GB2216720A

GB2216720A - Electric switch

Info

Publication number: GB2216720A
Application number: GB8903666A
Authority: GB
Inventors: Alan John Mitcham; David Huw Prothero
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1988-02-18
Filing date: 1989-02-17
Publication date: 1989-10-11
Anticipated expiration: 2009-02-17
Also published as: GB8903666D0; GB2216720B; GB8803786D0

Abstract

A short-circuiting switch for a railgun has a switching assembly (18) containing liquid metal sealed therein to provide a conductive path (30). The assembly (18) is slideable between a first position at which it short-circuits first and second conductors (12) and (14) and a second position at which it short-circuits second and third conductors (14) and (16). The motive force for moving the assembly (18) from the first to the second position is derived from electromagnetic forces generated when current flows through the path (30) between the conductors (12) and (14). The assembly passes through an intermediate position at which all three conductors are momentarily short-circuited, which reduces any tendency to arcing during the transition from the first to the second position. Thus, in the first position the switch provides a short-circuit across the DC power supply to the railgun to generate a build-up of current, whereas in the second position the switch delivers this current as a pulse through a projectile-launching armature between the rails of the railgun. <IMAGE>

Description

ELECTRICAL SWITCH AND ELE^TRO}1AGNETIC PROJECTILE LAUNCHER INCORPORA G THE SWITCH The invention relates to an electrical switch and electromagnetic projectile launcher incorporating the switch.

Electromagnetic projectile launchers (usually referred to.

as ttrailguns") utilise high direct currents (DC) to launch projectiles. The basic construction of a railgun (see Figure 2) comprises a power supply circuit having two generally parallel rails bridged by a projectile armature. In operation the rails are short-circuited until the current level required for launch is achieved whereupon the current is allowed to flow through the projectile armature. The projectile armature is accelerated to launch speed owing to the interaction of the current in the projectile armature with the magnetic field induced between the rails.

The typical requirements for the switch short-circuiting the rails during the current build up are: very low resistance (usually less than 10 microhms); high current bearing capability (usually of the order of 1 MA for periods of 200 ms); rapid commutation of the current (typically in 0.5ms or less); capacity for repeated operation; and capacity for current transfer without damage to itself.

It is an object of the present invention to provide an electrical switch and to provide an electromagnetic projectile launcher incorporating the switch.

According to one aspect of the present invention, an electrical switch comprises first, second and third conductor means and a switching assembly having conductive path means of liquid metal, said assembly being in sealed, sliding relationship with said conductor means and being movable between first and second positions, said conductive path means, in said first position of said assembly, short-circuiting said first and second conductor means and, in said second position of said assembly, shortciruiting said second and third conductor means and, in an intermediate position during movement of said assembly from said first to said second position, simultaneously short-circuiting said first and second and said second and third conductor means, respectively, movement of said assembly from said first to said second position being effected, at least in part, by electromagnetic forces generated when current flows through said conductive path means in said first position of said assembly.

Preferably, said conductive path means comprises first and second conductive paths, said first path, in said first position of said assembly, short-circuiting said first and second conductor means and said second path, in said second position of said assembly, short-circuiting said second and third conductor means.

Preferably, said second and third conductor means are substantially aligned with, but insulated from, one another and are generally parallel to said first conductor means, said switching assembly being located between, and linearly movable relative to, said first and said second and third conductor means. In a preferred embodiment of this form of the invention, said first conductor means is substantially cylindrical, said second and third conductor means are annular and are arranged coaxially about said first conductor means and said assembly is annular.

According to another aspect of the present invention, an electro-magnetic projectile launcher comprises an electrical power source for supplying direct current, a pair of substantially parallel rails, a projectile armature locatable between said rails for propulsion therealong by electromagnetic forces and an electrical switch constructed in accordance with the invention, said first and second conductor means of said switch being connected to said power source and said first and third conductor means of said switch being connected to said rails.

