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Design of Rare Earth Free Permanent Magnet Generators
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Wind power group)ORCID iD: 0000-0003-3341-6910
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Low speed permanent magnet (PM) synchronous generators (SGs) are commonly used in renewable energy. Rare earth (RE) PMs such as neodymium-iron-boron are a popular choice due to their high performance. In 2011 supply and cost issues were added to the previously existing environmental concerns regarding REPM raw materials as the world's major producer China imposed export restrictions. This thesis aims to investigate and propose design solutions for PMSGs that do not use REPMs. Two approaches are used: to design generators using the cheaper and more abundant ferrite PM materials, and to investigate how properties of new PM materials influence SG design.

A ferrite PM rotor is designed to replace a REPM rotor in an experimental 12 kW wind power generator. The new design employs a flux concentrating spoke type rotor to achieve performance similar to the old REPM rotor while using ferrite PMs. The ferrite PM rotor design is built. The air gap length, magnetic flux density in the air gap, PM remanence, and voltage at both load and no load are measured. The generator has lower no load voltage than expected, which is mainly explained by lower than specified remanence of the ferrite PMs in the prototype. With the measured remanence inserted into the calculations some discrepancy remains. It is found that the discrepancy can be explained by the magnetic leakage flux in the end regions of the spoke type rotor, which is not modeled in the two dimensional simulations used for the design calculations.

To investigate the influence of PM material properties three different PM rotor topologies are optimized for torque production using PM materials described by their remanence, recoil permeability, and demagnetization resistance. Demagnetization is considered using currents determined by a novel, winding design independent short circuit model. It is found that the spoke type rotor gives the highest torque of the three rotor topologies for low remanence materials as long as the PMs have sufficient demagnetization resistance. For high remanence materials the surface mounted PM rotor can give higher torque if the demagnetization resistance is high, but otherwise a capped PM rotor gives higher torque.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. , p. 75
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1746
Keywords [en]
permanent magnet generators, electrical machine design, ferrite permanent magnet
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-366344ISBN: 978-91-513-0510-3 (print)OAI: oai:DiVA.org:uu-366344DiVA, id: diva2:1265306
Public defence
2019-01-18, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Funder
ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 12-295Carl Tryggers foundation , 15:152Swedish Research Council, 2012-4706Available from: 2018-12-19 Created: 2018-11-22 Last updated: 2019-01-21
List of papers
1. A Complete Design of a Rare Earth Metal-Free Permanent Magnet Generator
Open this publication in new window or tab >>A Complete Design of a Rare Earth Metal-Free Permanent Magnet Generator
2014 (English)In: Machines, ISSN 2075-1702Article in journal (Refereed) Published
Abstract [en]

The price of rare-earth metals used in neodymium-iron-boron (NdFeB) permanent magnets (PMs) has fluctuated greatly recently. Replacing the NdFeB PMs with more abundant ferrite PMs will avoid the cost insecurity and insecurity of supply. Ferrite PMs have lower performance than NdFeB PMs and for similar performance more PM material has to be used, requiring more support structure. Flux concentration is also necessary, for example, by a spoke-type rotor. In this paper the rotor of a 12 kW NdFeB PM generator was redesigned to use ferrite PMs, reusing the existing stator and experimental setup. Finite element simulations were used to calculate both electromagnetic and mechanical properties of the design. Focus was on mechanical design and feasibility of construction. The result was a design of a ferrite PM rotor to be used with the old stator with some small changes to the generator support structure. The new generator has the same output power at a slightly lower voltage level. It was concluded that it is possible to use the same stator with either a NdFeB PM rotor or a ferrite PM rotor. A ferrite PM generator might require a larger diameter than a NdFeB generator to generate the same voltage.

