Zheda 2011-11-18
Recent Advances in Permanent Magnet Machines
Professor Z.Q. Zhu, Fellow IEEE
University of Sheffield
Sheffield, UK
Email: Z.Q.Zhu@Sheffield.AC.UK
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Current States and Trends
1980s
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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
1980s, in addition to switched reluctance machines
machines, significant
advances on
 NdFeB magnet
 IGBT
 Space-vector PWM
 Direct torque control / Flux weakening control
 Microprocessor/DSP
p
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 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
High Integration
 Power electronics/machines/controller
 System integration and design consideration
New PM Machine Topologies
 Transverse flux
 Soft magnetic composite
 Fractional
F ti
l slot/fault-tolerant
l t/f lt t l
t
 Hybrid PM/IM, PM/SR, PM/SynR, PM/WF
 PM on stator, including switched flux
 Variable flux
 Halbach
 Dual-mechanical ports/Magnetically geared
High Power
High Speed
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• PM machines used to be for low power applications
• High power density, high power
18 MW PM ship propulsion machine
by DRS Technologies, Inc
For ship propulsion
5MW, 147 rpm PM wind generator
by Prokon Nord
For wind power generation
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High speed definition is related to power
10krpm used to be “high speed”
100krpm is currently commercially available
500krpm,
p even 1000krpm,
p p
prototyped
yp
High power density/Light weight/High system efficiency
Automotive/aerospace/domestic appliance/Industrial
100krpm, SR motor
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100krpm, PM motor
Dyson vacuum cleaner
Sir James Dyson visited Sheffield
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2
Direct-drive
Direct-drive
Refrigerator compressor
• Result in large machine size
• High system reliability and efficiency
Reciprocating
tubular PM motor
 Integrated linear motor/compressor unit
 Increased efficiency
Siemens Wind Power SWT-3.0-101 direct drive wind turbines
Rated power: 3MW
Turbine concept: Direct drive, variable pitch control and variable speed
Generator: Outer rotor PMSG
6MW DD PM generator also available
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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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Fault-tolerant PM Machines
Fractional Slot PM Machines
BB
-BA
-AA
AA
-A
B
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A
-C
A
BB
-B
CC
-CC
-CC
CC
IPM machine with 22poles, 24-slots
CC
-CB
-CC
-B
B
C-A
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BB
AA
-AA
AA
-BB
Fractional number of slots per pole per phase
Non-overlapping concentrated winding
Short end-windings
High
g efficiency
y and torque
q density
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
g ((higher
g
winding
g inductance))
-BC
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-AB
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E’
E’
F
D’
A
A’
D
C’
B
C
Panasonic
B’
Open circuit
Bosch
Brose
Toyota/Asin
Rolls Royce
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E’
F
D’
A
A’
D
C’
B
C
F
F’
E
F’
E
F’
E
• Good fault-tolerant capabilities
A type of fractional slot machine
Only alternate teeth carry a concentrated coil
Each phase is electromagnetically, thermally, and
mechanicallyy ‘isolated’
No. of rotor magnet poles > fundamental stator mmf
pole no.
Torque produced by interaction of high-order stator
mmf harmonic with permanent magnet rotor
B’
Phase A excited
D’
A
A’
D
C’
B
C
B’
Phase A short-circuited
12
3
Halbach PM Machines
Flywheel Halbach Motor/Generator
Magnet with novel magnetisation: self-shielding
RIM
- Sinusoidal & cosinusoidal circumferential & radial magnetisations
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CONTAINMENT
PASSIVE
MAGNETIC
BEARING
ACTIVE
MAGNETIC
BEARING
Inherent sinusoidal airgap
field distribution
Negligible cogging torque
and essentially sinusoidal
back-emf waveform - skew
not required, nonoverlapping
pp g windings
g can
be employed
High magnetic loading,
even with large airgap
MOTOR/GENERATOR
 High airgap flux density
 Ironless machine
‘Halbach’
Magnetised Rim
‘Litz’ wire
conductors
 Low idle loss
Cooling
Channels
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Fault-Tolerant Halbach Machines
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Toyota Advanced i-REAL Personal Mobility Vehicle
• Aircraft electrical fuel pump
• Halbach magnetised fault-tolerant
PM machine with a higher airgap
fl density
flux
d
it
• Large airgap can be employed for
flooded rotor
4-phase, 8-slot, 6-pole, 2 segments per pole,
Halbach fault-tolerant machines
(Newcastle)
http://www.youtube.com/watch?v=lLB1Po5JxGI&feature=fvw
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Halbach motor made by Aisin Seiki Co. Ltd.
