EP0744793A1 - Alternators slip-rings and cylindric collectors made from sintered cupro-graphitic composite material - Google Patents

Abstract

A sliding contact in the form of a ring or a segment of a ring, used in the rotor of an electric motor or generator where it engages at least one stationary pick-up brush, comprises an electrically conducting sintered composite of 90-98 wt.% Cu or Cu alloy containing flakes of graphite of which more than 50% are preferentially inclined by at least 45 deg. to the rotary axis. Density is 6.5-8.5, and its electrical resistance parallel to the rotary axis is higher than perpendicular thereto by a ratio of more than 1.2. The resistance to flexing in the parallel direction is less than 0.8 of the resistance in the perpendicular direction. Also claimed are manufacture of the contact by cold-pressing of a mix of metal and graphite powders and solid lubricant in the axial direction, then sintering in a reducing atmos., and a motor/generator incorporating such a contact.

Description

Field of the invention

The present invention relates to the sliding electrical contacts of electrical machines, such as motors or alternators. The invention relates more specifically to the rotary contact parts of said sliding electrical contacts, such as alternator rings or cylindrical collectors of electric motors.

Said parts are fixed to the rotary element of the electric machine, generally at one of its ends, and are traversed by one or more brushes, so as to establish a sliding electrical contact, and to make current flow, between the electrical conductors integral with the rotary element and stationary electrical conductors. The brushes generally comprise a wearing part to which a connecting electrical conductor is fixed.

The invention relates in particular to rotary contact parts made of cupro-graphitic composite material intended to be used in combination with brushes containing carbonaceous materials.

State of the art

According to the prior art, the rotary contact part, which generally has a usable thickness which is considerably smaller than the usable length of the wear part of the brush, is made of a material having a high electrical conductivity and sufficient mechanical characteristics. to avoid in particular bursting in rotation. The brush wear part is most often made of a conductive material which is softer than the rotary contact part and which offers satisfactory tribological behavior and electrical contact properties, so as to avoid rapid wear of the sliding contact and develop a low voltage drop between the brush and the rotating contact piece.

It is well known to produce rotary contact parts in copper alloy, such as lightly alloyed copper or bronze, and the wear parts of brushes in a material containing a carbon material, such as amorphous carbon and natural graphites. or synthetic.

Rotary copper alloy contact parts are most often obtained by drawing or stamping, and the hardening caused by plastic deformation allows sufficient mechanical properties to be obtained on the finished product.

The use of rotary graphite contact parts is also known for producing motors intended to operate in immersion in aggressive environments with respect to copper, such as certain gasolines of motor vehicles.

In order to obtain a mechanical resistance of the rotary contact part greater than that of a rotary graphite contact part, it is known from German application DE 32 30 298 to produce said parts in metallographitic composite, in particular from a mixture of bronze or copper powders and graphite compacted by sintering. This approach has not given any known application to the sliding electrical contacts of electrical machines.

It is known to use brushes whose wearing part is in metallographitic composite, generally obtained by incomplete sintering of the metallic phase or by impregnation, with a proportion of copper generally less than 85%.

Problem

The reliability of an electric machine is a criterion which is becoming increasingly important in the choice of a technical solution. This criterion becomes decisive when an electrical machine is intended to be part of an often complex assembly, such as an appliance or a motor vehicle, since the premature failure of one of the components of the assembly reduces the reliability of any assembly and incurs additional maintenance costs.

The reliability criterion translates in particular into a service life which corresponds to the operating time of the machine, maintenance-free and unattended, until the first failure leading to a stop and requiring intervention.

However, the life of electric machines is often limited by the wear of sliding electrical contacts. In current applications based on the use of known materials, the known lifetimes are at most of the order of 1500 hours.

However, although such lifetimes are entirely satisfactory for a large number of applications, certain fields, such as that of motor vehicles, increasingly require lifetimes significantly greater than 1500 hours.

Subject of the invention

The main object of the present invention is a piece of rotary contact in graphite-copper composite, the wear rate of which with regard to brushes containing a carbonaceous material leads to lifetimes clearly superior to 1500 hours.

