advanced electrical machines for new & emerging applications
DESCRIPTION
electric MachinesTRANSCRIPT
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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 Applications10th/11th October 2007, Pori, Finland
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Drivers for advanced machines/actuators
Electrical machines and actuators
Energy efficiency Emissions
::
Drive-by-wire Fly-by-wire Embedded generation More-electric ships
::
Performance Functionality Reliability/maintainability Safety
:
Becoming more fierce Adoption of advanced
technologies::
Applies to all market sectors Automotive AerospaceMarine Consumer products
etc.
Technology development
Legislation
Competition
Consumer expectations
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Automotive: More-electric technologies
Automated manual transmission
Active vehicle suspension Electromechanical valve actuation
Adoption of more-electric technologies is significantly increasing electrical load
Load will soon exceed capability of present alternators
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Exhaust gas energy recovery Naturally aspirated engine Has potential to reduce size, or eliminate,
conventional alternator and load imposed on engine
Favours switched reluctance machine (SR) machine High temperature:
- ~900C at turbine- ~300C at machine rotor
High speed:- up to 80krpm
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SR machine design
Cedrat FLUX2D
Maximum speed: 80,000 rpmMaximum power: 6 kW Average power: 2.3 kW
3-phase 6:4 SR machine Fundamental electrical frequency is 5.3kHz at
80,000rpm
Design constrained by centrifugal stress and safety margin between max. speed and 1st critical speed
1st critical speed ~99,000rpm
Bearing
SR Rotor
Bearing
Turbine
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SR machine design Stranded conductor used to minimise high
frequency eddy current loss Coolant temperature in cooling jacket 90C
Temperature distribution at rated power Current density distribution with5-turns, 19-strand conductor
Frame
Stator
19 strandconductor
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Optimal control angle trajectories Switch-on (on) and dwell (dw) angles determine SR machine power and losses, peak/rms current
and VA rating of converter
Optimum on and dw for minimum loss at operating point x
Maximum efficiency
X
Motoring 2.3kW
Generating 2.3kW
Zero NetPower
Constant power contours at 80 krpm as on and dw varied
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SR machine
Specification Generator voltage 12V Generator efficiency >70%Water-cooled Location pre-catalyst Sensorless rotor position control Sealed for life bearings Length ~150mm, weight ~7kgMaximum output power 6kW @ 80krpm
Dynamometer testing
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Turbogenerator
Turbine, guide vane and base-plate Complete assembly
Turbine and generator sized for highest IC engine residency operating point Necessary to by-pass turbine when engine operating at peak power
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Turbogenerator control
Switched reluctance generator
Volute
Exhaust manifold
Exhaust throttle
Exhaust gas mass flow rate and temperature determine energy at turbine
Waste-gate valve regulates flow rate and protects system under fault condition
Waste-gate enables turbine to be by-passed so that engine can develop peak power without undue back-pressure
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Cold-air rig testingTIGERS turbine
Electronically controlledwaste-gate
Compressor air
TIGERS SR machine
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Engine dynamometer testing
Will enable influence of increased EBP on fuel consumption to be assessed
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Electrical torque-boosting of down-sized IC engine
0
50
100
150
200
250
300
0 1000 2000 3000 4000 5000
Speed (rpm)
T
o
r
q
u
e
(
N
m
) 3.0L NA1.8L TC
Benefits of down-sizing Reduction in fuel consumption Reduced emissions Lower weight Comparable performance at high engine speeds
Down-sized IC engine exhibits reduced torque at low engine speeds
