Electromagnetic coil and electromagnetic clutch comprising same

Abstract

Provided are: an electromagnetic coil capable of easily and reliably preventing insulation failure even when a bobbinless coil is adopted; and an electromagnetic clutch including the same. The present invention comprises: a magnetic coil case 11 having an annular housing groove 12; a coil 13 housed in the housing groove 12; and an electrically-insulating insulating cover 15 that is mounted in the housing groove 12 and prevents contact between the coil 13 and the coil case 11. An insulation failure of the coil 13 that can occur when the coil 13 comes into contact with the coil case 11 is reliably prevented. A side wall of the coil case 11 is provided with a retreat part 33 that increases in distance from the coil 13 toward an open end, and the insulating cover 15 is formed so as not to cover the retreat part 33.

Description

Electromagnetic coil and electromagnetic clutch equipped therewith

[0001] This invention relates to an electromagnetic coil used in electromagnetic clutches and the like, and to an electromagnetic clutch equipped with this electromagnetic coil.

[0002] In air conditioning compressors mounted on vehicles, an electromagnetic clutch is used to disconnect the vehicle's drive engine from the compressor. This electromagnetic clutch comprises a rotor rotatably mounted on the compressor housing and transmitting the rotation of the vehicle's engine; an excitation coil disposed radially inward of the rotor and capable of exciting a magnetic field on the rotor; an armature plate provided close to the axial end face of the rotor and attracted to the end face of the rotor by the magnetic field excited by the excitation coil; and a hub that transmits the rotation of the rotor to the compressor's drive shaft via the armature plate. Conventional excitation coils used a bobbin coil, in which a winding wire is wound around a bobbin made of resin or the like. This was housed in an annular storage groove formed in the coil case, and resin was further molded into the storage groove to ensure the coil's water resistance.

[0003] However, coils using such bobbins have the problem of increasing the overall size of the excitation coil because the bobbin size is larger than the coil itself. As a result, the electromagnetic clutch itself becomes larger and heavier, and is also disadvantageous in terms of manufacturing costs. Therefore, bobbinless excitation coils, in which the coil is not wound on a bobbin, have been proposed. For example, Patent Document 1 (Japanese Patent Publication No. 64-26032) discloses an excitation coil using a bobbinless coil in which the coil is wound without using a bobbin. The coil, which is wound in a ring shape, is restrained at several points of the ring portion with insulating tape to prevent unraveling. The insulating tape is wound to have a predetermined thickness, and by housing this coil in a storage groove of a coil case, the thickness of the insulating tape prevents contact between the housing and the coil, and the thickness of the insulating tape also creates a gap between the coil housing and the coil, and by molding resin through this gap, the resin is spread throughout the storage groove of the coil case, thereby making the coil water resistant.

[0004] Japanese Patent Application Laid-Open No. 64-26032

[0005] However, in the above configuration employing a bobbinless coil, although a configuration is adopted in which the coil wire is wound in a ring shape and an insulating tape is used to prevent disengagement, since the coil wire itself has flexibility, it is difficult to maintain a perfect circle. For this reason, there is a possibility that the portion where the insulating tape is not wound may contact the inner wall of the coil case. When such contact occurs, the enamel coating applied to the coil wire may peel off, and as a result, the coil wire may be electrically connected to the coil case, causing insulation failure. In order to solve this problem, means such as increasing the winding thickness of the insulating tape or increasing the winding positions of the insulating tape can be considered. However, when these means are taken, the manufacturing process becomes complicated, and the cycle time of coil manufacturing becomes long, which causes inconvenience.

[0006] The present invention has been made in view of such circumstances, and a main object thereof is to provide an electromagnetic coil that can simply and surely prevent insulation failure even when a bobbinless coil that does not use a bobbin is adopted, and an electromagnetic clutch provided with the same.

[0007] In order to achieve the above object, the electromagnetic coil according to the present invention includes: a magnetic coil case having an annular storage groove; a coil stored in the storage groove; and an electrically insulating insulating cover attached to the storage groove to prevent contact between the coil and the coil case.

