Patent Publication Number: US-2007095627-A1

Title: Externally-controlled fluid coupling

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application 2005-314527, filed on Oct. 28, 2005, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      This invention generally relates to an externally-controlled fluid coupling. More specifically, this invention pertains to an externally-controlled fluid coupling for a cooling fan for a vehicle engine.  
     BACKGROUND  
      As a conventional fluid coupling for a cooling fan, a fluid coupling, in which a valve provided in a communicating passage between an operation chamber and a storage chamber is operated by temperature change of a bimetal to control the amount of a fluid supplied to the operation chamber and in turn to control torque transmitted from a driving disk to a case, is known. JP2004-162911A discloses an externally-controlled fluid coupling including an electromagnet instead of a bimetal and an opening/closing valve, which opens/closes a hole of a supply passage, in which a fluid flows from a storage chamber to an operation chamber, by magnetic force exerted from the electromagnet to externally control the amount of the fluid flowing from the storage chamber to the operation chamber and in turn to control rotation of a fan. In the externally-controlled fluid coupling, because the opening/closing valve is provided only in the supply passage, in which the fluid flows from the storage chamber to the operation chamber, at the time when an engine stops, in other words, in a situation where a driving rotational member stops, the fluid stored in the storage chamber flows out to the operation chamber side through a return passage. As a result, there is a drawback that rotation of the fan may occur and cause noise at the next time of starting the engine.  
      A fluid coupling designed for overcoming the above drawback is disclosed in JPH04 (1992)-258529A. The fluid coupling includes two ring-shape electromagnets provided on the same shaft center. One of the electromagnets actuates a supply valve for opening/closing a supply passage, in which a fluid flows from a storage chamber to an operation chamber. The other one of the electromagnets actuates a return valve for opening/closing a return passage, in which the fluid flows from the operation chamber to the storage chamber. The externally-controlled fluid coupling has an advantage that the supply valve and the return valve can be controlled to open/close nonsynchronously. However, because two ring-shape electromagnets provided on the same shaft center are utilized, a diameter of an outer electromagnet becomes large, which increases weight thereof. Further, because an area of the outer electromagnet, through which magnetic flux passes, becomes large, a magnetic material (pulled member) provided at the side of the valve also need to be large for obtaining necessary pulling force. As a result, the valve itself becomes large, which tends to cause high cost and lowering reliability.  
      A need thus exists for a simply configured externally-controlled fluid coupling, which can control a flow rate of a fluid flowing in a supply passage and a return passage nonsynchronously. The present invention has been made in view of the above circumstances and provides such an externally-controlled fluid coupling.  
     SUMMARY OF THE INVENTION  
      According to an aspect of the present invention, an externally-controlled fluid coupling includes a driving disk fixed to a driving rotational shaft, a case member rotatably supported by the driving rotational shaft, a cover member hermetically connected to the case member to form an inner space between the case member and the cover member, a separating plate for separating the inner space into an operation chamber accommodating the driving disk and a storage chamber for storing a fluid, a supply passage for supplying the fluid from the storage chamber to the operation chamber, a return passage for returning the fluid from the operation chamber to the storage chamber, a supply valve for opening/closing the supply passage, a return valve for opening/closing the return passage and an electromagnetic actuating means for actuating the supply valve and the return valve to open/close on the basis of a control signal from a controller. The electromagnetic actuating means includes an electromagnet commonly utilized for actuating the supply valve and the return valve. The electromagnet generates a first power of magnetism for opening or closing the supply valve and a second power of magnetism for opening or closing the return valve. The values of the first and second powers of magnetism are different from each other so that the supply valve and the return valve open/close nonsynchronously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein:  
       FIG. 1  represents a cross-sectional view illustrating an externally-controlled fluid coupling according to an embodiment of the present invention;  
       FIG. 2  represents a front view illustrating the externally-controlled fluid coupling illustrated in  FIG. 1 ;  
       FIG. 3  represents a detail view illustrating a supply passage;  
       FIG. 4  represents a detail view illustrating a return passage;  
       FIG. 5  represents a detail view illustrating an electromagnetic actuating means;  
       FIGS. 6A, 6B  and  6 C represent explanatory diagrams typically illustrating valve configurations, of which biasing force of springs are different;  
       FIGS. 7A, 7B  and  7 C represent explanatory diagrams typically illustrating different valve control modes;  
       FIGS. 8A, 8B  and  8 C represent explanatory diagrams typically illustrating valve configurations of different areas, at which magnetic force is exerted;  
       FIGS. 9A and 9B  represent explanatory diagrams typically illustrating supply passages, of which diameters of exits are different; and  
       FIGS. 10A and 10B  represent explanatory diagrams typically illustrating supply passages, of which the number of exits to the operation chamber is different. 
