Abstract:
An electromagnetic retarder apparatus with a built-in exciter having a retarder exciter inside thereof for generating braking torque by an eddy current produced in an eddy-current cylinder as a plurality of magnetic poles that are alternately magnetized to N and S poles by a field current caused to flow by a voltage generated in the exciter are caused to rotate in a relation to the eddy-current cylinder disposed at a location facing the magnetic poles; the eddy-current cylinder comprises a core formed by laminating a magnetic material, and short-circuiting method provided on the laminated core in the axial direction thereof for causing the generated eddy current to flow therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electromagnetic retarder with a built-in exciter, and more specifically to an electromagnetic retarder with a built-in exciter having such a construction that an eddy-current cylinder for generating braking torque by eddy current is formed by laminating a magnetic material, and an exciter core divided into a plurality of segments is used as the exciter. 
     2. Description of the Prior Art 
     FIG. 19 is a longitudinal sectional diagram illustrating the essential part of an electromagnetic retarder with a built-in exciter of a conventional type, viewed in the direction of arrows AOB of FIG.  20 . FIG. 20 is a right-hand side view of FIG.  19 . 
     In FIGS. 19 and 20, a support disc  1  that is of a dish- or cup-shape on both sides thereof is rotatably provided between a flange  2 - 1  on the side of the output-shaft of a transmission  2  and a flange  3 - 1  on the side of a propeller-shaft  3 , as shown in FIG.  19 . The support disc  1  is formed into a shape of an open-ended dish or cup by flange members on both sides of the rim thereof, and coaxially fitted between the flange  2 - 1  on the side of the output shaft and the flange  3 - 1  on the side of the propeller shaft  3  with bolts  4  and nuts  5 . 
     An eddy-current cylinder  7  is disposed coaxially with the support disc  1  via a mounting disc  6  outside the support disc  1 . The eddy-current cylinder  7  is made of an iron material. In some cases, the mounting disc  6  may be formed integrally with the eddy-current cylinder  7 . 
     A cylindrical support member  8  made of a magnetic material is disposed in a space formed by the outside of the support disc  1  having on both side thereof members formed into an open-end disc or cup shape, and the eddy-current cylinder  7 . One end of the cylindrical support member  8  is fixedly fitted to a support plate  9  having a recess at the center thereof. The support plate  9  is fixedly fitted to an end of the transmission  2  with bolts  10 . In this case, too, the support plate  9  may be formed integrally with the support member  8 . 
     A pole core  11  made of a magnetic material is fitted with bolts  12  to the outer circumferential surface of the cylindrical support member  8  disposed in a space formed by the outside of the support disc  1  having on both side thereof members formed into an open-end disc or cup shape, and the eddy-current cylinder  7 . An air gap is formed between the pole core  11  and the inner circumferential surface of the eddy-current cylinder  7 . A field coil  13  is wound on the pole core  11 . An exciter core  14  is fixedly fitted to the inner circumferential surface of the support member  8 , and an exciter coil  15  is wound on a slot provided on the exciter core  14 . 
     On the outer circumferential surface of the support disc  1  having on both sides thereof members formed into an open-end dish- or cup-shape, provided at equal intervals are permanent magnets  16  formed into an arc-segment shape, for example, arranged in alternately different polarities. An air gap is formed between the permanent magnet  16  and the exciter core  14 . The exciter coil  15  wound on the exciter cores  14  and the permanent magnets  16  constitute an exciter. 
     The a-c voltage generated in the exciter coil  15  is rectified by rectifying means, and a field current flows in the field coil  13  by turning on a retarder main switch. 
     Numeral  17  refers to a heat shield plate for shielding the radiant heat from the eddy-current cylinder  7  caused by the heat as an eddy-current loss to inhibit temperature rise in the field coil  13 . Numeral  18  refers to a radiating fin for dissipating the heat generated in the eddy-current cylinder  7  as an eddy-current loss into the atmosphere. 
