Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Commutator (electric) wikipedia , lookup
Brushed DC electric motor wikipedia , lookup
Manchester Mark 1 wikipedia , lookup
Stepper motor wikipedia , lookup
Magnetic core wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Electric motor wikipedia , lookup
Brushless DC electric motor wikipedia , lookup
Aspects of Permanent Magnet Machine Design Christine Ross February 7, 2011 Grainger Center for Electric Machinery and Electromechanics Outline • Permanent Magnet (PM) Machine Fundamentals • Motivation and Application • Design Aspects – PM Material – PM Rotor Configurations – Manufacturing Processes • Design Tools 2 Permanent Magnet (PM) Machine Fundamentals • Focus on electronically 3-phase controlled PM AC synchronousstator windings machines – Rotor magnetic field is supplied by PMs – Stator windings are sinusoidally distributed windings, excited by sine-wave currents • “Brushless DC” machines can also use PMs Laminated stator 4-pole PM rotor Cross-section of surface-mounted PM machine 3 PM Machine Theory • Output torque is proportional to power • Control instantaneous torque by controlling magnitude of phase currents f 60 N RPM p P T rm speed N in RPM supply frequency f number of pole pairs p output torque T output power P rotor speed rm in rad/s 4 PM Machine Control • Instantaneous torque control – – – – Servo performance 0.1-10 kW Fast dynamic response Smooth output torque Accurate rotor position sensor information needed Back-emf e ωrm Single-phase equivalent circuit 5 PM Machine Control • Flux-weakening control – Constant power drives – Traction, washing machines, starter/alternators – Require constant output power over a speed range – To operate above rated speed while maintaining rated terminal voltage, reduce flux by controlling magnetizing current Ideal flux-weakening characteristics T Iarm torque T magnetic flux armature current Iarm V terminal voltage V magnetic flux [1] Soong angular speed in rad/s 6 Motivation for PM Machine • Motivation for PM machines: – High efficiency (at full load) – High power density – Simple variable-frequency control – Rotor excited without current • No rotor conductor loss and heat – Magnet eddy current loss is lower than iron loss and rotor cage loss 7 PM Machine Disadvantages • Magnet cost • New magnet manufacturing processes • Magnet sensitivity to temperature and demagnetization • Little control of magnet field – Always have no-load spinning losses – Without control, over speed means over voltage – fault management issues 8 PM Machine Applications • AC PM machines – Servo control systems – Precision machine tools – IPM – washing-machines, air conditioning compressors, hybrid vehicle traction • DC PM machines – Lower cost variable-speed applications where smoothest output torque is not required – Computer fans, disk drives, actuators • Industrial applications where constant speed is necessary IPM washing-machine motors [5] Hendershot and Miller 9 Design Specifications • Electrical • Environmental – – – – Ambient temperature Cooling system Structure Vibration • Mechanical outputs – Torque – Speed – Power • Key features of machines – – – – – – – – – – Flux linkage Saliency, inductances Assembly process Magnet cost Number of magnets Simplicity of design Field weakening Reluctance torque Field control Line start, no inverter 10 PM Material • Soft magnetic material Remnant Flux Density B (steel) – small B-H loop Coercivity H • Hard magnetic material – (PM) – large B-H loop • Choose magnets based on high Br and r c Hc 11 PM Material PM Br (T) Hc (kA/m) Alnico5-7 1.3 60 Ferrite 0.4 300 NdFeB (sintered) 1.1 Sm2Co7 (sintered) 1.0 [2] Miller Cost Resistivity (µΩ-cm) Max. Working Temp. (ºC) 47 > 500 low >10,000 250 450 850 medium 150 80-200 310-350 750 Higher than NdFeB 86 250-350 700-800 Arnold Magnetics Curie Temp. (ºC) 12 [3] Hendershot and Miller PM Material • Chinese dependency • No shortage – Mountain Pass, CA – Idaho – Nd is about as common as Cu Arnold Magnetic Technologies 13 PM Machine Rotor Configurations • Surface-mounted PM rotor – Maximum magnet flux linkage with stator – Simple, robust, manufacturable – For low