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

Electrical machines and actuators

Energy efficiencyEmissions

::

Drive-by-wireFly-by-wireEmbedded generation‘More-electric’ ships

::

PerformanceFunctionalityReliability/maintainabilitySafety

:

Becoming more fierceAdoption of advancedtechnologies

::

Applies to all market sectorsAutomotiveAerospaceMarineConsumer productsetc.

Technology development

Legislation

Competition

Consumer expectations

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

Exhaust gas energy recoveryNaturally aspirated engine Has potential to reduce size, or eliminate,

conventional alternator and load imposed on engine

Favours switched reluctance machine (SR) machineHigh temperature:

- ~900°C at turbine- ~300°C at machine rotor

High speed:- up to 80krpm

SR machine design

Cedrat FLUX2D

Maximum speed: 80,000 rpmMaximum power: 6 kWAverage power: 2.3 kW

3-phase 6:4 SR machineFundamental 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

SR machine designStranded conductor used to minimise highfrequency eddy current loss

Coolant temperature in cooling jacket 90°C

Temperature distribution at rated powerCurrent density distribution with5-turns, 19-strand conductor

Frame

Stator

19 strandconductor

Optimal control angle trajectoriesSwitch-on (θon) and dwell (θdw) angles determine SR machine power and losses, peak/rms current and VA rating of converter

Optimum θon and θdw 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 θon and θdw varied

SR machine

Specification

Generator voltage 12VGenerator efficiency >70%Water-cooledLocation – pre-catalystSensorless rotor position control Sealed for life bearingsLength ~150mm, weight ~7kgMaximum output power 6kW @ 80krpm

Dynamometer testing

Turbogenerator

Turbine, guide vane and base-plate Complete assembly

Turbine and generator sized for highest IC engine residency operating pointNecessary to by-pass turbine when engine operating at peak power

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

Cold-air rig testingTIGERS turbine

Electronically controlledwaste-gate

Compressor air

TIGERS SR machine

Engine dynamometer testing

Will enable influence of increased EBP on fuel consumption to be assessed

Electrical torque-boosting of down-sized IC engine

0

50

100

150

200

250

300

0 1000 2000 3000 4000 5000

Speed (rpm)

Torq

ue (N

m) 3.0L NA

1.8L TC

Benefits of down-sizingReduction in fuel consumptionReduced emissionsLower weightComparable performance at high engine speeds

Down-sized IC engine exhibits reduced torque at low engine speeds

Electrical torque and power requirementsTorque 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

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

Simulation of torque-boost systemDrive-away cycle Power and energy consumption

Acceleration from 0 to 100 km/h in 18 secondsGear shift at 2200 rpmRegenerative 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)

EST

Pow

er (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

EST

Ener

gy (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

3-phase PM brushless torque-boost machine

22-poles, 24-slots

Annular space envelope necessitatesa high pole numberInterior 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

3-phase, PM brushless torque-boost machineDynamometer testing

Total mass: 17.2kgPeak current @132Nm: 650AEfficiency @1069rpm, 132Nm: 93%Idling loss @3000rpm: 390W

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

α, β

d, q

PIController

Vα

Vβ

Vd

Vq

α, β

d, q

a, b, c

α, β

M

HybridObserver

+

- +

-

Sh

Ch

Id*

Iq*

Id

Iq

qError

dError

Iα

Iβ

IaIb

Ic

ha hb hc

ABC

SVPWMModulator

α, β

d, q

PIController

Vα

Vβ

Vd

Vq

α, β

d, q

a, b, c

α, β

M

HybridObserver

+

- +

-

Sh

Ch

Id*

Iq*

Id

Iq

qError

dError

Iα

Iβ

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

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, ω

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

Supercapacitor energy storage unit

36, 3500F, 2.7V max. supercapacitors

Efficiency map of torque-boost system

⎪⎪⎭

⎪⎪⎬

⎫

⎪⎪⎩

⎪⎪⎨

⎧

+=

∫∫

∫∫

d

d

c

c

Tdcdc

T

T

Tdcdc

midttitv

dttTt

dttTt

dttitv

)()(

)()(

)()(

)()(

21

ω

ωη

∫∫

=

c

d

Tdcdc

Tdcdc

scdttitv

dttitv

)()(

)()(η

∫∫

=

c

d

T

Test

dttTt

dttTt

)()(

)()(

ω

ωη

Average efficiency of machine & inverter

Average efficiency of supercapacitors

Average efficiency of torque-boostsystem

Free-piston energy converter

Battery

Traction drive

ICE

Generator

Floating piston – eliminates crankshaftPiston motion controlled by electrical machineFacilitates optimum combustion (HCCI/ACI)

Series hybrid vehicle

2-stroke unit

Piston Inlet port

Tubular permanent magnet machine Exhaust valve

Moving-magnet armature

Tubular electrical machineNo end-windings, high power density and volumetric efficiency

Phase A Phase B Phase C

Magnets

r

Supporting tube

Titaniumtube

Modular stator winding9-slot/10-pole/12-coilsLow cogging forceSinusoidal emf

Quasi-Halbach magnetised armature15-poles (10-poles active)Negligible flux on inner boreLow moving mass

Tubular electrical machinePhase winding Assembled machine excluding

water-cooled jacket

44 kW rated output power (4kN@11m/s)

