Patent Publication Number: US-2016238087-A1

Title: Electromagnetic Clutch

Description:
TECHNICAL FIELD 
     The present invention relates to an electromagnetic clutch and more particularly to an electromagnetic clutch suitable for intermittently transmitting power of an engine or motor of a vehicle to an in-vehicle driven device (for example, a compressor in an air conditioner for use in a vehicle). 
     BACKGROUND ART 
     As this type of electromagnetic clutches, for example, an electromagnetic clutch disclosed in Patent Document 1 has been known. The electromagnetic clutch disclosed in Patent Document 1 has an energization interrupting device configured to cut a cutting wire that forms a part of an electromagnetic coil to thereby forcibly interrupt electric power supply to the electromagnetic coil if a rotor temperature exceeds a predetermined temperature due to relative sliding between friction surfaces of a rotor and an armature. In this energization interrupting device, a thermally-actuated device is provided in the rotor and the cutting wire is provided in the electromagnetic coil unit. When the rotor temperature increases beyond the predetermined temperature, the thermally-actuated element is displaced by a predetermined distance toward the electromagnetic coil unit and then engaged with the cutting wire to cut the cutting wire. 
     REFERENCE DOCUMENT LIST 
     Patent Document 
     
         
         Patent Document 1: JP H01-210626 A 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Note that in the energization interrupting device provided with the thermally-actuated element and the cutting wire, the thermally-actuated element and the cutting wire should be positioned opposite to each other in a narrow space between the rotor and the electromagnetic coil unit. Thus, if it fails to precisely control a relative distance between the thermally-actuated element and the cutting wire in the axial direction of the electromagnetic clutch, or the direction in which the thermally-actuated element is displaced, the following problem occurs. That is, the energization interrupting device causes an operation error to unintentionally cut the cutting wire or fail to cut the wire. Since the thermally-actuated element is fixed to the rotor, its position in the axial direction of the electromagnetic clutch is defined at the design state according to the sizes of the rotor and bearing and the size of a housing of a driven device where the rotor is positioned and fixed. In addition, the displacement amount of the thermally-actuated element is determined in consideration of design factors such as materials and sizes. Regarding the position of the cutting wire in the axial direction of the electromagnetic clutch, the positional accuracy thereof varies depending on how the cutting wire is mounted to an end surface of the electromagnetic coil unit on the rotor side. If the cutting wire is not appropriately mounted, the position of the cutting wire varies largely in the axial direction of the electromagnetic clutch. As a result, the above operation error occurs and the reliability of the energization interrupting device lowers. 
     Regarding the energization interrupting device of the electromagnetic clutch disclosed in Patent Document 1, the document only remarks that winding end of an electromagnetic coil is engaged with a hook of a bobbin and used as a cutting wire. There is no description about the way to control the position of the cutting wire in the axial direction of the electromagnetic clutch. 
     The present invention has been made in view of the above problems and an object of the present invention is to provide an electromagnetic clutch that facilitates positional control of the cutting wire and enhances reliability of the energization interrupting device. 