[0008] Therefore, the coil stored in the storage groove is prevented from contacting the inner wall of the coil case by the insulating cover attached to the storage groove, so that insulation failure of the coil that may occur due to contact between the coil wire and the coil case is surely prevented. As a result, even when the coil is flexible and difficult to maintain a perfect circle, contact with the inner wall of the case is eliminated, and high insulation performance can be maintained.

[0009] Furthermore, the installation of an insulating cover eliminates the need for the conventional process of wrapping insulating tape around the entire circumference of the coil wire, thus simplifying the manufacturing process and enabling efficient coil production. As a result, the electromagnetic coil of the present invention not only improves reliability by preventing contact between the coil and the coil case, but also contributes to reducing manufacturing costs and shortening the manufacturing cycle time.

[0010] Furthermore, it is preferable to fill the storage groove with insulating resin and then resin-mold the coil. This resin molding integrates the coil with the insulating cover and coil case, preventing displacement of the coil due to vibration or external forces. In addition, the resin covering the entire coil improves moisture resistance, preventing a decrease in insulation performance due to moisture and dust from the external environment.

[0011] Furthermore, the coil case has a storage groove with a U-shaped cross-section that is open at one end, and the insulating cover fitted into the storage groove has an inner wall facing the inner side wall of the coil case, a bottom wall facing the bottom wall of the coil case, and an outer wall facing the outer side wall of the coil case, so that the entire coil case can be efficiently covered across the inner side wall, bottom wall, and outer side wall of the coil case. As a result, a reliable insulating layer is formed between the entire inner wall of the coil case and the coil, and even if the coil is flexible and does not easily maintain a perfect circle, contact with any part of the coil case is reliably avoided.

[0012] At least one of the inner and outer side walls of the coil case should have a retraction section that moves away from the coil as it approaches the open end, and the insulating cover should be formed so as not to cover the retraction section. With this configuration, the side wall of the coil case will be sufficiently far from the coil in the retraction section, so there will be no risk of contact between the coil and the coil case in that area, and insulation will not be necessary. Therefore, there will be no particular problem even if the insulating cover does not cover the retraction section. In addition, by not covering the retraction section, it is possible to keep the height of the side wall of the insulating cover low, making it possible to make the insulating cover smaller.

[0013] In this configuration, it is preferable to provide a recessed groove around the entire circumference of the side wall of the coil case in the retracted section. Since the retracted section with the groove is not covered by the insulating cover, the insulating resin will reliably enter the groove, and the resin that solidifies within the groove will act as an anchor, allowing the resin to be firmly fixed to the housing section.

[0014] Furthermore, it is preferable that the outer wall of the insulating cover has multiple reduced-diameter sections, which are radially reduced inward, spaced apart in the circumferential direction. This configuration makes it possible to secure a space between the reduced-diameter sections of the insulating cover and the coil case, and through this space, the resin can be reliably filled between the insulating cover and the coil case all the way to the bottom wall. As a result, it is possible to secure a large contact area between the insulating cover and the coil case and the resin, and to firmly fix the insulating cover and the coil case together.

[0015] Furthermore, it is preferable to provide positioning means that fit together on the bottom wall of the coil case and the bottom wall of the insulating cover. Such positioning means may consist of a protrusion formed on one of the bottom walls of the coil case and the insulating cover, and a recess formed on the other that fits together with the protrusion. With such a configuration, it becomes possible to easily position the insulating cover when installing it into the storage groove of the coil case.

