Abstract:
An electromagnetic release drive, particularly suitable for a residual current circuit breaker, includes a plunger loaded by a spring in a release direction, a permanent magnet configuration, a coil and a yoke. The coil generates in the yoke a magnetic flux opposed to the permanent magnet configuration when the coil is driven or released such that the spring force overcomes the attraction force of the permanent magnet configuration. The permanent magnet configuration and the pole shoe conducting the magnetic flux to the plunger are associated with the yoke and the plunger such that, in a first position, the plunger is located in the active range of the permanent magnet configuration and of the pole shoe and, in a second position, is located at least partly in the active range of the pole shoe. Thus, in a first position, both the magnetic flux of the coil and that of the permanent magnet configuration, the latter at least partly, run through the plunger and, in the second position of the plunger, a closed magnetic circuit is formed by the yoke, the plunger, the pole shoe, and the permanent magnet configuration. The invention achieves two advantages. First, the working point of the permanent magnet configuration is maintained even in the release position, i.e., the second position. Second, the permanent magnet configuration can be magnetized with the coil.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an electromagnetic release for a protective circuit breaker, in particular, for a residual current circuit breaker. 
     Conventionally, a release used for a network- or mains-voltage-independent residual current protective device was based upon a magnetic circuit on the compensation principle. A U-shaped magnetic yoke is provided. A coil is wound around one limb of the yoke. On the yoke there is a permanent magnet, and the two limbs of the yoke are covered by an armature, which is spring loaded into the disconnect or release position. The permanent magnetic acts such that the armature, in the quiescent state, is attracted against the free ends of the limbs of the yoke. If a fault current occurs, then the magnetic flux generated by the fault current acts against the flux generated by the permanent magnet, so that the spring overcomes the attraction force and pivots the hinged armature into the opening position. 
     In addition to such holding-magnet releases, blocking magnet releases have also been used, but these are used much less frequently. The coil winding is connected to a secondary winding of a summation current transformer, whose primary winding is formed by the live conductor. As soon as a fault current occurs, current is applied to the coil of the release in a conventional manner, and the release responds. 
     In the event of an adhesion layer being present between the armature bearing face and the surface of the pole, the excess force from the spring, which moves the armature in the disconnect direction, is sometimes inadequate to break the contact between the armature and the pole face, and, in this example, the release fails. 
     It is necessary for the pole surface to be polished in order to achieve an adequate magnetic adhesion force. The pole face and the air gap present are extremely critical variables. Therefore, for example, applying a protective layer as a measure against sticking cannot be used. Furthermore, the geometry of the configuration makes automated production increasingly difficult because the individual parts have to be produced with high precision and monitoring, and have to be assembled with a great deal of personal, i.e., manual, effort, under clean-room conditions. 
     Because sticking sometimes cannot be avoided, the user is recommended in general terms to operate a test push-button once a month in order to check the serviceability of the release. When the test push-button is actuated, a fault current is simulated, so that the release responds and the residual current circuit breaker opens. 
     Because regular testing of a residual current circuit breaker is often not performed, in particular, in a domestic household, consideration has been given to avoiding possible sticking of the hinged armature in the event of a fault current. To such an end, carrying out automatic testing with automatic opening has been proposed. Such automatic testing can be disadvantageous to the extent that current interruptions are produced as a result of the automatic opening of the circuit breaker. Such interruptions are mostly undesired and present problems, which will not be further discussed. 
     In addition, there are also additional devices associated with the release in the form of additional releases. The additional releases are configured, for example, as piezoelectric elements or as electromagnetic releases. However, such additional elements and additional releases increase the outlay on the production of a residual current circuit breaker. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an electromagnetic release that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that prevents sticking to the greatest possible extent, so that a release can be used readily even in a residual current circuit breaker for unlatching a switching mechanism. In particular, the electromagnetic release of the invention has fewer parts and has a simpler configuration. Accordingly, automatic production is improved and manufacture time and cost are reduced. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, an electromagnetic release, including a yoke, a spring disposed in the yoke and having a spring force, a plunger loaded by the spring in a release direction, a permanent magnet configuration associated with the yoke and the plunger and having a magnet flux, an attraction force, and an active range, a coil associated with the yoke and generating in the yoke a magnetic coil flux opposed to the magnet flux such that, when the magnetic coil flux is released, the spring force overcomes the attraction force of the permanent magnet configuration, and at least one pole shoe assigned to the yoke and the plunger and having an active range, the at least one pole shoe and the permanent magnet configuration conducting the magnetic coil flux to the plunger such that, in a first position of the plunger, the plunger is located in the active range of the permanent magnet configuration and in the active range of the at least one pole shoe, and both the magnetic coil flux and at least part of the magnet flux run through the plunger, and, in a second position of the plunger, the plunger is located at least partly in the active range of the at least one pole shoe, and the magnet flux runs through the yoke, the plunger, and the permanent magnet configuration. 
