Power transformer and electric machine

Abstract

The invention relates to a current transformer (1) for transmitting current to the rotor of an electric machine, comprising a rotor module (11) which is at least partially rotationally symmetrical about an axis of rotation (DA), wherein the rotor module (11) has at least two rotor grinding wheels (12a, 12b) which are designed in an annular shape and a stator module (13), wherein the stator module (13) has at least two counter grinding wheels (14a, 14b) which are designed in an annular shape, wherein the central axes of the counter grinding wheels (14a, 14b) are coaxial to each other and to the axis of rotation (DA) and the mean diameter of a first counter grinding wheel (14a) corresponds to the mean diameter of the first rotor grinding wheel (12a) and the mean diameter of a second counter grinding wheel (14b) corresponds to the mean diameter of the second rotor grinding wheel (12b).The stator module (13) comprises at least one actuating element (15) which presses the counter grinding wheels (14a, 14b) at least temporarily in the direction of the axis of rotation (DA) against the rotor grinding wheels (12a, 12b). The invention further relates to an electric machine and a method for adjusting a current transformer (1) during the operation of an electric machine.

Description

[0001] The invention relates to a current transformer for transmitting current to the rotor of an electrical machine, comprising a rotor module which is at least partially rotationally symmetrical to a rotational axis, wherein the rotor module has at least two rotor grinding wheels which are annular, and a stator module, wherein the stator module has at least two counter-grinding wheels which are annular, wherein the central axes of the counter-grinding wheels run coaxially to one another and to the rotational axis and the average diameter of a first counter-grinding wheel corresponds to the average diameter of the first rotor grinding wheel and the average diameter of a second counter-grinding wheel corresponds to the average diameter of the second rotor grinding wheel. The stator module comprises at least one actuating element which presses the counter-grinding wheels against the rotor grinding wheels at least temporarily in the direction of the rotational axis.The invention further relates to an electrical machine and a method for adjusting a current transformer during operation of an electrical machine.

[0002] Transformers or current transformers are used to transmit electrical current to the coils of a rotor of an electrical machine. The rotor with the coils rotates about its axis of rotation when the electrical machine is in operation. The stator of the electrical machine, on which the power supply for the rotor is also arranged, remains unchanged in its position during operation of the electrical machine. In order to transmit electrical current from the stationary stator to the moving rotor, a current transformer has at least two electrical contact points at which a sliding contact is established between a moving rotor module and a stationary stator module. For example, two slip rings can be arranged on the rotor axis, the contact surfaces of which point radially away from the axis of rotation.In this case, two carbon brushes can be fixedly mounted on the stator side, which are resiliently pressed in the radial direction to the axis of rotation against the slip rings rotating with the rotor. Such a power transmitter can be operated dry, i.e. without the use of coolant at the contact points. In this case, the friction generated by the brushes rubbing against the slip rings can cause the components of the transmitter to heat up considerably and thus be subject to increased wear. Furthermore, dry operation of a power transmitter without coolant usually requires the provision of a radial shaft seal on the rotor shaft to keep the coolant used in the electrical machine away from the power transmitter. Such a radial shaft seal is also subject to wear, generates additional losses, and also requires installation space in the electrical machine.Alternatively, it is possible to operate a power transformer in a wet state, i.e., using a coolant. This allows frictional heat to be dissipated more effectively from the contact points. However, the coolant can cause the transformer's brushes to float, resulting in a loss of contact between the brushes and the slip rings. This disrupts the operation of the electrical machine and can also lead to sparking, which in turn leads to increased wear on the power transformer or even damage to it.

[0003] WO2017 / 208626 A1 describes an electric machine used as a signal generator for controlling the valves of an internal combustion engine. This electric machine comprises a current transformer in which slip rings are mounted on a rotor. These slip rings have contact surfaces oriented axially relative to the rotor's rotational axis. Spring-mounted brushes are pressed onto these contact surfaces in the axial direction.

[0004] DE 10 2016 200 766 A1 describes an electric machine comprising a rotatably mounted rotor mounted on a hollow shaft. Electric current is transmitted to the rotor via a current transformer, which has at least one slip ring with a contact surface oriented axially to the rotational axis, against which a brush is pressed in the axial direction.

[0005] EP 2 426 793 A1 describes a slip-ring brush system for a current-excited synchronous motor. In this slip-ring brush system, two slip rings are arranged on the rotor of an electric machine, oriented axially to the rotational axis, and integrated into an insulating slip-ring unit. The slip rings are spaced apart from each other radially to the rotational axis. The slip rings each interact with a brush mounted on the stator, which is spring-mounted in the axial direction to the rotational axis.

[0006] The object of the invention is to propose solutions that enable the transmission of electrical current to a rotating rotor of an electrical machine to be made more reliable and with less wear.