     Means for Solving the Problems 
     In order to achieve the above object, the present invention provides an electromagnetic clutch, including: a rotor unit provided with a rotor that is rotated with power of a driving source, and rotatably supported to a boss formed on an end surface of a housing of a driven device; an armature unit provided with an armature that is magnetically attracted to the rotor when the rotor is excited, and fixed to a rotation shaft of the driven device, which passes through the boss; an electromagnetic coil unit including: a bobbin having first and second flanges on both sides of a cylindrical portion with an electromagnetic coil wound around an outer circumference of the cylindrical portion positioned between the flanges, the coil serving to excite the rotor in response to electric power supply; and a ring case provided with a circular bobbin container and accommodated in a circular recess formed in the rotor, the ring case being fixed to the end surface of the housing of the driven device with an opening edge of the bobbin container facing toward the rotor; and a thermally-actuated element attached to the rotor unit, and displaced toward the electromagnetic coil unit at over a predetermined temperature, the thermally-actuated element serving to cut a cutting wire portion that forms a part of the electromagnetic coil to forcibly interrupt electric power supply to the electromagnetic coil, with the wire being placed toward the electromagnetic coil unit across a movement area of the thermally-actuated element. In the clutch, the bobbin includes: first and second wall portions extending opposite to each other from the first flange formed on the opening edge in the bobbin container toward a bottom wall in the circular recess of the rotor where the thermally-actuated element is mounted; an inner abutment portion extending from an extension end of the first wall portion toward an inner opening edge of the bobbin container; and an outer abutment portion extending from an extension end of the second wall portion toward an outer opening edge of the bobbin container. The bobbin is accommodated into the bobbin container such that the inner abutment portion and the outer abutment portion abut inner and outer opening edges of the bobbin, respectively. The cutting wire portion is stretched between both of the wall portions at a predetermined distance from an end surface of each of the first and second wall portions. 
     Effects of the Invention 
     According to the electromagnetic clutch of the present invention, while the inner and outer abutment portions of the bobbin are engaged with an opening edge of the bobbin container of the ring case, the bobbin having the electromagnetic coil wound thereon is positioned and accommodated into the bobbin container. Thus, it is possible to define the position of the bobbin in the bobbin container in the axial direction of the electromagnetic clutch, and also to precisely position the cutting wire portion stretched between the first wall portion and the second wall portion, in the axial direction of the electromagnetic clutch. The reliability of the energization interrupting device can be enhanced as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an electromagnetic clutch according to an embodiment of the present invention. 
         FIG. 2  is a front view of a rotor unit. 
         FIG. 3  is a sectional view taken along the line A-O-A of  FIG. 2 . 
         FIG. 4  is a sectional view of an armature unit. 
         FIG. 5  is a sectional view of an electromagnetic coil unit. 
         FIG. 6  is a sectional view of a bobbin in the electromagnetic coil unit. 
         FIG. 7  is an enlarged view of a bridge wire viewed from the arrow A of  FIG. 5 . 
         FIG. 8  is a view of the bridge wire viewed from the arrow B of  FIG. 7 . 
         FIG. 9  is a view of the bridge wire viewed from the arrow C of  FIG. 7 . 
         FIG. 10  is an enlarged sectional view of the bridge wire taken along the line D-D of  FIG. 7 . 
         FIG. 11  is an explanatory operational view of an energization interrupting device under the condition that bimetal is not displaced. 
         FIG. 12  is a sectional view of the bimetal of  FIG. 11 . 
         FIG. 13  is an explanatory operational view of the energization interrupting device under the condition that the bimetal is displaced beyond a predetermined distance. 
         FIG. 14  is a sectional view of the bimetal of  FIG. 13 . 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. 
       FIG. 1  shows the structure of an electromagnetic clutch according to the embodiment of the present invention. An electromagnetic clutch  10  of this embodiment is incorporated, for example, in a compressor for an in-vehicle air conditioner and configured to intermittently transmit power from a vehicle engine or motor as a driving power source to the compressor as a driven device. In other words, the electromagnetic clutch  10  switchingly transmits/interrupts power from the engine or the motor to the compressor. The compressor is operated when power is transmitted from the engine or motor, and stops operation when power transmission is interrupted. The compressor of the present invention could be, for example, a swashplate type variable-displacement compressor. Here, it is possible to employ variable-displacement compressors of other types or fixed-displacement compressors of scroll type, vane type, etc. 
     In  FIG. 1 , the electromagnetic clutch  10  includes a rotor unit  20 , an armature unit  30 , and an electromagnetic coil unit  40  and in addition, an energization interrupting device  50 . 
     The rotor unit  20  is rotated with power of an engine or motor, and provided with a rotor  21 , a friction member  22 , and a bearing  23 . 