was adopted for the position control unit
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4
Concept of Hybrid PM Electrical Machines
Hybrid IM & PM - Line-Start PMMs
• 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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One of the earliest investigated PM machines
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PM into squirrel-cage rotor
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Self-start by induction motor action
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At synchronous
h
speed
d PM operation
ti
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During starting, PMs produce a braking torque &
significant torque oscillation
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Hybrid RM & PM - Inset and Interior PMMs
Hybrid RM & PM - Multi-Layer PMMs
Features:
• PM torque + reluctance torque
• Reduced PM flux without compromising
torque density
• Reduced magnet volume
Toyota Prius: overlapping distributed
stator winding / IPM rotor PMM
Inset
Radially
magnetised
Circumferentially
magnetised
Honda insight: nonoverlapping concentrated
stator winding / IPM rotor PMM
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Saliency ratio/reluctance torque significantly increased
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Wide speed operation is possible
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Overlapping stator winding usually employed
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More difficult to fabricate when no of layers very high, usually 2~3
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Potential significant rotor loss
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PM assisted synchronous reluctance machines are under intensive
investigation due to recent soaring price of rare earth magnet
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Recent Development of Magnetless Machines
Recent Development of Magnetless Machines
 Price for NdFeB magnets is soaring!
Synchronous Reluctance Machine
 PM free machine
 Utilising reluctance torque
 Inherently failure safe and no need to protect
converters from over voltage
 Possible lower torque density
 Potentially lower efficiency and power factor
PM Assisted Synchronous Reluctance Machine
= Synchronous reluctance motor + Ferrite magnet
or a small amount of rare earth magnet
 IPM machine technology
 With added ferrite magnets or a small amount of rare
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
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)
Hybrid RM & PM – PMMs with Q-Axis Flux Barrier
Hybrid SM & PM – PMMs Having PMs on Stator
Q-axis flux barrier:
• PM on stator
To reduce crossing coupling
saturation effect
• Salient pole stator
with nonoverlapping,
overlapping
concentrated
stator winding
• Reduced cross-coupling
magnetic saturation
• Minimal effect on d-axis flux
• Significantly improved
position estimation accuracy
• Significantly enhanced peak
torque capability
• Significantly reduced
acoustic noise
• Reduced reluctance torque or
even negative
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• Salient pole rotor
without winding
and magnet
Doubly salient
Flux reversal
• Reluctance action
• Negligible
reluctance torque
Switched flux
Switched flux PM machines exhibit significantly higher torque density
than doubly salient/flux reversal PM machines
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Switched Flux PM Machines
Switched Flux PM Machines
1-phase switched-flux PM (SFPM)
brushless machine (1955)
(a)
3-phase
1-phase
(b)
• Low cost application
• Limited angle: actuator
– Laws relay
Main flux is reversed when rotor rotates one
stator pole pitch, i.e. from (a) to (b).
