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Axial Field Permanent Magnet Machines with High Overload Capability for Transient Actuation Applications

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Axial Field Permanent Magnet Machines with High Overload Capability for Transient Actuation Applications THE UNIVERSITY OF SHEFFIELD Axial field permanent magnet machines with high overload capability for transient actuation applications By Jiangnan Gong A Thesis submitted for the degree of Doctor of Philosophy Department of Electronic and Electrical Engineering The University of Sheffield. JANUARY 2018 ABSTRACT This thesis describes the design, construction and testing of an axial field permanent magnet machine for an aero-engine variable guide vane actuation system. The electrical machine is used in combination with a leadscrew unit that results in a minimum torque specification of 50Nm up to a maximum speed of 500rpm. The combination of the geometry of the space envelope available and the modest maximum speed lends itself to the consideration of an axial field permanent magnet machines. The relative merits of three topologies of double-sided permanent magnet axial field machines are discussed, viz. a slotless toroidal wound machine, a slotted toroidal machine and a yokeless axial field machine with separate tooth modules. Representative designs are established and analysed with three-dimensional finite element method, each of these 3 topologies are established on the basis of a transient winding current density of 30A/mm2. Having established three designs and compared their performance at the rated 50Nm point, further overload capability is compared in which the merits of the slotless machine is illustrated. Specifically, this type of axial field machine retains a linear torque versus current characteristic up to higher torques than the other two topologies, which are increasingly affected by magnetic saturation. Having selected a slotless machine as the preferred design, further design optimization was performed, including detailed assessment of transient performance. A key feature of this design is the use of a solid (i.e. non-laminated) toroidal stator core. This provides a stator with increased mechanical robustness, improved heat transfer and a ready means of incorporating fixing points into the core. However, these advantages are gained at the expense of a significant eddy currents in the stator core. A series of three-dimensional, magneto-dynamic finite element simulations were performed. Although the resulting eddy current losses are excessive for continuous operation, the reduction in transient performance which results from the eddy currents is shown to be manageable. The loss analysis is supplemented by transient three-dimensional finite element thermal modelling. Three-dimensional mechanical analysis is performed in combination with analytical equation to analyse the stator and rotor plate deflection subject to axial attractive force. I The construction of a prototype double-sided axial field machine is described in this thesis which contains several interesting design features including a profiled rotor core to reduce mass, radially magnetised rotor magnets to produce torque from the axially oriented conductors on the inner edge of the toroidal winding. The testing of the machine is performed under a series of load points up to 75Nm to validate the predicted torque versus current density characteristics. II ACKNOWLEDGEMENT I would like to express my sincere gratitude to Professor Geraint Jewell, for his invaluable help, guidance, support and encouragement throughout my entire PhD study especially at the points when I encounter setbacks. I’m very lucky to have him as supervisor. Also, I would like to thank my second supervisor Dr Guang-Jin Li for all the helpful technical advice and encouragement he gave me. I would like to thank Rolls Royce for their financial support, and many thanks to Ellis Chong and Mark Boden for their generous support and advice over the years. I’m also grateful for the great support from the technical staff in Electrical Machines and Drives group, without their help, the prototype and test rig construction would not have happened. Thank you to all my friends in UTC and the wider EMD group, for all the technical discussions which I learned a lot from, and also for providing a delightful working environment. Finally, I would like to thank my family and friends for their love and care. In particular my parents and my husband, their consistent support and encouragement gave me great strength. III Contents ABSTRACT ............................................................................................................................ I ACKNOWLEDGEMENT .................................................................................................... III NOMENCLATURE ........................................................................................................... VIII CHAPTER 1. INTRODUCTION ...................................................................................... 1 MOTIVATION AND OBJECTIVE OF THE WORK...................................................................... 1 VARIABLE GUIDE VANE DRIVE ........................................................................................... 2 GENERAL FEATURES OF AXIAL FIELD MACHINES ............................................................... 6 REVIEW OF MAIN AXIAL FIELD PERMANENT MAGNET MACHINE TOPOLOGIES .................... 8 1.4.1 TORUS non slotted .............................................................................................. 11 1.4.2 TORUS slotted ..................................................................................................... 12 1.4.3 YASA topology ..................................................................................................... 14 THESIS STRUCTURE ......................................................................................................... 16 CHAPTER 2. BASELINE DESIGN AND ANALYSIS OF REFERENCE MACHINE TOPOLOGIES .................................................................................................................... 17 INTRODUCTION ................................................................................................................ 17 DESIGN METHODOLOGY .................................................................................................. 17 2.2.1 Finite element analysis ........................................................................................ 18 MATERIAL SELECTION ..................................................................................................... 19 2.3.1 Rotor permanent magnet materials ..................................................................... 19 2.3.2 Soft magnetic materials ....................................................................................... 23 INITIAL SIZING OF A NON-SLOTTED TOROIDAL MACHINE (TORUS-NS)........................... 27 2.4.1 Sizing equations for TORUS non-slotted machine .............................................. 27 2.4.2 Sizing equations for TORUS non-slotted machine .............................................. 28 2.4.3 Initial design synthesis ........................................................................................ 30 AIRGAP FLUX DENSITY INVESTIGATION FOR NON-SLOTTED TORUS MACHINE (TORUS-NS) ........................................................................................................................................ 32 2.5.1 One-dimensional model ...................................................................................... 32 2.5.2 Two-dimensional finite element model................................................................ 33 2.5.3 Three-dimensional finite element model ............................................................. 37 2.5.1 Pole number selection ......................................................................................... 43 IV OPTIMISATION OF COIL THICKNESS .................................................................................. 45 INITIAL SIZING OF A SLOTTED TOROIDAL MACHINE (TORUS-S) ...................................... 47 2.7.1 Sizing equations for TORUS slotted machine ..................................................... 47 SLOT-POLE COMBINATION FOR SLOTTED MACHINE .......................................................... 51 2.8.1 Analytical model.................................................................................................. 54 2.8.2 Comparison between analytical and finite element model ................................. 57 CHAPTER 3. COMPARISON OF TOPOLOGIES ...................................................... 62 INTRODUCTION ................................................................................................................ 62 OVERALL PERFORMANCE PREDICTION ............................................................................. 63 INVESTIGATION OF TRANSIENT CAPABILITY .................................................................... 71 3.3.1 Overload capability ............................................................................................. 71 3.3.2 Current density limitations .................................................................................. 73 MAGNET IRREVERSIBLE DEMAGNETIZATION ................................................................... 76 3.4.1 Magnet demagnetization for TORUS non-slotted machine ................................ 77 3.4.2 Magnet demagnetization for the slotted TORUS machine .................................. 81 EDDY CURRENT EFFECTS WITH SOLID STATOR CORES ...................................................... 84 CHAPTER 4. OPTIMIZATIONS AND DETAILED PERFORMACNE MODELLING ...................................................................................................................
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