     According to the invention, at least one permanent magnet and at least one pole shoe are assigned to the yoke and to the release plunger such that, in a first position, the plunger is located in the active range of the permanent magnet and of the pole shoe and, in a further position, is located only in the active range of the pole shoe. Accordingly, in the first position, both the magnetic flux from the coil and that from the permanent magnet, the latter at least partly, run through the plunger. In the second position, the magnetic field generated by the permanent magnetic runs through the plunger, the permanent magnet and the yoke, so that in the latter position a stable working point of the permanent magnet is maintained. 
     In accordance with another feature of the invention, the yoke has two yoke sections running parallel to each other, to which the plunger axis runs perpendicularly. The plunger reaches through one of the yoke sections (first yoke section), forming an air gap, whose width remains constant during the entire movement of the plunger. Thus, a change in the force on the plunger is avoided. 
     In accordance with a further feature of the invention, in its first position, the plunger bears against the inner face of the second yoke section. Due to the configuration of the release, the release force being sufficiently high, processes involving sticking of the plunger to the second yoke section, which could give rise to an ineffective release, are avoided. 
     In accordance with an added feature of the invention, to provide assistance, the plunger can be coated with an anti-adhesion layer on its actuating face facing the second yoke section. The layer may be made of a material that is as corrosion resistant as possible, in particular of nickel or a nickel alloy. 
     In accordance with an additional feature of the invention, the plunger can preferably have a ridge; the spring is then inserted between the pole shoe and the ridge. 
     In accordance with yet another feature of the invention, the yoke is a closed ring and has limbs disposed opposite the first of the two yoke sections, the coil is disposed inside the yoke, and the plunger, the permanent magnet configuration, and the at least one pole shoe are disposed inside the coil, the permanent magnet configuration bears against the inner face of the second of the two yoke sections, the at least one pole shoe is disposed coaxially with the permanent magnet configuration, the permanent magnet configuration and the at least one pole shoe accommodate the plunger therebetween in a quiescent state of the coil, and the plunger reaches through the limbs. 
     In accordance with yet a further feature of the invention, the yoke has at least a U-shape, at least one web, and at least one limb, the coil surrounds the at least one web, and the at least one limb forms the first of the two yoke sections and covers the end face of the plunger. 
     In accordance with yet an added feature of the invention, the U-shaped yoke has an integral further yoke piece, the at least one limb is two limbs forming two parallel yoke webs, the permanent magnet configuration and the plunger bear against one of the two yoke webs, and the plunger reaches through another of the two yoke webs. 
     According to a particularly advantageous refinement of the invention, the yoke can have a pot, into which the annular coil, the permanent magnet configuration constructed as an annular permanent magnet, the annular pole shoe, the spring constructed as a helical spring and the plunger can be inserted in the following way. The plunger is surrounded both by the permanent magnet and by the pole shoe and the spring. The pot is closed by a cover, through which the plunger reaches. In order to form the release, the cover serves as the first yoke section and the bottom of the pot forms the second yoke section. 
     In accordance with yet an additional feature of the invention, there is provided a sleeve of insulating material, the bottom of the pot has an inner side, and the at least one pole shoe and the permanent magnet are pressed against the inner side of the bottom of the pot with the sleeve of insulating material. 
     Another configuration considerably simplifies the manufacture of the release. It is possible to prefabricate the configuration of the permanent magnet configuration, pole shoe, coil former and coil and simply insert it into the pot. In accordance with still another feature of the invention, the at least one pole shoe and the permanent magnet are cast into a cylindrical body to form a coil former, such that the coil, the coil former, the at least one pole shoe, and the permanent magnet form a pre-assembled unit. 