• Smooth torque
• 3-ph, 12/10 stator/rotor pole
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Switched Flux PM Machines
Flux-linkage (mWb)
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D-axis
3
Q-axis
0
-3
-6
0
60
120
180
240
300
Rotor position (electrical degree)
360
Switched Flux PM Machines
• When rotor pole aligns
with one side stator tooth
– neg. max flux linkage
• When rotor pole aligns
with another side stator
tooth
– pos. max flux linkage
• When rotor pole/slot aligns
with magnet
– zero flux linkage
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Advantages
• Simple and robust rotor
• Easier to manage magnet
temperature rise
• Flux focusing / low cost ferrite
magnets may be used
• Sinusoidal back-emf waveform –
suitable for brushless AC
operation
Disadvantages
• Reduced copper area
• Low over-load capability due to
h
heavy
saturation
i
• Complicated stator
• Leakage outside stator
• High magnet volume
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Switched Flux PM Machines
Switched Flux PM Machines
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FE predicted
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Reduction of magnet usage:
Rated current
Torque (Nm)
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Number of magnet halved
but torque is increased
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4
Conventional, 12/10
3
E-core, 6/11
2
C-core, 6/13
3.5
Multi-tooth, 6/19
1
0
10
20
3
40
2.5
Q-axis current (A)
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Torque (Nm)
0
50
60
Measured
2
1.5
Rated current
1
Conventional
Multi-tooth
E-core
Conventional, 12/10
E-core, 6/11
C-core, 6/13
C-core
0.5
0
0
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High torque for starting
starting, at low
speeds and hill climbing, and
high power for high speed
cruising
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8
10
12
30
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16
18
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Variable Flux PM Machines
Rare Earth PM Machines
EV/HEV Torque/Power, Speed, &
Efficiency Requirements
High torque/power density
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Q-axis current (A)
Mismatch between Machine High Efficiency & Driving Cycles
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2
Advantages:
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High torque density
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High efficiency
Disadvantages:
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Wide operating speed range
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Irreversible demagnetisation
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High efficiency over wide
speed and torque ranges,
particularly at low torque
operation (partial load)
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Expensive magnet and limited resources
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Not adjustable flux
Variable flux
Need for system level design
consideration
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Variable Flux PM Machines
Mechanically Adjusted Variable Flux PM Machines
Means for varying flux:
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Mechanical
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Electric
Excitation flux path topology:
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Series
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Parallel
VFPM machines
Hybrid (PM + field coil)
Mechanically adjusted PM
Other types
Memory
Stator Armature winding
Armature
winding
Rotor Magnets C il excitation
Coil
it ti llocation:
ti
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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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Various Hybrid PM and Coil Excited VF PM Machines
Mechanically Adjusted Variable Flux PM Machines
Based on consequent-pole PMM
Based on hybrid stepper PMM
Based on Switched Flux PM machine
Based on
conventional PMM
Normal
Based on claw-pole PMM
Based on switched flux PMM
Based on doublysalient PMM
Closed
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Hybrid PM and Coil Excited Machines
A Novel Hybrid PM and Coil Excited Machine
Advantages:
F3
F6
Torque (Nm)
1
Disadvantages:
0.8
0.6
11-rotor poles, 2D FE
0.4
13-rotor poles, 2D FE
11-rotor poles, measured
0.2
 Complicated structure




F1
1.2
F4
Based on SFPM machine
 Easy to achieve constant power operation (flux weakening)
 Potentially enhanced low speed torque
 Reduced risk of high open
open-circuit
circuit back
back-emf
emf at high speed during
flux weakening
 High efficiency operation possible
13-rotor poles, measured
0
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
-20
-15
-10
-5
0
5
10
15
20
DC excitation current (A)
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Variable Flux Memory PM Machines
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(V. Ostovic)
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Dual-Mechanical Port Machines
Changing magnetization intensity
of Alnico type of magnets by a
very short pulse current via stator
windings and memorizing flux
densit level
density
le el in the rotor magnets
magnets.
