EP0876669A1

EP0876669A1 - Electrical actuator means

Info

Publication number: EP0876669A1
Application number: EP96938379A
Authority: EP
Inventors: Stephen Ernest Lane; Martin Snell
Original assignee: FKI PLC
Current assignee: Brush Holdings Ltd
Priority date: 1995-11-15
Filing date: 1996-11-15
Publication date: 1998-11-11
Also published as: AU7582796A; GB9523440D0; WO1997018569A1

Abstract

The actuator includes a parallelepiped housing (2) having bolted on end covers (4, 6) through which extends a non-magnetic plunger rod (8) biassed by a spring (12). The plunger rod (8) carries a soft iron sleeve moveable within an electrical coil positioned adjacent the end face (4) and a permanent magnet array positioned centrally of the housing (2). The housing (2) is penetrated by a slot (28) accommodating a bus bar (30) carrying current to an associated circuit breaker having a fixed jaw (32) and a moveable jaw (34). In operation, the electrical coil is energised to augment the flux from the permanent magnet array to overcome the spring (12) and move the plunger rod (8) to a closed position, where it is held magnetically. Energising the electrical coil with the opposite polarity effects movement of the plunger rod (8) to an open or tripped position. Current flowing in the bus bar (30) produces a magnetic flux acting to reduce the hold-on force such that an overload will also effect tripping. Manual operation of the actuator may be effected utilising a lever arm (40) and linkage (42) acting on the plunger rod (8) and sleeve to cause a sufficient movement as to overcome the magnetic flux of the permanent magnet array to open the actuator or to overcome the spring (12) to close the actuator.

Description

Electrical actuator means

This invention relates to an electrical actuator operating with magnetic latching and having holding forces and operating characteristics particularly suited for use in conjunction with electrical switchgear, particularly in relation to direct current circuit breakers such as vacuum circuit breakers, auto reclosers and contactors and for use on lower voltage air break equipment.

US Patent Specification No. 3 886 507 describes an electrically operable, magnetically latchable, actuator having an electrical wire wound coil of rectangular cross-section with an axially outer end face and an axially inner end face, a plunger moveable co-axially of the electrical wire wound coil between an open position and a closed position, a sleeve of magnetisable material secured to the plunger, a permanent magnet array of substantially rectangular cross-section perpendicular to the plunger longitudinal axis positioned axially adjacent the axially inner end face of the electrical wire wound coil having a central aperture receiving the sleeve of magnetisable material secured to a mid-portion of the plunger and biassing means biassing the plunger to the open position.

In an electrically operable, magnetically latchable, actuator of the form set out, according to the present invention, a main electrical current carrying bus bar having a portion extending in register with and adjoining at least one of the axial side faces of the permanent magnet array and corresponding side faces of the electrical wire wound coil is arranged such that flow of the main electrical current in the bus bar produces a magnetic flux effective to reduce the magnetic flux produced by the permanent magnet array.

The invention will now be described, by way of example, with reference to the accompanying, partly diagrammatic, drawings, in which:-

Figure 1 is a cross-sectional plan view of an electrical actuator operating with magnetic latching; and

Figure 2 is an isometric perspective view of the actuator, and an associated circuit breaker mechanism.

As shown in Figure 1, the actuator includes a parallelepiped housing 2 having bolted on end covers 4 and 6 through which extends a plunger rod 8 of non¬ magnetic material. One end of the plunger rod 8 is formed with a shoulder 10 providing a seating for a compression spring 12 bearing against an outer face of the end cover 6. The plunger rod 8 carries a soft iron plunger sleeve 14 formed with a stepped end face 16 of rectangular cross-section perpendicular to the longitudinal axis arranged to abut, in a closed position, a soft iron pole piece or armature 18 mounted on the end cover 4. An electrical coil 20 is positioned intermediate the housing 2 and the armature 18 and extends axially between an axially outer end face 19 adjacent the end cover 4 and an inner end face 21 beyond the armature 18. A permanent magnet array 22 in the form of rectangular cross-section blocks 24 is positioned intermediate the housing 2 and the plunger sleeve 14 adjacent the electrical coil 20 and spaced from the end cover 6. The magnet array 22 is secured in the housing 2 by means of a fixing plate and the magnetic flux is directed toward the plunger sleeve 14 through a soft iron core piece 26.