[0007] This object of the invention is achieved by a current transformer for transmitting current to the rotor of an electrical machine, comprising - a rotor module which is at least partially rotationally symmetrical to a rotational axis, wherein the rotor module is provided for fastening to the rotor shaft of an electrical machine, wherein the rotational axis is oriented coaxially to the rotational axis of the rotor during this fastening, wherein the rotor module has at least two rotor grinding wheels which are annular, wherein the central axes of the rotor grinding wheels are coaxial to one another and the average diameter of a first rotor grinding wheel is greater than the average diameter of a second rotor grinding wheel, - a stator module which is provided for attachment to the stator of the electrical machine, wherein, in this attachment, the stator module is arranged adjacent to the rotor module in the direction of the rotational axis, wherein the stator module has at least two counter-grinding wheels which are ring-shaped, wherein the central axes of the counter-grinding wheels run coaxially to one another and to the rotational axis, and the mean diameter of a first counter-grinding wheel corresponds to the mean diameter of the first rotor grinding wheel, and the mean diameter of a second counter-grinding wheel corresponds to the mean diameter of the second rotor grinding wheel, wherein the rotor grinding wheels each have a contact surface which is at least partially flat, which runs in a plane perpendicular to the rotational axis and is directed towards the stator module in the direction of the rotational axis, and the counter-grinding wheels each have a counter-contact surface which is at least partially flat,which runs in a plane perpendicular to the axis of rotation and is directed in the direction of the axis of rotation towards the rotor module, and the stator module comprises at least one actuating element which presses the counter grinding wheels at least temporarily in the direction of the axis of rotation against the rotor grinding wheels, so that the counter contact surfaces rest at least partially on the contact surfaces, so that a transmission of electrical current between the stator module and the rotor module is possible.

[0008] The current transformer according to the invention serves to transmit current to the coils of a rotating rotor of an electrical machine. The current transformer comprises two assemblies: A rotor module is intended to be arranged on a rotor of an electrical machine so that it rotates during operation. A stator module is intended to be attached to a stator of an electrical machine. When installed in an electrical machine, a sliding electrical contact is established at least temporarily between the rotor module and the stator module, which transmits current from the stator to the rotor. By temporarily is meant that in some embodiments the electrical connection between the rotor module and the stator module can be severed.

[0009] The rotor module is shaped rotationally symmetrically to a rotational axis in some areas. The rotational axis is an imaginary axis that is used to define the current transformer. When a current transformer is installed in an electrical machine, this rotational axis is aligned coaxially to the rotational axis of the rotor of the electrical machine. At least two rotor grinding wheels, which are ring-shaped, are arranged on the rotor module. These rotor grinding wheels are made of an electrically conductive material, for example steel or copper. The center axes of the two rotor grinding wheels are aligned coaxially to one another and to the rotational axis. The average diameter of a first rotor grinding wheel is larger than the average diameter of a second rotor grinding wheel, so that the first rotor grinding wheel encloses the second rotor grinding wheel.A mean diameter is understood to be a diameter which is present in the middle of the width of the annular rotor grinding wheel. The rotor grinding wheels are firmly connected to the remaining area or the other components of the rotor module in the direction of the axis of rotation and in the circumferential direction around the axis of rotation. The stator module comprises at least two counter grinding wheels which match the dimensions of the rotor grinding wheels on the rotor module. For this purpose, the mean diameter of a first counter grinding wheel corresponds to the mean diameter of a first rotor grinding wheel. The mean diameter of a second counter grinding wheel corresponds to the mean diameter of a second rotor grinding wheel. According to the invention, the counter grinding wheels which are arranged on the stator module are also annular. This annular shape extends in the circumferential direction around the axis of rotation.When installed in an electrical machine, the rotor module and the stator module are arranged opposite one another in the direction of the axis of rotation. The rotor grinding wheels have, at least in some regions, flat contact surfaces which are oriented in a plane perpendicular to the axis of rotation and point towards the stator module. Accordingly, the counter-grinding wheels also have, at least in some regions, a flat counter-contact surface which also runs in a plane perpendicular to the axis of rotation and points towards the rotor module. When installed, the contact surfaces are therefore arranged opposite the counter-contact surfaces in the direction of the axis of rotation. The stator module furthermore has at least one adjusting element which presses the counter-grinding wheels with the counter-contact surfaces axially against the rotor grinding wheel with the contact surfaces, at least temporarily, in the direction of the axis of rotation.In this way, the contact surfaces rest on the mating contact surfaces at least temporarily, and this contact enables the transmission of electrical current between a counter-grinding wheel and a rotor grinding wheel. The counter-grinding wheels are arranged in the stator module in a rotationally fixed manner in the circumferential direction around the axis of rotation. However, the counter-grinding wheels are axially movable in the direction of the axis of rotation, with the actuating element acting on this movement of the counter-grinding wheels. The actuating element can take on different designs, for example as a linearly acting spring, as an actuator, or as a combination of both. It is also conceivable to arrange at least one actuating element in the rotor module and not to provide an actuating element in the stator module, while arranging the counter-grinding wheel there so that it is axially immobile.A functional reversal of the mobility of the grinding wheels in the axial direction, in which the axial mobility is implemented in the rotor module instead of, as previously described, in the stator module, is thus also disclosed.

