Technical field

The present invention relates to slip rings for traction motors. In particular, it relates to slip rings cooled by means of a conductive core.

Background of the Invention

Traction motors generally require a DC current to be supplied to the rotor in order to create a magnetic field with its windings, and thus a rotary electrical connection is needed. Permanent magnets can be used to avoid the need for the rotary electrical connection, but they are not as strong as electromagnets and require rare elements, such as neodymium, so they are not practical for most applications. Rotary electrical connection can operate with a contactless energy transfer, but this approach is not suitable for motors with higher power, e.g., above 150 kW. Sliding contact energy transfer, i.e., slip ring contact, is thus used in most applications.

One of the downsides of slip rings is a wear of carbon brushes which maintain the sliding contact. The wear is exacerbated by a temperature increase caused by electrical resistance of the sliding contact. It is therefore advantageous to provide traction motor slip ring device with cooling systems for the brushes. A system for providing a passive cooling of the brushes is disclosed in document W02020148014 A1. In this document, brush holders with heat-sink elements are used to conduct the heat away from the brushes and thus prolong their lifetime. Intensity of the cooling is however limited, so it would be advantageous to provide a solution for improved cooling of a slip ring device. 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 traction motor, the slip ring device comprising at least two brushes, at least two contact rings, and a sleeve for fixed connection to the rotor of the traction motor, especially to the rotor shaft. The sleeve is preferably a separate part rigidly attached to the rotor. Each contact ring is provided with an electrical connector for connection to rotor windings and is in a sliding contact with a brush. Each brush can be spring-loaded to maintain the contact and can be connected to a power source. The slip ring device further comprises a core made of a material having thermal conductivity of at least 85 W m" ¹ K" ¹ , preferably at least 100 W m" ¹ K" ¹ , preferably at least 120 W ■m" ¹ K" ¹ , wherein the core is at least partially located inside the sleeve and extends at least partially inside at least one of the contact rings. The thermal conductivity can be e.g., as measured when temperature of the material is 25 °C. The conductivity of the core material is preferably higher than that of the material of the rotor shaft, e.g., iron or steel.

Heat from the at least one contact ring generated by a current passing through the sliding contact, which has a relatively high resistance, e.g., a hundred times higher than the brush itself, is thus transferred, at least partially, to the core. The contact ring is thus cooled, and so is the brush contacting it. The core can dissipate the heat to its surroundings. An active cooling is however preferable. Traction motors are generally provided with water-tight casings so there is no air circulation which could move the air from the inside of the casing to the outside and thus could remove the heat, so actively cooling the core can significantly improve the transfer of heat away from the sliding contact. Preferably, temperature of the brushes is kept under 100°C. The core can also transfer away heat from other parts of the slip ring device, e.g., from bearings. Further, the core increases the mechanical stability of the device and can help dampen vibrations. It preferably extends into all the contact rings so they can all be efficiently cooled. In some embodiments, however, the core might not extend into a certain contact ring and the heat from this ring might then be transferred to some extent to the core via other components, such as the sleeve or an electrical insulation.

The sleeve is the main structural part of the device and connects the core, the electrical connectors, and the contact rings, directly or indirectly, to the rotor. In can also provide contact surfaces for bearings or seals. Preferably the sleeve contains a circular outer surface for a bearing. It preferably contains a circular outer surface for a seal, e.g., a shaft seal or an oil seal. The sleeve can provide passages for a coolant, as described below. The core and the sleeve are preferably symmetrical around the rotor rotation axis or at least around a plane passing through the axis. They can e.g., have a cylindrical or conical shape, with diameter varying along their length. This helps eliminate the runout of the rotor shaft, which is a significant concern at the high rotation speeds traction motors generally operate at.

The core is preferably made of metal, especially aluminum or an alloy thereof. It can also be from copper or a copper alloy as they also have very good thermal conductivity properties. Aluminum is advantageous because it is light, cheap, and better dampens vibrations when compared to copper. The core is preferably rigidly fixed to the sleeve, e.g., by an interference or press fit. The core is preferably not hollow to ensure good thermal conductivity and mechanical stability, i.e., it is solid without cavities.

