Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-02-14
2002-06-04
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S045000
Reexamination Certificate
active
06400057
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to slip-ring configurations for electrical machines, such as electric motors and generators, having brushes made of carbon materials and slip-ring bodies, wherein the brushes are electrically conductively connected to slip rings of the slip-ring bodies. The invention also relates to a slip-ring body and a method for retooling slip-ring bodies.
Electric motors and generators with which electrical energy is converted into rotational energy or, conversely, rotational energy is converted into electrical energy, require a current supply to a rotatably disposed coil, which is connected in a force-locking or form-locking manner to a rotating shaft. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements. That usually takes place by way of slip rings which are connected to the rotating shaft, concentric therewith and conductively connected to stationary brushes, or by way of pairing brushes with so-called commutators or collectors. In addition to producing an electrical connection between the stationary part and the rotating part of the electrical machine, the commutators or collectors also effect commutation (in direct-current machines).
Usually, the slip rings and commutators are formed of metals such as copper or copper alloys such as, for example, bronze, tin bronzes, nickel bronze, silver or steel. The slip rings are connected by insulating fastenings to a hub (rotating shaft) to form slip-ring bodies, which are insulated with respect to the hub and with respect to each other. Electrically conductive brushes are disposed stationarily along the circumference of the slip rings and are held in contact with the surface of the slip rings by spring force. In the case of alternating-current motors and generators, slip rings are required individually or plurally for each phase.
The sliding contacts (brushes) generally are formed of carbon materials, possibly in combination with metals, for example metal graphite. In order to produce metal graphite, mixtures of metal powders, in particular copper, tin or lead, are pressed with graphite, in particular natural graphite, and subsequently hardened by calcining or sintering.
In the case of all of those material pairings, wear results from reciprocal movement as well as from transmission of somewhat high currents, in which case dust can form from abrasion. On one hand, that can lead to shortening of a creepage path because of dirt accumulation and thus to arcing. On the other hand, an eroding of contacting layers results. There is a necessity for replacing the brushes and subsequent treatment of a surface of the slip rings (machining or stripping off of defective spots such as grooves or the like). In that connection, additional maintenance intervals result, which are shorter than maintenance intervals of (roller) bearings, something which causes substantially increased maintenance costs, above all as a result of additional down-times.
It is therefore desirable to keep the abrasion as low as possible and thus to lessen the frequency of the maintenance work caused as a result, or to make it at most the same as the frequency of the maintenance work for the bearings and/or other wearing parts.
It is known from East German Patent DD 258 687 A1 and from a publication entitled VEM Journal 1975, pages 15 ff, that wear is very low in the case of a pairing of graphite brushes with slip rings made of graphite. However, that system has the disadvantage of only permitting small currents to be conducted through the graphite body of the slip rings because of its specific resistance, which is relatively high in comparison with metals. When high currents are conducted, the ohmic heat is unacceptably high. That can lead to damage to the system. In a slip ring, the introduction or removal of the current takes place by way of a metal conductor which extends parallel to the axis of rotation in such a manner that it is laterally offset with respect thereto and is electrically conductively connected to the body of the slip ring. The resistance inside a graphite slip ring is of a similar magnitude to the contact resistance between a slip ring and a brush. In the case of a constant induced current in the coil, that leads to periodic voltage fluctuations in a generator. In a motor, it leads to an uneven torque, depending on the path length of the current and thus the active resistance in the slip ring.
Another construction is known from East German Patent DD 248 909 A1. A slip ring having a metallic slip-ring base and a carbon sliding ring soldered onto it is described therein. The slip-ring base is provided with hollow spaces in order to be able to remove waste heat by ventilation on all sides. The side of the carbon sliding ring that faces the metallic slip-ring base has to be metallized in order to ensure a low contact resistance and permit a soldered joint. Thermal stresses occur as a result of the strong heating of the structure by the ohmic dissipated energy, as well as during soldering. An outer portion of the metallic slip-ring base is therefore preferably provided with recesses for compensation of thermal stresses.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a slip-ring configuration in electric motors and generators, a slip-ring body and a method for retooling slip-ring bodies, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type, which on one hand lead to as little wear as possible and which on the other hand allow a sufficiently high current load, in order to permit systems of that kind to also be used in the high-current range without an occurrence of strong temperature rises known from the prior art. A further object is to be able to retrofit existing machines having metallic slip rings, in such a way that wear becomes less, with as few parts as possible needing to be replaced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a slip-ring configuration for electric motors and generators, comprising a slip-ring body having slip rings to be electrically conductively connected to brushes made of carbon materials. The slip rings each have a given radius and a metallic ring of standard construction acting as a slip-ring base with a periphery. There is at least one electrically conductive sliding layer made of a graphite material. The at least one sliding layer has a thickness amounting to a maximum of 11% of the given radius and each of the at least one sliding layer is electrically conductively fastened to the periphery of the metallic slip-ring base of a respective one of the slip rings by gluing.
Accordingly, this object is achieved by a slip-ring construction which includes a metallic ring of standard construction as a slip-ring base and a sliding layer glued onto this slip-ring base. The sliding layer preferably is formed of a carbon material. If a carbon material is used, it is advantageous to use a graphite material, particularly preferably an isostatically pressed graphite material. Furthermore, the flexural strength of the graphite material should preferably amount to at least 30 MPa (=30 N/mm
2
) in order to ensure that the layer thickness of the carbon material can be kept sufficiently small. The result of this construction is, on one hand, that the contact-surface pairing has minimal wear, because the material of the friction partner of the brushes can be chosen in such a way that the abrasion between these materials which are moved against each other is considerably lower than that between a pairing of metals or a pairing with metal and carbon material for the brushes. On the other hand, as a result of this construction, the contact resistance between the metallic base of the slip ring and the sliding layer is centrosymmetrical.
In accordance with anoth
Hahn Ingolf
Meyer Aloysius
Stadie Klaus
Vesper Wolfgang
Greenberg Laurence A.
Lam Thanh
Locher Ralph E.
Ramirez Nestor
SGL Carbon AG
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