[0010] In the current transmitter according to the invention, two annular grinding wheels rest on one another over a large area when transmitting current between the rotor module and the stator module. In the current transmitter according to the invention, the contact surface of a rotor grinding wheel is essentially the same size as the mating contact surface of a counter-grinding wheel. Compared to a known solution in which one side of this pairing is formed by a brush with a smaller surface area, the contact surface according to the invention between a rotor grinding wheel and a counter-grinding wheel is significantly larger. In this way, the transmission of larger currents is possible than with known solutions. Furthermore, the large contact surface between the rotor module and stator module significantly reduces the risk of the two grinding wheels floating up and breaking contact when the current transmitter is operated with coolant.In this way, sparking is prevented and wear on the current transformer is reduced. Furthermore, according to the invention, the rotor grinding wheel and the counter grinding wheel are pressed against one another in the axial direction to the rotational axis of the rotor. This reduces the installation space required for installing the current transformer in an electrical machine compared to solutions in which carbon brushes are pressed against corresponding slip rings in the radial direction to the rotational axis. The current transformer according to the invention can be used both dry, without coolant, and wet, using a coolant. Optionally, it is possible to configure the actuating element so that the pressing force between the counter grinding wheels and the rotor grinding wheels can be adjusted.In this case, this contact force can be adjusted during operation of an electrical machine, ensuring a secure, low-wear contact is always maintained without generating unnecessary frictional power due to the sliding contact between the contact surface and the counter-contact surface. Furthermore, the current transformer according to the invention has a simple design and allows for easy maintenance and replacement of the rotor grinding wheel and / or counter-grinding wheel.

[0011] In one embodiment, the rotor grinding wheels are fixed axially, radially, and in the circumferential direction relative to the axis of rotation in a rotor module housing of the rotor module, and the counter-grinding wheels are fixed radially and in the circumferential direction relative to the axis of rotation in a stator module housing of the stator module, wherein the counter-grinding wheels are mounted so as to be movable axially relative to the axis of rotation in the stator module housing. The rotor module housing and the stator module housing are preferably made of an electrically insulating material, for example a plastic. The housings serve to position and fix the grinding wheels relative to one another. In a preferred embodiment, the rotor grinding wheels are completely fixed in the rotor module, whereas the counter-grinding wheels are fixed in the circumferential direction in the stator module but are axially movable in the direction of the axis of rotation.As previously described, this mobility can also be reversed so that the counter grinding wheels in the stator module are completely fixed and the rotor grinding wheels in the rotor module have one degree of freedom in the direction of the rotation axis.

[0012] In a further embodiment, the rotor grinding wheels and / or the counter grinding wheels have slots or holes which extend at least partially axially to the axis of rotation. In this embodiment, openings or perforations are arranged in the rotor grinding wheels and / or the counter grinding wheels. This allows the coolant to be removed more effectively from the contact surfaces and counter contact surfaces when the current transmitter is operated with liquid coolant. Slots and / or holes in the grinding wheels can absorb or remove coolant, further reducing the risk of the contact surfaces and counter contact surfaces floating up. The slots or holes extend at least partially in the direction of the axis of rotation. In addition, slots can also be arranged, for example, radially to the axis of rotation or in its circumferential direction.

[0013] In one embodiment, it is provided that the rotor module and / or the stator module has at least one coolant supply, which is fluidically connected to the contact surfaces and / or the mating contact surfaces and supplies coolant to the region in which the contact surfaces and the mating contact surfaces rest on one another. By providing such a coolant supply, coolant can be supplied particularly effectively to the region in which contact surfaces and mating contact surfaces rest on one another and in which heat is generated by sliding friction when the rotor or rotor module is rotating. The coolant supply improves the dissipation of the generated heat and thus prevents overheating of the current transformer. In this way, wear on the current transformer during operation is reduced.It is possible that the coolant supply is in fluid contact with the previously described slots or holes in the rotor grinding wheels and / or counter grinding wheels.

[0014] In a further embodiment, the adjusting element comprises a spring acting axially relative to the rotational axis, which is arranged between a stationary stator module housing and the counter grinding wheels mounted so as to be movable axially relative to the rotational axis. In this embodiment, the adjusting element comprises a spring acting in the direction of the rotational axis, which presses a counter grinding wheel against a rotor grinding wheel. This achieves a constant contact force between the counter contact surface and the contact surface.

