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Leijon, J., Sjölund, J., Ekergard, B., Boström, C., Eriksson, S., Temiz, I. & Leijon, M. (2018). Study of an Altered Magnetic Circuit of a Permanent Magnet Linear Generator for Wave Power. Energies, 11(1), Article ID 84.
Open this publication in new window or tab >>Study of an Altered Magnetic Circuit of a Permanent Magnet Linear Generator for Wave Power
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2018 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 11, no 1, article id 84Article in journal (Refereed) Published
Abstract [en]

The wave energy converter (WEC) studied and developed at Uppsala University in Sweden is a point absorbing buoy connected to a linear generator (LG) on the seabed. Previous studies have improved the sustainability of the generator, changing its magnets from Nd2Fe14B-magnets to ferrites. In this paper, the magnetic circuit of the linear generator is further studied. Ferrite magnets of two different types (Y30 and Y40) are studied along with different shapes of pole shoes for the system. The finite element method (FEM) simulations in a program called Ace are performed. The results show that a linear generator including both Y30 and Y40 magnets and shortened T-shaped pole shoes can generate a similar magnetic energy in the airgap as a linear generator only containing Y40 magnets and rectangular pole shoes. This shows that the magnetic circuit can be altered, opening up sizes and strengths of magnets for different retailers, and thereby possibly lowering magnet cost and transportation. This work was previously presented as a conference at the European Wave and Tidal Energy Conference (EWTEC) 2017 in Cork, Ireland; this manuscript has been carefully revised and some discussions, on magnet costs for example, have been added to this paper.

Keywords
wave energy converter (WEC), linear generator (LG), ferrite permanent magnets (PMs), renewable energy sources
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-348930 (URN)10.3390/en11010084 (DOI)000424397600084 ()
Funder
Swedish Research Council, 2015-03126Swedish Energy Agency, P42243-1
Available from: 2018-04-26 Created: 2018-04-26 Last updated: 2019-03-04Bibliographically approved
Eklund, P. & Eriksson, S. (2018). Winding Design Independent Calculation Method for Short Circuit Currents in Permanent Magnet Synchronous Machines. In: 2018 XIII International Conference on Electrical Machines (ICEM): . Paper presented at XIIIth International conference on Electrical Machines, September 3-6 2018, Alexandroupoli, Greece (pp. 1021-1027).
Open this publication in new window or tab >>Winding Design Independent Calculation Method for Short Circuit Currents in Permanent Magnet Synchronous Machines
2018 (English)In: 2018 XIII International Conference on Electrical Machines (ICEM), 2018, p. 1021-1027Conference paper, Published paper (Refereed)
Abstract [en]

When designing permanent magnet (PM) synchronous machines the demagnetizing effect of short circuit currents on the PMs needs to be considered. In some cases there can be a need to estimate the demagnetizing field from the winding without knowing the winding scheme. To do this a lumped parameter model of the dynamics of the magnetic field and armature current density distribution is proposed. Validation of the model using two different machines shows acceptable agreement. The proposed model is found to be useful for its particular purpose of determining the approximate short circuit current distribution in the armature without knowing the winding design.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-363290 (URN)10.1109/ICELMACH.2018.8506920 (DOI)
Conference
XIIIth International conference on Electrical Machines, September 3-6 2018, Alexandroupoli, Greece
Funder
Carl Tryggers foundation , 12-295
Available from: 2018-10-31 Created: 2018-10-31 Last updated: 2018-11-22Bibliographically approved
Rossander, M., Goude, A. & Eriksson, S. (2017). Critical Speed Control for a Fixed Blade Variable Speed Wind Turbine. Energies, 10(11), Article ID 1699.
Open this publication in new window or tab >>Critical Speed Control for a Fixed Blade Variable Speed Wind Turbine
2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 11, article id 1699Article in journal (Refereed) Published
Abstract [en]

A critical speed controller for avoiding a certain rotational speed is presented. The controller is useful for variable speed wind turbines with a natural frequency in the operating range. The controller has been simulated, implemented and tested on an open site 12 kW vertical axis wind turbine prototype. The controller is based on an adaptation of the optimum torque control. Two lookup tables and a simple state machine provide the control logic of the controller. The controller requires low computational resources, and no wind speed measurement is needed. The results suggest that the controller is a feasible method for critical speed control. The skipping behavior can be adjusted using only two parameters. While tested on a vertical axis wind turbine, it may be used on any variable speed turbine with the control of generator power.

