Material choice for a rotor in a switched reluctance high speed motor
Materialval för rotor i en variabel reluktans höghastighetsmotor
Christoffer Christiansen
Faculty of Health, Science and Technology Degree project for master of science in engineering, mechanical engineering 30 credit points Supervisor: Pavel Krakhmalev and Johan Fjällman Examiner: Jens Bergström Date: Spring semester 2017, 2017-05-24
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Abstract
With the increasing environmental impact from the automotive industry, electric vehicles become more and more popular. This combined with the great breakthroughs in fast electronics the switched reluctance motor (SRM) has again gained popularity in recent years. Due to its cheap and rugged construction it is a good alternative to the permanent magnet motors and to the induction motor. The ´two main problems holding the SRM back are torque ripple and the acoustic noise generated from it. A lot of research is currently being performed in order to find a solution to these issues.
This thesis has investigated different materials for the rotor in a high speed SRM. Different materials have been evaluated based on both mechanical and magnetic properties. This is done through simulations of the forces acting on the rotor combined with simulations of the magnetic field. The forces are simulate in the DASSULT SYSTEMS ABAQUS program and the magnetic field is simulate using AVL FIRE. Three different kinds of alloys are investigated, two different cobalt alloys are simulated as well as a silicon alloy with pure iron as a reference.
The results show that the material needs to have a yield strength of at least 349 MPa to withstand the forces affecting the rotor. And that by using the high purity cobalt-iron alloy the generated torque could be increased with up to 20.9%, but with a cost increase of 3151.9% compared to the silicon alloy.
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Sammanfattning
Med den ökande miljöpåverkan från fordonssektorn växer intresset för elektriska fordon. Det ökande intresset kombinerat med stora framgångar inom elektronik har väckt ett stort intresse för den variabla reluktansmotorn. Tack till dess pålitliga och billiga konstruktion är det en bra kandidat att ersätta permanent magnet motorn och induktionsmotorn. Det som tidigare hållit tillbaka den variabla reluktansmotor är fluktuationer i de genererade vridmomentet och de ljud som genereras ifrån detta. Mycket forskning pågår för att hitta lösningar till dessa problem.
Detta examensarbete har fokuserat på att undersöka olika material för att använda i rotorn för en variabel reluktanshöghastighetsmotor. Olika material har utvärderats med avseende på både mekaniska och magnetiska egenskaper. Detta har genomförts genom simuleringar av de krafter som påverkar rotorn kombinerat med simuleringar av magnetfälten i rotorn. Spänningarna i materialet har simulerats i DASSULT SYSTEMS ABAQUS och magnetfältet har simulerats i AVL FIRE. Tre olika legeringar testades, två kobolt legeringar och en kisel legering. Rent järn simulerades också för att agera som referens.
Resultaten visar att sträckgränsen för materialen måste vara över 349 MPa för att klara de påfrestningar som rotorn utsätts för. Genom att använda kobolt järn legeringen kunde vridmomentet ökas med 20.9%, med en ökad kostnad av 3151.9% jämfört med kisel järn legeringen.
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Acknowledgments
I would firstly like to thank Jonas Modin for giving me the opportunity to perform this thesis at AVL. I would also like to extend my gratitude to Dr. Johan Fjällman for his valuable supervision and guidance throughout the entire project. On Karlstad University I would like to thank my supervisor Professor Pavel Krakhmalev for always answering my questions, and helping me develop the idea for this thesis.
Lastly I would like to thank my friends and family for supporting me and always keeping me motivated.
Christoffer Christiansen 30th of June 13, 2017
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Content
1 INTRODUCTION ______1 1.1 BACKGROUND ______1 1.1.1 Switched reluctance motor ______1 1.2 PROBLEM STATEMENT ______3 2 THEORY OF MATERIAL ______4 2.1 MAGNETISM ______4 2.1.1 Ferromagnetism ______4 2.1.2 Hysteresis loop ______5 2.1.2.1 Hysteresis losses ______5 2.1.3 Hard vs soft ______6 2.1.4 Magnetic flux density ______7 2.2 ELECTRICAL STEEL______7 2.2.1 Composition ______7 2.2.2 Grain orientation ______8 2.2.3 Lamination thickness ______8 2.2.4 Effect of stresses ______9 2.2.5 Heat treatment ______9 2.3 DIFFERENT ALLOYS USED TODAY ______10 2.3.1 Low carbon steel ______11 2.3.2 Silicon steel ______11 2.3.3 Nickel alloys ______11 2.3.4 Cobalt alloys ______11 2.3.5 Density ______12 2.3.6 Price ______13 3 METHOD ______14 3.1 DESIGN OF MOTOR ______14 3.1.1 Calculations of aimed torque______14 3.1.2 Rotor type ______16 3.1.3 Calculations of rotor poles ______16 3.1.4 Rotor diameter ______17 3.1.5 Rotor pole arc/pitch ______18 3.1.6 Stator pole arc/pitch ______19 3.1.7 Rotor inner diameter and axis diameter ______19 3.1.8 Air gap ______19 3.1.9 Axis mounting ______19 3.1.10 Final rotor design ______20 3.2 SIMULATIONS ______21 3.2.1 Abaqus ______21 3.2.1.1 Mesh ______21 3.2.1.2 Boundary conditions and load ______21 3.2.1.3 Von Mises Stress ______22 3.2.1.4 Isotropic material ______22 3.2.2 AVL FIRE ______23 3.2.2.1 Maxwell stress tensor ______23 3.2.2.2 Mesh ______23 V
3.2.2.2.1 Mesh refinement______23 3.2.2.2.2 Cell selections and faces ______25 3.2.2.3 Time step ______26 3.2.2.4 Control strategy ______26 3.2.2.5 Fill factor ______27 3.2.2.6 Current density ______27 3.2.2.7 B (H) curve ______27 3.2.2.8 Lamination thickness ______28 4 RESULTS ______29 4.1 ABAQUS 1 ______29 4.2 AVL FIRE ______34 4.3 ABAQUS 2 ______37 5 DISCUSSION ______40 5.1 FUTURE WORK ______42 6 CONCLUSION ______43 7 REFERENCES ______44
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LIST OF TABLES Table 1.1 - Typical motor configurations ...... 3 Table 2.1 - Relative properties for differant alloys [21] ...... 10 Table 3.1 - Motor specifications for various personal EV ...... 15 Table 3.2 - Base speed vs max speed of different SRM ...... 16 Table 4.1 – Results from calculation of discretization error ...... 34 Table 5.1 - Comparison between different alloys ...... 41 Table 5.2 - Comparison of power output and price for equally sized motors ...... 41
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NOMENCLATURE
M ...... Magnetization [T]
Mr ...... Remanent magnetization [T]
Mr ...... Saturated magnetization [T] H ...... Coercive force [A/m] B ...... Magnetic flux density [T] ...... Permeability [henry/m]