Device discharging ground currents, particularly in wind turbines

Abstract

Devices for discharging ground currents, more particularly in wind turbines, from electrical machines. The devices have a grounding ring mounted on a rotor of the electrical machine and a sliding contact connectable to ground and that is in contact with the grounding ring. The sliding contact element has a carbon brush with a bulk density between 1.1 and 1.4 g/cm.sup.3, and the carbon brush contains a metal inclusions composed of silver, the silver proportion being between 1 and 8%.

Claims

1. A device for discharging ground currents, particularly in wind turbines, from electric machines with a grounding ring arranged on a rotor of the electric machine and with a sliding contact element connected to ground and in contact with the grounding ring, wherein the sliding contact element comprises a carbon brush having a bulk density from 1.2 to 1.28 g/cm.sup.3, and the carbon brush comprises metal inclusions of silver, wherein the silver proportion is between 2 and 5%.

2. A carbon brush with metal inclusions for a sliding contact element which can be brought in contact with a grounding ring of an electric machine in a wind power plant, wherein the carbon brush has a bulk density from 1.2 to 1.28 g/cm.sup.3, and a silver proportion between 2 and 5%.

Description

(1) FIG. 1 shows a device for discharging ground currents, generally designated with 10. The device 10 includes a grounding ring 12 which is mounted on a shaft 14. The shaft 14 is part of a rotor of an electric machine, in particular of a rotor-fed asynchronous generator for wind turbines. Design and operation of rotor-fed asynchronous generators are generally known, so that a detailed description thereof can be omitted in the context of the present disclosure.

(2) A sliding contact element 16 having a carbon brush with metal inclusions 18 is associated with the grounding ring 12. The carbon brush with metal inclusions 18 is connected to ground potential via an illustrated line 20.

(3) The carbon brush with metal inclusions 18 is in contact with the grounding ring 12. Such an arrangement is known in the art.

(4) According to the invention, the carbon brush with metal inclusions 18 has a bulk density from 1.2 to 1.28 g/cm.sup.3. Furthermore, the metal-graphite brush 18 is impregnated with a silver proportion between 2.5 and 5%.

(5) Due to the low density and the proportion of silver, a low voltage drop, i.e. a low contact resistance between the carbon brush with metal inclusions 18 and the ground ring 12, is achieved in addition to the constant coefficient of friction.

(6) To achieve the silver impregnation, a silver solution is injected into the carbon brush with metal inclusions 18 with the low bulk density, whereafter the silver is precipitated by a chemical-thermal process. The silver is hereby not melted. The proportion of silver plays only a minor role for the electrical conductivity of the carbon brush with metal inclusions 18. A silver proportion of 5% in the metal-graphite brush 18 lowers the voltage drop across the device 10 by 50%, whereas a silver proportion of 2.5% lowers the voltage drop in the device 10 by 25% compared to a carbon brush with metal inclusions that is not impregnated with silver.

(7) The silver impregnation thus provides further advantages in addition to the low bulk density of the carbon brush with metal inclusions 18 already provided by the invention.

(8) In particular, a high pulse current strength is achieved, which is this example 0 A/cm.sup.2 at open circuit, and 20 A/cm.sup.2 under load. Due to the low density, the carbon brush with metal inclusions 18 has high elasticity, resulting in a constant friction coefficient. The low density can be achieved, for example, by low ash content, such as 0.8% in the employed carbon brush with metal inclusions.

(9) In summary, it can be stated that a stable coefficient of friction in combination with the low silver content in the brush material and a low voltage drop is achieved as a result of the highly elastic brush material for the carbon brush with metal inclusions 18 with low density. This enables a low-maintenance operational behavior of the device for discharging ground currents with low wear of the carbon brush with metal inclusions 18 and low wear of the grounding ring 12.

LIST OF REFERENCE NUMBERS

(10) 10 device 12 grounding ring 14 shaft 16 sliding contact element 18 carbon brush with metal inclusions 20 line