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Recent Advances in Permanent Magnet Machines

Zheda 2011-11-18

Professor Z.Q. Zhu, Fellow IEEE

University of Sheffield UKUniversity of Sheffield, UKEmail: Z.Q.Zhu@Sheffield.AC.UK

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• Before 1980s, pioneering work on PM machines primarily based on ferrite magnets, and also SmCo magnets for aerospace/military application

• In 1980s in addition to switched reluctance machines significant

1980s

In 1980s, in addition to switched reluctance machines, significant advances on

NdFeB magnet IGBT Space-vector PWM Direct torque control / Flux weakening control Microprocessor/DSPp Surface-mounted PM (SPM) and Interior PM (IPM) machine

topologies• Over last 30 years, many new and novel PM machine topologies

developed

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• High Torque• High Power• High Speed• Direct Drive

Current States and Trends

• High Integration Power electronics/machines/controller System integration and design consideration

• New PM Machine Topologies Transverse flux Soft magnetic composite F ti l l t/f lt t l t Fractional slot/fault-tolerant Hybrid PM/IM, PM/SR, PM/SynR, PM/WF PM on stator, including switched flux Variable flux Halbach Dual-mechanical ports/Magnetically geared

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High Power

• PM machines used to be for low power applications• High power density, high power

5MW, 147 rpm PM wind generator by Prokon Nord

For wind power generation

18 MW PM ship propulsion machine by DRS Technologies, Inc

For ship propulsion7

High Speed

• High speed definition is related to power• 10krpm used to be “high speed”• 100krpm is currently commercially available• 500krpm, even 1000krpm, prototypedp p p yp• High power density/Light weight/High system efficiency• Automotive/aerospace/domestic appliance/Industrial

100krpm, SR motor 100krpm, PM motorDyson vacuum cleaner 8Sir James Dyson visited Sheffield

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Direct-drive

• Result in large machine size• High system reliability and efficiency

Siemens Wind Power SWT-3.0-101 direct drive wind turbinesRated power: 3MWTurbine concept: Direct drive, variable pitch control and variable speedGenerator: Outer rotor PMSG6MW DD PM generator also available

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Increased efficiency Integrated linear motor/compressor unit

Reciprocating tubular PM motor

Direct-drive

Refrigerator compressor

• crank for rotary-to-linear motion conversion eliminated• high efficiency PM tubular motor with Halbach magnetisation• Soft magnetic composite is used• stroke and frequency adaptively matched to thermal load

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Fractional Slot PM Machines

• Fractional number of slots per pole per phase• Non-overlapping concentrated winding• Short end-windings• High efficiency and torque density

-AA AA-BA

-AA

A-CCC

-CC

CC

-CB

BB

-BB

BB

-BC

-CC

CC

-CC

-AA AA-BA

-AA

A-CCC

-CC

CC

-CB

BB

-BB

BB

-BC

-CC

CC

-CC

IPM machine with 22-poles, 24-slots

g y q y• Low cogging torque• Reduced reluctance torque, or negligible reluctance torque• Higher mmf harmonics• High rotor iron and magnet loss• High noise and vibration• Improved flux-weakening (higher winding inductance)

C-A

AA-AA AA -AB

-BB

BB

-BBC-A

AA-AA AA -AB

-BB

BB

-BB

g ( g g )• Good fault-tolerant capabilities

Toyota/Asin Rolls RoyceBoschPanasonic Brose

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Fault-tolerant PM Machines

• A type of fractional slot machine• Only alternate teeth carry a concentrated coil• Each phase is electromagnetically, thermally, and

mechanically ‘isolated’y• No. of rotor magnet poles > fundamental stator mmf

pole no.• Torque produced by interaction of high-order stator

mmf harmonic with permanent magnet rotor

E

E’ FF’E

E’ FF’ E

E’ FF’

Open circuit Phase A excited Phase A short-circuited

A

A’

CC’

D

D’

BB’

A

A’

CC’

