Technical field

The present invention relates to cooling of slip rings, especially of slip rings for a doubly- fed induction generators, commonly used in wind turbines.

Background of the Invention

Doubly-fed induction generators, or DFIGs, used in wind turbines generally comprise a rotor with windings connected to a converter via a slip ring device, and a stator connected to the power grid. The converter can be used to control frequency of the current supplied to the rotor windings which enables the generator to output current with a constant frequency regardless of rotational speed of the rotor, i.e., regardless of wind speed. A DFIG is used in most wind turbines over 3 MW.

Slip ring devices usually comprise multiple contact rings and multiple brushes for passing current to or from the contact rings. There are usually multiple brushes per contact ring and the brushes are spring-loaded to ensure constant contact between the contact rings and the brushes. Each such spring should ensure that each brush is forced towards the contact ring with substantially the same force for each brush over the entire lifetime of the brush. However, this sliding contact remains one of the main sources of heat in the slip ring device due to its relatively high electrical resistance. Since brushes are damaged by high temperatures, cooling of slip ring devices, especially of the brushes and contact rings, is necessary to avoid frequent costly and inconvenient brush replacements.

In many applications, passive cooling, e.g., by providing a heat sink attached to a brush holder, is sufficient. DFIGs however usually require active cooling, e.g., by an air fan. A DFIG with a cooling fan for maintaining air circulation around a slip ring device is for example described in document CN113937959A.

Providing for an air circulation around the slip ring device might however not be sufficient for maintaining temperatures below 100 °C, which is generally considered necessary to prevent thermal damage of the brushes. It would therefore be advantageous to provide a doubly-fed induction generator with an improved mechanism for cooling of the slip ring device.

Summary of the Invention

The shortcomings of the solutions known in the prior art are to some extent eliminated by a slip ring device for a doubly-fed induction generator, the slip ring device comprising multiple contact rings electrically insulated from each other, multiple brushes, and a cooling fan. Each contact ring is in a sliding contact with at least one brush and is provided with an electrical conductor for connection to rotor windings. Electrical current can thus be conducted from the brushes (e.g., connected to the power grid via a converter) through the sliding contact to the contact rings, then through each contact ring to the electrical conductor and then to the rotor windings, where the current is used to induce rotating magnetic field needed for power generation, with rotation speed not dependent on rotation speed of the rotor. There is at least one groove on each contact ring’s brush-contacting surface. The slip ring device further comprises a non-empty set of cooling passages for air moved by the cooling fan, each passage from the set passing through the multiple contact rings, and for each passage from this set, at least one groove on each of the multiple contact rings extends to the passage.

Slip ring contact rings having grooves, especially helical grooves, are known in the art. The grooves serve as an escape channel for air trapped in front of a brush contacting given brush-contacting surface due to the high rotational speeds of DFIG contact ring surfaces (e.g., 30 meters per second). Without an escape channel, the air dragged by the contact ring surface and trapped in front of the brush would tend to lift the brush out of contact with the contact ring, thus severing the sliding electrical contact between the brush and the contact ring and limiting the slip ring device’s operation. The grooves may also prevent the current between a brush and a contact ring from constantly passing through only a small area on the brush, which would excessively wear this brush area out. The groove severs, at times, the electrical contact between this specific small area and the contact ring, so the current is forced to pass through another part of the brush.

In the present invention, by appropriately choosing the length and depth of the groove and/or the size and position of the cooling passages, these elements intersect each other, i.e., the groove extends into the cooling passages and/or the passages run through the groove(s). The air, which is forced to move through the cooling passages by the fan, then partially escapes through the open parts of the bottom of the groove(s) towards the brushes, cooling them in the process. The air which leaves the passage(s) through the groove(s) can than flow for example around the brushes or brush holder in the direction of the axis of rotation, or along the device’s chassis, and can leave the device through the same outlet as the air continuing through the passages. Providing additional air outlets in the chassis along the axis of rotation, through which the air moving perpendicularly to the axis can leave the device, might also be advantageous in some embodiments.

