GB2122031A - Electromagnetic release device - Google Patents

Abstract

An electromagnetic release device, e.g., for use in a circuit breaker, has a magnetic circuit, an annular permanent magnet 18 to provide magnetic attraction to hold a movable armature 16 in the magnetic circuit against a spring 56 and annular coils 20, 22 energisable by direct current to produce magnetic flux opposing the attraction to allow the spring 56 to move the armature 16 to an actuating position. The magnetic circuit includes a hollow screen 10, 14 and a cylindrical core 12 within the screen about which the magnet 18 and the coils 20, 22 are arranged. The screen is effective to screen the coils against extraneous magnetic fields. One form of screen comprises a cylinder 10, an end-piece 14 at one end of the cylinder 10 and the armature 16 at the other end of the cylinder 10. <IMAGE>

Description

SPECIFICATION Electromagnetic actuators The invention relates to electromagnetic actuators particularly, though not exclusively, for tripping circuit-interrupters in circuit breakers.

In UK Patent Specification No. 1236916, an electromagnetic actuator for a circuit-interrupter is described in which in a magnetic circuit permanent magnets provide a magnetic attraction to hold a movable armature in the circuit and in which, upon operation, coils are used to produce an opposing magnetic flux in the circuit so that the armature is movable by a spring to perform a trip function. The magnetic circuit of the actuator is formed by a core and two U-shaped members positioned on opposite sides of the core to one another.

The actuator described in Specification No.

1236916 suffers from a number of disadvantages. For example, the coils are not effectively screened and voltages may be induced in the coils by extraneous magnetic fields. Such induced voltages may result in inadvertent operation of the device even though a fault on the electrical system which would require operation of a circuit-interrupter has not arisen. Other disadvantages are that the members forming the magnetic circuit only afford protection against damage to the coils in the plane including the members and the core and that the coils are wound square to fit the square-section core and are, therefore, relatively difficult to wind and have a relatively high resistance as compared to circular coils.

It is an object of the invention to provide an electromagnetic actuator in which the coil means are screened against extraneous magnetic fields.

According to the invention an electromagnetic actuator comprises a magnetic circuit, a permanent magnet to provide magnetic attraction to hold a movable armature in the circuit against a spring and coil means energisable by direct electric current to produce magnetic flux opposing the attraction to allow the spring to move the armature to an actuating position, the magnetic circuit including a hollow screen and a cylindrical core within the screen, the magnet and the coil means being annular and being arranged about the core, and the screen being effective to screen the coil means against extraneous magnetic fields.

Preferably, the screen comprises a cylinder, an end-piece at one end of the cylinder and the armature at the other end of the cylinder.

Preferably, the coil means comprises two coils positioned one on either side of the permanent magnet and electrically connected in series to one another.

Preferably, the core is hollow and accommodates the spring means and a guide member to one end of which the armature is mounted, the guide member being slidable in the core to ensure that the movement of the armature is rectilinear.

An electromagnetic actuator will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is an elevation, partly in section of the actuator; Figure 2 is an horizontal section on line Il-Il in Figure 1; Figure 3 is a vertical section on line Ill-Ill in Figure 2; and Figures 4 and 5 are simplified versions of Figure 3 in which the magnetic flux has been indicated for the unoperated and the operated conditions of the actuator, respectively.

The electromagnetic actuator (see Figures 1 to 3) has a magnetic circuit formed by: a hollow cylinder 10; a hollow cylindrical core 12; a square base plate 1 4 fixed to the cylinder 10 at a first end thereof; and an armature disc 1 6 at the second end of the cylinder 10. These components are all made of radiometal, for example. The actuator also has an annular permanent magnet 18 and coil means consisting of two annular coils 20 and 22 positioned one on either side of the magnet 18, the magnet 18 and the coils 20 and 22 being located on the core 1 2 within the cylinder 10. The coils 20 and 22 are electrically connected in series to one another.

The cylinder 10 is slotted at 24 along its full length. The sides of the slot 24 can be sprung apart during assembly of the actuator to facilitate positioning of the cylinder 10 around the magnet 18 mounted on the core 12. Leads 26 for the coils 20 and 22 extend through the slot 24. The slot 24 is sealed with epoxy resin material, for example, after the assembly of the actuator has been completed.