Keywords
vertical axis wind turbine, variable speed, control, optimal torque, critical speed, speed exclusion zone, natural frequencies, eigenfrequencies
National Category
Energy Systems
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-331782 (URN)10.3390/en10111699 (DOI)000417046500018 ()
Funder
StandUpStandUp for Wind
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2018-03-09Bibliographically approved
Larsson, S. & Eriksson, S. (2017). Investigation of Permanent Magnet Demagnetization in Synchronous Machines During Multiple Short-Circuit Fault Conditions. Energies, 10(10), Article ID 1638.
Open this publication in new window or tab >>Investigation of Permanent Magnet Demagnetization in Synchronous Machines During Multiple Short-Circuit Fault Conditions
2017 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 10, article id 1638Article in journal (Refereed) Published
Keywords
Permanent magnets, Demagnetization, FEM, Multiple fault conditions
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-303509 (URN)10.3390/en10101638 (DOI)000414578400193 ()
Funder
Swedish Research Council, 2010-3950StandUp
Available from: 2016-10-14 Created: 2016-09-20 Last updated: 2018-02-15Bibliographically approved
Eklund, P., Sjolund, J., Eriksson, S. & Leijon, M. (2017). Magnetic End LeakageFlux in a Spoke Type Rotor Permanent Magnet SynchronousGenerator. In: : . Paper presented at ICEMD 2017 : International Conference on Electrical Machines and Drives, March 26 - 27, 2017 Madrid, Spain.
Open this publication in new window or tab >>Magnetic End LeakageFlux in a Spoke Type Rotor Permanent Magnet SynchronousGenerator
2017 (English)Conference paper, Published paper (Other academic)
Abstract [en]

The spoke type rotor can be used to obtain magnetic flux concentration in permanent magnet machines. This allows the air gap magnetic flux density to exceed the remanent flux density of the permanent magnets but gives problems with leakage fluxes in the magnetic circuit. The end leakage flux of one spoke type permanent magnet rotor design is studied through measurements and finite element simulations. The measurements are performed in the end regions of a 12 kW prototype generator for a vertical axis wind turbine. The simulations are made using three dimensional finite elements to calculate the magnetic field distribution in the end regions of the machine. Also two dimensional finite element simulations are performed and the impact of the two dimensional approximation is studied. It is found that the magnetic leakage flux in the end regions of the machine is equal to about 20 % of the flux in the permanent magnets. The overestimation of the performance by the two dimensional approximation is quantified and a curve-fitted expression for its behavior is suggested.

Keywords
End effects, End leakage flux, Permanent magnet machine, Spoke type rotor
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-366658 (URN)
Conference
ICEMD 2017 : International Conference on Electrical Machines and Drives, March 26 - 27, 2017 Madrid, Spain
Funder
Swedish Research Council, 2012-4706ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 12-295
Available from: 2018-11-22 Created: 2018-11-22 Last updated: 2019-03-20Bibliographically approved
Rossander, M., Goude, A. & Eriksson, S. (2017). Mechanical torque ripple from a passive diode rectifier in a 12 kW vertical axis wind turbine. IEEE transactions on energy conversion, 32(1), 164-171
Open this publication in new window or tab >>Mechanical torque ripple from a passive diode rectifier in a 12 kW vertical axis wind turbine
2017 (English)In: IEEE transactions on energy conversion, ISSN 0885-8969, E-ISSN 1558-0059, Vol. 32, no 1, p. 164-171Article in journal (Refereed) Published
Abstract [en]