[0016] As described above, the electromagnetic coil and electromagnetic clutch equipped therewith according to the present invention are designed so that an electrically insulating cover is fitted into the storage groove of a magnetic coil case having an annular storage groove, and the coil stored in the storage groove is reliably prevented from coming into contact with the coil case by the insulating cover. Therefore, even when a bobbinless coil is used, insulation failures and conductivity failures can be easily and reliably avoided. Furthermore, by providing a retractable portion on at least one of the inner and outer side walls of the coil case, which moves away from the coil as it approaches the open end, it is possible to form a portion that is sufficiently separated from the coil, and by lowering the height of the side wall of the insulating cover, the entire insulating cover can be made smaller. In addition, by providing a recess in the retractable portion and filling it with resin, the resin solidified in the recess acts as an anchor, which improves the fixing strength of the resin mold.

[0017] Furthermore, by providing a diameter-reducing section on the outer wall of the insulating cover, a space is formed between the coil case and the insulating cover by this section, allowing the insulating resin to be injected. This ensures that the entire coil is reliably molded, further improving the coil's fixation and moisture-proof performance. Moreover, by providing a positioning means that fits into the bottom wall of the coil case and the bottom wall of the insulating cover, the positioning of the insulating cover when it is installed in the storage groove can be performed easily and accurately, thereby improving the precision and efficiency of manufacturing. As described above, according to the present invention, it is possible to reliably improve insulation performance by preventing contact between the coil and the coil case, as well as improve the efficiency of the manufacturing process, reduce manufacturing costs, and further improve the reliability and durability of the entire electromagnetic clutch.

[0018] This figure shows an example configuration of an electromagnetic clutch equipped with an electromagnetic coil according to the present invention. This figure shows an example configuration of an electromagnetic coil according to the present invention, viewed from the open end of the coil case. This is a cross-sectional view of the electromagnetic coil shown in Figure 2, cut along line A-A. This is a cross-sectional view of the electromagnetic coil shown in Figure 2, cut along line B-B. This is a cross-sectional view of the electromagnetic coil shown in Figure 2, cut along line C-C. This figure shows a coil case used in the electromagnetic coil according to the present invention, viewed from the bottom wall side. This figure shows a coil case used in the electromagnetic coil according to the present invention, viewed from the open end side. This figure shows a coil case used in the electromagnetic coil according to the present invention, cut along line A-A in Figure 4A. This figure shows an insulating cover used in the electromagnetic coil according to the present invention, viewed from the bottom wall side. This figure shows an insulating cover used in the electromagnetic coil according to the present invention, viewed from the bottom wall side. This figure shows an insulating cover used in the electromagnetic coil according to the present invention, viewed from the open end bottom wall side. This is a cross-sectional view of the insulating cover used in the electromagnetic coil according to the present invention, cut along line A-A in Figure 5A. This diagram illustrates the process of assembling the electromagnetic coil. This diagram illustrates the flow of insulating resin when filling the storage groove, and illustrates the flow of resin in the part where a diameter reduction section is not provided. This diagram illustrates the flow of insulating resin when filling the storage groove, and illustrates the flow of resin in the part where a diameter reduction section is formed.

[0019] Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that the embodiments shown in the accompanying drawings are examples of the present invention, and the present invention is not limited to these embodiments.

[0020] First, an electromagnetic clutch 1 equipped with an electromagnetic coil according to the present invention will be described.

[0021] As shown in Figure 1, the electromagnetic clutch 1 comprises an electromagnetic coil 10 (described later), a drive-side rotor 2 made of a magnetic material, a driven-side hub 3 that can rotate relative to the rotor 2, and an armature plate 4 made of a magnetic material, connected to the hub 3 and positioned opposite the side end face of the rotor 2 with a gap between them.

[0022] The electromagnetic coil 10 is fixed to the housing 5 (for example, the housing of a compressor) via a fixing plate 8 and is housed inside the rotor 2. When energized, the electromagnetic coil 10 forms a magnetic circuit via the rotor 2 and the armature plate 4, causing the armature plate 4 to be electromagnetically attracted to the rotor 2.

[0023] The hub 3 is mounted with respect to the rotating shaft 6, which protrudes outward from the housing 5, in such a way that its relative rotation is restricted.

