GB2095042A

GB2095042A - Fiber brushes and method of making fiber brushes

Info

Publication number: GB2095042A
Application number: GB8127972A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1981-01-29
Filing date: 1981-09-16
Publication date: 1982-09-22
Also published as: DE3138690A1; FR2498831A1; JPS57129151A

Abstract

A fiber brush 1 for a dynamoelectric machine formed of insulated conductive fibers bonded with adhesive, one end of the fibers being stripped of insulation and joined into a conductive mass to which a conductive bar 7 is joined. Several bundles of fibers and bars are disposed side by side with insulation 17 between them to form a brush. Figure 5 shows a jig 31 around which the insulated wire 5 is wrapped and held in place by cap plates 35 so that it can be cut to form the flattened fiber bundles. <IMAGE>

Description

SPECIFICATION Fiber brushes and method of making fiber brushes This invention relates to a fiber brush for a dynamoelectric machine and in particular to stranded electrical brushes having each strand coated with an insulating material and bound together with an abradable material.

Electrical brushes are utilized in electrical machinery to transfer current between moving portions of the machine and stationary portions thereof and are normally made of monolithic slabs of carbon or composites of carbon and highly conductive metals. In the early stages of development of electrical machinery, stranded wire was gathered together in bundles which resembled a paint brush and utilized to transfer current between the stationary and moving parts of the electrical machinery; hence, they were given the name "brushes", a name which continues to be utilized even though the brush changed from a stranded structure to a monolithic structure.

The efficiency of high current, low voltage DC machinery depends to a large measure on the performance of the brush systems, which transfer current from the rotating to the stationary portions of the machines. In order to reduce the resistance losses and improve the overall efficiencies of these systems, sintered metallic graphite brushes containing 50 to 75% silver or copper have replaced conventional carbon or electrogaphic brushes. These brushes have about one-tenth the resistance of the conventional carbon brush; however, the low resistance in conjunction with bar leakage inductance creates a switching problem at the trailing end of the brush where the rotor bars break contact.This problem is known as metal depletion, a condition which occurs due to high temperature rise at the interface surface where the brush leaves the bar, the temperature rise- being sufficient to melt metal from the metal graphite composite brush structure. Depletion occurs first at the trailing .edge of the brush where the power density reaches a maximum and then moves from the trailing edge towards the undepleted region.

Thus, in effect, the electrical trailing edge of the brush moves away from the physical trailing edge into the brush face. This continues to occur until power dissipated within the high resistance depletion zone becomes an appreciable fraction of the total power dissipated during the switching interval. At this point, the depletion zone stabilizes at a fixed distance from the trailing edge of the brush.

According to the present invention, a fiber brush, for a dynamoelectric machine, said fiber brush comprising a plurality of conductive fibers coated with an electrically insulating material, said fibers being disposed side by side in a substantially flat bundle in which adjacent fibers are stuck to one another by an adhesive material, one end of said fibers being stripped of insulation, said stripped end of said fibers being joined into a conductive mass, a conductive bar electrically and mechanically joined to said conductive mass, a plurality of said bundles and bars being disposed side by side with insulating layers disposed between adjacent bundles and insulating material disposed to electrically separate adjacent conductive bars to form a brush having a precise cross-sectional dimension.

Conveniently, a conductive bar is electrically and mechanically joined to the conductive mass and a plurality of the bundles and bars are disposed side by side with abradable insulating layers disposed between adjacent bundles. Insulating material is also disposed to electrically separate adjacent conductive bars to form a fiber brush of stable cross-sectional dimensions.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a brush; Figure 2 is a partial sectional view of a segment of the brush; Figure 3 is a partial sectional view of a segment of the brush; Figure 4 is a partial sectional view of the brush; Figure 5 is an isometric view of a jig utilized in forming brush segments; Figures 6 through 9 are partial sectional views of the jig showing steps utilized in fabricating a brush segment; and Figure 10 is an isometric view of another jig.

Figure 1 shows a fiber brush 1 for a dynamoelectric machine (not shown). The brush 1 comprises a plurality of segments 3 electrically isolated from each other, but joined in a unitized brush.

