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Demagnetization Studies on Permanent Magnets: Comparing FEM Simulations with Experiments
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In a world where money often is the main controlling factor, everything that can be tends to be more and more optimized. Regarding electrical machines, developers have always had the goal to make them better. The latest trend is to make machines as efficient as possible, which calls

for accurate simulation models where different designs can be tested and evaluated. The finite element method is probably the most popular approach since it makes it possible to, in an easy and accurate way, get numerical solutions to a variety of physics problems with complex geometries and non-linear materials.

This licentiate thesis includes two different projects in which finite element methods have had a central roll. In the first project, the goal was to develop a simulation model to be able to predict demagnetization of permanent magnets. It is of great importance to be able to predict if

a permanent magnet will be demagnetized or not in a certain situation. In the worst case, the permanent magnets will be completely destroyed and the machine will be completely useless. However, it is more probable that the permanent magnets will not be completely destroyed and that the machine still will be functional but not as good as before. In a time where money is more important than ever, the utilization has to be as high as possible. In this study the demagnetization risk for different rotor geometries in a 12 kW direct driven permanent magnet synchronous generator was studied with a proprietary finite element method simulation model. The demagnetization study of the different rotor geometries and magnet grades showed that here is no risk for the permanent magnets in the rotor as it is designed today to be demagnetized. The project also included experimental verification of the simulation model. The simulation model was compared with experiments and the results showed good agreement.

The second project treated the redesign of the rotor in the generator previously mentioned. The goal was to redesign the surface mounted NdFeB rotor to use a field concentrating design with ferrite permanent magnets instead. The motivation was that the price on NdFeB magnets has fluctuated a lot the last few years as well as to see if it was physically possible to fit a ferrite rotor in the same space as the NdFeB rotor. A new rotor design with ferrite permanent magnets was presented together with an electromagnetic and a mechanical design.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 43 p.
Series
UURIE / Uppsala University, Department of Engineering Sciences, ISSN 0349-8352 ; 338-14L
Keyword [en]
Permanent magnet, demagnetization, simulation, FEM, Comsol Multiphysics, VAWT
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-236301OAI: oai:DiVA.org:uu-236301DiVA: diva2:765099
Presentation
2014-06-05, Å4101, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2010-3950
Available from: 2014-11-25 Created: 2014-11-17 Last updated: 2014-11-25Bibliographically approved
List of papers
1. 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: 2017-12-05Bibliographically approved
2. 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
3. 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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