Abstract:
A communication device which has high cooling efficiency and can be reduced in size and weight. The communication device in which a plurality of electronic circuit boards are mounted to a back wiring board (BWB) includes a casing containing the electronic circuit boards as a unit, an intake fan unit for introducing air into the casing to cool the electronic circuit boards, and a discharge fan unit having a plurality of blower fans each arranged such that air intake and discharge directions thereof form an angle of nearly 90 degrees, the blower fans discharging the air introduced into the casing. Combined use of an axial fan and the blower fans permits air to be introduced into and discharged from the casing from rear and front sides, respectively, of the casing, whereby spaces that the fan units occupy in the communication device can be reduced.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    This invention relates to a communication device provided with a plurality of electronic circuit boards, such as a platform information communication device, a cooling fan unit and an operation control method for the cooling fan unit, and more particularly, to a communication device comprising multiple electronic circuit boards mounted to a back wiring board (BWB).  
           [0003]    (2) Description of the Related Art  
           [0004]    A conventional cooling structure for an electronic unit is disclosed in Japanese Unexamined Patent Application No. 05-259673, for example. According to this publication, a cooling system is constructed by mounting a cooling fan to an electronic unit constituted by a combination of intense heat-generating communication devices, such as electronic exchanges, such that air is forced to flow through the interior of the electronic unit. In this cooling system, the intense heat-generating communication devices are provided with fins, in order to reduce the size of the heat radiator of the electronic unit and thereby reduce the size and weight of the unit, and cooling air is forced to flow along the fins by an axial fan.  
           [0005]    [0005]FIG. 19 is a side view schematically showing the construction of a communication device using a conventional cooling system.  
           [0006]    The cooling system for the communication device  1  has axial fans arranged on cooling air inlet and outlet sides, respectively, and is generally called push-pull type. A fan unit  100  constituted by two axial fans  101  and  102  serves to suck cooling air coming from an air inlet  110  arranged at a lower portion of the device into the interior of the communication device  1 . A discharge fan unit  200  is adapted to dispel heat to outside from an air outlet  210  arranged at an upper portion of the communication device  1 , and is constituted by two axial fans  201  and  202 .  
           [0007]    A plurality of electronic circuit boards (not shown), for which the cooling system is provided, are inserted as a plug-in unit (PIU) into the communication device  1  from the front side thereof such that the boards are directed vertically between the two fan units  100  and  200  and arranged side by side. Arrow A appearing in the left part of FIG. 19 indicates the direction of insertion of the electronic circuit boards.  
           [0008]    In the conventional cooling system, the four axial fans  101 ,  102 ,  201  and  202  are operated at all times to function as a push-pull type fan system so that heat can be dispelled to outside from the communication device  1 . Thus, the communication device  1  needs to include spaces for containing the two fan units  100  and  200 , respectively, a space for containing a shielding plate  111  which allows air to be introduced into the intake fan unit  100  from the air inlet  110  at the lower portion of the front of the communication device, and a space for containing another shielding plate  211  which guides the air from the discharge fan unit  200  toward the air outlet  210  at the upper portion of the back of the communication device.  
           [0009]    In the push-pull type cooling system constituted by axial fans as described above, the cooling air is sucked in and discharged both in a horizontal direction, as indicated by arrows B and C, respectively, in FIG. 19. Accordingly, the communication device  1  inevitably has an increased size which is larger than the width or height of the electronic circuit boards by an extra amount corresponding to the thicknesses of the axial fans of the fan units  100  and  200  and the spaces allotted to the air inlet  110  and outlet  210 . Also, in the case of the conventional communication device  1  in which the electronic circuit boards as a plug-in unit are positioned horizontally and arranged one above another, the cooling air cannot be smoothly sucked in and discharged when a plurality of such communication devices are mounted on a rack.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is to provide a communication device having high cooling efficiency and permitting reduction of size and weight thereof, a cooling fan unit and an operation control method for the cooling fan unit.  
           [0011]    To achieve the object, there is provided a communication device having a plurality of electronic circuit boards mounted to a back wiring board (BWB). The communication device comprises a casing containing the electronic circuit boards as a unit, an intake fan unit for introducing air into the casing to cool the electronic circuit boards, and a discharge fan unit including a blower fan arranged such that air intake and discharge directions thereof form an angle of nearly 90 degrees, the blower fan discharging the air introduced into the casing.  