Place, publisher, year, edition, pages
Basel, Switzerland: Multidisciplinary Digital Publishing Institute, 2014
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-236639 (URN)10.3390/machines2020120 (DOI)
Available from: 2014-11-20 Created: 2014-11-20 Last updated: 2018-11-22
2. Air Gap Magnetic Flux Density Variations due to Manufacturing Tolerances in a Permanent Magnet Synchronous Generator
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
3. Winding Design Independent Calculation Method for Short Circuit Currents in Permanent Magnet Synchronous Machines
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
4. The influence of permanent magnet material properties on generator rotor design
Open this publication in new window or tab >>The influence of permanent magnet material properties on generator rotor design
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Due to the price and supply insecurities for rare earth metal-based permanent magnet (PM) materials a search for new PM materials is ongoing.The properties of a new PM material are not known yet, but a span of likely parameters can be studied.In this paper, it is investigated how the remanence and recoil permeability of a PM material affect its usefulness in a low speed, multi-pole, PM synchronous generator.Demagnetisation is also considered.The investigation is carried out by optimising three different rotor topologies for maximum torque production for different PM material parameters and a fixed PM maximum energy.The rotor topologies used: surface mounted PM rotor, spoke type PM rotor and an interior PM rotor with radially magnetised PMs.The three different rotor topologies have their best performance for different kinds of materials.The spoke type PM rotor is the best at utilising low remanence materials as long as they are sufficiently resistant to demagnetisation.The surface mounted PM rotor works best with very demagnetisation resistant PM materials with a high remanence, while the radial interior PM rotor is preferable for high remanence materials with low demagnetisation resistance.

Keywords
Permanent magnet, generator, electrical machine design
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-366342 (URN)
Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2018-11-22
5. Experimental Evaluation of a Rare Earth-Free Permanent Magnet Generator
Open this publication in new window or tab >>Experimental Evaluation of a Rare Earth-Free Permanent Magnet Generator
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Low speed, high torque machines are used in wind turbines where the turbine rotor is directly connected to the generator. A permanent magnet synchronous generator using high-energy rare-earth permanent magnets (PMs) is one common choice for this application, but rare-earth PMs have supply insecurities and cost risks. A rare-earth free PM generator, using ferrite PMs in a spoke-type rotor, for use in a 12 kW experimental wind turbine is built and tested. Voltages and currents at load and no load are measured, as well as the magnetic field in the end regions of the machine.The measurements are compared to two-dimensional finite element design calculations. Simulations of the three-dimensional magnetic field in the end regions are also made. The generator can deliver the required power at nominal speed and has low harmonic content in the output. The measured voltage is lower than expected, requiring a higher current than calculated for the rated power. Three-dimensional magnetic field simulations show that there are leakage flux paths in the end-regions that the two-dimensional design calculations overlook, explaining the discrepancy between simulations and measurements.

Keywords
Electrical machines, experiment, rare earth free permanent magnet, permanent magnet machines, permanent magnet generator
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-366343 (URN)
Funder
ÅForsk (Ångpanneföreningen's Foundation for Research and Development), 12-295Carl Tryggers foundation , 15:152
Available from: 2018-11-20 Created: 2018-11-20 Last updated: 2018-11-22
6. Development of Power Electronics Based Test Platform for Characterization and Testing of Magnetocaloric Materials
Open this publication in new window or tab >>Development of Power Electronics Based Test Platform for Characterization and Testing of Magnetocaloric Materials
Show others...
2015 (English)In: Advances in Electrical Engineering, ISSN 2356-6655, Vol. 2015, no 670624, p. 7-Article in journal (Refereed) Published
Abstract [en]

Magnetocaloric effects of various materials are getting more and more interesting for the future, as they can significantly contribute towards improving the efficiency of many energy intensive applications such as refrigeration, heating, and air conditioning. Accurate characterization of magnetocaloric effects, exhibited by various materials, is an important process for further studies and development of the suitable magnetocaloric heating and cooling solutions. The conventional test facilities have plenty of limitations, as they focus only on the thermodynamic side and use magnetic machines with moving bed of magnetocaloric material or magnet. In this work an entirely new approach for characterization of the magnetocaloric materials is presented, with the main focus on a flexible and efficient power electronic based excitation and a completely static test platform. It can generate a periodically varying magnetic field using superposition of an ac and a dc magnetic field. The scale down prototype uses a customized single phase H-bridge inverter with essential protections and an electromagnet load as actuator. The preliminary simulation and experimental results show good agreement and support the usage of the power electronic test platform for characterizing magnetocaloric materials.

Place, publisher, year, edition, pages
Hindawi Publishing Corporation, 2015
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-245221 (URN)10.1155/2015/670624 (DOI)
Projects
Magnetocaloric
Available from: 2015-02-25 Created: 2015-02-25 Last updated: 2018-11-22
7. Determining demagnetisation risk for two PM wind power generators with different PM material and identical stators
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
8. Magnetic End LeakageFlux in a Spoke Type Rotor Permanent Magnet SynchronousGenerator
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

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