In the known generator designs in which an inner or outer stator acts as the armature, the stator is loaded with a plurality of windings or coils, and a corresponding rotor is loaded with magnets or magnet pole pieces. The armature itself is usually made by stacking laminate sheets of a suitable material such as electrical steel or silicon steel, and arranging these about a supporting structure. A laminate sheet is pre-cut or punched in the desired shape according to the required number of stator windings and stator teeth, and, when mounted on the supporting structure, will extend radially outward. In the case of very large electric motors or generators, a laminate sheet can comprise an arc segment of the stator, with a number of armature teeth protrusions that define the winding slots. Each winding is arranged in a slot formed between adjacent stator teeth. A laminate sheet serves to fulfill various important functions such as reducing eddy currents, lowering hysteresis, and providing a path for magnetic flux. For this reason, a laminate sheet is usually realized as an uninterrupted area with a main body and a number of stator tooth protrusions. Ideally, the metal of the laminate sheet provides the magnetic flux with an uninterrupted path through the body of the laminate sheet.
During operation of a generator such as a wind turbine generator, high levels of electrical current are induced in the windings, and these become very hot as a result. A high temperature in the windings is undesirable for a number of reasons. The resistance of the windings increases at higher temperatures, with a detrimental effect on the generator's output power. Also, the heat from the generator is passed to the magnets and can have an adverse effect on their performance. Other components in the generator, such as electrical circuitry for auxiliaries, can also be affected by the high temperatures. Therefore, much effort is invested in attempting to cool the generator.
In some prior art approaches to cool the windings, a cooling fluid is brought as close as possible to the stator windings. For example, a cooling airflow can be directed into the air-gap between rotor and stator, so that some heat can be transported away by the cooling airflow. Heat dissipation elements such as cooling fins may be mounted onto an exterior surface, for example onto the outer surface of an outer rotor, in an effort to transfer heat away from the magnets that have in turn been heated by the windings across the air-gap. Other cooling techniques involve arranging a heat exchanger in an interior space of the generator in an attempt to reduce the heat in the generator. However, these approaches are all limited to some extent by the generator design, specifically by constraints that affect the stator design. For example, in most designs it is not possible to access the hot windings directly since each winding is closely packed between adjacent stator teeth. In other designs it is not possible to access the windings or the stator supporting structure for cooling purposes. Effectively, the narrow airgap and the laminate stack of the stator inhibit access to the hot windings. Therefore, the effectiveness of the known designs is limited by the inability to draw heat away from the windings as well as the supporting structure. In one approach, channels are formed to extend through the stator teeth between the generator airgap and the stator interior, and the purpose of the channels is to allow cooling air to pass through the body of the stator. However, to be effective in cooling the hot stator, a large number of such channels are required. Since the channels effectively remove metal from the body of the stator, the magnetic flux is adversely affected. Furthermore, such channels are formed in the stator teeth and are therefore not close to the stator windings, so that their effectiveness at drawing heat away from the winding is limited.