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Demagnetization and Fault Simulations of Permanent Magnet Generators
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Wind Power)ORCID iD: 0000-0001-7424-3310
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Permanent magnets are today widely used in electrical machines of all sorts. With their increase in popularity, the amount of research has increased as well. In the wind power project at Uppsala University permanent magnet synchronous generators have been studied for over a decade. However, a tool for studying demagnetization has not been available. This Ph.D. thesis covers the development of a simulation model in a commercial finite element method software capable of studying demagnetization. Further, the model is also capable of simulating the connected electrical circuit of the generator. The simulation model has continuously been developed throughout the project. The simulation model showed good agreement compared to experiment, see paper IV, and has in paper III and V successfully been utilized in case studies. The main focus of these case studies has been different types of short-circuit faults in the electrical system of the generator, at normal or at an elevated temperature. Paper I includes a case study with the latest version of the model capable of handling multiple short-circuits events, which was not possible in earlier versions of the simulation model. The influence of the electrical system on the working point ripple of the permanent magnets was evaluated in paper II. In paper III and VI, an evaluation study of the possibility of creating a generator with an interchangeable rotor is presented.  A Neodymium-Iron-Boron (Nd-Fe-B) rotor was exchanged for a ferrite rotor with the electrical properties almost maintained.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 59 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1444
Keyword [en]
Demagnetization, Permanent magnet, Finite Element Method, Synchronous generators, Wind power
National Category
Engineering and Technology Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-303517ISBN: 978-91-554-9733-0OAI: oai:DiVA.org:uu-303517DiVA: diva2:1038990
Public defence
2016-12-09, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-11-16 Created: 2016-09-20 Last updated: 2016-11-28
List of papers
1. Investigation of Permanent Magnet Demagnetization in Synchronous Machines During Multiple Short-Circuit Fault Conditions
Open this publication in new window or tab >>Investigation of Permanent Magnet Demagnetization in Synchronous Machines During Multiple Short-Circuit Fault Conditions
(English)Article in journal (Refereed) Submitted
Keyword
Permanent magnets, Demagnetization, FEM, Multiple fault conditions
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-303509 (URN)
Available from: 2016-10-14 Created: 2016-09-20 Last updated: 2016-10-20
2. Permanent Magnet Working Point Ripple in Synchronous Generators
Open this publication in new window or tab >>Permanent Magnet Working Point Ripple in Synchronous Generators
(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-303518 (URN)
Available from: 2016-10-19 Created: 2016-09-20 Last updated: 2016-10-20
3. 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, 593-597 p.Article 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. 

Keyword
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: 2016-10-20Bibliographically approved
4. Experimental Verification of a Simulation Model for Partial Demagnetization of Permanent Magnets
Open this publication in new window or tab >>Experimental Verification of a Simulation Model for Partial Demagnetization of Permanent Magnets
2014 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 12, 7401105Article in journal (Refereed) Published
Abstract [en]

This article aims to verify a FEM simulation model for demagnetization of permanent magnets. The model is designed to determine the remaining magnetization within the permanent magnet after it has been exposed to high demagnetizing fields and/or temperature. An experimental setup was built and a permanent of SmCo type was experimentally tested and the results have been compared to simulation results. The results show a good agreement between results from simulationand results from experiments. A maximal deviation of 3 % of the simulation results in relation to the experimental results were achieved for most part of the magnet. During the calibration of the simulation model it was found that the coercivity had to be significantly lowered compared to the permanent magnets reference value to match simulation results to the experimental results.

Keyword
Simulation, FEM, Permanent magnet, Demagnetization, SmCo
National Category
Engineering and Technology Physical Sciences
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-236307 (URN)10.1109/TMAG.2014.2339795 (DOI)000349445500018 ()
Funder
Swedish Research Council, 2010-3950
Available from: 2014-11-18 Created: 2014-11-17 Last updated: 2016-10-20Bibliographically approved
5. Study of demagnetization risk for a 12 kW direct driven permanent magnet synchronous generator for wind power
Open this publication in new window or tab >>Study of demagnetization risk for a 12 kW direct driven permanent magnet synchronous generator for wind power
2013 (English)In: Energy Science & Engineering, ISSN ISSN 2050-0505, Vol. 1, no 3, 128-134 p.Article in journal (Refereed) Published
Abstract [en]

One of the main aspects when designing a permanent magnet (PM) generator is to choose suitable PMs, both in terms of achieving the required flux in the generator but also of withstanding high demagnetizing fields, that is, having sufficiently high coercivity. If the coercivity is too low, the magnets are at risk of demagnetizing, fully or partially, at the event of a short circuit and/or an increase in temperature. This study aims to determine the risk of demagnetization for a 12 kW direct driven permanent magnet synchronous generator. Furthermore, as the prices on PMs have increased drastically the last few years the possibility to use smaller and/or cheaper PMs of different grades has been investigated. A new proprietary finite element method (FEM) model has been developed, which is also presented. The study is based on simulations from this FEM model and is focused on NdFeB magnets. Results show that the reference magnet can withstand a two-phase short circuit at both the temperatures tested and in both geometries. The use of cheaper magnets, smaller air gap and in the event of a two-phase short circuit often results in partial irreversible demagnetization. However, magnets with lower coercivity are easier demagnetized.

Keyword
Demagnetization, NdFeB, permanent magnet, simulation, synchronous generator
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-212671 (URN)10.1002/ese3.16 (DOI)000209695200003 ()
Funder
Swedish Research Council, 2010-3950
Available from: 2013-12-13 Created: 2013-12-13 Last updated: 2016-11-17Bibliographically approved
6. 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: 2016-10-20

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