Embodiments of the present invention relate to a carbon brush for transmitting high currents, having a connecting element for connecting an electrical connection track and a consumable contact element with a contact surface that is designed to contact a slip ring device, particularly a commutator.
Carbon brushes of the kind described in the introduction are often referred to as “industrial carbon brushes” and are used particularly in situations that require transmission of high currents, often in excess of 20 ampere/cm2. In this context, carbon brushes of such kind are used in motors and generators, and use of the generators with these carbon brushes is becoming increasingly popular in wind turbines.
Because of the high outputs the industrial carbon brushes are required to transmit is such cases, it is imperative to add sufficient fractions of metallic inclusions to the carbon, to help to reduce contact resistances as much as possible and so minimise the heating of the carbon brushes caused by electrical resistance. To this end it has proven advantageous to manufacture the carbon brushes, which are usually produced as moulded parts, from a particulate moulding compound consisting mostly of a suitable metal, particularly silver. In practice, the silver component in industrial carbon brushes is frequently as high as 60% and more. As a result, the production costs for carbon brushes are considerable due to the quantity of silver alone, without taking into account the production costs associated with any given process.
The known industrial carbon brushes are manufactured as essentially homogeneous moulded parts, the composition of which is consistent for both the contact element that contacts the slip ring device during operation and the connecting element to which the electrical connection track is connected. Consequently, this means that a significant portion of the silver in the carbon brush, which is consumed together with the carbon in the contact element as the carbon brush is worn down, is also present in the connecting element, where the specific material properties of the silver are not required, even though they are essential for interacting with the carbon in the contact element. The connecting element in the carbon brush serves instead to establish an electrically conductive connection with the connection track. Once the contact element has been used up, the connecting element is usually disposed of, together with the silver it contains.
Accordingly, the methods used until now for manufacturing industrial carbon brushes have entailed using a quantity of silver, not all of which is needed for operating the carbon brushes.
An object of the embodiments of the present invention is therefore to suggest a carbon brush for transmitting high currents which enables the carbon brush to function reliably and at the same time is associated with lower manufacturing costs than the known carbon brushes.
The carbon brush according to the embodiments of the invention for transmitting high currents includes a moulded body the composition of which is varied according to the different functions of the brush areas, in which the connecting element serves to provide a connection with an electrical connection track and the contact element serves to ensure uninterrupted contact with the material of the counter-contact element or the slip ring device, and unlike the connecting element must ensure flush contact with the material of the counter-contact through wear.
In the carbon brush according to the embodiments of the invention, the connecting element and the contact element are combined in a layer transition zone aligned perpendicularly to the direction of wear of the carbon brush and form a one-piece moulded body having different compositions that are adapted to the different functions of the connecting element and the contact element. Accordingly, the carbon fraction in the contact element is greater than the carbon fraction in the connecting element, because the carbon is important for creating the desired low-friction contact. Besides its carbon content, the contact element also includes a metal fraction, which is different from the metal fraction in the connecting element, so that the metal fractions in each may be optimised for the different functions of the metal fractions in the contact element and the connecting element. Thus, a metal may be selected for the contact element that both enables generation of a desirable low contact resistance, and also has relatively low abrasive properties so as to minimise its negative effect on the advantageous properties of the carbon for the purposes of establishing a low-friction contact. For the connecting element, which is separated from the consumable part of the carbon brush by the layer transition zone, and for which wear-related properties are therefore of secondary importance, a metal fraction may thus be used that has both sufficient mechanical strength to fulfill its mechanical connecting function and good electrical conductivity for creating the electrical connection with the connection track, without regard for its abrasive properties.
The carbon brush according to the embodiments of the invention is thus defined as a carbon brush in which areas are separated by the layer transition zone, each of which areas may be optimised with respect to their essential functions independently of the other area. In this way, it is also possible to significantly reduce the proportion of silver in the carbon brush compared with the known carbon brush because in the design of the embodiments of the invention, the silver content is then only needed for the contact element. Accordingly, the remaining part of the brush, which must be disposed of and essentially consists only of the connecting element when a carbon brush has been worn down to the layer transition zone, contains no silver at all.
In a preferred embodiment of the carbon brush according to the invention, the contact element has a metal content composed mostly of silver and the connecting element has a metal content composed mostly of copper. In this way, it is possible to create a carbon brush that is optimised with regard to selecting the respective metal for the contact element and the connecting element. Of course other metals, now known or later discovered, may be used that satisfy the mechanical and electrical requirements of the elements.
The fraction of metal in the contact element may be about 50% or more depending on the requirement for the carbon brush. Depending on this requirement, the metal content in the contact element may be up to 100% of a copper material or a copper alloy.
A particularly high-strength embodiment, capable of withstanding extremely high mechanical loads, is created if the connecting element is made from about 100% copper or a copper alloy.
If, as in a particularly advantageous embodiment of the carbon brush, the contact element next to the layer transition zone has an indicator area formed opposite a consumable area with different composition and provided with a contact surface, which indicator area has a metal component that differs from the consumable area, it is possible to form a consumable or maintenance indicator on the carbon brush as a multilayer construction is worn down without the need to provide special, separate contact devices. In this embodiment, it is simple to exploit the advantageous effect that is created when the contact surface is formed in the indicator area, causing the resistance to change because the metal component of the indicator area differs from that of the consumable area, This change in resistance may be used as a signal to indicate that the carbon brush needs to be replaced.
Regardless of the formation of an indicator area on the carbon brush, in any case it is advantageous if the connection element has a flat connecting surface for creating a soldered or welded connection to the electrical track, so that it is possible to create a simple, flat attachment to the connecting track with a soldered or welded connection without the need for special retaining devices on the connection element.
In the following, preferred embodiments of the invention will be explained with reference to the drawing.
a to 4c show a method for manufacturing the carbon brush of
In the case of the embodiment shown in
Contact element 11 has a contact area 16 adjacent with a contact surface 14 of a commutator ring 15 indicated in
In the embodiment of the carbon brush 20 shown in
As is shown in
Then, slide 32 is withdrawn from the moulding tool (
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Number | Date | Country | |
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20100133951 A1 | Jun 2010 | US |