FIELD OF THE INVENTION
The present invention generally relates to the technique of grounding shaft currents or stray current generated on the rotors of steam turbine and compressors leading to damages of bearings on which the rotors rests. In particular, the invention relates to a shaft grounding brush with high conductive metallic bristles to enhance the life of the brush. More particularly, the present invention relates to an improved electroscavenging system for grounding stray currents in industrial compressors and turbines.
BACKGROUND OF THE INVENTION
It is known that stray current or shaft currents that are generated by residual magnetic fields in the stator or rotor assembly are minimized by shaft grounding brushes. An arm extension for the brush holder is employed for holding and providing electrical contact to the assembly for drawing electrical charge and grounding it. The extension part of the brush holder plays an important role in performance and endurance of scavenging electrostatic and electrical current to the system. The functions of an electrical scavenger is to receive micro and heavy currents generated during operation through the bristles of the brush to the mains (ground). To avoid wear of the rotor surface by the brush holder, a soft non ferrous high conductive material is used. The purpose of the brush holder is to hold the bristles intact during operation and to collect currents from the bristles to pass it to the arm extension and thereby to the ground and the earthing mains. Due to regular buildup of electrostatic charges on the rotor including the casings of the compressors, magnetization occurs on the casing. Stray currents generated due to magnetization may cause over-burning of the

bristles due to poor conductivity of the brush holders. As a result, the brush wears at a much faster rate, reducing their useful life, affecting the rotor dynamics as well as shut down schedules of the machines
Improvement of grounding brush life is always a priority issue and in the past, considerable work has been done in developing high performance bristles to improve wear life. Problems are encountered with prior art low conductive brush holder in particular to the brushes playing the important role in drawing electrical current through the bristles and passing it down to the mains earth ground. The available system as described in patent No. US 4378138 is able to ground the electrical charge but the bristle and bristle holder have lower conductivity and suffer from poor life.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved electroscavenging system for grounding stray currents in industrial Compressors and Turbines.
Another object of the present invention is to propose a method of producing a highly conductive bristles holder to enhance the life of brush.
SUMMARY OF THE INVENTION
According to the present invention a copper bearing holder material is provided for improving current carrying capacity of the electroscavenging system.

The produced brush is enabled to improve the stray currents pickup from rotor of steam turbines and compressors and additionally enhances the life of the brushes. A method is provided to produce an electroscavenging system to perform under dry condition as well as under the influence of corrosive lubricants: The brush is put on the steam turbine rotor running at a speed of 9800 rounds per minute (RPM) under the influence of corrosive lubricants. Life of the grounding brush is improved according to the invention by five times in comparison to the available grounding brush.
One feature of the invention is a grounding brush holder, which can be constructed to adapt any size of diameter of the shaft and may be positioned on the compressor, turbine or other machine to be held against the shaft and allow the electrical discharge to outside of the machine to ground to the earthing mains of power plant. Another feature of the brush is the capability of resting orr the shaft by overcoming the centrifugal force of the rotating rotor and thereby maintaining a constant pressure head on the shaft. Another feature is a high conductive grounding brush holder to ensure minimum wear and continuous contact with the shaft. A further feature of the invention is a thin segmental housing of the brush that makes possible positioning of a brush arm to fit in a small opening distance in the machinery and hence requiring a minimal change in design of the machinery.
The brush is mounted on the rotor axially and parallel to the rotation of the rotor. The bristles are so angled to reduce a direct impact on the rotor material. A brush support made up of thin parallel insulating plates supporting between them, a brush arm, and a plurality of holes are made successively to extend the arm based on the distance between the casing and the rotor shaft. The brush

arm is freely suspended for carrying weight of the brush so as to exert uniform pressure on the bristles enabling all the bristles to be engaged simultaneously. Another feature is a ground wire attachment which is independent of any external wire and avoids disjoining of the arm extension wire during regular servicing and routine checkup. The wear of the brush can be automatically verified by a vibration monitoring system, which is normally installed in any captive power plant.
According to the invention, the brush holder can be formed of copper non ferrous grade and the arm extension of annealed metal for the inventive electric scavenging system. The arm extension and the brush holder is made of high current carrying non-magnetic material. The arm extension holder is used for inserting bristles produced of economical material. The holders have hardness less than the rotor shaft, and the holder is enabled to act as a heat sink for both the bristle and arm extension. Weight of various components of the brush holder by selection of different materials is synchronized to exert uniform pressure on the bristles. The bristles are plugged into the brush holder in such a way so as to enable smooth pick up of electrostatic charge generated on the rotor. An additional cooling mechanism is provided through maintenance of the radial gap between the brush and the arm extension. The material for the bristle holder is selected to avoid damage of the shaft of the rotor due to rubbing. Anti-corrosive bristles holder material is selected to improve current conducting capacity. The high conducting arm extension is also designed to match the conductivity of the brush holder to increase flow of low currents at high temperatures. The arm extension and brushes are capable to conduct currents even at high temperature essentially up to 100°C.