The influence of passive rectification on the mechanical torque of a permanent magnet generator for a directly driven vertical axis wind turbine has been studied. Passive diode rectification introduce electromagnetic torque ripple from the generator. The conversion of electromagnetic torque ripple into mechanical torque ripple and rotational speed ripple has been modeled, analytically evaluated, and simulated. The simulations have been compared to measurements on an open site 12 kW prototype. A parameter study with the model illustrates the impact of shaft torsional spring constant, generator rotor inertia, generator inductance, and dc-link capacitance. The results show that the shaft and generator rotor can be an effective filter of electromagnetic torque ripple from diode rectification. The measured mechanical torque ripple amplitude on the prototype is less than +/- 0.9% of nominal turbine torque. The measurements compare well with the simulations.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-272363 (URN)10.1109/TEC.2016.2626783 (DOI)000396130300016 ()
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2017-10-18Bibliographically approved
Frost, A. E., Ulvgård, L., Sjökvist, L., Eriksson, S. & Leijon, M. (2017). Partial Stator Overlap in a Linear Generator for Wave Power: An Experimental Study. Journal of Marine Science and Engineering, 5(4), Article ID 53.
Open this publication in new window or tab >>Partial Stator Overlap in a Linear Generator for Wave Power: An Experimental Study
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2017 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 5, no 4, article id 53Article in journal (Refereed) Published
Abstract [en]

This paper presents a study on how the power absorption and damping in a linear generator for wave energy conversion are affected by partial overlap between stator and translator. The theoretical study shows that the electrical power as well as the damping coefficient change quadratically with partial stator overlap, if inductance, friction and iron losses are assumed independent of partial stator overlap or can be neglected. Results from onshore experiments on a linear generator for wave energy conversion cannot reject the quadratic relationship. Measurements were done on the inductance of the linear generator and no dependence on partial stator overlap could be found. Simulations of the wave energy converter's operation in high waves show that entirely neglecting partial stator overlap will overestimate the energy yield and underestimate the peak forces in the line between the buoy and the generator. The difference between assuming a linear relationship instead of a quadratic relationship is visible but small in the energy yield in the simulation. Since the theoretical deduction suggests a quadratic relationship, this is advisable to use during modeling. However, a linear assumption could be seen as an acceptable simplification when modeling since other relationships can be computationally costly.

Place, publisher, year, edition, pages
MDPI AG, 2017
Keywords
Wave Power, linear generator, stator overlap, active area, end-stop forces
National Category
Energy Engineering Marine Engineering
Identifiers
urn:nbn:se:uu:diva-329833 (URN)10.3390/jmse5040053 (DOI)000423694200009 ()
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2018-03-19Bibliographically approved
Sjökvist, S., Rossander, M. & Eriksson, S. (2017). Permanent Magnet Working Point Ripple in Synchronous Generators. The Journal of Engineering, 2017(5), 158-161
Open this publication in new window or tab >>Permanent Magnet Working Point Ripple in Synchronous Generators
2017 (English)In: The Journal of Engineering, ISSN 1872-3284, E-ISSN 2051-3305, Vol. 2017, no 5, p. 158-161Article in journal (Refereed) Published
Abstract [en]

Permanent magnets (PMs) are today widely used in electrical machines of all sorts. With their increase in popularity, the amount of research has increased as well. In this study, the magnetic flux density ripple of the working point of the PMs in a 100 kW PM synchronous generator has been investigated for three different load cases: no load, AC load, and DC load. The PMs will be subjected to a shift in working point as a consequence of the characteristics of the electrical loading. This study is based on finite element method simulations where the ripple of the magnetic flux density in the PMs was recorded at three positions within a PM. The slot harmonic of 7.5 times the electrical frequency (fel)" role="presentation">(fel) was present in the results for all load cases, but mainly at the surface of the PM, as expected. Results showed an unexpected harmonic of 1.5fel" role="presentation">1.5fel , assumed to be an undertone of the slot harmonics. The 6fel" role="presentation">6fel harmonic for the DC load case was significantly higher than for the AC load case and is caused by the current fluctuation during passive rectification. For the studied machine, the added harmonics in the magnetic field due to passive rectification are less than the slot-related harmonics.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-303518 (URN)10.1049/joe.2016.0309 (DOI)000398521800001 ()
Funder
Swedish Research Council, 2010-3950Carl Tryggers foundation , 15:152
Available from: 2016-10-19 Created: 2016-09-20 Last updated: 2019-01-25Bibliographically approved
Eklund, P. & Eriksson, S. (2016). Air Gap Magnetic Flux Density Variations due to Manufacturing Tolerances in a Permanent Magnet Synchronous Generator. In: 2016 XXII INTERNATIONAL CONFERENCE ON ELECTRICAL MACHINES (ICEM): . Paper presented at 22nd International Conference on Electrical Machines (ICEM), SEP 04-07, 2016, Lausanne, SWITZERLAND (pp. 93-99). IEEE
Open this publication in new window or tab >>Air Gap Magnetic Flux Density Variations due to Manufacturing Tolerances in a Permanent Magnet Synchronous Generator
2016 (English)In: 2016 XXII INTERNATIONAL CONFERENCE ON ELECTRICAL MACHINES (ICEM), IEEE, 2016, p. 93-99Conference paper, Published paper (Refereed)
Abstract [en]