     In accordance with yet an additional feature of the invention, the permanent magnet has at least one of the group consisting of a circumferential ridge and a groove holding the permanent magnet on the coil former in a form-fit. 
     In accordance with again another feature of the invention, the plunger is moveable away from the one of the two yoke webs at most to place the end face of the plunger essentially in an area of the at least one pole shoe to ensure a flux through the at least one pole shoe, the plunger, and the yoke. This configuration provides a further advantage. If, the end of the plunger is located in the area of the pole shoe when the release, serving as a residual current release, has reached its release position, then the working point of the permanent magnet remains approximately constant in any possible position, because, in any possible position, a magnetic flux through the permanent magnet, the pole shoe, the plunger and the yoke is ensured. 
     In accordance with again a further feature of the invention, the permanent magnet configuration has an axial length, and a distance the end face of the plunger assumes from the one of the yoke webs when driven is greater than an axial length of the permanent magnet configuration. 
     In accordance with again an added feature of the invention, a released position is defined by the plunger being essentially located only in an area of the at least one pole shoe, and in the released position the permanent magnet configuration is magnetized by a current pulse through the coil. An advantage of this feature is provided by the released position of the coil, wherein a flux through the permanent magnet is generated, so that the permanent magnet can be magnetized by a pulse originating from the coil. As a result, it is no longer necessary to install the permanent magnet in the premagnetized state or to magnetize it from the outside in special, complicated devices. Instead, the permanent magnet is magnetized only when it has been mounted in the release. 
     In accordance with a concomitant feature of the invention, there is also provided a residual current circuit breaker electromagnetic release. 
     Other features that are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an electromagnetic release, it is, nevertheless, not intended to be limited to the details shown since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, cross-sectional side view of an electromagnetic release according to the invention in a first, attracted position; 
     FIG. 2 is a diagrammatic, cross-sectional side view of the release according to FIG. 1 in a second extended position; 
     FIG. 3 is a diagrammatic, cross-sectional side view of another embodiment of the release of FIG. 1 in the second extended position; 
     FIG. 5 is a diagrammatic, cross-sectional side view of a further embodiment of the release of FIG. 1; 
     FIG. 4 is a diagrammatic, cross-sectional plan view of the release of FIG. 5 along the line IV—IV; 
     FIG. 6 is a diagrammatic, cross-sectional plan view of the release of FIG. 5 along the line VI—VI; 
     FIG. 8 is a diagrammatic, cross-sectional side view of another embodiment of the release of FIG. 5; 
     FIG. 7 is a diagrammatic, cross-sectional plan view of the release of FIG. 8 along the line VII—VII; 
     FIG. 9 is a diagrammatic, cross-sectional plan view of the release of FIG. 8 along the line IX—IX; 
     FIGS. 10 and 11 are schematic, cross-sectional side views of another embodiment and explain the action of the electromagnetic releases according to FIGS. 1 to  9 ; 
     FIG. 12 is a diagrammatic, cross-sectional side view of a mold for manufacturing the coil former; 
     FIG. 13 is a diagrammatic, cross-sectional side view of the release before insertion into the yoke; and 
     FIG. 14 is a diagrammatic, partial, cross-sectional side view of the release. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. 
     Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown an electromagnetic release having a yoke  10  with a bowl-like pot  11  that is closed by a cover  12 . In the interior of the pot  11 , adjoining the inner wall, there is an annular coil  13  that surrounds a plastic sleeve  14 , a permanent magnet  15 , and a pole shoe  16 . The permanent magnet  15  is also annular, and it is seated directly on the bottom  17  of the pot  11 . The permanent magnet  15  is adjoined by and touches the pole shoe  16 . The pole shoe has two sections  18  and  19  of different internal diameter. The sleeve  14  holds the permanent magnet  15  and the pole shoe  16  in place against the bottom  17 . The permanent magnet  15  and the annular pole shoe  16  surround a plunger  20  that bears against the inner face of the bottom  17  of the pot with one end face and, with its other end, projects out of the cover  12  from an opening  21 . The plunger  20  has a circumferential annular ridge  22 . Between the annular ridge  22  and the pole shoe  16  there is a helical compression spring  23 . The internal diameter of the section  19  of the pole shoe  16  results in the formation of an air gap  24  between the inner face of the section  19  and the outer face of the plunger. 