D

D’

BB’

A

A’

CC’

D

D’

BB’

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Halbach PM Machines

Magnet with novel magnetisation: self-shielding

- Sinusoidal & cosinusoidal circumferential & radial magnetisations

• Inherent sinusoidal airgapfield distribution

• Negligible cogging torque and essentially sinusoidal back-emf waveform - skew not required, non-overlapping windings can pp g gbe employed

• High magnetic loading, even with large airgap

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PASSIVEMAGNETICBEARING

RIM CONTAINMENT

Flywheel Halbach Motor/Generator

ACTIVEMAGNETICBEARING MOTOR/GENERATOR

‘Litz’ wireconductors

‘Halbach’Magnetised Rim High airgap flux density

Ironless machine

CoolingChannels

Low idle loss

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• Aircraft electrical fuel pump

• Halbach magnetised fault-tolerant PM machine with a higher airgapfl d it

Fault-Tolerant Halbach Machines

flux density

• Large airgap can be employed for flooded rotor

4-phase, 8-slot, 6-pole, 2 segments per pole,Halbach fault-tolerant machines

(Newcastle)

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Toyota Advanced i-REAL Personal Mobility Vehicle

http://www.youtube.com/watch?v=lLB1Po5JxGI&feature=fvw

Halbach motor made by Aisin Seiki Co. Ltd. was adopted for the position control unit

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Concept of Hybrid PM Electrical Machines

• Concept of hybrid electrical machines is to uniquely integrate PMs into various conventional electrical machine technologies in order to fully utilize their synergies

• Many novel hybrid PM machine topologies have been developed

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Hybrid IM & PM - Line-Start PMMs

• One of the earliest investigated PM machines• PM into squirrel-cage rotor• Self-start by induction motor action

At h d PM ti• At synchronous speed PM operation • During starting, PMs produce a braking torque &

significant torque oscillation

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Hybrid RM & PM - Inset and Interior PMMs

Features:• PM torque + reluctance torque• Reduced PM flux without compromising

torque densitytorque density• Reduced magnet volume

Toyota Prius: overlapping distributed stator winding / IPM rotor PMM

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Honda insight: non-overlapping concentrated

stator winding / IPM rotor PMM

Inset Radiallymagnetised

Circumferentially magnetised

Hybrid RM & PM - Multi-Layer PMMs

• Saliency ratio/reluctance torque significantly increased

• Wide speed operation is possible

• Overlapping stator winding usually employed• Overlapping stator winding usually employed

• More difficult to fabricate when no of layers very high, usually 2~3

• Potential significant rotor loss

• PM assisted synchronous reluctance machines are under intensive investigation due to recent soaring price of rare earth magnet

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Price for NdFeB magnets is soaring!

Recent Development of Magnetless Machines

Synchronous reluctance machines and PM assisted synchronous reluctance machines become attractive and under extensive investigation

ABB have developed synchronous reluctance machine for industrial applications. It shows improved efficiency over conventional induction machines

(ABB)

Recent Development of Magnetless Machines

Synchronous Reluctance Machine PM free machine Utilising reluctance torque Inherently failure safe and no need to protect

converters from over voltage

IPM machine technology With added ferrite magnets or a small amount of rare

PM Assisted Synchronous Reluctance Machine= Synchronous reluctance motor + Ferrite magnet

or a small amount of rare earth magnet

converters from over voltage Possible lower torque density Potentially lower efficiency and power factor

earth magnets, power density, efficiency and power factor improved, but may be lower than conventional IPM machine employing rare earth magnets (e.g. 75%)

Excellent high speed power capability Ferrite magnets may experience demagnetisation

problem which can be solved by improving the design of flux barriers and iron bridges

Hybrid RM & PM – PMMs with Q-Axis Flux Barrier

Q-axis flux barrier:

