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Alasdair McDonald & Markus Mueller Edinburgh University Henk Polinder Delft University of Technology Structural Mass in Direct-drive.

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Presentation on theme: "Alasdair McDonald & Markus Mueller Edinburgh University Henk Polinder Delft University of Technology Structural Mass in Direct-drive."— Presentation transcript:

1 Alasdair McDonald & Markus Mueller Edinburgh University Henk Polinder Delft University of Technology Structural Mass in Direct-drive Permanent Magnet Electrical Generators

2 Direct-drive electrical generators McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Direct-drive  Advantages and challenges  Reducing generator mass  Normal methods, looking at ‘active’ material  What is structural mass? How can we include it?  How can we reduce structural mass in the generators.

3 Advantages of direct-drive McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Reliability and maintenance  Efficiency Direct drive Geared DFIG Power Efficiency

4 Direct-drive difficulties McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Power = torque x speed Tω  Gearbox steps up speed and steps down torque ωT  Need large radius, R or large shear stress, τ  Large and heavy generator l R τ  Torque = 2 x π x R 2 x l x τ T ω T ω  Direct-drive means that generator must produce large torque

5 Reducing generator mass McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Active mass  Optimise within each generator topology  Compare different generator topology types  But active mass is only third of total mass  Structural mass  Model structural mass, integrate into optimisation  Generator topologies with reduced forces  Structural shape optimisation  Lightweight materials

6 Optimise within each generator topology McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators Copper Steel Permanent Magnet  Maximise the torque density by reducing copper, steel and permanent magnet material

7 Choose different topology types McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Permanent magnet and electrically-excited topologies  Axial-flux, radial-flux and transverse-flux topologies

8 Address structural mass McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Structural mass maintains airgap clearance between rotor and stator, does not play an active role in electromagnetic circuits  A number of forces at play:  Shear stress  Normal stress  Forces from rotor blades  Weight of active material  Thermal stresses

9 Shear stress McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Shear stress is the useful torque-producing force  Rotor structure must transmit torque from rotor shaft to airgap  Stator structure also experiences shear stress  Increasing shear stress gives smaller machine

10 Normal stress McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators Copper Steel PM B φ g B g 2 2μ2μ o q =  Normal stress, q acts to close air gap clearance  It can be an order of magnitude bigger than shear stress  Shear stress = f ( B g ); Normal stress = f ( B g 2 )

11 Normal stress modelling McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Simple analytical models used early on in design process  Axial-flux machines: rotor and stator structure modelled as circular discs  Radial-flux type machines: rotor and stator structures modelled as disc and wheel structures Stress, q

12 Forces from the rotor blades McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Yaw error, wind shear, aeroelastic vibrations, blade weight  Depends on connection to rotor blades and bearing arrangement  “Static” and dynamic  Effects shear and normal stress  Ripe for further investigation, not modelled here

13 Gravity McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Weight of active and structural material can deflect rotor and stator structures  Can be modelled in disc and wheel structures

14 Centripetal and thermal effects McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Centripetal acceleration can deflect rotor cylinder into airgap.  Differences in rotor and stator temperature rise can lead to closing/opening of airgap.

15 Active and structural mass McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators Active mass Structural mass

16 Air-cored machines McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators

17 Air-cored machines McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Axial-flux machines  2, 3 and 5 MW machines  2 different aspect ratio, k r = 0.7 and k r = 0.9

18 Iron- and air-cored machines McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Iron-cored machines:  B g = 0.9 T, shear stress = 45 kN/m 2 and normal stress = 320 kN/m 2. Small radius.  Air-cored machines:  B g = 0.5 T, shear stress = 25 kN/m 2 and normal = 100 kN/m 2. Larger radius.

19 Air-cored results McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  For greater power ratings, structural mass is a greater percentage of total mass. Structural mass will become more important.  When including structural mass, k r = 0.7 is lighter and cheaper than when k r = 0.9. Need to include structural mass in decision making.  Air-cored machines are lighter than iron-cored machines.  Air-cored machines are larger.  Air-cored machines have about 50% more copper, iron and PM than the iron-cored machines and are more expensive.

20 Evolutionary methods McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators Rotor Stator ? ?

21 Use of lightweight materials McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  Composite materials are used in wind turbine blades and other weight sensitive structures  Why not use materials with high E/ ρ in electric machine structures?

22 Summary McDonald, Mueller & Polinder, Structural mass in direct- drive permanent magnet electrical generators  We can reduce generator mass  Using normal methods, looking at ‘active’ material  By addressing structural mass. Shown how this can be linked to the active material design.  Including structural mass changes our decision making.  Air-cored generators, structural optimisation and lightweight materials may help further reduce generator mass.


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