           [0012]    Also, to achieve the above object, there is provided a cooling fan unit for cooling a plurality of electronic circuit boards electrically connected by a BWB. The cooling fan unit comprises a fan unit casing containing a blower fan, a control circuit board arranged parallel with the BWB of the cooling fan unit and facing the BWB, for controlling operation of the blower fan, and a plug-in connector arranged in a predetermined position of the control circuit board such that the plug-in connector is connected to a connector on the BWB when the fan unit casing is mounted to a cooling position of the electronic circuit boards.  
           [0013]    Further, to achieve the above object, there is provided an operation control method for a cooling fan unit having a blower fan which is fitted in a casing containing an electronic circuit board for cooling interior of the casing. The cooling fan unit operation control method comprises the step of temporarily increasing starting torque of the blower fan only for a short period of time immediately after operation of the cooling fan unit is started, to cause rotation speed of the blower fan to reach a steady-state value in an early stage after start of the blower fan.  
           [0014]    The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic plan view of a communication device equipped with a cooling system which has a push-pull arrangement constituted by blower fans and an axial fan;  
         [0016]    [0016]FIG. 2 is a perspective view showing external appearance of the communication device;  
         [0017]    [0017]FIG. 3 is a diagram showing distribution of wind velocity vectors in the internal space observed when the rotation speeds of the blower fans are set to an identical value;  
         [0018]    [0018]FIG. 4 is a diagram illustrating adjustment of wind velocity vectors by means of a baffle plate;  
         [0019]    [0019]FIG. 5 is a diagram showing suction vectors observed when the axial fan is stopped;  
         [0020]    [0020]FIG. 6 is a perspective view showing external appearance of a fan unit including three blower fans;  
         [0021]    [0021]FIG. 7 is a side view of the fan unit, showing discharge vectors of the respective blower fans;  
         [0022]    [0022]FIG. 8 is a diagram showing distribution of wind velocity vectors observed when the rotation speeds of the blower fans are set to different values;  
         [0023]    [0023]FIG. 9 is a sectional plan view showing an attachment/detachment mechanism for the blower fans;  
         [0024]    [0024]FIG. 10A is a sectional side view showing the fan unit before a plug-in connector is connected and FIG. 10B is a partial enlarged sectional view of FIG. 10A;  
         [0025]    [0025]FIG. 11 is a sectional side view showing the fan unit after the plug-in connector is connected;  
         [0026]    [0026]FIG. 12 is a block diagram illustrating the function of a booster circuit;  
         [0027]    [0027]FIG. 13 is a graph showing change in rotation speed of the blower fan equipped with the booster circuit;  
         [0028]    [0028]FIG. 14 is a block diagram showing an exemplary booster circuit;  
         [0029]    [0029]FIG. 15 is a flowchart showing a procedure for rotation control of the axial fan and the blower fans;  
         [0030]    [0030]FIG. 16 is a diagram showing a sequential control circuit for driving the blower fans and the axial fan;  
         [0031]    [0031]FIG. 17 is a graph showing temperature sequences;  
         [0032]    [0032]FIG. 18 is a table showing the rotation speeds of the blower fans and axial fan in individual operation modes; and  
         [0033]    [0033]FIG. 19 is a side view showing a schematic construction of a communication device using a conventional cooling system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Embodiments of the present invention will be hereinafter described with reference to the drawings.  
         [0035]    The invention will be described first with reference to a communication device in which a plug-in unit is mounted horizontally.  
         [0036]    [0036]FIG. 1 is a schematic plan view of a communication device equipped with a cooling system which has a push-pull arrangement constituted by blower fans and an axial fan.  
         [0037]    The cooling system of the communication device  1  differs in construction from the conventional cooling system in that it includes three blower fans  11 ,  12  and  13  as a pull-side fan unit  10 . On the cooling air inlet side, however, a fan unit  20  including an axial fan  21  is used. The fan unit  20  is arranged on the right side of the back of the communication device  1 , and the fan unit  10  including the blower fans  11 ,  12  and  13  is inserted in the left-hand portion of the communication device  1  from the back thereof.  
