Abstract:
An improved fan useful for ventilating applications in motor vehicles features a modular structure, which facilitates quick replacement of any components likely to fail. A first module ( 110 ) is intended for permanent installation on the part that is to be cooled. A second module ( 110 ) is configured for quick engagement to and disengagement from the first module. The second module preferably comprises a hub ( 22; 362 ), an internal stator ( 60; 332 ) mounted on the hub, and one or more struts ( 74; 344 ) connecting the hub to a cylindrical casing part ( 76; 336 ) which surrounds but is spaced from the outside of the fan wheel ( 46; 348 ). The struts form a lattice ( 112 ) which can be easily grasped for swapping out the second module when repair or replacement becomes necessary. The fan has a Hall sensor ( 50 ) and a control circuit ( 156 ) which regulates fan speed according to PWM (Pulse Width Modulation) or DC voltage signals ( 164 ) supplied from outside and has means ( 186; 244 ) for generating a fault signal in the event of a fault state, and for sending the fault signal out on a control line ( 90 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The invention concerns, inter alia, an equipment fan having a fan wheel that is driven by an external-rotor motor whose internal stator is mounted on a hub. The invention preferably concerns a fan of this kind that can communicate with an external control device via a control line (“bus”).  
       BACKGROUND  
       [0002]     Equipment fans are often installed in inaccessible locations where subsequent replacement of the fan, e.g. for a repair, is very difficult. This applies in particular to land and water vehicles and aircraft.  
       SUMMARY OF THE INVENTION  
       [0003]     It is therefore an object of the invention to provide a modular fan structure which facilitates quick replacement of any failing components.  
         [0004]     According to the invention, this object is achieved by providing a housing containing non-wearing components, which releasably engages a replaceable module including an external rotor, fan wheel, a hub, an internal stator mounted on the hub, and at least one strut connecting the hub to a cylindrical casing. In a fan of this kind, the housing can be mounted on an object that is to be ventilated, since it usually contains only mechanical parts that are not subject to wear. The component having the fan wheel, external-rotor motor, and casing part, on the other hand, can easily be detached from said housing as necessary, and repaired or replaced with a new component of identical type. An exchange of this kind can be made in a very short period of time, so that damage due to failure of a fan does not result in extended downtime of the equipment being cooled by it.  
         [0005]     Another manner of achieving the stated object is to equip the motor with at least one signal line, through which control signals can be fed from outside to the motor, and through which a fault signal can be fed back from the motor to the outside, so that something can be done about the fault state. It enables rapid fault detection, and thus efficient replacement of a defective fan once a fault has been detected.  
         [0006]     Further details and advantageous refinements of the invention are evident from the exemplary embodiments, which are described below and depicted in the drawings, but which are not to be construed as a limitation of the invention.  
     
    
     BRIEF FIGURE DESCRIPTION  
       [0007]      FIG. 1  is an enlarged section through the right half of a first exemplary embodiment of a fan according to the invention;  
         [0008]      FIG. 2  is a plan view, viewed in the direction of arrow II of  FIG. 1 ;  
         [0009]      FIG. 3  is a side view of housing part  110  of  FIG. 4 , viewed in the direction of arrow III of  FIG. 4 ;  
         [0010]      FIG. 4  is a plan view of housing part  110 , viewed in the direction of arrow IV of  FIG. 5 ;  
         [0011]      FIG. 5  is a side view of housing part  110 , viewed in the direction of arrow V of  FIG. 4 ;  
         [0012]      FIG. 6  is a side view of the complete fan, viewed in the direction of arrow VI of  FIG. 7 ;  
         [0013]      FIG. 7  is a plan view of the complete fan, viewed in the direction of arrow VII of  FIG. 6 ;  
         [0014]      FIG. 8  is a side view of the complete fan, viewed in the direction of arrow VIII of  FIG. 7 ;  
         [0015]      FIG. 9  is a side view of the complete fan, viewed in the direction of arrow IX of  FIG. 7 ;  
         [0016]      FIG. 10  is a block diagram of a preferred circuit for remote control of a fan according to the invention via a control line (bus);  
         [0017]      FIG. 11  is a circuit diagram similar to  FIG. 10 , with further details;  