     Correspondingly, there is a further air gap  25  between the inner face of the opening  21  and the outer face of the plunger  20 . The internal diameter of the permanent magnet  15  then corresponds to the internal diameter of the section  18  of the pole shoe  16 . 
     The permanent magnet  15  produces a magnetic flux. Depending on the alignment of the north and south poles, the main part  26  of the magnetic flux runs from the permanent magnet  15  into the pole shoe  16 , through the air gap  24  into the plunger  20 , from there into the yoke  10  and the bottom  17  of the yoke, and back to the permanent magnet  15 . Between the plunger  20  and the bottom of the yoke  10  there is a very small working air gap  27 . Due to the magnetic flux  26  of the permanent magnet  15 , the plunger  20  is attracted towards the bottom  17  of the pot  11 . 
     If a current flows through the coil  13 , a magnetic flux  28  is generated. The magnetic flux  28  runs from the bottom  17  of the pot into the plunger  20 , through the plunger  20  into the cover  12 , and back again to the bottom  17  of the pot through the side wall  29  of the bottom of the pot. In other words, given appropriate polarization, the magnetic flux  28  acts counter to the permanent magnet flux  26  in the plunger  20 . As a result, the flux  26  generated by the permanent magnet  15  is cancelled, and the spring  23  (under compression) moves the plunger in the direction of the arrow P until the ridge  22  comes to bear against the inner face of the cover  12 . See plunger position in FIGS. 2 and 3. Therefore, the end of the plunger  20 , which initially bears on the bottom  17  of the pot, has been moved away from the bottom of the pot and is located approximately still in the area of the permanent magnet  15 . The working air gap  27  is then sufficiently great so that the permanent magnet  15  does not move the plunger back again towards the bottom of the pot. 
     It is also possible to dimension the plunger  20  or its travel such that the inner end of the plunger ends at the step  30  at which the section  18  merges into the section  19 , see FIG.  3 . 
     FIG. 5 shows an annular, rectangular yoke  50  having two longitudinal webs  51  and  52  running parallel to each other and connected to each other at one respective end by a transverse web  53 . Disposed at the other end of the longitudinal web  51  is a limb  54 , and disposed at the other end of the longitudinal web  52  is a limb  55 . The limbs  54 ,  55  run towards each other perpendicular to the longitudinal webs  51 ,  52  and end at a specific distance from each other. A coil  56  is inside the longitudinal webs  51  and  52 . The coil  56  has a coil axis running parallel to the longitudinal webs  51  and  52 . Inside the coil  56 , in each case adjacent to the latter and bearing against the inner face of the transverse web  53 , are two permanent magnets  57  and  58 , each having a rectangular cross section whose width corresponds to the width of the transverse web  53 . See FIG.  4 . 
     The permanent magnets  57 ,  58  are adjoined respectively by pole shoes  59 ,  60  that respectively have two sections  61 ,  62  and  63 ,  64  similar to pole shoe  16 . Sections  61 ,  62  are further remote from the transverse web  53  of the yoke  50  and from the permanent magnets  57 ,  58 . The thickness of the sections  63 ,  64 , as measured in the direction of the transverse web  53 , is smaller than the thickness of the sections  61 ,  62  and corresponds to the thickness of the permanent magnets  57 ,  58 . See FIG.  5 . 
     Between the permanent magnets  57 ,  58  and the pole shoes  59 ,  60  is a rectangular plunger  65  having a width corresponding to the width of the yoke  50 . The rectangular shape of the plunger is such that, between the sections  61  and  62 , an air gap is formed that, with regard to its dimensions, corresponds approximately to the air gap  27 . The plunger  65  projects beyond the limbs  54  and  55 . The ends of the limbs  54 ,  55  respectively form with the plunger  65  an air gap that similarly corresponds to the air gap  25 . The plunger  65  has extensions  66 ,  67  projecting in the direction of the transverse web  53 . Between the pole shoes  59  and  60  and the extensions  66 ,  67  there is a compression spring  68  that loads the plunger permanently in the direction of the arrow P, in other words, out of the yoke  50 . 