[0024] Furthermore, the rotor 2 is an annular member centered on the centerline CL of the rotation shaft 6, and is composed of a pulley to which rotational power is transmitted via a belt from an external power source such as an engine. This rotor 2 is rotatably supported on the boss portion 5a of the housing 5 via a radial bearing 7.

[0025] The armature plate 4 is made of a magnetic material and is fixed to the hub 3 via an elastic member 9, and is positioned opposite the side end face of the rotor 2 with a gap in between. The armature plate 4 has a friction surface 4a that faces the side end face of the rotor 2. The friction surface 4a can come into close contact with the side end face of the rotor 2 by electromagnetic attraction due to the deformation of the elastic member 9.

[0026] Therefore, when the rotor 2 is belt-driven by an external power source, and the electromagnetic coil 10 is energized, the armature plate 4 is electromagnetically attracted (closely attached) to the rotor 2 and rotates together with the rotor 2, and the rotating shaft 6 rotates via the hub 3 which is integrated with the armature plate 4.

[0027] The electromagnetic coil 10 is fixed to a fixing plate 8 attached to the housing 5 and housed in a manner that ensures a predetermined clearance between it and the rotor 2 (pulley). Specifically, the rotor 2 has a housing recess 51 formed in an annular shape around the center line CL on the end face opposite to the end face facing the armature plate 4. The electromagnetic coil 10 is fixed to the fixing plate 8 positioned to face the open end of the housing recess 51 and is inserted into the housing recess 51 such that a gap is formed between it and the inner wall of the housing recess 51. Therefore, the rotor 2 can rotate without contacting the electromagnetic coil 10.

[0028] The electromagnetic coil 10 is formed in an annular shape with respect to the center line CL of the rotation axis 6, and as shown in Figures 2 and 3, it comprises a magnetic coil case 11 with an annular storage groove 12, a coil 13 housed in the storage groove 12, and an electrically insulating cover 15 fitted to the storage groove 12 to prevent contact between the coil 13 and the coil case 11.

[0029] As shown in Figure 4, the coil case 11 is formed in a U-shape with an open end on the side opposite to the fixing plate 8, and comprises an inner side wall 11a, an outer side wall 11b, and a bottom wall 11c that connects the ends of these side walls 11a and 11b and closes the end on the fixing plate 8 side, and an annular storage groove 12 is defined by these side walls 11a and 11b and the bottom wall 11c.

[0030] As shown in Figure 3A, a through-hole 22 is formed at a predetermined location in the bottom wall 11c, and a grommet 21 for inserting the external wiring 20 is fitted into it. This through-hole 22 is positioned to align with the through-hole 8a provided in the fixing plate 8.

[0031] The insulating cover 15 fitted into the storage groove 12 is formed in an annular shape, as shown in Figure 5, and comprises an inner wall 15a facing the inner side wall 11a of the coil case 11, an outer wall 15b facing the outer side wall 11b of the coil case 11, and a bottom wall 15c facing the bottom wall 11c of the coil case 11 and connecting the inner wall 15a and the outer wall 15b.

[0032] A through-hole 15d is formed in the bottom wall 15c of the insulating cover 15 at a position aligned with the through-hole 22, and as shown in Figure 3B, a boss portion receiver 16 is formed protruding from the inside of the insulating cover 15 to receive a boss portion 23 that has been machined to accommodate a screw-fastening portion 11d (to which the terminals provided at the ends of the coil wires are screwed) provided on the surface side of the coil case 11. Furthermore, to prevent the insulating cover 15 from rotating relative to the coil case 11, a positioning projection 17 is formed protruding outward (towards the bottom wall 11c of the coil case 11), as shown in Figure 3C. This positioning projection 17 is formed in two locations on the bottom wall 15c of the insulating cover 15, separated by a phase of 180 degrees around the center line CL of the rotation axis 6, and is formed in a semicircular cross-section with the side facing the center line (CL) of the rotation axis 6 cut off to avoid interference with the curved portion (R portion) on the inner periphery of the bottom wall 15c of the insulating cover 15. When the insulating cover 15 is formed by thermoforming, a cylindrical protrusion 18 and a pair of semicircular recesses 19 with a semicircular cross-section are formed on the back of the boss receiving 16 and the positioning protrusion 17 on the inner surface (inner surface of the bottom wall 15c).

