GB2145581A - Sliding contact arrangement - Google Patents

Abstract

A sliding electrical contact arrangement for high power high frequency use in, for example, contact welding or induction heating apparatus has a movable conductor bar (32) with a flexible metal foil skin (34) for engaging a fixed conductor plate (26). The conductor bar and its skin extend transversely to the direction of movement of the bar, and the skin encloses a space (35) for cooling fluid, which may be pressurised to urge the skin uniformly against the plate. This arrangement may form part of a variable inductor coupled in series on one of a pair of bus bars between a generator and a welding station, the inductor comprising a shorted stub in the form of two plates (26, 28) projecting perpendicularly from the bus bar with an adjustable position shorting bar between the plates, the bar having a metal foil skin portion on both sides thereof to engage the opposing faces of the plates. <IMAGE>

Description

SPECIFICATION Sliding contact arrangement This invention relates to a sliding electrical contact or contacts for high frequency high power equipment, for example contact welding or induction heating apparatus.

In such equipment it is often necessaryto provide a variable position connection between two conductors, for instance to allow movement of one conductor relative to the other orto vary a contact position.

Conventional sliding contacts generally have an insufficiently low contact resistance for high currents or excessive inductance for high frequency applications.

In our co-pending application filed with this applicalion, we disclose an impedance matching system for low impedance loads in which a variable inductance is used to transform the series resistance of a load to a higher resistance for improved matching to the output of a high frequency generatoroperating at, for example, 400 kHz. The small inductance required, typically lessthan 0.02 ,aH, may beformed by interrupting a supply bus bar with a shorted stub of variable length. It is an object of this invention to provide a continuously variable shorting connection for such a requirement, having low contact resistance and residual inductance.

According to this invention there is provided a sliding contact arrangement having a first conductor having a contact surface and second conductor which is linearly slidable relative to the contact surface of the first conductor, the second conductor extending transversely to the direction of relative movement and having a flexible portion for engaging the contact surface of the first conductor. Preferably the second conductor comprises a transverse bar with a flexible metal foil skin enclosing a fluid tight space on that side ofthe barfacing the contact surface.Means are providedforsupplying cooling fluid under pressure to this space so that heat generated in the contact area can be quickly dissipated,thefluid pressure also preferably serving to urge the foil against the contact surface along the length ofthe bar.

The invention can be applied to the variable length stub described above by fixing foil skin sections to opposite sides of the bar so that the bar can be fitted between two parallel flat strip conductors forming the stub with the foil sections engaging the respective inner faces of the conductors. By moving the bar towards orawayfrom the junction ofthestrip conductorwith the supply bus bars,theseries inductance ofthe stub can be varied from virtually zero upto a required maximum value.

The major advantages ofthe sliding contact arrangement ofthis invention are its high current capacity clue to its large contact area and efficient cooling, and its very low residual inductance resulting from the relative short path length at high frequencies.

The invention will now be described by way of example with reference to the drawings in which: Figure lisa simplified perspective viewofa variable inductance stub on a pair of bus bars; Figure 2 is a cross-section of part of the stub of Figure 1; Figure 3 is a sectional view looking in the direction of the arrow III in Figure 2; and Figure 4 is a cross-section of a single sliding contact arrangementforconnectingtwo relatively movable conductors.

Referring to Figure 1, a pair of bus bars 10 and 12 for contactwelding or induction heating apparatus comprise two shallow back-to-back channel sections mounted in close proximity to each otherfor minimum inductance. Bus bars ofthisconfiguration are capable of carrying large alternating currents. Flat strip sections maybe used instead of channel sections.

Avariable series inductance is formed by a stub 14 having a shorting bar 16 which is movable towards or away from the base ofthe stub rotation of a threaded sleeve 17 on a threaded bar 18 attached to the rearface of the shorting bar 16, well away from the current flow at the front face. Carbon-free hoses 20 and 22 supply cooling waterto interior spaces in the bar 16. The inductanceofthestubl4can beadjustedtosubstan- tiallyzero by moving the bar 16 towards the bus bars with thefrontface 24 ofthe barflush with the inner surface of the bus bar 12. The actual variation of inductance per unit length ofthe stub is dependent on the width of the bus bars and the gap between the faces of the stub.

The construction ofthe shorting bar 16 and its relationship with the two plates 26 and 28 of the stub are shown in more detail in Figures 2 and 3. The stub plates are made from copperfor low losses, machined flatontheiropposing surfaces for maximum contact with the shorting bar 16, and water cooled by pipes 30 soldered to the outside faces. When frequent move mentoftheshorting bar is anticipated, the insidefaces ofthe plates are plated with rhodium or silver to reduce wear, although this does increase losses slightly. When used as a preset adjustment, no plating is necessary.

The shorting bar 16 has a rectangularsection body 32 made from brass, with a thin foil 34 of copper, beryllium copper or other suitable flexible conducting material clamped to the front face ofthe body 32 by a clamping plate 36 (shown only in Figure 2, not in Figure 3). The foil is soldered to the body 32 at its sides and rear edges to provide a watertight envelope on the upper and lowerfacesofthe bar, thereby enclosing spaces 38 and 40 which communicate with an inlet passage 42 and an outlet passage 44 drilled in the body 32 (see Figure 3). This arrangement ensures that high velocity cooling water can be brought as close as possible to the contact area where the foil 34 contacts the inner faces of the plates 26 and 28. Back pressure is applied to the cooling circuit, giving a pressure under the foil of 30 to 40 p.s.i., which distorts the foil slightly and holds it in very close contact with the stub plates overthe whole width of stub, resulting in extremely low contact resistance.

