Magnax direct-drive generators will allow avoiding troublesome gearboxes
A small startup in Belgium is finalizing the development of a prototype generator, which is lighter, smaller, and more efficient than conventional designs of similar output. Inventor and Magnax founder Peter Leijnen say his axial flux 100-kW prototype weights about 850 kg, which is significantly less than a traditional direct-drive generator that typically weighs 4,000 kg or more. The advantages of the new technology include higher efficiency (96% to 97%, depending on the size of the machine), reliability, and scalability with relatively low production costs. While the prototype will be installed in a medium-scale, direct-drive wind turbine, Leijnen adds that Magnax’ focus is on developing megawatt-sized machines for large-scale wind turbines.
“The idea for a new generator came when a client asked me to develop a new type of wind turbine, which requires a direct-drive generator and significantly lower weight,” says Leijnen. After some market research, he concluded that no existing generator would fit the requirements. “So, I decided to build one. Previous research indicated that the target could only be met by the so-called ‘axial-flux’ topology that works for generators and motors.”
With equal diameter, the Magnax design is about half the length and a third of the weight of a conventional generator of similar output. The three-unit design is a concept illustration.
Fortunately, Ghent University, which is close to where Leijnen lives, had been researching axial-flux technology over the last six years. Still, a considerable challenge lay ahead in turning the technology into a design that could be produced with a high degree of automation on an industrial scale, and with power ratings up to several MWs.
Based on the calculations and electromagnetic simulations from the University, Leijnen’s team designed and built the 100-kW prototype “in two years of hard-core R&D and prototyping,” he says.
Despite their electromagnetic superiority over radial-flux machines, axial-flux designs pose serious production challenges. Leijnen says that is the main reason most generators today use a radial flux.
The first challenge, he says, was finding a way to accurately fix and position the stator teeth and the windings in the stator, which is difficult because there are high magnetic forces acting between the rotor and the stator. The air gap must be kept uniform and small.
The large hollow shaft on Magnax Machines provides ample space for the integration of a load-bearing system, which is independent of the generators internal bearing. This allows designing a load-bearing system that fits the application. In addition, says Leijnen, the axial air gap, means even deflections of the bearing system do not affect the air gap. It is also possible to use the internal Magnax Machine bearing alone in cases where only torque loads are applied.
“The second challenge lay in cooling the windings, which is difficult because they are deep within the stator, and between the two rotor disks. The third challenge is that traditional axial-flux machines are notoriously difficult to manufacture and it’s next to impossible to highly automate their production. Radial-flux machines are easier to manufacture because they use more or less the same principles of well-known induction generators.” Leijnen says the new, patented mechanical concept solves the challenges and enables highly automated manufacturing.
Electromagnetic calculations on axial-flux machines, he adds, are particularly difficult and require a huge amount of FEM simulation and analytical calculation. “Parameters such as terminal voltage, iron losses, eddy-current losses in the permanent magnets, copper losses, torque ripple, and cogging torque must be calculated before you can even start on the mechanical design. Over the past six years, Ghent University has been gaining expertise in this field, specifically for axial-flux machines. So they created the electromagnetic model, which I then translated into a mechanical design using our new concept,” says Leijnen.
He says he’s convinced that wind power will remain the largest fraction of renewable energy generation, even with solar rising exponentially. “With innovations like these, wind-generated power will eventually become cheaper than fossil energy, even with sub-megawatt turbines in standalone applications. Offshore wind has even greater potential. Technical advances will enable the construction of ever-larger turbines, thus further driving down energy costs. Deployment on such a large scale and over such vast spaces as the ocean will necessitate low or even no-maintenance components, and our generators, being direct-drive and having encapsulated windings, will be hard to beat on this front, too.”
Radial fluxes and hence forces, project radially in most conventional generators. Magnax proposed an axial flux design which presents several advantages and design challenges.
So what is next for Magnax? “From a product development perspective, I have a clear view of how we can further innovate with these machines. Not only in the wind industry but also for other uses such as elevators and aircraft,” he says.
Leijnen adds that since the concept is proven, his co-founder Daan Moreels is preparing a go-to-market plan. “It will take some time to find the ideal use cases and co-creation opportunities, but we’ll continue to further leverage industrial innovation with direct-drive motor and generator technology that outperforms conventional technology in weight, efficiency, reliability, and cost-effectiveness. This way we support the global transition to fully renewable power generation and ultra-efficient machines.” Moreels adds that the Magnax generator has passed prototype tests that can be viewed working in this video: https://goo.gl/VYQGQq