Significant advantages, such as
simple structure/high
efficiency/large starting
torque/wide operation speed
range
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Manyy topologies
p g
p
possible
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Significant potential
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Toshiba washing machine
employing memory motor for
washing and dry spinning
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Three parts: one stator, two
rotors and two airgaps
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Two rotors can be assigned
g
arbitrary , having independent
mechanical speeds and torque
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Dual-rotor PM machine is a
special type of dual-mechanicalport machine
•
May be considered as a doublefed wound-field
o nd field rotor ind
induction
ction
machine with a permanent
magnet rotor sandwiched
between its stator and rotor
(Ohio - Prof. L.Y. Xu)
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Dual-Mechanical Port PM Machine for HEV Application
Dual rotor PM machines
(with different rotor speeds)
Magnetically Geared PM Machine
Stationary polepieces
housing
Low-speed
pm rotor
water channels
stator
outer rotor
inner rotor
permanent magnets
air gaps
shaft
axial air holes
High-speed
g p
pm
p rotor
Disadvantages:
Complex mechanical structure and need
of brushes/slip rings
 Zero wear and no lubrication
 Low maintenance/high reliability
 Inherent overload protection /no
jamming
(HIT - Prof. P. Zheng )
 Integrated magnetic gear and PM machine
 Within same space envelop of PM machine
 High speed machine potentially for low
speed application
Magnomatics Ltd: Sheffield University spin-off company
promoting commercial exploitation of such system
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Summary
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Sheffield
 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
y in different market sectors,, including
g aerospace,
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
City of Sheffield – a city of innovation
• One of the origins of industrial revolution
• Origin of modern football clubs
• Excellent environment
• Low living
g cost
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% off its
it research
h activity
ti it is
i ‘internationally
‘i t
ti
ll recognised
i d or hi
higher’
h ’ [RAE,
[RAE 18
Dec. 2008]
• Host of National Central Facility for III-V Semiconductors
Challenges and Opportunities
 System
y
level design
g considerations are essential for different applications
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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 First Class Researchers and Excellent Research
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Electrical Machines and Drives Research Group
Electrical Machines and Drives Research Group
Headed by Prof. Z.Q. Zhu, IEEE, Fellow
Academic staff members
Dr Kais Atallah
Dr Martin Foster
P f
Professor
Geraint
G i t Jewell
J
ll
Professor Shankar Madathil
Mr Ken Mitchell
Dr David Stone
Professor Jiabin Wang
Professor Qiang Zhu
Professor
Lecturer/Senior Lecturer
Lecturer/Senior Lecturer
Academic
Academic Related
Research Associates
Postgraduates
1 of 3
Main Research Groups in
Dept of Electronic & Electrical Engineering
(Electrical Machines)
(Power Electronics)
(El t i l M
(Electrical
Machines)
hi
)
(Power Electronics)
(Electrical Machines)
(Power Electronics)
(Electrical Machines /control)
(Electrical Machines /control)
under recruitment
under recruitment
under recruitment
8+3
1
14
50
Visitors
Technicians
Secretarial/Administrative
Total:
Host
The Rolls-Royce University Technology Centre in ‘Advanced
Electrical Machines & Drives’
Sheffield Siemens Wind Power Research Centre (S2WP)
Birthplace
3 university spin
spin-off
off companies
2
4
1
83
Annual Research Income (average over last 3 years)
~£3M
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Rolls-Royce UTC in ‘Advanced Electrical Machines & Drives’
Research Activities
• Establish in November 2003
• Focussed largely on electric systems for aerospace gas-turbines
(with some marine applications)
• Director: Professor G.W. Jewell
• Directly employs 3.5 research staff and 8 PhD students
• Encompass all facets of permanent magnet machines and
drive systems (power electronics, electromagnetic devices,
electrical systems and control systems)
• 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.)
Power electronics
LP spool generator
HP spool starter/generator
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美的集团威灵上海研发中心
Sheffield Siemens Wind Power Research Centre (S2WP)
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
500m2 later
500
l t this
thi year))
•
To develop most reliable, innovative and efficient
onshore and offshore wind turbine generator systems
for global market
•
Academic director: Prof. Z.Q. Zhu
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Collaborating with The Electrical Machines and Drives
Group, including access to Group’s expertise and
state-of-the-art facilities
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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
电机研发部
驱动控制部
行政部
从 事 综 合运营、人力、
财 务 、 行政后勤等工作
•
研发中心
从 事 驱 动控制研发设计
Siemens external competence centre, established in
2009
从 事 电 机本体研发设计
•
•
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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
•
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
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
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