As shown in Figures 2 and 3, the segments 3 -comprise a plurality of individually insulated conductive fibers 5 or copper wires coated with an electrically insulating material, the fibers being disposed side by side in a substantially flat bundle in which adjacent fibers are stuck to one another by an adhesive material. One end of the fibers is stripped of insulation and joined into a conductive mass by soldering. The segments 3 also comprise a T-shaped conductive bar 7 having a cross member 9 and a stem 11. The cross member 9 of the T-shaped bar 7 has a step 12 adjacent to its outer margin, and the stripped end of the flattened bundle of conductive fibers 5 is electrically and mechanically joined thereto by soldering.

The stem 11 of the T-shaped bar 7 has in its base a plurality of holes 1 3 which receive resistive wires 1 5 made of a resistive material such as Nichrome, which are silver-soldered in place to provide a good mechanical and electrical juncture resistive to the relatively high temperatures possible at this juncture.

Each flattened bundle of conductive fibers 5 and attached T-shaped bar 7 is electrically isolated or insulated from the adjacent flat bundle and T-shaped bar by insulating paper 17 or other abradable insulating material. Each cross member 9 of the T-shaped bars 7 has a plurality of holes 19 in their distal ends for receiving dowel pins 21 made of Micarta or other electrically insulating material. The T-shaped bars 7 and flattened fiber bundles 5 have insulated papers 17 disposed between adjacent bundles, and the periphery of the bundles are wrapped in insulating paper 23 while the periphery of the T-shaped bars are wrapped in insulating tape 25 such as Kapton, forming a unitized brush having a controlled cross-sectional dimension.

A spring saddle 26 is disposed on each side of the stems 11 on the cross members 9. The spring saddle 26 is preferably made of graphite and is electrically insulated from the T-shaped bar 7 by a sheet 27 of insulating elastomer material.

The length of the Nichrome wires 1 5 can be varied to provide the desired resistance to each brush segment to minimize depletion at the trailing end of the brush.

The brush 1 is an Insulated fiber brush that has controlied dimensional cross section, and wears like a monolithic solid brush by combining abradable insulating material and binders and yet exhibits the electrical characteristics of an insulated stranded brush.

Figure 5 shows a jig 31 having a plurality of rectangular-shaped trough portions 33 disposed so that insulated wire 5 wrapped around the jig 31 and held in place by cap plates 35 can be cut to form the flattened fiber bundles- of insulated wire used to form the brush segments 3.

The method of making the brush 1 follows Figures 6, 7, 8 and 9, and comprise the steps of laying an abradable insulating material such as paper 17 in the bottom of the troughs 33 (Figure 6); disposing a strip of gauze 37 on the sidewalls of the trough 33 so that it extends a short distance above and below the side walls (Figure 7); wrapping a predetermined number of turns of insulated wire 5 around the jig 31 filling the troughs 33 and adding adhesive to the insulated wire 5 as it is deposited in the troughs 33 (Figure 8); folding the gauze 37 over the turns of insulated wire 5 and adding paper insulation 17 over the top of the windings (Figure 9); placing the cap plate 35 over the troughs 33 in the jig 31, squeezing out excessive adhesive and generally fixing the cross-sectional dimension of the winding; placing the assembly in an oven at room temperature and raising the temperature about 30C a minute until it reaches 1 900C and maintaining it at this temperature for approximately two hours, after which the assembly is allowed to cool to room temperature; cutting the insulated wires 5 to form flattened bundles of wire which are removed from the jig 31 and trimmed leaving about a half inch of wire 5 extending from one end of the flattened bundles; dipping the exposed wire in caustic maintained at approximately 3850C to remove all insulation from this end of the wire 5; rinsing this end of the wire 5 well with water to remove ali caustic; dipping the stripped end of the wires 5 in a flux and then dipping the ends of the wire in a 60/40 solder to tin them and form a conductive mass on one end of the flattened bundle; trimming the soldered end to about one-quarter inch and checking to make sure that the solder penetrated the bundle; tinning the steps 12 of the crosses 9 of the T-bars 7, utilizing flux and 60/40 solder; soldering the tinned ends of the flattened fiber bundles to the steps 12 on the crosses 9 of the T-bars 7 to form a mechanical and electrical juncture; removing excessive solder; applying insulation to the soldered portion of the fiber bundles and the T-bars to form the insulated brush segments 3; stacking the insulated brush segments 3 and joining a predetermined number of them together, utilizing insulated pins 21 which fit through holes 1 9 in the cross portions 9 of the T-shaped bars 7; wrapping the predetermined number of joined brush segments 3 with insulating paper 23 and adhesive and placing them in a jig 41 as shown in Figure 10, which squeezes out excessive adhesive and fixes the cross-sectional dimension of the brush 1; placing the jig 41 and brush 1 in an oven and curing in the manner stated hereinbefore by raising the temperature at the rate of 30C per minute until it reaches 1 900C and then maintaining at that temperature for approximately two hours then removing the brush from the jig 41; inserting Nichrome wires 15 in the holes 13 in the base of the stem 11 of the T-shaped bars 7; fastening the wires 1 5 to the stem 11 by silver-soldering them together; and finally wrapping the T-bars 7 with Kapton tape 25 to complete the brush.

