Cooling system for slip rings and brushes of electric machines and electric machine comprising said cooling system

Abstract

Cooling system for slip rings (2) and brushes of electric machines, comprising a rotating shaft (3) configured to accommodate at least one slip ring (2) thereon, wherein the slip ring (2) comprises a brush holder assembly (1), the cooling system characterized in that the rotating shaft (3) comprises: - an axial bore (4) configured for the circulation of a cooling fluid, - a set of radial bores (5) located at least in a cross-section of the rotating shaft (3) configured to be longitudinally adjacent to at least one slip ring (2), the radial bores (5) being configured for communication between the axial bore (4) and the outside of the rotating shaft (3) for the flow of the cooling fluid from the axial bore (4) to the outside of the shaft (3), - a deflector element connected to the radial bores (5) of the rotating shaft (3) and configured to be longitudinally adjacent to at least one slip ring (2), configured for the orientation of the fluid Cooling towards the brush holders (1), - the deflector element comprises a static deflector ring (6), configured not to rotate with the rotating shaft (3), concentric to the rotating shaft (3) comprising radial holes (7) in fluid communication with the radial holes (5) of the rotating shaft (3), and in that the cooling system further comprises a cooling fluid drive element in communication with the axial hole (4).

Description

Cooling system for slip rings and brushes of electric machines and electric machine comprising said cooling system Object of the invention The present invention relates to a cooling system for slip rings and brushes of electric machines. Background of the invention When conducting electrical currents, the rings and brushes experience energy losses and, consequently, heat generation, which in some cases damages these components. Cooling these elements is usually achieved with one or more fans, either coupled to the shaft of the electrical machine or independently driven by electric motors. It is a problem in many electrical machines that the brushes and rings near the fan cool better than those further away. Therefore, the rings and brushes in electrical machines often present problems of overheating due to current circulation and insufficient cooling. As mentioned previously, it is common practice for these devices to be cooled by a fan driven by the shaft where the slip rings are mounted. In other cases, an external fan driven by electric motors is installed. There are also some machines that have vacuum cleaners to extract hot air from the brush area. In some cases, for cooling purposes, the brushes and rings often have slots to allow air to circulate. However, the airflow is not the same in all rings and brushes, and in many electrical machines breakdowns occur due to excessive heating of some of the brushes, brush holders and rings, as discussed above. In the article "Thermal design and development of actively cooled brushes for compact homopolar generators", by D. Makel published in the journal IEEE Transactions on Magnetics, other cooling methods that are not widely used industrially are known, such as water cooling in channels, or the direct evaporation of a fluid directly on the surface of the brushes. Description of the invention The present invention aims to improve the cooling of the rings and brushes, by means of a radial airflow that comes from inside the rotating shaft and is directed at each of the brush holders of the machine. The cooling system of the invention comprises a rotating shaft configured to accommodate at least one slip ring. The slip ring, in turn, comprises a brush holder assembly. The rotating shaft comprises: - an axial orifice configured for the circulation of a cooling fluid, - a set of radial holes located at least in one cross-section of the rotating shaft, configured to be longitudinally adjacent to at least one slip ring. The radial holes are configured to allow communication between the axial hole and the outside of the rotating shaft for the flow of cooling fluid from the axial hole to the outside of the shaft. The cooling system also includes a cooling fluid pumping element in communication with the axial orifice. According to the above, the invention employs a drive element that drives air or refrigerant gas through the axial hole in the rotating shaft. This air or gas exits radially through the rotating shaft via the radial holes. This improves the cooling of the brushes, especially those furthest from the end of the shaft. The present invention also relates to an electric machine comprising a rotating shaft and at least one slip ring mounted on the rotating shaft. The slip ring comprises a brush holder assembly. The electric machine includes a cooling system as previously described. A method for cooling slip rings and brushes of electric machines is also possible, comprising the following steps: - driving by means of a driving element of a cooling fluid through an axial bore of a rotating shaft configured for the location thereon of at least one slip ring, wherein the slip ring comprises a set of brush holders, - directing the cooling fluid through a set of radial holes located at least in one cross-section of the rotating shaft attached longitudinally to at least one slip ring, so that the cooling fluid passes from the axial hole to the outside of the rotating shaft in the area adjacent to the slip ring. The method and system of the present invention is of particular relevance in wind generators based on doubly fed induction machines. Brief description of the figures A series of figures, non-limiting examples, are briefly described here to help to better understand the invention. Figure 1 shows a perspective view of an example embodiment of the cooling system of the invention in a machine with twelve brush holders and three rings, specifically with four brush holders per ring evenly distributed at 90 degrees. Figure 2 shows a perspective view of the same embodiment as Figure 1, where one of the three rings and its four brush holders has been removed. Two deflectors corresponding to the removed ring have also been removed from the rotating shaft. Description of preferred figures and embodiments The numerical references for the figures are indicated below. (1) Brush holder; (2) Rings; (3) Axis; (4) Axial hole in the shaft; (5) Radial holes in the shaft; (6) Deflector rings; (7) Radial holes in the deflectors; (8) Air or refrigerant gas inlet; Next, we