Advanced electrical machines for new and emerging applications
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Advanced electrical machines for new and emerging applications J. Wang and D. Howe University of Sheffield Nordic Seminar on ‘Advanced Magnetic Materials and their Applications’ 10th/11th October 2007, Pori, Finland Drivers for advanced machines/actuators Technology development ¾ Drive-by-wire ¾ Fly-by-wire ¾ Embedded generation ¾ ‘More-electric’ ships : : Legislation ¾ Energy efficiency ¾ Emissions : : ¾ Applies to all market sectors Automotive Aerospace Marine Consumer products etc. Electrical machines and actuators Competition ¾ Becoming more fierce ¾ Adoption of advanced technologies : : Consumer expectations ¾ Performance ¾ Functionality ¾ Reliability/maintainability ¾ Safety : Automotive: ‘More-electric’ technologies ¾ Adoption of ‘more-electric’ technologies is significantly increasing electrical load ¾ Active vehicle suspension ¾ Electromechanical valve actuation ¾ Automated manual transmission ¾ Load will soon exceed capability of present alternators Exhaust gas energy recovery ¾ Naturally aspirated engine ¾ Has potential to reduce size, or eliminate, conventional alternator and load imposed on engine ¾ Favours switched reluctance machine (SR) machine High temperature: - ~900°C at turbine - ~300°C at machine rotor High speed: - up to 80krpm SR machine design ¾ Maximum speed: 80,000 rpm ¾ Maximum power: 6 kW ¾ Average power: 2.3 kW ¾ Design constrained by centrifugal stress and safety margin between max. speed and 1st critical speed Turbine SR Rotor Cedrat FLUX2D Bearing ¾ 3-phase 6:4 SR machine Fundamental electrical frequency is 5.3kHz at 80,000rpm Bearing 1st critical speed ~99,000rpm SR machine design ¾ Stranded conductor used to minimise high frequency eddy current loss ¾ Coolant temperature in cooling jacket 90°C Frame Stator 19 strand conductor Current density distribution with 5-turns, 19-strand conductor Temperature distribution at rated power Optimal control angle trajectories ¾ Switch-on (θon) and dwell (θdw) angles determine SR machine power and losses, peak/rms current and VA rating of converter ¾ Constant power contours at 80 krpm as θon and θdw varied Motoring 2.3kW Zero Net Power X Generating 2.3kW Ma x effi imum cien cy Optimum θon and θdw for minimum loss at operating point x SR machine ¾ Dynamometer testing ¾ Specification Generator voltage 12V Generator efficiency >70% Water-cooled Location – pre-catalyst Sensorless rotor position control Sealed for life bearings Length ~150mm, weight ~7kg Maximum output power 6kW @ 80krpm Turbogenerator ¾ Turbine, guide vane and base-plate ¾ Complete assembly ¾ Turbine and generator sized for highest IC engine residency operating point ¾ Necessary to by-pass turbine when engine operating at peak power Turbogenerator control ¾ Exhaust gas mass flow rate and temperature determine energy at turbine ¾ Waste-gate valve regulates flow rate and protects system under fault condition Exhaust manifold Exhaust throttle Volute Switched reluctance generator Waste-gate enables turbine to be by-passed so that engine can develop peak power without undue backpressure Cold-air rig testing TIGERS turbine Electronically controlled waste-gate TIGERS SR machine Compressor air Engine dynamometer testing ¾ Will enable influence of increased EBP on fuel consumption to be assessed Electrical torque-boosting of down-sized IC engine ¾ Benefits of down-sizing Reduction in fuel consumption Reduced emissions Lower weight Comparable performance at high engine speeds ¾ Down-sized IC engine exhibits reduced torque at low engine speeds 300 Torque (Nm) 250 3.0L NA 1.8L TC 200 150 100 50 0 0 1000 2000 3000 Speed (rpm) 4000 5000 Electrical torque and power requirements ¾ Torque deficit can be provided by electrical torque-boost machine Torque-boost machine can also start engine and provide regenerative braking Speed Torque Power No load speed Max. torque 1069 rpm 132 Nm Max. power 1704rpm 104.5 Nm 14.78 kW 18.65 kW 3000 rpm Typical operating points Super-capacitor based torque-boost system CAN I/O Master Controller ECU CAN DC/DC Supercapacitor Unit Converter Alternator Battery Power Electronic Converter Clutch Down-Sized IC Engine Gearbox Starter (Optional) Torque