Patent Publication Number: US-2016245345-A1

Title: Electromagnetic Clutch And Compressor

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 of an air conditioner for use in a vehicle). 
     BACKGROUND ART 
     As this type of electromagnetic clutches, for example, the one disclosed in Patent Document 1 has been known. The electromagnetic clutch disclosed in Patent Document 1 is provided with an electromagnetic coil unit including a rotor driven with power from a power source, an armature provided opposite to the rotor and connected to a rotation shaft of a driven device, and a bobbin around which an electromagnetic coil is wound. The electromagnetic coil unit causes magnetic attraction between the rotor and the armature in response to electric power supply. The electromagnetic clutch is further provided with a thermally-actuated member that can be displaced toward the electromagnetic coil by sensing the temperature. The member is installed in an electromagnetic unit of the rotor. If heat is generated due to relative sliding between friction surfaces of the rotor and the armature, and the temperature exceeds a predetermined level, the thermally-actuated member attached to the grounded rotor serves to cut a wire extending as a part of the electromagnetic coil in a bridge shape and stretched across an area where the thermally-actuated member moves along with the rotor rotation. Thus, electric power supply to the electromagnetic coil is forcibly interrupted. 
     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 the bridge wire stretched across the area where the thermally-actuated member moves along with the rotor rotation is formed on the ground side of the electromagnetic coil. Accordingly, if the thermally-actuated member cuts the bridge wire, no excessive current flows therein. 
     However, the electromagnetic clutch has a possibility that, after the bridge wire is cut, the tip end of wire remaining on the side (positive side) opposite to the ground side would contact the rotating thermally-actuated member or rotor and be grounded thereby. As a result, even if the bridge wire is cut, current flows in the electromagnetic coil and the armature is reattracted to the rotor. 
     The present invention has been made in view of the above circumstances and an object of the present invention is to provide an electromagnetic clutch that can prevent an armature from being reattracted to a rotor after a bridge wire is cut. 
     Means for Solving the Problems 
     In order to achieve the above object, the present invention provides an electromagnetic clutch, including: a rotor rotated with a power of a driving source; an armature provided opposite to the rotor and connected to a rotation shaft of a driven device; an electromagnetic coil unit that includes an electromagnetic coil, and causes magnetic attraction between the rotor and the armature upon electric power supply to the electromagnetic coil; and a power interruption device that forcibly interrupts electric power supply to the electromagnetic coil. In the electromagnetic clutch, the power interruption device includes: a thermally-actuated member attached to the rotor and displaced at over a predetermined temperature; and a bridge wire stretched across an area where the thermally-actuated member moves along with rotation of the rotor and extending as a part of ground wire of the electromagnetic coil. The power interruption device cuts the bridge wire such that the thermally-actuated member is displaced to collide with the bridge wire, and the bridge wire has a cutting region apart from a position where the displaced thermally-actuated member collides with the bridge wire. 
     Effects of the Invention 
     In the above electromagnetic clutch, the bridge wire extending as a part of ground wire of the electromagnetic coil is cut at the cutting region apart from a position where the displaced thermally-actuated member collides with the bridge wire. Owing to this structure, after the bridge wire is cut, the tip end of the wire remaining on the side (positive side) opposite to the ground side can be kept from contacting the rotor or thermally-actuated member that are generally grounded. This makes it possible to prevent current from into the electromagnetic coil after the bridge wire is cut and thus prevent the armature from being reattracted to the rotor after the cutting of the bridge wire. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an electromagnetic clutch according to 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 a sectional view taken along the line D-D of  FIG. 7 . 
         FIG. 11  is an enlarged view of a portion E of  FIG. 7 . 
         FIG. 12  is an explanatory operational view of a power interruption device under the condition that bimetal is not displaced. 
         FIG. 13  is an explanatory operational view of a power interruption device under the condition that bimetal is displaced beyond a predetermined distance. 
         FIG. 14  is a sectional view of the bimetal of  FIG. 12 . 