Claims

1. A fiber brush for a dynamoelectric machine, said fiber brush comprising a plurality of conductive fibers coated with an electrically insulating material, said fibers being disposed side by side in a substantially flat bundle in which adjacent fibers are stuck to one another by an adhesive material, one end of said fibers being stripped of insulation, said stripped end of said fibers being joined into a conductive mass, a conductive bar electrically and mechanically joined to said conductive mass, a plurality of said bundles and bars being disposed side by side with insulating layers disposed between adjacent bundles and insulating material disposed to electrically separate adjacent conductive bars to form a brush having a precise cross-sectional dimension.

2 A fiber brush as claimed in Claim 1 including an insulating abradable wrapper binding the side 'by-side bundles.

3. A fiber brush as claimed in Claim 2 including insulating fasteners for fastening said conductive bars together.

4. A fiber brush as claimed in any one of Claims 1 to 3, wherein each flattened bundle has an insulating layer of abradable material disposed on opposite sides thereof.

5. A fiber brush as claimed in any one of Claims 1 to 4 in which resistive wires electrically are connected to a plurality of said conductive bars.

6. A fiber brush as claimed in any one of Claims 1 to 5, wherein the stripped ends of the fibers are joined into a conductive mass by solder.

7. A fiber brush as claimed in any one of Claims 1 to 6, wherein the abradable insulating layer is paper.

8. A fiber brush as claimed in any one of the Claims 3 to 7, wherein the insulating.fasteners are Micarta pins.

9. A fiber brush as claimed in any one of Claims 1 to 8 and including blocks disposed on said bars for receiving forces, which bias said fiber brushes into electrical contact.

10. A fiber brush as claimed in Claim 9, wherein the blocks are formed of graphite and are electrically insulated from said bars.

11. A method of making a fiber brush for a dynamoelectric machine as claimed in any one of Claims 1 to 10, in which the steps include wrapping insulated fine wire around a jig having at least one rectangular-shaped trough portion to form a flattened bundle of insulated wire within the trough portion, adding adhesive to the wire within the trough portion, clamping the wires within the trough portion to bring the wires in close proximity and squeeze out excessive adhesive, setting the adhesive, cutting the wires on both sides of the trough to form a flattened bundle of wires of generally uniform length and a predetermined cross section, stripping insulation from one end of the wires forming the flattened bundle, joining the stripped ends into a conductive mass, joining the conductive mass to a conductive bar both mechanically and electrically, stacking a plurality of flattened bundles and bars side by side with a layer of abradable insulating material disposed between adjacent flattened bundles, and wrapping the stacked bundles with an abradable insulating material to form a fiber brush having a controlled cross section.

12. A method as claimed in Claim 11 including the steps of placing a sheet of abradable insulating material in the bottom of the trough.

13. A method as claimed in Claim 12 including the steps of placing a sheet of abradable insulating material over the wire disposed in the trough prior to clamping the wire.

14. A method as claimed in Claims 12 or 13 including the step of fastening the conductive bars together so that they are not in electrical contact.

1 5. A method as claimed in any one of Claims 11 to 14 including the step of connecting high resistive wires to the conductive bars.

1 6. A fiber brush, for a dynamoelectric machine, constructed and adapted for use substantially as hereinbefore described and illustrated with reference to the accompanying drawings.