    
    
     DETAILED DESCRIPTION  
      An embodiment of the present invention will be explained with reference to drawing figures.  FIG. 1  represents a cross-sectional view illustrating an externally-controlled fluid coupling according to the embodiment of the present invention.  FIG. 2  represents a schematic front view illustrating the same, in which a rotational shaft, or the like, is partially omitted.  FIG. 1  is a cross-sectional view taken on line of I-I of  FIG. 2 .  
      The fluid coupling includes a driving rotational shaft  1 , to which driving force is transmitted from an engine (not illustrated), and a housing  2  as a driven rotational member. A cooling fan for the engine is attached to a peripheral portion of the housing  2 . A driving disk  3  is fixed to an end portion of the driving rotational shaft  1 . The housing  2  is configured from a ring-plate-shape case member  2   a  supported by the driving rotational shaft  1  through a first bearing  4   a  and rotatable about the driving rotational shaft  1  and a cover member  2   b  hermetically connected to the peripheral portion of the case member  2   a  by a screw and a seal for forming an inner space  5  accommodating the driving disk  3  between the cover member  2   b  and the case member  2   a . In the inner space  5 , for example, viscous fluid such as silicon oil, or the like, is stored. A ring-shaped electromagnetic actuating means  6 , which will be detailed later, is provided between the driving rotational shaft  1  and the case member  2   a.    
      The inner space  5  is divided by a ring-shape separating plate  7  in a direction perpendicular to the rotational driving shaft  1  into two sections, namely an operation chamber  5   a  accommodating the driving disk  3  and a storage chamber  5   b  for storing a fluid. Configuration members of the electromagnetic actuating means  6  are provided in the storage chamber  5   b . A labyrinth portion  8 , which functions as a torque transmitting portion, is formed in the operation chamber  5   a  at a position where the driving disk  3  and the case member  2   a  face each other.  
      As detailed in  FIG. 3 , a communicating hole is formed at the case member  2   a  from a position facing the storage chamber  5   b  of the case member  2   a  to a position facing the operation chamber  5   a  in an area of the labyrinth portion  8 . The communicating hole serves as a supply passage  9  for supplying the fluid from the storage chamber  5   b  to the operation chamber  5   a . In the embodiment of the present invention, two supply passages  9 , in other words, a first supply passage  9   a  and a second supply passage  9   b , are provided so that the second supply passage  9   b  is located at a position shifted from that of the first supply passage  9   a  by an angle of 180° in a peripheral direction, in other words, so that the first supply passage  9   a  and the second supply passage  9   b  are axially symmetric.  
      As detailed in  FIG. 4 , a communicating hole is formed at the case member  2   a  from a position facing the operation chamber  5   a  in an area outside the labyrinth portion  8  of the case member  2   a  to a position facing the storage chamber  5   b . The communicating hole serves as a return passage  10  for returning the fluid from the operation chamber  5   a  to the storage chamber  5   b . In the embodiment of the present invention, two return passages  10 , in other words, a first return passage  10   a  and a second return passage  10   b , are provided so that the second return passage  10   b  is located at a position shifted from that of the first return passage  10   a  by an angle of 180° in a peripheral direction, in other words, the first return passage  10   a  and the second return passage  10   b  are axially symmetric. In the meantime, the first return passage  10   a  and the second return passage  10   b  are provided at an intermediate position between the first supply passage  9   a  and the second supply passage  9   b  in a peripheral direction respectively.  
