KARLÉN DAVID (SE)
JOHANSSON ERIK (SE)
SIMON REINHARD (CH)
| DE3538684A1 | 1986-05-07 | |||
| US5876862A | 1999-03-02 | |||
| US4672008A | 1987-06-09 | |||
| US20060239854A1 | 2006-10-26 |
ANONYMOUS: "Doduco - WERKSTOFF INFORMATION Silber-Zinnoxid (Ag/SnO ) 2", 30 April 2011 (2011-04-30), www.doduco.net, pages 1 - 8, XP055156977, Retrieved from the Internet
| CLAIMS 1 . An electrical contact tip (5) for switchi ng applications, the contact tip (5) comprises a body comprising a first layer (7a) and a second layer (7b), the first layer (7a) is arranged on the second layer (7b) and is adapted to come in contact with a correspondi ng contact tip (5) during switching operations, wherein the first layer (7a) and the second layer (7b) consist of Ag- composites comprising one or more elements, compounds or al- loys, characterized in that the hardness of the first layer (7a) is lower than the hardness of the second layer (7b). 2. The electrical contact tip (5) according to claim 1 , wherein the hardness of the second layer (7b) is at least 1 .2 times higher than the hardness of the first layer (7a). 3. The electrical contact tip (5) according to any of claim 1 and 2, wherein the hardness of the first layer (7a) is in the range of 50 to 140 Vickers Hv1 and the hardness of the second layer (7b) is in the range of 60 to 150 Vickers Hv1 . 4. The electrical contact tip (5) according to any of the previous claims, wherein the resistivity of the first layer (7a) is lower than the resistivity of the second layer (7b). 5. The electrical contact tip (5) according to any of the previous claims, wherein the resistivity of the second layer (7b) is at least 1 .2 times higher than the resistivity of the first layer (7a). 6. The electrical contact tip (5) according to any of the previous claims, wherein the resistivity of the first layer (7a) is in the range of 1 .7 - 10 8 to 2.6- 1 0 8 Ω- m and the resistivity of the sec- ond layer (7b) is in the range of 1 .9 0"8 to 2.8 0 8 Ω- m. 7. The electrical contact tip (5) according to any of the previous claims, wherein the thickness of the first layer (7a) is smaller than the thickness of the second layer (7b). 8. The electrical contact tip (5) according to any of the previous claims, wherein the thickness of the first layer (7a) is between 10 and 40% of the thickness of the second layer (7b). 9. The electrical contact tip (5) according to any of the previous claims, wherei n the content of Ag in the Ag-composite of the first layer (7a) is higher than in the Ag-composite of the second layer (7b). 10. The electrical contact tip (5) according to any of the previous claims, wherei n the content of Ag in the Ag-composite of the first layer (7a) is in the range between 70 and 96 wt.% and the content of Ag in the Ag-composite of the second layer (7b) is in the range between 40 and 92 wt. %. 1 1 . The electrical contact tip (5) according to any of the previous claims, wherei n the one or more elements, compounds or alloys of the Ag-composite of the first layer (7a) and the second layer (7b) are selected from the group of Ag , Al , Fe, Sn, C, Cu, Cr, Mo, Ni , Co, W, CdO, Sn02, ZnO, Fe203, WC, MoC, ZrC, Ti B2, ZrB2, AgMo, AgCo, AgNi , AgMo, AgCu , AgCr, AgCo, l n203, 12. The electrical contact tip (5) according to any of the previous claims, wherein the first layer (7a) comprises a contact zone that comprises serrations. 13. The electrical contact tip (5) according to any of the previous claims, wherein the first layer (7a) and the second layer (7b) are produced by means of sintering compressed powder mixtures representing the chemical composition of the first layer (7a) and the second layer (7b). 14. The electrical contact tip (5) according to any of the previous claims, wherei n the body further comprises a third layer (7c) arranged on an opposite side of the second layer (7b), which third layer (7c) has the purpose of attaching the electrical contact tip (5) to an electrical conductor (3). 15. An electrical switching device comprising an electrical contact tip (5) according to any of claim 1 -14. |
FI ELD OF THE I NVENTI ON
The present i nvention relates to an electrical contact tip for switchi ng applications, in particular for low voltage applications. The contact tip comprises a body comprising a first layer and a second layer. The first layer is arranged on the second layer and is adapted to come i n contact with a corresponding contact tip during switching operations. The first layer and the second layer consist of Ag-composites comprising one or more elements, compounds or alloys. The present invention also relates to an electrical switchi ng device comprising the electrical contact tip.
PRIOR ART Switchi ng devices, in particular low voltage contactors, have traditionally been used for different load switching applications and the contact material of the electrical contact tip has been chosen for these duties. However, there is an increasing demand for switching devices in applications where there are not many load switching operations. These are for instance isolation and by-pass applications.