proceed to describe in more detail the examples of embodiment of the invention together with the figures introduced previously, which are preferred embodiments that do not limit the invention. The cooling system shown comprises a rotating shaft (3) configured to accommodate at least one slip ring (2), specifically three slip rings are shown. The slip ring (2) comprises a brush holder assembly (1), specifically in Figures 1 and 2 it comprises four brush holders (1) per slip ring (2). The rotating shaft (3) comprises: - an axial bore (4) configured for the circulation of a cooling fluid, - a set of radial bores (5) located at least in a cross-section of the rotating shaft (3) configured to be located longitudinally adjacent to the slip rings (2). The radial bores (5) being configured for communication of the axial bore (4) with the outside of the rotating shaft (3) for the flow of the cooling fluid from the axial bore (4) to the outside of the shaft (3). The cooling system also includes a coolant pumping element in communication with the axial orifice (4), not shown. The coolant pumping element could be a fan. The coolant could be air or a refrigerant gas. Figure 1 shows an example of the system of the invention in a machine with twelve brush holders (1) and three slip rings (2). That is, the machine comprises four brush holders (1) per slip ring (2). In this figure, the brush holders (1) supporting the brushes in contact with the slip rings (2) mounted on the rotating shaft (3) can be seen. The rotating shaft (3) has an axial hole (4) that allows the entry of air or refrigerant gas through the rotating shaft (3). The air or refrigerant gas is driven by a drive element. On the rotating shaft (3) there is a series of radial holes (5) that allow the fluid that has previously entered through the rotating shaft (3) to exit in a radial direction. In one embodiment, the cooling system comprises a deflector element connected to the radial holes (5) of the rotating shaft (3). The deflector element is configured to be located longitudinally adjacent to at least one slip ring (2). The deflector element is configured to direct the cooling fluid towards the brush holders (1). To better cool the brushes, the fluid should be directed towards the brush holders (1), for which deflector elements are used. In the embodiment shown in the figures, the deflector element comprises a deflector ring (6) concentric to the rotating shaft (3) comprising radial holes (7) in fluid communication with the radial holes (5) of the rotating shaft (3). In one embodiment, the deflector ring (6) is static, i.e., it does not rotate with the rotating shaft (3). In one embodiment, the deflector ring (6) comprises as many radial holes (7) as the slip ring (2) has brush holders (1). The radial holes (7) are configured to face the brush holders (1). Therefore, the deflector rings (6) have radial holes (7), as many as there are brush holders (1) per slip ring (2). Thus, the fluid exits from the inside of the rotating shaft (3) through the radial holes (5) and subsequently through the radial holes (7) of the deflector ring (6), impacting the brush holders (1). In the embodiment shown, the slip ring (2) is configured to comprise four brush holders (1) distributed at 90° and the deflector ring (6) comprises four radial holes (7) distributed at 90°. In one embodiment, the rotating shaft (3) comprises more radial holes (5) than the deflector ring (6) has radial holes (7). Optionally, the cooling system may comprise two baffle rings (6), each configured to be located adjacent to one face of a slip ring (2). That is, each of the slip rings (2) would have two baffle rings (6). Figure 2 shows the same system as described in Figure 1, but in the case of Figure 1 one of the slip rings (2) has been removed to allow a clear view of the fluid path. Figure 2 shows a perspective view of the embodiment example of Figure 1, where one of the three slip rings (2) and its four brush holders (1) are disassembled. The two deflector rings (6) corresponding to the disassembled slip ring (2) have also been removed from the rotating shaft (3). This reveals the radial holes (5) in the rotating shaft (3). These holes (5) are equidistant on both sides of the slip ring (2). Figure 2 also shows one of the deflector rings (6) which has as many radial holes (7) as brush holders (1), and they are located opposite the brush holders (1). According to the above description, the cooling method for slip rings (2) and brushes of electric machines would comprise the following stages: - driving by means of a driving element of a cooling fluid through an axial bore (4) of a rotating shaft (3) configured for the location thereon of at least one slip ring (2), wherein the slip ring (2) comprises a set of brush holders (1), - directing the cooling fluid through a set of radial holes (5) located at least in a cross-section of the rotating shaft (3) longitudinally attached to at least one slip ring (2), so that the cooling fluid passes from the axial hole (4) to the outside of the rotating shaft (3) in the area attached to the slip ring (2). In one embodiment, the method further comprises the step of directing the cooling fluid after its passage through the set of radial holes (5) by a deflector element connected to the radial holes (5) of the rotating shaft (3) located longitudinally attached to at least one slip ring (2) for the orientation of the cooling fluid towards the brush holders (1). In the embodiment in which the cooling system comprises the baffle ring (6), the cooling fluid passes through a set of radial holes (5) and subsequently through the radial holes (7) of the baffle ring (6). If the system comprised two baffle rings (6) each attached to one face of a slip ring (2), the cooling fluid would pass through a set of radial holes (5) and subsequently through the radial holes (7) of one of the baffle rings (6) located on each face of the slip ring (2). The invention also relates to an electric machine comprising a rotating shaft (3) and at least one slip ring (2) mounted on the rotating shaft (3), wherein the slip ring (2) comprises a brush holder assembly (1) and comprising a cooling system according to any one of the above embodiments. As discussed above, the method and system of the present invention are of particular relevance in wind turbine generators based on doubly fed induction machines. These machines have three slip rings (2) on the rotating shaft (3), since they are three-phase, with several brushes on each slip ring (2), housed in brush holders (1).