         [0038]    A plurality of electronic circuit boards, for which the cooling system is provided, are illustrated as a plug-in unit  30  which is inserted into an internal space  70  of the communication device  1  from a direction indicated by arrow A such that the electronic circuit boards are mounted horizontally inside a casing  7 . The communication device  1  has an air duct  61  communicating with an air inlet  73 , and cooling air indicated by air flow vector B is introduced from the fan unit  20  into the air duct  61  and the internal space  70  of the casing  7 . The internal space  70  of the casing  7  is an area where the electronic circuit boards are accommodated when the plug-in unit  30  is inserted in the communication device  1 , and the air duct  61  is formed adjacent to the internal space  70  of the casing  7 . A baffle plate  2  is arranged in the air duct  61  in the vicinity of the fan unit  20  to divide the air flow introduced into the casing  7 , as described later. Another baffle plate  3 , separate from the baffle plate  2 , is arranged in the air duct  61  at a distance from the fan unit  20 .  
         [0039]    A back wiring board (BWB)  4 , on which power supply lines etc. are wired, is arranged in the internal space  70  and fixed to the communication device  1  near the back thereof. The BWB  4  permits a plurality of electronic circuit boards to be horizontally mounted thereto at predetermined intervals, and a part thereof is cut away to admit the air from the fan unit  20 . The back of the communication device  1  is covered with a cover  5  which is attached to the casing  7  through a pivot  51  to be opened and closed. A space  40  for wiring is defined between the cover  5  and the rear side of the BWB  4 .  
         [0040]    The cooling air introduced into the casing  7  from the air inlet  73  at the back of the device passes between the electronic circuit boards and is drawn horizontally from the air duct  61  by the blower fans  11 ,  12  and  13  of the fan unit  10 . The fan unit  10  is removably inserted from the back of the device into an air duct  62  located on the left side of the casing  7  as viewed from the front thereof, and is positioned in the air duct  62  such that a predetermined gap G exists between the fan unit  10  and the internal space  70 . An attachment/detachment mechanism of the fan unit  10  will be described in detail later.  
         [0041]    Arrows D 1 , D 2  and D 3  appearing in FIG. 1 indicate magnitudes of wind velocity vectors associated with the blower fans  11 ,  12  and  13 , respectively. The magnitudes of the wind velocity vectors D 1 , D 2  and D 3  are proportional to the quantities of air drawn by the respective blower fans  11 ,  12  and  13  of the fan unit  10 . Also, the directions of the wind velocity vectors D 1 , D 2  and D 3  in which air is drawn by the fan unit  10  are at right angles with the direction of the air flow vector B in which air is introduced from the axial fan  21  into the air duct  61 . The blower fans  11 ,  12  and  13  of the fan unit  10  have the function of discharging air heated inside the communication device  1  from an air outlet  74  formed on the left side of the front of the device  1  in a direction indicated by air flow vector C.  
         [0042]    Each of the electronic circuit boards has a baffle plate  30 P formed at a predetermined position thereof to regulate the flow of cooling air passing between the boards. Also, fixtures  71  and  72  for fixing the communication device  1  to a rack are secured to left and right portions of the back of the casing  7  by screws or the like.  
         [0043]    [0043]FIG. 2 is a perspective view showing external appearance of the communication device.  
         [0044]    Electronic circuit boards  31  to  36  constituting a six-tier plug-in unit are inserted into the communication device  1  from the front thereof. In this embodiment, heat generated by the electronic circuit boards  31  to  36  is dispelled by the three blower fans  11 ,  12  and  13  through the air outlet  74  formed on the left side of the front of the casing  7 .  
         [0045]    Thus, in the embodiment described above, the blower fans  11 ,  12  and  13  are used in combination with the axial fan  21  to constitute a push-pull type cooling system, whereby the extra lower and upper spaces required in the conventional device for containing the shielding plates  111  and  211  can be omitted from the communication device  1 . Also, the baffle plates  2  and  3 , the air duct  61 , etc. permit the cooling air taken in from the fan unit  20  to flow uniformly through the internal space  70  at a constant flow rate, and accordingly, increase in the temperature inside the communication device  1  can be prevented without fail.  