         FIG. 15  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 in a compressor constituting an in-vehicle air conditioner and configured to transmit/interrupt 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 of the present invention is provided with the electromagnetic clutch. An embodiment of the clutch is described later. While the compressor is operated in response to power transmission from the engine or motor, the compressor stops operation in response to power interruption. The compressor of the present invention could be, for example, a swashplate type variable-displacement compressor. Note that 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 , an electromagnetic coil unit  40 , and a power interruption device  50 . 
     The rotor unit  20  is provided with a rotor  21  rotated with a power of an engine or motor, and rotatably supported to an end surface of the compressor. The rotor unit is composed of the rotor  21 , a friction member  22 , and a bearing  23 . 
     The rotor  21  has a circular shape. The inner circumference of the rotor  21  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 on which a belt for transmitting a rotational force from the engine or motor is stretched. 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, 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 . Arc-shaped slits  21   e  and  21   f  are formed on the end surface portion  21   c  so as to divert magnetic flux generated in the electromagnetic coil  42 . In addition, between the arc-shafted slits  21   e  and  21   f  in 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, 12, and 13 ) where a bimetal  51  of the power interruption device  50  is attached as described below. A friction surface  21   c   1  is defined as an end surface of the end surface portion  21   c  opposite to the bottom wall of the circular recess  21   d . The rotor  21  is grounded although not shown. 
     The friction member  22  is made of a circular nonmagnetic material to increase friction coefficient and attached to the friction surface  21   c   1  of the rotor  21 . 
     Discussing the structure of the bearing  23 , its inner ring 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  on the outer circumference of the boss  1   a  as the end surface of the front housing  1 . 
     The armature unit  30  transmits/interrupts power from an engine or motor to a compressor according to the movement of the armature  33 , which collides with or retracts from the rotor  21  in response to electric power supply/interruption with respect to the electromagnetic coil  42 . As shown in  FIG. 4 , the armature 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 . 
     The rubber unit  32  includes an inner ring  32   a , an outer ring  32   b , and a circular rubber  32   c  interposed between the inner ring  32   a  and the outer ring  32   b  and fixed to the inner ring  32   a  and the outer ring  32   b  by cure adhesion. 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 provided opposite to the rotor  21  and connected to the rotation shaft  2  of the compressor. To be specific, the armature  33  is a circular plate member with one end surface forming a friction surface  33   a , which faces the friction surface  21   c   1  of the rotor  21  at a predetermined interval. The armature  33  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  33  constitutes a magnetic circuit together with the rotor  21 . When electric power is supplied to the electromagnetic coil  42 , the armature is magnetically attracted to the rotor  21 . Meanwhile, when magnetic attraction power is extinguished due to power interruption, the armature moves away from the rotor  21 . 
     The electromagnetic coil unit  40  includes the electromagnetic coil  42  and generates a magnetic field in response to electric power supply to the coil to thereby cause magnetic attraction between the armature  33  and 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 for accommodating the bobbin  41 , an annular disc-like fixing member  44  forming the other end surface of the electromagnetic coil unit  40 , and a connecting portion  45  for connecting an external power source 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  around which the electromagnetic coil  42  is wound. The outer cylindrical portion  43   a  and the 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  and an end surface  43   b   1  of the inner cylindrical portion  43   b  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 . The electromagnetic coil  42  is wound around an area surrounded by the flanges  41   b  and  41   c . The bobbin  41  also has an inner wall  41   d  and an outer wall  41   e , which 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 in almost all the circumference of the proximal end of the first flange  41   b . Similarly formed in almost all the circumference thereof is an inner flange  41   f , which extends radially inwardly from the tip end thereof. Further, 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 flange  41   g  extends radially outwardly from the tip end thereof. Referring to  FIG. 8  that illustrates the structure viewed from the arrow B of  FIG. 7 , the outer wall  41   e  of the bobbin  41  has a first slit  41   e   1  with a predetermined distance h 2  (corresponding to the thickness of the outer flange  41   g ) from the tip end (upper surface of the outer flange  41   g ). Moreover, as shown in  FIG. 9  that illustrates the structure viewed from the arrow C of  FIG. 7 , the inner wall  41   d  of the bobbin  41  has a second slit  41   d   1  with the depth h 2  (corresponding to the thickness of the inner flange  41   f ) that is the same as the first slit  41   e   1 , from the tip end (upper surface of the inner flange  41   f ), and a third slit  41   d   2  extending from the tip end (upper surface of the inner flange  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 flange  41   f , and the outer flange  41   g , which are integrally formed of a resin material, for example, a polyamide resin. Owing to this structure, as will be described below, a bridge wire  52  can be easily obtained using the winding end of the electromagnetic coil  42  wound around the bobbin  41 . 