[0033] The bottom wall 11c of the coil case 11 to which the insulating cover 15 is attached is provided with a boss portion 23 that fits inside the boss portion receiver 16 of the insulating cover 15, and a recess 24 with a circular cross-section that receives the positioning projection 17 of the insulating cover 15. The assembly tolerance of the boss portion 23 of the coil case 11 and the boss portion receiver 16 of the insulating cover 15 is set to be large to account for molding errors. The assembly tolerance of the pair of positioning projections 17 of the insulating cover and the pair of recesses 24 of the coil case 11 is also set to be large, but the fitting of the boss portion 23 and boss portion receiver 16, and the pair of positioning projections 17 and recesses 24 suppresses circumferential rattle of the insulating cover 15 after it has been attached to the coil case 11, and defines the circumferential position of the insulating cover 15.

[0034] The outer wall 15b of the insulating cover 15 is capable of contacting the outer peripheral side wall 11b of the coil case 11, and the inner wall 15a of the insulating cover 15 is capable of contacting the inner peripheral side wall 11a of the coil case 11. Multiple (four in this example) reduced-diameter portions 31, which are reduced in diameter toward the radially inward direction, are provided on the outer peripheral side wall 11b of the insulating cover 15 at intervals in the circumferential direction. These reduced-diameter portions 31 are formed to a predetermined width, and when the insulating cover 15 is attached to the coil case 11, a space 32 is formed between the reduced-diameter portions 31 and the outer peripheral side wall 11b of the coil case 11.

[0035] Furthermore, the inner side wall 11a of the coil case 11 is provided with a retraction portion 33 that moves away from the coil 13 as it approaches the open end. This retraction portion 33 is formed around the entire inner surface of the coil case 11. In this embodiment, in the area up to partway towards the open end, it is shaped like a frustoconical cone, where the distance from the coil 13 gradually increases along the axial direction, and in the area from there to the open end, it is cylindrical, extending parallel to the center line of the coil case (a shape in which the distance from the coil is kept constant). Note that instead of a frustoconical shape, it is also possible to form a stepped or stepped shape to widen the distance from the coil 13 as it approaches the open end.

[0036] Given the shape of the coil case 11, the inner wall 15a of the insulating cover 25 is formed to be low in height from the bottom wall 15c so as not to cover the retractable portion 33. In this example, the outer wall 15b of the insulating cover is formed to be high enough to completely accommodate the coil, and the inner circumferential side wall is formed to be at the height of the boundary position where the retractable portion 33 begins to form on the inner circumferential side wall 11a of the coil case 11. Furthermore, in the retractable portion 33, and especially in this example, the cylindrical portion following the frustoconical portion of the retractable portion, is provided with a recessed groove 34 extending around its entire circumference toward the outer circumferential surface of the coil case 11.

[0037] In the above configuration, to form the electromagnetic coil 10, as shown in Figure 6, the coil case 11 is set on an assembly stand (not shown) with its opening facing upwards, and the insulating cover 15 is fitted into the storage groove 12 with its opening facing upwards. At this time, the through hole 22 of the coil case 11 and the through hole 15d of the insulating cover 15 are aligned, the circumferential position of the insulating cover 15 is finely adjusted, the boss portion 23 provided on the bottom wall 15c of the insulating cover 15 is fitted into the boss portion receiver 16, and the positioning projection 17 is fitted into the recess 24. As a result, the circumferential position of the insulating cover 15 is defined within the housing and then fixed without shifting position.