Boost Electrical Machine Simulation of torque-boost system ¾ Drive-away cycle ¾ Power and energy consumption EST Power & Energy 2.5E+04 1.6E+05 2.0E+04 1.4E+05 EST Power (W) EST Energy (J) 1.0E+04 1.0E+05 5.0E+03 8.0E+04 0.0E+00 6.0E+04 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 -5.0E+03 4.0E+04 -1.0E+04 2.0E+04 -1.5E+04 0.0E+00 -2.0E+04 -2.0E+04 Time (sec) Acceleration from 0 to 100 km/h in 18 seconds Gear shift at 2200 rpm Regenerative braking with gear shift from 5th to 3rd Number of supercaps (3500F) Max. DC link voltage(V) Min. DC link voltage (V) Energy required during Acc. (kJ) Regen energy (kJ) Net energy consumption (kJ) Rms torque (Nm) 1.2E+05 36 93 68 146 146 0 70 EST Energy (J) EST Power (W) 1.5E+04 3-phase PM brushless torque-boost machine ¾ Annular space envelope necessitates a high pole number ¾ Interior magnet rotor - Reluctance torque in addition to excitation torque ¾ Modular stator winding - Each phase comprises concentrated coils wound on adjacent teeth - Short end-windings ¾ Number of stator slots close to number of rotor poles - Virtually zero cogging torque without skew 22-poles, 24-slots 3-phase, PM brushless torque-boost machine ¾ Dynamometer testing Total mass: 17.2kg Peak current @132Nm: 650A Efficiency @1069rpm, 132Nm: 93% Idling loss @3000rpm: 390W Machine control strategies Idem Error + ¾ Brushless DC control for cranking - PI Controller A B C Modulator M ha hb hc Ia IphA I IphB Ib Current Mixer Ic IphC Idealised brushless dc machine phase current waveforms I d* I q* dError + - Vd PI Error Controller Vq q + α, β d, q Vα SVPWM Modulator A B C Vβ M Id Iq ¾ Brushless AC control for speeds above 500 rpm α, β d, q Iα Iβ a, b, c α, β ha hb hc Ia Ib Ic Sh Hybrid Ch Observer Schematic of torque-boost test system Temperature Measurement T, ω EST machine Dynamometer Power Analyser 4-Q SuperDC Capacitor Power 3-phase Inverter Vac,Iac Coolant Temperature & flow control 120V/500A supply Bank Vdc,Idc DSP Control Board CAN Link Labview interface via CAN ¾ DC bus-voltage VDC from supercapacitor varies ¾ When VDC is sufficient to supply required current, max. torque/ampere control is employed ¾ When back-emf > VDC, field-weakening control is employed Supercapacitor energy storage unit ¾ 36, 3500F, 2.7V max. supercapacitors Efficiency map of torque-boost system ¾ Average efficiency of machine & inverter η mi ⎧ 1 ⎪⎪ = ⎨ 2⎪ ⎪⎩ ∫ ∫ v dc (t )idc (t )dt Tc ∫ + ∫ v ω (t )T (t )dt ⎫ Td ω (t )T (t )dt Tc Td ⎪⎪ ⎬ dc (t )i dc (t ) dt ⎪ ⎪⎭ ¾ Average efficiency of supercapacitors η sc ∫ = ∫ v dc (t )idc (t )dt Td v dc (t )idc (t )dt Tc ¾ Average efficiency of torque-boost system η est ∫ = ∫ ω (t )T (t )dt Td Tc ω (t )T (t )dt Free-piston energy converter ¾ Series hybrid vehicle Generator Traction drive ICE Floating piston – eliminates crankshaft Piston motion controlled by electrical machine Facilitates optimum combustion (HCCI/ACI) Piston Inlet port Battery Moving-magnet armature ¾ 2-stroke unit Tubular permanent magnet machine Exhaust valve Tubular electrical machine ¾ No end-windings, high power density and volumetric efficiency Phase A Phase B Phase C Titanium tube r Supporting tube Magnets ¾ Modular stator winding 9-slot/10-pole/12-coils Low cogging force Sinusoidal emf ¾ Quasi-Halbach magnetised armature 15-poles (10-poles active) Negligible flux on inner bore Low moving mass Tubular electrical machine ¾ Phase winding ¾ Assembled machine excluding water-cooled jacket ¾ 44 kW rated output power (4kN@11m/s) Stator mmf space harmonic distribution ¾ 5th harmonic interacts with magnets to produce thrust force ¾ Induced eddy currents at 44kW, 11m/s In magnets ¾ 2, 5, 8, … forward travelling harmonics & 1, 4, 7, … backward travelling harmonics induce eddy currents in armature In titanium tube Design optimisation Optimum τmr/τp = 0.625 Optimum τp/Re = 0.25 0.95 1 0.94 0.95 0.93 0.9 0.92 0.85 0.91 0.8 0.9 0.75 Drive system efficiency Machine efficiency 0.89 Power factor 0.88 0.87 0.5 Output 0.7 0.55 0.6 0.65 0.7 Rm/Re Power = 44 kW 0.65 0.75 0.8 0.6 0.85 Power factor ¾ Optimum Rm/Re for max. machine efficiency is significantly different to that for max. system efficiency (and min. converter VA rating) Efficiency ¾ Main design parameters: Rm/Re, τp/Re, τmr/τp Free-piston energy