     The action of the embodiments of FIGS. 4 to  6  is the same as in FIGS. 1 to  3 . The permanent magnets  57  and  58  generate a non-illustrated magnetic flux through the pole shoes  59 ,  60  and the plunger  65  as far as the transverse web  53 . When the coil  56  is energized then—depending on the direction of the current—a flux is produced through the plunger  65 , running counter to the flux generated by the permanent magnets  57 ,  58 . The energized flux reduces the attraction force on the plunger generated by the permanent magnets. Accordingly, the force of the compression spring is overcome and the plunger  65  is forced out of the yoke in the direction of the arrow P until the projections  66  and  67  come to bear against the inner faces of the limbs  54  and  55 . 
     In the embodiment according to FIGS. 7 to  9 , instead of a virtually closed yoke, the yoke  80  has a longitudinal web  81  with a limb  82 ,  83  at each of its ends. A coil  84  surrounds the longitudinal web  81 . The coil  84  is adjoined by a permanent magnet  85  and the permanent magnet  85  is adjoined by a pole shoe  86  that, in terms of its shape, corresponds to the pole shoe  59 . Also provided is an armature  87  or plunger  87  (corresponding to the armature  65 ) having one end covered by the limb  82  and another end projecting beyond the limb  83 . A projection  88  is provided on the plunger  87 . The projection  88  is oriented towards the coil  84 . Between the pole shoe  86  and the projection  88  is a compression spring  89  that has the same action as the compression spring  23 ,  68 . In the FIGS. 7 to  9  embodiment, many types of spring are possible, for example a spiral spring. 
     The action of the embodiment according to FIGS. 7 to  9  is the same as that of FIGS. 4 to  6 . A difference being that the yoke is U-shaped and not closed. 
     FIGS. 10 and 11 show the action in a schematic illustration. A yoke  100  has a first yoke web  101  surrounded by a coil  102 . The yoke  100  has a figure-eight shape and a further transverse web  103 , in which is disposed a permanent magnet  104 . The central web  105  of the figure-eight shape has a working air gap  106 . The state illustrated in the embodiment of FIG. 10 shows the magnetic flux  107  originating from the coil  102  canceling the flux  108  originating from the permanent magnet  104  in the area of the working air gap  106  so that the plunger located in the area of the central web  105  can be moved by a suitable spring. The fundamental basic structure illustrated by FIG. 10 is implemented in a solution in the embodiments of FIGS. 1 to  9 , with the preferred embodiment being the configuration according to FIG.  3 . 
     The assembly of the electromagnetic release is very simple: the pot is manufactured, the coil is put into the pot, and the permanent magnet and the pole shoe as well as the sleeve are put into the coil in sequence, so that the permanent magnet is located between the bottom of the pot and the pole shoe. The plunger is then inserted, runs through the pole shoe, and, in the quiescent state, is attracted towards the bottom of the pot. 
     In the embodiment of FIG. 3, the magnetic flux  28  originating from the coil  13  flows through the plunger  20 , the pole shoe  19 , the permanent magnet  15  into the bottom  17  of the pot, through the side walls of the pot  11  to the cover  12 , and, from there, into the plunger  20 . Thus, virtually the entire magnetic flux  28  generated by the coil runs completely through the permanent magnet  15 . With respect to the distance D and to the length L, the magnetic flux between the plunger  20  and the bottom  17  of the pot can be made to be opposed by a high magnetic resistance. Essentially, D should always be greater than L. As a result, the permanent magnet  15  can be magnetized to its working point by the magnetic flux  18  originating from the coil  13  and, because the magnetic flux originating from the permanent magnet  15  always runs through the coil  13 , the working point of the permanent magnet is changed only insignificantly. In other words, it remains essentially stable. Based upon the configuration of FIG. 3, which also applies to FIG. 2, the action of the permanent magnet  15  is also maintained. FIG. 11 shows the schematic configuration: the magnetic flux  107  that originates from the coil runs completely or virtually completely through the permanent magnet  104  because of the high magnetic resistance in the working air gap  106 A. Thus, the permanent magnet  104  can be magnetized by the flux  107  (or  28 ), and the working point of the permanent magnet  104  also remains stable. 