Since the contact rings with the passages rotate at relatively high speeds (e.g., 2000 rpm), the centrifugal force acting on the air inside of the passages pushes the air out through the grooves in significant volumes, and cooling of the brushes as well as of the contact rings is considerably improved. The improved cooling then improves lifespan of the brushes or contact rings. A generator provided with the slip ring device according to the invention can thus be in operation longer without needing maintenance or brush replacement.

The grooves can have any shape and size, i.e., length, width, angle with respect to the axis of rotation, number of turns or pitch in case of a helix etc., and there can be any number of them. These parameters would affect the intensity of cooling, but also for example the conductivity/resistance of the brush-contact ring sliding electrical contact, strength of the contact ring etc. Since the current passing between the brush(es) and a contact ring can have magnitude e.g., 1 or 2 kA, limiting the contact area between each brush and the contact ring by excessive grooving could actually increase the temperatures by increasing electrical resistance of the sliding contact. A skilled person designing the grooves should therefore also consider this factor. Similarly, the size and number of the cooling channels affects not only the cooling intensity but also electrical resistance between the outer surface of each contact ring and the inner part connected to the electrical conductor(s).

For each passage from the set, the at least one groove extending into the passage on each of the multiple contact rings may have a depth larger than the distance between the passage and the brush-contacting surface of the contact ring. In other words, each groove extends into the cooling passage(s) by its depth, instead of e.g., by its length (e.g., if a circular groove was drilled perpendicular to the axis of rotation).

Preferably, each passage from the set is parallel to axis of rotation of the slip ring device for at least the part of its length which passes through the multiple contact rings. The cooling air flowing in these passages is thus not obstructed by any curves or turns of the passages, so the flow rate of the air is improved. Direction of the flow can be towards the rotor or away from it, depending on placement and shape of the fan and direction of rotation. The improved cooling effect of the invention is generally not affected by the flow direction.

Preferably, a layer of electrically insulating material is provided between the contact rings, wherein each layer comprises cut-outs for passing of the air, the cut-outs being aligned with the cooling passages. The air is thus not obstructed by the insulating material. The layer of insulating material preferably has a higher radius in places where it surrounds the electrical conductors and lower radius anywhere else, especially in places where the cooling passage are placed.

The slip ring device preferably further comprises a deflector for directing the cooling air into the cooling passage and into contact with the brushes and/or the brushcontacting surfaces of the contact rings. Without the deflector, the air might tend to flow along the chassis of the device/generator, where there is the least resistance to the air flow. In order to achieve a maximum brush cooling, it is thus advantageous to prevent the air from flowing in areas which do not need cooling, i.e., areas not adjacent to the brush-contacting surfaces of the contact rings or the contact ring-contacting surfaces of the brushes. The deflector might for example be formed as several thin plates, e.g., up to 6 mm thick, placed perpendicularly to the axis of rotation at several places along the axis. Each plate is shaped complementarily to the device’s chassis, brush holder, contact rings etc., such that it forces the air to flow along the surfaces that need cooling in as large amounts as possible.

Preferably, the set of cooling passages comprises at least three passages, wherein the passages have circular cross-sections. Circular passages are easier to make, e.g., to drill, and they limit the electrical current flow through the contact ring less than e.g., passages with elongated (bean shaped) cross sections with equal total passage cross section area.

Preferably, the groove(s) extending into the passage on each of the multiple contact rings has/have a helical shape, with axis of the helix coinciding with the axis of rotation. Helical shape is advantageous for the escape of the air and also for preventing the current from passing through only a small part of the brush, as described above.

The shortcomings of the solutions known in the prior art are to some extent also eliminated by a doubly-fed induction generator comprising the slip ring device of the invention. Such a DFIG can comprise a rotor with windings connected to a converter via the slip ring device, and a stator connectable to the power grid.