         [0018]      FIG. 12  is a plan view of an equipment fan  320  according to a second exemplary embodiment of the invention, viewed in the direction of an arrow XII of  FIG. 13 ;  
         [0019]      FIG. 13  is a side view, viewed in the direction of arrow XIII of  FIG. 12 ;  
         [0020]      FIG. 14  is a plan view, viewed in the direction of arrow XIV of  FIG. 13 ;  
         [0021]      FIG. 15  is a side view, depicted partly in section, which depicts the routing of the electrical connecting lines; and  
         [0022]      FIG. 16  shows a preferred exemplary embodiment of apparatus  150  of  FIG. 11 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  shows a greatly magnified section through the right half of an external-rotor motor  20 , the left half being essentially mirror-symmetrical thereto. To save drawing space, fan blade  46  and strut  74  are shown broken away. The motor has a hub  22 , made of a suitable plastic, that is configured integrally with a bearing support tube  24  in which an upper ball bearing  26 , a spacer  28  for the outer races, and a lower ball bearing  30  are arranged, which ball bearings support central shaft  32  of an external rotor  34 . The inner races of ball bearings  26 ,  30  are braced against one another by a compression spring  36  that is arranged between the inner race of ball bearing  26  and a rotor part  38 . The latter, as depicted, is mounted at the upper end of shaft  32  and carries a ferromagnetically soft ring  40  in which a rotor magnet  42  is arranged. Extending around ring  40  is an annular part  44  made of plastic, which is configured integrally with five fan blades  46 . Opposite lower end  48  of rotor magnet  42 , a Hall IC (Integrated Circuit)  50  is arranged on a circuit board  52  that carries electronic components for controlling motor  20  and for fault reporting. Hall IC  50  controls the current in motor  20  and serves as the sensor for its rotation speed.  
         [0024]     Central shaft  32  has, at its lower end, an annular groove  54  into which a holding part  56 , which is immobilized by means of a leaf spring  58  in bearing support tube  24 , resiliently engages.  
         [0025]     An internal stator  60  is mounted on the outer side of bearing support tube  24 . The stator has a lamination stack  62  in which a winding  68  is mounted by means of a coil carrier  64 ,  66 . One terminal  70  of winding  68  is depicted. It is soldered to a pin  72  that is mounted in coil former  66 .  
         [0026]     Hub  22  is configured integrally with struts  74  which join hub  22  to a substantially cylindrical casing part  76  that surrounds fan blades  46  radially with a spacing (cf.  FIG. 2 ). Struts  74  form a protective lattice that is depicted in  FIGS. 2 and 7  and that also serves as a grasping aid for inserting motor  20  into a housing ( FIGS. 3 through 5 ) or removing it therefrom.  
         [0027]      FIG. 2  shows a plan view in the direction of arrow II of  FIG. 1 . It is evident that six struts  74  are mounted on hub  22 , and join hub  22  to casing part  76 . Hub  22 , struts  74 , and casing part  76  are configured as an integral plastic part. Approximately at their midpoints, struts  74  are joined to one another by an annular strut  80  on which are applied an arrow  82  for the opening direction and an arrow  84  for the closing direction, as well as corresponding labels (OPEN, CLOSE).  
         [0028]     Three connecting lines  86 ,  88  (+ and −) and  90  (control line) are soldered on in the region of hub  22 , and guided from there via a T-shaped clamp part  92  on the outer side of casing part  76  and a further clamping part  94 , also on the outer side of casing part  76 , to a connector plug  96 . Also located on the outer side of casing part  76  are four radially protruding pegs  98  which serve as snap-lock pegs and are here arranged at equal spacings of 90 degrees.  
         [0029]     The module depicted in  FIGS. 1 and 2 , made up of external-rotor motor  20 , fan blades  46 , and tubular casing  76 , is labeled  100 . It constitutes a replaceable module which, in the event of a fault, can be quickly replaced as a complete unit with no need to remove the fan housing for that purpose.  
         [0030]      FIG. 4  is a plan view of the open side of a fan housing  110 . The latter has at its bottom a protective lattice  112  that is configured integrally with housing  110 , and it has a substantially cylindrical opening  114  for receiving the cylindrical casing part  76  ( FIG. 2 ). The contour of housing  110  is substantially square, e.g. having the standard dimensions 80×80 mm, but a thin-walled casing part  116  in which opening  114  is configured protrudes locally beyond this square contour. Openings  118 A,  118 B,  118 C,  118 D for the reception of pegs  98  ( FIG. 2 ) are provided in these protruding parts  116 A through  116 D.  