      As further detailed in  FIG. 3 , a supply valve  20  configured from a belt-shape spring member  21  is provided at an opening of the supply passage  9  at the side of the storage chamber  5   b . An end portion of the spring member  21  is formed as a sealing surface for closing the opening of the supply passage  9  by biasing force of the spring member  21 . A base end portion of the spring member  21  is fixed to the case member  2   a . The spring member  21  is cantilevered, and biasing force according to curving of the spring member  21  functions to close the opening of the supply passage  9 . A pulled portion  22  made of magnetic material is provided at a middle portion of the spring member  21 . In a situation where magnetic force generated by the electromagnetic actuating means  6  is exerted to the pulled portion  22 , the spring member  21  is moved against the biasing force, and the supply valve  20  is opened from a closed state. Here, the supply valve  20  is provided at each of the first supply passage  9   a  and the second supply passage  9   b . A supply valve  20  provided at the first supply passage  9   a  will be referred to as a first supply valve  20   a  and a supply valve  20  provided at the second supply passage  9   b  will be referred to as a second supply valve  20   b , as necessity arises. As further detailed in  FIG. 4 , a return valve  30  is provided at an opening of the return passage  10 . A configuration of the return valve  30  is similar to that of the supply valve  20 . The return valve  30  includes a spring member  31  and a pulled portion  32 . The return valve  30  is provided at each of the first return passage  10   a  and the second return passage  10   b . A return valve  30  provided at the first return passage  10   a  will be referred to as a first return valve  30   a  and a return valve  30  provided at the second return passage  10   b  will be referred to as a second return valve  30   b , as necessity arises.  
      For opening the supply valve  20  and the return valve  30  closed by biasing force of respective spring members  21  and  31  against the biasing force, the electromagnetic actuating means  6  exerts magnetic force to the pulled portions  22  and  32  of the supply valve  20  and the return valve  30 . A configuration of the electromagnetic actuating means  6  will be explained with reference to  FIG. 5 . The configuration of the electromagnetic actuating means  6  is substantially common to all of the supply valves  20  and the return valves  30 . Accordingly, explanation will be made taking an example from the first supply valve  20   a.    
      As can be grasped with reference to  FIGS. 1, 2  and  5 , the electromagnetic actuating means  6  includes one ring-shape electromagnet  60  provided coaxially with the driving rotational shaft  1 , a circular yoke  61  having a ring groove for accommodating the electromagnet  60 , a first ring  63  fitted to outside of an outer race of the first bearing  4   a  and attached to the case member  2   a , a second ring  64  provided outside the first ring  63  and attached to the case member  2   a  so that the second ring  64  is magnetically connected with the pulled portion  22  of the first supply valve  20   a , a bracket ring  65  assembled with the second ring  64  through the second bearing  4   b  and connected to the circular yoke  61  and a valve-pulling portion  66  fixed to an outer peripheral surface of the first ring  63  so that the valve-pulling portion  66  faces the pulled portion  22  of the first supply valve  20   a . The circular yoke  61 , the first ring  63 , the second ring  64 , the bracket ring  65  and the valve-pulling portion  66  are made of magnetic material. As a result, a closed magnetic circuit, in which the electromagnet  60  is a source of magnetism, is configured from the circular yoke  61 , the first ring  63 , the second ring  64 , the pulled portion  22  and the bracket ring  65 . In a situation where electricity is applied to the electromagnet  60  on the basis of a control signal of a controller  90 , the pulled portion  22  is pulled toward the valve-pulling portion  66 . Then, the first supply valve  20   a  is opened. In the meantime, the first supply valve  20   a , the second supply valve  20   b , the first return valve  30   a  and the second return valve  30   b  are positioned on a circumference, which is coaxial with a center of the ring-shape electromagnet  60  and of which a diameter is the same as a diameter of a central circumference of the ring-shape electromagnet  60 , to face the electromagnet  60  so that the first supply valve  20   a , the second supply valve  20   b , the first return valve  30   a  and the second return valve  30   b  can be actuated to open/close by one common electromagnet  60 .  