The slip ring device advantageously further comprises a cooling space for passing of a coolant therethrough, wherein at least part of the core extends into the cooling space. The extending part can have any suitable length; among other things, this length has impact on the intensity of cooling. The part of the core extending into the cooling space can be e.g., 2-75 mm long, preferably 5-50 mm, e.g., 5-30 mm. The cooling space can be defined by the sleeve and/or the core, preferably it leads between them. The core is thus actively cooled which increases heat transfer from the contact rings. The coolant can be air, water, oil, or any other suitable medium. The intensity of the core cooling can be regulated by making the extending part larger or smaller or providing it with increased surface area. Preferably the coolant is oil. There can be a seal, e.g., an O-ring, provided between the core and the sleeve, to prevent any medium from getting towards the contact rings.

The slip ring device, especially the sleeve, can further comprise at least one cooling channel leading into the cooling space. This is especially advantageous when the motor is provided with a coolant circuit running through the rotor shaft. The channels can then transfer the coolant from the shaft to the outside sleeve surface. Preferably, the coolant channel(s) are non-parallel to the rotation axis. The rotation of the device than pumps the coolant with centrifugal force. The rotation of the rotor can be e.g., over 10000 RPM, e.g., 17000 RPM, so the pumping effect can be significant. Preferably, the channels lead to the cooling space in such an arrangement that the length of the core part extending into the cooling space has impact on the flow through the cooling space and the coolant channel(s). The core can thus be used to decrease the flow or the speed of the coolant to prevent it from e.g., abrasing other components in its way. This way of coolant speed decreasing is preferable to e.g., limiting cross-sections of the channels, since drilling small-diameter channels is generally complicated or at least relatively expensive.

Preferably, the cooling space is adapted for connection to a coolant circuit of the traction motor. The core is thus cooled with the same medium as the traction motor itself. Preferably, the medium is liquid. Generally, the liquid medium has operation temperature around 50°C, so there is a large enough temperature gradient to effectively cool the contact rings, and through them the brushes.

The part of the core extending into the cooling space may comprise elements shaped for increasing surface area. Such elements might be e.g., blades, fins, ribs, or any other protrusions or recesses. The cooling intensity can thus be regulated with these shape elements. This is especially advantageous when the core extends into a hollow air-containing space, e.g., at the end of the motor casing, since the elements might make the air circulate and thus provide active cooling even in a closed space not otherwise adapted to provide air circulation. The core can thus be adapted to (additionally) transfer heat in the direction away from the rotor, as opposed to the direction towards the rotor, where the heat is transferred when the motor coolant circuit is used to cool the core.

The core is preferably separated from the at least one contact ring it extends into by a layer of electrically insulating material. This layer is thin enough to not completely hinder heat transfer but thick enough to provide sufficient electrical insulation. Any insulation material known in the art can be used, especially resin- or polymer-containing materials. Thickness of the layer can be e.g., 0.1 -2 mm, especially 0.5-1 .5 mm, depending on the materials used, voltage, etc. The core and/or the sleeve is preferably also insulated from the electrical connectors, e.g., by use of the same insulation material. Preferably, the connectors are placed inside the sleeve and then insulated.

Preferably the core extends into each contact ring and the contact rings are supported by the core. This provides for the best heat transfer from all the rings. The contact rings can be supported on the core with an insulation in-between, as described above. The sleeve thus supports the rings only indirectly - the rings can be placed on a part of the core extending from the sleeve, which means the sleeve can be shorter and thus significantly cheaper to make.

Preferably, at least one brush, more preferably each brush, is provided with a heattransfer element for transferring heat away from the brush, the heat-transfer element being connected to the brush. The sliding contact between brushes and contact rings is thus cooled from both sides which further increases the cooling intensity and decreases the temperature of the brushes. The at least one brush might further comprise a brush holder wherein the brush and the heat-transfer element are supported by the brush holder and electrically insulated from each other by the brush holder. The heat-transfer elements of all the brushes can then be connected to a common heat sink without short-circuiting the power supply.

The drawbacks of the know solutions are further at least partially eliminated by a traction motor which comprises the slip ring device described above. The slip ring device is connected to the rotor shaft via its sleeve and is preferably connected via its cooling space to a coolant circuit of the motor.