The impact of manufacturing tolerances on the performance of a permanent magnet synchronous generator is investigated. A generator with a flux concentrating spoke-type rotor, with ferrite permanent magnets, is used in the investigation. Measurements of the air gap magnetic flux density, the air gap length, as well as the magnetization and size of the permanent magnets have been performed. Correlations are calculated and causalities are discussed. It is found that the permanent magnets used are below tolerance in remanent magnetic flux density, that the air gap length is smaller than specified, and that the resulting air gap magnetic flux density is lower than specified. From the results it can be concluded that the design should be made with tolerances in mind and that quality control of parts, especially of PM magnetization, is important for machine performance.

Place, publisher, year, edition, pages
IEEE, 2016
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-292962 (URN)10.1109/ICELMACH.2016.7732511 (DOI)000390884900012 ()978-1-5090-2538-1 (ISBN)
Conference
22nd International Conference on Electrical Machines (ICEM), SEP 04-07, 2016, Lausanne, SWITZERLAND
Funder
Swedish Research Council, 2012-4706ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 12-295
Available from: 2016-05-16 Created: 2016-05-11 Last updated: 2018-11-22Bibliographically approved
Sjökvist, S., Eklund, P. & Eriksson, S. (2016). Determining demagnetisation risk for two PM wind power generators with different PM material and identical stators. IET Electric Power Applications, 10(7), 593-597
Open this publication in new window or tab >>Determining demagnetisation risk for two PM wind power generators with different PM material and identical stators
2016 (English)In: IET Electric Power Applications, ISSN 1751-8660, E-ISSN 1751-8679, Vol. 10, no 7, p. 593-597Article in journal (Refereed) Published
Abstract [en]

Ways to utilise ferrite permanent magnets (PMs), in a better way has been in focus the last couple of years since the use of neodymium-iron-boron (NdFeB) PMs has been debated. While ferrite PMs offer a low-cost alternative to rare- earth PMs, it is a trade-off for lower energy density. Depending on the type of PM and if the PMs are surface mounted or buried, the risk of demagnetisation during a fault condition can vary significantly between machines. In this study, the demagnetisation risk of two electrically similar generators with identical stators has been studied during several short- circuit faults at different temperatures. The study is simulation-based, and the results show that the generator with the ferrite rotor will suffer from a small but not significant amount of demagnetisation in the worst, three-phase-neutral, short-circuit case at a temperature of 5°C, whereas the NdFeB PMs will suffer from partial demagnetisation if a fault occurs at 120°C. For operational temperatures between 20 and 60°C both generators will sustain a short-circuit event. 

Keywords
PM, Electrical machines, Demagnetization, Synchronous generators, Wind power, Comsol Multiphysics
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity; Engineering Science
Identifiers
urn:nbn:se:uu:diva-291371 (URN)10.1049/iet-epa.2015.0518 (DOI)000381407900001 ()
Funder
Swedish Research Council, 2010-3950StandUp
Available from: 2016-05-02 Created: 2016-05-02 Last updated: 2018-11-22Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-8598-2565

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