Description of the invention

According to the invention, the rotary contact part, such as a cylindrical ring or collector, intended for the rotary element of an electric machine, such as a motor or an alternator, is made of a cupro composite material. - sintered graphite and is characterized in that said composite material comprises between 90 and 98% by weight of copper or copper alloy, in that the effective density of said composite material is between 6.5 and 8.5, and in that that the graphite flakes are strongly oriented relative to the axis of symmetry of said rotary contact piece, that is to say that the main axis P of more than 50% of the graphite flakes is inclined by less than 45 ° with respect to said axis of symmetry, so that the anisotropy of the electrical resistivity and of the flexural strength is very pronounced, that is to say that the Rho ratio of electrical resistivity rho || / rho ⊥ is greater than 1.2 and the ratio R of flexural strength R || / R⊥ is less than 0.8, where || denotes the direction parallel to said axis of symmetry and ⊥ the perpendicular direction.

The axis of symmetry of the rotary contact piece corresponds to the axis of rotation of the rotary element of the electric machine.

The main axis P of the graphite flakes, which is substantially perpendicular to the apparent plane of the flakes, corresponds to the average orientation of the crystallographic axes c perpendicular to the basal planes of the graphite particles of the flakes.

Preferably, the particles of graphite flakes have a maximum dimension of less than 200 μm and at least 90% of said particles have a maximum dimension of less than 100 μm. Larger particles cause too many faults and too much internal mechanical stress.

Preferably, the connection of the rotary contact parts to the electrical conductors of the rotary element is carried out using one or more electrical connection conductors fixed to the contact parts. The connection conductors can be of any known conductive material, such as copper and its alloys or aluminum and its alloys.

According to a variant of the invention, the contact parts, to which one or more connection conductors are advantageously fixed, form a separate assembly which can be manufactured individually and then fixed on the shaft of the rotary element. This assembly is preferably held by a support piece of insulating material, such as a polymer resin possibly loaded, which allows to ensure a satisfactory positioning of the contact pieces relative to each other and relative to the axis of rotation.

The Applicant has surprisingly discovered that, as the examples show, the rotary contact part according to the invention leads to significantly longer service lives of the sliding contact than those of the prior art.

This finding finds no known explanation and in fact goes against the usual teaching of the state of the prior art, in particular as regards the mechanical characteristics of the materials. The increase in lifespan could be linked to a modification of the mechanisms tribological under electric current and electrical phenomena on contact.

The rotary contact parts according to the invention have the advantage of replacing the parts of the prior art without major modification of electric machines and of producing lower voltage drops on contact, less vibration, a lower noise level. and less electromagnetic interference, and allow easy electrical junctions with the electrical conductors according to known techniques, such as welding or sealing.

The second object of the present invention is a method of economical manufacture of the rotary contact part of the first object of the invention.

• préparation d'un mélange de poudre de graphite, de poudre de cuivre ou d'alliage de cuivre, et d'au moins un lubrifiant solide,
• formation d'une pièce moulée crue par compression axiale à froid du mélange dans un moule, de telle manière que l'axe de compression coincide avec l'axe de symétrie de ladite pièce,
• frittage de la pièce moulée crue sous atmosphère réductrice.

The method of the invention comprises the following steps:

• preparation of a mixture of graphite powder, copper powder or copper alloy, and at least one solid lubricant,
• forming a raw molded part by cold axial compression of the mixture in a mold, so that the compression axis coincides with the axis of symmetry of said part,
• sintering of the raw molded part under a reducing atmosphere.

Preferably, the particles of graphite flakes have a maximum dimension of less than 200 μm and at least 90% of said particles have a maximum dimension of less than 100 μm. Preferably, the maximum dimension of the particles of the copper powder is close to that of the graphite particles and less than 200 μm, and at least 90% of the copper particles have a maximum dimension less than 100 μm. Too different dimensions of graphite and copper particles cause in particular a lowering of the mechanical characteristics and a greater porosity. Too large copper particles lead to a unacceptable number of defects and significant mechanical stresses.

The particles of said copper-based powder preferably have an irregular surface morphology, that is to say of dendritic structure or the like, such as that obtained by electrolytic means.

It may be advantageous to use copper of electrolytic origin.

The solid lubricant (s) of the mixture is (are) chosen from known solid lubricants, such as stearates.

The proportion of solid lubricant is preferably less than 5% by weight so as to ensure satisfactory lubrication during shaping without, however, leaving too much porosity during sintering.

The compression pressure is preferably between 150 and 350 MPa so as to ensure sufficient compression, without requiring difficult compression conditions.

The sintering temperature is preferably between 500 and 1050 ° C. A temperature below 500 ° C gives incomplete sintering and a temperature above 1050 ° C causes significant softening of the copper particles, or even their melting, and leads in particular to heterogeneities in the distribution of the graphite particles. It is advantageous to carry out the sintering operation at a temperature between 700 and 900 ° C., for reasons of cost and speed. The holding time at the sintering temperature is chosen, preferably between 1 and 5 hours, so as to ensure complete sintering while avoiding recrystallizations secondary and the appearance of defects and constraints.