     The rotor  21  has a circular shape. The inner circumference thereof is rotatably supported to the outer circumference of a boss  1   a  as an end surface of a front housing  1  of the compressor by means of the bearing  23 . On the outer circumference of the rotor  21 , formed are grooves to which a belt for transmitting a rotational force from the engine or motor is hooked. More specifically, as shown in  FIGS. 2 and 3 , the rotor  21  is integrally constituted of an outer cylindrical portion  21   a  having the outer circumference with the belt grooves formed therein, an inner cylindrical portion  21   b  having the inner circumference, and an end surface portion  21   c  connecting the outer cylindrical portion  21   a  and the inner cylindrical portion  21   b . The outer cylindrical portion  21   a , the inner cylindrical portion  21   b , and the end surface portion  21   c  are formed of a ferromagnetic material (specific example: iron material). These members form a circular recess  21   d  that accommodates an electromagnetic coil  42  in the electromagnetic coil unit  40  as described below. Arc-shaped slits  21   e  and  21   f  are formed in the end surface portion  21   c  and used to divert magnetic flux generated in the electromagnetic coil  42 . In addition, between the arc-shafted slits  21   e  and  21   f  on a bottom end surface portion  21   c   2  of the end surface portion  21   c  in the circular recess  21   d , formed is a circular groove  21   g  (see  FIGS. 2, 11 , and  12 ) where a bimetal  51  of the energization interrupting device  50  as described below is attached. A friction surface  21   c   1  is an end surface of the end surface portion  21   c  opposite to the bottom end surface portion  21   c   2  of the circular recess  21   d . The friction member  22  is made of a circular nonmagnetic material to increase friction coefficient and attached to the friction surface  21   c   1 . Discussing the structure of the bearing  23 , as shown in  FIG. 1 , the inner ring side thereof is positioned to the outer circumference of the boss  1   a  of the front housing  1  and fixed thereto with a snap ring  4 . The bearing rotatably supports the rotor  21  onto the outer circumference of the boss  1   a  as a part of the end surface of the front housing  1 . 
     The armature unit  30  transmits a power from an engine or motor to a compressor when an armature  33  is magnetically attracted to the rotor  21  in response to electric power supply to the electromagnetic coil  42 . As shown in  FIG. 4 , the unit includes a hub  31 , a rubber unit  32 , and the armature  33 . 
     The hub  31  is provided with a flange portion  31   a  and fixed to the tip end of a rotation shaft  2  of the compressor by means of a nut  5  (see  FIG. 1 ). The rubber unit  32  includes an inner ring  32   a , an outer ring  32   b , and a circular rubber  32   c  interposed by cure adhesion between the inner ring  32   a  and the outer ring  32   b . The inner ring  32   a  is fixed to the flange portion  31   a  of the hub  31  with a rivet  34 . The armature  33  is a circular plate member with one end surface that serves as a friction surface  33   a  facing the friction surface  21   c   1  of the rotor  21  at a predetermined interval. The armature is fixed to the outer ring  32   b  of the rubber unit  32  with a rivet  35  and elastically supported by the circular rubber  32   c . The armature  33  is formed of a ferromagnetic material (specific example: iron material). The armature constitutes a magnetic circuit together with the rotor  21 . In response to electric power supply to the electromagnetic coil  42 , the armature is magnetically attracted to the rotor  21 . Meanwhile, when magnetic attraction power is extinguished due to interruption of electric power supply, the armature moves away from the rotor  21 . 
     The electromagnetic coil unit  40  generates a magnetic attraction power by magnetizing the rotor  21 . The unit includes a bobbin  41 , the electromagnetic coil  42  wound around the bobbin, a ring case  43  having a circular recess that serves as a bobbin container for accommodating the bobbin  41 , an annular disc-like fixing member  44  fixed to the ring case  43  and configured to form the other end surface of the electromagnetic coil unit  40 , and a connecting portion  45  for connecting an external power supply in the vehicle and the electromagnetic coil  42 . 