     The operation of the conventional type of the retarder with a built-in exciter having the aforementioned construction will be described in the following. 
     As the output shaft, that is, the flange  2 - 1  on the side of the output shaft of the transmission  2  is rotated, the support disc  1 , the permanent magnet  16 , the mounting disc  6  and the eddy-current cylinder  7  are also rotated en bloc. At this time, the rotation is also transmitted to the flange  3 - 1  on the side of the propeller shaft  3 . 
     By turning on the retarder main switch to activate the retarder, a d-c voltage obtained by rectifying the a-c voltage generated in the exciter coil  15  is applied to the field coil  13  to cause a field current to flow. As a result, the pole core  11  is magnetized to N and S poles alternately, and an eddy current is produced in the eddy-current cylinder  7 . A braking torque is generated in the direction opposite to the rotation of the eddy-current cylinder  7  between the eddy current and the field formed by the pole core  11 , applying a braking action to the rotation of the flange  2 - 1  on the side of the output shaft. 
     FIG. 21 is a front view of the eddy-current cylinder of the conventional type in which the mounting disc and the eddy-current cylinder are formed integrally, FIG. 22 is a partial cross-sectional view of the side part of FIG.  21 . In the figures, the eddy-current cylinder  39  is made of a magnetic material, such as iron, has inclined radiating fins  40  on the outer circumferential surface thereof, and is equivalent to the mounting disc  6  and the eddy-current cylinder  7  described in FIGS. 19 and 20. The outside and inside diameters of the eddy-current cylinder  39  are Lo and Lr, respectively, and the thickness of the core (equal to the thickness of the eddy-current cylinder  7  in FIG. 19) is t 2 , as shown in FIG.  22 . 
     However, the fins of the eddy-current cylinder  7  and the eddy-current cylinder  39 , the radiating fin  18 , and the inclined radiating fin  40  shown in FIGS. 21 and 22 as used in the electromagnetic retarder with a built-in exciter of the conventional type shown in FIGS. 19 and 20 have been machined with a gear hobbing machine after machined with a lathe. 
     The conventional manufacturing method for manufacturing the eddy-current cylinders  7  and  39  having the radiating fins  18  and the inclined radiating fins  40  has had low manufacturing yield, and involved long hours for machining the radiating fins  18  and the inclined radiating fins  40 , leading to increased manufacturing cost. 
     Furthermore, the conventional type of the electromagnetic retarder with a built-in exciter requires splash-proof specifications to prevent water splashes during the travel of a truck from falling on the retarder, lowering the insulation of the exciter. Although there can be a method of covering the exciter coil and other parts with resin, etc. after assembly with the conventional type of the electromagnetic retarder with a built-in exciter, this method could lead to lowered productivity. To cope with this, a method of dividing the exciter core into sections, providing water-proofing measures to each section and assembling the water-proofed sections into one piece has been proposed. This method also has the risk of increasing magnetic resistance, adversely affecting the performance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an eddy-current cylinder for electromagnetic retarders having such a construction that braking torque characteristics are improved and the manufacture of eddy-current cylinders is made easy, thereby manufacturing cost is reduced, by producing a core by laminating a magnetic material, and providing short-circuiting means allowing eddy current to flow therein in the axial direction of the eddy-current cylinder. 
     It is another object of the present invention to provide an exciter for electromagnetic retarders having such a construction that water proofness is imparted to the exciter coil of the retarder, and an exciter core is divided into a plurality of pieces to facilitate the assembling of the retarder with a built-in exciter while preventing the exciter performance from deteriorating due to the division of the exciter core. 
     It is a further object of the present invention to provide an electromagnetic retarder with a built-in exciter having the aforementioned eddy-current cylinder and exciter for the retarder. 