      As typically illustrated in  FIGS. 6A, 6B  and  6 C, in the embodiment of the present invention, an outer shape of the spring member  21  of the first supply valve  20   a  is the same as that of the spring member  21  of the second supply valve  20   b . The spring member  21  of the first supply valve  20   a  has a hole  21   a  at a middle portion thereof. On the other hand, the spring member  21  of the second supply valve  20   b  does not have such a hole  21  a at a middle portion (vicinity of a bended portion) thereof. Accordingly, biasing force for closing the first supply valve  20   a  is smaller than that for closing the second supply valve  20   b . As a result, pulling force required for opening the first supply valve  20   a  is smaller than that required for opening the second supply valve  20   b . The spring member  31  of the first return valve  30   a  is the same as that of the second return valve  30   b . Outlines of the spring members  31  of the first return valve  30   a  and the second return valve  30   b  are the same as that of the spring member  21  of the supply valve  20 . However, each of the spring members  31  of the first return valve  30   a  and the second return valve  30   b  has a hole  3   1   a  larger than the hole  21  a of the first supply valve  20   a  at a middle portion (vicinity of a bended portion) thereof. Accordingly, biasing force for closing the return valve  30  is smaller than that for closing the first supply valve  20   a  and the second supply valve  20   b . As a result, pulling force required for opening the return valve  30  is smaller than that required for opening the first supply valve  20   a  and the second supply valve  20   b.    
      As described above, pulling force required for opening the second supply valve  20   b , the first supply valve  20   a  and the return valve  30  becomes smaller stepwise. Such a configuration of the spring members enables various valve control modes for the controller  90 , which controls current inputted to the electromagnet  60  to control the supply valve  20  and the return valve  30 . The control modes will be explained.  
      In a situation where current inputted to the electromagnet  60  is zero current (approximate current value, at which spring members are not moved), the first supply valve  20   a , all of the second supply valve  20   b  and the return valve  30  are maintained to close. Such a valve state of the fluid coupling will be referred to as a first state. In a situation where current inputted to the electromagnet  60  is a predetermined small current value (current value, at which power of magnetism for generating magnetic force for moving only the spring member  31  of the return valve  30  to open is generated), only the return valve  30  is opened, and the first supply valve  20   a  and the second supply valve  20   b  are maintained to close. Such a valve state of the fluid coupling will be referred to as a second state. In a situation where current inputted to the electromagnet  60  is a predetermined medium current value (current value, at which power of magnetism for generating pulling forces for moving only the spring member  31  of the return valve  30  and the spring member  21   a  of the first supply valve  20   a  to open is generated), the return valve  30  and the first supply valve  20   a  are opened, and only the second supply valve  20   b  is maintained to close. Such a valve state of the fluid coupling will be referred to as a third state. In a situation where current inputted to the electromagnet  60  is a predetermined large current value (current value, at which power of magnetism for generating pulling forces for moving all of the spring member  31  of the return valve  30 , the spring member  21   a  of the first supply valve  20   a  and the spring member  21   b  of the second supply valve  20   b  to open is generated), all of the return valve  30 , the first supply valve  20   a  and the second supply valve  20   b  are opened. Such a valve state of the fluid coupling will be referred to as a fourth state.  
      Operations of the externally-controlled fluid coupling configured as described above will be explained. In a state where an engine starts and the driving rotational shaft  1  is rotating, in a situation where a large current described above is applied to the electromagnet  60  on the basis of the control signal of the controller  90 , the valve state becomes the fourth state. The first and second supply valves  20   a  and  20   b  and the first and second return valves  30   a  and  30   b  are opened. As a result, the fluid flows to the operation chamber  5   a  (labyrinth portion  8 ) from the storage chamber  5   b  through the first supply passage  9   a  and the second supply passage  9   b . The fluid having flown into the operation chamber Sa flows through the labyrinth portion  8  and flows back to the storage chamber  5   b  through the first return passage  10   a  and the second return passage  10   b  with a help from a function of a pump mechanism (an obstacle for the fluid generally called a dam, made of elastic material, or the like) provided at a most outer peripheral portion of the inner space  5 . Such a circulation of the fluid functions to transmit rotation of the driving disk  3 , which is rotating integrally with the driving rotational shaft  1 , to the housing  2  to rotate the housing  2 , and as a result to rotate the fan, which cools a radiator and the engine. Further, in a situation where temperature lowering of radiator fluid, or the like, is detected by a sensor (not illustrated), the controller  90  emits a corresponding control signal to apply the middle current described above to the electromagnet  60 . In this situation, the valve state becomes the third state. The first supply valve  20   a  and the first and second return valves  30   a  and  30   b  are maintained to open, and the second supply valve  20   b  is closed. As a result, the fluid flows to the operation chamber  5   a  from the storage chamber  5   b  through only the first supply passage  9   a , and the amount of circulating fluid reduces. Accordingly, rotational frequency of the housing  2 , in other words, the fan, reduces. At the time of stopping the engine, because electricity is not applied to the electromagnet  60 , the valve state becomes the first state and the first and second supply valves  20   a  and  20   b  and the first and second return valves  30   a  and  30   b  are closed. In other words, because all of the return passages  10  are closed, back flow of the fluid from the storage chamber  5   b  to the operation chamber  5   a  can be inhibited. Accordingly, at the next time of starting the engine, transmission of rotation of the driving rotational shaft  1  to the housing  2 , which leads to rotation of the fan, can be inhibited. In the meantime, in a situation where the valve state is the second state, the fluid can preferably be returned from the operation chamber  5   a  to the storage chamber  5   b.    