         [0031]      FIG. 3  depicts opening  118 A which is at the right side in  FIG. 4 , and which transitions laterally into a latch opening  120 A that has on the one side a resilient latch tongue  122 A and on the other side a resilient latch tongue  124 A.  
         [0032]      FIG. 5  depicts opening  118 B that is at the bottom in  FIG. 4 . It transitions laterally into a latch opening  120 B that has on the one side a resilient latch tongue  122 B and on the other side a resilient latch tongue  124 B. The other openings  118 C and  118 D are identical in configuration to opening  118 B, and the reference characters used for them are therefore identical, but have the letters C and D, respectively, added.  
         [0033]     In order to receive lines  86 ,  88 , and  90 , T-shaped part  92 , and clamping part  94 , cylindrical opening  114  has a radial enlargement  126  that extends over an angle of approximately 20 degrees. The cover of this enlargement is labeled  130  and is depicted in  FIG. 3 . Latching members  132  for the mounting of plug  96  are located next to this cover ( FIG. 2 ).  
         [0034]     Housing  110  has, at its corners, holes  136  for permanent mounting of this part onto a component that is to be cooled, e.g. a transmitter device; and it has two projecting pegs  138  for precisely fitted retention.  
         [0035]     Housing  110  is permanently installed on the part that is to be cooled. Module  100  ( FIG. 2 ) can then be inserted, after installation, into housing  110  and removed therefrom again if necessary, e.g. for repair.  
         [0036]      FIGS. 6 through 9  show the fan in its complete state and at approximately actual size. Module  100  is inserted into housing  110  and latched therein. This is done by pushing pegs  98  axially into openings  118 A- 118 D and then rotating module  100  a few degrees clockwise in the direction of arrow  84  (CLOSE). Pegs  98  thus snap into latch openings  120 A- 120 D, as shown clearly by  FIGS. 6, 8 , and  9 . Plug  96  is then snapped onto latching members  132 , as depicted in  FIGS. 6 through 9 .  
         [0037]     Removal of module  100  from housing  110  proceeds in the opposite sequence, i.e. module  100  is rotated a few degrees counterclockwise in the direction of arrow  82 , and then pulled axially out of housing  110 .  
         [0038]     As depicted in  FIG. 7 , a mark  122  is provided on casing part  76  and a mark  124  on casing part  116 C, and marks  122 ,  124  point toward one another when module  100  is correctly latched. This permits easy visual inspection at the acceptance check.  
         [0039]     For rotation of module  100 , the openings between radial struts  74  and annular strut  80  are configured so that a person&#39;s fingers can be introduced into these openings and the protective lattice can be used as a grasping aid. Be it noted that protective lattice  112  depicted in  FIG. 4  is arranged on one side of the complete fan, and protective lattice  74 ,  80  is arranged on the other side of the fan, so that the latter has a protective lattice on both sides, the two protective lattices preferably being made of plastic. Protective lattice  112  is configured integrally with housing  110 , and protective lattice  74 ,  80  integrally with tubular casing  76  and hub  22 .  
         [0040]      FIG. 10  shows an associated circuit. Motor  20  is depicted schematically on the right. It generates, by means of an apparatus  150 , i.e tacho-generator, a signal that corresponds to the actual rotation speed n ist , which is applied to a rotation speed controller  152 . Motor  20  is connected, in series with an output stage  154 , between lines  86  (+) and  88  (ground).  
         [0041]     In  FIG. 10 , output stage  154  is depicted symbolically as an npn transistor In  FIG. 11 , it is constituted by the two transistors  224 ,  226 . Motor  20  is controlled by a control device  156  that serves in general to make available an actuating signal for motor  20  and to evaluate a fault signal from motor  20 . Control device  156  can supply a PWM (Pulse Width Modulation) signal or a DC voltage control signal as the actuating signal.  
         [0042]     What serves to control the rotation speed of motor  20  is thus a DC voltage signal, or a PWM signal  164 , that is delivered by control device is  156  via control line  90  to motor  20 , converted there by a filter  158  into a DC voltage on a line  159 , and conveyed to rotation speed controller  152  as target value n soll . Alternatively, control can also be accomplished by means of a DC voltage that is conveyed to input  90  and can have values, for example, between 2 and 7 V. DC voltage n soll  on line  159  increases as the pulse duty factor pwm of PWM signal  164  rises. The following conditions apply:  
                                                       pwm &lt; 10%   Fan off           pwm = 30-85%   Working range of motor 20           pwm &gt; 95%   Fan off.                      