     In disclosed embodiments, the eddy-current cylinder comprises a core formed by laminating a magnetic material, and short-circuiting means provided in the laminated core in the axial direction of the cylinder for allowing the generated eddy current to flow therein. The retarder exciter comprises an exciter coil wound on bobbins disposed on the outside thereof facing a plurality of permanent magnets via an air gap, cylindrical exciter cores molded by a molding material together with the bobbins and the exciter coil, cylindrical pole cores provided integrally with the exciter cores on the outer periphery of the exciter cores and having field coils and a plurality of magnetic poles alternately magnetized to N and S poles by a field current caused to flow in the field coils by a voltage generated in the exciter coil, and an eddy-current cylinder provided outside the pole cores facing the pole-core magnetic poles via an air gap; the cylindrical exciter cores molded by a molding material together with the bobbins and the exciter coil being divided into a plurality of pieces to form molded exciter cores; the molded shape of both ends of the divided molded exciter core pieces is such that the overhanging part of the molding material above the bobbins is tapered toward the ends of the molder exciter core with respect to the inside diameter surface of the molded exciter core, with the corner part thereof at both ends being chamfered and the amount of overhand being gradually decreased; the end faces at both ends of each of the divided molded exciter core pieces being formed in such a state that the exciter core surface protrudes from the molded surface; the molded shape of the groove width portion at the exciter coil insertion hole of the bobbin of the molded exciter core has such a construction that the overhang portion of the molding material above the bobbin is tapered toward the center of said groove width with respect to the inside diameter surface of the molded exciter core; with the amount of overhang being gradually decreased; both ends of said exciter coil wound on said molded exciter core are drawn through holes on a collar, of a shouldered embedded construction, mounted on a collar mount protruding from the outer circumferential surface of said molded exciter core in the axial direction; areas around the embedded part of said collar being molded by a molding material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of assistance in explaining a method for manufacturing an eddy-current cylinder according to the present invention. 
     FIG. 2 is a diagram of a part of the eddy-current cylinder of assistance in explaining various eddy-current short-circuiting methods for allowing the eddy current of the eddy-current cylinder to flow therein. 
     FIG. 3 is a diagram comparing output torque values between an eddy-current cylinder embodying the present invention and the conventional type of the eddy-current cylinder. 
     FIG. 4 is a front view of an example of the assembled eddy-current cylinder of the present invention in which a mounting cylinder is incorporated. 
     FIG. 5 is a partial cross-sectional view of the side part of FIG.  4 . 
     FIG. 6 is a front view of an example of a stamped core. 
     FIG. 7 is a front view of another example of the assembled eddy-current cylinder of the present invention in which a mounting cylinder is incorporated. 
     FIG. 8 is a partial cross-sectional view of the side part of FIG.  7 . 
     FIG. 9 is a front view showing an example of the eddy-current cylinder used in assembling the eddy-current cylinder of FIG.  7 . 
     FIG. 10 is a longitudinal sectional view showing an example of the retarder with a built-in exciter in which the exciter according to the present invention is used. 
     FIG. 11 is a partially cutaway longitudinal sectional view of FIG. 10 showing the essential part, partially cut away for the sake of clarity. 
     FIG. 12 is a diagram showing the molded shape of an example of the molded exciter core that is divided into four pieces. 
     FIG. 13 is the right-hand side view of FIG.  12 . 
     FIG. 14 is an enlarged view of the part encircled and labeled A in FIG.  13 . 
     FIG. 15 is a diagram of assistance in explaining the assembled state of the divided parts. 
     FIG. 16 is an enlarged cross-sectional view of an example of each groove of the molded exciter core. 
     FIG. 17 is a diagram of assistance in explaining the shape of the thermally expanded molded part around the groove. 
     FIG. 18 is a cross-sectional view of the collar mounting portion, viewed in the direction shown by the arrows I—I of FIG.  13 . 
     FIG. 19 is a longitudinal sectional view of the essential part of the electromagnetic retarder with a built-in exciter of the conventional type, viewed in the direction shown by the arrows AOB of FIG.  20 . 
     FIG. 20 is a right-hand side view of FIG.  19 . 
     FIG. 21 is a front view of the eddy-current cylinder of the conventional type in which the mounting disc and the eddy-current cylinder are formed integrally. 