To reduce crossing coupling saturation effect

• Reduced cross-coupling

• Reduced cross-coupling magnetic saturation

• Minimal effect on d-axis flux

• Significantly improved position estimation accuracy

• Significantly enhanced peak torque capability

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torque capability

• Significantly reduced acoustic noise

• Reduced reluctance torque or even negative

Hybrid SM & PM – PMMs Having PMs on Stator

• PM on stator • Salient pole stator

with non-overlappingoverlapping, concentrated stator winding

• Salient pole rotor without winding and magnet

Doubly salient Flux reversal

• Reluctance action• Negligible

reluctance torqueSwitched flux

24Switched flux PM machines exhibit significantly higher torque density than doubly salient/flux reversal PM machines

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1-phase switched-flux PM (SFPM) brushless machine (1955)

Switched Flux PM Machines

(a) (b)

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Main flux is reversed when rotor rotates one stator pole pitch, i.e. from (a) to (b).

(a) (b)

3-phase1-phase

Switched Flux PM Machines

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• Low cost application• Limited angle: actuator

– Laws relay

• Smooth torque• 3-ph, 12/10 stator/rotor pole

0

3

6

kage

(mW

b) D-axisQ-axis

• When rotor pole aligns with one side stator tooth– neg. max flux linkage

• When rotor pole aligns with another side stator

Switched Flux PM Machines

-6

-3

0 60 120 180 240 300 360

Rotor position (electrical degree)

Flux

-lin with another side stator

tooth– pos. max flux linkage

• When rotor pole/slot aligns with magnet– zero flux linkage

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Switched Flux PM Machines

• Simple and robust rotor• Easier to manage magnet

• Reduced copper area• Low over-load capability due to

h i

Advantages Disadvantages

• Easier to manage magnet temperature rise

• Flux focusing / low cost ferrite magnets may be used

• Sinusoidal back-emf waveform –suitable for brushless AC operation

heavy saturation• Complicated stator• Leakage outside stator• High magnet volume

operation

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Reduction of magnet usage:

Switched Flux PM Machines

Number of magnet halved but torque is increased

E-coreConventional Multi-tooth

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C-core

5

6

7

8

(Nm

)

FE predictedRated current

Switched Flux PM Machines

2.5

3

3.5

m)

0

1

2

3

4

0 10 20 30 40 50 60

Q-axis current (A)

Torq

ue

Conventional, 12/10

E-core, 6/11

C-core, 6/13Multi-tooth, 6/19

Measured

300

0.5

1

1.5

2

0 2 4 6 8 10 12 14 16 18

Q-axis current (A)

Torq

ue (N

Conventional, 12/10E-core, 6/11C-core, 6/13

Rated current

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Mismatch between Machine High Efficiency & Driving Cycles

• High torque/power density

• High torque for starting at low

EV/HEV Torque/Power, Speed, & Efficiency Requirements

High torque for starting, at low speeds and hill climbing, and high power for high speed cruising

• Wide operating speed range

• High efficiency over wide speed and torque ranges, particularly at low torque operation (partial load)

Need for system level design consideration

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Variable Flux PM Machines

Rare Earth PM Machines Advantages:

• High torque density

• High efficiency

Disadvantages:

• Irreversible demagnetisation

• Expensive magnet and limited resources

• Not adjustable flux

Variable flux

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Variable Flux PM Machines

Means for varying flux:

• Mechanical

• Electric

Excitation flux path topology:

• Series

• Parallel

C il it ti l ti

VFPM machines

Hybrid (PM + field coil)

Mechanically adjusted PM

Other typesMemory

Coil excitation location:

• Stator

• Rotor

Series flux paths

Parallel flux paths

Rotor PM’s + stator coils

Rotor PM’s + rotor coils

Stator PM’s + stator coils

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Mechanically Adjusted Variable Flux PM Machines

Stator  

Armature windingArmature winding 

Rotor  

Magnets 

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Mechanically Adjusted Variable Flux PM Machines

Based on Switched Flux PM machine

Normal

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Based on consequent-pole PMM

