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 …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

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.

Multilevel inverters: a survey of topologies, controls, and applications

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.

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Recent Advances and Industrial Applications of Multilevel Converters

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

In pulse modulators applying pulse transformers reset circuits are used to achieve optimal utilization of the core material, which results in lower costs, downsized pulse transformer/system volume and therefore in an improved pulse behavior due to the smaller parasitics. Because of its simplicity the most common method to reset the core is a DC reset circuit, where a DC current is used to premagnetize the core. However, the DC reset circuit - even with an optimal design-leads to significant losses in the freewheeling path. By applying an active reset method the losses due to the passive reset circuit can be reduced significantly. So far, only the theoretical behavior of the active reset circuit has been examined. Therefore, the detailed design and measurement results are presented in this paper. Furthermore, a new control method for achieving symmetrical flux swings in the core is presented.

Design and control of an active reset circuit for pulse transformers

State of the art active magnetic bearings are controlled using a current reference, a current observer or the current as a state variable in a state-space controller. Using the current to express the state of a magnetic bearing has several disadvantages such as an unstable zero, hidden information such as stray flux, eddy currents, magnetization or hysteresis of the iron core material. Above all, it is in a nonlinear relation with both the generated force and the air gap. In this paper a novel method is presented to directly and linearly control the force of a magnet using a force controller that is based on a flux estimator. The method is verified with simulations as well as with measurements. It decouples the magnet and position controllers, eases the modeling of non-ideal parameters of magnetic bearings, reduces noise, has a larger range of displacement and behaves intrinsically linear.

Novel observer based force control for active magnetic bearings

Bearingless motors offer contactless levitation and rotation through a hermitically closed process chamber and satisfy the increasing demand for ultra high purity spin processes in chemical, biochemical and semiconductor industry. Most of these processes require a high acceleration capability for minimal process times and a large air gap in order to fit in a mechanical stable chamber wall. The combination of these requirements possibly causes saturation effect issues that may occur depending on the specific drive claw shape. In this paper, saturation effects which reduce the effective torque of the motor are investigated for two commonly known drive claw shapes. The investigations are exemplarily carried out for the Bearingless 2-Level Motor (B2M), since it allows the isolated examination of saturation effects caused by the drive flux due to the axial and circumferential separation of the bearing and drive units. The paper visualizes the flux density distribution and the flux trajectory change at the presence of saturation for the presented drive claw shapes by means of 3D finite element simulations. One drive claw type is found to be the more applicable concept and is subsequently optimized with respect to maximum torque by variation of the drive claw width. Finally, the superior performance of the designed claw is proven by the comparison of simulation results with measurement data of an experimental prototype.

Saturation effects in high acceleration bearingless motors

Rotary–linear electric machines can perform coupled rotary and linear motion. In addition, they can have magnetic bearings (MBs) integrated and magnetically coupled with the rotary, linear, or rotary–linear machine operation. Since rotary–linear machines with MBs have not been thoroughly analyzed in the literature, the models that provide understanding of their operation and give basis for the control system implementation are not entirely covered. Hence, in this article, an enhanced complex space vector-based model of the rotary–linear machine with MBs is derived and expressions for the torque, thrust force, and MB force are given. The rotary–linear machine complex space vector of the voltage, current, or flux linkage is defined using the proposed transformation with two complex frames: one related to the rotation and MBs and another to the linear motion. This results in complex space vectors with two complex units; however, the techniques used for a conventional complex space vector calculation can also be applied to the proposed complex space vector description. This is also experimentally validated on a hardware prototype of a magnetically levitated linear tubular actuator (MALTA), whose position control system is designed and implemented based on the enhanced space vector modeling approach, with the dynamic operation of the MALTA, including linear motor operation with an axial stroke of 10mm and a mechanical frequency of 17Hz.

Enhanced Complex Space Vector Modeling and Control System Design of Multiphase Magnetically Levitated Rotary–Linear Machines

The efficiency limit for an inductive power transfer between two flat half-filled disc coils is derived based on a model for the eddy current losses in the coils and the losses due to electromagnetic radiation. Analytic approximations for the coupling factor of the coils and eddy current losses are proposed and experimentally verified. It is shown that the approximative terms allow us to express the maximum efficiency of the coil pair analytically. If the strand diameter of the coil is sufficiently small, the efficiency depends only on the strand diameter, diameter of the coils, and the gap between the coils—but not on the operating frequency. Therefore, increasing the frequency does not result in higher efficiency but allows to reduce the coil thickness.

Johann W. Kolar

Papers

Design and control of an active reset circuit for pulse transformers

Novel observer based force control for active magnetic bearings

Saturation effects in high acceleration bearingless motors

Enhanced Complex Space Vector Modeling and Control System Design of Multiphase Magnetically Levitated Rotary–Linear Machines

Inductive Power Transfer Efficiency Limit of a Flat Half-Filled Disc Coil Pair