     The release illustrated is used, in particular, as a release in a residual current circuit breaker. A particular advantage is achieved, that is, the prevention of sticking by the end face of the plunger  20  to the bottom  17  of the pot. Therefore, the magnitude of the working air gap—in contrast to conventional holding-magnets or blocking-magnet releases, in which the corresponding parts in contact with each other have to be produced extremely precisely and accurately—is not so critical. Instead, the free end face of the plunger, which comes to bear against the bottom  17  of the pot, can also be coated with an anti-adhesion layer. Such a layer reliably avoids the situation where, for a magnetic release configured in accordance with the invention, a malfunction of a residual current circuit breaker occurs. The anti-adhesion layer used can be a layer of corrosion resistant material, for example Ni or a nickel alloy. 
     An already pre-magnetized permanent magnet can also be incorporated. Thus, the configuration according to the invention achieves a situation where the working point of the permanent magnet remains approximately constant in any possible position of the plunger. Furthermore, there is an added advantage allowing the permanent magnet to be magnetized in the installed state, partial magnetization being carried out in the embodiment according to FIG. 2, and leading to the permanent magnet being magnetized further and further, since as a result its magnetic resistance becomes lower. 
     In order to manufacture the internal components of a release, use can be made of a pot-like mold  120  surrounding an internal space  121 . See FIGS. 12 to  14 . The bottom  122  of the mold  120  is located at one end, shown to the right of FIG.  12 . The free end  123  is or can be closed by a cover  124 , on whose side facing the internal space  121  is an integrally molded mandrel  125  projecting as far as the bottom  122  and ending at a short distance from the bottom  122 . The mandrel  125  has two sections  126 ,  127  with different diameters. The diameter of the section  126  adjoining the cover  124  is greater than the other section  127 . The diameter of the section  126  corresponds to the internal diameter of the annular permanent magnet  15 . See FIG. 1 or  2 . The transition from the section  126  to the section  127  is stepped and matched to the internal contour of the pole shoe  16  (see FIG. 1) so that the section  19  of the pole shoe  16  is matched to the external diameter of the section  127  of the mandrel  125 . The step on the mandrel  125  corresponds to the step on the section  19  of the pole shoe  16 . Disposed between the pole shoe  16  and the bottom  122  is an intermediate sleeve  128  that bears closely against the bottom  122  and against the pole shoe  16 , ensuring that no gaps remain between the cover  124  and the permanent magnet  15  or between the permanent magnet  15  and the pole shoe  16 , through which the compound of the coil former can penetrate inwards. On its outer face, the permanent magnet  15  has a circumferential groove  129 . In the area of the bottom  122  and in the area of the cover  124 , the inner wall of the internal space  121  widens. In the area of the bottom  122 , the internal space  121  has a widening  130 , and a return  131  in the area of the cover. 
     If, after the mold has been assembled, with the introduction of the intermediate sleeve  128  and the fitting of the cover  124  with the mandrel  125 , the internal space  121  is potted with a suitable curing material, then the internal space  121  forms the coil former. Material of the coil former  132  engages in the circumferential groove  129  on the permanent magnet and, in this way, ensures that during the demolding operation the permanent magnet  15  does not fall out but is firmly held within the coil former  132 . The pole shoe  16  is then held firmly between the permanent magnet  15  and the coil former. 
     FIG. 13 illustrates the coil former  132  with the flange webs  133  and, 134 , the permanent magnet  15 , and the pole shoe  16 . In the embodiment, the intermediate sleeve  128  has been removed so that, between the pole shoe and the end on the right of the coil former  132 , at which the flange web  130  is located, the accommodation space  22   a  for the spring  23  remains. The coil former  132  is wound with the coil  135 . Therefore, a unit is formed from the coil former, permanent magnet  15 , pole shoe  16 , and coil  135 , and can be inserted into the pot-like yoke  11 . See FIG.  14 . The spring  23  is inserted into the space between the end of the coil former  132  having the flange  133 , and, after that, the plunger  20  with the ridge  22  is inserted through the spring  23  and the pole shoe  16  and the permanent magnet  15 . After the pot  11  has been closed by the cover  12 , from which the plunger  20  projects, the release has been completed.