The body of the contact tip is typically arranged of a composite material of silver metal oxide (Ag-MeO). The choice of contact tip material is a compromise between several opposing requirements, such as low contact resistance, low erosion wear and good welding properties.
It is desired to use the same contact material of the contact tip for both by-pass and isolation applications as well as for load switchi ng applications. In isolation/by-pass applications low con- tact resistance to achieve low losses and hence less thermal problems is the main criteria while in load switching applications the life time of the contact tip is the most important parameter. The later depends mainly on the erosion properties of the con- tact tip material .
Accordi ngly, a problem with prior art contact tips for use i n both by-pass/isolation applications and load switching applications is that the material properties are not optimized for either of the applications.
To get a low enough contact resistance the materials in prior art contact tips contain typically 86 mass% silver, 12% tin oxide and 2% bismuth oxide. This gives a relatively good compromise be- tween erosion resistance and low contact resistance. A harder material with less silver could give lower erosion rates but would at the same time i ncrease the contact resistance. A softer contact material would do the opposite. US4672008 discloses an electrical contact provided with a coating adapted to prevent formation of segregation or depletion layer on the outer surface of the contact. The thin layer could be produced by powder metallurgical sintering . US20060239854 discloses a contact comprising an outer layer adapted to reduce the abrasion of the contact and enable the contact to be used i n heavy loads. The outer layer has a higher hardness than the inner layer of the contact. OBJ ECTS AND SUMMARY OF THE I NVENTION
A first object of the present invention is to provide an electrical contact tip that has improved properties for use in both bypass/isolation switching applications and load switching applica- tions compared with prior art contact tips. A second object of the invention is to provide an electrical contact tip that can be produced more cost effective than prior art contact tips.
This object is obtai ned by an electrical contact tip for switching applications, the contact tip comprises a body comprising a first layer and a second layer, the first layer is arranged on the second layer and is adapted to come in contact with a corresponding contact tip during switching operations, wherein the first layer and the second layer consist of Ag-composites comprising one or more elements, compounds or alloys. The contact tip is characterized in that the hardness of the first layer is lower than the hardness of the second layer.
The electrical contact tip is arranged for switching applications, in particular for low voltage applications below 1000 V. The contact resistance and the erosion resistance are dependent on the hardness of the body of the contact tip, wherein a high hardness provides high erosion resistance and high contact resistance, and vice versa.
The first layer has the function of providing low contact resistance. A low contact resistance is of particular importance i n by-pass/isolation switching applications. The second layer has the function of providing high erosion resistance. A high erosion resistance is of particular importance in load switching applications.
In isolation/by-pass applications the contact erosion is more or less negligible and the first layer provides low contact resistance through out the life of the contact tip. I n load switching applications on the other hand , the low contact resistance is of less importance and the first layer will be worn off early in life of the contact tip. Thereafter, the second layer is exposed and provides high erosion resistance for enduring load switching appli- cations. Accordingly, the invention provides contact tip that enables use in both by-pass/isolation switchi ng applications and load switching applications with improved performance compared with prior art contact tips.
Accordi ng to an embodiment of the invention , the hardness of the second layer is at least 1 .2 times higher than the hardness of the first layer.
Accordi ng to an embodiment of the invention , the hardness of the first layer is in the range of 50 to 140 Vickers Hv1 and the hardness of the second layer is in the range of 60 to 1 50 Vickers Hv1 .
Accordi ng to an embodiment of the invention , the resistivity of the first layer is lower than the resistivity of the second layer. A low resistivity is of importance in particular for the first layer in order to provide low contact resistance when used in bypass/isolation switchi ng applications.
Accordi ng to an embodiment of the invention , the resistivity of the second layer is at least 1 .2 times higher than the resistivity of the first layer.
Accordi ng to an embodiment of the invention , the resistivity of the first layer is in the range of 1 .7 1 0 8 to 2.6- 1 0 8 Ω- m and the resistivity of the second layer is in the range of 1 .9- 1 0 "8 to 2.8- 10 8 Ω- m.
Accordi ng to an embodiment of the invention , the thickness of the first layer is smaller than the thickness of the second layer.
A relatively small thickness of the first layer compared with the second layer is desired as the erosion for by-pass/isolation applications is lower than the erosion for load switching applications. Accordi ng to an embodiment of the invention , the thickness of the first layer is between 10 and 40% of the thickness of the second layer. Accordi ng to an embodiment of the invention , the content of Ag in the Ag-composite of the first layer is higher than in the Ag- composite of the second layer.