Various Hybrid PM and Coil Excited VF PM MachinesBased on hybrid stepper PMM

Based on

Based on claw-pole PMM

Based on switched flux PMM

Based on conventional PMM

Based on doubly-salient PMM

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Easy to achieve constant power operation (flux weakening) Potentially enhanced low speed torque Reduced risk of high open-circuit back-emf at high speed during

Hybrid PM and Coil Excited Machines

Advantages:

Reduced risk of high open circuit back emf at high speed during flux weakening

High efficiency operation possible

Complicated structure Torque density likely reduced

Disadvantages:

Torque density likely reduced Limited flux enhancing capability due to magnetic saturation Extra DC source required, or Extra mechanical means required for mechanically adjusted flux

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Based on SFPM machine

A Novel Hybrid PM and Coil Excited Machine

F1F3

1

1.2

F4

F6

0

0.2

0.4

0.6

0.8

1

Torq

ue (N

m)

11-rotor poles, 2D FE

13-rotor poles, 2D FE

11-rotor poles, measured

13-rotor poles, measured

-20 -15 -10 -5 0 5 10 15 20

DC excitation current (A)

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Variable Flux Memory PM Machines

• Changing magnetization intensity of Alnico type of magnets by a very short pulse current via stator windings and memorizing flux densit le el in the rotor magnetsdensity level in the rotor magnets.

• Significant advantages, such as simple structure/high efficiency/large starting torque/wide operation speed range

• Many topologies possibley p g p

• Significant potential

• Toshiba washing machine employing memory motor for washing and dry spinning

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(V. Ostovic)

• Three parts: one stator, two rotors and two airgaps

• Two rotors can be assigned

Dual-Mechanical Port Machines

garbitrary , having independent mechanical speeds and torque

• Dual-rotor PM machine is a special type of dual-mechanical-port machine

• May be considered as a double-fed o nd field rotor ind ction

(Ohio - Prof. L.Y. Xu)

fed wound-field rotor induction machine with a permanent magnet rotor sandwiched between its stator and rotor

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Dual rotor PM machines(with different rotor speeds) housing

water channels

outer rotor

stator

Dual-Mechanical Port PM Machine for HEV Application

axial air holesshaft

permanent magnets

air gaps

inner rotorouter rotor

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(HIT - Prof. P. Zheng )

Disadvantages: Complex mechanical structure and need of brushes/slip rings

Low-speed pm rotorStationary pole-

pieces

Low-speed pm rotorStationary pole-

pieces

Magnetically Geared PM Machine

High-speed pm rotorHigh-speed pm rotor

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g p pg p p

Integrated magnetic gear and PM machine Within same space envelop of PM machine High speed machine potentially for low

speed application

Zero wear and no lubrication Low maintenance/high reliability Inherent overload protection /no

jamming

Magnomatics Ltd: Sheffield University spin-off company promoting commercial exploitation of such system

Over last 30 years, significant advances on magnetic materials, electrical machines, power electronics, microprocessors and controls

PM brushless machines have high power/torque density and high efficiency

Now used extensively in different market sectors, including aerospace,

Summary

y , g p ,automotive, marine, industry, power generation, consumer products: such as lawn mower/vacuum cleaner/refrigerator/air conditioner

Numerous new and novel PM machine topologies have been developed and are still emerging

Challenges and Opportunities

System level design considerations are essential for different applicationsy g pp

Rare-earth PMs still relatively expensive

Concerns about their resources and prices

“Magnetless” machines, using no magnet or less magnet, are being seriously investigated

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City of Sheffield – a city of innovation• One of the origins of industrial revolution• Origin of modern football clubs• Excellent environment• Low living cost

Sheffield

g

University of Sheffield – engineering matters• One of Top 10 UK research lead universities• One of the largest and strongest Engineering Faculties in the UK

Department of Electronic and Electrical Engineering• Historically gained the top 5* ratings in successive Research Assessment

