technology designs aspects of pm machines

1Challenge the future

TechnologyDesigns aspects of PM machines

Henk Polinder


Structure

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


What is a permanent magnet?

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

rmrmm BHB 0


Permanent magnet BH curves


Permanent magnets: demagnetization / temperature


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)


Structure



Classification

• DC mechanical commutator• Iron armature• Hollow rotor• Disc armature

• AC electronic commutation PMSM / Brushless DC• Surface mounted / embedded magnets• Distributed / concentrated windings


Brushed DC

• Iron armature• Disc armature• Hollow rotor


PM AC motor


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


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


Rotor layouts

1 surface mounted magnets2 inset magnets3 embedded magnets4 embedded magnets

Embedded: -Flux weakening-Flux concentration


Concentrated fractional pitch windings

• Reduces cost• Increases losses in back iron

and magnets


Range extender: concentrated coils, embedded magnets


Structure



Force density

2kN/m5025dF

dggdgggggg FVFlrFrTP 22 2

dgggg F

PlrV

2

2


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


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?


Structure



Calculation methods

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


Analytical machine model

2s r

AA J Bt

- Magnetic vector potential- 2 dimensional - Boundary conditions


FEM: Range extender


Structure



Issues

• Demagnetisation (earlier)• Losses, mainly for fractional pitch windings• Availability of magnet material and magnet cost• Fault tolerance• Design for specific applications


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??


Direct drive generators in wind turbines


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


Linear PM generator Archimedes Wave Swing


Wheel motor Nuna

• High efficiency• No gear losses• 100 km/h @ 2 kW solar


HISPEM: 200 kW, 45000 rpm

• High power density


HISPEM fault tolerant• 5 or 7 phase• 75 kW• 60000 rpm


Conclusions

Main reasons to use PM machines:• High efficiency• High force density

Main issues• Risk of demagnetisation• Availability of materials and cost

technology designs aspects of pm machines

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