A lower content of Ag is necessary for the second layer com- pared with the first layer. Thereby, the cost of manufacturing the contact tip of the invention is reduced compared with prior art contact tips in that the Ag constitutes a significant portion of the manufacturing cost. Accordi ng to an embodiment of the invention , the Ag-composites comprise metallic Ag-matrix with the one or more elements, compounds or alloys distributed in the Ag-matrix. The Ag-matrix consists of Ag or an Ag-based alloy, and possible impurities. Accordi ng to an embodiment of the invention , the content of Ag in the Ag-composite of the first layer is in the range between 70 and 96 wt.% and the content of Ag in the Ag-composite of the second layer is in the range between 40 and 92 wt. %. Accordi ng to an embodiment of the invention , the one or more elements, compounds or alloys of the Ag-composite of the first layer and the second layer are selected from the group of Ag , Al , Fe, Sn, C, Cu , Cr, Mo, Ni , Co, W, CdO, Sn0 2 , ZnO , Fe 2 0 3 , WC, MoC, ZrC, Ti B2, ZrB2, AgMo, AgCo, Ag Ni , AgMo, AgCu , AgCr, AgCo, l n 2 0 3 , Bi 2 0 3 , W0 3 , MoOs, CuO.
Accordi ng to an embodiment of the invention , the first layer comprises a contact zone that comprises serrations. The serrations have the function of improving the electrical contact be- tween the contact tip and a corresponding contact tip in switching applications. Accordi ng to an embodiment of the i nvention , the first layer and the second layer are produced by means of sintering compressed powder mixtures representing the chemical composition of first and second layers. The use of a powder metallurgical process has the advantage that the first layer and the second layer can be produced with high quality.
Accordi ng to an embodiment of the invention , the body further comprises a third layer arranged on an opposite side to the first layer on the second layer, which third layer has the purpose of attachi ng the electrical contact tip to an electrical conductor.
The second layer has two sides opposite to each other, the first layer is attached on one side of the second layer and the third layer is attached on the other side of the second layer.
Accordi ng to an embodiment of the invention , the third layer consists of a material suitable for brazi ng .
The object of the invention is further obtained by an electrical switchi ng device comprising an electrical contact tip according to any of claim 1 -14. BRI EF DESCRI PTI ON OF TH E DRAWI NGS
The invention will now be explained more closely by the descri ption of different embodiments of the invention and with reference to the appended figures.
Fig . 1 shows an electrical contact comprising an electrical contact tip according to an embodiment of the invention .
Fig . 2 shows a cross section of the contact tip in fig . 1 . Fig . 3 shows a graph of the hardness and the conductivity of a first layer and a second layer of the contact tip in fig . 1 .
Fig . 4 shows a flow chart of a powder metallurgy process for produci ng a contact tip according to the invention .
DETAI LED DESCRI PTI ON OF PREFERRED EM BOD I MENTS OF THE I NVENTION Fig . 1 shows an electrical contact 1 comprising a conductor 3 and an electrical contact tip 5 according to an embodiment of the invention . The contact tip 5 is attached at one end of the conductor 3. The contact tip 3 is adapted to be used in low voltage switching applications of a switching device, i n particular voltage below 1000 V.
The contact tip 5 comprises a body comprising a first layer 7a, a second layer 7b and a third layer 7c. Fig . 2 relates to a cross section of the contact tip 5, where the three layers 7a, 7b 7c are disclosed . The first layer 7a is arranged on the second layer 7b. The second layer 7b is arranged on the third layer 7c.
The first layer 7a is adapted to come in contact with a correspondi ng contact tip 5 duri ng switching operations in a switching device. The material of the first layer 7a has properties that are suitable for by-pass/isolation switching applications, where a low contact resistance is desired but the erosion resistance is of less importance. The second layer 7b is adapted to come in contact with a correspondi ng contact tip 5 duri ng switching operations in case the first layer 7a has been worn off. The second layer 7b has properties that are suitable for load switching applications, where a high erosion resistance is desired but the contact resistance is of less importance. The third layer 7c has the function of attachi ng the contactor tip to the conductor 3. For example, the third layer 7c consists of a material suitable for brazing . The first layer 7a and the second layer 7b comprise an Ag- composite comprising a metallic matrix of Ag or an Ag-alloy and one or more elements, compounds or alloys distributed in the matrix. I n an embodiment the elements or compounds constitute grains of one or more metal oxide. The elements, compounds or alloys of the Ag-composite of the first layer 7a and the second layer 7b may in particular be selected from the group of Ag , Al , Fe, Sn, C, Cu, Cr, Mo, Ni , Co, W , CdO, Sn0 2 , ZnO, Fe 2 0 3 , WC, MoC, ZrC, Ti B2, ZrB2, Ag Mo, AgCo, AgNi , Ag Mo, AgCu , AgCr, AgCo, ln 2 0 3 , Bi 2 0 3 , W0 3 , Mo0 3 , CuO .