converter ¾ Efficiency map of machine/converter Switching frequency 25kHz ¾ Prototype Flywheel energy storage/peak power buffer Vehicle Management Unit Flywheel unit Vehicle control Motor/ Generator Road Power Power Vehicle drive train Inverter Drive Motor Power Power Energy Store Power Electronics ¾ Potential benefits Handles peak power during acceleration/deceleration Enables kinetic energy recovery Primary energy source independent from high power demand (enhanced efficiency, extended lifetime, etc) Improved vehicle performance/response ¾ From 2009, kinetic energy recovery systems will be permitted on Formula 1 cars Max. energy released per lap ≤400kJ Max. power in or out ≤60kW Flywheel energy storage/peak power buffer ¾ Specific energy storage capability determined by tensile strength and density of flywheel material and geometry E= σ k J / kg ρ where σ = design stress of material ρ = density k = shape factor ¾ Flywheel shapes ¾ Solid disc/interface/shaft Compatible with: Isotropic material (eg. Maraging steel) Max. specific energy ⎛⎜ E ⎞⎟ = 0.6 δ J / m3 ⎝M⎠ ρ Fibre composite materials have highest tensile strength to density ratio, and rim shaped flywheel provides highest specific energy capability ¾ Annular rim Compatible with: Anisotropic material (eg. Kevlar) Max. specific energy ⎛⎜ E ⎞⎟ = 0.5 δ J / m3 ⎝M⎠ ρ Composite flywheel ¾ Typical specification ¾ Concept Peak power: ~40kW Continuous power: ~30kW Max speed: ~60krpm Stored energy: ~1.5MJ (~400Wh) Operating speed range: ~60krpm→30krpm Recoverable energy: ~1MJ (~300Wh) ¾ Kinetic/electrical energy conversion Annular carbon fibre composite flywheel rim Integral magnetic bearing system Integral permanent magnet brushless dc machine PM brushless motor/generator Halbach magnetised Air-cored Water-cooled Containment Passive Magnetic Bearing Active Magnetic Bearing Rim Cooling ducts Litz wire (648 strands/conductor) Motor/Generator Demonstrator flywheel unit Carbon fibre composite rim and rotating components of bearings and electrical machine Flywheel with end-cap of containment removed Flywheel in containment together with power electronic converter, magnetic bearing amplifiers/controller, coolant system Central hub comprising stationary components of bearing system and electrical machine Flywheel unit in safety vessel Aerospace: Current aircraft systems ¾ Aircraft loads supplied by combination of hydraulic, pneumatic, electrical and mechanical power ‘More-electric’ aircraft systems ¾ Use of electrical power alone will enable global optimisation and system level performance improvements IN: Fuel Electrical start OUT: Thrust Electricity Typically: 10% weight reduction 13% lower engine thrust 9% reduction in fuel - corresponding emissions reduction Cabin Air Electrical Wing Anti-ice Electrically Driven Hydraulics and/or Electromechanical Fuel Air Auxiliary Power Unit Electricity (Hotel mode only) Cabin Air ‘More-electric’ aircraft engine ¾ Electrical machines integrated into engine, for starting/generating and power transfer between spools ¾ Potentially the only means of delivering future power requirements (>1MW for large aircraft) ¾ Future power generation/starting ¾ Current power generation Power electronics LP spool generator Radial take-off shaft HP spool starter/generator Complex gear System • Heavy • High maintenance • High losses Electrical generator • • • • • • • Simplified engine architectures Eliminates take-off shafts Reduces engine size with respect to aerodynamic drag Enhanced functionality –wind-milling capability Allows energy transfer between spools Improved efficiency Reduced maintenance ‘More-electric’ aero-engine : HP spool starter-generator Representative specification for large civil turbo-fan engine Rotor inner bore Axial length (overall) Maximum power Starting torque Maximum operating speed Over-speed capability Ambient temperature 200mm 100mm 100-150kW 150-250Nm 13,500rpm 110% 350-400oC Conventional SR topology ¾High temperature environment favours switched reluctance machine ¾Rotor is subjected to extreme mechanical loading – severely constrains maximum rotor diameter Modular SR topology Modular rotor 8-phase 24/18 pole Single-piece rotor 4-phase 24/18 pole Series of rotor modules attached to a nonmagnetic, high-strength