It may be advantageous to seal the electrical conductors in the rotary contact part during the compression phase.

It is advantageous to fix one or more electrical connection conductors to the rotary contact parts during the step of forming the green molded part. Preferably, the connecting conductor is fixed by axial compression of said mixture around the conductor.

According to an advantageous variant of the invention, after the steps of forming the green molded parts and of sintering according to the invention, the contact parts are assembled so as to form a separate rigid assembly, such as a manifold assembly, which can then be fixed on the shaft of a rotating element.

The manufacturing process of the invention has the advantage of not requiring the addition of organic binders or metals. The method according to the invention also has the advantage of producing rotary contact parts with the desired dimensions or requiring only simple complementary machining.

Description of the figures

The invention will be better understood with the aid of the following figures, which are given for information only and are in no way limiting.

FIG. 1 illustrates diagrammatically the configuration of a sliding electrical contact of an electrical machine, with the rotary element (1), at least one rotary contact part (2) and at least one brush, of which only the room wear (3) without the holding system. The axis of rotation (4) of the rotary element here coincides with the axis of symmetry of the rotary contact piece (2). The direction of rotation indicated is arbitrary. The directions t, a and r correspond respectively to the direction tangential to the rotary contact piece, to the axial direction parallel to the axis of rotation and to the radial direction with respect to the same axis of rotation and with respect to the sliding electrical contact. .

Figure 2 a) schematically illustrates the rotary element (1) of an electric motor rotating around an axis of rotation (4). The sliding electrical contact comprises brushes, of which only the wearing parts (3) are shown, and a cylindrical collector (5) comprising several blades (6). FIG. 2 b) schematically illustrates the rotary element (1) of an alternator rotating around an axis of rotation (4). The sliding electrical contact comprises brushes, of which only the wearing parts (3) are shown, and rings (7).

Figure 3 schematically illustrates an axial section of a rotary contact piece (2), which comprises flakes (9), made of graphite particles (8) and whose main axis P is inclined at an angle A relative to any axis (40) parallel to the axis of symmetry.

FIG. 4 shows the micrograph of an axial section of a rotary contact piece according to the invention.

FIG. 5 schematically represents a longitudinal section of a rotary motor element, or rotor, which rotary element comprises a shaft (10), a drive armature (11) and a manifold assembly (12). The armature (11) comprises a coil (13) and most often a strip iron (14). The manifold assembly (12) comprises the manifold blades (15), which constitute individual rotary contact parts, a conductor (16) for connection to the winding. (13) and a support piece (17) made of insulating material. Only two winding connections are shown to simplify the figure.

FIG. 6 illustrates, in axial half-section, an embodiment of the manufacturing method according to the invention which makes it possible to obtain sets of separate contact parts (20) which can be subsequently assembled on rotary elements.

According to a first variant, a first step comprises the formation of a number N of connection pieces made of conductive material (step a). The mixture according to the invention is then compressed, according to the invention, around a part of the connecting piece (step b). After the sintering step, the contact pieces (15) provided with the connection conductor (16) are assembled and held by a support piece of insulating material (17) (step c). The contact parts are then electrically separated from each other, by any known means, such as by machining, so as to create the space (18) necessary between the blades (step d) and to obtain a finished part (20). .

According to another variant, a first step comprises the formation of an initial part comprising the connection conductors (step a '). The mixture is then compressed, according to the invention, around a part of this part (step b '). After the sintering step, the support piece (17) is formed (step c ') and the contact pieces are electrically separated from each other, by any known means, such as by machining, so as to create the spacing (18) necessary between the blades (step d) and to obtain a finished part (20). The operation of electrically separating the contact pieces can be carried out in part before the step of forming the support piece.

Examples EXAMPLE 1:

12V alternator rings have been produced according to the prior art and according to the invention.

Alternator rings according to the prior art have been produced by machining tubular sections of non-annealed copper.

Rings according to the invention were produced according to the method of the invention from electrolytic copper powder and natural graphite. The maximum dimensions of the copper and graphite particles were comparable and less than 200 µm and at least 90% of the particles were of maximum dimensions less than 100 µm.

The solid lubricant was zinc stearate (about 0.4% by weight in all cases).