         [0046]    In cases where the cooling system is continuously operated, a problem described below arises in connection with a fan control circuit, as known in the art. The fan control circuit has the function of monitoring the rotation speed of each fan to detect deterioration of the fan motor and outputting an alarm if the rotation speed drops below a minimum speed, as described later. Also, at the start of the fan motor, the alarm signal needs to be masked so as not to be generated unnecessarily. The masking time is conventionally set to about 1.5 sec, which is sufficiently long for the axial fan  21 . However, the blower fans  11 ,  12  and  13  require a longer time for their motor rotation to become stable than in the case of the axial fan. Accordingly, an additional fan control circuit separate from that for the axial fan  21  is prepared and a longer alarm masking time is set therein so that an alarm may not be output at the start of the fan motor.  
         [0047]    [0047]FIG. 3 shows distribution of wind velocity vectors in the internal space observed when the rotation speeds of the blower fans are set to an identical value, and FIG. 4 illustrates adjustment of wind velocity vectors by means of the baffle plate.  
         [0048]    In the conventional push-pull type cooling system (FIG. 19) constituted by a plurality of fan units, the push-side fan unit  100  and the pull-side fan unit  200  are operated at different rotation speeds. Within the fan unit constituted by a plurality of fan motors like the fan unit  10 , however, the fan motors are controlled so that they may always be operated at the same rotation speed.  
         [0049]    In the case where the three blower fans  11 ,  12  and  13  constituting the fan unit  10 , as shown in FIG. 3, are operated at the same rotation speed, the blower fan  13  closest to the air outlet  74  shows the greatest wind velocity vector D 3 , the middle blower fan  12  shows the second greatest wind velocity vector D 2 , and the blower fan  11  farthest from the air outlet  74  shows the smallest wind velocity vector D 1 .  
         [0050]    The differences in magnitude among the wind velocity vectors D 1 , D 2  and D 3  are attributable to unevenness of air flow rate in the interior of the casing  7  and make it difficult to dispel heat generated by the horizontally mounted plug-in unit  30  to outside without fail, possibly causing local increase of temperature inside the communication device  1 . Accordingly, as shown in FIG. 4, the baffle plate  2  is arranged in the aforementioned position of the air duct  61  communicating with the air inlet  73 , to divide the air flow introduced into the casing  7 . In the illustrated example, the baffle plate  2  is positioned such that, provided the opening area of the inlet side of the fan unit  20  is 100, the area of the fan unit opening into the internal space  70  of the casing  7  is 85 while the area of the fan unit for diverting the air flow into the air duct  61  is 15. Consequently, even when the three blower fans  11 ,  12  and  13  are operated at the same speed, the wind velocity vectors D 1 , D 2  and D 3  can be adjusted so as to have an identical magnitude, and this was confirmed by the experiments.  
         [0051]    [0051]FIG. 5 shows suction vectors observed when the axial fan is stopped.  
         [0052]    Even while the axial fan  21  is stopped, cooling air flows into the communication device  1  from the air inlet  73  through spaces between the blades of the axial fan  21 , as shown in FIG. 5. Usually, the axial fan  21  is not always operated and the blower fans  11 ,  12  and  13  alone are operated. The axial fan is controlled such that only when the temperature in the communication device  1  rises to a level higher than or equal to a predetermined temperature or when the fan unit  10  is detached for replacement of blower fans, for example, the axial fan is temporarily operated. With the fan unit  20  constituted by the axial fan  21 , therefore, cooling air can be sucked in through the air inlet  73  even while the axial fan is stopped.  
         [0053]    Since cooling air can be taken in through the spaces between the blades of the axial fan  21  even while the axial fan  21  is at rest, no additional air inlet needs to be formed separately from the air inlet  73 . When the internal temperature is so high that the axial fan  21  is operated, cooling air is introduced from the same air inlet  73 .  
         [0054]    The cooling fan unit of the present invention will be now described taking the fan unit  10  of the communication device  1  as an example.  
         [0055]    [0055]FIG. 6 is a perspective view showing external appearance of the fan unit with three blower fans, and FIG. 7 is a side view of the fan unit and shows discharge vectors of the respective blower fans.  
