GB2192097A

GB2192097A - Electromagnetic actuator

Info

Publication number: GB2192097A
Application number: GB08713638A
Authority: GB
Inventors: Frank Mclean Logie
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1986-06-27
Filing date: 1987-06-11
Publication date: 1987-12-31
Also published as: GB8615791D0; GB8713638D0; GB2192097B

Abstract

The actuator has a "C" or "E" core, and an armature plate 13; the outer limbs 12 of the core having extensions 14 with faces 15 which define air gaps with the edges 16 of the armature. This arrangement increases pull with only a small increase in mass. <IMAGE>

Description

SPECIFICATION Electromagnetic devices This invention relates to electromagnetic devices of the kind comprising a "C" or "E" core and a plate-like armature for movement towards the pole faces defined by the limbs of the core when winding means carried on the core is energised.

In the case of a "C" core the areas of the two pole faces are equal but in the case of an "E" core the area of the centre pole face defined by the centre limb of the core is substantially twice that of the area of each outer pole face defined by an outer limb of the core. This is because the magnetic flux flowing in each outer limb is substantially half that which flows in the centre limb assuming of course that the flux densities in the individual air gaps are substantially equal.

In order to increase the magnetic pull on the armature it is a simple matter to increase the magnetic flux in the core and armature providing the material of the core and armature can carry the extra flux without saturation. However, in view of the possibility of saturation this method of increasing the pull is limited in application.

Another method of increasing the magnetic pull without increasing the flux density in the core is to increase the area of the pole faces and the core section. If the increase in the pole face area is achieved by increasing the width of the core, the "width" being considered as the distance between the opposite sides of the core, the width of the armature must also be increased by a corresponding amount with a consequent increase in the weight of the armature in proportion to the increase in width. In this case the thickness of the armature can remain the same.

If on the other hand the increase in area is achieved by increasing the length of each pole face then in the case of a "C" core the armature length must be increased by twice the increase in length of the individual pole faces.

In the case of the "E" core the increase in length is four times the increase in length of each outer pole face because the centre pole face must have its length increased by twice the amount of each outer pole face. Morevoer, in the case of the "C" core and the "E" core the thickness of the armature must be increased to cater for the additional magnetic flux.

The object of the invention is to provide an electromagnetic device of the kind specified in a simple and convenient form.

According to the invention in an electromagnetic device of the kind specified there is associated with the limbs or the outer limbs, core extensions respectively, each core extension acting as a return path for part of the flux flowing in the associated limb of the core, the flux flowing into the armature through an air gap defined between the extension and an edge face of the armature, said winding means including windings about each of said limbs or said outer limbs or said core extensions.

In the accompanying drawings: Figure 1 is a perspective view of a conventional "E" core type electromagnetic device, and Figure 2 is a view similar to Fig. 1 showing the modification of the "E" core device in accordance with the invention, the two figures being to a different scale.

With reference to Fig. 1 there is illustrated a conventional electromagnetic device which comprises an "E" core generally indicated at 10 having a centre limb 11 and outer limbs 12. Also forming part of the device is an armature 13 which is of substantially rectangular form and having a thickness appropriate to the flux which is passed through it during the use of the device. The core and the armature can be of laminated construction. Conventionally the centre core 11 is provided with a winding not shown which when energised, causes flux to flow in the centre limb, which flux is divided equally between the outer limbs. In order therefore to maintain the same flux density in the outer limbs, the areas of the pole faces defined by the outer limbs are substantially half the area of the pole face difined by the centre limb.

In order to increase the pull on the armature it is proposed to increase the flux which fiows in the outer limbs and as shown in Fig. 2, the core is modified by the provision of extensions 14 to the outer limbs 12 respectively.

The extensions 14 are of generally "C" shaped form and define faces 15 presented to but spaced by a small working gap the end edges faces 16 of the armature respectively.

In providing the extensions 14 the area of each outer limb 12 and its pole face is increased to that of the centre limb 11. This increase results in an increase in length 1 of each outer limb and as a result an increase in the length of the armature 13 by an amount equal to twice the increase in length of each outer limb, the width w of the limbs and the armature remaining the same.

With this arrangement the flux flowing in each limb is substantially equal and the thickness of the armature remains the same, however, the pull on the armature is substantially increased with only a small increase in its mass due to the additional length.

It will be noted that the area of each pole face defined by an outer limb 12 is substantially equal to the area of the pole face defined by the centre limb and this is because it is arranged that the flux flowing in each limb is substantially equal. In order to provide the flux a winding is provided around the centre limb as in the example of Fig. 1, together with additional windings around the outer limbs, the additional windings contributing half the flux flowing in the outer limbs. Instead of the additional windings being wound about the outer limbs, they can be wound about the extension portions.

It will be appreciated that the modification can be effected in such a manner as to produce the same pull on the armature, and the weight of the armature can be reduced by reducing its thickness.

Whilst in theory it is possible to increase the area of the pole faces defined by the outer limbs without increasing the length of the armature, this would involve a reduction in the size of the gaps between the centre limb 11 and the outer limbs 12. The size of the gaps is usually critical for other reasons namely flux leakage and winding area and in practice therefore it is usually not possible to reduce the gaps.

In the case of a device having a "C" core the provision of the extensions enables the flux flowing in the armature between the limbs to be reduced thereby allowing a reduction in thickness of the armature with a consequent reduction in its weight for the same pull. If it is desired to increase the magnetic pull then the areas of the limbs and the pole faces defined thereby must be increased. As a result the length of the armature must be increased but it may not be necessary to increase the thickness of the armature in fact depending on the magnetic flux flowing in the limbs it may still be possible to reduce the thickness of the armature by virtue of the fact that the portion of the armature intermediate the limb will carry only half of the flux flowing in the limbs.

Claims

1. An electromagnetic device comprising a "C" or "E" core and a plate like armature for movement towards the pole faces defined by the limbs of the core when winding means carried on the core is energised and core extensions associated with the limbs or the outer limbs respectively, each core extension acting as a return path for part of the flux flowing in the associated limb of the core, the flux flowing into the armature through an air gap defined between the extension and an edge of the armature, said winding means including windings about each of said limbs or said outer limbs or said core extensions.

2. A device according to Claim 1 in which in the case of an "E" core the flux flow in each of the outer limbs is substantially equal to the flux flow in the centre limb of the core.

3. An electromagnetic device comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.