Claims

1. Cooling system for slip rings (2) and brushes of electric machines, comprising a rotating shaft (3) configured for the location thereon of at least one slip ring (2), wherein the slip ring (2) comprises a brush holder assembly (1), the cooling system characterized in that the rotating shaft (3) comprises: - an axial bore (4) configured for the circulation of a cooling fluid, - a set of radial bores (5) located at least in a cross-section of the rotating shaft (3) configured to be located longitudinally adjacent to at least one slip ring (2), the radial bores (5) being configured for communication of the axial bore (4) with the outside of the rotating shaft (3) for the flow of the cooling fluid from the axial bore (4) to the outside of the shaft (3),- a deflector element connected to the radial holes (5) of the rotating shaft (3) and configured to be located longitudinally adjacent to at least one slip ring (2), configured to direct the coolant fluid towards the brush holders (1), - the deflector element comprises a static deflector ring (6), configured not to rotate with the rotating shaft (3), concentric to the rotating shaft (3) comprising radial holes (7) in fluid communication with the radial holes (5) of the rotating shaft (3), and in that the cooling system further comprises a coolant pumping element in communication with the axial hole (4).

2. Cooling system for slip rings (2) and brushes of electric machines, according to claim 1,characterized in that the deflector ring (6) comprises as many radial holes (7) as the slip ring (2) has brush holders (1), and these holes are configured to face the brush holders (1).

3. Cooling system for slip rings (2) and brushes of electrical machines, according to claim 2, characterized in that the slip ring (2) is configured to comprise four brush holders (1) distributed at 90° and the deflector ring (6) comprises four radial holes (7) distributed at 90°.

4. Cooling system for slip rings (2) and brushes of electrical machines, according to any one of the preceding claims, characterized in that the rotating shaft (3) comprises more radial holes (5) than the deflector ring (6) has radial holes (7).

5. Cooling system for slip rings (2) and brushes of electrical machines, according to any one of the preceding claims,characterized in that it comprises two deflector rings (6), each configured to be located adjacent to a face of a slip ring (2).

6. Cooling system for slip rings (2) and brushes of electric machines, according to any one of the preceding claims, characterized in that the coolant is air or refrigerant gas.

7. Cooling system for slip rings (2) and brushes of electric machines, according to any one of the preceding claims, characterized in that the driving element of the coolant is a fan.

8. Electric machine comprising a rotating shaft (3) and at least one slip ring (2) mounted on the rotating shaft (3), wherein the slip ring (2) comprises a brush holder assembly (1), characterized in that it comprises a cooling system according to any one of the preceding claims.