.1ectrica1 switches ancl Le~,romagnetic l)ru ect i lr launchers will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-section of a switch constructed in accordance with the invention; Figure 2 is a schematic circuit diagram showing the basic principle of an electromagnetic projectile launcher; Figure 3 is a view similar to Figure 1 of another embodiment of a switch constructed in accordance with the invention which has been incorporated into a launcher; Figures 4 to 6 are schematic cross-sections of a further embodiment of a switch constructed in accordance with the invention which has been incorporated into a launcher, the figures showing respectively the initial, intermediate and final configurations of the switch during operation; and Figures 7 and 8 are similar views to that shown in Figure 4 but of yet further embodiments of switches constructed in accordance with the invention.

In the drawings, like reference numerals have been used for like parts.

Referring to Figure 1, the switch 10 has first, second and third conductors 12, 14 and 16 respectively, of copper for example and a switching assembly 18 in sealed, sliding relationship with the conductors 12, 14, 16.

The conductors 12, 14 and 16 are arranged generally parallel to one another, the switching assembly 18 being sandwiched between the first conductor 12 and the second and third conductors 14 and 16 for movement between a first position (shown in full outline) and a second position (shown in ghost outline). The conductors 12, 14 and 16 form part of or are connected to first and second circuits (not shown) as indicated by arrows 20 and 22, res[)tctiviy.

The first conductor 12 is common to both circuits and constitutes a continuous curren @ath through the switch 10. The first conductor 12 is electrically separated from the conductors 14, 16 by insulation 24. It is also electrically separated from the switching assembly 18 when the assembly 18 is in its second position by insulation 26.

The second and third conductors 14 and 16 are respectively connected to the first and second circuits and are electrically separated from one another by insulation 28.

The switching assembly 18 has a conductive path 30 of liquid metal. The path 30, in the first position (full outline) of the assembly 18, short-circuits the first and second conductors 12 and 14 and, in the second position (ghost outline), short-circuits the second and third conductors 14 and 16.

The liquid metal forming the conductive path 30 of the assembly 18 is trapped in an insulated frame 32 which carries the necessary seals to retain the liquid metal in the frame 32 and between the conductors 12, 14, 16. The frame 32 can take any suitable form provided allowance is made for expansion of the liquid metal owing to ohmic heating. For example, the frame 32 can be closed loop, a resiliently-biased member (not shown) being provided within the loop to allow for the expansion of the liquid metal.

The motive force for moving the switching assembly 18 from its first position to its second position is derived from the electromagnetic forces generated when current flows between the conductors 12 and 14 through the conductive path 30. Whilst in many applications the electromagnetic forces will move the assembly 18 with the required speed to commutate the current, it is within the scope of the present invention to supplement that motive force, e.g.

by mechanical means such as springs, if necessary.

The assembly 18, in its first position, is restrained against movement arising from the electromagnetic forces by any suitable latching or restraining means (not shown). For example, piston rods of piston-and-cvlinder arrangements can act, in an extended .osi.ion, as stops for the assembly 1 ,. In tI0r3t instance, nQ asse . I is r ed tt e by recractior he ;* > ull rails to a withdrawn position. Also, subsequent extension of the piston rods will then "reset" the switch 10 by moving the assembly 18 back to its first position.

Energy-dissipating means such as a damper arrangement (not shown) are also provided to bring the assembly to rest in its second position.

The length, in the direction of travel, of the conductive path 30 is selected such that the leading edge (during travel of the assembly 18 from its first to its second position) of the path 30 is in engagement with the conductor 16 before the trailing edge of the path 30 has electrically disengaged from the conductor 12 owing to the insulation 26. In this arrangement, arcing on disengagement of the assembly 18 from the conductor 12 is minimised.

In a preferred form of the switch .18, such arcing is minimised even further by the provision in the conductor 12 of a block 34 of high resistivity material, eg carbon, immediately adjacent to the insulation 26. As the assembly 18 moves from its first position, current starts to flow through the block 34 and, owing to the high resistivity of the block 34, a significant voltage is developed between the conductors 12 and 14 which enhances the speed at which commutation of the current to the conductor 16 occurs.

The edge of the block 34 may be insulated at 36 to prevent excessive temperature rises in the block 34 owing to the current density concentrations which would otherwise occur at that edge during commutation. The dimensions of the block 34 are selected such that complete commutation of the current has substantially occurred by the time contact between the block 34 and the conductive path 30 has ceased during travel of the assembly from its first to its second position.