         [0056]    As shown in the figures, the fan unit  10  communicating with the air outlet  74  of the communication device  1  is constructed such that the three blower fans  11 ,  12  and  13  are arranged at different levels, or in a staggered fashion.  
         [0057]    Let it be assumed that the plug-in unit  30  mounted horizontally to the communication device  1  contains electronic circuit boards in such a manner that the lower the board is located in the internal space  70 , the more electricity it consumes. The gap G is provided between the plug-in unit  30  horizontally positioned in the casing  7  and the fan unit  10 , as shown in FIG. 1. In addition, the three blower fans  11 ,  12  and  13  are arranged in a staggered fashion so that the cooling air introduced into the casing  7  may flow concentratedly toward the lower electronic circuit boards  31 ,  32 , etc., whereby the electronic circuit boards can be efficiently cooled. Also, since no extra space is needed above and below the fan unit  10 , the vertical size of the communication device  1  can be reduced with ease.  
         [0058]    As shown in FIG. 7, the blower fans  12  and  13  are arranged such that they discharge air in the same direction as the air outlet  74  with which the fan unit  10  communicates, and the blower fan  11  alone is arranged such that the air discharge direction thereof is at a right angle with that of the fan unit  10 . The blower fan  11  is arranged in this manner in order to permit reduction in the vertical size of the fan unit  10 , and to this end, an outlet duct  11   a  is arranged in communication with the outlet of the blower fan  11  to cause the discharged air to be directed to the air outlet  74  with which the fan unit  10  communicates.  
         [0059]    Thus, to keep the temperature in the communication device  1  at a constant value by means of the multiple blower fans  11 ,  12  and  13 , the blower fans are arranged in a staggered fashion, rather than in alignment, whereby not only the cooling target can be concentratedly cooled but the blower fans  11 ,  12  and  13  can be efficiently arranged inside the fan unit  10 .  
         [0060]    [0060]FIG. 8 shows distribution of wind velocity vectors observed when the rotation speeds of the blower fans are set to different values.  
         [0061]    In the fan unit  10  shown in FIG. 8, the blower fans  11 ,  12  and  13  are operated at rotation speeds R 1 , R 2  and R 3  (R 1 &gt;R 2 &gt;R 3 ), respectively. In this case, the wind velocity vectors D 1 , D 2  and D 3  associated with the three blower fans  11 ,  12  and  13  can be made equal even if the baffle plate  2  shown in FIG. 1 is not provided. Accordingly, where the blower fans  11 ,  12  and  13  of the fan unit  10  are operated at different rotation speeds, the interior of the casing  7  can be easily cooled by the cooling air flowing at a uniform rate therein, compared with the communication device of FIG. 1.  
         [0062]    [0062]FIG. 9 is a sectional plan view showing the attachment/detachment mechanism for the blower fans.  
         [0063]    The fan unit  10  is inserted into the left-hand air duct  62  of the casing  7  from the back of the communication device  1 , as mentioned above, and comprises a fan unit casing  14  containing the three blower fans  11 ,  12  and  13 , and a control circuit board  15  arranged on that surface of a rear end portion of the fan unit casing  14  which faces the BWB  4  in parallel therewith. A plug-in connector  16  for receiving external signal is arranged at a predetermined position on the control circuit board  15 , to allow the operation of the blower fans to be controlled in accordance with the external signal. The control circuit board  15  is arranged parallel with the BWB  4 , and therefore, as the fan unit  10  is inserted into the casing  7  up to a predetermined position, the plug-in connector  16  automatically connects with a connector  41  of the BWB  4 .  
         [0064]    Namely, the electronic circuit boards, which are to be cooled by the fan unit  10  and which are not shown in FIG. 9, are inserted from the front side of the BWB  4  and are electrically connected by the BWB  4 . The connector  41  is arranged on the rear side of the BWB  4  at a location connectable with the plug-in connector  16 , and accordingly, as the fan unit casing  14  is inserted in a direction indicated by arrow H until the rear end face thereof becomes flush with the fixture  71 , the two connectors are connected to each other and electrical connection thereof is established. Also, since the control circuit board  15  and the BWB  4  are arranged parallel with each other, the control circuit board  15  with a large size can be used depending on the area of the BWB  4 , thus permitting the plug-in connector  16  of the control circuit board  15  and the connector  41  of the BWB  4  to be located with a higher degree of freedom.  