 
         [0043]     If connection  90 ′ from control device  156  to control line  90  is interrupted, rotation speed controller  152  would continuously receive a signal that would correspond to a PWM signal  164  having a pulse duty ratio of 100%, and motor  20  would run at maximum speed. To prevent this, a switching member  160  is provided that blocks output stage  154  in such a case, so that motor  20  receives no current and is shut off. The same is true of a pulse duty factor &gt;95% that is conveyed to control line  90 , and is also interpreted as a shutoff signal.  
         [0044]     If the fan is used in a motor vehicle, terminal  86  is connected to the positive pole of the vehicle battery (not depicted). Terminal  86  is connected to a filter  166  for EMI (electro-magnetic interference) protection, and a diode  168  is provided for protection against incorrect connection to the battery. Also provided is a capacitor  170  that supplies motor  20  with reactive power.  
         [0045]     A stabilized voltage of e.g. +7.7 V is generated on line  174  by way or an internal constant-voltage source  172 , and is filtered by a capacitor  176 . Hall IC  50 , which is controlled by permanent-magnet rotor  42  ( FIG. 1 ) and in turn controls output stage  154  via a connection  177  as a function of the position of said rotor, is connected to line  174 .  
         [0046]     A PTC (Positive Temperature Coefficient) resistor  180 , whose output signal is conveyed via a line  182  to rotation speed controller  152  and controls the latter to a rotation speed of zero if the temperature of motor  20 /output stage  154  exceeds a value that is critical for all components, e.g. 115 degrees C., is provided in thermal communication with motor  20  and output stage  154  (or with the two transistors  224 ,  226  in  FIG. 11 ).  
         [0047]     Provided in the connection from output stage  154  to ground  88  is a measuring resistor  184  at which there occurs, during operation, a voltage which is dependent on the current i of motor  20  and is conveyed to a control member  186 .  
         [0048]     If the voltage at resistor  184  becomes too high, control member  186  then generates at an output  188  a signal which blocks output stage  154  for e.g. 13 seconds, and it generates at an output  190  a signal which is conveyed to an npn transistor  192  and makes the latter conductive.  
         [0049]     The emitter of transistor  192  is connected to ground  88 , and its collector to control line  90 ; i.e. when transistor  192  is conductive, control line  90  acquires approximately the potential of ground  88 .  
         [0050]     In control unit  156 , line  90 ,  90 ′ is connected via a resistor  194  to the collector of an npn transistor  196  whose emitter is connected to ground  88  and to whose base the depicted PWM signal  164  is conveyed during operation.  
         [0051]     When control line  90  is connected through transistor  192  to ground  8  . . . , the effect is the same as if PWM signal  164  had a pulse duty ratio of 0%, and motor  20  is shut off. The same is true when a DC control voltage conveyed to input  90  assumes a value of zero.  
         [0052]     In this context, the collector of transistor  196  is connected via a resistor  198  to a node  200 , and the latter is connected to ground  88  via a resistor  202  and a capacitor  204  connected in parallel therewith.  
         [0053]     In normal operation, capacitor  204  becomes charged by the pulses of PWM signal  164  (for which see  FIG. 11 ). The result is to produce a non-zero positive potential at node  200 . If, however, transistor  192  becomes conductive because motor current i is continuously too high, the potential of node  200  is then reduced, and a FAULT signal is produced as a result.  
         [0054]     PWM pulses  164  thus travel via control line  90  to rotation speed controller  152 ; and in the event of malfunctions, the fact that transistor  192  becomes conductive allows a fault signal to travel in the opposite direction from motor  20  to control device  156 .  
         [0055]     To prevent an excessively high current i from flowing when motor  20  is started, the voltage at resistor  184  is also conveyed to a control member  208  which, when it responds, limits current i in output stage  154  to a defined value. Control member  186  is deactivated during starting, i.e. only starting current limiter  208  is active at that time.  