     As shown in  FIG. 5 , the ring case  43  has a circular recess integrally formed by an outer cylindrical portion  43   a , an inner cylindrical portion  43   b , and an end surface portion  43   c  that connects the outer cylindrical portion  43   a  and the inner cylindrical portion  43   b . The recess accommodates the bobbin  41  having the electromagnetic coil  42  wound therearound. The recess is inserted into a circular recess  21   d  of the rotor  21  in a relatively rotatable manner with its opening facing toward the rotor  21 . The outer cylindrical portion  43   a  and an inner cylindrical portion  43   b  are coaxial with the axial line of the rotation shaft  2  of the compressor. The end surface portion  43   c  is orthogonal to the axial line of the rotation shaft  2 . An end surface  43   a   1  of the outer cylindrical portion  43   a  (outer circumference side opening edge) and an end surface  43   b   1  of the inner cylindrical portion  43   b  (inner circumference side opening edge) extend flush with each other and orthogonally to the axial line of the rotation shaft  2 . The outer cylindrical portion  43   a , the inner cylindrical portion  43   b , and the end surface portion  43   c  are formed of a ferromagnetic material (for example, iron material) to constitute a magnetic circuit. 
     As shown in  FIG. 6 , the bobbin  41  includes the cylindrical portion  41   a , and a first flange  41   b  and a second flange  41   c , which extend radially outwardly from each end of the cylindrical portion  41   a  opposite to each other. The electromagnetic coil  42  is wound around the outer circumference of the cylindrical portion  41   a  formed between the flanges  41   b  and  41   c . The bobbin  41  also has an inner wall  41   d  as a first wall portion and an outer wall  41   e  as a second wall portion, which face each other and extend from a proximal end and a tip end of the first flange  41   b  respectively toward a bottom wall  21   c   2  of the circular recess  21   d  of the rotor  21 . The inner wall  41   d  is formed at substantially the entire circumference of the proximal end of the first flange  41   b . Similarly formed at substantially the entire circumference thereof is an inner abutment portion  41   f  that extends radially inwardly from the tip end thereof (end of the extension) such that the portion can abut an inner opening edge of the circular recess as the bobbin container. Further, as shown in  FIG. 7 , the outer wall  41   e  is formed only in the vicinity of a predetermined portion of the tip end of the first flange  41   b  (the winding end of the electromagnetic coil  42 ). An outer abutment portion  41   g  extends radially outwardly from the tip end (end of the extension) such that the portion can abut the outer opening edge of the circular recess as the bobbin container. In addition, as indicated by the dashed circle in  FIG. 8 , the outer wall  41   e  of the bobbin  41  has a first slit  41   e   1  with a predetermined distance from the tip end (upper surface of the outer abutment portion  41   g ), in other words, a predetermined depth h 2  (corresponding to the thickness of the outer abutment portion  41   g ). Moreover, as indicated by the dashed circle in  FIG. 9 , the inner wall  41   d  of the bobbin  41  has a second slit  41   d   1  with a predetermined distance from the tip end (upper surface of the inner abutment portion  41   f ), in other words, the same depth h 2  as the first slit  41   e   1  (corresponding to the thickness of the inner abutment portion  41   f ), and a third slit  41   d   2  extending from the tip end (upper surface of the inner abutment portion  41   f ) down to a first flange surface  41   b   1 . The bobbin  41  includes the cylindrical portion  41   a , the first flange  41   b , the second flange  41   c , the inner wall  41   d , the outer wall  41   e , the inner abutment portion  41   f , and the outer abutment portion  41   g , which are integrally formed of a plastic material, for example, a polyamide resin. 