         [0065]    [0065]FIG. 10A is a sectional side view showing the fan unit before the plug-in connector is connected. The figure shows only the rear end portion of the fan unit casing  14  taken along line X-X in FIG. 9.  
         [0066]    The fan unit  10  has a floating plate  17  slidably holding the control circuit board  15 . The floating plate  17  has circular holes  17   h  of predetermined size formed near the respective four corners thereof, and is held so as to be slidable in a plane parallel to the BWB  4  by means of the circular holes  17   h  and bolts  18   a  and  18   b  fixed to the fan unit casing  14  with spacers (not shown) therebetween. Also, guide pins  42  and  43 , each tapered at a distal end portion thereof, protrude from the BWB  4 , and guide holes  17   g  are formed through the floating plate  17  at locations corresponding to the respective guide pins  42  and  43 .  
         [0067]    As seen from the enlarged part shown in FIG. 10B, the guide holes  17   g  have a size corresponding to the diameter of the guide pins  43 . Accordingly, as the fan unit casing  14  is moved in a direction indicated by arrow H in the figure to be fitted into the casing  7  of the communication device  1 , the floating plate  17  is first allowed to slide freely in a direction indicated by arrow J and the plug-in connector  16  of the control circuit board  15  is finally positioned with respect to the connector  41  of the BWB  4 .  
         [0068]    [0068]FIG. 11 is a sectional side view showing the fan unit after the plug-in connector is connected.  
         [0069]    When the plug-in connector  16  connects with the connector  41  of the BWB  4 , the floating plate  17  to which the control circuit board  15  is fixed is positioned by the two guide pins  42  and  43 . Because of the spacers provided between the circular holes  17   h  and the respective bolts  18   a  and  18   b , there is play between the floating plate  17  and the fan unit casing  14 . The play serves to absorb errors caused during the assembling of the communication device  1  and of the fan unit casing  14 , and also permits the plug-in connector to be positioned relative to the connector  41  as the guide pins  42  and  43  are inserted through the respective guide holes  17   g  of the floating plate  17 . This not only lightens the labor associated with the connection of the connectors when the fan unit  10  is mounted, but prevents the plug-in connector  16  from developing defects such as bending of its connector pins.  
         [0070]    An operation control method for the cooling fan unit of the present invention will be now described taking, as an example, the aforementioned fan unit  10  for cooling the communication device  1 .  
         [0071]    [0071]FIG. 12 is a block diagram illustrating the function of a booster circuit.  
         [0072]    As shown in the figure, positive and negative power supplies  142  and  143  supply electric power to blower fan motors  10 M, a rotation speed control circuit  151 , and a booster circuit  152 . The rotation speed control circuit  151  controls the rotation speeds of the blower fan motors  10 M to a constant speed and is supplied with a boost signal from the booster circuit  152 .  
         [0073]    When the fan unit  10  starts to cool the communication device  1 , the aforementioned alarm masking time should preferably be made identical with that for the axial fan. Accordingly, the booster circuit  152  associated with the fan unit  10  is switched on for several hundred milliseconds immediately after the start of power supply so that a voltage higher than that for normal operation may be supplied to the blower fan motors  10 M only for the on-period, to thereby increase the rotation speeds up to a stable speed in a short period of time.  
         [0074]    [0074]FIG. 13 shows change in rotation speed of the blower fan provided with the booster circuit, wherein the horizontal axis indicates time and the vertical axis indicates rotation speed.  
         [0075]    Symbol t0 indicates the operation start time, and t1 indicates the alarm masking time for ordinary axial fans. In the case of an axial fan, the rotation speed reaches r2 at the time t2 and is stabilized, but in the case of a blower fan, the rotation speed does not even reach specified speed r0 (minimum guaranteed speed) at the time t1 and reaches stable rotation speed r2 as late as time t3. Accordingly, by switching on the booster circuit  152  shown in FIG. 12 up to the time t2, it is possible to shorten the time required until the rotation speed of the blower fan is stabilized. After the time t2 at which the stable rotation speed r2 is reached, the booster circuit  152  is switched off to supply the blower fans with a normal voltage. As a consequence, the alarm masking time for the blower fans can be set to the same length as that for the axial fan.  