[0015] In a further embodiment, it is provided that the actuating element comprises an actuator which acts axially to the axis of rotation and is connected to a stationary stator module housing and to the counter-grinding wheels, wherein the actuator is configured to move or position the counter-grinding wheels relative to the stator module housing and the rotor grinding wheels axially to the axis of rotation and / or the actuator is configured to adjust a pressing force of the counter-grinding wheels on the rotor grinding wheels. An actuator is a component which moves a counter-grinding wheel in the direction of the axis of rotation or adjusts its pressing force against a rotor grinding wheel. This makes it possible, on the one hand, to adjust the position of a counter-grinding wheel in the direction of the axis of rotation. It is also possible to move the counter-grinding wheel away from the rotor grinding wheel so that there is a distance between the two grinding wheels.Alternatively or additionally, it is possible to adjust the contact force between the two grinding wheels, for example, depending on the speed of the rotor module or the rotor of the electric machine. This embodiment with an actuator can also be combined with the previously described embodiment, in which the actuating element comprises a spring. For example, a spring and an actuator can be mechanically connected in series.

[0016] In one embodiment, the actuator comprises an electromagnetic actuator, a hydraulic actuator, a pneumatic actuator, or a piezoelectric actuator. The actuator of the previously described embodiment can be based on different physical principles. For example, the position of a counter-grinding wheel can be adjusted using an electromagnetic actuator. By applying different currents or voltages to such an electromagnetic actuator, the position of the counter-grinding wheel can be continuously adjusted.

[0017] The object of the invention is further achieved by an electrical machine, in particular a separately excited synchronous machine, with a stator and a rotor rotatably mounted relative to the stator, in which a current transformer according to one of the previously described embodiments is arranged between the stator and the rotor, wherein the rotor module is fastened to a rotor shaft of the rotor, wherein the axis of rotation is oriented coaxially to the axis of rotation of the rotor and the stator module is fastened to the stator, wherein the center axes of the annular counter grinding wheels are oriented coaxially to the axis of rotation and the axis of rotation of the rotor and, in the direction of the axis of rotation, the contact surfaces of the rotor grinding wheels are arranged opposite the counter contact surfaces of the counter grinding wheels and the contact surfaces rest at least temporarily on the counter contact surfaces.

[0018] The electrical machine according to the invention comprises at least one current transformer according to one of the previously described embodiments. The current transformer serves as an interface for transmitting electrical current from the stator of the electrical machine to its rotor. The large contact area between the contact surfaces and the mating contact surfaces in the current transformer enables stable and low-wear current transmission within the electrical machine. Due to the small size of the current transformer, it can be easily installed in the electrical machine, simplifying its design.

[0019] In one embodiment of the electric machine, it is provided that the stator module has an adjusting element which comprises an actuator acting axially to the axis of rotation, which actuator is connected to a stationary stator module housing and to the counter-grinding wheels, and in a drive position, the actuator positions the counter-grinding wheels such that the counter-contact surfaces bear against the contact surfaces, in particular wherein the pressing force is adjustable, and in a freewheeling position, the actuator positions the counter-grinding wheels such that there is a distance between the counter-contact surfaces and the contact surfaces in the direction of the axis of rotation. In this embodiment, the current transmitter of the electric machine comprises at least one adjusting element which adjusts the relative position of at least one counter-grinding wheel to a rotor grinding wheel.When this actuating element is activated, an actuator arranged therein moves the counter-grinding wheel in the direction of the axis of rotation. This allows electrical contact to be established between the contact surfaces and the counter-contact surfaces in a drive position, whereby current is transferred to the rotor of the electric machine. However, it is also possible to activate the actuator in such a way that it positions at least one counter-grinding wheel so that there is a distance between a contact surface and a counter-contact surface. In this way, current transfer between the rotor and stator is no longer possible. However, in the freewheeling position with the rotor rotating, no sliding friction occurs in the current transmitter between the rotor module and the stator module. This means that no frictional power needs to be generated by the rotating rotor, which reduces the power loss of the electric machine in the freewheeling position.Alternatively, an actuator can be arranged between the stator module housing and the stator. When activated, this actuator moves the entire stator module in the direction of the rotation axis away from the rotor module, thus interrupting the electrical and mechanical contact between the contact surfaces and the mating contact surfaces. In this alternative embodiment, the actuating element in the stator module can be formed, for example, by a spring acting in the direction of the rotation axis, which, when the actuator is activated, is moved away from the rotor module together with the stator module housing and the mating grinding wheels.