Exercises (RAEs) over recent decades 95% f it h ti it i ‘i t ti ll i d hi h ’ [RAE 18• 95% of its research activity is ‘internationally recognised or higher’ [RAE, 18 Dec. 2008]

• Host of National Central Facility for III-V Semiconductors

First Class Researchers and Excellent Research

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Electrical Machines and Drives Research Group

Academic staff membersDr Kais Atallah (Electrical Machines)Dr Martin Foster (Power Electronics)P f G i t J ll (El t i l M hi )

Headed by Prof. Z.Q. Zhu, IEEE, Fellow

Professor Geraint Jewell (Electrical Machines)Professor Shankar Madathil (Power Electronics)Mr Ken Mitchell (Electrical Machines)Dr David Stone (Power Electronics)Professor Jiabin Wang (Electrical Machines /control)Professor Qiang Zhu (Electrical Machines /control)Professor under recruitmentLecturer/Senior Lecturer under recruitmentLecturer/Senior Lecturer under recruitmentLecturer/Senior Lecturer under recruitment

Academic 8+3Academic Related 1Research Associates 14Postgraduates 50

Visitors 2Technicians 4Secretarial/Administrative 1Total: 83

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Electrical Machines and Drives Research Group

1 of 3Main Research Groups inDept of Electronic & Electrical Engineering

HostThe Rolls-Royce University Technology Centre in ‘Advanced Electrical Machines & Drives’

Sheffield Siemens Wind Power Research Centre (S2WP)

Birthplace3 university spin off companies3 university spin-off companies

Annual Research Income (average over last 3 years)~£3M

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• Encompass all facets of permanent magnet machines and drive systems (power electronics, electromagnetic devices, electrical systems and control systems)

Research Activities

• Capability for producing and testing demonstrator systems

• Strong industrial collaboration, particularly in automotive, wind power and aerospace sectors (e.g. Toyota, Nissan, Fiat, BMW, Siemens, Rolls Royce, Goodrich etc.)

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• Establish in November 2003• Focussed largely on electric systems for aerospace gas-turbines

(with some marine applications)• Director: Professor G.W. Jewell

Rolls-Royce UTC in ‘Advanced Electrical Machines & Drives’

• Directly employs 3.5 research staff and 8 PhD students

LP spool generatorPower electronics

HP spool starter/generator

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• Siemens external competence centre, established in 2009

• Will be R&D centre for Siemens Wind Power• Based at Kroto Innovation Centre at University of

Sheffield (currently 210m2 and will be expanded to 500 2 l t thi )

Sheffield Siemens Wind Power Research Centre (S2WP)

500m2 later this year)• To develop most reliable, innovative and efficient

onshore and offshore wind turbine generator systems for global market

• Academic director: Prof. Z.Q. Zhu• Collaborating with The Electrical Machines and Drives

Group, including access to Group’s expertise and state-of-the-art facilities 50:50 Siemens : University employed researchers• 50:50 Siemens : University employed researchers

• Currently 15 researchers + 5 offered, to be 50 with PhD qualification within 3 years

• Cover electromagnetic design and analysis, as well as mechanical and control aspects

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• 主任 诸自强教授

• 上海张江-国家级高新技术开发区

• 5年内，成为国内一流的电机及其驱动控制研发机构，并在国际上具备一定的影响力

美的集团威灵上海研发中心

研发中心

电机研发部 驱动控制部 行政部

• 10年内，成为国际一流的电机及其驱动控制研发机构

• 成为美的电机产业战略发展的技术驱动力

• 开拓高新产品领域，打造自主的现代电机核心技术

• 前瞻性应对传统产品升级换代，提升现有产品技术竞争力

从事电机本体研发设计

从事驱动控制研发设计

从事综合运营、人力、

财务、行政后勤等工作

品技术竞争力

• 培育高层次、高水平的技术人才

• 5 年内，投入2亿, 人员总数为80-100 人

• 由海内外博士、硕士构成

• 每年公派一名优秀员工赴英国谢菲尔德大学攻读博士学位