         [0076]    [0076]FIG. 14 is a block diagram showing an exemplary booster circuit.  
         [0077]    In the booster circuit  152 , a series circuit of resistors R 1  and R 2 , a series circuit of a resistor R 4  and a transistor Q 1 , and a series circuit of a transistor Q 2  and a resistor R 5  are connected between positive and negative power supplies  142  and  143 . The transistor Q 1  has its base connected to the junction point between the resistors R 1  and R 2 , as well as to capacitors C 1  to C 3  and a resistor R 3 . The transistor Q 2  has its base connected to the junction point between the resistor R 4  and the transistor Q 1 , and outputs at its emitter the boost signal through a diode D 11 .  
         [0078]    By using the booster circuit  152 , it is possible to temporarily increase the starting torque of the blower fans only for a short period immediately after the start of operation of the cooling fan unit. Thus, where the blower fans are fitted into the casing containing electronic circuit boards, the rotation speeds of the blower fans can be made to reach a steady-state value in an early stage after the start of the fans, thereby shortening the time needed for the blower fans to reach the specified rotation speed r0 as well as the time required until the rotation speeds are stabilized at the start of the fans. Therefore, the booster circuit which is thus very simple in construction and is inexpensive has only to be incorporated into the cooling fan unit for cooling the communication device. Since a control circuit almost identical with that for the axial fan which is very often used in this type of device can be used for blower fans that require a longer starting time, the device can be reduced in size and cost.  
         [0079]    [0079]FIG. 15 is a flowchart showing a procedure for rotation control of the axial fan and the blower fans.  
         [0080]    In the cooling system whose discharge fan unit  10  is constituted by the blower fans  11 ,  12  and  13  as in the communication device  1  shown in FIG. 1, only the intake fan unit  20  for introducing cooling air into the casing  7  is operated for a short period of time after the start of operation of the cooling system, and after a predetermined pressure difference is created between the inside and outside of the casing  7 , operation of the blower fans  11 ,  12  and  13  is started, whereby the time needed for the blower fans to reach a steady-state rotation speed after the start thereof can be shortened, as in the case of using the aforementioned booster circuit.  
         [0081]    In Step ST 1  in FIG. 15, the main power supply is turned on. Then, in Step ST 2 , electric power is supplied only to the axial fan, to rotate the axial fan motor first, in Step ST 3 .  
         [0082]    At this point of time, the blower fans are not rotating, and therefore, cooling air is just introduced into the casing  7  of the communication device  1  without being discharged. Accordingly, after a lapse of about 1.5 seconds by which the axial fan reaches a stable rotation speed, the pressure in the casing  7  is relatively high compared with the outside pressure (Step ST 4 ). While in this state, electric power is supplied to the blower fans (Step ST 5 ), to rotate the blower fan motors (Step ST 6 ). Consequently, the difference in pressure between the inside and outside of the casing  7  serves as a mechanical boosting function to accelerate the blower fans, so that the blower fan motors reach the specified rotation speed in a short time.  
         [0083]    The following describes a method of switching operation modes of the cooling system in accordance with environmental conditions of the communication device, such as ambient temperature of the device and operating states of the fans, after the operation of the fan units constituting the cooling system is stabilized.  
         [0084]    [0084]FIG. 16 shows a sequential control circuit for driving the blower fans and the axial fan.  
         [0085]    As seen from FIG. 16, positive and negative power supplies  142  and  143  supply electric power to the blower fan motors  10 M and a rotation speed control circuit  151 , and also to the axial fan motor  20 M through a switching circuit  20 S. The blower fan motors  10 M are connected to a rotation speed detection circuit  153 . The detection circuit  153  and a specified rotation speed circuit  154  are connected to a rotation speed comparator circuit  155  thereby to monitor the operation states of the blower fan motors.  