[0020] The object of the invention is finally achieved by a method for adjusting a current transformer during operation of an electrical machine according to the previously described embodiment, wherein the electrical machine comprises at least one speed sensor which detects the speed of the rotor and a control is provided which is connected to the speed sensor and the actuator of the stator module, the method comprising the steps A) Determination of the rotor speed by the speed sensor, B) Calculation of an optimal contact force of the counter grinding wheels on the rotor grinding wheels by the control system, in particular based on a characteristic curve and the determined speed of the rotor, C) Setting the calculated, optimal contact force in the current transformer, by the control and the actuator.

[0021] The method according to the invention serves to optimally adjust the current transformer of an electrical machine depending on the current operating state of the electrical machine. To determine the current operating state of the electrical machine, at least one speed sensor is provided, which determines the rotor speed. This rotor speed is then transmitted to a controller. The controller serves to perform calculations and to control and adjust at least one actuator of a control element.

[0022] In a first process step A), the speed sensor determines the rotor speed. Process step A) is preferably performed continuously. The speed is transmitted from the speed sensor to the controller.

[0023] In a second process step B), the controller determines the optimal drive force of the counter grinding wheel on the rotor grinding wheel. The previously determined rotor speed is included in this calculation. The calculation can, for example, use a characteristic curve stored in the controller that represents a relationship between the current rotor speed and the associated optimal contact force. For example, the optimal contact force can be increased as the rotor speed increases in order to prevent contact from being broken. Alternatively, other signals or events can also be included in the calculation of the optimal contact force. For example, a torque sensor can be provided on the rotor shaft to determine the torque that the electric machine should generate.In a case where no torque is to be generated by the electric motor, the optimal contact force can, for example, be set to zero. Such a zero contact force can be achieved by eliminating any mechanical and electrical contact between the contact surfaces and the mating contact surfaces.

[0024] In a third method step C), the previously calculated optimal drive force is transmitted to the actuator by the controller. The controller configures the actuator so that it generates the optimal drive force in the current transformer. In the previously described case, in which the optimal drive force is zero, the actuator can be configured so that it moves the counter-grinding wheel into the freewheeling position, in which there is a distance between the contact surfaces and the counter-contact surfaces. In other operating states, the optimal pressure force between contact surfaces and counter-contact surfaces is adjusted by the actuator, in particular continuously. The method according to the invention has the advantage that the electric machine is always operated with the optimal pressure force between counter-grinding wheels and rotor grinding wheels.This ensures safe electrical operation and adjusts the drive force to prevent excessive wear on the current transformer. This can significantly improve the reliability and service life of an electrical machine.

[0025] Features, effects, and advantages disclosed in connection with the current transformer are also deemed to be disclosed in connection with the electrical machine and the method. The same applies in reverse: features, effects, and advantages disclosed in connection with the electrical machine and the method are also deemed to be disclosed in connection with the current transformer.

[0026] The invention is schematically illustrated by means of embodiments in the drawings and will be further described with reference to the drawings. They show: Fig. 1 in a schematic, sectional and perspective view an embodiment of a current transformer according to the invention, Fig. 2 shows a schematic, sectional side view of the current transformer Fig. 1 in a first state and Fig. 3 shows a schematic, sectional side view of the current transformer Fig. 1 in a second state.

[0027] Fig. Figure 1 shows a schematic, sectional and perspective view of an embodiment of a current transformer 1 according to the invention. Fig. 1 shows a current transformer 1 on its own, i.e., without an electrical machine. Oriented to the rear left, a rotor module 11 can be seen in a sectioned state, which is largely rotationally symmetrical to a rotation axis DA. When the current transformer 1 is installed in an electrical machine, the rotation axis DA is aligned coaxially to the rotation axis of the rotor of the electrical machine. The rotor module 1 comprises a hollow shaft that is rotationally symmetrical to the rotation axis DA. This shaft can be used to attach the rotor module 11 to the rotor shaft. Oriented to the front right, the rotor module 11 comprises an annular end that is connected to the shaft. The shaft and the end are made of an electrically insulating material, for example a plastic. The rotor module 11 comprises two coaxially arranged, annular rotor grinding wheels 12a, 12b.The center axes of these rotor grinding wheels 12a, 12b are aligned and positioned both coaxially to one another and coaxially to the rotational axis DA. The average diameter of the first rotor grinding wheel 12a, arranged on the outside, is larger than the average diameter of the second rotor grinding wheel 12b, arranged on the inside. The rotor grinding wheels 12a, 12b are partially surrounded by insulating material of the closure and are thus electrically insulated from one another. Both rotor grinding wheels 12a, 12b each have a flat contact surface, which here is ring-shaped and runs perpendicular to the rotational axis DA. These contact surfaces are directed in the direction of the rotational axis DA toward the stator module 13. It is possible for the two rotor grinding wheels 12a, 12b to be removed from the closure of the rotor module 11 and thus to be easily replaced.The rotor grinding wheels 12a, 12b are arranged in the rotor module 11 in a rotationally fixed manner in the circumferential direction about the rotation axis DA. Furthermore, the rotor grinding wheels 12a, 12b are arranged axially fixed in the rotor module 11 in the direction of the rotation axis DA.