The cylinder 10 has external threaded formations 28 and 30 positioned adjacent respective first and second ends thereof. The formation 28 has a steel clamping ring 32 screwed onto it and the formation 30 has a brass cap 34 screwed onto it. The ring 32 is squareshaped and has threaded holes at each corner for receiving screws 36 which pass through holes in the corners of the base plate 1 4. The heads of the screw 36 sit in recesses 38 provided in the plate 14.

The base plate 14 has two threaded holes 40, for receiving screws to mount the actuator in, for example, a circuit-breaker (not shown) and a central stepped aperture 42. The aperture 42 receives a brass screw 44 which is screwed into the end of the core 12 to secure the core 12 to the plate 14. A shim 46 of copper foil is positioned between the plate 1 4 and the core 12 to ensure that a fixed gap is provided in the magnetic circuit. As the cylinder 10 and the core 12 are of the same length, a corresponding gap is produced at 48 between the second end of the cylinder 10 and the armature 16. The armature 1 6 engages the free end of the core 12 which has a flange to retain the coil 22 relative to the core 12.

The armature 1 6 has four holes 50 to permit air flow during movement of the armature 1 6. The armature 1 6 is located centrally by a pin 54, of aluminium alloy for example, which is a loose fit in an aperture in the closed end of a tube 52, also of aiuminium alloy for example. The tube 52 is slidable in the hollow core 1 2 to ensure that the armature 1 6 moves rectilinearly only. A compression spring 56 is located in the hollow core 12 and has one end within the tube 52. The spring 56 biases the armature 1 6 away from the second end of the cylinder 1 0. The core 1 2 and the tube 52 are both split (indicated at 58 in Figure 2) to permit air flow during movement of the armature 1 6.

The brass cap 34 has a central aperture in which is fixed a brass bush 60. The bush 60 has a bearing sleeve 62 lined with, for example, polytetrafluoroethylene. A striker or drive shaft 64, of alum:nium alloy for example, slides in the sleeve 62. The striker 64 is movable by the armature 16.

The end of the striker 64 adjacent the armature 16 has a flange 66 on which is seated a ring 68 of, for example, neoprene or polyvinylchloride to cushion the shock of the striker 64 hitting the bush 60 when the striker 64 is moved by the armature 16. A ring 70 of the same material is fixed inside the cap 34 to cushion the shock of the armature 1 6 hitting the cap 34. If the armature 1 6 and the striker 64 complete a full stroke, they wouid occupy the positions shown in ghost outline in Figure 3.

The magnet 1 8 is formed from particles of ferrite materials or rare earth cobalt alloys embedded in an orientated manner in resilient rubber or plastics material. The rubber or plastics material is produced either as an annular block or in strip form. The magnet 1 8 can be either a block or it can be several strips overlying one another.

The block is slotted or opposite ends of the strips do not meet so that the magnet 18 has a slot 72.

The slot 72 enables the magnet 1 8 to accommodate compressive forces which may arise when the cylinder 10 is positioned over the magnet 18 and also possibly accommodates the emergence of the coil lead at the start of the winding adjacent the core 12.

The magnet 1 8 may be pre-magnetised but is preferably magnetised in situ by the coils 20 and 22 to give radially inner and outer pole faces, typically south and north, respectively. To magnetise the magnet 18 8 in situ, the two adjacent leads 26 of the coils 20 and 22 (which are connected to one another as shown at 74 in Figures 4 and 5) are connected to the negative terminal of a direct current (dc) circuit (not shown) and the two remote leads 26 of the coils 20 and 22 are both connected to the positive terminal of the source.

The presence of the magnet 1 8 results in the formation of first and second subsidiary magnetic circuits. The first subsidiary circuit includes the base plate 1 4 and the second subsidiary circuit, when completed, includes the armature 1 6 which is held by magnetic attraction at the second end of the cylinder 10 and the adjacent end of the core 12. The two subsidiary circuits have similar magnetic reluctances.

The operation of the actuator will now be described with reference to Figures 4 and 5.