The housing 2 is penetrated by a lateral slot 28 extending parallel to a side face of the housing and through which extends a twin strap copper conductor or bus bar 30 carrying a direct current supply subject to the action of an associated circuit breaker (shown in an isometric view in Figure 2) .

As shown in Figure 2, the twin strap copper conductor 30 is connected to a fixed jaw 32 of a circuit breaker mechanism associated with the actuator. A moveable jaw 34 is connected to the plunger 8 and carries a further portion 36 of twin strap copper conductor or bus bar. An air blast discharge head 38 is provided to discharge an arc extinguishing blast of air or other gas upon the moveable jaw 34 being moved from a closed position contacting the fixed jaw 32 to an open position as shown in Figure 2. A lever arm 40 and linkage 42 is mounted on the housing end cover 6 for effecting manual opening or closing of the actuator.

In operation, the actuator is moved to a closed position by applying an electrical pulse to the coil 20 such that the magnetic flux produced by the coil causes the plunger sleeve 14 to move toward the armature 18, the magnetic flux produced by the coil 20 being in the same direction as, and augmenting, the flux arising from the permanent magnet array 22 to overcome the spring 12.

Soon after the plunger sleeve 14 reaches the closed position the coil electrical pulse is interrupted. However, the plunger sleeve 14 remains in the closed position since the attracting force between the plunger sleeve and the armature 18 is of sufficient magnitude as to hold the spring 12 compressed and overcome the magnetic flux arising from current flow through the bus bar 30.

To move the actuator to an open position an electrical pulse is applied to the coil 20 such as to produce a magnetic flux opposing the flux arising from the permanent magnet array 22 thereby substantially reducing the net flux at the armature with a corresponding reduction in the holding force. The reduction is such that the spring force exerted by the compression spring 12 will exceed the force arising from magnetic flux and the plunger will be urged to an open position. As the plunger sleeve 14 moves away from the armature 18 the magnetic flux flowing through the armature and the plunger sleeve spreads out or leaks. This, coupled with the increased air gap at the faces, causes a rapid reduction in the residual magnetic attraction force. The plunger 8 driven by the spring 12 thus accelerates toward the open position by virtue of the increasing net effect of the spring 12. This rapid movement induces a back electro-motive force in the coil 20, momentarily acting against any further increase in the current induced magnetic flux in the coil and acting to prevent the magnetic attraction force from re¬ establishing.

In a fully open position the magnetic attraction force between the plunger sleeve 14 and the armature 18 is such as to prevent the electrical pulse (if still maintained) from urging the plunger sleeve 14 toward the closed position. In addition, the effect of the magnetic force between the plunger sleeve 14 and the armature 18 serves to increase the effectiveness of the actuator during the closing operation by restraining movement of the plunger sleeve 14 until the magnetic flux due to the coil has reached an appropriate magnitude. This magnitude is selected as to ensure that the full closing of the actuator is always achieved.

Current flowing in the bus bar 30 produces an additional magnetic flux which, with the appropriate nominal direction of current flow or polarity, will generate magnetic fluxes acting to reduce the hold-on force by saturation of parts of the magnetic circuit and by opposing the holding flux arising from the permanent magnet array 22. The parameters of the coil 20 and permanent magnet array 22 are selected to be such as to generate a sufficient hold-on force with normal electrical current flow in the bus bar 30.