Description of drawings

A summary of the invention is further described by means of exemplary embodiments thereof, which are described with reference to the accompanying drawings, in which:

Fig 1. Shows a sectional view of a slip ring device according to the invention, wherein the sectional plane passes through the axis of rotation of the device and through one cooling passage, and wherein it can be seen that a helical groove on a brush-contacting surface of each contact ring cuts through the cooling passage.

Fig 2. Shows a detailed view of the cooling passage from fig. 1 and of the groove extending into this passage. Fig 3. Shows the slip ring device from fig. 1 , wherein it can be seen that the insulation material between the slip rings has several cut-outs aligned with the cooling passages so that the air can pass through without being obstructed by the insulating material.

Fig 4. Shows the slip ring device from the front, i.e., when viewed in the direction of the axis of rotation, wherein the distribution of the cooling passages around the perimeter of the contact rings can be seen in this figure.

Fig 5. Shows a schematic drawing of the slip ring device as depicted in fig. 4 provided with a chassis, a brush holder with five brushes, and a deflector.

Fig 6. Shows a schematic partially sectional drawing of the slip ring device in another embodiment, wherein there are two helical grooves on each contact ring, the two grooves having opposite orientation and intersecting each other at multiple points.

Exemplary Embodiments of the Invention

The invention will be further described by means of exemplary embodiments with reference to the respective drawings.

The subject of the present invention is a slip ring device for a doubly-fed induction generator (DFIG). This device comprises several contact rings 1, e.g., made of brass or stainless steel, and at least one brush 9 for each contact ring 1_. Each contact ring 1 is in a sliding contact with the respective brushes 9, forming a rotary electrical connection for supplying current to rotor windings. Multiple brushes 9 for each contact ring 1, e.g., five brushes 9 as depicted in fig. 1 , are preferable. A layer of electrically insulating material 7 is provided between each pair of adjacent contact rings 1, and at least one electrical conductor 2, e.g., an insulated wire, is attached to each contact ring 1 in order to connect it to the rotor windings. For example, Bulk moulding compound (BMC) can be used as the insulating material 7. Each contact ring 1 further comprises at least one groove 3 on its outer surface to prevent air trapped in front of or under the brushes 9 from lifting the brushes 9, which would interrupt the sliding contact between the brush 9 and the contact ring 1. The device can be fixed, e.g., by an interference fit or welding, to the shaft of the DFIG rotor.

The slip ring device further comprises a cooling fan, preferably with rotational axis coinciding with the axis 6 of rotation of the slip ring device, e.g., a set of blades attached to the rotor, one of the contact rings 1 or the insulation of the contact ring 1. etc. The device also comprises a set of cooling passages 5 for the air stirred by the fan. This set comprises at least one, preferably at least three, e.g., at least eight, passages 5, each leading through all the contact rings T Preferably, the passages 5 have a straight portion passing through all the contact rings 1 and through the cooling fan. Rotational speed of the fan can depend on the rotation speed of the rotor and on number of poles of the rotor. The amount of air passing through the device, e.g., through the cooling passages 5 and around brushes 9 and contact rings 1, depends on fan design, its rotational speed, size of air inlet(s) and outlet(s), optional air filter(s), cross section of each passage 5, and number of the passages 5 etc. Preferably, all those parameters are chosen, or some have values required for providing other functions of the device or the DFIG and the remaining parameters are chosen accordingly, to ensure cooling sufficient to keep the temperature of the brushes 9 and of the brush-contacting surfaces 4 of the contact rings 1 below 105 °C, preferably below 100 °C. The number of passages 5 and their size can thus be chosen for each slip ring device or generator individually, based on their other constructional features. The limiting factor for number and size of the passages 5 is mainly conductivity of the contact rings 1, i.e., their ability to pass high currents from their outer surface towards the electrical conductor(s) 2 with as little resistance as possible, which decreases with the number of passages 5 made in each contact ring T A minimum thickness, when measured perpendicularly to the axis 6 of rotation, of insulation necessary between the contact rings 1 might be another limiting factor for location and diameter of the cooling passages 5.