     FIG. 22 is a partial cross-sectional view of the side part of FIG.  21 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a diagram of assistance in explaining the method for manufacturing an eddy-current cylinder embodying the present invention. 
     In the figure, an eddy-current cylinder  20  is formed by helically winding a continuously stamped strip of core  22  of a predetermined shape to a predetermined number of layers. The core  22  is made of a continuously stamped strip of a magnetic material, such as silicon or electrical steel, with the one side edge thereof having projections  23  that form radiating fins  21  when helically wound, and the other side edge thereof formed into a straight line or a partially notched straight line. The continuously stamped strip of the core  22  is helically wound around a cylindrical core jig (not shown), for example, with the straight-line side edge thereof facing inward, to form an eddy-current cylinder  20  with radiating fins  21 . 
     The inside diameter of the core  22  at this time is a size of a helix formed by the helically wound core  22 , and needless to say, the core  22  is formed by helically winding a continuous strip of a magnetic material. The radiating fins  21  may be formed into a shape of vertical parallel lines, instead of a shape of inclined parallel lines, as shown in FIG.  1 . 
     FIG. 2 is a diagram of a part of the eddy-current cylinder to explain various eddy-current short-circuiting methods. 
     The eddy-current short-circuiting method using the eddy-current cylinder shown in FIG. 2 (A) is such that short-circuiting means for short-circuiting each layer of the core  22  are provided on the inside diameter surface  20 - 1  of the eddy-current cylinder  20  in the axial direction thereof, for example, by forming a plurality of current carrying parts  24  on the inside diameter surface  20 - 1  of the eddy-current cylinder  20  in the axial direction of the eddy-current cylinder  20  to electrically short-circuiting each layer of the core  22 . Means for electrically short-circuiting the inside diameter surface  20 - 1  of each layer of the core  22  include peeling the oxide film on the current carrying parts  24  of the core  22 , partially plating the current carrying parts  24  of the core  22 , metal spraying the current carrying parts  24  of the core  22 , forming weld beads on the current carrying parts  24  of the core  22  by welding, etc. In addition, there can be a method of fixedly fitting a thin cylinder made of an iron material to the entire inside diameter surface  20 - 1  formed by each layer of the core  22  of the eddy-current cylinder  20 , as shown in FIG. 5, which will be described later. 
     The eddy-current short-circuiting method of the eddy-current cylinder  20  shown in FIG. 2 (B) is such that short-circuiting means for short-circuiting each layer are provided on the part of the eddy-current cylinder  20  corresponding to the core  22 , for example, by using a plurality of rivets  25 , made of an electrically conductive material, such as iron, copper, aluminum, etc. Holes are provided on the core  22  in the axial direction of the eddy-current cylinder  20 , that is, in the direction shown by arrows C-D of FIG. 1, for example, and rivets  25  are inserted into the holes and upsetting the rivets in the holes to form an electrically short-circuited loop. 
     The short-circuiting means provided on the eddy-current cylinder  20  may be of such a construction that a short-circuiting circuit of a cage type, for example, is formed to allow the eddy current generated in the eddy-current cylinder  20  to flow therein, and need not short-circuit all the layers of the core  22 . 
     An eddy-current cylinder  20  having good manufacturing yield, improved workability and therefore reduced manufacturing cost, formed by helically winding a continuous stamped strip of core  22  of a shape shown in FIG. 1 can be used in place of conventional types of eddy-current cylinders  7  shown in FIGS. 19 and 20. 
     By using an eddy-current cylinder  20  according to the present invention, braking torque can be improved since a-c magnetic resistance is reduced by the laminated core  22 , and the eddy current can be increased as the eddy current flows in a short-circuiting circuit formed by short-circuiting means provided on the eddy-current cylinder  20 . 
     FIG. 3 is a diagram comparing output torque values between an eddy-current cylinder embodying the present invention and an eddy-current cylinder of a conventional type. 