To effect manual opening of the actuator, manual force is applied through the lever arm 40 and associated linkage 42 to the plunger 8 sufficient to overcome the magnetic flux arising from the permanent magnet array 22 holding the plunger sleeve 14 in magnetic contact with the armature 18. As indicated, once such magnetic contact is broken the magnetic force reduces as the air gap is increased and the compression spring 12 will act to urge the plunger to the open position.

Similarly, with such an arrangement, to effect manual closure of the actuator, manual force is applied through the lever arm 40 and associated linkage 42 to the plunger 8 sufficient to overcome the effect of the compression spring 12. As the air gap is thereby reduced, so the effect of the magnetic flux produced by the permanent magnet array 22 will be increased up to a maximum upon the plunger sleeve 14 making magnetic contact with the armature 18. In order to embrace a range of actuation force requirements, the permanent magnet array 22 is composed of a stack of blocks of some seven millimetre thickness sheet, of a material such as isotropic neodymium ferro boron, bolted together. By varying the thickness of the stack of blocks, so the magnetic flux arising is varied. The spring rate of the compression spring 12 and the form and winding of the electrical coil 20 are selected to produce the required characteristics for the actuator.

Isotropic neodymium ferro boron magnets exhibit a relatively high resistance to demagnetisation, that is, a high intrinsic coercivity, and are thus virtually unaffected by the opposing magnetic flux produced by the coil 20 during movement of the actuator to the open position.

In addition, the utilisation of magnets in the form of flat blocks gives rise to a permanent magnet array

22 of relatively large surface area, giving efficient magnetic coupling across the plunger sleeve 14 and the air gap. This allows for the use of relatively thin magnetic material of comparatively low remanence and coercivity whilst minimising the reluctance of the electrical circuit providing the electrical pulse to the actuator.

The plunger sleeve 14 is of parallelepiped form giving a cross-sectional external perimeter as large as possible for a required cross-sectional area and thereby achieving a high leakage flux even at small gaps. The stepped end face 16 of the plunger sleeve serves to maximize magnetic flux by minimizing gap reluctance during movement of the actuator to the closed position. The stepped end face 16 also gives rise to a rapid reduction in the magnetic attraction force during initial movement of the plunger sleeve to the open position since the magnetic flux is only concentrated whilst the faces actually make contact.

Since, in the closed position of the actuator a relatively low reluctance path is offered to the magnetic flux in the magnetic circuit, movement of the actuator to the open position upon appropriate energisation of the coil 20 is effected relatively rapidly, an important consideration where the actuator is used in conjunction with a protective circuit. In addition, actuation with such an arrangement is achieved with tripping pulses of a magnitude of only a few amps (say three amps) although, of course, higher electrical current pulses equally also effect actuation.

Actuators of conventional design, having two coils, one to effect movement to an open position, the other to effect movement to a closed position, or with a coil positioned externally of a permanent magnet array, require provision for the electrical pulse achieving opening movement to have an electrical current value of up to approximately half the value of the electrical current required to effect movement of the actuator to a closed position. In the present arrangement, initiation of movement to the open position is achieved with an electrical current of approximately only one tenth of the electrical current required to effect movement of the actuator to the closed position.

The present invention, by utilising but a single electrical coil axially adjacent a permanent magnet array rather than the two coils of conventional designs gives rise to an actuator exhibiting economy in cost and space as well as a reduction in complexity in design and operation. In a conventional design of magnetic actuator having two coils, it is found that, upon applying an electrical pulse to one of the coils, a current tends to be induced in the other of the coils tending to produce a magnetic flux in opposition to the flux produced in the first coil, leading to complex designs. In actuators in which the electrical current pulses are controlled by electronic switches it is not practicable to place the coil not being energised in open circuit since dangerous over voltages can be produced. In some instances, upon attempting manual operation of a two coil actuator whilst a current path exists in either coil, incorrect operation may occur.