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Electrical torque and power requirements Torque deficit can be provided by electrical torque-boost machine
Max. torque Max. powerSpeed 1069 rpm 1704rpmTorque 132 Nm 104.5 NmPower 14.78 kW 18.65 kW
No load speed 3000 rpm
Typical operating points
Torque-boost machine can also start engineand provide regenerative braking
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Super-capacitor based torque-boost system
Alternator
Clutch
Gearbox
ECU
CAN
Down-Sized ICEngine
Master ControllerI/O
CAN
Torque BoostElectrical Machine
Alternator
Clutch
Starter(Optional)
Gearbox
ECU
Battery
CAN
DC/DCConverter
Master ControllerI/O
CAN
Supercapacitor Unit
Power ElectronicConverter
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Simulation of torque-boost system Drive-away cycle Power and energy consumption
Acceleration from 0 to 100 km/h in 18 seconds Gear shift at 2200 rpm Regenerative braking with gear shift from 5th to 3rd
EST Power & Energy
-2.0E+04
-1.5E+04
-1.0E+04
-5.0E+03
0.0E+00
5.0E+03
1.0E+04
1.5E+04
2.0E+04
2.5E+04
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
Time (sec)
E
S
T
P
o
w
e
r
(
W
)
-2.0E+04
0.0E+00
2.0E+04
4.0E+04
6.0E+04
8.0E+04
1.0E+05
1.2E+05
1.4E+05
1.6E+05
E
S
T
E
n
e
r
g
y
(
J
)
EST Power (W)
EST Energy (J)
Number of supercaps (3500F) 36Max. DC link voltage(V) 93Min. DC link voltage (V) 68Energy required during Acc. (kJ) 146Regen energy (kJ) 146Net energy consumption (kJ) 0Rms torque (Nm) 70
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3-phase PM brushless torque-boost machine
22-poles, 24-slots
Annular space envelope necessitatesa high pole number Interior magnet rotor
- Reluctance torque in addition toexcitation torque
Modular stator winding- Each phase comprises concentrated
coils wound on adjacent teeth- Short end-windings
Number of stator slots close tonumber of rotor poles
- Virtually zero cogging torquewithout skew
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3-phase, PM brushless torque-boost machine Dynamometer testing
Total mass: 17.2kgPeak current @132Nm: 650AEfficiency @1069rpm, 132Nm: 93%Idling loss @3000rpm: 390W
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Machine control strategiesModulator
PIController
CurrentMixer
M+
-
Error
IaIb
Ic
ha hb hc
ABC
Idem
I
ModulatorPI
Controller
CurrentMixer
M+
-
Error
IaIb
Ic
ha hb hc
ABC
Idem
I
, d, q
PIController
V
V
Vd
Vq
, d, q
a, b, c
,
M
HybridObserver
+
- +
-
Sh
Ch
Id*
Iq*
Id
Iq
qError
dError
I
I
IaIb
Ic
ha hb hc
ABC
SVPWMModulator
, d, q
PIController
V
V
Vd
Vq
, d, q
a, b, c
,
M
HybridObserver
+
- +
-
Sh
Ch
Id*
Iq*
Id
Iq
qError
dError
I
I
IaIb
Ic
ha hb hc
ABC
SVPWMModulator
Brushless DC control for cranking
Brushless AC control for speeds above 500 rpm
Idealised brushless dc machine phase current waveforms
IphA
IphB
IphC
Idealised brushless dc machine phase current waveforms
IphA
IphB
IphC
IphA
IphB
IphC
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Schematic of torque-boost test system
Dynamo-meter
CoolantTemperature & flow control
3-phase Inverter
120V/500A
4-Q DC Power
supply
TemperatureMeasurement
EST machine
PowerAnalyser
Vac,Iac
T,
Vdc,Idc
Labview interface via CAN
DSP Control Board
CANLink
Super-Capacitor
Bank
DC bus-voltage VDC from supercapacitor variesWhen VDC is sufficient to supply required current, max. torque/ampere control is employedWhen back-emf > VDC, field-weakening control is employed
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Supercapacitor energy storage unit
36, 3500F, 2.7V max. supercapacitors
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Efficiency map of torque-boost system
+=
d
d
c
c
Tdcdc
T
T
Tdcdc
midttitv
dttTt
dttTt
dttitv
)()(
)()(
)()(
)()(
21
=
c
d
Tdcdc
Tdcdc
scdttitv
dttitv
)()(
)()(
=
c
d
T
Test
dttTt
dttTt
)()(
)()(
Average efficiency of machine & inverter
Average efficiency of supercapacitors
Average efficiency of torque-boostsystem
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Free-piston energy converter
Battery
Traction drive
ICE
Generator
Floating piston eliminates crankshaft Piston motion controlled by electrical machine Facilitates optimum combustion (HCCI/ACI)
Series hybrid vehicle
2-stroke unit
Piston Inlet port
Tubular permanent magnet machine Exhaust valve