     In FIG. 3, mark ◯ represents the initial braking torque value for the eddy-current cylinder according to the present invention, and mark X that for the eddy-current cylinder of the conventional type. The eddy-current cylinder of the present invention shown by mark ◯ is of the construction shown in FIGS. 4 and 5, which will be described below. 
     FIG. 4 is a front view of an assembled eddy-current cylinder embodying the present invention in which a mounting cylinder is incorporated. FIG. 5 is a partial cross-sectional view of the side part of FIG.  4 . In the figures, the assembled eddy-current cylinder  34  is such that the outer circumferential surface of a mounting cylinder  36  is integrally and fixedly fitted to the inner circumferential surface of the eddy-current cylinder  20  shown in FIG.  1 . 
     The outside and inside diameters of the assembled eddy-current cylinder  34  of the present invention Lo and Lr are formed in the same size as those of the eddy-current cylinder of the conventional type shown in FIG. 22, and the thickness of the core of the assembled eddy-current cylinder  34  of the present invention is formed in the same size as the thickness t 2  of the eddy-current cylinder  39  of the conventional type shown in FIG.  22 . 
     The eddy-current cylinder of the electromagnetic retarder with a built-in exciter according to the present invention comprises a core  22  formed by helically winding a continuous strip of a magnetic material to a predetermined number of layers to form the cylinder, with one side edge of the strip having projections  23  to form radiating fins  21  when helically wound and the other side edge having a shape of straight line and facing inward, and short-circuiting means, such as a current carrying part  24  shown in FIG. 2 (A), provided on the laminated eddy-current cylinder  20  in the axial direction of the cylinder to allow the generated eddy current to flow therein, so that braking torque is improved and the manufacturing process is made easy to reduce manufacturing cost. 
     Although the above description is concerned with an eddy-current cylinder formed by helically winding a continuous strip of core  22 , similar effects can be achieved, though manufacturing yield might be somewhat lowered, by laminating core sheets  35  that are stamped into a predetermined shape to form the eddy-current cylinder  20 , as shown in FIG. 6, in place of the eddy-current cylinder  20  formed by helically winding a continuous strip of core  22 . 
     FIG. 7 is a front view of another example of an assembled eddy-current cylinder according to the present invention in which a mounting disc is incorporated, and FIG. 8 is a partial cross-sectional view of the side part of FIG.  7 . In the figures, the assembled eddy-current cylinder  30  is such that a mounting disc  32  is welded integrally to a side of the eddy-current cylinder  31  shown in FIG.  9 . The eddy-current cylinder  31  has such a construction that a predetermined number of core sheets stamped into a predetermined shape are laminated. 
     When the outside and inside diameters of the assembled eddy-current cylinder  30  in which the mounting disc  32  is fixedly fitted to the eddy-current cylinder  31  are Lo and Lr, respectively, (see FIG.  8 ), and the thickness of the core of the eddy-current cylinder  30  is formed into t 2  (t 2  in FIG.  8 ), the initial braking torque is as shown by the characteristics marked by ◯ in FIG.  3 . Needless to say, the assembled eddy-current cylinder  30  has short-circuiting means as shown in FIG. 2 in the laminated core constituting the eddy-current cylinder  30 . Numeral  33  refers to a radiating fin. 
     FIG. 10 is a partial longitudinal sectional view of an example of the retarder with a built-in exciter in which the exciter according to the present invention is used. FIG. 11 is a partially cutaway longitudinal sectional view of FIG. 10 showing the essential part. 
     In FIGS. 10 and 11, like parts are indicated by like numerals used in FIGS. 19 and 20. The construction of the electromagnetic retarder with a built-in exciter of FIGS. 10 and 11 is remarkably different from that of the electromagnetic retarder with a built-in exciter shown in FIGS. 19 and 20 in that the exciter core  114  is divided into four pieces; an exciter coil  15  being wound on each of the quartered exciter core pieces via a bobbin  120 , and these members being molded by a molding material to form exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4 . 
