Technology Designs aspects of PM machines - EMVT

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TechnologyDesigns aspects of PM machines

Henk Polinder

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Structure

1 Permanent magnets2 Classification of PM machines3 What is different from other types of machines4 Calculation methods5 Issues6 Conclusions

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What is a permanent magnet?

• Source of magnetomotive force• Makes magnetic field without a current

rmrmm BHB 0

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Permanent magnet BH curves

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Permanent magnets: demagnetization / temperature

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Permanent magnet properties

???1,4-0,55-0,12-10001,4NdFeB

1000,5-0,03-0,02-7501,0SmCo

200,5-0,25-0,05-1301,2Alnico

21012+0,34-0,2-2500,4Ferrite

Cost (€/kg)

ρ

(μΩm)dHcB/dT

(%/K)dBr/dT(%/K)

HcB(kA/m)

Br(T)

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Structure

1 Permanent magnets2 Classification of PM machines3 What is different from other types of machines4 Calculation methods5 Issues6 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

1 surface mounted magnets2 inset magnets3 embedded magnets4 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

1 Permanent magnets2 Classification of PM machines3 What is different from other types of machines4 Calculation methods5 Issues6 Conclusions

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Force density

2kN/m5025dF

dggdgggggg FVFlrFrTP 22 2

dgggg F

PlrV

2

2

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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

1 Permanent magnets2 Classification of PM machines3 What is different from other types of machines4 Calculation methods5 Issues6 Conclusions

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Calculation methods

• 1D analytical approximations• 2D analytical modelling• Numerical: FEM

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Analytical machine model

2s r

AA J Bt

- Magnetic vector potential- 2 dimensional - Boundary conditions

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FEM: Range extender

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Structure

1 Permanent magnets2 Classification of PM machines3 What is different from other types of machines4 Calculation methods5 Issues6 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

25 > 250NdFeB (€/kg)

2446Active material weight (ton)

8.047.88Annual energy yield (GWh)

312 > 794447Generator cost (k€)

PM excitationElectrical excitation

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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