     The electromagnetic coil unit  40  is securely insulated by pouring a resin through the space between the ring case  43  and the bobbin  41  accommodated into the circular recess of the ring case  43 . As shown in  FIG. 5 , the bobbin  41  is accommodated in the ring case  43  in such a way that the outer abutment portion  41   g  of the bobbin  41  abuts against the end surface  43   a   1  of the outer cylindrical portion  43   a  of the ring case  43 , and the inner abutment portion  41   f  abuts the end surface  43   b   1  of the inner cylindrical portion  43   b  of the ring case  43 . In this way, the bobbin  41  is positioned to the circular recess of the ring case  43  and thus accommodated and fixed thereto. The fixing member  44  fixed to the end surface of the end surface portion  43   c  opposite to the bottom wall of the circular recess is positioned to the end surface of the front housing  1  and fixed thereto with the snap ring  3  as shown in  FIG. 1  and hence, the electromagnetic coil unit  40  is fixed to the end surface of the front housing  1 . 
     When heat is generated due to relative sliding between the rotor  21  and the armature  31 , the energization interrupting device  50  forcibly interrupts electric power supply to the electromagnetic coil  42 . The energization interrupting device  50  is provided with thermally-actuated elements, for example, the bimetal  51  and the bridge wire  52  serving as a cutting wire portion that forms a part of the electromagnetic coil  42 . 
     The bimetal  51  is formed in a substantially rectangular shape and accommodated in the circular groove  21   g  formed in the bottom wall  21   c   2  of the circular recess  21   d  in the rotor  21 . One end thereof is fixed with a rivet  53 , and the other end faces toward the rotation direction of the rotor  21 . Note that the bimetal  51  could be fixed by any other fixing member such as a bolt. Since the bimetal  51  is accommodated and positioned in the circular groove  21   g , when engaged with the bridge wire  52 , the bimetal  51  can be prevented from tilting to the left or right relative to the rotation direction of the rotor  21  in response to the reaction force of the bridge wire  52 . If sensing the temperature higher than a predetermined level, the bimetal  51  is displaced beyond a predetermined distance toward the electromagnetic coil unit  40 . The bimetal  51  is preferably a snap action type that starts inverted motion at a predetermined temperature. The snap action type bimetal is hardly displaced at a temperature lower than an inverted motion temperature (temperature causing inverted motion) but is largely displaced at over the inverted motion temperature. By utilizing the inverted motion, the bridge wire  52  is cut. In general, in a compressor for an in-vehicle air conditioner, the electromagnetic clutch  10  could increase the temperature up to 150° C. Taking this temperature into account to set the inverted motion temperature for interrupting electric power supply to the electromagnetic coil  42 , the temperature is appropriately set to 180° C. to 190° C. 
     The bridge wire  52  is obtained from the winding end (the ground side of the electromagnetic coil  42 ) of the electromagnetic coil  42  wound around the bobbin  41 . The wire is stretched on one end surface of the electromagnetic coil unit  40  opposite to the bottom wall  21   c   2  in the circular recess  21   d  of the rotor  21  such that the wire crosses an area where the bimetal  51  moves along with the rotation of the rotor  21  (movement area of the bimetal  51 ) and also is engaged with the bimetal  51  displaced beyond a predetermined distance. More specifically, as shown in  FIGS. 7 to 10 , the winding end of the electromagnetic coil  42  wound around the bobbin  41  is inserted into the first slit  41   e   1  from one side (radially outer portion of the bobbin  41 ) of the outer wall  41   e , the other side of which faces the inner wall  41   d . The inserted wire crosses a space surrounded by the outer wall  41   e , the inner wall  41   d , and the first flange  41   b . Then, the wire is inserted into the second slit  41   d   1  so as to stretch between the slits. The wire is thus positioned and supported to the end surfaces of both the slits  41   e   1  and  41   d   1 . The stretched wire serves as the bridge wire  52 . Subsequently, the wire is inserted into the third slit  41   d   2  of the inner wall  21   d  from one side (radially inner portion of the bobbin  41 ) of the inner wall  21   d , the other side of which faces the outer wall  41   e . The inserted wire is guided along a guide wall  41   b   2  (see  FIG. 10 ) formed on the first flange surface  41   b   1  of the first flange  41   b  toward the outer wall  41   e  across the first flange surface  41   b   1  of the first flange  41   b . The wire is routed outwardly in the radial direction of the bobbin  41 . Thus, the wire is stretched between the outer wall  41   e  and the inner wall  41   d  over the first flange  41   b . The thus-formed wire serves as the bridge wire  52 . The inner abutment portion  41   f  and the outer abutment portion  41   g  are formed at the same height from the first flange surface  41   b   1  of the first flange  41   b . The first slit  41   e   1  and the second slit  41   d   1  are formed at the same depth h 2 . Thus, the bridge wire  52  is stretched in parallel to the first flange surface  41   b   1  of the first flange  41   b  at a predetermined height. 