      As described above, in this fluid coupling, different valve opened/closed states of, in particular, the first supply valve  20   a  and the second supply valve  20   b , can be set by controlling current applied to the electromagnet  60  commonly utilized. Accordingly, the controller  90  can have different valve control modes. For example, as shown in a graph of  FIGS. 7A, 7B  and  7 C, following modes can serve as examples. 
      a) Three-step (sequential) control mode: from a state where both of the first supply valve  20   a  and the second supply valve  20   b  are closed, at first only the first supply valve  20   a  is opened, and after that the second supply valve  20   b  is opened.     b) Two-step (simultaneous) control mode: from a state where both of the first supply valve  20   a  and the second supply valve  20   b  are closed, the first supply valve  20   a  and the second supply valve  20   b  are opened simultaneously.     c) Linear control mode (arbitrary acceleration of rotation): from a state where both of the first supply valve  20   a  and the second supply valve  20   b  are closed, at first only the first supply valve  20   a  is opened, and after that the second supply valve  20   b  is intermittently opened by pulse width modulation (PWM) control. Optimum selection from such various control modes enables proportional rise in rotational frequency of the fan for required airflow. In addition, optimum selection from such various control modes can vary a response time until the fan achieves necessary fan rotational frequency.    

      In the embodiment described above, variation of biasing force (spring constant) at the time when each valve moves to open produced valve states from the first state to the fourth state on the basis of one common electromagnet  60  as described above. Instead, in a second embodiment of the present invention, the same effect can be obtained from constant biasing force and different size of areas, at which magnetic force is exerted, between the valve-pulling portion  66  of the electromagnetic actuating means  6  and the pulled portions  22  and  32  of respective valves. In other words, the pulled portions  22  and  32  are configured so that, even in a situation where current applied to the electromagnet  60  is the same, magnetic force exerted to the pulled portions  22  and  32  from the valve-pulling portion  66  is different. Thus, the valve states from the first state to the fourth state are produced.  
      As an example thereof, as typically illustrated in  FIGS. 8A, 8B  and  8 C, the valve-pulling portions  66 , which face the pulled portions  32  of the first return valve  30   a  and the second return valve  30   b  respectively, is configured so that, the areas, at which magnetic force is exerted, are sufficiently large and the first return valve  30   a  and the second return valve  30   b  are opened in a situation where electricity, of which a level of current is the small current value described above, is applied to the electromagnet  60 . Further, the valve-pulling portion  66 , which faces the pulled portion  22  of the first supply valve  20   a , is configured so that the area, at which magnetic force is exerted, is slightly smaller and the first supply valve  20   a  is opened in a situation where electricity, of which a level of current is the medium current value described above, is applied to the electromagnet  60 . Further, the valve-pulling portion  66 , which faces the pulled portion  22  of the second supply valve  20   b , is configured so that the area, at which magnetic force is exerted, is still smaller and the second supply valve  20   b  is opened in a situation where electricity, of which a level of current is the large current value as described above, is applied to the electromagnet  60 . By doing so, all of operations and effects of the valve control explained in the first embodiment can also be obtained in the fluid coupling according to the second embodiment.  
      Variation examples of two embodiments described above will be explained. 