The difference in properties of the first layer 7a and the second layer 7b is characterized in that the hardness of the first layer 7a is lower than the hardness of the second layer 7b. Furthermore, the conductivity of the first layer 7a is higher than the hardness of the second layer 7b, and accordingly the contact resistance of the first layer 7a is lower than the second layer 7b, as can be seen in fig . 3.
The hardness of the first layer 7a and second layer 7b is de- pendent on the content of Ag in the Ag-composite, wherein the content of Ag in first layer 7a is higher than in the Ag-composite of the second layer 7b. Accordingly, the hardness of the first layer 7a and the second layer 7b is adjusted by adj usting the relationship between the content of Ag and the content of ele- ments, compounds or alloys in the Ag-composites.
The content of Ag i n the Ag-composite of the first layer 7a is preferably in the range between 70 and 96 wt.% and the content of Ag in the Ag-composite of the second layer 7b is preferably in the range between 40 and 92 wt. %. By adjusting the first layer 7a and the second layer 7b accordi ng to above, the first layer 7a receives a lower contact resistance than the second layer 7b and the second layer 7b receives a higher erosion resistance than the first layer 7a.
The first layer 7a is suitable for use in by-pass/isolation switching applications because of its low contact resistance. The erosion resistance of the first layer 7a is low compared to the second layer 7b. However, in by-pass/isolation switching applica- tions the erosion of the contact tip is neglectable in view of the life time of such switching device.
A contact zone of first layer 7a, adapted to be i n direct contact with a corresponding contact tip 5, is preferably provided with serrations for improving the electrical contact with the correspondi ng contact tip 5.
The second layer 7b is suitable for use in load switchi ng applications because of its high erosion resistance. The contact re- sistance of the second layer 7b is high compared to the first layer 7a. However, in load switching applications the contact resistance is of less importance for the performance of the switching device. I n load switchi ng applications the first layer 7a will be worn off early in life of the switching device and thereafter the second layer 7b will be outer surface of the contact tip 5 that comes into contact with a corresponding contact tip 5 of the switchi ng device.
Accordi ngly, the combination of the first layer 7a and the second layer 7b improves the contact tip 5 for use in both bypass/isolation switching applications and load switching applications compared with prior art contact tips 5.
The hardness of the second layer 7b is preferably at least 1 .2 times higher than the hardness of the first layer 7a. For example, the hardness of the first layer 7a is in the range of 50 to 140 Vickers Hv1 and the hardness of the second layer 7b is in the range of 60 to 150 Vickers Hv1 .
Moreover, the resistivity of the second layer 7b is preferably at least 1 .2 times higher than the resistivity of the first layer 7a. For example, the resistivity of the first layer 7a is in the range of 1 .7 10 "8 to 2.6- 1 0 8 Ω- m and the resistivity of the second layer 7b is in the range of 1 .9- 1 0 8 to 2.8- 1 0 8 Ω- m. It is sufficient for by-pass/isolation applications that the thickness of the first layer 7a is smaller than the thickness of the second layer 7b. The thickness of the first layer 7a is preferably between 10 and 40% of the thickness of the second layer 7b. Preferably, the contact tip 5 of the invention is manufactured by means of a powder metallurgy process. The starting raw powders for the first layer 7a, the second layer 7b and the third layer 7c are either metals or a combination of metals and metal oxides. Fig . 4 shows a flow chart of a powder metallurgy process for producing a contact tip 5 according to the invention .
In a first step 1 10 of the powder metallurgy process the raw powders are su bjected to powder conditioning , which typically consists of several sub-steps of chemical powder treatment, mixing , milling , granulation , and sieving . Three different granulated powders are produced , which are to be formed into the first layer 7a, the second layer 7b and the third layer 7c.
In a second step 120 of the powder metallurgy process the pow- der of the respective layers 7a , 7b, 7c are subjected to uniaxial pressing . A uniaxial pressing die is first filled with a layer of a first zone of powder representing the first layer 7a, then it is subsequently filled by another layer of a second zone of powder representing the second layer 7b on top of it, and finally it is subsequently filled by a third zone of powder representing the third layer 7c. For the filling operation three different powder feeds and die filling shoes are used . The cavity in the die is generated by lowering the lower piston by a distance equal to the individual layer thickness needed for the formation of first, second and third zone. Thereafter, a graded compact is formed by uniaxial pressing . Preferably, also serrations in the first layer 7a are formed by using an upper piston with an inverted serrated surface geometry. The net-shaped geometry of the body of the contact tip 5 is preferably formed in one pressing step. In a final step 130 of the powder metallurgy process the graded compact is subjected to thermal treatment. The net-shaped graded compact is thermally treated at temperatures below 1200°C i n either reducing (H2) atmosphere or partly under oxidizing (02) conditions i n order to develop the final material composition and a dense microstructure.
The present invention is not limited to the disclosed embodiments but may be modified within the framework of the claims.