     As shown in  FIG. 10 , an inclined surface  41   b   3  is formed above the first flange surface  41   b   1  of the first flange  41   b  of the bobbin  41  such that the inclined surface slopes up toward the rotation direction of the bimetal  51 . The terminal end of the inclined surface  41   b   3  and the first flange surface  41   b   1  of the first flange  41   b  form a step serving as the guide wall  41   b   2 . An electromagnetic coil portion  47  is guided along the step to cross above the first flange surface  41   b   1  of the first flange  41   b  from the radially inner portion to the radially outer portion of the bobbin  41 . The step height, i.e., the height of the inner wall  41   b   2  from the first flange surface  41   b   1  is set equal to or slightly larger than the outer diameter of the electromagnetic coil portion  47 . 
     Here, a brief description is given of the general operation of intermittently transmitting power to the compressor by means of the electromagnetic clutch  10  and the operation of the energization interrupting device  50 . If electric power is supplied to the electromagnetic coil  42  of the electromagnetic coil unit  40  under the condition that the rotor  21  is rotated with a rotational force from the engine, the rotor  21  is excited, and the generated electromagnetic force makes the armature  33  magnetically attracted to the rotor  21 . Then, the armature  33  is rotated in sync with the rotor  21 . The rotational force of the armature  22  is transmitted to the rotation shaft  2  of the compressor by way of the rubber unit  32  and the hub  31  to thereby operate the compressor. If the electric power supply to the electromagnetic coil  42  of the electromagnetic coil unit  40  is interrupted in this state, the rotor  21  is demagnetized, and the armature  33  is retracted from the rotor  21  due to a restoring force of the rubber  32   c . No rotational force of the rotor  21  is transmitted to the armature  33 . As a result, the rotation shaft  2  stops rotating and the compressor stops the operation. In the normal state, the temperature of the end surface portion  21   c  of the rotor  21  does not reach the predetermined temperature at which the bimetal  51  is displaced over the predetermined distance. As shown in  FIGS. 11 and 12 , the bimetal  51  is rotated integrally with the rotor  21  without contacting the bridge wire  52 . 
     On the other hand, if an excessively larger torque than usual acts on the rotation shaft  2  due to, for example, damaged inner parts of the compressor, the contact surfaces of the rotor  21  and the armature  33  slide on each other to generate the friction heat, resulting in rapid temperature rise at the end surface portion  21   c  of the rotor  21 . When the temperature of the end surface portion  21   c  increases rapidly, as shown in  FIGS. 13 and 14 , the free end of the bimetal  51  is displaced toward the electromagnetic coil unit  40 . If the temperature exceeds the predetermined temperature, the free end of the bimetal  51  is displaced beyond the predetermined distance and engaged with the bridge wire  52  to cut the bridge wire  52 . As a result, electric power supply to the electromagnetic coil  42  is forcibly interrupted and the armature  33  is retracted from the rotor  31 . This makes it possible to prevent the engine from an excessive load, protect the belt against any damage, and ensure driving safety of the vehicle. 