Moving-magnet armature
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Tubular electrical machine No end-windings, high power density and volumetric efficiency
Phase A Phase B Phase C
Magnets
r
Supporting tube
Titaniumtube
Modular stator winding 9-slot/10-pole/12-coils Low cogging force Sinusoidal emf
Quasi-Halbach magnetised armature 15-poles (10-poles active) Negligible flux on inner bore Low moving mass
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Tubular electrical machine Phase winding Assembled machine excluding
water-cooled jacket
44 kW rated output power (4kN@11m/s)
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Stator mmf space harmonic distribution 5th harmonic interacts with magnets to
produce thrust force Induced eddy currents at 44kW, 11m/s
In magnets In titanium tube
2, 5, 8, forward travelling harmonics & 1, 4, 7, backward travelling harmonics induce eddy currents in armature
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Design optimisationMain design parameters: Rm/Re, p/Re, mr/p Optimum Rm/Re for max. machine efficiency
is significantly different to that for max. system efficiency (and min. converter VA rating)
0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.87
0.88
0.89
0.9
0.91
0.92
0.93
0.94
0.95
Rm/Re
E
f
f
i
c
i
e
n
c
y
Drive system efficiency Machine efficiency
Power factor
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
P
o
w
e
r
f
a
c
t
o
r
Output
Power = 44 kW
Optimum mr/p = 0.625Optimum p/Re = 0.25
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Free-piston energy converter Efficiency map of machine/converter Switching frequency 25kHz
Prototype
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Flywheel energy storage/peak power buffer
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
From 2009, kinetic energy recovery systems will be permitted on Formula 1 carsMax. energy released per lap 400kJMax. power in or out 60kW
Motor/Generator
PowerElectronics
EnergyStore Inverter
DriveMotor
VehicleManagement Unit
Vehicle controlFlywheel unit
Vehicle drive train
RoadPower
P
o
w
e
r
P
o
w
e
r
P
o
w
e
r
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Flywheel shapes
Flywheel energy storage/peak power buffer Specific energy storage capability determined by tensile strength and density of
flywheel material and geometry
where = design stress of material = densityk = shape factor
kg/JkE =
3m/J6.0ME
=
Solid disc/interface/shaftCompatible with: Isotropic material (eg. Maraging steel)Max. specific energy
Annular rimCompatible with: Anisotropic material (eg. Kevlar)Max. specific energy 3m/J5.0
ME
=
Fibre composite materials have highest tensile strength to density ratio, and rim shaped flywheel provides highest specific energy capability
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Composite flywheel Concept Typical specification
Peak power: ~40kW Continuous power: ~30kWMax speed: ~60krpm Stored energy: ~1.5MJ (~400Wh) Operating speed range: ~60krpm30krpm Recoverable energy: ~1MJ (~300Wh)
Kinetic/electrical energy conversion PM brushless motor/generator Halbach magnetised Air-coredWater-cooled Annular carbon fibre composite flywheel rim Integral magnetic bearing system
Integral permanent magnet brushless dc machine
Motor/Generator
ContainmentRimPassive
MagneticBearing
ActiveMagneticBearing
Litz wire(648strands/conductor)
Cooling ducts
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Demonstrator flywheel unit
Carbon fibre compositerim and rotating components
of bearings and electricalmachine
Flywheel with end-capof containment removed
Central hub comprising stationary components of bearing system and electrical machine
Flywheel in containment together with power electronic converter, magnetic bearing amplifiers/controller, coolant system
Flywheel unit in safety vessel
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Aerospace: Current aircraft systems Aircraft loads supplied by combination of hydraulic, pneumatic, electrical and mechanical power
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More-electric aircraft systems Use of electrical power alone will enable global optimisation and system level performance
improvements
AuxiliaryPower Unit
Cabin Air
