Technology
Designs aspects of PM machines
Henk Polinder
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Structure
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2
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Permanent magnets
Classification of PM machines
What is different from other types of machines
Calculation methods
Issues
Conclusions
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What is a permanent magnet?
• Source of magnetomotive force
• Makes magnetic field without a current
Bm   0  rm H m  Br
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Permanent magnet BH curves
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Permanent magnets: demagnetization
/ temperature
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Permanent magnet properties
Br
HcB
dBr/dT dHcB/dT
ρ
(%/K)
(%/K) (μΩm)
Cost
(€/kg)
(T)
(kA/m)
Ferrite
0,4
-250
-0,2
+0,34
1012
2
Alnico
1,2
-130
-0,05
-0,25
0,5
20
SmCo
1,0
-750
-0,02
-0,03
0,5
100
NdFeB
1,4
-1000
-0,12
-0,55
1,4
???
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Structure
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2
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Permanent magnets
Classification of PM machines
What is different from other types of machines
Calculation methods
Issues
Conclusions
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Classification
• DC mechanical commutator
• Iron armature
• Hollow rotor
• Disc armature
• AC electronic commutation PMSM / Brushless DC
• Surface mounted / embedded magnets
• Distributed / concentrated windings
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Brushed DC
• Iron armature
• Disc armature
• Hollow rotor
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PM AC motor
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Classification
• Brushes / brushless
• Brush wear / inverter cost
• Air gap winding / teeth
• cogging / force density
• Radial flux / axial flux
• available space / cost
• Rotating / linear
• performance / cost
• Brushless DCM / PMSM
• torque ripple
• Surface mounted magnets / embedded
• flux weakening
• Distributed / fractional pitch concentrated windings
• cost / losses
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PMSM or BDCM
PMSM:
- distributed windings
- sinusoidal voltage
- sinusoidal currents
- continuous position sensor
- smooth force
BDCM:
- concentrated windings
- trapezoidal voltage
- rectangular currents
- 6 step position sensor
- force ripple
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Rotor layouts
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surface mounted magnets
inset magnets
embedded magnets
embedded magnets
Embedded:
-Flux weakening
-Flux concentration
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Concentrated fractional pitch
windings
• Reduces cost
• Increases losses in back iron
and magnets
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Range extender: concentrated
coils, embedded magnets
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Structure
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Permanent magnets
Classification of PM machines
What is different from other types of machines
Calculation methods
Issues
Conclusions
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Force density
P   g T   g rg F  2 g rg2l g Fd  2 gVg Fd
Fd  25  50 kN/m 2
P
Vg  r l 
2 g Fd
2
g g
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Differences with other machines
Permanent magnets make it possible to
• use smaller pole pitches
• use fractional pitch concentrated windings
• use larger air gaps
• position with higher accuracy
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Advantages and disadvantages
Advantages of PM machines compared to alternatives:
• more efficient
• higher power density
• higher accuracies
• high speeds
Disadvantages
• limited field weakening
• risk of demagnetisation
• cost?
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Structure
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2
3
4
5
6
Permanent magnets
Classification of PM machines
What is different from other types of machines
Calculation methods
Issues
Conclusions
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Calculation methods
• 1D analytical approximations
• 2D analytical modelling
• Numerical: FEM
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Analytical machine model
- Magnetic vector potential
- 2 dimensional
- Boundary conditions




A
2
 A  
  J s    Br
t
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FEM: Range extender
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Structure
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2
3
4
5
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Permanent magnets
Classification of PM machines
What is different from other types of machines
Calculation methods
Issues
Conclusions
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Issues
•
•
•
•
•
Demagnetisation (earlier)
Losses, mainly for fractional pitch windings
Availability of magnet material and magnet cost
Fault tolerance
Design for specific applications
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Availability of NdFeB material
• Between 1990 and 2005, magnet prices dropped by roughly a
factor of 10
• The permanent magnet crisis (2010/2011)
• Over 95% of rare earth materials mined in China
• Large demand
• Renewable energy generation
• Electric mobility
• China protects market
• Long term
• Materials also found at other places
• Mining is being developed
• Cost??
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Direct drive generators in wind
turbines
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Direct drive: PM and alternatives
Electrical excitation PM excitation
Generator cost (k€)
447
312 > 794
Annual energy yield (GWh)
7.88
8.04
Active material weight (ton) 46
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NdFeB (€/kg)
25 > 250
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Linear PM generator
Archimedes Wave
Swing
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Wheel motor
Nuna
• High efficiency
• No gear losses
• 100 km/h @ 2 kW solar
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HISPEM: 200 kW, 45000 rpm
•
High power density
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HISPEM fault tolerant
•
•
•
5 or 7 phase
75 kW
60000 rpm
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Conclusions
Main reasons to use PM machines:
• High efficiency
• High force density
Main issues
• Risk of demagnetisation
• Availability of materials and cost
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