US3305808A - Arrangement for the protection of actuating electromagnets operated by alternating current against overheating - Google Patents

Description

Feb. 21, 1967 G. WIDL 3,305,808

ARRANGEMENT FOR THE PROTECTION OF ACTUATING ELECTROMAGNETS OPERATED BY ALTERNATING CURRENT AGAINST OVERHEATING Filed May 18, 1965 2 Sheets-Sheet 1 FIG! INVENTOR Garlarc! Feb. 21, 1967 ARRANGEMENT FOR THE PR OPERATED BY ALTERN G. WIDL OTECTION OF ACTUATING ELECTROMAGNETS ATING CURRENT AGAINST OVERHEATING Filed May 18, 1965 2 Sheets-Sheet 2 INVENTOR CV' bin my L/ United States Patent B 9 Claims. Cl. 335-243 The present invention relates to an arrangement for the protection of actuating electromagnets operated by alternating current against overheating.

Actuating electromagnets, or control magnets, as for instance used in hydraulic arrangements for controlling the position of a valve member or the like include a yoke of cylindrical or cup-shaped configuration carrying a mag net winding and a movable armature connected to the member to be moved and attracted into the winding to a greater or lesser extent during energizing of the latter to change thereby the position of the member which is to be controlled by the electromagnet.

Electromagnets which are operated by alternating current draw at the beginning of their working stroke a greater current than at the end of the working stroke in which the air gap between armature and yoke is reduced or in which the armature may abut against the yoke, whereby the impedance of the magnet winding of the electromagnet greatly increases. If during operation of such an actuating magnet, which for instance may be used to control a hydraulic arrangement, the armature due to jamming of the member connected thereto cannot be moved, the magnet winding will draw, when energized, constantly a great current which will lead to an overheating of the magnet and possibly destruction of the winding.

It is one object of the present invention to overcome this disadvantage in the operation of actuating electromagnets which are operated by alternating current.

It is an additional object of the present invention to provide for an arrangement including an actuating electromagnet which is operated by alternating current and in which overheating of the electromagnet is positively prevented.

It is an additional object of the present invention to provide for an arrangement of the aforementioned type which is constructed of relatively few and simple parts so that the arrangement can be manufactured at reasonable cost and will stand up trouble-free under extended use.

With these objects in view, the arrangement according to the present invention for the protection of actuating electromagnets operated by alternating current against overheating, mainly comprises support means having an abutment face, an electromagnet including a yoke normally supported on the abutment face of the support means and being slidably guided by the support means movable away from and towards the abutment face, and a magnet winding carried by the yoke and adapted to be connected to a source of alternating current for energizing the electromagnet. The arrangement includes further an armature separated by an air gap from the yoke to be moved along a working stroke, when the magnet is energized by an alternating current, whereby the current drawn by the winding is greater at the beginning and smaller at the end of the working stroke, connecting means engaging the armature and being adapted to be connected to an element to be moved by the armature during movement of the latter along the working stroke, and spring means operatively connected to the yoke and arranged and constructed to yieldably press the yoke against the abutment face of the support means with a force which at any position of the magnet and the armature relative to each other is smaller than the magnetic force in the air gap, but greater than the force to be exerted by the magnet to move under normal operating conditions the armature through its working stroke, whereby when after energizing of the winding the armature is prevented from moving along the working stroke due to jamming or the like of the element connected thereto, the yoke and the winding carried thereby may move against the force of the spring means relative to the armature to a position in which the current drawn by the winding is reduced to prevent thereby overheating of the latter. The aforementioned spring means may be constructed so that the spring force changes in a non linear manner with the change of the position of the yoke and the spring means may for instance be in the form of one or a plurality of conical coil springs. To assure that in the moment of energizing the magnet winding the full force developed by the magnet Will act on the armature, it is preferred to provide some damping means to dampen the movement of the yoke or to add additional weight to the yoke to increase the inertia thereof.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal cross section of one embodiment of the arrangement according to the present invention;

FIG. 2 is a partial cross sectional view of an arrangement similar to that shown in FIG. 1 but showing a different spring;

FIG. 3 is a longitudinal cross section through another embodiment according to the present invention; and

FIG. 4 is a transverse cross section taken along the line IV-IV of FIG. 3.