         [0056]     Line  188  is connected to the output of controller  152 , to the output of current limiter  208 , and to a diode member  209 . If controller  152 , control member  186 , or current limiter  208  generates a low potential at its output, diode member  209  then becomes conductive, reduces the voltage on line  177 , and thereby blocks output stage  154  completely or partially, so that either motor  20  receives zero current or (during starting) motor current i is limited.  
         [heading-0057]     Manner of Operation of  FIG. 10   
         [0058]     The target rotation speed of motor  20  is defined by means of a DC voltage (in this case 2-7 V) at input  90  or by means of pulse duty ratio pwm of PWM signal  164 . As long as the latter is less than 10%, motor  20  is stationary. In the range from 30 to 85%, the rotation speed increases. At a pulse duty ratio above 95%, the motor is switched off by way of switching member  160 , as already described.  
         [0059]     At startup, motor current i is limited by control member  208  to a defined maximum value, by the fact that diode member  209  correspondingly reduces the control signal for output stage  154  if starting current i becomes too high.  
         [0060]     If motor  20  becomes jammed, current i rises sharply; this overcurrent causes control member  186 , via diode member  209  and output stage  154 , to shut off motor  20  for e.g. 13 seconds and then to switch motor  20  on for e.g. two seconds in order to attempt a restart of the motor. This periodic switching on and off prevents motor  20  and its output stage  154  from overheating if motor  20  is prevented from rotating.  
         [0061]     The periodic signal generated in this context by control member  186  is also conveyed via line  190  to npn transistor  192 , and causes the latter to switch on and off periodically. As a result, the potential at point  90  also changes periodically and is transferred via control line  90 ′ to control device  156 , where it generates the FAULT signal already described.  
         [0062]      FIG. 11  shows a brushless motor  20  having two stator winding phases  220 ,  222  that are each connected in series with a power transistor  224  and  226 , respectively. For commutation, these are controlled in the usual way via their bases by Hall IC  50  ( FIG. 10 ); this is not depicted in  FIG. 11 . The base of transistor  224  is connected to the anode of a diode  228 , and that of transistor  226  to the anode of a diode  230 . The cathodes of diodes  228 ,  230  are connected to a line  232 . Line  232  is connected to the collectors of two npn transistors  234 ,  236  whose emitters are connected to ground  88 .  
         [0063]     When one of transistors  234 ,  236  becomes conductive, a connection is created from the base of transistors  224 ,  226  to ground, so that these transistors are blocked and motor  20  no longer receives current. If one of transistors  234 ,  236  becomes only partially conductive, it then reduces the base current of transistors  224 ,  226  so that motor current i correspondingly decreases. This occurs in the context of current limiting, principally when motor  20  is started.  
         [0064]     The emitters of transistors  224 ,  226  are connected to ground  88  via a node  240  and measuring resistor  184 . The potential at node  240  is conveyed via a resistor  242  to the base of transistor  236 , so that the latter acts as a current limiter: as the voltage at resistor  184  increases, transistor  236  becomes increasingly conductive and thereby limits motor current i, for example to a maximum value of approximately 0.5 A at startup.  
         [0065]     The potential at node  240  is also conveyed to the positive input of an operational amplifier  244 , whose negative input is connected to a node  246  that is connected via a resistor  248  to ground  88  and via PTC resistor  180  and a resistor  250  to line  174 .  
         [0066]     Output  252  of operational amplifier  244  is connected via a capacitor  254  (e.g. 2.2 uF) to the positive input, via a resistor  256  (e.g. 100 kOhm) to node  246 , via a resistor  258  to the base of transistor  234 , via a capacitor  260  (e.g. 1 nF) to ground  88 , and via a resistor  262  to the base of transistor  192 . The base of transistor  234  is also connected via a resistor  264  to ground  88 .  
         [0067]     If motor current i becomes continuously too high due to mechanical jamming of motor  20 , operational amplifier  244  switches its output  252  to High; as a result, transistor  234  becomes conductive and, as described, cuts off current to motor  20 . At the same time, transistor  192  is also switched on via resistor  262  and produces a low potential on control line  90 .  
         [0068]     Once operational amplifier  244  has switched over, it remains in that state for approximately 13 seconds because of the effect of capacitor  254  and then switches back into the state in which its output is low, so that transistors  192  and  234  are again blocked and motor  20  once again receives current. If the latter is still jammed, it is switched on for approx. two seconds and, if it does not start, is again made currentless for 13 seconds.  