The grounding brush is positioned to contact a shaft surface and are grounded external to the machine. These brushes are desirably held by gravity against the shaft.
The construction of the casing and housing of the grounding brush of the invention is as under :
(1) Holes are made on the extension rod for holding the brush of different sizes.
(2) Arm extension material used is steel/copper grade.

(3) Non-Magnetic Steel casing for holding a complete grounding kit.
(4) Free Suspension over which the arm swings under the effect of gravity.
(5) Hy-Lam insulating material for avoiding contact of the arm extension to steel casing.
(6) Connecting means is provided on the tail piece of the arm extension for connecting cable which extends to the outer insulating block provided with female connector so that external male connector can be plugged in to connect to earth ground.
(7) A flexible high conducting wire is used for carrying current
generated on the brush to outside earth mains.

Other features and advantages of the invention may be seen by referring to the following description with the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Brush Holder of a Grounding Brush-High conductive copper grade according to the invention.
Figure 2 Side view of the Brush with circular Hole of the invention.
Figure 3 Brush-Holder of the invention in which bristles are mounted at oblique angle.
Figure 4 Segmented piece of a brush holder with holes provided parallel to the brush direction according to the invention.
Figure 5 Segmented piece of a brush holder in inverted position.
Figure 6 Segmented piece of a brush with four holes for bristle packing and two holes for joining pieces to form a complete Brush according to the invention.
Figure 7 Showing construction of casing and housing of a grounding brush of the invention.
Figure 8 Showing a front view of arm extension means.

DETAILED DESCRIPTION OF THE INVENTION
According to the invention, an arm extension means is provided to the grounding brush device which is mounted on the housing of a compressor or steam turbine and the brush device extends through a slot made in the housing in a direction substantially tangential to a cylindrical or conical surface on the rotor. The grounding brush device resting on this surface being substantially horizontal. The brush device has a thin, relatively flat substantially rectangular casing with flat space sidewalls as shown in figure 6. This casing is welded to a mounting flange and having locating holes for holding laminates and casing of components together as shown in figure 7.
Within the casing, is a support structure for the brush holder/arm extension. An insulating pad in the form of strips/flaps is housed at both sides of the walls of the brush housing. These strips can be made of any electrically insulating and non-conducting material e.g. Hy-Lam. The flaps are housed in another insulating block to provide necessary strength to hold firmly. This block is outside the rotor casing and is fixed on the metal housing with screws.
The brush is formed of a high conductivity material such as copper to improve the conductivity of bristles holder as shown in figure 1. The conductive brush holder is made of non-magnetic material to avoid generation of eddy currents due to magnetic flux cutting. The brush comprises a plurality of segments enjoined at predetermined angle to form a desired size. An annular space for example, 2 mm in dia. is provided within the segments to allow screws to hold the bristles firmly as a single brush (see figure 6). The brush is configured to compensate the eddy current losses generated on the bristles. The brush Holder is so constructed to have uniform pressure on individual bristles as shown in

figure 3. To further improve the performance and to restrict the complete wear, the length of the bristles is increased for example, 50 mm. Additional length of the bristles causes the stiffness of the bristles to increase and proper contact with the rotor is maintained throughout the operation. The diameter of each bristle is optimized for example, in the range of 200-250 micron and packed as bunch to avoid burning of the bristles during high voltage and current flow in operation. The brush holder is divided into sub section allowing increase or reduction in length as desired as shown in figures 6 and 7. As per the requirement, contact angle of the bristles through the sub-sections of the holder can be altered. A strip to the entire length of the brush dimension can be put on to integrate the brush holder as shown in figure 5. The attachment to the brush holder has at least two holes parallel to the arm extension.
The developed electrostatic scavenging arm and the brush holder allows enhancement in the life of the brushes with better pick up of electrostatic charge through the machinery as well as reduced mechanical vibration for longer intervals. Also the efficiency is increased by at least 500% and overall wear (erosion) of the bristles is substantially reduced.
The arm Extension to the brush holder is further modified to receive stray currents and ground them to earthing mains. Electrical contact between the arm extension rod and the ground mains might get disconnected during repeated removal of the electro scavenging system from casing. A mechanism is developed to isolate the grounding wire from the electrical contact to the outside of the holder. The grounding brush holder is wired at the end of the arm extension and to an insulating block. The block is then provided with male/female connector. The brush is electrically connected by a lead wire to the liner plates. This mechanism has the advantage of isolating the inside wiring