Referring now to the drawings, and more specifically to FIG. 1 of the same, it will be seen that the arrangement for the protection of an actuating electromagnet operated by alternating current against overheating comprises support means, shown in FIG. 1 as a substantially cylindrical hollow housing 10, having in the region of the bottom end thereof an annular abutment face 10. Arranged within the housing 10 is an electromagnet including a substantially cup-shaped yoke 11 having a bottom wall 11 and at its end opposite the bottom wall an inwardly extending rim portion 18 having an upper annular face extending substantially normal to the axis of the yoke, and an annular magnet winding located in the yoke between the bottom wall 11' and the rim portion 18. The winding 12 is adapted to be connected to a source of alternating current in a manner not shown in the drawing. The yoke 11 is slidably guided in the housing 10 movab'ly. away from and toward the abutment face 10'. The connection of the magnet winding 12 to the source of alternating current has to be made in such a manner so as not to impede any movement of the yoke 11 and the winding carried thereby in axial direction. The arrangement includes further a T-shaped armature 16 having a stem portion extending through the rim portion 18 partly in the cylindrical space defined by the winding 12 and having outside the yoke a transverse portion 17 projecting to opposite sides of the stem portion spaced from the upper face of the rim portion 18 of the yoke 11. Connecting means in form of a rod 19 are slidably guided in a central bore of the bottom wall 11' of the yoke and engage with the upper end thereof the bottom face of the stem portion of the armature 16. The lower end of the rod 19 is adapted to be connected in any suitable manner to an element, not shown in the drawing, to be moved during energizing of the electromagnet.

When the winding 12 is energized by alternating current, the armature 16 will under normal operating conditions be moved along its working stroke in which the stem portion of the armature moves further into the winding 12 and in which the transverse portion 17 approaches the rim portion 18 of the yoke, whereby the impedance of the winding 12 increases so that the current drawn by the electromagnet is considerably reduced at the end of the working stroke of the armature.

If, for some reason, for instance due to jamming of the element connected to the rod 19, the armature cannot be moved when the winding is energized, the winding would draw continuously relatively great current which would lead to an overheating and destruction of the winding.

To prevent such an occurrence the arrangement includes further spring means, shown in FIG. 1 as two coil springs 13 and 14, which abut with the lower ends against the upper surface of the yoke 11 and with their upper ends against a cover which is by means not shown in the drawing fixedly connected to the housing 10. The springs 13 and 14 are dimensioned so as to press the bottom wall 11' of the yoke against the abutment face 10 of the housing 18 with a force which at any position of the yoke and the armature relative to each other is smaller than the magnetizing force between yoke and armature, but greater than the force which under normal operating conditions is necessary to move the rod 19.

Due to the provision of the springs 13 and 14, and due to the specific dimensioning of the springs, the yoke 11 will, when the winding 12 is energized and when the armature 16 is prevented from moving, move against the force of the springs 13 and 14 relative to the stationary armature 16 so that the gap between the portions 17 of the armature 16 and the portion 18 of the yoke 11 is reduced and the impedance of the winding 12 thereby considerably increased so that the current passing through the winding will be considerably reduced and overheating of the winding 12 thereby positively prevented. In dimensioning the springs 13 and 14 it has to be also taken into account that the magnetizing force acting between armature 16 and yoke 11, which has to move in the event the armature 16 is kept stationary, the yoke against the force of the spring means, has to overcome not only the force of the springs 13 and 14 but has to overcome also the inertia of the yoke 11 and the winding 12 carried thereby.

Instead of substantially cylindrical coil springs 13 and 14, as shown in FIG. 1, it is also possible to use.substantially conical coil springs between the cover 15 and the yoke 11 and one such conical coil spring 14 is shown in FIG. 2. It is to be understood that the embodiment only partially shown in FIG. 2 corresponds in all other aspects to the embodiment as shown in FIG. 1. A conical coil spring as shown in FIG. 2 is advantageous since the relationship between deflection of such a coil spring and the force developed by the coil spring during increased deflection corresponds substantially to the increase of the magnetic force developed by the electromagnet during relative movement of armature and yoke relative to each other in a direction in which the air gap is reduced.

To assure thatin the moment of energizing the winding of the electr-omagnet the full force developed by the magnet will act on the armature and the rod 19 connected thereto, it is advantageous to delay any possible movement of the yoke by damping means or to increase the inertia of the yoke by adding additional weight thereto.