The powders of electrolytic copper and natural graphite and zinc stearate were mixed, in different proportions, according to known techniques. Raw moldings were formed by axial compression of the mixture in a mold under a pressure of 195 MPa. The density of the raw parts was approximately 7.2. The raw moldings were sintered at 850 ° C for 3 hours, after a temperature rise to 50 ° C / h, under a reducing atmosphere comprising about 40% hydrogen and 60% nitrogen. A micrographic section of one of the parts obtained is shown in Figure 4.

The electrical resistivity was measured using the 4-point method. The flexural strength is measured according to the 3-point method, with 36mm x 20mm x 11.3 mm test pieces, with a spacing of 27 mm between the two lower contact points.

The lifetime tests were carried out on a test bench under real conditions of use. In all cases, the intensity was 3.5A, the temperature 100 ° C, the speed of rotation of 10000 rpm. The alternator was in continuous operation during the test.

The tests were carried out with LCL C7364 metallographitic brushes compressed in the radial direction r. The brushes had a section of 4.6mm x 6.4mm and a usable length of 10mm.

The results obtained are collated in Table 1. D corresponds to the density. % Cu and% C correspond respectively to the proportion of copper and natural graphite, by weight, of the sintered part. The service life corresponds to the time elapsed since the alternator was started and the first failure related to the complete wear of one of the brushes and / or one of the rings. TABLE 1 Case % Cu %VS Rho R D Lifetime 1- 100 0 1.0 > 1 8.9 1510 h 2- 100 0 1.0 0.9 8.6 1480 h 3- 95 5 2.1 0.4 7.7 3050 h 4- 93 7 2.7 0.3 6.9 3100 h 5- 85 15 2.2 0.4 6.5 1220 h

Case 1 concerns alternator rings produced according to the state of the art. Cases 2 to 5 relate to rings produced by sintering according to the manufacturing process of the invention, with proportions of copper and graphite corresponding to the invention in cases 3 and 4. Each case corresponds to 3 tests on different alternators.

These results show that the lifespan is clearly greater than 1500 hours for the alternator rings of the invention.

EXAMPLE 2:

Cylindrical manifolds of 12V auxiliary motor have been produced according to the prior art and according to the invention.

The collectors according to the prior art were produced by machining tubular sections of non-annealed copper.

The collectors according to the invention were produced from electrolytic copper powder and natural graphite according to the manufacturing process of Example 1, with the exception of the following points. The compression pressure was 220 MPa. The sintering temperature was 700 ° C and the sintering time 4 hours. The orientation of the graphite flakes was comparable to that of Example 1.

The lifetime tests were carried out on a test bench under real conditions of use. In all cases, the current was 23.0 A, the voltage 11.75 V and the rotation speed of 2500 rpm. Engine operation was continuous during the test.

The tests were carried out with LCL C7273 metallographic brushes compressed in the tangential direction t. The brushes had a cross section of 8 mm x 9 mm and a usable length of 10 mm.

The results obtained are collated in Table 2. The notation is the same as that of example 1. The lifetime corresponds to the time elapsed since the starting of the motor and the first failure linked to the complete wear of one of the brushes and / or one collectors.

Case 1 concerns collectors made according to the state of the art. Cases 2 and 3 correspond to collectors according to the invention. Each case corresponds to 3 tests on different engines. TABLE 2 Case % Cu %VS Rho R D Lifetime 1- 100 0 1.0 > 1 8.9 1490 h 2- 95 5 2.1 0.4 7.7 2760 h 3- 93 7 2.7 0.3 6.9 3300 h

These results show that the lifespan is clearly greater than 1500 hours for the collectors according to the invention.

EXAMPLE 3:

Cylindrical collectors of vacuum cleaner motor 1000W at 230 V have been produced according to the prior art and according to the invention.

The collectors according to the prior art were produced by assembling individual blades obtained by machining from a drawn profile of OFHC copper.

The collectors according to the invention were produced from electrolytic copper powder and natural graphite according to the manufacturing process of Example 1, with the exception of the following points. The compression pressure was 240 MPa.

The sintering temperature was 900 ° C and the sintering time 2.5 hours. The orientation of the graphite flakes was comparable to that of Example 1.

The lifetime tests were carried out on a test bench under real conditions of use. In all cases, the intensity was 5 A and the rotation speed 25000 rpm. The engine was started cyclically, 30 s off and 30 s running during the entire test.

The tests were carried out with LC-A149 grade carbon brushes compressed in the tangential direction t. The brushes had a section of 6.3 mm x 11.3 mm and a usable length of 20 mm.

The results obtained are collated in Table 3. The notation is the same as that of Example 1. The lifetime corresponds to the time elapsed since the engine was started and the first failure linked to the complete wear of the '' one of the brushes and / or one of the collectors.