      1) In a first variation example, as typically illustrated in  FIGS. 9A and 9B , a diameter of the exit of the first supply passage  9   a  is set to a value different from that of the second supply passage  9   b . For example, the diameter of the exit of the second supply passage  9   b  (refer to  FIG. 9B ) is set to be larger than that of the first supply passage  9   a  (refer to  FIG. 9A ), thus creating different supply passage flow rates. By this, a wide difference can be produced in the amount of circulation of the fluid between a state (third state) where the fluid can flow only in the first supply passage  9   a  and a state (fourth state) where the fluid can flow in both of the first supply passage  9   a  and the second supply passage  9   b . Accordingly, a wide difference in rotational frequency of the fan can be obtained.     2) In a second variation example, one exit to the operation chamber  5   a  is provided at the first supply passage  9   a  at an inner peripheral side of the labyrinth portion  8  (refer to  FIG. 10A ), and two exits to the operation chamber  5   a  are provided at the second supply passage  9   b  at inner and outer peripheral sides of the labyrinth portion  8  (refer to  FIG. 10B ). By doing so, in a situation where the fluid is permitted to flow in the second supply passage  9   b  in addition to the first supply passage  9   a  (fourth valve state), large torque transmitting function is generated in the labyrinth portion  8 , which can heighten rotational speed of the fan, thus resulting in rapid cooling.    

      An externally-controlled fluid coupling according to the two embodiments and the variation examples of the present invention were explained. The embodiments and the variation examples can be applied solely or in arbitrary combinations. Further, examples of application of the externally-controlled fluid coupling technique are not limited only to the embodiments and variation examples described above. Configurational and functional variations can be made for various kinds of configuration elements, such as supply passages, return passages, supply valves and return valves provided thereat and an electromagnetic actuating means, within a frame of the invention.  
      According to a first aspect of the present invention, an externally-controlled fluid coupling includes a driving disk fixed to a driving rotational shaft, a case member rotatably supported by the driving rotational shaft, a cover member hermetically connected to the case member to form an inner space between the case member and the cover member, a separating plate for separating the inner space into an operation chamber accommodating the driving disk and a storage chamber for storing a fluid, a supply passage for supplying the fluid from the storage chamber to the operation chamber, a return passage for returning the fluid from the operation chamber to the storage chamber, a supply valve for opening/closing the supply passage, a return valve for opening/closing the return passage and an electromagnetic actuating means for actuating the supply valve and the return valve to open/close on the basis of a control signal from a controller. For independently controlling a flow rate of the fluid flowing in the supply passage and the return passage in a simple configuration, in the externally-controlled fluid coupling according to the aspect, the electromagnetic actuating means includes an electromagnet commonly utilized for actuating the supply valve and the return valve. The electromagnet generates a first power of magnetism for opening or closing the supply valve and a second power of magnetism for opening or closing the return valve. The values of the first and second powers of magnetism are different from each other so that the supply valve and the return valve open/close nonsynchronously (in arbitrary timings including simultaneous).  
      In this configuration, because power of magnetism generated by one common electromagnet causes to open/close the supply valve and the return valve, configuration of the electromagnetic actuating means can be simple. Further, because the electromagnet generates the first power of magnetism for opening or closing the supply valve and the second power of magnetism for opening or closing the return valve and the values of the first and second powers of magnetism are different from each other, the supply valve and the return valve can open/close nonsynchronously, such that generation of a first predetermined power of magnetism causes to open only the return valve and generation of a second predetermined power of magnetism causes to open not only the return valve but also the supply valve. By doing so, balances between the amount of the fluid returned from the operation chamber to the storage chamber and the amount of the fluid supplied from the storage chamber to the operation chamber can be varied. Accordingly, plural control results, such as multistage control of output rotational frequency, can be available.  
      For nonsynchronously opening/closing the supply valve and the return valve, according to a second aspect of the present invention, the supply valve includes a spring for exerting biasing force to close the supply valve, the return valve includes a spring for exerting biasing force to close the return valve, each of the supply valve and the return valve is opened in a situation where magnetic force exerted by the electromagnetic actuating means (electromagnet) is larger than the biasing force and the biasing force is differently set between the spring of the supply valve and that of the return valve. Biasing force can be easily changed by change of a spring constant determined from a cross-sectional shape, or the like. Accordingly, cost for manufacturing such a valve can be restricted to be low. For example, in a situation where a valve, which is opened from a closed state by weak magnetic force, and another valve, which is opened from a closed state by strong magnetic force, are prepared, selective generation of two strong/weak magnetic force correspondent thereto enables to sequentially open/close the supply valve and the return valve. Further, in a situation where there are plural supply valves and a return valve, or in a situation where there are a supply valve and plural return valves, or in a situation where there are plural supply valves and plural return valves, appropriate selection of a spring constant of each valve enables various output rotational frequency controls.  