Such an arrangement is shown in FIGS. 3 and 4, and this arrangement may include a closed substantially cylindrical housing 20 having a bottom wall 20' and a top wall 20" which is formed at the inner surface thereof with a central projecting portion which may have the shape of a spherical segment as shown in FIG. 3. Arranged within the housing 20 is a substantially U-shaped yoke 11" having a bottom portion and at its upper end a pair of inwardly extending portions. A magnet winding 12 is carried by the yoke 11" between the aforementioned portionsthereof. Fastened to the bottom portion of the yoke 11 is a substantially cylindrical piston 28 which is provided at its cylindrical surface with a sealing ring 30 which sealingly engages the inner cylindrical surface of the housing 28. A movable armature 16' having a central stern portion extending into the winding 12 and a transverse portion outside the yoke 11" and projecting with opposite ends beyond the stem portion of the armature spaced from the inwardly extending portions of the yoke. Connecting means in form of a rod 19 abut with an upper end thereof against the bottom surface of the stem portion of the armature 16' and the rod 19 extends with clearance through a bore 32 formed through the bottom portion of the yoke 11" and through the piston 28 and with a lower portion thereof through a bore 34 in the bottom wall 20' of the housing. The space within the housing about the aforementioned elements located therein is filled with a damping fluid 26, for instance oil. Appropriate sealing means, not shown in the drawing, are provided in the bore 34 about the bottom portion of the rod 19 to prevent any oil which can pass through the clearance between rod 19 and the bore 32 from flowing out of the housing 20.

The arrangement shown in FIGS. 3 and 4 includes also spring means operatively connected to the yoke 11 for resiliently holding the latter, respectively the piston 28 connected thereto against the bottom wall 20' of the housing. The spring means are shown in FIG. 3 as a leaf spring 22 connected by means of a screw or the like to the spherical projection on the top wall 20" of the housing so as to project with lateral portions thereof freely from the connected portion. The opposite ends of the spring 22 are preferably slightly curved, as shown in FIG. 3. A pair of rods 24 extend parallel to the axis of the armature 16 and the opposite ends of the rods 24 are respectively located in the curved ends of the spring 22 and in appropriate cavities at the upper end of the yoke 11" to transmit the spring force to the latter. The spring 22 is dimensioned so as to develop a force of the same magnitude as described in connection with FIG. 1 and for the same purpose as set forth above. Obviously, instead of the leaf spring 22 shown in FIG. 3, a pair of coil springs as shown in FIG. 1 or conical coil springs as shown in FIG. 2 could also be used in the arrangement illustrated in FIGS. 3 and 4.

The arrangement illustrated in FIGS. 3 and 4 will operate in substantially the same manner as described'in connection with FIG. 1. In dimensioning the spring 22 it has to be taken into account that when the armature 16' is prevented from moving during energizing of the winding 12', the magnetic force developed by the magnet has not only to overcome the pressure of the spring 22 to move the yoke 11 to a position in which the inwardly extending portions of the yoke 11" will engage the transverse portion of the armature 16', but that the magnetic force has also to overcome the resistance of the damping arrangement and the increased inertia of the yoke 11" due to the weight of the piston 28 attached thereto. During upward movement of the yoke 11", oil in the housing 20 will move from the space above the piston 28 through the clearance between the rod 19 and the bore 32 into the gradually enlarging space below the piston 28.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of overheating protection for actuating electromagnets operated by alternating current differing from the types described above.

While the invention has been illustrated and described as embodied in an arrangement for the protection of an actuating electromagnet operated by alternating current against overheating, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. Arrangement for the protection of actuating electromagnets operated by alternating current against overheating comprising, in combination, support means having an abutment face; an electromagnet including a yoke normally supported on said abutment face of said support means and being slidably guided by said support means movably away from and toward said abutment face, and a magnet Winding carried by said yoke and adapted to be connected to a source of alternating current for energizing said electromagnet; an armature separated by an air gap from said yoke to be moved along a working stroke when said magnet is energized by an alternating current, whereby the current drawn by said winding is greater at the beginning and smaller at the end of said working stroke; connecting means engaging said armature and being adapted to be connected to an element to be moved by said armature during movement of the latter along said working stroke; and spring means operatively connected to said yoke and arranged and constructed to yieldably press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter.

2. Arrangement for the protection of actuating electromagnets operated by alternating current against overheating comprising, in combination, support means having an abutment face; an electromagnet including a yoke normally supported on said abutment face of said support means and being slidably guided by said support means movably away from and toward said abutment face, and a magnet winding carried by said yoke and adapted to be connected to a source of alternating current for energizing said electromagnet; an armature separated by an air gap from said yoke to be moved along a working stroke when said magnet is energized by an alternating current, whereby the current drawn by said winding is greater at the beginning and smaller at the end of said working stroke; connecting means engaging said armature and being adapted to be connected to an element to be moved by said armature during movement of the latter along said working stroke; and conical coil spring means operatively connected to said yoke and arranged and constructed to yieldably press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter.