The circuit diagram for a typical electromagnetic projectile launcher, ie "railgun", is shown generally at 40 in Figure 2. The railgun 40 iias an electrical power supply c)nsis.inT of a homopolar direct current (DC) generator 42; a clost@@ switch 44; a storage inductor 46 (which may be integral with the generator); and a switch 48. Two parallel conducting rails 50 are connected to the supply across the switch 48. A projectile armature 52 is located between the rails 50 and is designed to propel a projectile 54. In general, the projectile armature 52 may be of metal or other conducting material, insulated at 56 from the projectile 54, or of plasma.

In operation, the switch 48 is set to short-circuit the rails 50 and the switch 44 is closed to charge the inductor 46.

Once the required current level has been achieved, the switch 48 is operated to divert the current through the projectile armature 52.

The armature 52 is then propelled by electromagnetic forces along the rails 52 to launch the projectile 54.

In accordance with one aspect of the invention, in a railgun, the switch 48 is a switch 10 constructed in accordance with the invention.

Referring to Figure 3, the switch 48A which is similar to the switch 10 described above with reference to Figure 1, is incorporated in a railgun circuit. The switching assembly 18 is shown intermediate its first and second positions, commutation of the current being almost completed.

In this embodiment, however, the liquid metal is divided between two conductive paths 30A and 30B. Path 30A, in the first position of the assembly, short-ciruits the conductors 12 and 14 to circulate the current around the generator circuit and path 30B, in the second position of the assembly, short-circuits conductors 14 and 16 so that current passes, via rails 50, through the projectile armature 52. The amount of liquid metal in each conductive path 30A, 30B can be "tuned" to the current-carrying requirements of the respective short-circuits effected by the paths 30A, 30B.

In the embodiment shown in Figures 4 to 6, the structure of the switch 48B is similar to the structure of the switch described it reference to Figure 3, except that the conductor 1 is a solid ~ 1inder nicil terminates in a central stub I 2A cei!nected t,, one of the rails 50 and the conductors 14, 16, the assembly 18, the insulation 24, 26 and the carbon material 34 are all annular components.For convenience, the rails 50 are shown normal to the longitudinal axis 38 of the conductor 12 However, any convenient angular relationship between the conductors 12, 14, 16 and the rails 50, including axial as shown in Figure 3, can be adopted.

Figure 4 shows the assembly 18 in its first position. Upon release of the assembly 18, the electromagnetic forces arising from the current flowing through the conductive path 30A drives the assembly 18 towards its second position which is shown in Figure 6.

Figure 5 showns the assembly 18 in an intermediate position in which commutation of the current to the conductor 16 is about to commence.

In the embodiment shown in Figure 7, the radial thickness of the conductive path 30A has been reduced as compared to the radial thickness of the path 30B. Consequently, both the mass and the acceleration applied to the assembly 18 is reduced thereby reducing the impact forces experienced by the switch 48C as the assembly 18 is brought to rest in its second position.

In the embodiment shown in Figure 8, the switching assembly 18 of the switch 48D is shown in its second position. In this embodiment, the electromagnetic forces generated in the first conductive path 30A, which are used to move the assembly 18 from its first to its second position, are also used, in the second position, to drive the liquid metal constituting the path 30A from the frame 32 as indicated by the arrows 42. The liquid metal is driven into a chamber or cavity 44 formed between the central stub 12A of the first conductor 12 and the conductors 14, 16 and is then removed from the switch 10.

A solenoid 40 is located about the switch 10 adjacent the cavity 44. The axial field generated by the solenoid 40 interacts with the residual current in the liquid metal and causes it to follow a spiral path.

Resulting arcs which may be struck between the carbon material 34 and an inner hollow cylindrical extension 16A of the third conductor 16 or between the first conductor 12 and the extension 16A will also be caused to rotate in the magnetic field and, as a result, extinguishment of, those arcs will be assisted in a manner similar to the extinguishment of arcs in "rotary arc circuit breakers". Extinguishment of any such arc may be further enhanced by providing a gas blast along the direction of the axis 38 and out of the cavity 44. The gas can be introduced directly into the cavity 44 and/or can-be introduced into frame 32 behind the liquid metal of the path 30A as it is expelled therefrom.