         [0069]     If motor  20  becomes too hot because of overload and/or elevated ambient temperature (in summer), the resistance of PTC resistor  180  becomes high; the result is that the potential at node  246  drops and also that transistors  192  and  234  are switched on, and motor  20  is made currentless until the temperature at PTC resistor  180  has once again decreased sufficiently.  
         [0070]     Rotation speed controller  152  operates by comparing signals n ist  and n soll . It has for that purpose an operational amplifier  152 K to which these signals are conveyed. If the rotation speed of motor  20  is too high, output  270  of operational amplifier  152 K then becomes high, and that signal is transferred via a resistor  272  to the base of transistor  236 , makes it conductive, and thereby influences transistors  224 ,  226  so that motor current i (and thus the rotation speed of motor  20 ) decreases.  
         [0071]     Control line  90  is connected via a resistor  276  to line  174  and via a resistor  278  to a node  280  that is connected via a capacitor  282  to ground  88  and via a resistor  284  to the negative input of operational amplifier  152 K. That negative input is also connected via a resistor  286  to ground.  
         [0072]     Control line  90  is connected via a resistor  290  to the base of a pnp transistor  292  whose emitter, like the emitter of a pnp transistor  294 , is connected to line  174 .  
         [0073]     The collector of transistor  292  is connected via a resistor  296  to ground  88 , and via a capacitor  298  to its base. That base is also connected via a resistor  300  to the collector of transistor  294 , which is connected via a resistor  302  to the base of transistor  236 .  
         [0074]     When transistor  294  is conductive, it conveys a base current to transistor  236  and thereby blocks transistors  224 ,  226  so that motor  20  receives no current.  
         [0075]     As long as the pulse duty ratio of the PWM signal (cf.  164  in  FIG. 10 ) on control line  90  is in the range from 30 to 85%, capacitor  282  is continuously discharged by the PWM pulses to a sufficient extent that transistor  292  is kept conductive by the potential on control line  90  and consequently blocks transistor  294 .  
         [0076]     If the pulse duty ratio of the PWM signal on control line  90  exceeds a value of 95%, or if control line  90 ′ ( FIG. 10 ) is interrupted (which corresponds in effect to a pulse duty ratio of 100%), capacitor  282  is charged to a higher voltage that is determined by resistors  276 ,  278 ,  284 ,  286 ; as a result, transistor  292  is blocked, and transistor  294  becomes conductive and shuts off motor  20  in the manner described.  
         [0077]     An interruption of control line  90 ′ ( FIG. 10 ) therefore causes motor  20  to come to a stop, whereas without circuit  160  it would run at maximum speed.  
         [0078]     In this fashion it is possible to transfer signals via control line  90  in both directions, i.e. signals which control motor  20  (PWM signals  164  or a control DC voltage) in the direction toward motor  20 , and a fault signal (if motor  20  is rotating too slowly or is being prevented from rotating) in the opposite direction.  
         [0079]      FIGS. 12 through 15  show a second exemplary embodiment of an equipment fan  220  according to the present invention, which here is very small and has an outside diameter of approx. 4 cm. In  FIGS. 12 through 14 , a common reference scale of 1 cm is indicated by way of example in order to illustrate typical size relationships.  
         [0080]     Exactly as in the case of the fan shown in  FIGS. 1 through 9 , here again equipment fan  320  is assembled from two parts, namely an outer housing  322  which is equipped externally with a flange  324  that is configured integrally with a protective lattice  326 , and which has a substantially cylindrical opening  328  into which the actual fan  330  is inserted and locked.  
         [0081]     Fan  330  has a hub  332  that is connected via three struts  334  to a tubular outer part  336  whose outer side  338  fits with a sliding fit into opening  328 .  
         [0082]     Provided on outer side  328  with a 180-degree spacing are two radially projecting pegs  340 , of which only one is depicted (in  FIG. 13 ); provided in outer housing  322  to receive them are two guide openings  342  which in plan view (as in  FIG. 13 ) are approximately L-shaped, i.e. proceeding from a lateral orifice, this opening extends first axially and then radially in a portion  344  that tapers toward its end into a latch opening into which (as shown in  FIG. 13 ) peg  340  can be snap-locked. A wall portion  346  can yield elastically upon snap-locking or unsnapping. This solution is obviously simpler than the one shown in  FIGS. 1 through 9 .  