3. Arrangement for the protection of actuating elec tromagnets operated by alternating current against overheating comprising, in combination, support means having an abutment face; an electromagnet including a yoke normally supported on said abutment face of said support means and being slidably guided by said support means movably away from and toward said abutment face, and a magnet winding carried by said yoke andadapted to be connected to a source of alternating current for energizing said electromagnet; an armature separated by an air gap from said yoke to be moved along a working stroke when said magnet is energized by an alternating current, whereby the current drawn by said winding is greater at the beginning and smaller at the end of said working stroke; connecting means engaging said armature and being adapted to be connected to an element to be moved by said armature during movement of the latter along said working stroke; spring means operatively connected to said yoke and arranged and constructed to yieldably press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said Working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter; and damping means cooperating with said yoke for damping the movement thereof.

4. Arrangement for the protection of actuating electromagnets operated by alternating current against overheating comprising, in combination, support means having an abutment face; an electromagnet including a yoke normally supported on said abutment face of said support means and being slidably guided by said support means movably away from and toward said abutment face, and a magnet winding carried by said yoke and adapted to be connected to a source of alternating current for energizing said electromagnet; an armature separated by an air gap from said yoke to be moved along a working stroke when said magnet is energized by an alternating current, whereby the current drawn by said winding is greater at the beginning and smaller at the end of said working stroke; connecting means engaging said armature and being adapted to be connected to an element to be rnoved by said armature during movement of the latter along said working stroke; spring means operatively connected to said yoke and arranged and constructed to yieldably press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other.is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said Working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter; and weight means fixed to said yoke for increasing the inertia thereof.

5. Arrangement for the protection of actuating electromagnets operated by alternating current against overheating comprising, in combination, hollow housing means having in the region of one end thereof an inwardly extending shoulder forming an abutment face and a cover at the other end thereof; an electromagnet including a substantially cup-shaped yoke having a bottom wall normally supported on said abutment face and at an end thereof opposite said bottom wall an inwardly extending rim portion having an outer annular face, and a magnet winding located in said yoke between said bottom wall and said rim portion thereof and adapted to be connected to a source of alternating current, said yoke being slidably guided in said housing movably away from and toward said abutment face; a T-shaped armature separated by an air gap from said yoke and having a stem portion extending through said rim portion of said yoke into said winding and having outside said yoke a transverse portion projecting to opposite sides of said steam portion spaced from said outer annular face of said rim portion, said armature moving, when said electromagnet is energized, along a working stroke in which the transverse portion thereof moves toward said annular face whereby the current drawn by said winding is reduced as said transverse portion approaches said annular face; connecting means engaging said stem portion of said armature and being adapted to be connected to an element to be moved by said armature during movement of the latter along said working stroke; and spring means abutting with opposite ends thereof against said cover of said housing means and said outer annular face of said rim portion of said yoke, said spring means being dimensioned so as to press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter.

6. Arrangement for the protection of actuating electromagnets operated by alternating current against overheating comprising, in combination, hollow housing means having in the region of one end thereof an inwardly extending shoulder forming an abutment face and a cover at the other end thereof; an electromagnet including a substantially cup-shaped yoke having a bottom Wall formed with a central bore therethrough and being normally supported on said abutment face and at an end thereof opposite said bottom wall an inwardly extending rim portion having an outer annular face, and a magnet winding located in said yoke between said bottom wall and said rim portion thereof and adapted to be connected to a source of alternating current, said yoke being slid- .ably guided in said housing movably away from and toward said abutment face; a T-shaped armature separated by an air .gap from said yoke and having a stern portion extending through said rim portion of said yoke into said winding and having outside said yoke a transverse portion projecting to opposite sides of said stem portion spaced from said outer annular face' of said rim portion, said armature moving, when said electromagnet is energized, along a working stroke in which the transverse portion thereof moves toward Said annular face whereby the current drawn by said winding is reduced as said transverse portion approaches said annular face; connecting means including a rod slidably guided in said central bore of said bottom wall of said yoke and engaging with one end thereof the end of said stem portion of said armature facing said bottom wall and being adapted to be connected at the other end thereof to an element to be moved by said armature during movement of the latter along said working stroke; and spring means abutting with opposite ends thereof against said cover of said housing means and said outer annular face of said rim portion of said yoke, said spring means being dimensioned so as to press said yoke against said abutment face with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said working stroke due to jamming or the like of the element connected thereto, said yoke and said winding carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter.