[0038] Subsequently, the coil 13, which has the coil wires bundled in a ring shape and partially wrapped with insulating tape at appropriate points in the circumferential direction to prevent unraveling, and to which accessories such as diodes are connected, is inserted into the storage groove 12 of the coil case 11. At this time, the coil 13 is housed so as to rest on the boss support 16 provided on the bottom wall 11c of the coil case 11 and on accessories such as diodes placed between the bottom wall 15c of the insulating cover 15. As a result, the coil is housed in a state where it is floating above the bottom wall 15c of the insulating cover 15, but the lower surface of the coil 13 is supported at multiple points by the boss support 16 and accessories, so the inconvenience of the coil tilting is prevented. After housing the coil 13 in the insulating cover 15 as described above, accessories such as a thermal fuse β are installed on the upper surface of the coil 13, and then an electrically insulating thermosetting resin (insulating resin) α is poured through the opening of the coil case 11 and then heated to solidify it.

[0039] Therefore, with the above configuration, the housed coil 13 is only partially bundled with insulating tape without using a bobbin, and since the coil wire itself is flexible, it becomes difficult to maintain a perfect circle in the parts where the insulating tape is not wrapped. However, the coil 13 is positioned inside the insulating cover 15, so contact with the coil case 11 is reliably avoided. As a result, there is no risk of wear or peeling of the enamel coating on the surface of the coil wire due to contact with the coil case, and the occurrence of insulation failure caused by the coil 13 contacting the coil case 11 is prevented. Thus, in the above configuration, by reliably eliminating contact between the coil 13 and the coil case 11, it is possible to stably maintain the insulation performance of the coil even when a bobbinless coil is used.

[0040] Furthermore, while conventional methods required wrapping insulating tape around the entire circumference of the coil, the present invention has a configuration in which an insulating cover is pre-installed in the storage groove and the coil is housed within it. This eliminates the need to increase the thickness of the insulating tape or the number of wrapping locations, significantly simplifying the manufacturing process and enabling reductions in manufacturing costs and shortening the manufacturing cycle time. Moreover, in the above configuration, since the insulating cover 15 is provided with a diameter-reducing portion 31, the insulating resin α can easily flow through the space 32 formed between the coil case 11 and the diameter-reducing portion 31 to the bottom walls 11c, 15c, thereby ensuring a large contact area between the insulating resin α and the coil case 11 and insulating cover 15. As a result, after the insulating resin α solidifies, the resin exerts a strong fixing force, enabling the entire coil to be held stably.

[0041] Furthermore, the coil case 11 is provided with a retraction section 33 that moves away from the coil 13 as it approaches the open end, so that the coil 13 avoids contact with the coil case 11 at the retraction section 33. For this reason, there is no particular problem even if the height of the inner wall 15a of the insulating cover 15 is reduced so that it does not cover the retraction section. Rather, since the height of the inner wall 15a of the insulating cover 15 can be reduced, it becomes possible to miniaturize the insulating cover 15. In addition, since the retraction section 33 is not covered by the insulating cover 15 and a groove 34 is provided in this retraction section 33, the insulating resin α can be reliably filled into the groove during resin molding, and an anchoring effect can be achieved by the solidification of the insulating resin α. As a result, the bonding strength between the insulating resin α and the coil case 11 is further improved, and the coil will not shift position even under vibration or external force, maintaining long-term durability and reliability.

[0042] Furthermore, by forming a retracted portion 33 that is not covered by the insulating cover 15, it becomes possible to fill the space between the bottom surface of the coil 13 and the insulating cover 15 with insulating resin around its entire circumference. That is, as shown in Figure 7A, in the portion of the insulating cover where the reduced diameter portion 31 is not formed, it becomes possible to flow insulating resin α between the coil 13 and the bottom wall 15c through the gap between the coil 13 and the outer wall 15b of the insulating cover 15, and as shown in Figure 7B, in the portion where the reduced diameter portion 31 is formed, the insulating resin α that has flowed under the coil in the portion where the reduced diameter portion 31 is not formed moves in the circumferential direction, and it becomes possible to guide the insulating resin α under the coil through the space between the retracted portion 33 and the coil 13, so that insulating resin α can be reliably filled inside the insulating cover around its entire circumference.