speeds, magnets are bonded to hub of soft magnetic steel – Higher speeds – use a retaining sleeve – Inset – better protection against demagnetization; wider speed range using flux-weakening; increases saliency; but also increases leakage Inset magnets Surface bread-loaf magnets 14 PM Machine Rotor Configurations • Interior-mounted PM (IPM) rotor • IPM Advantages – Extended speed range with lower loss – Increases saliency and reluctance torque – Greater field weakening capability A. O. Smith 15 PM Machine Topology • SMPM: – More mechanically robust – Magnet losses can be an issue (not shielded by rotor iron); reduce by segmenting magnets axially or radially or increasing magnet resistivity • IPM: – Better demagnetization withstand Characteristic SMPM IPM Saliency No Yes Field Weakening Some Good Controller Standard More Complex [4] Klontz and Soong 16 PM Manufacturing Practices • Realistic manufacturing tolerances – Key parameters – stator inner diameter, rotor outer diameter, no load current, winding temperature – Issues with core steels – laser cutting, punched laminations, lamination thickness – Issues with magnets – dimensions, loss of strength due to thermal conditioning Hybrid Camry PM synchronous AC motor/generator ecee.colorado.edu – High speed practice and limits – rotor diameter limits speed 17 PM Machine Design Process • Design and simulate motor and driver – Separately – Combined • Analytical, lumped-circuit, and finite-element design tools • Different tools are used to trade-off understanding of the design, speed, and accuracy Finite element meshing, flux lines and B for SMPM machine A.O. Smith 18 Analytical Design Tools • Broad simplifying approximations – Equivalent circuit parameters • Use for initial sizing and performance estimates • Performance prediction • Limitations – Does not initially account for local saturation – Requires tuning with FE results 19 Analytical Design Tools • Core losses – Hysteresis loss – Eddy current loss – Anomalous loss – depends on material process, impurities • Problems with core loss prediction – Stator iron loss: based on knowledge of stator tooth flux density waveforms – Usually assumes sinusoidal time-variation and one-dimensional spatial variation – Flux waveforms have harmonic frequency and rotational component – Use dB/dt method for eddy-current term, frequency spectrum method • Torque, efficiency, inductance [4] Klontz and Soong 20 Lumped-Circuit Design Tools • Non-linear magnetic material modeling of simple geometries • Need a good understanding of magnetic field distribution to partition • Fast to solve, good for optimization • Limitations Lovelace, Jahns, and Lang – Requires tuning with FE results 21 Finite-Element Modeling and Simulation Tools Average Magnitude Magnetic Flux Density linear run 4 0.045 3.5 0.04 3 2.5 0.035 y (m) • Important aspects – model saturation • More accurate • Essential when saturation is significant • Limitations 2 0.03 1.5 0.025 – Meshing – Only as accurate as model design – 2D, 3D – Not currently used as a design tool due to computational intensity 1 0.5 0.02 0.015 0.02 0.025 0.03 x (m) 0.035 0.04 0.045 Nonlinear magnetostatic FE average magnetic flux density solution for machine with solid rotor 22 Ideal Design Tool • Easy to set up • Models all significant aspects of machine that affect performance – magnetic saturation • Efficiently simulates transient conditions and steady-state operation 23 References [1] W.L. Soong, “Design and Modeling of Axially-Laminated Interior Permanent Magnet Motor Drives for Field-Weakening Applications,” Ph.D. Thesis, School of Electrical and Electronic Engineering, University of Glasgow, 1993. [2] T.J.E. Miller, “Brushless Permanent-Magnet and Reluctance Motor Drives”, Oxford Science Publications, 1989. [3] J.R. Hendershot and T.J.E. Miller, “Design of Brushless Permanent-Magnet Motors”, Magna Physics Publishing and Oxford University Press, 1994. [4] K. Klontz and W.L. Soong, “Design of Interior Permanent Magnet and Brushless DC Machines – Taking Theory to Practice” course notes 2010. [5] J.R. Hendershot and T.J.E. Miller, “Design of Brushless Permanent-Magnet Motors”, Motor Design Books, 2010. Questions? 24