7. Arrangement for the protection of actuating elee tromagnets operated by alternating current against overheating comprising, in combination, a closed housing having a cylindrical wall, a top wall and a bottom wall formed with a central bore therethrough; an electromagnet including a yoke having a transverse bottom portion facing said bottom wall and a pair of inwardly extending portions at one end thereof opposite said transverse bottom portion, and a magnet winding carried by said yoke between said transverse bottom portion and said inwardly extending portions thereof; a piston fixed to said transverse bottom portion of said yoke and carrying a sealing ring slidably engaging said cylindrical wall of said housing, said piston and said transverse portion of said yoke being formed with a central bore therethrough aligned with said bore through said bottom wall of said housing, said piston having a bottom face normally abutting against said bottom Wall of said housing and said yoke being slidably guided by said piston in said housing movably away from and toward said bottom wall of the latter; a T-shaped armature separated by an air gap from said yoke and having a stern portion extending into said winding and outside said yoke a transverse portion projecting to opposite sides of said stem portion spaced from said inwardly extending portions of said yoke, said armature moving, when said electromagnet is energized, along a working stroke in which said transverse portion thereof moves toward said inwardly extending portions of said yoke, whereby the current drawn by said winding is reduced as said transverse portion approaches said inwardly extending portions; connecting means including a rod extending with clearance through said bore of said transverse portion of said yoke and said piston and in a sealed manner through said central bore in said bottom wall of said housing, said rod engaging with one end thereof said stem-shaped portion of said armature and being adapted to be connected at the other end thereof to an element moved by said armature during movement of the latter along said working stroke; a damping fluid filling the space in said housing about said magnet and said armature; and spring means operatively connected to said yoke and arranged and constructed to resiliently press said piston carried by said yoke against said bottom wall of said housing with a force which at any position of said magnet and said armature relative to each other is smaller than the magnetic force in said air gap, but greater than the force to be exerted by said magnet to move under normal operating conditions said armature through its working stroke, whereby when after energizing of said winding said armature is prevented from moving along said working stroke due to jamming or the like of the element connected thereto, said yoke, said winding and piston carried thereby may move against the force of said spring means relative to said armature to a position in which the current drawn by said winding is reduced to prevent thereby overheating of the latter, during movement of said yoke and said piston carried thereby said damping fluid may pass through said clearance between said rod and the inner surface of said bore through said piston from one to the other side of the latter.

8, An arrangement as set forth in claim 7, wherein said spring means is a leaf spring fixed at a substantial-1y central portion thereof to said top wall and having a pair of 10 housing, and including a pair of rods respectively arranged spaced from opposite ends of said transverse portion of said T-shaped armature and engaging with opposite ends thereof said free ends of said leaf spring and said one end of said yoke.

9. An arrangement as set forth in claim 7, wherein said damping fluid is oil.

References Cited by the Examiner UNITED STATES PATENTS 1,194,132 3/1916 Brown 335-240 2,692,354 10/1954 Fisher 335-240 BERNARD A. GILHEANY, Primary Examiner.

opposite free ends spaced from said top wall of said 15 G. HARRIS, Assistant Examiner.

Claims (1)

1. ARRANGEMENT FOR THE PROTECTION OF ACTUATING ELECTROMAGNETS OPERATED BY ALTERNATING CURRENT AGAINST OVERHEATING COMPRISING, IN COMBINATION, SUPPORT MEANS HAVING AN ABUTMENT FACE; AN ELECTROMAGNET INCLUDING A YOKE NORMALLY SUPPORTED ON SAID ABUTMENT FACE OF SAID SUPPORT MEANS AND BEING SLIDABLY GUIDED BY SAID SUPPORT MEANS MOVABLY AWAY FROM AND TOWARD SAID ABUTMENT FACE, AND A MAGNET WINDING CARRIED BY SAID YOKE AND ADAPTED TO BE CONNECTED TO A SOURCE OF ALTERNATING CURRENT FOR ENERGIZING SAID ELECTROMAGNET; AN ARMATURE SEPARATED BY AN AIR GAP FROM SAID YOKE TO BE MOVED ALONG A WORKING STROKE WHEN SAID MAGNET IS ENERGIZED BY AN ALTERNATING CURRENT, WHEREBY THE CURRENT DRAWN BY SAID WINDING IS GREATER AT THE BEGINNING AND SMALLER AT THE END OF SAID WORKING STROKE; CONNECTING MEANS ENGAGING SAID ARMATURE AND BEING ADAPTED TO BE CONNECTED TO AN ELEMENT TO BE MOVED BY SAID ARMATURE DURING MOVEMENT OF THE LATTER ALONG SAID WORKING STROKE; AND SPRING MEANS OPERATIVELY CONNECTED TO SAID YOKE AND ARRANGED AND CONSTRUCTED TO YIELDABLY PRESS SAID YOKE AGAINST SAID ABUTMENT FACE WITH A FORCE WHICH AT ANY POSITION OF SAID MAGNET AND SAID ARMATURE RELATIVE TO EACH OTHER IS SMALLER

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