     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 in the circular recess of the ring case  43 . As shown in  FIG. 5 , the bobbin  41  is positioned and fixed in the circular recess of the ring case  43  such that the outer flange  41   g  of the bobbin  41  is positioned to the end surface  43   a   1  of the outer cylindrical portion  43   a  of the ring case  43 , and the inner flange  41   f  is positioned to the end surface  43   b   1  of the inner cylindrical portion  43   b  of the ring case  43 . The electromagnetic coil unit  40  is fixed to the end surface of the front housing  1  such that the fixing member  44  fixed to the end surface of the end surface portion  43   c , which is 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 . 
     When heat is generated due to relative sliding between the rotor  21  and the armature  31 , the power interruption device  50  forcibly interrupts electric power supply to the electromagnetic coil  42 . The device is provided with thermally-actuated members, for example, the bimetal  51  and the bridge wire  52 . 
     The bimetal  51  is provided on the rotor, and displaced toward the electromagnetic coil unit  40  if sensing the temperature higher than a predetermined level. More specifically, the bimetal  51  is formed in substantially a rectangular shape and accommodated in the circular groove  21   g  formed in the bottom wall  21   c   2  of the circular recess  21   d  of 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 . Since the bimetal  51  is accommodated and positioned in the circular groove  21   g , if the bimetal  51  collides with (engaged with) and cuts the bridge wire  52 , the bimetal 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 . 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 largely displaced at the inverted motion temperature or higher. By utilizing the inverted motion, the bridge wire  23  is cut. In general, in a compressor for an in-vehicle air conditioner, the electromagnetic clutch  10  may increase the temperature up to 150° C. Taking the temperature into consideration, the inverted motion temperature for interrupting electric power supply to the electromagnetic coil  42  is set to 180° C. to 190° C. Here, the bimetal  51  is fixed with the rivet  53  but any other fixing member such as a bolt is applicable. 
     The bridge wire  52  is a part of ground wire of the electromagnetic coil  42 . The bridge wire is, for example, obtained using the winding end of the electromagnetic coil  42  wound around the bobbin  41 . The bridge wire  52  is inserted into the circular recess  21   d  of the rotor  21 , and stretched over one end surface of the rotor unit  20  opposite to the bottom wall  21   c   2  thereof so as to cross an area where the bimetal  51  moves along with the rotation of the rotor  21  (movement area of the bimetal  51 ) and also to collide with (engage with) the bimetal  51  displaced beyond a predetermined distance. More specifically, as shown in  FIGS. 7 to 11 , 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 , and positioned and supported on the end surfaces of both the slits  41   e   1  and  41   d   1 . 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 surface  41   b   1  of the first flange  41   b  toward the outer wall  41   e  over the surface  41   b   1  of the first flange  41   b . Then, the wire is routed outwardly in the radial direction of the bobbin  41 . In this way, the bridge wire  52  extends over the first flange  41   b  and is stretched between the outer wall  41   e  and the inner wall  41   d.    