In addition to effecting opening of the actuator by applying an appropriate electrical current to the electrical coil 20, in the event of an overload or fault current flowing in the bus bar 30 exceeding a predetermined magnitude, the magnetic flux thereby generated is arranged to be of sufficient magnitude as to sufficiently oppose the hold-on force generated by the magnetic flux arising from the permanent magnet array 22 that the spring force exerted by the compression spring 12 will prevail and the plunger 8 will be moved to the open position by the effect of the spring 12. Thus, a direct acting instantaneous trip arrangement operative on the occurrence of an electrical current overload is provided effective in addition to, and electrically independently of, the arrangement whereby opening of the actuator is achieved by supplying a control current to the coil 20.

Claims

1. An electrically operable, magnetically latchable, actuator having an electrical wire wound coil (20) of rectangular cross-section with an axially outer end face

(19) and an axially inner end face (21) , a plunger (8) moveable co-axially of the electrical wire wound coil

(20) between an open position and a closed position, a sleeve of magnetisable material (14) secured to the plunger (8) , a permanent magnet array (22) of substantially rectangular cross-section perpendicular to the plunger longitudinal axis positioned axially adjacent the axially inner end face (21) of the electrical wire wound coil (20) having a central aperture receiving the sleeve (14) of magnetisable material secured to a mid-portion of the plunger (8) , biassing means (12) biassing the plunger (8) to the open position, characterised in that a main electrical current carrying bus bar (30) having a portion extending in register with and adjoining at least one of the axial side faces of the permanent magnet array (22) and corresponding side faces of the electrical wire wound coil (20) is arranged such that flow of the main electrical current in the bus bar (30) produces a magnetic flux effective to reduce the magnetic flux produced by the permanent magnet array (22) .

2. An electrically operable, magnetically latchable, actuator as claimed in Claim 1, characterised in that a tubular armature (18) extends co-axially of the coil (20) with an axially outer end face adjacent the axially outer end face (19) of the coil (20) and an axially inner end face located axially intermediate the axially outer end face (19) of the coil (20) and the axially inner end face (21) of the coil (20) .

3. An electrically operable, magnetically latchable, actuator as claimed in Claim 2, characterised in that an end face (16) of the sleeve (14) of magnetisable material registering with the armature (18) is of stepped form.

4. An electrically operable, magnetically latchable, actuator as claimed in any preceding Claim, characterised in that the permanent magnet array (22) comprises a multiplicity of flat, substantially rectangular cross-section, block magnets (24).

5. An electrically operable, magnetically latchable, actuator as claimed in Claim 4, characterised in that the block magnets (24) are of isotropic neodymium ferro boron material.

6. An electrically operable, magnetically latchable, actuator as claimed in any preceding Claim, characterised in that the sleeve (14) of magnetisable material secured to a mid-portion of the plunger (8) is of substantially parallelepiped form.

7. An electrically operable, magnetically latchable, actuator as claimed in any preceding Claim, characterised in that a housing (2) of the actuator is formed with a parallel-sided slot (28) penetrating the housing and accommodating a portion of the bus bar (30) .

8. An electrically operable, magnetically latchable, actuator as claimed in Claim 7, characterised in that a fixed jaw portion (32) of a circuit breaker mechanism associated with the actuator is mounted on the housing (2) and electrically connected to the portion of the bus bar (30) extending through the slot (28) .

9. An electrically operable, magnetically latchable, actuator as claimed in Claim 8, characterised in that a moveable jaw portion (34) of the circuit breaker mechanism is connected to the plunger (8) and is electrically connected to a further portion (36) of the bus bar (30) .

10. An electrically operable, magnetically latchable, actuator as claimed in Claim 9, characterised in that means (38) are provided to discharge an arc extinguishing blast of gas between the fixed and moveable jaw portions (32, 34) upon the moveable jaw portion (34) being moved from a closed position contacting the fixed jaw portion (32) to an open position.

11. An electrically operable, magnetically latchable, actuator as claimed in any one of Claims 7 to 10, characterised in that a lever arm (40) and linkage (42) is mounted on the housing (2) and is manually operable to effect axial movement of the plunger (8) in either axial direction.