1. 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

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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.

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

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

17. 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

18. 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.

19. 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.

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

21. 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?

22. 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.