     The inner flange  41   f  and the outer flange  41   g  are formed at the same height from the 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, i.e., depth h 2 . Thus, the bridge wire  52  is stretched in parallel to the surface  41   b   1  of the first flange  41   b  at a predetermined height. By controlling the following: the height h 1  from the end surface (reference surface) of the fixing member  44  near the front housing  1  to the end surfaces  43   a   1  and  43   b   1  of the outer cylindrical portion and the inner cylindrical portion of the ring case  43 , respectively; the depth h 2  from the end surfaces of the outer wall  41   e  and the inner wall  41   d  to the end surfaces of the first slit  41   e   1  and the second slit  41   d   1 , respectively; and the thickness of each of the inner flange  41   f  and the outer flange  41   g , the bridge wire  52  can be precisely positioned in the axial direction of the electromagnetic clutch. Similarly, the surface  41   b   1  of the first flange can be precisely positioned. 
     As shown in  FIGS. 12 and 13 , a cutting region  54  of the bridge wire  52  is defined apart from the position where the displaced bimetal  51  collides with the bridge wire  52  by a predetermined distance L and also defined on the side (positive side/coil side) of the bridge wire  52  opposite to the ground side (i.e., negative side). In this embodiment, as shown in  FIG. 11 , an area of the cutting region  54  of the bridge wire  52  is set smaller in section than the other areas of the bridge wire  52 . The distance L is preferably 1 mm or more, for example. 
     Here, if the bridge wire  52  is stretched such that the wire would collide with the bimetal  51  face to face, the following situation is conceivable. That is, the bridge wire  52  collides with the whole bimetal  51  at a time, and the bimetal  51  is largely displaced and fails to cut the bridge wire  52 . To prevent such a situation, in this embodiment, the bridge wire  52  is inclined so that the cutting region could collide with the bimetal  51  ahead of the ground side. More specifically, as shown in  FIG. 7 , the bridge wire  52  is included at an angle θ relative to a line K passing the center in the radial direction of the bobbin  41  and the first slit  41   e   1  so that the cutting region (first slit  41   e   1  side) could get closer to the bimetal  51 . The angle θ is set to, for example, 20° to 60°. 
     In this embodiment, a support portion  55  (see  FIG. 11 ) is formed using the end surface of the first slit  41   e   1 . The support portion supports the cutting region  54  of the bridge wire  52  and its vicinities reversely to the rotation direction of the bimetal when the bimetal  51  collides with the bridge wire  52 . The first slit  41   e   1  is slightly wider than the outer diameter of the wire as shown in  FIG. 11 . Thus, if the displaced bimetal  51  collides with the bridge wire  52 , the cutting region  54  is shifted along the rotation direction of the bimetal  51 . At this time, the cutting wire  54  is made closer to the radially inner portion of the bobbin  41  than the first slit  41   e , i.e., closer to the bimetal  51  than the support portion  55 . 
     Next, a brief description is given of how the thus-configured electromagnetic clutch  10  transmits/interrupts power to the compressor. 
     Discuss first an operation of transmitting/interrupting power under a normal state in which the power interruption device is not operating. In this state, an air conditioner control device in the vehicle makes control to supply/interrupt electric power to the electromagnetic coil unit  40 . 
     First, a rotational force from the engine rotates the rotor  21 . If electric power is supplied to the electromagnetic coil  42  in this state, the electromagnetic coil unit  40  magnetizes the rotor  21 . The generated magnetic force makes the friction surface  33   a  of the armature  33  attracted to the friction surface  21   c  of the rotor  21 . The frictional force generated therebetween rotates the armature  33  in sync with the rotor  21 . The rotational force of the armature  33  is transmitted to the rotation shaft  2  by way of the rubber unit  32  and the hub  31 . The compressor starts operating along with the rotation of the rotor  21 . In contrast, if the air conditioner control device in the vehicle interrupts electric power supply to the electromagnetic coil  42  of the coil unit  40 , the rotor  21  is demagnetized, and the restoring force of the rubber  32   c  makes the friction surface  33   a  of the armature  33  retract from the friction surface  21   c   1  of the rotor  21 . No rotational force of the rotor  21  is transmitted to the armature  33 , and the rotation shaft  2  stops rotating to terminate the operation of the compressor. In such a state that electric power is supplied to the electromagnetic coil  42 , the rotor  21  is excited, the magnetic force makes the friction surface  33   a  of the armature  33  attracted to the friction surface  21   c   1  of the rotor  21 , and the armature  33  is rotated in sync with the rotor  21 , the temperature of the end surface portion  21   c  of the rotor  21  does not reach the inverted motion temperature of the bimetal  51 . As shown in  FIGS. 12 and 14 , the bimetal  51  passes above the bridge wire  52  without colliding with the bridge wire  52 . 