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 present invention and

Fig 2. Shows a schematic drawing of rotary part of the device of Fig. 1 . Embodiments of the Invention

The invention will be further described by means of exemplary embodiments with reference to the respective drawings. Subject of the invention is a slip ring device, especially for a traction motor. The slip ring device comprises at least two brushes 3 and at least two contact rings 4 for sliding contact with the brushes 3. Contact rings 4 are carried by a sleeve 1 with is adapted to be fixed, i.e., immovably connected, to a shaft of a rotor 2 of a traction motor. The contact rings 4 can be e.g., made of bronze. The sleeve 1 can for example be made of steel. The brushes 3, e.g., made of graphite or carbon, are static with respect to the rotor 2, but they can be slideably attached and spring loaded to maintain contact with the rings, as is usual in the state of the art. The brushes 3 can for example be attached to a bearing shield of the motor, while the sleeve 1 and the contact rings 4 rotate together with the rotor 2. Each contact ring 4 is provided with an electrical connector 5 which connects it to windings (not shown) of the rotor 2 when the slip ring device is mounted to the motor. Metal sheets for windings of the rotor 2 surrounding the shaft are depicted in Fig. 1. A heat-conducting, preferably aluminum, core 6 is provided inside the sleeve T The core 6 extends into at least one of the contact rings 4. The core 6 conducts heat away from the contact rings 4, provides mechanical stability to the slip ring device, and can help with dampening vibrations. An embodiment of the device according to the invention is illustrated in Fig. 1 and Fig. 2.

The sleeve 1 preferably has a circular perimeter with diameter varying along its length, as can be seen in Fig. 2. The axis of the sleeve 1 is aligned with the axis of the rotor 2 and also with the axis of the core 6 in order to make weight distribution as symmetrical around axis of rotation as possible. The sleeve 1 can be connected to the rotor 2 e.g., by pressing and/or welding, the core 6 can be fixed to the sleeve 1 for example also by pressing, i.e., interference fit. Preferably, the traction motor is liquid cooled, and the slip ring device, especially the sleeve 1, partially defines a coolant circuit. The sleeve 1 can define a cooling space 7 and coolant channels 10 through which the coolant can circulate. The core 6 extends into the cooling space 7 and the transfer of heat away from the contact rings 4 is thus improved. Oil can be used as the coolant, e.g., the coolant circuit can then be common for the motor, the slip ring device and also for a transmission of the motor. The oil can then also contact bearings 9. On the opposite end of the sleeve 1, facing away from the motor, the core 6 can protrude from the sleeve 1 and provide a support for the contact rings 4. All the contact rings 4 are thus located around the core 6. The contact rings 4 and the electrical connectors 5 are separated from each other, the core 6, and the sleeve 1 by an electrically insulating material 8. The layer of insulation between the core 6 and the contact rings 4 is preferably as thin as possible while maintaining sufficient electrical insulation, in order to not impede heat transfer more than necessary. For example, a so- called BMC (Bulk Moulding Compound) can be used as the electrically insulating material 8. BMC is a polymer composite comprising glass fiber and a resin with other optional additives. Typically, the fibers are 2-12 mm in length and constitute 10-30 % of the compound. Resin is typically polyester or vinylester resin. Any other insulating material 8 can alternatively be used. The thickness of the layer can be e.g., 0.1 -2 mm, preferably at least 0.5 mm, such as 1 -1 .3 mm, to secure good enough electrical insulation.

Channels for the electrical connectors 5 can also be provided inside the sleeve 1_. The electrical connectors 5 can be e.g., made of copper, preferably with a small amount of zircon, e.g., up to 1 or 2 %. Insulated electrical connectors 5 can be used, but preferably they are insulated from the sleeve 1 after they are inserted therein to make the insertion easier and avoid damaging the insulating during the insertion. BMC can be used to insulate the connectors 5, together with insulating the contact rings 4. Alternatively, a sealant, e.g., silicone sealant, can be used for insulation.