[0043] As described above, according to the electromagnetic coil 10 of this embodiment, even when a bobbinless coil 13 is used, the installation of the insulating cover 15 makes it possible to reliably prevent contact between the coil 13 and the coil case 11 while allowing the coil wire to bend, thus making it possible to reliably prevent insulation failure with a simple structure.

[0044] In the above configuration, an example where the retreat portion 33 is provided on the inner peripheral side wall 11a of the coil case 11 has been shown. However, the retreat portion 33 may be provided on the outer peripheral side wall 11b of the coil case 11, or may be provided on both the inner peripheral side wall 11a and the outer peripheral side wall 11b. Therefore, the heights of the inner side wall 15a and the outer side wall 15b of the insulating cover 15 may be appropriately adjusted according to the retreat portion 33.

[0045] 1 electromagnetic clutch 2 rotor 3 hub 4 armature plate 10 electromagnetic coil 11 coil case 11a inner peripheral side wall 11b outer peripheral side wall 11c bottom wall 12 storage groove 13 coil 15 insulating cover 15a inner side wall 15b outer side wall 15c bottom wall 17 positioning convex portion 24 concave portion 33 retreat portion 34 concave groove α insulating resin

Claims

1. An electromagnetic coil (10) comprising: a magnetic coil case (11) having an annular storage groove (12); a coil (13) housed in the storage groove (12); and an electrically insulating insulating cover (15) mounted on the storage groove (12) to prevent contact between the coil (13) and the coil case (11).

2. The electromagnetic coil (10) according to claim 1, characterized in that the storage groove (12) is filled with insulating resin (α) and the coil (13) is resin-molded.

3. The electromagnetic coil (10) according to claim 1, characterized in that the storage groove (12) is formed in a U-shape in cross-section with one end open, and the insulating cover (15) fitted into the storage groove (12) comprises an inner wall (15a) facing the inner side wall (11a) of the coil case (11), an outer wall (15b) facing the outer side wall (11b) of the coil case (11), and a bottom wall (15c) facing the bottom wall (11c) of the coil case (11).

4. The electromagnetic coil (10) according to claim 3, characterized in that at least one of the inner side wall (11a) and the outer side wall (11b) of the coil case (11) is provided with a retractable portion (33) that moves away from the coil (13) as it approaches the open end, and the insulating cover (15) is formed so as not to cover the retractable portion (33).

5. The electromagnetic coil (10) according to claim 4, characterized in that the retracted portion (33) is provided with a recessed groove (34) extending around the entire circumference of the side walls (11a, 11b).

6. The electromagnetic coil (10) according to claim 3, characterized in that the outer wall (15b) of the insulating cover (15) is provided with a plurality of reduced-diameter portions (31) that are spaced apart in the circumferential direction, with the diameter being reduced toward the radially inward direction.

7. The electromagnetic coil (10) according to claim 3, characterized in that the bottom wall (11c) of the coil case (11) and the bottom wall (15c) of the insulating cover (15) are provided with positioning means that fit together with each other.

8. The electromagnetic coil (10) according to claim 7, characterized in that the positioning means comprises a positioning projection (17) formed on one of the bottom wall (11c) of the coil case (11) and the bottom wall (15c) of the insulating cover (15), and a recess (24) formed on the other that fits with the positioning projection.

9. An electromagnetic clutch (1) comprising: an electromagnetic coil (10) according to any one of claims 1 to 8; a drive-side rotor (2) made of a magnetic material; a driven-side hub (3) rotatable relative to the rotor (2); and an armature plate (4) made of a magnetic material, connected to the hub (3), and positioned opposite the side end face of the rotor (2) with a gap between them, wherein the electromagnetic coil (10) is housed inside the rotor (2) so as to be able to generate an attractive force that causes the armature plate (4) to be electromagnetically attracted to the rotor (2) by energizing the coil (13).

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