The second conductor 14 is provided with suitable filling and vent holes, indicated generally at 46, to permit liquid metal to be introduced into the frame 32 to form the first conductive path 30A when the assembly 18 is in its first position.

As the insulation 26 is absent in this embodiment, additional support for the switching assembly 18 is provided by the extension 16A of the third conductor 16.

In switches constructed in accordance with the invention, it is preferred to use metals for the or each conductive path which are liquid at or close to ambient temperatures thereby avoiding complications involving pre-heating of the switch to make the metal liquid. Examples of suitable metals are mercury and some alloys of mercury, sodium/potassium alloys and alloys of gallium and indium.

Claims

CLALMS

1. An electrical switch comprising first, second and third conductor means and a switching assembly having conductive path means of liquid metal, said assembly being in sealed, sliding relationship with said conductor means and being movable between first and second positions, said conductive path means, in said first position of said assembly, short-circuiting said first and second conductor means and, in said second position of said assembly, short-circuiting said second and third conductor means and, in an intermediate position during movement of said assembly from said first to said second position, simultaneously shortcircuiting said first and second and said second and third conductor means, respectively, movement of said assembly from said first to said second position being effected, at least in part, by electromagnetic forces generated when current flows through said conductive path means in said first position of said assembly.

2. A switch according to claim 1, in which said conductive path means comprises first and second conductive paths, said first path, in said first position of said assembly, short-circuiting said first and second conductor means and said second path, in said second position of said assembly, short-circuiting said second and third conductor means.

3. A switch according to claim 2, in which said paths are dimensioned in accordance with the current-carrying requirements of the respective short-circuits effected by said paths.

4. A switch according to any one of the preceding claims, in which said second and third conductor means are substantially aligned with, but insulated from, one another and are generally parallel to said first conductor means, said switching assembly being located between, and linearly movable relative to, said first and said second and third conductor means.

5. A switch according to claim 4, in which said first conductor means is substantially cylindrical, in which said second and third conductor means are annular and are arranged coaxially about said first conductor means and in which said assembly is annular.

6. A switch according to any one of the preceding claims, in which, during movement of said assembly from said first to said second position, said conductive path means disengages from said first conductor means at a section thereof comprised by high resistivity material.

7. A switch according to claim 6, in which the trailing edge of said section, relative to the direction of movement of said assembly from said first to said second position, being insulated thereby to avoid abnormally high current densities in said section.

8. A switch according to claim 2 or to any one of claims 3 to 7 as dependent on claim 2, in which a flow path for liquid metal is provided whereby, as said assembly completes its movement to said second position, liquid metal is expelled from said assembly via said flow path by said electromagnetic forces.

9. A switch according to claim 8, in which said flow path comprises a chamber for receiving liquid metal from said assembly, said switch further comprising a solenoid adjacent said chamber whereby said liquid metal, or plasma arcs arising therefrom, is constrained to flow in a spiral pattern in said chamber under the influence of a magnetic field generated by said solenoid.

10. An electrical switch according to claim 1 substantially as hereinbefore described with reference to Figures 1 and 3 to 8 of the accompanying drawings.

11. An elecrtical switch according to claim 1 substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

12. An electrical switch according to claim 1 substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

13. An electrical switch according to claim 1 substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.

14. An electrical switch according to claim 1 substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.

15. An electrical switch according to claim 1 substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

16. An electromagnetic projectile launcher comprising an electrical power source for supplying direct current, a pair of substantially parallel rails, a projectile armature locatable between said rails for propulsion therealong by electromagnetic forces and an electrical switch as claimed in any one of the preceding claims, said first and second conductor means of said switch being connected to said power source and said first and third conductor means of said switch being connected to said rails.

17. An electromagnetic projectile launcher according to claim 16 substantially as hereinbefore described with reference to the accompanying drawings.