from the outside and as a result, pressure on the electrical connection joint to the arm extension and an outside wire is avoided. When the casing has been bolted in position in the housing, the cap and the associated components are placed in operative relation on and losses and within the casing. Since the liner plates and the brush are non magnetic, therefore, the eddy current losses wear of the bristle is minimal. After the associated components are in position, the cap is bolted to a flange as shown.
REFERENCES
1. John S. Sohre et al, Shaft Brush for Turbomachinery, US 4378138 (1983): As the title suggests this patent claims on design of Brush assembly for grounding electric current from shaft. There is no mention of brush holder design or composition.
2. William L Hansen & Ira N. Hurst et al., Brush Construction for Electrical Rotating Machinery, US 2342936 (1942): The invention deals with improved design used for fast moving motors of small capacity where rate of carbon were brush is more. The main application is in electrical motors for smooth current inducing to rotate shaft.
3. Electrolux limited of Electric work et al., Improvements in and relating to Brush Holder for Dynamo-Electric Machines, UK 456774 (1936): In this patent Brush Holder design specified for carbon block holding and is claiming for motors electric source.

4. Edmund 0 Schweitzer et al, Brush for dynamo electric machine and the like US 753676 (1915): In this patent a mechanism to hold carbon brush for dynamo is mentioned.
5. Robert W. Stanley et al, Brush Holder, US 136201 (1918) : In this patent brush holder to hold two brushes with mechanism has been demonstrated for electric motors.
6. Apart from the above patent few text books, various journal and conference papers have been referred which have not be mentioned in this report.

WE CLAIM:
1. An improved electroscavenging system for grounding stray currents in
industrial Compressors and Turbines, comprising :
a grounding brush holder with a casing located to contact shaft surface of a rotary machine having a housing, and horizontally being held under gravity on the shaft and grounded external to the machine; an extendable brush arm interposed between a brush support made of a thin insulating pads/flaps and provided with a plurality of axially extended holes to allow the brush arm to freely suspend under gravity and extend corresponding to the distance between the casing and the rotor shaft to hold the brush;
a plurality of bristles each having optimized diameters and is angularly fixed on the brush holder to reduce direct impact on the rotor material; an insulating block provided with at least one each male and female connector, the brush being connected by a flexible high conducting wire via the arm extension to the insulating block;
wherein the brush holder is formed of high conductivity material such as high copper alloy with low amount of metallic alloying elements to improve current carrying capacity including avoidance of eddy current generation due to magnetic flux cutting.
2. The system as claimed in claim 1, wherein a radial gap is maintained
between the brush and the arm extension to achieve an auto-cooling
effect.

3. The system as claimed in claim 1, wherein the brush holder has a hardness less than that of the machine shaft to allow the holder to act as a heat sink for both the bristles and arm extension.
4. The system as claimed in claim 1, wherein the assembly of the bristles on the brush holder enables effective pick-up of electrostatic charges generated on the rotor.
5. The system as claimed in any of the preceding claims, wherein the brush holder is formed of a plurality of segments enjoined at predetermined angle to produce a desired size of the brush.
6. The system as claimed in claim 1, wherein the arm extension is made of annealed metal.

ABSTRACT

The invention relates to an improved electroscavenging system for grounding stray currents in industrial Compressors and Turbines, comprising : a grounding brush holder with a casing located to contact shaft surface of a rotary machine having a housing, and horizontally being held under gravity on the shaft and grounded external to the machine; an extendable brush arm interposed between a brush support made of a thin insulating pads/flaps and provided with a plurality of axially extended holes to allow the brush arm to freely suspend under gravity and extend corresponding to the distance between the casing and the rotor shaft to hold the brush; a plurality of bristles each having optimized diameters and is angularly fixed on the brush holder to reduce direct impact on the rotor material; an insulating block provided with at least one each male and female connector, the brush being connected by a flexible high conducting wire via the arm extension to the insulating block; wherein the brush holder is formed of high conductivity material such as high copper alloy with low amount of metallic alloying elements to improve current carrying capacity including avoidance of eddy current generation due to magnetic flux cutting.