Electrical Wing Anti-ice
Electrically Driven Hydraulics and/orElectromechanical
Fuel
Air
Electricity (Hotel mode only)Cabin Air
IN: FuelElectrical start
OUT: ThrustElectricity
Typically:10% weight reduction13% lower engine thrust9% reduction in fuel
- corresponding emissions reduction
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More-electric aircraft engine Electrical machines integrated into engine, for starting/generating and power transfer between spools Potentially the only means of delivering future power requirements (>1MW for large aircraft) Current power generation Future power generation/starting
Complex gear System Heavy High maintenance High losses
Radial take-off shaftHP spool starter/generator
LP spool generatorPower electronics
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
Electrical generator
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More-electric aero-engine : HP spool starter-generator
Representative specification for large civil turbo-fan engine
Rotor inner bore 200mmAxial length (overall) 100mmMaximum power 100-150kWStarting torque 150-250NmMaximum operating speed 13,500rpmOver-speed capability 110%Ambient temperature 350-400oC
High temperature environment favours switched reluctance machineRotor is subjected to extreme mechanical loading
severely constrains maximum rotor diameter
Conventional SR topology
Single-piece rotor4-phase 24/18 pole
Modular rotor8-phase24/18 pole
Series of rotor modules attached to a non-magnetic, high-strength hub
Modular SR topology
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More-electric aero-engine : HP spool starter-generator
Clockwise motoring torque from starting position shown:
BA GF DC AH FE CB HG ED BA GF
Modular switched reluctance machine- Two-phases on adjacent teeth excited simultaneously
Similar benefits to conventional short flux path machines in terms of iron loss Non-continuous back-iron limits feasible combinations of rotor poles, stator poles and
phases
Laminated cobaltiron rotor pole
modules
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More-electric aero-engine : LP shaft generator Favours permanent magnet machine
equipped with Samarium Cobalt magnets Requires fault-tolerance
Conventional 3-phase permanent magnet machines
Non-overlapping (concentrated) winding 33-slots / 22-poles
Phase C
Phase B Phase A
Ambient temperature ~150oCSpeed range ~1000 3000rpmMaximum power 250kWWind-milling power 25kW
Overlapping (distributed) winding66-slots / 22-poles
Non-overlapping (concentrated)winding 20-slots / 24-poles
Fault-tolerant 5-phase permanentmagnet machine
Higher phase number Coils wound on alternate teeth Negligible mutual coupling between
phases Coil inductance limits short-circuit
current to rated value
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More-electric aero-engine : LP shaft generator Fault-tolerant permanent magnet machine
Magnetic field distributions
Open-circuit Phase A short-circuit (Negligible mutual coupling with other phases)
5-phase, 40-slots, 28-poles,4-coils per phase
Prototype
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More-electric aero-Engine : LP shaft generator
Fault-tolerant permanent magnet machine
5-phase, 40-slots, 28-poles 20-coils (4-coils/phase)
Terminal short-circuit fault on phase A at rated torque
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More-electric aircraft : Flight control surface actuation Electromechanical actuator
Electrohydrostatic actuator
PM brushless motorGearbox
Ballscrew
End-effector
M PValveBlock
ActuatorAccumulator
PositionController
MotorElectronics MM PP
ValveBlock
ActuatorAccumulator
PositionController
MotorElectronics
pump
motor
powerelectronics
accumulator
actuator
valveblock
Source: Liebherr GmbH
pump
motor
powerelectronics
accumulator
actuator
valveblock
Source: Liebherr GmbH
Integrated variable-speed motor/fixed displacement pump
Flight controlsurfaces
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Consumer products : Current refrigerator compressor technology
Reciprocating compressor driven by rotary motor (1-ph induction motor) via crank mechanism
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%)
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.