In Figure 4, the actuator is shown in a primed condition. The striker 64 has been pushed in to move the armature 1 6 against the action of the spring 56 to complete the magnetic circuit. The armature 1 6 is held by magnetic attraction at the second end of the cylinder 10. The magnetic flux from the magnet 18 in both of the subsidiary circuits is shown in broken arrows. The outer leads of the coils 20 and 22 are connected to the positive and negative terminals of the dc circuit as indicated. The adjacent leads are mereiy connected to one another as shown at 74.

When a fault condition occurs, the dc circuit is energised and the coils 20 and 22 produce a magnetic flux in the magnetic circuit of the actuator (shown in full arrows in Figure 5). The flux produced by the coils 20 and 22 opposes the flux in the second subsidiary circuit due to the magnet 1 8. The attractive force experienced by the armature 1 6 is reduced to an extent such that the spring 56 moves the armature 16 to the position shown in Figure 5. The movement of the armature 1 6 causes the striker 64 to move and, for example, operate a latch mechanism in a circuit-breaker. The flux from the permanent magnet is shunted through the first subsidiary circuit.

The dc circuit is pulsed for 50 milliseconds, for example, following which the actuator can be reset. If the fault is not cleared, the control equipment will pulse the dc circuit again and the actuator will operate. In other arrangements, the dc circuit can remain energised until the fault is cleared and the actuator will not reset whilst the fault is present.

Some details of a typical actuator are given below: Overall length of actuator 85 millimetres (mm) Maximum diameter of actuator 44 mm Length of stroke 12 mm Energy output 0.2 Joule dc circuit-power 12 millivoltamperes (mVA) The energy output of the actuator can be varied over a range simply by varying the length of the stroke. Energy outputs outside than range can be obtained by suitable selection of other spring sizes and corresponding changes to the magnet and coil characteristics.

The above-described actuator is very sensitive and the cylinder 10, the plate 14 and the armature 1 6 form a hollow screen which screens the coils from induced voltages by effectively casing the coils in magnetic material. The overall cover provided by the magnetic material also provides greater protection to the coils against accidental damage and the design optimises (as compared to the device described in Specification No. 1236916 which typically has a dc power requirement of around 100 mVA) the coil manufacture and resistance. The above-described actuator also avoids having to use a separate dust cover.

Modifications (not shown) are possible within the scope of the invention. For example, the cylinder may be slotted at each end only to accommodate the coil leads; a single coil having the requisite number of turns may be located on the core adjacent the armature; the magnet could be cast from aluminium-nickel-cobalt alloy in which instance it could be made up from a number of segments; the fixed gaps in the magnetic circuit can be at any suitable interfaces between components; the striker could be pivotally mounted; the spring could be external of the magnetic circuit; the striker could be fixed to the armature; the slots in the core and guide tube could be replaced by suitably positioned holes; and the plate and the armature need not be plate like, for example they could be bar-shaped.

Claims (6)

Claims

1. An electromagnetic actuator comprising a magnetic circuit, a permanent magnet to provide magnetic attraction to hold a movable armature in the circuit against a spring and coil means energisable by direct electric current to produce magnetic flux opposing the attraction to allow the spring to move the armature to an actuating position, the magnetic circuit including a hollow screen and a cylindrical core within the screen, the magnet and the coil means being annular and being arranged about the core, and the screen being effective to screen the coil means against extraneous magnetic fields.

2. An actuator according to claim 1, in which the screen comprises a cylinder, an end-piece at one end of the cylinder and the armature at the other end of the cylinder.

3. An actuator according to claim 1 or claim 2, in which the coil means comprises two coils positioned one on either side of the permanent magnet and electrically connected in series to one another.

4. An actuator according to any preceding claim, in which the core is hollow and accommodates the spring means and a guide member to one end of which the armature is mounted, the guide member being slidable in the core to ensure that the movement of the armature is rectilinear.

5. An actuator according to any preceding claim, in which a cover encloses the armature, the cover having an aperture through which extends, and is guided by, a striker or drive member movable away from the core by the armature.

6. An electromagnetic actuator according to claim 1 substantially as described herein with reference to the accompanying drawings.