      For opening/closing the supply valve and the return valve nonsynchronously, according to a third aspect of the present invention, the supply valve includes a spring for exerting biasing force to close the supply valve, the return valve includes a spring for exerting biasing force to close the return valve, each of the supply valve and the return valve is opened in a situation where magnetic force exerted by the electromagnetic actuating means (electromagnet) is larger than the biasing force and an area of the supply valve, the area at which magnetic force is exerted by the electromagnetic actuating means, is set to be different from an area of the return valve, the area at which magnetic force is exerted by the electromagnetic actuating means. In this configuration, biasing force of the springs are substantially the same between the valves. Instead, a size of the area, at which magnetic force is exerted, of the valves are varied so that magnetic force exerted to the valves, in other words, magnetic pulling force for operating the valves against the biasing force, becomes different even in a situation where power of magnetism generated by the electromagnetic actuating means is the same. For example, in a situation where a valve to which 100% of magnetic pulling force is exerted on the basis of a predetermined power of magnetism generated by the electromagnetic actuating means and a valve to which 50% of magnetic pulling force is exerted on the basis of the predetermined power of magnetism generated by the electromagnetic actuating means are prepared, appropriate generation of appropriately selected two different power of magnetism enables to open/close the valves sequentially. Here also, in a situation where plural supply valves and a return valve are provided, or in a situation where a supply valve and plural return valves are provided, or in a situation where plural supply valves and plural return valves are provided, appropriate selection of a size of the area, at which magnetic force is exerted, of each valve (in other words, selection of magnetic pulling force), enables to various output rotational frequency controls.  
      In a situation where the valve configuration described above, in which the supply valve and the return valve are opened/closed nonsynchronously with use of one common electromagnet, is employed, according to a fourth aspect of the present invention, the controller for giving a control signal to the electromagnetic actuating means includes a simultaneous mode to open/close the supply valve and the return valve simultaneously and a sequential mode to open/close the supply valve and the return valve sequentially. By doing so, stepwise control of output rotation or variable control of acceleration/deceleration can be easily available. At this time, according to a fifth aspect of the present invention, it is preferable that the power of magnetism generated by the electromagnetic actuating means is selected between maximum and minimum values in the simultaneous mode and the power of magnetism generated by the electromagnetic actuating means is selected stepwise in the sequential mode.  
      For realizing various output rotational frequency controls, according to a sixth aspect of the present invention, an externally-controlled fluid coupling includes a driving disk fixed to a driving rotational shaft, a case member rotatably supported by the driving rotational shaft, a cover member hermetically connected to the case member to form an inner space between the case member and the cover member, a separating plate for separating the inner space into an operation chamber accommodating the driving disk and a storage chamber for storing a fluid, plural supply passages for supplying the fluid from the storage chamber to the operation chamber, a return passage for returning the fluid from the operation chamber to the storage chamber, plural supply valves for opening/closing the supply passages respectively, a return valve for opening/closing the return passage, an electromagnetic actuating means for actuating the supply valves and the return valve on the basis of a control signal from a controller. An electromagnet generates a first power of magnetism for opening or closing one of the plurality of supply valves and a second power of magnetism for opening or closing another of the plurality of supply valves. The values of the first and second powers of magnetism are different from each other so that the plurality of supply valves open/close nonsynchronously (in arbitrary timings including simultaneous).  
      In this configuration, according to a seventh aspect, the plural supply passages are provided, plural supply valves are provided for opening/closing the supply passages respectively and the plural supply valves are opened/closed nonsynchronously. By doing so, the optimum amount of the fluid supplied from the storage chamber to the operation chamber can be set according to operational conditions.  
      For various control of the amount of the fluid supplied from the storage chamber to the operation chamber with use of supply passages nonsynchronously controlled by the respective supply valves, according to an eighth aspect of the present invention, one of the plural supply passages has an inner diameter different from that of another of the plural supply passages. Further, for supplying the fluid from the storage chamber to different suitable areas of the operation chamber, one of the plural supply passages includes plural exits to the operation chamber. By doing so, reliable output rotation control can be performed.  
      The principles, preferred embodiment and mode of operation of the present invention, have been described in the foregoing specification. However, the invention that is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents that fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.