     As will be described in detail in FIG.  12  and thereafter, the quartered molded exciter core pieces  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  is such that the molded shape at both ends of the quartered molded exciter core pieces  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  has such a construction that the overhanging portion above the bobbin  120  with respect to the inside diameter surface of the molded exciter core is tapered toward the ends of the molded exciter core pieces  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4 ; with the corner of the ends chamfered and the amount of overhang of the molding material  123  being gradually decreased, that the divided end faces at both ends of the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  are formed into a shape that the exciter core surface protrudes from the molded surface; the molded shape of each groove into which the exciter coil  15  wound on the bobbin  120  of the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3 , and  121 - 4  has such a construction that the overhanging portion above the bobbin  120  with respect to the inside diameter surface of the molded exciter core is tapered toward the centers of the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4 ; with the amount of overhang of the molding material  123  being gradually decreased, and that both ends of the exciter coil  15  wound on the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  are drawn through holes on collars  122  (see FIG. 11) of a shouldered embedded construction, provided on collar mounting portions protruding from the outer circumferential surface of the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4 ; with areas around the embedded portions are molded with a molding material. 
     In FIGS. 10 and 11, numeral  130  refers to a band for fixing permanent magnets  16  to prevent them from falling off due to centrifugal force. 
     All these constructions help prevent magnetic characteristics and water-proofness from deteriorating as a result of the quartering of the exciter core  114 . 
     FIG. 12 is a diagram showing the molded shape of a quartered exciter core piece, FIG. 13 is a right-hand side view of FIG. 12, and FIG. 14 is an enlarged diagram of a portion encircled by A in FIG.  13 . 
     In FIGS. 12 and 13, the molded exciter core  121 - 1  is such that an exciter core  114 , s bobbin provided on each groove (corresponding to a slot) of the exciter core  114 , and an exciter coil  15  (not shown in FIGS. 12 and 13) wound on the bobbin  120  provided on the exciter core  114  are molded by a molding material  123  made of a synthetic resin, such as silicone. The inside diameter surface X and the outside diameter surface Y of the exciter core  114  are exposed from the molding material  123 , and the end faces  114 - 1  at both ends of the divided exciter core piece  114  are molded in a protruded state from the molded surface Z on the end face of the molded exciter core  121 - 1 , as shown in FIG. 14 which is an enlarged diagram of a portion encircled by A in FIG.  13 . 
     The molded shape at both ends of the molded exciter core  121 - 1  is such that the overhanging portion of the molding material  123  above the bobbin  120 , that is, a portion L shown in FIG. 14 is molded in the state of tapering toward the end of the molded exciter core  121 - 1  (by angle α with respect to the inside diameter surface X of the molded exciter core  121 - 1 ); with the corner portion C extending to the tapered portion L being chamfered. 
     As a result, when the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  are assembled into a cylindrical shape, as shown in FIG. 10, a gap  127  is formed between the molded exciter cores  121 - 1  and  121 - 2  as the end faces  114 - 1  of the exciter cores  114  protruding from the molded surfaces Z butt against each other, as shown in FIG. 15 which is a diagram of assistance in explaining the assembled state of the divided molded exciter core pieces. As a result of the gap  127  formed between the molded exciter cores  121 - 1  and  121 - 2 , the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  can be easily press-fitted into the cylindrical yoke of a support plate  9 . Furthermore, the magnetic resistance of the magnetic path can be prevented from increasing and performance can be maintained since the molded exciter cores  121 - 1 ,  121 - 2 ,  121 - 3  and  121 - 4  can be press-fitted without no gap between the end faces  114 - 1  thereof. 
     The gap  127  serves as a relief for the thermal expansion of the molding material  123  to prevent the mutual interference of the molding materials of the molded exciter cores  121 - 1  and  121 - 2 . 