         [0083]     Fan  330  has five fan blades  348  that are mounted on an external rotor  360 . Three lines  364 ,  366 ,  368  are provided for electrical connection of internal stator  362 ; they lead in this case to an electronic system (not depicted) outside fan part  330 , since with such a small equipment fan the electronics would not have enough room in fan  330  itself. As  FIG. 15  shows, lines  364 ,  366 ,  368  are guided around two holding parts  370 ,  372  (on the outer side of tube  338 ) to a plug  374 . A label is designated  376 .  
         [0084]     For the reception of lines  364 ,  366 ,  368  and holding parts  370 ,  372 , outer housing  322  is here again equipped with a radial enlargement  380  whose cover is labeled  382 . Its radial extension allows fan part  330  to rotate in outer housing  322  to the extent necessary for locking and unlocking.  
         [0085]     In the interest of brevity, the reader is referred to the first exemplary embodiment ( FIGS. 1 through 9 ) for an explanation of the manner of operation of the second exemplary embodiment ( FIGS. 12 through 15 ). In the context of the second exemplary embodiment as well, fan part  330  can very easily be inserted into and removed from outer housing  322 , which in many cases represents a considerable simplification upon installation.  
         [0086]     Numerous variations and modifications are of course possible in the context of the present invention. For example, latch protrusions  94  can be provided on the inner side of opening  114 , and casing part  76  can have corresponding latch openings. In the context of  FIGS. 10 and 11 , functions that are not desired by the customer can be omitted, and additional functions can alternatively be added.  
         [0087]      FIG. 16  shows an embodiment for generating a signal corresponding to the actual rotation speed n ist  (cf.  FIGS. 10 and 11 ). Identical or identically functioning parts are labeled with identical reference characters.  
         [0088]     Circuit  150  comprises an amplification member in the form of a pnp transistor  400  (preferably BC856B) whose base is connected via a resistor  402  (preferably 1 kOhm) to positive line  86 ; an outcoupling apparatus  404 ,  406  in the form of two diodes  404 ,  406  (preferably BAV70), whose anodes are connected respectively to the sides of stator winding phases  220 ,  222  opposite to the side connected to positive line  86  and whose cathodes are connected to a node  408 ; a resistor  410  (preferably 39 kOhm) which is arranged between node  408  and the emitter of transistor  400 ; and a smoothing apparatus in the form of a capacitor  414  (preferably 100 nF), which capacitor  414  is arranged between the base and collector of transistor  400 . The collector of transistor  400  is connected via a resistor  418  (preferably 36 kOhm) to ground line  88 , in which context a rotation-speed-dependent voltage that is proportional to the rotation speed can be picked off at a node  412  between the collector of transistor  400  and resistor  418 .  
         [0089]     The base of transistor  400  is connected via resistor  402  to positive line  86 . As soon as one of transistors  224 ,  226  (for example, transistor  224 ) opens during operation, phase  220  operates in generator mode; and because of the voltage proportional to rotation speed n ist  that is induced in stator winding phase  220 , which voltage is added to the potential of positive line  86 , the potential at node  408  becomes greater than the potential on positive line  86 .  
         [0090]     As a result, transistor  400  (operating as an amplification member) becomes conductive, and a current flows through resistor  410 , transistor  400 , and resistor  418  to ground line  88 .  
         [0091]     This current has a ripple corresponding to the voltage induced in stator winding phase  220 . That ripple is eliminated by an alternating current feedback using capacitor  414 , so that a direct current which is proportional to the rotor rotation speed flows through resistor  418  to ground line  88 . A potential proportional to the rotor rotation speed is thus obtained at node  412 .  
         [0092]     The diode voltage of diode  420  is added to the potential at node  412  via diode  420  and resistor  422 , and the result is conveyed via output n ist  to operational amplifier  152  (cf.  FIG. 11 ).  
         [0093]     The advantage of this circuit  150  is that it functions independently of the magnitude of operating voltage  86  being used, and supplies a signal n ist  that is proportional to the instantaneous rotation speed of motor  20 .  
         [0094]     It will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the inventive concept. For example, features of one embodiment could be combined with features of another embodiment. Therefore, the invention is not limited to the specific embodiments shown and described, but rather is defined by the following claims.