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.

Performance Comparison of Bearingless Motor Topologies in Exterior Rotor Construction

Abstract: This paper presents several feasible slot/pole combinations for a bearingless motor in exterior rotor construction. Due to the limited available stator space, the slot number of the stator has to be chosen small in order to provide sufficient winding space. The characteristics of each possible topology are discussed in great detail and for each setup the optimal design parameters are derived using 3D-FEM analysis. In the end, a fair comparison between the presented topologies is undertaken and the optimal motor topology is implemented in a real-size prototype setup. Introduction For the chemical, pharmaceutical and biotechnological industry sector, the qualitative refinement of high-purity mixing is a mandatory prerequisite to improve both research and production processes. State-of-the art mixing systems either use magnetic couplings or a long shaft passing through a seal to transmit the rotation energy from an outer motor into the process tank. With a bearingless motor [1-8], the common drawbacks of additional bearings inside the process tank and of pinch-off areas due to mechanical contact can be eliminated. Moreover, due to the large possible air gap, clean-in-place and sterilization-in-place processes [9] are facilitated. The bearingless motor is completely free of wear as well as free of lubrication and thus promises low maintenance cost and long life time. The bearingless motor in exterior rotor construction can be advantageously employed for high-purity mixing when only the rotor is placed inside the process tank, whereas the stator and all control and power electronics are located outside, separated by the tank wall (cf. Fig. 1). The impeller can then be mounted directly onto the rotor, which levitates and rotates inside the tank in a contactless manner. With this construction type, flow-low zones are avoided and complete tank drainage through the bottom outlet is not disturbed. Moreover, high mixing torque can be produced with a compact setup. Tank bottom Tank outlet Stator Tank wall Rotor with impeller (levitated) Air gap Fig. 1: Open view of the tank bottom. The stator is placed inside an indentation reaching into the tank bottom, but still outside of the tank wall. Only the rotor with the impeller blades is then mounted inside the process liquid and is levitated around the stator. This paper will focus on different exterior rotor bearingless motor topology possibilities. In a first step, three feasible topologies are discussed and compared analytically. In a second step, their torque and bearing performances are compared based on a 3D-FEM analysis. The goal is to come up with high-torque motors that show stable bearing behavior under working conditions. In the end, the simulation results of the most promising topology are verified with the performance of prototype setup. Exterior rotor bearingless motor topologies The most characteristic topology differences for bearingless motors result from the choice of the rotor pole pair number p and the stator slot number q. Once this choice is made, mainly geometrical variations influence the final design of each motor. For the exterior rotor construction type, the outer rotor diameter (which usually results from the required tank volume and the specific application) limits the motor size in the radial directions. Therefore, the stator space will be very limited since the whole stator iron and the windings need to be placed inside the hollow rotor ring. For this reason, only stator configurations with small slot numbers can be considered because the ratio of iron material to winding material would become unfavorably large otherwise. The proposed topologies are assembled with combined concentrated windings [10], so that there is one separate coil per stator tooth and it contributes to the generation of both torque and bearing forces. The required drive and bearing currents can be digitally controlled independent of each other and will then be superimposed mathematically prior to applying them to the stator coils. The rotor of the bearingless motor consists of a back iron ring and of permanent magnets which are magnetized in radial direction in alternating order. With a very small slot number of three, a permanent magnet motor could be built. However, it is not possible to come up with a bearingless motor with this design, because drive and bearing would influence each other so that they cannot be controlled independently. The four-slot/twelve-pole motor from Fig. 2(a) consists of a two-phase bearing combined with a single-phase drive. It can be controlled with an inverter consisting of four full-bridges,

Two position current controller of power electronic system - with synchronisation of switching condition changes with phase offset

Abstract: The invention relates to a method for controlling parallel-connected, two-point current-regulated power electronic subsystems 12 and 13, in which case each subsystem has a current-stabilizing inductance 9 or 10, respectively, an electronic switching apparatus 7 or 8, respectively, and a hysteresis switching element 30 or 31, respectively, which establishes the switching state of this switching apparatus. The electronic switching apparatuses are actuated with respect to the output of the hysteresis switching elements 30 and 31 identically for both subsystems, delayed via delay elements 34 and 35. Symmetrical coupling 21 and 22 of the subsystems 12 and 13 ensures that the signal profile of the control errors that occur at the input of the hysteresis switching elements is different from that in decoupled operation, such that, after switching the leading system, the rate of change of the signal which occurs at the input of the hysteresis switching element of the lagging system is increased/reduced and thus, once the time delay between switching of the hysteresis switching element and actuation of the associated electronic switching apparatus has elapsed for the lagging system, a value of the control error is higher or lower than the control error of the leading system, corresponding to an increase/reduction in the phase offset of the switching state change of the subsystems, and the phase offset at each switching threshold is thus further increased/reduced until antiphase/in-phase operation is reached, or the switching state changes of the subsystems are synchronized in antiphase/phase-synchronous operation of the subsystems, with the current elements, averaged over time, being guided along a nominal value 11 predetermined by higher-level system parts or setting apparatuses.

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.