In order to improve cooling of the contact rings 1 and brushes 9, the distance of the cooling passages 5 from the outer surface of the contact rings 1 and/or the depth of the grooves 3 on the outer surfaces of the contact rings 1 are selected such that on each contact ring 1_, at least one groove 3 extends into each cooling passage 5. The air flowing through the passage 5 can therefore be radially pushed outwards, by the centrifugal force caused by rotation of the contact rings 1, into the grooves 3. The cooling air is therefore being blown on the ring-contacting surface of the brushes 9 and the brushes 9 are thus cooled on this surface, where their temperature is the highest.

Each contact ring 1. preferably contains one helical groove 3, preferable having multiple turns. For example, each contact ring 1. can have an outer diameter of 300- 400 mm and width of 80-150 mm, and for such ring, the groove 3 can have width of 1 - 5 mm, e.g., 2 mm, and depth of 5-20 mm, e.g., 10-15 mm. The pitch of such groove 3 can be e.g., 15-30 mm. Generally, however, these features and components can have any size, e.g., as chosen by a skilled person for a specific DFIG. In some embodiments, each contact ring 1. can have multiple helical grooves 3, e.g., two grooves 3 shifted relative to each other by half a pitch, or multiple helical grooves 3 placed next to each other, e.g., each groove s having only one turn. Diameter of each cooling passage d can be e.g., 30-60 mm. Alternatively, the groove(s) 3 can have a different than helical shape, e.g., they can be circular or straight, e.g., aligned with the axis 6 of rotation of the device or angled with respect to the axis. If multiple grooves 3 are used on each contact ring 1, they are preferably placed symmetrically or regularly around the circumference of the contact rings 1, such that the weight distribution of the rings remains as symmetrical as possible.

In fig. 1 and especially in the detail in fig. 2, the depth D of the groove 3 and the distance d of the cooling passage 5 from the brush-contacting surface 4 can be seen. Since D > d, the groove 3 extends into the passage 5 I the passage 5 passes through the groove 3. Parts of the bottom of the groove 3, where the bottom is open into the passages 5, thus serve as ventilation holes for cooling the brushes 9 and also the contact rings 1 by the air forced through the cooling passage 5 by the fan and to the groove 3 by the centrifugal force.

The layers of insulating material 7 separating the contact rings 1 from each other or from other components, e.g., 20-100 mm thick, preferably comprise cut-outs 8 aligned with the cooling passages 5, so that the air can flow in each passage 5 between the rings in a straight line, without being significantly obstructed by the insulating material 7. As can be seen in figs. 1 and 3, the insulating material 7 can be disc-shaped piece of material, having smaller radius where there are cut-outs 8 in the disc (see the bottom side of fig. 1 ) and larger radius around the electrical conductors 2 connecting the contact rings 1 to the rotor windings (see the top side of fig. 1 ). Three layers of the insulating material 7 with cut-outs 8 are visible in fig. 3.

The slip ring device can comprise a chassis 12 surrounding the device, e.g., being fixed to the stator of the generator or enclosing the whole generator. The brushes 9 can then be fixed to the chassis 12, preferably via a brush holder 11 . In order to force the air moved by the cooling fan into contact with surfaces on the slip ring device having the highest temperature, i.e., the outer surfaces of the contact rings 1 and the ring-contacting surfaces of the brushes 9, a deflector 10 is preferably provided inside of the chassis 12. The deflector 10 can for example be a plate of electrically insulating material 7 or several such plates fixed to the chassis 12 along the axis 6 of rotation of the slip ring device. These plates surround the contact rings 1 and their insulations and ensure that the cooling air passes through the cooling passages 5 or along the outer surface of the contact rings 1 and brushes 9, instead of e.g., passing along the chassis 12 where cooling is not needed. Preferably, the deflector 10 is made of glass fiber reinforced polymer, e.g., a glass cloth or fabric impregnated with thermosetting epoxy resin. In some embodiments, the deflector 10 can be made e.g., from metal sheets. Each of the plates forming the deflector 10 can for example have a thickness of 1 -3 mm, e.g., 2 mm.