Case 1 concerns collectors made according to the state of the art. Cases 2 and 3 correspond to the collectors of the invention. Each test corresponds to 3 tests on different engines. TABLE 3 Case % Cu %VS Rho R D Lifetime 1- 100 0 1.0 > 1 8.9 600 h 2- 98 2 2.1 0.3 7.9 1210 h 3- 95 5 2.1 0.4 7.0 1360 h

There are therefore relative increases in the duration of life in the same proportions as for the auxiliary motor of example 2.

Claims (21)

Rotary electrical contact piece for a rotary element of an electrical machine made of a sintered cupro-graphitic composite material and characterized in that said composite material comprises between 90 and 98% by weight of copper or copper alloy, in that the effective density of said composite material is between 6.5 and 8.5, and in that the graphite flakes are strongly oriented with respect to the axis of symmetry of said rotary contact piece, that is to say that the main axis P of more than 50% of the graphite flakes is inclined by less than 45 ° relative to the axis of symmetry of said rotary contact piece, so that the anisotropy of the electrical resistivity and of the flexural strength is very pronounced, ie the Rho ratio of electrical resistivity rho || / rho ⊥ is greater than 1.2 and the ratio R of flexural strength R || / R⊥ is less than 0.8, where || denotes the direction parallel to said axis of symmetry and ⊥ the perpendicular direction. Rotary contact piece according to claim 2, characterized in that the graphite is of natural origin. Rotary contact piece according to either of Claims 1 and 2, characterized in that the particles of graphite flakes have a maximum dimension of less than 200 µm and in that at least 90% of said particles have a maximum dimension of less than 100 µm. Method of manufacturing rotary contact parts according to one of claims 1 to 3, characterized in that it comprises the following steps: - preparation of a mixture of graphite powder, copper or copper alloy powder, and at least one solid lubricant, forming a green molded part by cold axial compression of the mixture in a mold, such that the compression axis coincides with the axis of symmetry of said part, - sintering of the raw molded part under a reducing atmosphere. A method of manufacturing rotary contact parts according to claim 4, characterized in that the particles of said copper or copper alloy powder have a dendritic or similar surface morphology. Method of manufacturing rotary contact parts according to one of claims 4 and 5, characterized in that the copper is of electrolytic origin. Method for manufacturing rotary contact parts according to one of Claims 4 to 7, characterized in that the maximum particle size of the copper powder is close to that of the graphite particles and less than 200 µm, and in that '' at least 90% of the copper particles have a maximum dimension of less than 100 µm. Method for manufacturing rotary contact parts according to one of Claims 4 to 7, characterized in that the proportion of solid lubricant is less than 5% by weight. Method for manufacturing rotary contact parts according to one of Claims 4 to 8, characterized in that the solid lubricant is a stearate. Method of manufacturing rotary contact parts according to one of claims 4 to 9, characterized in that the compression pressure is between 150 and 350 MPa. Method of manufacturing rotary contact parts according to one of Claims 4 to 10, characterized in that the sintering temperature is between 500 and 1050 ° C. Method for manufacturing rotary contact parts according to one of Claims 4 to 11, characterized in that the sintering temperature is between 700 and 900 ° C. Method for manufacturing rotary contact parts according to one of Claims 4 to 12, characterized in that the time required to maintain the sintering temperature is between 1 and 5 hours. Method of manufacturing rotary contact parts according to one of claims 4 to 13, characterized in that the electrical conductors are sealed in the rotary contact part during the compression phase. Manufacturing method according to one of claims 4 to 13, characterized in that at least one electrical connection conductor is fixed to the rotary contact parts by axial compression of said mixture around the connection conductor during the compression phase. Set of rotary contact pieces characterized in that it comprises a support piece of insulating material and rotary contact pieces according to one of claims 1 to 3. Assembly according to claim 16, characterized in that at least one electrical connection part is fixed to each contact part. Method of manufacturing the assemblies according to claim 17, characterized in that the connection piece (s) are fixed to the rotary contact pieces by axial compression of the said mixture around the connection pieces during the compression phase. A method of manufacturing assemblies according to claim 17, characterized in that it comprises the manufacture of an initial piece of conductive material, the axial compression of said mixture around a part of this initial piece and the electrical separation between the pieces of rotary contacts by any known means. Electric machine characterized in that it comprises at least one rotary contact part according to one of claims 1 to 3. Electric machine characterized in that it comprises an assembly according to one of claims 16 and 17.

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