Internal spring loaded plungers 46,48,50 and 52 (only four are shown) are housed in the body 32 at intervals along its length to hold the foil 24 against the stub plates when no water pressure is applied, to provide rigidityto the assemblyfor adjustment and setting up purposes.

At high frequencies the current in the stub is almost entirely confined by electrical skin effect to those surfaces ofthe stub plates and the shorting bar indicated bythe dotted line 54 in Figure 2. The current skin depth is typically 0.1 mm in copper at kHz, so the relative thinness ofthe foil 34 does not contribute significant losses at this frequency. The ends of the body 32 are shaped where the foil 34 is soldered at its edges to give a curved profile with the maximum projection occurring nearerthe front face than the rear face ofthe bar 16. The spring loaded plungers 46to 52 are also positioned with this in mind. Since the current is confined to the frontface ofthe bar, cooling passage branches 56 (Figure 2) are drilled close beneath the frontsurfaceofthe body 16.

A second embodiment of the invention is shown in Figure 4. Two flat strip conductors 58 and 60 are connected by a transverse bar 62 similar to that ofthe variable inductor of Figures 1 to 3. In this case, the foil 34 extends on one side ofthe bar only and engages the lowersurface ofthe conductor 60, the bar itself being clamped to an end flange ofthe movable conductor 58.A bearing surface 64supports the bar 62.

Claims (14)

1. Ahigh current sliding contact arrangement comprising first and second conductors having respective first and second contact surfaces, the second conductor being movable relative to the first conduc torwith the second contactsurface in sliding engagement with the first contact surface, wherein the second conductor extends transversely relative to the direction of relative movement ofthe two conductors, and the second contact surface is formed on a resilient portion ofthe second conductorto define a region of contact between the two contact surfaces which extends transverselyto the direction of relative movement.

2. A contact arrangement according to claim 1, wherein the second conductor has an internal transversely extending cooling fluid passage beneath the second contact surface.

3. A sliding contact arrangement comprising: a first conductor having a planarcontactsurface; a second conductorwhich is movable relative to the first conductor and includes a flexible electrically conductive skin extending transversely to the direction of relative movement ofthe two conductors, the skin enclosing a space, which when pressurised, urges the skin against said planar contact surface to define a continuous transversely extending area of contact between the two conductors.

4. An arrangement according to claim 3, wherein the second conductor has a conductor body with an internal fluid passageway, and wherein the skin comprises a metal foil secured to the conductor body to form a sealed enclosure between thefoil and the body, which enclosure communicates with the passagewayto allow pressurisation via the passageway.

5. An arrangementaccording to claim 4,wherein the enclosure communicates with an inlet passageway and an outlet passagewayto allow circulation of cooling fluid through the enclosures.

6. A high current sliding contact arrangement comprising: a first conductor having a planar contact surface, a second conductorwhich is movable relative to the firstconductorand includesaflexible electrically conductive skin arranged to be resiliently urged against the planar contact surface, said skin enclosing a space having an inlet and outletforthe circulation of cooling fluid.

7. An arrangement according to claim 6, wherein the second conductorcomprisesa rigid conductor body, the skin being bonded to the conductor body, and a plurality of springs located between the conductor body and the skin for urging the skin againstthe planarcontact surface ofthefirst con- ductor.

8. An arrangement according to claim 7, wherein the conductor body comprisesametal ba bar mounted transversely relative to the direction of relative movement ofthe conductors, the arrangementfurther comprising screw adjustment means coupled to the barforsetting the relative positions ofthe conductors.

9. Avariable inductor including: first and second conductive plates mounted in a parallel spaced apart relationship; a conductor element in sliding engagementwith both plates; and adjustment means for moving the conductor ele ment towards or away from two respective and mutually adjacent edges of the plates to vary the series inductanceformed between said edges by the plates and the conductor element, the conductor element comprising a conductor body coupled to said adjustment means, and flexible metallic skin portions attached to the conductor body and arranged to be urged resiliently against the conductive plates thereby to define respective areas of contact with each plate which extend transversely relative to the direction of movement ofthe conductor element.

10. Avariable inductor according to claim 9, wherein the skin portion enclose a sealed enclosure for the passage of cooling fluid.

11. Avariable inductoraccording to claim 10, further including cooling means attached to outer faces ofthe conductive plates.

12. Avariable inductor according to claim 9, wherein the skin portions are each part of a single metal foil element having a first transversely extending rear edge bonded to the conductor body, an intermediate portion covering a face ofthe conductor body facing said adjacent edges of the plates, a second transversely extending rear edge bonded to the conductor body, and opposite side edges secured to the conductor body.

13. Asliding contact arrangement constructed and arranged substantially as herein described and shown in the drawings.

14. Avariable inductor constructed and arranged substantially as herein described and shown in the drawings.