Stator mmf space harmonic distribution5th 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

Design optimisationMain design parameters: Rm/Re, τp/Re, τmr/τp 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

Effic

ienc

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

Pow

er fa

ctor

Output

Power = 44 kW

Optimum τmr/τp = 0.625Optimum τp/Re = 0.25

Free-piston energy converterEfficiency map of machine/converter

Switching frequency 25kHzPrototype

Flywheel energy storage/peak power buffer

Potential benefitsHandles peak power during acceleration/decelerationEnables kinetic energy recoveryPrimary energy source independent from high power demand (enhanced efficiency,extended lifetime, etc)

Improved vehicle performance/responseFrom 2009, kinetic energy recovery systems will be permitted on Formula 1 cars

Max. energy released per lap ≤400kJMax. power in or out ≤60kW

Motor/Generator

PowerElectronics

EnergyStore Inverter Drive

Motor

VehicleManagement Unit

Vehicle controlFlywheel unit

Vehicle drive train

RoadPower

Pow

er

Pow

er

Pow

er

Flywheel shapes

Flywheel energy storage/peak power bufferSpecific energy storage capability determined by tensile strength and density of flywheel material and geometry

where σ = design stress of materialρ = densityk = shape factor

kg/JkEρσ

=

3m/J6.0ME

ρδ

=⎟⎠⎞

⎜⎝⎛

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

ρδ

=⎟⎠⎞

⎜⎝⎛

Fibre composite materials have highest tensile strength to density ratio, and rim shaped flywheel provides highest specific energy capability

Composite flywheelConcept Typical specification

Peak power: ~40kWContinuous power: ~30kWMax speed: ~60krpmStored energy: ~1.5MJ (~400Wh)Operating speed range: ~60krpm→30krpmRecoverable energy: ~1MJ (~300Wh)

Kinetic/electrical energy conversionPM brushless motor/generatorHalbach magnetisedAir-coredWater-cooledAnnular carbon fibre composite flywheel rim

Integral magnetic bearing systemIntegral permanent magnet brushless dc machine

Motor/Generator

ContainmentRimPassive

MagneticBearing

ActiveMagneticBearing

Litz wire(648strands/conductor)

Cooling ducts

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

Aerospace: Current aircraft systemsAircraft loads supplied by combination of hydraulic, pneumatic, electrical and mechanical power

‘More-electric’ aircraft systemsUse 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

‘More-electric’ aircraft engineElectrical machines integrated into engine, for starting/generating and power transfer between spoolsPotentially 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

‘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

‘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 lossNon-continuous back-iron limits feasible combinations of rotor poles, stator poles and phases

Laminated cobaltiron rotor pole

modules

‘More-electric’ aero-engine : LP shaft generator

Favours permanent magnet machine equipped with Samarium Cobalt magnetsRequires 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 numberCoils wound on alternate teethNegligible mutual coupling between phasesCoil inductance limits short-circuit current to rated value

‘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

‘More-electric’ aero-Engine : LP shaft generator

Fault-tolerant permanent magnet machine

5-phase, 40-slots, 28-poles20-coils (4-coils/phase)

Terminal short-circuit fault on phase A at rated torque

‘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

Consumer products : Current refrigerator compressor technology

Reciprocating compressor driven by rotary motor (1-ph induction motor) via crank mechanism

Piston stroke fixed by crankSignificant friction loss in compressorOn/off duty cycle of fixed-speed compressor determined by refrigerator temperature setting and loadOverall 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

Direct-drive linear compressorFor max. efficiency, displacement and flow rate, electricalsupply frequency should coincide with mechanical resonant frequency

where KT = total equivalent spring stiffnessm = total moving mass

Reduces friction loss associated with crankEnables soft start/stop (low noise)Facilitates continuous variable cooling capacity, by varyingfrequency (over narrow range) and stroke (although smallamplitude stroke compromises volumetric efficiency)

95% efficiency (electrical-mechanical) achievable

mK

21f T

r π=

Quasi-Halbach magnetised motor

Employs trapezoidal radially and axially magnetised magnetsCross-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

Control of direct-drive linear compressorThe mechanical resonant frequency is:

mTK

21

rf π=

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

Linear compressor controlTotal 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 T

r

Stroke (m)

Freq

uenc

y (H

z)

Stiff

ness

(Nm

)

Stroke (m)

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-bridgeResonant frequency tracked by varying supply frequency and searching for max. power point (MPP)

MPP

Experimental resultsInitial 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

- Single-stage helical gear - External - Internal

Industrial : Magnetic gears

Mechanical Magnetic

Transmitted torque density50 - 150 kNm/m3

Generally requires lubrication/coolingGenerates noise/vibrationLimited life

SN

SN

Poor utilisation of magnetsLow torque transmission capability

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 transmissionPole-pieces modulate fields produced by pm rotors, resulting in asynchronous space harmonic fieldsHighest asynchronous space harmonic utilised for torque transmission when ph = ns – plGear ratio =

Torque transmission capability ~70 kNm/m3

ns = 27ph = 4pl = 23

h

l

pp

High performance magnetic gearsOnly 3 components2 are free to rotate, the 3rd is earthed

Prototype 5.75:1 gear

• Torque density: 78kNm/m3

Zero wear and no lubricationLow maintenance/high reliabilityInherent overload protection/no jamming

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

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

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