[0028] The current transformer 1 further comprises a stator module 13 oriented to the front right, which is also largely rotationally symmetrical about the rotation axis DA. Fig. 1, the stator module 13 is shown in a position relative to the rotor module 11 that corresponds to the relative position of both modules when installed in an electrical machine. The stator module 13 is arranged adjacent to the rotor module 11 in the direction of the axis of rotation DA. The stator module 13 comprises a stator module housing made of electrically insulating material, for example plastic, which is intended for attachment to the stator of an electrical machine. Two counter-grinding wheels 14a, 14b, which are ring-shaped, are arranged in this stator module housing. The two counter-grinding wheels 14a, 14b are arranged in a rotationally fixed manner relative to the stator module housing in the circumferential direction around the axis of rotation DA. However, the two counter-grinding wheels 14a, 14b are mounted in the stator module housing of the stator module 13 so as to be axially movable in the direction of the axis of rotation DA.The rotor module 13 is oriented relative to the stator module 11 in the illustrated state and also when installed in an electrical machine such that the two center axes of the counter-grinding wheels 14a, 14b are oriented coaxially to the axis of rotation DA and thus also coaxially to the center axes of the rotor grinding wheels 12a, 12b. Both counter-grinding wheels 14a, 14b have a flat counter-contact surface oriented towards the rotor module 11. The average diameter of a first counter-grinding wheel 14a corresponds to the average diameter of the first rotor grinding wheel 12a. Furthermore, the average diameter of a second counter-grinding wheel 14b corresponds to the average diameter of a second rotor grinding wheel 12b. To transmit electrical current from the stator module 13 to the rotor module 11, the counter-grinding wheels 14a, 14b are pressed at least temporarily against the rotor grinding wheels 12a, 12b in the direction of the axis of rotation DA.In a pressed-on state, the mating contact surfaces lie flat on the contact surfaces. The stator module 13 comprises at least one adjusting element 15, which presses the counter-grinding wheels 14a, 14b against the rotor grinding wheels 12a, 12b. In the illustrated embodiment, a plurality of adjusting elements 15 are provided, which are arranged in the direction of the rotation axis DA between a respective counter-grinding wheel 14a, 14b and the stator module housing of the stator module 13. These adjusting elements 15 are symbolized by cylindrical elements in the illustration, but can also have a different shape. The adjusting elements 15 are designed to be variable in their length in the direction of the rotation axis DA. In the illustrated embodiment, each adjusting element 15 comprises an actuator acting axially to the rotation axis DA. This actuator is configured to move the counter-grinding wheels 14a, 14b in the direction of the rotation axis DA toward the rotor module 11 or away from it.Thus, by actuating the actuators of the adjusting elements 15, the relative position of the counter-grinding wheels 14a, 14b to the stator module housing and to the oppositely arranged rotor module 11, in particular to its rotor grinding wheels 12a, 12b, can be adjusted. Furthermore, the actuators make it possible to adjust the pressing force of the counter-grinding wheels 14a, 14b on the rotor grinding wheels 12a, 12b. In addition to or as an alternative to an actuator, the adjusting elements 15 can also comprise a spring acting axially to the rotational axis DA. Such a spring presses the counter-grinding wheels 14a, 14b against the rotor grinding wheels 12a, 12b through its elastic restoring force. The adjusting elements 15 can be designed and configured such that in a drive position, the counter-grinding wheels 14a, 14b bear against the rotor grinding wheels 12a, 12b, but in a free-running position, there is a distance between the contact surfaces and counter-contact surfaces.Details on the drive position are in . Fig. 2 and details of the freewheel position are in Fig. 3. However, the adjusting elements 15 can also be designed such that they comprise only a spring acting axially relative to the rotational axis DA, which permanently presses the counter grinding wheels 14a, 14b against the rotor grinding wheels 12a, 12b. In this case, a freewheeling position is neither possible nor provided.

[0029] In the illustrated embodiment, the current transformer 1 comprises two rotor grinding wheels 12a, 12b and two counter grinding wheels 14a, 14b. However, it is also possible for the current transformer 1 to comprise a different number of rotor grinding wheels 12a, 12b and counter grinding wheels 14a, 14b. Furthermore, it is possible for the rotor grinding wheels 12a, 12b to be movably mounted in the rotor module 1 in the direction of the rotation axis DA and to be positionable by at least one adjusting element 15. In this case, the counter grinding wheels 14a, 14b arranged in the stator module 13 are arranged axially immovably in the stator module housing.

[0030] Fig. 2 shows a schematic, sectional side view of the current transformer 1 from Fig. 1 in a first state. In this illustrated first state, the two counter-grinding wheels 14a, 14b rest against the two rotor grinding wheels 12a, 12b. The adjusting elements 15 press the counter-grinding wheels 14a, 14b in the direction of the rotation axis DA against the rotor grinding wheels 12a, 12b. In this first illustrated state, current can be transmitted between the stator module 13 and the rotor module 11 via the contact surfaces and counter-contact surfaces that lie against one another. The illustrated first state corresponds, in the installed state of the current transformer 1 in an electrical machine, to the drive position in which the rotor is supplied with current and thus operation or driving of the electrical machine is possible. In this drive position, there is sliding contact between the counter-grinding wheels 14a, 14b and the rotor grinding wheels 12a, 12b when the rotor of the electrical machine rotates.This sliding contact creates sliding friction in the circumferential direction around the rotational axis DA, which generates frictional heat. To overcome this sliding friction, frictional power must be generated during the rotation of the rotor, which results in power loss during rotation.

[0031] operation of the electrical machine. In some operating conditions where no current transmission to the rotor is required, this frictional power can be avoided by inserting the current transformer into the Fig. 3 shown freewheel position.

[0032] Fig. 3 shows a schematic, sectional side view of the current transformer 1 from Fig. 1 in a second state. In the Fig. The second state shown in Figure 3 is the same current transformer 1 as in the Fig. 1 and Fig. 2. In the Fig. However, in the second state shown in Figure 3, there is a distance between the counter grinding wheels 14a, 14b and the rotor grinding wheels 12a, 12b in the direction of the rotation axis DA. Thus, in this state, the counter contact surfaces do not lie against the contact surfaces. In the installed state of the current transformer 1, the second state shown corresponds to the freewheeling position. In this freewheeling position, there is no sliding contact between the counter grinding wheels 14a, 14b and the rotor grinding wheels 12a, 12b, as a result of which no sliding friction occurs and therefore no frictional power has to be applied to rotate the rotor relative to the stator. To transfer to the freewheeling position, the counter grinding wheels 14a, 14b were Fig.2, the adjusting elements 15 move the counter grinding wheels 14a, 14b in the direction of the rotational axis DA to the right, away from the rotor module 11. In the embodiment shown, the counter grinding wheels 14a, 14b are arranged completely within the stator module housing in the freewheeling position. In the embodiment shown, the counter grinding wheels 14a, 14b are moved by an actuator belonging to each adjusting element 15 reducing its length in the direction of the rotational axis DA. If the counter grinding wheels 14a, 14b are to be brought back into contact with the rotor grinding wheels 12a, 12b, the actuators are controlled accordingly so that they increase their length and thereby press the counter grinding wheels 14a, 14b to the left against the rotor module 11 in the direction of the rotational axis DA. LIST OF REFERENCE SYMBOLS: 10 current transformers 11 Rotor module 12a, 12b Rotor grinding wheel 13 Stator module 14a, 14b Counter grinding wheel 15 Control element DA rotation axis

Claims

[1] Current transformer (1) for transmitting current to the rotor of an electrical machine, comprising - a rotor module (11) which is formed at least partially rotationally symmetrical to a rotational axis (DA), wherein the rotor module (11) is provided for fastening to the rotor shaft of an electrical machine, wherein the rotational axis (DA) is oriented coaxially to the rotational axis of the rotor during this fastening, wherein the rotor module (11) has at least two rotor grinding wheels (12a, 12b) which are annular, wherein the central axes of the rotor grinding wheels (12a, 12b) run coaxially to one another and the average diameter of a first rotor grinding wheel (12a) is greater than the average diameter of a second rotor grinding wheel (12b), - a stator module (13) which is provided for fastening to the stator of the electrical machine, wherein, in this fastening, the stator module (13) is arranged adjacent to the rotor module (11) in the direction of the axis of rotation (DA), wherein the stator module (13) has at least two counter-grinding wheels (14a, 14b) which are annular, wherein the central axes of the counter-grinding wheels (14a, 14b) run coaxially to one another and to the axis of rotation (DA), and the mean diameter of a first counter-grinding wheel (14a) corresponds to the mean diameter of the first rotor grinding wheel (12a) and the mean diameter of a second counter-grinding wheel (14b) corresponds to the mean diameter of the second rotor grinding wheel (12b), wherein the rotor grinding wheels (12a, 12b) each have a contact surface which is at least partially planar and which runs in a plane perpendicular to the axis of rotation (DA) and is directed in the direction of the axis of rotation (DA) towards the stator module (13), and the counter grinding wheels (14a, 14b) each have a counter contact surface which is at least partially planar and which runs in a plane perpendicular to the axis of rotation (DA) and is directed in the direction of the axis of rotation towards the rotor module (11) and the stator module (13) comprises at least one actuating element (15) which presses the counter grinding wheels (14a, 14b) at least temporarily in the direction of the axis of rotation (DA) against the rotor grinding wheels (12a, 12b), so that the counter contact surfaces rest at least partially on the contact surfaces, so that a transmission of electrical current between the stator module (13) and the rotor module (11) is possible. [2] Current transformer (1) according to claim 1, wherein the rotor grinding wheels (12a, 12b) are fixed axially, radially and in the circumferential direction to the axis of rotation (DA) in a rotor module housing of the rotor module (11) and the counter grinding wheels (14a, 14b) are fixed radially and in the circumferential direction to the axis of rotation (DA) in a stator module housing of the stator module (13), wherein the counter grinding wheels (14a, 14b) are mounted movably axially to the axis of rotation (DA) in the stator module housing. [3] Current transformer (1) according to one of the preceding claims 1 or 2, in which the rotor grinding wheels (12a, 12b) and / or the counter grinding wheels (14a, 14b) have slots or holes which extend at least partially axially to the axis of rotation (DA). [4] Current transformer (1) according to one of the preceding claims 1 to 3, in which the rotor module (11) and / or the stator module (13) has at least one coolant supply which is fluidically connected to the contact surfaces and / or the mating contact surfaces and supplies coolant to the region in which the contact surfaces and the mating contact surfaces rest on one another. [5] Current transformer (1) according to one of the preceding claims 1 to 4, in which the adjusting element (15) comprises a spring acting axially to the axis of rotation (DA), which is arranged between a stationary stator module housing and the counter grinding wheels (14a, 14b) mounted so as to be movable axially to the axis of rotation (DA). [6] Current transmitter (1) according to one of the preceding claims 1 to 5, wherein the actuating element (15) comprises an actuator acting axially to the axis of rotation (DA), which actuator is connected to a stationary stator module housing and to the counter grinding wheels (14a, 14b), wherein the actuator is configured to move or position the counter grinding wheels (14a, 14b) relative to the stator module housing and the rotor grinding wheels (12a, 12b) axially to the axis of rotation (DA) and / or the actuator is configured to adjust a pressing force of the counter grinding wheels (14a, 14b) on the rotor grinding wheels (12a, 12b). [7] Current transformer (1) according to claim 6, wherein the actuator comprises an electromagnetic actuator, a hydraulic actuator, a pneumatic actuator or a piezoelectric actuator. [8] Electrical machine, in particular a separately excited synchronous machine, with a stator and a rotor mounted so as to be rotatable relative to the stator, in which a current transformer (1) according to one of the preceding claims is arranged between the stator and the rotor, wherein the rotor module (11) is fastened to a rotor shaft of the rotor, wherein the axis of rotation (DA) is oriented coaxially to the axis of rotation of the rotor and the stator module (13) is fastened to the stator, wherein the central axes of the annular counter-grinding wheels (14a, 14b) are oriented coaxially to the axis of rotation (DA) and the axis of rotation of the rotor and in the direction of the axis of rotation (DA) the contact surfaces of the rotor grinding wheels (12a, 12b) are arranged opposite the counter contact surfaces of the counter-grinding wheels (14a, 14b) and the contact surfaces rest at least temporarily on the counter contact surfaces. [9] Electrical machine according to claim 8, wherein the stator module (13) has an actuating element (15) which comprises an actuator acting axially to the axis of rotation (DA), which actuator is connected to a stationary stator module housing and to the counter grinding wheels (14a, 14b), and in a drive position the actuator positions the counter grinding wheels (14a, 14b) such that the counter contact surfaces bear against the contact surfaces, in particular wherein the pressing force is adjustable, and in a freewheeling position the actuator positions the counter grinding wheels (14a, 14b) such that there is a distance between the counter contact surfaces and the contact surfaces in the direction of the axis of rotation (DA). [10] Method for adjusting a current transformer (1) during operation of an electrical machine according to claim 9, wherein the electrical machine comprises at least one speed sensor which detects the speed of the rotor and a controller is provided which is connected to the speed sensor and the actuator of the stator module (13), the method comprising the steps A) Determination of the rotor speed by the speed sensor, B) Calculation of an optimal contact force of the counter grinding wheels (14a, 14b) on the rotor grinding wheels (12a, 12b) by the control system, in particular on the basis of a characteristic curve and the determined speed of the rotor, C) Setting of the calculated, optimal contact force in the current transformer (1), by the control and the actuator.

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