hub ‘More-electric’ aero-engine : HP spool starter-generator ¾ Modular switched reluctance machine - Two-phases on adjacent teeth excited simultaneously Clockwise motoring torque from starting position shown: BA ⇒ GF ⇒ DC ⇒ AH ⇒ FE ⇒ CB ⇒ HG ⇒ ED ⇒ BA ⇒ GF Laminated cobalt iron rotor pole modules ¾ Similar benefits to conventional ‘short flux path’ machines in terms of iron loss ¾ Non-continuous back-iron limits feasible combinations of rotor poles, stator poles and phases ‘More-electric’ aero-engine : LP shaft generator Ambient temperature ~150oC Speed range ~1000 – 3000rpm Maximum power 250kW Wind-milling power 25kW ¾ Conventional 3-phase permanent magnet machines ¾ Favours permanent magnet machine equipped with Samarium Cobalt magnets ¾ Requires fault-tolerance ¾ Fault-tolerant 5-phase permanent magnet machine Higher phase number Coils wound on alternate teeth Negligible mutual coupling between phases Coil inductance limits short-circuit current to rated value Phase C Phase B Phase A Overlapping (distributed) winding 66-slots / 22-poles Non-overlapping (concentrated) winding 33-slots / 22-poles Non-overlapping (concentrated) winding 20-slots / 24-poles ‘More-electric’ aero-engine : LP shaft generator ¾ Fault-tolerant permanent magnet machine Magnetic field distributions Open-circuit 5-phase, 40-slots, 28-poles, 4-coils per phase Phase A short-circuit (Negligible mutual coupling with other phases) Prototype ‘More-electric’ aero-Engine : LP shaft generator ¾ Fault-tolerant permanent magnet machine 5-phase, 40-slots, 28-poles 20-coils (4-coils/phase) ¾ Terminal short-circuit fault on phase A at rated torque ‘More-electric’ aircraft : Flight control surface actuation ¾ Electromechanical actuator PM brushless motor Gearbox End-effector Flight control surfaces Ballscrew ¾ Electrohydrostatic actuator actuator Integrated variable-speed motor/ fixed displacement pump Position Controller Motor Electronics M P pump motor valve block Valve Block accumulator Source: Liebherr GmbH Accumulator Actuator power electronics Consumer products : Current refrigerator compressor technology ¾ Reciprocating compressor driven by rotary motor (1-ph induction motor) via crank mechanism Inlet Piston stroke fixed by crank Significant friction loss in compressor On/off duty cycle of fixed-speed compressor determined by refrigerator temperature setting and load Overall efficiency relatively low (~70%) Hermetically sealed compressor Variable-speed operation provides variable cooling capacity and improves efficiency (~85%). However, continuous operation down to low speeds not possible due to lubrication problems, etc. Direct-drive linear compressor ¾ For max. efficiency, displacement and flow rate, electrical supply frequency should coincide with mechanical resonant frequency fr = 1 KT 2π m where KT = total equivalent spring stiffness m = total moving mass ¾ Reduces friction loss associated with crank ¾ Enables soft start/stop (low noise) ¾ Facilitates continuous variable cooling capacity, by varying frequency (over narrow range) and stroke (although small amplitude stroke compromises volumetric efficiency) ¾ 95% efficiency (electrical-mechanical) achievable Quasi-Halbach magnetised motor Stroke (mm) (nominal) 10.5 Frequency (Hz) (nominal) 50 RMS voltage (V) 230 RMS current (A) 0.5 Outer diameter of stator (mm) 100 Axial length (mm) 50 Pole-pitch (mm) 25 Trapezoid angle (degree) 45 Air-gap length (mm) 0.8 Magnet thickness (mm) 5.0 Magnet remanence (T) 1.14 ¾ Employs trapezoidal radially and axially magnetised magnets ¾ Cross-sectional area of radially magnetised magnets increases with radius - increases radial flux density in airgap ¾ Cross-sectional area of axially magnetised magnets reduces with radius - increases flux which passes through axially magnetised magnets, rather than mild steel tube. ¾ Force density increased Control of direct-drive linear compressor ¾ The mechanical resonant frequency is: fr = KT m 1 2π where KT = total equivalent spring mass m = total moving mass Linear motor Compressor Suction valve Ps Discharge valve Pd Coils Suspension springs ¾ Total equivalent spring stiffness: KT = k + kg + kc where k = stiffness of suspension springs kc = equivalent stiffness of cogging force kg = equivalent stiffness