The air for cooling can enter the chassis 12 through an air inlet opening provided in the chassis 12, e.g., on one side of the chassis 12 or at the bottom, and after passing through the cooling passages 5, the air can leave the chassis 12 through a similarly placed outlet opening. The inlet and/or the outlet is preferably provided with at least one air filter, e.g., to prevent dust from getting into the chassis 12.

As depicted in fig. 5, the deflector 10 might surround the rotating part of the device at part of its circumference only, because the rest of the circumference is surrounded by the brush holder 11 , which also limits the amount of air that can pass without cooling the brushes 9 or the contact rings T Preferably, the brush holder 11 is placed substantially above the contact rings 1 and the deflector 10 is below the brush holder 11 and the contact rings 1_. This arrangement limits the extent to which gravity causes different brushes 9 on the same contact ring 1 to be pushed towards the ring with different pressures (see fig. 5 with the brushes 9 being distributed above the contact rings 1). In some embodiments the deflector 10 can be a part of the brush holder 11 or of the chassis 12. The number of brushes 9 can be different in different embodiments, however, preferably, they are placed above a horizontal plane passing through the axis 6 of rotation.

The amount of cooling passages 5 is chosen such that there is enough of them to keep the brushes 9 at low enough temperatures without obstructing transfer of electrical current through the contact rings 1, i.e., form the outer surface, where the current is supplied by the brushes 9, towards the middle part of the ring, where the electrical conductors 2 to the rotor windings are provided (see fig. 3 for placement of the electrical conductors 2). Preferably, the cooling passages 5 are circular, as circular holes are easier to manufacture and the current can then be effectively passed between the passage-forming holes in each contact ring 1_. The rings can also comprise balancing holes, which do not have to extend through the whole thickness of the ring, and which are made in order to remove material as needed in order to provide a symmetrical weight distribution to the contact rings 1_.

Exemplary embodiment of the distribution of the passages 5 and also of the balancing holes can be seen in fig. 4. There are three groups of three passages 5 placed along the outer perimeter of the device. The middle passage 5 in each of these groups is smaller than the outer ones in order to provide enough space for the balancing holes. In this figure, the distance d between the channels and the brushcontacting surface(s) 4 of the contact rings 1 can also be seen as the distance between the outermost point of each passage 5 and the closest point of the brush-contacting surface 4. The smaller middle passage 5 can be placed such that its distance from the outer surface is the same as for the outer passages 5, but it can also be further away from the brush-contacting surface 4.

In other embodiments, the passages 5 can all have the same diameter, can be distributed equidistantly around the perimeter, can have noncircular, e.g., oval or semicircular, shape etc.

The grooves 3 on the contact rings 1 can also intersect each other. For example, two helical grooves 5 can be provided on each contact ring 1_, wherein the grooves 5 have different orientations, i.e., one is left-handed, the other is right-handed. There are thus multiple points of intersection between the grooves 5 on each contact ring 1_. Such an embodiment is depicted in Fig. 6. This configuration can further improve the cooling of the brushes 9, as more air would pass through the grooves 5. Any other features of this embodiment can be, for example, as described for any embodiment above.

The subject of the invention is also a DFIG comprising a rotor and a stator, wherein the rotor is supplied power or supplies power via the slip ring device according to the present invention, e.g., as described above.

List of reference numbers

1 - contact ring

2 - electrical conductor

3 - groove

4 - brush-contacting surface

5 - passage

6 - axis of rotation

7 - insulating material

8 - cut-out

9 - brush

10 - deflector

11 - brush holder

12 - chassis