The outer surface of the sleeve 1 can have several different diameters forming contact surfaces for bearings 9 or seals. In the depicted exemplary embodiment, an oil seal 1 1 is provided to keep the oil circuit used for cooling closed, e.g., to keep the oil away from the brushes 3. A dust lip 12 is also used, in contact with another sleeve 1 surface, to keep the oil seal 11 clean. The dust lip 12 can also be a part of the oil seal 1 1 . On the other side of the oil seal 1 1 , a bearing 9 is provided which is one of the two bearings 9 used to secure the rotor 2, the other being at the opposite end of the motor. As can be seen in Figs. 1 a 2, coolant channels 10 are connecting the cooling space 7 and the inside of the rotor 2 shaft with the area around the bearing 9. The coolant channels 10 are provided at an angle with respect to the axis of rotation. The sleeve 1 thus also serves as a centrifugal pump which keeps the oil circulating. The length of the core 6 which extends into the cooling space 7 can than regulate the flow through the coolant channels 10 and thus can regulate the speed of the flowing oil. The length of the core 6 extending into the cooling space ?, and also its shape, e.g., a presence of any fins, also regulates the intensity of cooling of the core 6. The length can be, e.g., 3-30 mm, preferably 5-20 mm.

The core 6 can generally be made of any material with thermal conductivity at least 85 W m ^-1 K" ¹ , preferably at least 100 W m" ¹ K" ¹ , measured e.g., at 25°C. Aluminum, copper or alloys thereof are preferably used. In some embodiments, it can also be made from a composite material, however making the core 6 from a single piece of metal is in most cases the cheapest and most practical option. The electrical connectors 5 can alternatively be from any conductive material, e.g., copper, bronze etc. Copper alloys are the most advantageous because of their electrical and mechanical properties.

In some embodiments the motor might not have a liquid cooling circuit, or the sleeve 1 might not be connected to such a circuit. In such embodiments, the core 6 can then cool the contact rings 4 only passively, i.e., without transferring the heat to any medium. In other such embodiments the motor can be air cooled and so can be the core 6. The core 6 can also be adapted to transfer heat in the other direction than described above, i.e., away from the rotor 2. For example, the end of the core 6 facing away from the rotor 2 can extend into an air-filled space, which can be e.g., a closed space inside the motor casing. This end is then preferably provided with shape elements, e.g., fins, blades, or other protrusions, for making the air circulate and thus serve as a coolant. In some embodiments, the core 6 can be adapted to both transfer the heat towards the rotor 2, especially to its coolant, and to transfer the heat away from the rotor 2, especially via air-circulating shape elements. Water or other liquid medium can be used instead of the oil in some embodiments.

In some embodiments, the core 6 can be electrically connected to a contact ring 4 and to winding(s) of the rotor 2 and thus serve as the electrical connector 5. The core 6 can then be divided into two parts and serve as both + and - electrical connector 5. Having the core 6 insulated and providing separate connectors is however generally easier to manufacture. The sleeve 1 can be a part of the rotor 2, i.e., it can be from the same peace of material as the output shaft of the rotor 2.

It might be advantageous to provide cooling not only to the contact rings 4 but also to the brushes 3. Each brush 3 or at least one of them can then be provided with a heattransfer element, e.g., made of aluminum or any other material suitable for creating the core 6, placed to a proximity of the brush 3 in order to conduct heat away from it. The heat-transfer element can be in direct contact with the brush 3, but preferably they are insulated from each other. The brush(es) 3 can be held in place by a brush 3 holder. Such a holder can e.g., be made of an electrical insulant and can keep the brush 3 in place in contact with the respective contact ring 4. It can delimit a sliding movement of the brush 3 and contain a spring to push the brush 3 towards the contact ring 4. It can also comprise an electrical lead connecting the brush 3 to a power source. The heat-transfer element can then be also supported by the holder. The insulated space between the element and the brush 3 can be, e.g., 0.1 -2 mm, as described above for insulation for the core 6. Any electrically insulating but preferably heat conductive material(s) can be used as insulating material in the present invention. The same material, e.g., BMC or any other suitable plastic or polymer, can be used for insulation of the brush 3 holder. The heat-transfer element can be actively cooled, and/or it can dissipate heat into its surroundings. A common heat sink can be provided for heat-transfer elements of all the brushes 3 or for each brush 3 separately. A subject of the present invention is also a traction motor comprising the slip ring device of the invention. The traction motor is preferably an automobile traction motor. Any motor, especially an excited synchronous motor, can however also be provided with the slip ring device of the invention, regardless of e.g., number of poles, power or intended application of the motor.

List of reference numbers

1 - Sleeve

2 - Rotor

3 - Brush

4 - Contact ring

5 - Electrical connector

6 - Core

7 - Cooling space

8 - Insulating material

9 - Bearing

10 - Coolant channel

11 - Oil seal

12 - Dust lip