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Aspects of Permanent Magnet Machine Design
Christine Ross February 7, 2011 Grainger Center for Electric Machinery and Electromechanics
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Outline Permanent Magnet (PM) Machine Fundamentals
Motivation and Application Design Aspects PM Material PM Rotor Configurations Manufacturing Processes Design Tools
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Permanent Magnet (PM) Machine Fundamentals
Focus on electronically controlled PM AC synchronous 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 3-phase stator windings Laminated stator Four magnets – two pole-pairs, p = 2 Between the slots are the teeth which carry the flux through the winding region; Flux is predominantly confined to the teeth and links the coils of the winding 4-pole PM rotor Cross-section of surface-mounted PM machine
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PM Machine Theory Output torque is proportional to power
Control instantaneous torque by controlling magnitude of phase currents Go into d-q equivalent circuit? Torque production
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PM Machine Control Instantaneous torque control
Servo performance kW Fast dynamic response Smooth output torque Accurate rotor position sensor information needed Single-phase equivalent circuit
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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 [1] Soong
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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
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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 Limited by flux density of PM Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air-gap flux density and eventually, core saturation. Uncontrolled air-gap flux density leads to over voltage and poor electronic control reliability. A persistent magnetic field imposes safety issues during assembly, field service or repair, such as physical injury, electrocution, etc. In all cases, high performance permanent magnet materials are always expensive or virtually cartel controlled by a single country. The mining of high performance permanent magnet materials is environmentally demanding and as a result, the use of permanent magnets is by no means environmentally friendly. Air-gap depth tolerance improves only 20% over other electric machines before magnetic leakage becomes the same concern for any electric machine. High performance permanent magnets, themselves, have structural and thermal issues.
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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 Differing functionality, precision, automation in motion control IPM washing-machine motors [5] Hendershot and Miller
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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 Saliency – winding inductances vary as a function of rotor position; No saliency – if remove magnets, rotor has no tendency to align itself with stator when current is flowing (no reluctance torque) After this slide, add about general PM design procedure?
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PM Material Soft magnetic material (steel) – small B-H loop
Hard magnetic material – (PM) – large B-H loop Choose magnets based on high Br and Hc Remnant Flux Density Br Coercivity Hc
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PM Material Arnold Magnetics PM Br (T) Hc (kA/m) Cost
Resistivity (µΩ-cm) Max. Working Temp. (ºC) Curie Temp. (ºC) Alnico5-7 1.3 60 47 > 500 Ferrite 0.4 300 low >10,000 250 450 NdFeB (sintered) 1.1 850 medium 150 80-200 Sm2Co7 (sintered) 1.0 750 Higher than NdFeB 86 Energy product – largest value of product of B and H, measure of magnet performance Add curie temperature? Because of high coercivity of high performance permanent magnet materials, such as neodymium, air-gap depth is more tolerable, which puts lower structural constraints on frame and bearing assemblies. Ferrite – low-voltage DC motors Samarium-Cobalt magnets (late 1960s) – higher power levels Neodymium-Iron-Boron (NdFeB) magnets (early 1980s) – increased the cost-effectiveness of high-energy magnets Demagnetization: Thermal effects – as temperature increases NdFeB: both Br and Hc reduce, most prone to demag at high temp Ferrite: Br reduce, Hc increase, most prone to demag at low temp Can improve by using higher Hc magnets, increase magnet thickness or airgap [2] Miller [3] Hendershot and Miller Arnold Magnetics
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PM Material Chinese dependency No shortage Mountain Pass, CA Idaho
Nd is about as common as Cu Arnold Magnetic Technologies
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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 In embedded type, including small region larger than air gap of non-magnetic material between magnets causes the leakage to not increase Surface bread-loaf magnets
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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 Reulctance torque comes from the shape of the steel lamination, not from the magnets Total torque is a combination of reluctance torque and magnet alignment torque A. O. Smith
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PM Machine Topology SMPM: IPM: 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 Note that inverter limits the performance of torque and speed with current and voltage What is saliency, why can it be useful or good? – when rotor is unexcited, rotor will rotate if the stator is excited Why is field weakening useful – wide range of applications requiring wide speed range at constant power; field-weakening capability allows IPM to achieve this IPM controller more complex – nonlinear relationship between torque and current, Obtain maximum torque, phase-shift the current by an angle that depends on the load, even without magnetic saturation IPM magnets less susceptible to eddy-current losses Characteristic SMPM IPM Saliency No Yes Field Weakening Some Good Controller Standard More Complex [4] Klontz and Soong
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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 High speed practice and limits – rotor diameter limits speed Hybrid Camry PM synchronous AC motor/generator ecee.colorado.edu
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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
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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
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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
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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 Requires tuning with FE results Lovelace, Jahns, and Lang
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Finite-Element Modeling and Simulation Tools
Important aspects – model saturation More accurate Essential when saturation is significant Limitations Meshing Only as accurate as model design – 2D, 3D Not currently used as a design tool due to computational intensity Nonlinear magnetostatic FE average magnetic flux density solution for machine with solid rotor
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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
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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?
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