     In FIG. 13, numeral  124  denotes a lead wire of the exciter coil  15 ; the lead wires  124  from the both ends of the exciter coil  15  wound on the bobbin  120  are drawn out of a collar  122 . The collar  122  is mounted on a collar mount  123 - 1  formed in the state of protruding from the outer circumferential surface of the molded exciter core  121  shown in FIG. 10 in the axial direction. The construction of the collar  122  will be described in detail later, referring to FIG.  18 . 
     FIG. 16 is an enlarged cross-sectional view of an example of a groove portion (corresponding to a slot) of the molded exciter core. 
     The molded shape of a groove width S of the molded exciter core  121 - 1  is such that the overhanging portion of the molding material  123  above the bobbin  120  is formed in the state of tapering toward the center of the groove width S with respect to the inside diameter surface X of the molded exciter core  121 - 1 ; with the amount of overhang being gradually decreased. That is, the molded shape of the groove width S portion at the exciter-coil insertion hole of the molded exciter core  121 - 1  is formed symmetrically with respect to the center of the groove width S. Now, let us discuss one side of the groove width S portion. The overhanging portion of the molding material  123  above the bobbin  120 , that is, the portion L shown in FIG. 16 is molded in the state of tapering toward the center of the groove width S of the molded exciter core  121 - 1  (by angle α with respect to the inside diameter surface X of the molded exciter core  121 - 1 ), and the center portion D extending to the tapered portion L is formed into a recess. 
     FIG. 17 is a diagram of assistance in explaining the shape of the molding material on the groove portion when it is expanded by heat. 
     When the molding material  123  is formed as shown in FIG. 16, the molded shape of the groove width S portion at the exciter-coil insertion hole of the molded exciter core  121 - 1  is deformed by the thermal expansion of the molding material  123 , that is, the inside diameter surface X is deformed as shown by solid lines in FIG.  17 . 
     The molded shape of the groove width S portion at the exciter-coil insertion hole of the molded exciter core  121 - 1  shown in FIG. 16 is formed at five locations shown by V in FIG.  13 . 
     FIG. 18 is a partial cross-sectional view of the portion where the collar is mounted, viewed in the direction shown by arrows I—I in FIG.  13 . 
     The lead wire  124  of the exciter coil  15  wound on the molded exciter core  121 - 1  is drawn out of the hole of the collar  122  mounted on the collar mounting portion  123 - 1  of a shouldered embedded construction, protruding from the outer circumferential surface of the molded exciter core  121 - 1  in the axial direction. Areas around the embedded portion of the collar  122  are molded in a built-up state by a molding material  126 , as shown in FIG.  18 . 
     Since the collar  122  has a shouldered embedded construction, and areas around the molded portion of the collar  122  are molded in a built-up state by the molding material  126 , the lead wires  124  at both ends of the exciter coil  15  can be moved freely, and even when the movement of the lead wires  124  is transmitted to the collar  122 , the shouldered embedded construction of the collar  122  and the built-up molding material  126  prevent the collar  122  from falling off, thereby maintaining water-proofness. 
     As described above, the present invention makes it possible to improve braking torque characteristics, easily manufacture an eddy-current cylinder, and therefore reduce manufacturing cost by adopting the construction where the eddy-current cylinder is formed by laminating a magnetic material and short-circuiting means are provided inside the cylinder to allow eddy current to flow therein. 
     The yield of the magnetic material can be improved and manufacturing cost can be further reduced by using a method for manufacturing the eddy-current cylinder with radiating fins by helically winding a continuous strip of a magnetic material with one side edge thereof having projections to form radiating fins when helically wound and the other side edge formed into a straight-line shape. 
     By adopting a construction in which the exciter core of the electromagnetic retarder with a built-in exciter is divided into a plurality of pieces, and the adverse effect of the increase in the magnetic resistance of the magnetic path resulting from the division of the exciter core on performance, and deterioration of water-proofness can be prevented, the exciter core can be divided into a plurality of pieces and a retarder exciter having good water-proofness can be achieved. Division of the exciter core into a plurality of pieces helps improve material yield.