     Discussing next the case where the power interruption device is operating, if an excessive torque much larger than a normal value acts on the rotation shaft  2  due to, for example, damaged inner parts of the compressor under the state shown in  FIGS. 12 and 14 , relative sliding occurs between the friction surface  21   c   1  of the rotor  21  and the friction surface  33   a  of the armature  33 . The temperature of the end surface portion  21   c  of the rotor  21  suddenly rises due to heat generated between the friction surfaces. 
     Following the sudden temperature rise in the end surface portion  21   c , the other end of the bimetal  51  (opposite side to one end fixed with the rivet  53 ) is gradually displaced toward the electromagnetic coil unit  40 . If the temperature exceeds the inverted motion temperature, as shown in  FIGS. 13 and 15 , the other end of the bimetal  51  is largely displaced beyond a predetermined distance and then collides with (engaged with) the bridge wire  52 . An excessive tension acts on the cutting region  54  formed on the first slit  41   e   1  side to thereby cut the bridge wire  52  at the cutting region  54 . Consequently, it is possible to forcibly interrupt electric power supply to the electromagnetic coil  42 , retract the friction surface  33   a  of the armature  33  from the friction surface  21   c   1  of the rotor  21 , keep the engine from receiving an excessive load, protect the belt, and ensure driving safety of the vehicle. 
     In the thus-structured electromagnetic clutch  10 , the cutting region  54  is defined apart by a predetermined distance L from a position where the displaced bimetal  51  collides with the bridge wire  52  and also defined on the side opposite to the ground side of the bridge wire  52 . Accordingly, after the bridge wire  52  is cut, the tip end of wire (piece) remaining on the side opposite to the ground side never contacts the grounded rotor  21  or bimetal  51 . In other words, no current flows into the electromagnetic coil after the bridge wire is cut. Thus, the armature is never reattracted to the rotor after the cutting of the bridge wire. 
     Since the cutting region  54  is formed near the first slit  41   e   1 , the tip end of the piece of the bridge wire  52  opposite to the ground side hardly moves. Further, the resin-made outer wall  41   e  having the first slit  41   e   1  secures insulation against the tip end of the piece of the bridge wire  52  on the side opposite to the ground side. 
     In this embodiment, the cutting region  54  of the bridge wire  52  is set smaller in section than the other regions. When the bridge wire  52  is pulled upon collision with the bimetal  51 , the bridge wire  52  is easily cut at the cutting region  54  to thereby realize reliable cutting. This makes it possible to inhibit the bimetal  51  from being largely displaced upon collision with the bridge wire  52  and accordingly avoid any damage of the bimetal  51 . 
     Further, in this embodiment, the bridge wire  52  is placed at such angle as makes the cutting region  54  (first slit  41   e   1  side) collide with against the bimetal  51  ahead of the ground side. As a result, the edge of the bimetal  51  on the cutting region  54  side collides with the bridge wire  52  and thus, the bimetal  51  itself is less displaced and its tip end easily slips under the bridge wire  52 . In addition, the tension can securely act on the cutting region  54  on the side (positive side) opposite to the ground side. 
     In this embodiment, the support portion  55  is formed to support the cutting region  54  and its vicinities of the bridge wire  52  reversely to the rotation direction upon collision of the bimetal  51  against the bridge wire  52 . The tension securely acts on the cutting region  54  on the support portion  55  as fulcrum and thus, the wire can be cut at the cutting region  54  more reliably. This embodiment shows an example the cutting region  54  is defined closer to the bimetal  51  than the support portion  55 . However, the present invention is not limited thereto. The cutting region  54  may collide with the support portion  55 . 
