servo actuation of gearboxes, and in robot arms and/or end effectors. In order to design a compact LiRA with high dynamics, two different LiRA topologies are compared in this paper. A fair comparison is obtained by fixing the outer dimensions of the examined LiRA topologies (diameter and length). Electrical loading of the machines is determined using a simple thermal model, and an optimization based on finite element analysis is carried out. Performance criteria, such as circumferential and axial accelerations as well as maximum axial forces of all analyzed machines are compared. Moreover, metrics like total machine mass and volume of the used permanent magnets are included in the evaluation.

Contactless Picking of Objects Using an Acoustic Gripper

Abstract: Acoustic levitation forces can be used to manipulate small objects and liquids without mechanical contact or contamination This work presents analytical models based on which concepts for the controlled insertion of objects into the acoustic field are developed This is essential for the use of acoustic levitators as contactless robotic grippers Three prototypes of such grippers are implemented and used to experimentally verify the lifting of objects into an acoustic pressure field Lifting of high-density objects (ρ > 7 g/cm3) from acoustically transparent surfaces is demonstrated using a double-sided acoustic gripper that generates standing acoustic waves with dynamically adjustable acoustic power A combination of multiple acoustic traps is used to lift lower density objects (ρ≤025g/cm3) from acoustically reflective surfaces using a single-sided arrangement Furthermore, a method that uses standing acoustic waves and thin reflectors to lift medium-density objects (ρ≤1g/cm3) from acoustically reflective surfaces is presented The provided results open up new possibilities for using acoustic levitation in robotic grippers, which has the potential to be applied in a variety of industrial use cases

Magnetic equivalent circuit of MF transformers: modeling and parameter uncertainties

Abstract: Medium-frequency (MF) transformers are extensively used in power electronic converters. Accordingly, accurate models of such devices are required, especially for the magnetic equivalent circuit. Literature documents many different methods to calculate the magnetizing and leakage inductances of transformers, where, however, few comparisons exist between the methods. Furthermore, the impact of underlying hypotheses and parameter uncertainties is usually neglected. This paper analyzes nine different models, ranging from simple analytical expressions to 3D detailed numerical simulations. The accuracy of the different methods is assessed by means of Monte Carlo simulations and linearized statistical models. The experimental results, conducted with a $$100\,{\hbox {kHz}}$$ / $$20\,{\hbox {kW}}$$ MF transformer employed in a $$400\,{\hbox {V}}$$ DC distribution system isolation, are in agreement with the simulations (below 14% inaccuracy for all the considered methods). It is concluded that, considering typical tolerances, analytical models are accurate enough for most applications and that the tolerance analysis can be conducted with linearized models.

Bearingless Permanent-Magnet Motor with 4/12 Slot-Pole Ratio for Bioreactor Stirring Applications

Abstract: This article presents a novel topology for a bearingless permanent-magnet motor. This disk-shaped motor, featuring an exterior rotor, can be advantageously employed in delicate bioreactor processes. Both torque and bearing forces originate inside this magnetically levitated motor and are generated with concentrated combined windings. Using 3-D-FEM analysis, the optimal machine sizing parameters are evaluated with the goal to maximize torque while providing sufficient bearing forces to allow a precise and stable operation. The results from the sizing optimization and from the force and torque analysis have been implemented and tested with a real-size prototype setup.

Exploration of the Design and Performance Space of a High Frequency 166 kW/10 kV SiC Solid-State Air-Core Transformer

Abstract: With the availability of 10 kV SiC MOSFETs with low Zero Voltage Switching (ZVS) losses, Medium-Voltage (MV) converters, e.g., Solid-State Transformers (SST), capable of operation at very high switching frequencies become feasible. However, the optimization of MV and Medium-Frequency (MF) transformer of the dc–dc converter stage of a high power SST reveals that only limited improvements in efficiency and weight result for switching frequencies exceeding 50 kHz. Therefore, air-core transformers are expected to enable a realization with lower weight and, at the same time, simplified insulation coordination. This paper presents a comprehensive exploration of the design and performance spaces (efficiency, mass, volume) of a conventional, i.e., a magnetic-core based, and an air-core transformer employed in a resonant dc–dc converter with input and output voltages of 7 kV and a rated power of 166 kW. As a result, comparable efficiencies are achievable for both transformers (99.3% and 99.0%), but the SST with air-core transformer at a switching frequency of 103.6 kHz features 41% of the mass (10.3 kg) of a conventional transformer (24.9 kg at a switching frequency of 48.5 kHz). Accordingly, air-core transformers are of special interest for future weight-critical SST applications, e.g., in More Electric Aircraft and More Electric Ships.

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.