of compressed refrigerant ¾ For max. efficiency, supply frequency needs to track fr Linear compressor control ¾ Total effective gas stiffness kT varies with operating condition (stroke, evaporator/ambient/condenser temperatures) Frequency (Hz) Stiffness (Nm) Evaporator/ambient/condenser temperatures Stroke (m) Stroke (m) ⎛ KT ⎞ ⎟ also varies ¾ Hence, mechanical resonant frequency ⎜⎜ f r = ⎟ ⎝ m ⎠ ¾ For max. efficiency, supply frequency needs to track fr Linear compressor control ¾ Piston stroke controlled by varying current supplied from PWM H-bridge ¾ Resonant frequency tracked by varying supply frequency and searching for max. power point (MPP) • fr occurs at dP/df = 0 MPP • Perturbation frequency df = 0.025Hz • Perturbation period = 0.2s (~10 cycles) Experimental results ¾ Initial supply frequency: 46Hz Variation of rms current: 0.2, 0.3, 0.25, 0.3 A Variation of tracked resonant frequency: 43.35, 42.8, 42.5, 42.8 Hz Variation of input/output powers Variation of piston stroke Industrial : Magnetic gears ¾ Mechanical ¾ Magnetic - Single-stage helical gear - External - Internal S N S ¾ Transmitted torque density 50 - 150 kNm/m3 ¾ Generally requires lubrication/cooling ¾ Generates noise/vibration ¾ Limited life N ¾ Poor utilisation of magnets ¾ Low torque transmission capability High performance magnetic gears ¾ Principle of operation Stationary polepieces Radial flux density waveform Radial flux density waveform Low-speed pm rotor Space harmonic spectrum Space harmonic spectrum 4 pole-pair high-speed rotor 27 pole-pieces 27 static polepieces ns = no. of pole-pieces ph = pole-pairs on high-speed rotor pl = pole-pairs on low-speed rotor 23 pole-pair low-speed rotor 4 pole-pair high-speed rotor 5.75:1 gear ratio High-speed pm rotor All the magnets contribute to torque transmission Pole-pieces modulate fields produced by pm rotors, resulting in asynchronous space harmonic fields Highest asynchronous space harmonic utilised for torque transmission when ph = ns – pl Gear ratio = pl ph Torque transmission capability ~70 kNm/m3 ns = 27 ph = 4 pl = 23 High performance magnetic gears ¾ Only 3 components ¾ 2 are free to rotate, the 3rd is earthed ¾ Prototype 5.75:1 gear • Torque density: 78kNm/m3 ¾ Zero wear and no lubrication ¾ Low maintenance/high reliability ¾ Inherent overload protection/no jamming Other magnetic gear topologies ¾ Rotary: axial-field ¾ Linear: radial-field Axially magnetised permanent magnets High-speed pm armature Low-speed armature with ferromagnetic rings Low-speed pm rotor High-speed pm rotor Ferromagnetic pole-pieces Radially magnetised ring magnets Stationary pm armature Harmonic gears ¾ Mechanical ¾ Magnetic permanent magnets Circular-spline Flexible-spline stator back-iron (coupled to low-speed shaft) Wave-generator high-speed rotor (wave generator) (driven by high-speed shaft) bearing low-speed rotor back-iron • Oval wave-generator with outer ball bearing coupled to high-speed shaft • Flexible-spline teeth engage with teeth of circular-spline in a continuous rolling manner, and is coupled to low-speed shaft • Since flexible-spline has 2 fewer teeth than circular spline, each complete revolution of wave-generator causes a 2 tooth displacement of flexible-spline relative to circularspline • Gear ratio = No. of teeth on circular spline Difference in no. of teeth on circular / flexible splines (2) • High-speed rotor is equivalent to wave-generator, and deforms flexible low-speed rotor which rotates independently within a rigid outer cylindrical stator • Time-varying sinusoidal variation of airgap length modulates field produced by magnets on low-speed rotor and results in a dominant asynchronous space harmonic which interacts with magnets on stator (& vice-versa) Conclusions ¾ Many novel electromagnetic machine and actuator concepts are under development, both for near-term applications (eg. hybrid vehicles) and applications which are still embryonic and on the long-term horizon (eg.‘more-electric’ aircraft engines) ¾ ‘More-electric’ actuation technologies feature prominently in technology roadmaps for most market sectors ¾ Many design challenges remain, and there are significant opportunities for innovation ¾ There are also many challenges for magnetic materials development
