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The generator demonstrator on the test rig. The motor used for the drive is on the right.
© Fraunhofer IWES
Gearless wind turbines
22.11.2017

CAD model of the large-scale generator (cross-sectional view). Construction in half-section: rotor, stator, shaft, axis, bearing. Magnified view (zoom) of the active parts and their connection (with cross bolts and eye bolts) to the inactive structure.
© Fraunhofer IWES

In the experimental generator, the stator (left) and rotor (right) are joined together.
© Fraunhofer IWES

Generator and support structure
© Fraunhofer IWES

Ring generators are slimming down

Gearless wind turbines have weight issues. The weight of a generator increases disproportionately with its output. With the current concepts, it will be difficult to pass the 10-megawatt threshold -a class of turbines that is of particular interest to offshore wind farms. In order to solve this problem, researchers are developing a new type of ring generator with a larger diameter but significantly lower weight. A scaled down test generator showed some promising results on the test rig.

The economic pressure on newly planned onshore and offshore wind farms is rising. Scientists and manufacturers are therefore looking for ways to reduce the costs and effort involved in wind turbines, control and grid feeding, as well as repairs and maintenance. In the future, larger, more powerful wind turbines than those currently available on the market would be of particular interest to offshore wind farms because they have lower specific investment and operation costs. However, while the performance of a gearless wind turbine following current generator concepts increases more or less quadratically, the tower head mass - the generator is responsible for up to 35 percent of that mass - increases almost cubically. Wind turbines should have longer maintenance and repair intervals in order to limit the high costs involved for sea-based wind turbines. Manufacturers need other generator concepts to let gearless systems cross the 10-megawatt threshold.

In the MagnetRing research project, scientists from the Fraunhofer Institute for Wind Energy and Energy System Technology (Fraunhofer-Institut für Windenergie und Energiesystemtechnik, IWES) and industrial partners are developing and testing a concept for a novel ring generator with a larger diameter and a comparatively low weight. As a result, the nacelle mass is reduced to one third of that of conventional gearless systems. The scientists developed and refined an electromagnetic, structural mechanical and thermal concept for the generator. Important components included newly developed active air-gap and inverter controls. The researchers constructed a scaled down model of the ring generator with an output of 180 kW and tested it on the test rig. The results are being incorporated in the concept used to develop a 10-MW generator.

Higher generator output thanks to larger ring

In order to increase the output of a wind turbine, longer rotor blades can be used, the turbine can be installed on a higher tower or a higher rotational speed of the blades can be preset. A gearless system offers another option: a ring generator with a larger diameter allows for a higher output. With the planned 10-MW generator, the ring will have a diameter of 15 m, while that of large, commercially available ones is about 11 m in diameter. With the new design, the ring runs inside the nacelle, held in place by a supporting structure. It is designed as an internal pole alternator. The solenoid coils are installed on the fixed stator, while the permanent neodymium magnets are integrated in the rotor revolving around the interior. Due to design improvements, the permanent magnets in the new generator are much smaller and therefore lighter than those in conventional generators. The weight advantages are even greater when compared with separately excited generators.

From an electrotechnical point of view, the air gap between the stator and the rotor ought to be as small as possible. This improves electrical induction, i.e. power generation. On the other hand, the gap has to be large enough to prevent contact and therefore any damage during operation. The gap compensates for thermal changes in component size and axial fluctuations, for example as caused by gusts of wind. The air gap in the new generator was reduced significantly. The generator circuit of the generator demonstrator is divided into 12 segments, each of which can be controlled separately and each has its own inverter. The planned large-scale generator will have 48 segments. This will allow for a segmental amplification and attenuation of field strengths to compensate for vibrations.

The ring and the support structure are made of a sturdy material and are thus designed to be inherently rigid. Otherwise, the ring would ovally deform over time due to the attraction of the generator mass and the torque acting on the structure. The developers accept the weight increase associated with this design. A high-strength aluminium alloy was chosen for the generator demonstrator. Only two components are made of steel. In general, it is also possible to build the ring generator using steel components. Weight and cost are the deciding factors when it comes to choosing either aluminium or steel. Aluminium is lighter, but costs are higher.

Testing with a motor drive

The generator demonstrator, which was sized down to a diameter of 2 m and an output of 180 kW could be built using only aluminium due to its dimensions. On the test rig, an engine replaced the rotor blades to drive the generator. The model generator is mechanically and electrically fully functional and it is equipped with a complete control system. The rotor of the generator is mounted freely on the shaft so that it can be tilted during operation. This made it possible to test vibrations and the effectiveness of the damping concepts on the test rig. The generator demonstrator showed good results on the test rig in terms of control, current quality, power production and vibrational damping.

Transferring the results

The researchers are using the results to refine and transfer the concept to the 10-MW generator. The results on the test rig have unfolded new options. The segmentation of the generator circuit with separate controls allows for a direct connection of the generator to the medium-voltage network, without an intermediate transformer. This would save costs and minimise energy losses. Another option is to improve the partial load characteristics of wind turbines by controlling the segments. To achieve this, the electromagnetic and structural dynamics interactions would have to be researched even more extensively. Furthermore, the generator should be tested on a virtual wind turbine in the next test phase. In addition to technical feasibility, an economic operation of the entire wind turbine must also be taken into account. Another aspect is to investigate and develop effective cooling concepts for this type of large-scale generator. These concepts are needed to further increase peak performance.

(mi)

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Project management
Fraunhofer IWES

Industrial partner
Krämer Energietechnik GmbH

Industrial partners
Siemens AG