         [0086]    A comparator  156  is connected with a threshold circuit  157  for outputting a temperature T2, to monitor the temperature input from a temperature sensor  44  for detecting the ambient temperature of the communication device  1 . Specifically, when the ambient temperature is higher than the temperature T2, the rotation speed control circuit  151  is instructed to operate the blower fans at a high rotation speed r2, and when the ambient temperature is lower than a temperature T1 (=T2−ΔT), the control circuit  151  is instructed to rotate the blower fans at a low rotation speed r1.  
         [0087]    The switching circuit  20 S is connected with a comparator  158  and the rotation speed comparator circuit  155 , to perform on/off control of the axial fan motor  20 M. The comparator  158  is connected with a threshold circuit  159  for outputting a temperature T4, thereby to monitor the temperature input from the temperature sensor  44  for detecting the ambient temperature of the communication device  1 . Specifically, when the ambient temperature is higher than the temperature T4, the switching circuit  20 S switches the operation of the axial fan to be operated at a predetermined rotation speed r3, and when the ambient temperature is lower than a temperature T3 (=T4−ΔT), the switching circuit switches the operation of the axial fan to be stopped. ΔT represents temperature hysteresis of the temperature sensor  44  and is usually set to about 5° C.  
         [0088]    [0088]FIG. 17 shows temperature sequences. In the figure, the horizontal axis indicates ambient temperature of the communication device, and the vertical axis indicates rotation speeds of individual fans constituting the fan units.  
         [0089]    In the case where the cooling fan unit is constituted by the axial fan  21  for introducing cooling air into the casing  7  of the communication device  1  in which the electronic circuit boards are contained and the blower fans  11 ,  12  and  13  for discharging the air introduced in the casing  7 , as shown in FIG. 1, the communication device  1  needs to be cooled in accordance with the ambient temperature of the device  1 . Specifically, in a normally air-conditioned environment (at temperatures below T2 (° C.)), the blower fan motors alone are rotated at low speed (rotation speed r1) in the state shown in FIG. 5, and in this case, the axial fan motor can be kept stopped.  
         [0090]    However, in an environment (at temperature T2 (° C.) and above) in which the air conditioning is not normal, the blower fan motors must be controlled to be rotated at high speed (rotation speed r2).  
         [0091]    Further, when the air conditioning is not normal and at the same time the ambient temperature is higher than or equal to a predetermined temperature (temperature T4 (° C.)), the axial fan motor is also controlled to be rotated at a predetermined speed (rotation speed r3) with the blower fan motors kept rotating at high speed.  
         [0092]    If a blower fan motor stops because of malfunction or the rotation speed thereof drops to or below the minimum speed r0, it is necessary that the axial fan motor be rotated at the predetermined speed (rotation speed r3) to cool the interior of the casing  7 , without regard to the ambient temperature of the communication device  1 .  
         [0093]    [0093]FIG. 18 is a table showing the rotation speeds of the blower fans and axial fan in individual operation modes.  
         [0094]    The table indicates four operation modes, as a method of controlling the operation of the cooling fan unit in case of emergency, and the rotation speeds of the individual fans in the respective modes. First to third modes (FAIL1 to FAIL3) are modes applicable to the case where the blower fan motor associated with any one of the three blower fans  11 ,  12  and  13  constituting the discharge fan unit has stopped or the rotation speed thereof has dropped to or below the minimum rotation speed. In these modes, the axial fan  21  is controlled to be rotated at the predetermined speed (rotation speed r3), and at the same time the administrator is notified of a given warning so that the malfunctioning blower fan may be replaced.  
         [0095]    During the replacement, the communication device  1  has no blower fans arranged therein. In this case, the cooling fan unit is operated in a different operation mode (REMOVE), as shown in the table, such that the axial fan  21  is rotated at the predetermined speed (rotation speed r3) In the illustrated example, the time period for which the stopped state is maintained is limited to three minutes.  
         [0096]    As described above, the communication device of the present invention requires no shielding plates, unlike the conventional push-pull type cooling system using only axial fans. Since the spaces for such shielding plates can be omitted, a space-saving communication device can be provided.  
         [0097]    Also, with the cooling fan unit and its operation control method according to the present invention, the operation of the fans can be properly controlled in various environments in accordance with the operating states of the fans. Accordingly, a communication device or the like can be efficiently cooled and also the power consumption of the cooling system can be reduced.  
         [0098]    The foregoing is considered as illustrative only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.