A new magnetic worm geared wind turbine generator for 10MW and beyond
Dave Rodger, Hong Cheng Lai
Bathwick Electrical Design Ltd, Bath, UK
A recent survey1 reveals that one of the top impact factors for reducing LCOE (levelised cost of energy) in offshore wind is believed to be larger turbine capacity. Hence the average offshore nameplate capacity is predicted to be around 11MW by 2030. However, the downside of increased capacity is usually an unwanted
increase in drive train mass and space envelope. Here we propose a new type of generator based on magnetic worm gearing which is lightweight, modular and compact.
The new magnetic gearing system:
Extensive component testing and experimentation have derisked the machine.
Modelling predicts a generator mass of around 70t for a 10MW generator.
1 ForecastingWind Energy Costs and Cost Drivers June 2016 Lawrence Berkeley National Laboratory LBNL- 1005717
The new machine is the magnetic equivalent of a worm gear meshing a toothed wheel. The mechanical worm gear is very well known to result in very high gear ratios, one complete turn of the worm gives a one tooth progression of the wheel, but usually the drawback in this application would be the need for close tolerances
and high friction leading to low efficiency. In some configurations backdriving (where the slow wheel drives the worm fast) is impossible because the efficiency is below 50%.
The use of non contact magnetic gearing removes the mechanical friction between worm and wheel, resulting in high efficiency and no wearing surface.
Magnetic gearing is of little interest for wind turbine generator applications if the magnetic gear machine is of a comparable size to the direct drive machine which it is intended to replace. The only figure of merit which can be consistently used to compare the size of different electrical machines is the airgap shear stress,
the achievable force per square metre developed at the interface between the spinning and stationary parts of the machine. Typical values are as follows: Air cooled permanent magnet machine (no special cooling) about 40kN/m/m. Superconducting 2 wind turbine generator 179kN/m/m. Worm 3 magnetic gear >500kN/m/m
2 Superconductivity for Large Scale Wind Turbines R. Fair ; W. Stautner et al Oct 2012 DOE/EE0005143
3 Calculated and measured on BEDL’s test rig
A new machine which represents a fairly radical departure from the accepted wind turbine drive train will be described. Since it is smaller and lighter than comparable drive trains, it has the potential to be useful in extending the name plate capacity of wind turbines. Other advantages include:
• Each individual generator or group of generators could have its own converter.
• An electrical fault in a modular generator or converter need not bring down the entire machine.
Delegates will be exposed to a new type of wind turbine drive train which is relatively compact, lightweight, easier to maintain and scalable towards 20MW ratings.