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.

An Active Magnetic Damper Concept for Stabilization of Gas Bearings in High-Speed Permanent-Magnet Machines

Abstract: The successful application of ultrahigh-speed electrical-drive systems in industrial products is currently limited by lacking high-speed bearing technologies permitting high reliability and long lifetime. Promising bearing technologies for high rotational speeds are contactless bearing concepts such as active magnetic bearings or gas bearings. While magnetic bearings usually are major electromechanical systems with substantial complexity, gas bearings allow compact realizations with high load capacity and stiffness; however, poor dynamic stability has been limiting their use at high rotational speeds. Following a hybrid bearing approach with an aerodynamic gas bearing for load support, a small-sized active magnetic damper concept is proposed to enable the stable high-speed operation of the gas bearing with a minimum of additional complexity and costs. As for the effective stabilization of the gas bearing, a high-quality displacement measurement is essential, and a new eddy-current-based rotor-displacement self-sensing concept employing an auxiliary signal injection and rotor displacement measurement circuit is presented. A hardware implementation of the proposed concept is shown providing high-resolution measurement signals.

Novel ZVS S-TCM Modulation of Three-Phase AC/DC Converters

Abstract: For three-phase AC-DC power conversion, the widely-used continuous current mode (CCM) modulation scheme results in relatively high semiconductor losses from hard-switching each device during half of the mains cycle. Triangular current mode (TCM) modulation, where the inductor current reverses polarity before turn-off, achieves zero-voltage-switching (ZVS) but at the expense of a wide switching frequency variation (15× for the three-phase design considered here), complicating filter design and compliance with EMI regulations. In this paper, we propose a new modulation scheme, sinusoidal triangular current mode (S-TCM), that achieves soft-switching, keeps the maximum switching frequency below the 150 kHz EMI regulatory band, and limits the switching frequency variation to only 3×. Under S-TCM, three specific modulation schemes are analyzed, and a loss-optimized weighting of the current bands across load is identified. The 2.2 kW S-TCM phase-leg hardware demonstrator achieves 99.7% semiconductor efficiency, with the semiconductor losses accurately analytically estimated within 10% (0.3 W). Relative to a CCM design, the required filter inductance is 6× lower, the inductor volume is 37% smaller, and the semiconductor losses are 55% smaller for a simultaneous improvement in power density and efficiency.

Modeling and comparison of machine and converter losses for PWM and PAM in high-speed drives

Abstract: The interaction of the machine and the converter is becoming increasingly important, especially for high-speed drives, mainly due to the effect of the converter modulation on the machine losses. The allocation of the losses to different components of the drive system needs to be known in order to choose the ideal machine and modulation match. In this paper, individual models are introduced for calculating the rotor, copper, core and inverter losses, taking the modulation type into account. These models are developed considering two typical high-speed permanent-magnet synchronous motor topologies (slotted and slotless machines) driven by PulseAmplitude-Modulation (PAM) and Pulse-Width-Modulation (PWM) converters. The models are applied to two off-the-shelf machines and a converter operating both with PAM and PWM. The test bench used to experimentally verify the models is also described and the model results are compared to the measurements. The results show that PAM produces a higher overall efficiency for the high-speed machines considered in this work. However, PWM can be used to move the losses from the rotor to the converter at the expense of decreasing the overall drive efficiency. The possible benefits of these results are discussed.

Common mode EMC input filter design for a three-phase buck-type PWM rectifier system

Abstract: The EMC input filter design for a three-phase PWM rectifier is usually separated into the design of the differential (DM) and of the common mode (CM) stage. While for the DM filter part design rules and procedures are well-known and the parameters are easier to derive, the CM filter design is often based on trial-and-error methods and/or on the experience of the designer. In this work, a comprehensive design procedure for a CM EMC input filter is performed exemplarily for a three-phase three-switch buck-type PWM rectifier with an integrated boost output stage. A simplified model of the CM noise propagation is developed and the relevant parasitic impedances are identified. A capacitive connection from the star-point of the DM input filter to the capacitive centre point of the rectifier output voltage is proposed and the effect of this measure concerning CM EMC is verified. Finally, a two stage CM filter is designed and the compliance to the conducted emission requirements of CISPR 22 class A is verified through measurements on a 5 kW prototype.

An Optimized 5 kW, 147 W/in 3 Telecom Phase-Shift DC-DC Converter with Magnetically Integrated Current Doubler

Abstract: In the last decade there has been a tremendous growth in the number of data centers due to the increasing demand for internet services. At the same time, the cost for energy and materials have increased because of reducing resources and increased demand. That has caused a change in the driving forces for new power supply development, with more consideration on power density and efficiency. The commonly used DC-DC converter in the power supply unit (PSU) for data centers and telecom applications are full bridge phase-shift converters since they meet the demands of high power levels and concurrently efficient power conversion as well as a compact design. The constant operating frequency allows a simple control and EMI design. To develop a new converter with higher power density and/or high efficiency the designer has a lot of degrees of freedom. An optimization procedure, based on comprehensive analytical models, has been developed and leads to the optimal parameters (e.g. switching frequency or transformer design) to achieve the most compact and/or efficient design. In this paper an volume optimized 400V/48V phase-shift DC-DC converter with current doubler rectifier based on analytical models is constructed. The power density of the converter is increased by integrating the output inductors in the transformers core. The intrinsic voltage ringing of the rectifier diodes is damped by a lossless magnetic snubber, which feeds ringing energy to the output. Experimental results prove the theoretical analytical models and the design procedure.

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.