An Example for a Direct Drive System

Direct drive

The drive system of modern wind energy converters is based on a simple principle: Fewer rotating components reduce mechanical stress and at the same time increase the technical service life of the equipment. Maintenance and service costs for the wind turbine are lower (fewer wearing parts, no gear oil change, etc.) and operating expenses are reduced.

The rotor hub and annular generator are directly connected to each other as a fixed unit without gears. The rotor unit is mounted on a fixed axle, the so-called axle pin. Compared to conventional geared systems that have a large number of bearing points in a moving drive train, the direct drive system has only two slow-moving roller bearings. The reason for this is the low speed of the direct drive.

Direct Drive of E-48 - © Enercon

Annular generator

The annular generator is of primary importance in the gearless system design. Combined with the rotor hub it provides an almost frictionless flow of energy, while the gentle running of fewer moving components guarantees minimal material wear. Unlike conventional asynchronous generators, the annular generator is subjected to minimal mechanical wear, which makes it ideal for particularly heavy loads and a long service life. Time-consuming repair work and the associated down periods are thus prevented.

Improved adaptation of the pole shoes to slow rotation of ENERCONís annular generator. - © Enercon

ENERCON’s annular generator is a low-speed synchronous generator with no direct grid coupling. Output voltage and frequency vary with the speed and are converted for output to the grid via a DC link and an inverter achieving high speed variability.

Stator and rotor

According to ENERCON’s service life requirements the copper winding in the stator, the stationary part of the annular generator, known as closed, single-layer basket winding is produced in insulation class F (155°C). It consists of individual round wires that are gathered in bundles and varnish insulated. ENERCON does the copper winding by manually. In spite of increasing automation in other manufacturing areas, preference has been given to manual labour in this case for good reason. It ensures that the materials used are fully tested. Furthermore, a special processing method allows continuous windings to be produced. Each wire strand is continuous from start to finish.

Stators: A special processing method allows continuous windings to be produced.
Each wire strand is continuous from start to finish. - © Enercon

The magnetic field of the stator winding is excited via so-called pole shoes. These are located on the disk rotor, the mobile part of the ENERCON annular generator. Since the shape and position of the pole shoes have a decisive influence on the noise emission of the annular generator, ENERCON's Research & Development Department has dedicated particular attention to this aspect. The result: Improved adaptation of the pole shoes to slow rotation of ENERCON’s annular generator means that no noise is generated.

Temperature behaviour

ENERCON’s annular generator features optimised temperature control. The hottest areas in the annular generator are constantly monitored by a large number of temperature sensors. The sensors’ activation temperature is well below constant temperature resistance of the insulating materials used in the generator. This prevents temperature overload.

Power flow concept

The power flow concept reduces mechanical stress - © Enercon

In addition to the direct drive, mechanical stress is further reduced by a specially developed power flow concept which transmits dynamic loads directly away from the rotor unit to the tower. In order to avoid possible excessive stress at welded seams, only cast parts are used in the vicinity of major components.

Text und Photos: