Layered carbon brush for an electric motor

Abstract

Layered carbon brush (20) for an electric motor (100) with at least one first carbon brush part (10) and a second carbon brush part (19), which are electrically insulated from each other by means of an intermediate non-conductive connecting layer (18) for the mechanical connection of the carbon brush parts (10, 19), characterized in that each of the carbon brush parts has its own interface for connection to a connecting line for the electrical supply of current, and that the first carbon brush part (10) is made of a different material than the second carbon brush part (19).

Description

The invention relates to a laminated carbon brush for an electric motor, having at least a first carbon brush part and a second carbon brush part, which are connected by means of a non-conductive connecting layer arranged therebetween for the mechanical connection of the carbon brush parts.Carbon brushes for electric motors, also referred to as brush carbon or motor carbon, serve to produce a sliding contact with the substantially cylindrical commutator of an electric motor.In particular in electric motors which are operated at high speeds, such as washing machine motors which are operated with alternating current, it is customary to use carbon brushes designed as multi-layer carbon brushes.The published patent application DE 10 2014 214 221 A1 already discloses a production method for a two-layer carbon brush. In this case, a homogeneously formed connecting layer with a curing adhesive mass directly adjoining connecting contact surfaces of the carbon brush parts is provided between two carbon brush parts. This from the prior art is also shown in FIG. 1 below.A further solution for a production method of multilayer carbon is known from EP 3 018 772 B1, in which the electrically conductive material or a material containing it as filler and the electrically insulating material are introduced layer by layer into a mold in a sequence corresponding to the layer profile of the multilayer carbon brush to be produced, then pressed and subsequently heat treated. Comparable production methods are also known from the publications DE 10 209 199 A1, WO 2007 012 453 A1, WO 2007 090 435 A1 and DE199 135 99 A1.Finally, DE 10 2010 038832 A1 discloses a multilayer carbon brush having a carbon body which has two carbon layers which are separated from one another by an insulation layer and are arranged in a composite structure and which form a contact surface in a contact section with their end faces opposite a connection conductor side of the carbon body for bearing against a commutator of an electric motor. In the installed state, the resulting contact section has a contact cross section which has a width a parallel to the axis of the commutator and a height t tangential to the circumference of the commutator, wherein, at a height t greater than or equal to 4.5 mm and less than 5.5 mm, the width a is at least 2.8 times the height t. Thus, in this solution, a very specific ratio of the dimensioning of the contact surface is predefined.The document DE 10 2013 103 480 A1 discloses a multi-layer carbon brush for establishing a sliding contact with a commutator of an electric motor, which comprises a body and a shunt. The body includes a first electrically conductive layer, a second electrically conductive layer, and a separation layer disposed between the first and second conductive layers and electrically separating the first and second conductive layers. The shunt is coupled to the first conductive layer. The first conductive layer has a thickness greater than a thickness of the second conductive layer, and the separation layer has a lateral series resistance greater than the series resistance of the first and second conductive layers. Since only the first conductive layer is electrically coupled via the shunt, the second layer is not electrically connected to the conductive terminal of the electric motor. The body acts in a similar manner to a capacitor in this solution.A comparable solution is also shown in US 2007 / 0042650 A1.In contrast, it is an object of the present invention to improve known multi-layer carbon brushes for use in an electric motor.This object is achieved by a layered carbon brush having the features of claim 1. This is characterized in that the first carbon brush part is produced from a different material than the second carbon brush part. Such a layered carbon brush can be used both in motors with a single direction of rotation (the drive shaft of the electric motor) and in motors with two directions of rotation (the drive shaft). It is thus conceivable to use the layered carbon brush according to the claim in electric motors of washing machines, for example, or in electric motors of power tools in the kitchen or construction sector. In this case, the motor-driven drive shaft of a hammer drill or a kitchen mixer can have a main direction of rotation, but can also rotate in an opposite direction (two directions of rotation with a main direction of rotation), while the drive shaft can be driven in a single direction of rotation by, for example, an angle grinder, an electrically driven saw, a multi-tool and the like.Both carbon brush parts are provided via their own interface for connection to a connecting line for supplying current and are thus electrically active.Suitable materials for the carbon brush parts are basically electrically conductive materials, wherein the specific electrical resistance of the carbon brush part can be different depending on the material selected. Further influencing factors in the selection of a suitable material or a suitable material composition can be a desired hardness, transverse rupture strength and the like.By means of the layered carbon brush, current is to be conducted through the carbon brush to the commutator (also collector or current inverter). The commutator is usually formed with fins in the circumferential direction. The carbon brushes are looped on the plates, which connect the stationary supply lines to the rotating coils of the rotor of the electric motor. Each lamination is connected to the beginning of one coil and the end of another coil of a commutator winding. During operation, undesired cross currents can be established between the lamellae when a plurality of lamellae are simultaneously contacted with a carbon brush.A known disadvantage when using carbon brushes in electric motors is the excessive spark formation at the commutator (brush fire). Two effects cause electrical breakdowns to occur at the carbon brushes, which become visible as sparks:On the one hand, under load, the neutral zone rotates out of the center of the pole gap, as a result of which the induction voltage is not zero at the time of the current reversal. Precisely at this moment, the current flow in the outgoing coil connected to the commutator segments is separated, so that break-off sparks are produced, which are called brush fires.Furthermore, there are motors in which the brushes are so thick (the thickness here denotes the tangential extent with respect to the substantially circular cross section of the commutator) that they contact a plurality of plates of the commutator for a short time and thus cause both cross currents between the plates and a short-term short circuit which also entails a brush fire. Brushers have several disadvantageous consequences: on the one hand, it causes soiling of the lamellae and, on the other hand, leads to increased wear of the carbon brushes, since the heat which arises in the process promotes wear of the carbon brushes.In practice, therefore, a known solution consists in using as many lamellae and armature windings as possible in order to reduce the current density at the carbon brushes. At the same time, however, this increases the number of simultaneously contacted lamellae with a brush of constant thickness, as a result of which more undesired transverse currents can occur.In the assembled state of the electric motor, the coated carbon brush is installed in the electric motor in such a way that the coating direction runs substantially tangentially to the circumferential contour of the cylindrical commutator or collector. In this way, even in the case of known two-layer carbon brushes, the individual thicknesses of the carbon brush parts are halved compared to the total thickness by the insulation layer provided between the carbon brush parts, which electrically isolates the individual carbon brush parts from one another, and thus both the transverse currents occurring and the current density are reduced. However, the reduction of the thicknesses also undesirably increases the electrical resistance in the individual carbon brush parts.Compared to this known solution, the above-described undesired effects when selecting suitable materials can be reduced by selecting two carbon brush parts from different materials.Thus, the first carbon brush part (the first layer) rising in the main rotation direction of a rotating motor part (the rotating commutator) of the electric motor may be made of a first conductive material having a reduced electric resistance, thereby providing a higher current density in the first carbon brush part. However, the trailing second carbon brush part may be made of a second conductive material, which has, for example, a higher electrical resistance compared to the first material, whereby a reduced current density is provided at the second carbon brush part and so-called carbon brush fires may in turn be reduced.Alternatively or additionally, this object can also be achieved by a layered carbon brush having the features of claim 2. This is characterized in that the first carbon brush part has a first material thickness and the second carbon brush part has a second material thickness, wherein the first material thickness and the second material thickness are different.In this way, the known undesirable effects of occurring transverse currents and carbon brush fires can be countered in turn, for example by the first carbon brush part (the first layer) running up in the main direction of rotation of the rotating part (of the rotating commutator) of the electric motor having a first greater material thickness (in the tangential direction with respect to the outer periphery of the commutator).As a result, the first carbon brush part can, for example, in turn have a lower electrical resistance, as a result of which a higher current density can be provided, while the trailing second carbon brush part can have a second material thickness which is reduced in comparison with the first material thickness. In this way, the second carbon brush part can in turn have a higher electrical resistance than the first carbon brush part, as a result of which a reduced current density is provided and so-called carbon brush fires can in turn be reduced.Both aspects of the invention according to claims 1 and 2 thus achieve the same technical object. These can be used individually or in combination with one another in an electric motor.According to a further aspect of the invention, there is provided an electric motor having the features of claim 5.According to the invention, each of the electrically insulated carbon brush parts has its own interface for connection to a connecting line for the electrical supply of current. It is known from the prior art that each carbon brush part can have a centrally arranged interface (cf. FIG. 1 ). In contrast to this, the interfaces in the present invention can be arranged decentrally symmetrically to the connection layer (as a plane of symmetry). This variant is particularly suitable for carbon brush parts which have the same thickness but are produced from a different material.In the production of the laminated carbon brushes, the individual carbon brush parts can be bonded to one another with adhesive as a connecting layer or the laminated carbon brushes are produced in a pressing method, as is known from the prior art. Subsequently, the layered carbon brushes can be provided with interfaces or connection points for connecting the connecting lines and can be precisely brought to size during the final processing. For better contacting of the coated carbon brush with the power connection, the latter can have a bore as an interface, into which a connecting line, for example a copper cable, is inserted and pressed in ("combated"), or it can be soldered directly to a bronze leaf spring in the region of its interface.In order to simplify production, and in particular to be able to dispense with checking the orientation of the joined layered carbon brushes (i.e. the carbon brush parts which are already connected to one another by means of the non-conductive connecting layer) when providing the interfaces or connection points, in particular the bores, these can alternatively also be arranged diametrically on the joined layered carbon brush, with the result that, irrespective of whether the carbon brush part which is later installed in the electric motor and is arranged on the leading side is arranged on the left side or on the right side, the connection points are always arranged at the correct point.The diametrical arrangement is defined with respect to the surface of the laminated carbon brush which has the interfaces. In this case, for example, in the assembled state of the layered carbon brush, an interface can be arranged in an upper left corner (viewed in plan view of the surface containing the interfaces) and a further interface can be arranged in a lower right corner. The distance to the outer edges of the layered carbon brush can be predefined in such a way that, independently of the different material thickness (thickness) of the individual carbon brush parts, a corresponding interface (for example in the form of a bore) is provided on each carbon brush part.Furthermore, in a development, independently of the specific configuration and arrangement of the interfaces for the connecting lines, it can be provided that the layered carbon brush has a shut-off element.The brushes wear during operation and become shorter. Therefore, they are often accommodated in metallic guides in which they are pressed onto the commutator by means of a spring. If a predetermined threshold value for the abrasion is exceeded, the shut-off element serves for automatically protecting the electric motor from damage by the abraded carbon brushes. The shut-off element made of an electrically non-conductive material interrupts the current flow (the transmission to the commutator) and consequently leads to an automatic shut-off of the electric motor. Shut-off elements of this type are already known from the prior art, for which reason a detailed description is omitted here.According to a further development of the invention, it can be provided that the shut-off element is arranged centrally for a particularly uniform wear of the carbon brush parts with respect to the total material thickness of the layered carbon brush.Alternatively, however, it can also be provided that the shut-off element is arranged eccentrically with respect to the total material thickness of the layered carbon brush for a particularly reliable protection of the electric motor, in particular centrally in the carbon brush part which is arranged on a side running on a (main) rotating shaft of the rotating motor part in the assembled state of the electric motor.In addition, it should be noted that terms such as "comprising" "having" or "with" do not exclude other features or steps. Further, terms "a" or "the" that indicate a singular number of steps or features do not exclude a plurality of features or steps and vice versa.Further features and advantages of the invention are evident from the following description of the exemplary embodiments of the invention and from the dependent claims.The invention is described in more detail below with reference to the attached figures. The figures show several features of the invention in combination with each other. Naturally, however, the skilled person is also able to consider them separately from one another and, if appropriate, to combine them to form further meaningful sub-combinations without the need to make him novel.They show schematically: FIG. 1 shows a layered carbon brush according to the prior art; FIG. 2 shows a laminated carbon brush according to the invention according to a first embodiment; FIG. 3 shows a laminated carbon brush according to the invention according to a second embodiment; and FIG. 4 shows a laminated carbon brush according to the invention according to a third embodiment in a state in which it is installed in an electric motor.The figures generally show a layered carbon brush designated by the reference numeral 20.FIG. 1 shows a known embodiment according to DE 10 2014 214 221 A1, namely a layered carbon brush 20 which is composed of the two carbon brush parts 10 and 19, wherein a non-conductive connecting layer 18 for the mechanical connection of the carbon brush parts 10 and 19 is provided between the two carbon brush parts 10 and 19.The connecting layer 18 can be formed, for example, as described in the prior art, from a curing adhesive composition of a reactive 1-component PUR hot melt. Alternative solutions for the configuration of the connecting layer 18 are of course likewise conceivable in connection with the invention according to FIGS. 2, 3 to 4.The embodiment of FIG. 1 known from the prior art is used in particular in the motors of washing machines. The present invention can also be used in the electric motors of further machines driven by electric motors, for example in the household, kitchen or building sector.As already explained above, a layered carbon brush according to the invention, as shown in FIGS. 2, 3 to 4, can be used in motors with a single rotational direction (the drive shaft of the electric motor) as well as in motors with two rotational directions (the drive shaft). Thus, for example, the motor-driven drive shaft of a machine tool, such as a hammer drill, or of a kitchen appliance, such as a kitchen blender, can have a main rotational direction D, but can also rotate in an opposite direction (two rotational directions with a main rotational direction D), while the drive shaft can be driven by, for example, an angle grinder, an electrically driven saw, a multi-tool and the like in a single rotational direction (main rotational direction D).Unlike in the prior art, in the configuration according to the invention of FIG. 2, the first carbon brush part 10, which is arranged in an assembled state of an electric motor 100 (cf. FIG. 4 ) on an upstream side with respect to the main direction of rotation D of a moving part of the electric motor 100, in particular of a commutator 110 of the electric motor, is formed from a different material than the second carbon brush part 19.In particular, when selecting the material, it can be ensured that the first carbon brush part 10 has a lower specific (electrical) resistance than the second carbon brush part 19.In an analogous manner, in the second configuration according to the invention of FIG. 3, the first carbon brush part 10, which is arranged in an assembled state of an electric motor 100 (cf. FIG. 4 ) on an upward-running side with respect to the main direction of rotation D of a moving part of the electric motor 100, in particular of the commutator 110 of the electric motor, can have a greater thickness T 1 than the second carbon brush part 19 with its thickness T 2. The thickness T or T1 and T2 denotes the tangential extension relative to the substantially circular cross section of the commutator 110 (in the assembled state of the electric motor 100). In this way, even with the same material of the two carbon brush parts 10 and 19, the specific (electrical) resistance of the first carbon brush part 10 can be reduced in comparison with the specific (electrical) resistance of the second carbon brush part 19.The thickness T1 in relation to the total thickness T=T1+T2 is greater than 50%, in particular 60% to 90%, preferably 60% to 80%.FIG. 4 shows a highly schematic view of an electric motor 100 or its commutator 110, which is contacted by means of a coated carbon brush 20 according to FIG. 3. As mentioned above, the commutator as the rotating part of the electric motor 100 rotates in a main rotational direction D about a rotational axis (not shown). This main direction of rotation is also indicated in FIGS. 2 and 3 by an arrow denoted by D.FIG. 4 also shows a further embodiment with respect to the layered carbon brush 20, which is composed in a manner analogous to the second embodiment of FIG. 3 of two carbon brush parts 10 and 19 with different thicknesses T 1 and T 2.As shown in FIGS. 2, 3 to 4, the layered carbon brushes according to the present invention can be formed for each carbon brush part with an interface 12, 14 for connection to connecting lines 120, 140 in the form of carbon ropes or strands (cf. FIG. 2 ). In this case, the interfaces can be arranged mirror-symmetrically with respect to the connecting layer 18 and centrally or eccentrically (cf. FIG. 2 ), as in the prior art. Alternatively, however, as shown in FIGS. 3 and 4, a diametrical arrangement of the interfaces 12, 14 is also conceivable. In the production, however, in the case of a solution as shown in FIG. 2, when the interfaces 12, 14 are provided, attention must be paid to the correct orientation with regard to the later installation direction of the coated carbon brush 20 in the electric motor 100. Due to the diametrical arrangement of FIGS. 3 and 4, this checking step is omitted during production, in particular when setting the interfaces 12, 14, for example in the form of bores.Furthermore, FIGS. 2, 3 to 4 also show a shut-off element 150, which, in the illustration shown in FIGS. 3 and 4, is arranged eccentrically (i.e. the distance y of the shut-off element 150 from the outer surface 19 aof the second carbon brush part is greater than or less than the distance x of the shut-off element 150 from the outer surface 10 aof the first carbon brush part) with respect to the total thickness T of the layered carbon brush 20. This can in turn be arranged, for example, centrally with respect to the thickness T 1 of the first carbon brush part 10 (cf. FIG. 4 ) or coincident with the connecting layer 18 (cf. FIG. 3 ).

Claims

Laminated carbon brush (20) for an electric motor (100), having at least a first carbon brush part (10) and a second carbon brush part (19), which are connected to one another in an electrically insulated manner by means of a non-conductive connecting layer (18) arranged therebetween for the mechanical connection of the carbon brush parts (10, 19), characterized in that each of the carbon brush parts has its own interface for connection to a connecting line for the electrical supply with current, and in that the first carbon brush part (10) is produced from a different material than the second carbon brush part (19).Laminated carbon brush (20) for an electric motor (100), having at least one first carbon brush part (10) and one second carbon brush part (19), which are connected to one another in an electrically insulated manner by means of a non-conductive connecting layer (18) arranged therebetween for the mechanical connection of the carbon brush parts (10, 19), characterized in that each of the carbon brush parts has its own interface for connection to a connecting line for the electrical supply with current, and in that the first carbon brush part (10) has a first material thickness (T1) and the second carbon brush part (19) has a second material thickness (T2), wherein the first material thickness (T1) and the second material thickness (T2) are different.The layered carbon brush (20) according to claim 2, characterized in that the first material thickness (T1) is greater than the second material thickness (T2), wherein the first carbon brush part (10) with the first material thickness (T1) is arranged on the leading side with respect to a (main) rotational speed of a rotating motor part in an assembled state of the electric motor (100).The layered carbon brush (20) according to claim 2 or 3, characterized in that the first material thickness (T1) is greater than the second material thickness (T2), wherein the first carbon brush part (10) with the first material thickness (T1) has a lower specific resistance than the second carbon brush part (19) with the second material thickness (T2).Electric motor (100), in particular for operating an electric machine tool, comprising a stator, a rotor and a commutator (110) having a number of layered carbon brushes (20) according to one of the preceding claims.Electric motor (100) according to Claim 5, characterized in that the connecting lines (120, 140) of the carbon brush parts (10, 19) have, in particular, carbon ropes or strands for electrically supplying the carbon brush parts (10, 19) with current, wherein the interfaces (12, 14) of the carbon brush parts (10, 19) for connection to the respective connecting line (120, 140) are arranged diametrically opposite one another in the assembled state of the carbon brush parts (10, 19).Electric motor (100) according to Claim 5 or 6, characterized in that the coated carbon brush (20) has a shut-off element (150).Electric motor (100) according to Claim 7, characterized in that the shut-off element (150) is arranged centrally with respect to the total material thickness (T) of the coated carbon brush (20).Electric motor (100) according to Claim 7, characterized in that the shut-off element (150) is arranged eccentrically with respect to the total material thickness (T) of the laminated carbon brush (20), in particular centrally in the carbon brush part (10) which is arranged on a side running on a (main) outer side of the rotating motor part in the assembled state of the electric motor (100).

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