Inlet
Hermeticallysealed compressor
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Direct-drive linear compressor For max. efficiency, displacement and flow rate, electrical
supply frequency should coincide with mechanical resonant frequency
where KT = total equivalent spring stiffnessm = total moving mass
Reduces friction loss associated with crank Enables soft start/stop (low noise) Facilitates continuous variable cooling capacity, by varying
frequency (over narrow range) and stroke (although smallamplitude stroke compromises volumetric efficiency)
95% efficiency (electrical-mechanical) achievable
mK
21f Tr =
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Quasi-Halbach magnetised motor
Employs trapezoidal radially and axially magnetised magnets Cross-sectional area of radially magnetised magnets increases with radius
- increases radial flux density in airgap Cross-sectional area of axially magnetised magnets reduces with radius
- increases flux which passes through axially magnetised magnets, rather than mild steel tube. Force density increased
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
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Control of direct-drive linear compressor The mechanical resonant frequency is:
mTK
21
rf =
where KT = total equivalent spring massm = total moving mass
Total equivalent spring stiffness:
KT = k + kg + kc where k = stiffness of suspension springskc = equivalent stiffness of cogging forcekg = equivalent stiffness of compressed refrigerant
Compressor Linear motor
Suction valve
Dischargevalve
Coils Suspensionsprings
Ps
Pd
For max. efficiency, supply frequency needs to track fr
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Linear compressor control Total effective gas stiffness kT varies with operating condition (stroke, evaporator/ambient/condenser
temperatures)Evaporator/ambient/condenser temperatures
Hence, mechanical resonant frequency also varies
For max. efficiency, supply frequency needs to track fr
=
mKf Tr
Stroke (m)
F
r
e
q
u
e
n
c
y
(
H
z
)
S
t
i
f
f
n
e
s
s
(
N
m
)
Stroke (m)
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Linear compressor control
fr occurs at dP/df = 0
Perturbation frequency df = 0.025Hz Perturbation period = 0.2s (~10 cycles)
Piston stroke controlled by varying current supplied from PWM H-bridge Resonant frequency tracked by varying supply frequency and searching for max. power point (MPP)
MPP
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Experimental results 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
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- Single-stage helical gear - External - Internal
Industrial : Magnetic gears
Mechanical Magnetic
Transmitted torque density50 - 150 kNm/m3
Generally requires lubrication/cooling Generates noise/vibration Limited life
SN
SN
Poor utilisation of magnets Low torque transmission capability
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High performance magnetic gears
Principle of operationLow-speed pm rotor
High-speed pm rotor
Stationary pole-pieces
Low-speed pm rotor
High-speed pm rotor
Stationary pole-pieces
Radial flux density waveform
Space harmonic spectrum
4 pole-pair high-speed rotor
Radial flux density waveform
Space harmonic spectrum
4 pole-pair high-speed rotor
Radial flux density waveform
Space harmonic spectrum
27 pole-pieces
Radial flux density waveform
Space harmonic spectrum
27 pole-pieces
5.75:1 gear ratio
23 pole-pair low-speed rotor
4 pole-pair high-speed rotor
27 static pole-pieces
ns = no. of pole-piecesph = pole-pairs on high-speed rotorpl = pole-pairs on low-speed 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 =
Torque transmission capability ~70 kNm/m3
ns = 27ph = 4pl = 23
h
l
pp
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High performance magnetic gears Only 3 components 2 are free to rotate, the 3rd is earthed
Prototype 5.75:1 gear
Torque density: 78kNm/m3
Zero wear and no lubrication Low maintenance/high reliability Inherent overload protection/no jamming
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Other magnetic gear topologies
Rotary: axial-field Linear: radial-field
Stationary pm armature
High-speed pmarmature
Ferromagnetic pole-pieces
Low-speed pmrotor
Axially magnetised permanent magnets
High-speed pmrotor
Radially magnetisedring magnets
Low-speed armaturewith ferromagnetic rings
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Harmonic gears
Circular-splineFlexible-spline(coupled to low-speed shaft)
Wave-generator (driven by high-speed shaft)
Mechanical Magnetic
High-speed rotor is equivalent to wave-generator,and deforms flexible low-speed rotor which rotatesindependently within a rigid outer cylindrical stator
Time-varying sinusoidal variation of airgap lengthmodulates field produced by magnets on low-speedrotor and results in a dominant asynchronous space harmonic which interacts with magnets on stator(& vice-versa)
Oval wave-generator with outer ball bearing coupled to high-speed shaft
Flexible-spline teeth engage with teeth of circular-splinein 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 2tooth displacement of flexible-spline relative to circular-spline
Gear ratio )2(/.
.splinesflexiblecircularonteethofnoinDifference
splinecircularonteethofNo=
high-speed rotor(wave generator)
statorpermanent magnets
bearing
back-iron
low-speed rotor
back-iron
high-speed rotor(wave generator)
statorpermanent magnets
bearing
back-iron
low-speed rotor
back-iron
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Conclusions
Many novel electromagnetic machine and actuator concepts are underdevelopment, 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)
More-electric actuation technologies feature prominently in technology roadmaps for most market sectors
Many design challenges remain, and there are significant opportunitiesfor innovation
There are also many challenges for magnetic materials development