     While the bimetal is used as a thermally-actuated member in this embodiment, other thermally-actuated members such as shape memory alloy are applicable. Although the bridge wire  52  is formed using the winding end of the coil wound around the bobbin  41  in this embodiment, other types of wire are applicable with no limitation in place of the coil wound around the bobbin  41 . 
     Further, the above embodiment shows an example where the electromagnetic clutch is attached to the compressor used for the in-vehicle air conditioner, but the electromagnetic clutch of the present invention can be used for other purposes. 
     Moreover, in this embodiment, the cutting region  54  is formed apart from a position where the displaced bimetal  51  collides with the bridge wire  52 , on the side opposite to the ground side of the bridge wire  52 . However, the present invention is not limited thereto. The cutting region  54  has only to be apart from a position where the displaced bimetal  51  collides with the bridge wire  52 . Accordingly, even if the cutting region  54  is formed on the side opposite to the ground side of the bridge wire  52 , after the bridge wire  52  is cut, the tip end of wire (piece) remaining on the side (positive side) opposite to the ground side out of the cut bridge wire  52  can be prevented from contacting the grounded rotor  21  or thermally-actuated member  51 . This structure inhibits the armature from being reattracted to the rotor after the bridge wire is cut. In this case, the bridge wire  52  may be coated with an insulating material to achieve insulation against the thermally-actuated member  51  even if portions other than the tip end of the wire (piece) remaining on the side opposite to the ground side contacts the member. 
     The preferred embodiments of the present invention have been discussed above, but the present invention is not limited to the above embodiment and various modifications and changes are applicable within the technical scope of the present invention. 
     Note that the basic configuration of the electromagnetic clutch according to the present invention is as follows. That is, the electromagnetic clutch includes: a rotor rotated with power of a driving source; an armature provided opposite to the rotor and connected to a rotation shaft of the driven device; an electromagnetic coil unit that includes an electromagnetic coil, and causes magnetic attraction between the rotor and the armature upon electric power supply to the electromagnetic coil; and a power interruption device that forcibly interrupts electric power supply to the electromagnetic coil. In the clutch, the power interruption device includes: a thermally-actuated member attached to the rotor, and displaced at over a predetermined temperature; and a bridge wire stretched across an area where the thermally-actuated member moves along with rotation of the rotor and extending as a part of ground wire of the electromagnetic coil. The device cuts the bridge wire such that the thermally-actuated member is displaced to collide with the bridge wire. The bridge wire has a cutting region apart from a position where the displaced thermally-actuated member collides with the bridge wire and also on the side opposite to the ground side of the bridge wire. The electromagnetic clutch having the above basic configuration can be expressed as follows. That is, the electromagnetic clutch includes: a rotor rotated with power of a driving source; an armature provided opposite to the rotor and connected to a rotation shaft of the driven device; an electromagnetic coil unit that includes an electromagnetic coil, and causes magnetic attraction between the rotor and the armature upon electric power supply to the electromagnetic coil; and a power interruption device that forcibly interrupts electric power supply to the electromagnetic coil. In the clutch, a thermally-actuated member displaced at over a predetermined temperature is attached to the rotor, and ground wire of the electromagnetic coil serves as a bridge wire that is stretched across an area where the thermally-actuated member moves along with rotation of the rotor. The displaced thermally-actuated member collides with the bridge wire and thereby cuts the bridge wire. The bridge wire is cut in a position closer to the side opposite to the ground side relative to a position where the displaced thermally-actuated member collides with the bridge wire. 
     REFERENCE SYMBOL LIST 
     
         
           10  . . . Electromagnetic clutch 
           21  . . . Rotor 
           33  . . . Armature 
           40  . . . Electromagnetic coil unit 
           42  . . . Electromagnetic coil 
           50  . . . Power interruption device 
           51  . . . Bimetal (thermally-actuated member) 
           52  . . . Bridge wire 
           54  . . . Cutting region 
           55  . . . Support portion 
           2  . . . Rotation shaft of driven device (compressor)