     According to the electromagnetic clutch  1  of this embodiment, the outer abutment portion  41   g  and the inner abutment portion  41   f  of the bobbin  41  abut against the end surface  43   a   1  of the outer cylindrical portion  43   a  of the ring case  43  and the end surface  43   b   1  of the inner cylindrical portion  43   b , by which the bobbin  41  is positioned and accommodated in the circular recess of the ring case  43 . The inner abutment portion  41   f  and the outer abutment portion  41   g  have the same height from the first flange surface  41   b   1  of the first flange  41   b  and also, the depth from the end surface of the outer wall  41   e  to the bottom of the first slit  41   e   1  is the same (depth h 2 ) as that from the end surface of the inner wall  41   d  to the bottom of the second slit  41   d   1 . Thus, the bridge wire  52  is stretched in parallel to the first flange surface  41   b   1  at a predetermined height from the first flange surface  41   b   1  of the first flange  41   b . By precisely controlling the height h 1  (see  FIG. 5 ) from the end surface (reference surface) where the fixing member  44  is attached on the front housing  1  side, up to the end surfaces  43   a   1  and  43   b   1  of the outer cylindrical portion  43   a  and the inner cylindrical portion  43   b , respectively, of the ring case  43 , it is possible to precisely position the bridge wire  52  in the axial direction of the electromagnetic clutch. Similarly, the first flange surface  41   b   1  of the first flange  41   b  of the bobbin  41  can be precisely positioned in the circular recess of the ring case  43  as the bobbin container. The above structure enables precise control on a relative distance between the bimetal  51  and the bridge wire  52  whose positions in the axial direction of the electromagnetic clutch can be determined at the design stage. 
     Consider the possibility that, if the bimetal  51  is largely displaced, the displaced end portion of the bimetal  51  abuts the electromagnetic coil portion  47  that crosses over the first flange surface  41   b   1  of the first flange  41   b  of the bobbin  41  and the bimetal  51  is damaged thereby. In this embodiment, since the first flange surface  41   b   1  of the first flange  41   b  has the inclined surface  41   b   3 , the displaced end portion is guided along the inclined surface  41   b   3  and thus goes over the electromagnetic coil portion  47 . Therefore, the displaced end portion of the bimetal  51  is never engaged with the electromagnetic coil portion  47  and the bimetal can be protected. This realizes precise control on a relative distance between the bimetal  51  and the bridge wire  52  in the axial direction of the electromagnetic clutch, making it possible to protect the bimetal  51  even when the bimetal  51  is largely displaced and also to considerably improve the reliability of the energization interrupting device. 
     The inner wall  41   d , the outer wall  41   e , the inner abutment portion  41   f , and the outer abutment portion  41   g  integrally form the bobbin  41 . Thus, the bridge wire  52  can be easily obtained in the process for winding the electromagnetic coil  42  around the bobbin  41 . This realizes cost reduction of the electromagnetic clutch  1  even though the energization interrupting device is provided. 
     While the above embodiment shows an example where the bimetal is used as a thermally-actuated element, other thermally-actuated elements such as shape memory alloy are applicable. Further, although the above embodiment shows an example where the electromagnetic clutch is attached to the compressor used for the in-vehicle air conditioner, the electromagnetic clutch can be used for the other purposes without any limitation. 
     REFERENCE SYMBOL LIST 
     
         
           1  . . . Housing 
           2  . . . Rotation shaft 
           10  . . . Electromagnetic clutch 
           20  . . . Rotor unit 
           21  . . . Rotor 
           21   d  . . . Circular recess 
           30  . . . Armature unit 
           33  . . . Armature 
           40  . . . Electromagnetic coil unit 
           41  . . . Bobbin 
           42  . . . Electromagnetic coil 
           41   a  . . . Cylindrical portion 
           41   b  . . . First flange 
           41   c  . . . Second flange 
           41   d  . . . Inner wall 
           41   e  . . . Outer wall 
           41   f  . . . Inner abutment portion 
           41   g  . . . Outer abutment portion 
           41   b   2  . . . Guide wall 
           41   b   3  . . . Inclined surface 
           41   d   1  . . . Second slit 
           41   e   1  . . . First slit 
           50  . . . Energization interrupting device 
           51  . . . Bimetal 
           52  . . . Bridge wire (cutting wire portion)