Alasdair McDonald & Markus Mueller Edinburgh University Structural Mass in Direct-drive Permanent Magnet Electrical Generators Alasdair McDonald & Markus Mueller Edinburgh University Henk Polinder Delft University of Technology alasdair.mcdonald@ed.ac.uk

Direct-drive electrical generators McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Direct-drive 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.

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

Direct-drive difficulties McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Direct-drive difficulties T ω Gearbox steps up speed and steps down torque Power = torque x speed T ω Direct-drive means that generator must produce large torque Torque = 2 x π x R2 x l x τ Need large radius, R or large shear stress, τ Large and heavy generator l R τ

Reducing generator mass McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Reducing generator mass 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

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

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

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

Shear stress Shear stress is the useful torque-producing force McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Shear stress 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

Normal stress Normal stress, q acts to close air gap clearance McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Normal stress B 2 φ q = g B 2μ g o Normal stress, q acts to close air gap clearance It can be an order of magnitude bigger than shear stress Shear stress = f (Bg); Normal stress = f (Bg2) Copper Steel PM

Normal stress modelling McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Normal stress modelling 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 Stress, q

Forces from the rotor blades McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Forces from the rotor blades 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

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

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

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

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

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

Iron- and air-cored machines McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Iron- and air-cored machines Iron-cored machines: Bg = 0.9 T, shear stress = 45 kN/m2 and normal stress = 320 kN/m2. Small radius. Air-cored machines: Bg = 0.5 T, shear stress = 25 kN/m2 and normal = 100 kN/m2. Larger radius.

McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Air-cored results For greater power ratings, structural mass is a greater percentage of total mass. Structural mass will become more important. When including structural mass, kr = 0.7 is lighter and cheaper than when kr = 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.

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

Use of lightweight materials McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Use of lightweight materials 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?

Summary We can reduce generator mass McDonald, Mueller & Polinder, Structural mass in direct-drive permanent magnet electrical generators Summary 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.