Abstract:
In order to protect a brushless motor, heat generated at a switching device, in response to an abnormality, activates a protection device. The protection device includes a connecting line communicating between a source supply line and the switching device. The protection device is provided near the switching device such that it can sense heat at the switching device with a high degree of sensitivity. Thus, when excessive heat is generated at the switching device, the protection device reaches a specific temperature, whereby the source supply line becomes cut off from the switching device, thereby immediately stopping the operation of the brushless motor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a brushless motor that is mainly used in an air blower in an air conditioning system for vehicles. 
     Brushless motors in the prior, art include the one disclosed in Japanese Unexamined Utility Model Publication No. H2-139473. This brushless motor is provided with a rotor having a field magnet, a stator that generates a rotating magnetic field for the rotor, and a means for exciting the stator. Thus, in this brushless motor, a rotating magnetic field is generated by sequentially exciting a plurality of curved end portions arranged on the stator in a radial direction. The rotor is caused to rotate by the magnet provided on the rotor being repeatedly attracted and repulsed relative to the rotating magnetic field. 
     In this brushless motor, the means for exciting includes exciting coils that are wound around the stator core, and a means for switching the direction of the current running through the exciting coils. A plurality of field effect transistors (FETS) are normally used as the means for switching. These FETs control the direction of the current running to the exciting coils by controlling signals applied to the gate terminals of the individual FETs. Since a relatively large current runs through the exciting coils, a great quantity of heat is generated, normally reaching up to approximately 150° C., which necessitates that a heat radiator to be provided at each FET. 
     However, if a problem occurs in the brushless motor itself, e.g. an abnormality in the rotation rate, a circuit error or the like caused by excessive load, it is necessary to protect other normally operating portions from the problem. In particular, it is necessary to cut off communication between the source supply line and the exciting coils to stop the supply of power to the exciting coils so that the rotation of the brushless motor stops. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a brushless motor having a means for protection that protects the brushless motor by causing the means for switching to be cut off from the source supply line when heat is generated in a circuit due to an error and, in particular, when heat is generated at a field effect transistor, a power transistor or the like that is used as the means for switching. 
     Accordingly, the brushless motor according to the present invention comprises a rotating shaft, a rotor secured to the rotating shaft, a plurality of magnets provided at an internal circumferential surface of the rotor, a stator that generates a rotating magnetic field for the magnets, and exciting coil wound around the stator, a control circuit board having a plurality of means for switching the direction of the current supplied to the exciting coils, and a case housing that stores the control circuit. The brushless motor further comprises a means for protection provided on the control circuit board near the plurality of means for switching, which includes a connecting line between the source supply line and the means for switching, which connecting line melts and disconnects at a specific temperature to cut off the source supply line from the means for switching. 
     Thus, since the means for protection includes the connecting line that communicates between the source supply line and the means for switching, and is provided near the means for switching, the means for protection can sense heat generated at the means for switching with a high degree of sensitivity. Consequently, if the means for switching generates heat at or exceeding a specific level, causing the means for protection to become heated to a specific level, the communication between the source supply line and the means for switching is cut off, immediately stopping the operation of the brushless motor to achieve the object described above. 
     Moreover, according to the present invention, the case housing comprises a first case through which the rotating shaft passes, with the rotor and the stator mounted externally thereto. The control circuit board is mounted in the first case, and the means for heat discharge is located at the means for switching. The case housing further comprises a second case that interlocks with the first case to cover the control circuit board from below. The means for protection is provided on the control circuit board in the vicinity of the means for heat discharge and the means for switching, and extends toward the second case. 
     Thus, since the means for protection extends toward the second case, and is located on the control circuit board, the maintenance on the means for protection can be performed easily by simply removing the second case after the circuit is shut down by the means for protection. 
     Furthermore, according to the present invention, the means for protection includes a plate which is formed from an elastic conductive material and, in an elastically deformed state, the plate is fixed at its two ends to the source supply line and the side of the means for switching through solder that melts at a specific temperature. 
     Thus, when the solder is melted by the het generated at the means for switching, the plate cuts off communication between the source supply line and the means for switching due to the restoring force of the plate, thereby achieving the object described above. The plate is formed of a copper ally having a specific elastic coefficient, and it is desirable to use solder that melts within the range of approximately 160°˜180° C. 
     Moreover, the means for switching is only required to be capable of turning on off the current by a specific signal. Normally, power transistors or field effect transistors are employed as the means for switching. It is particularly desirable to use field effect transistors. In addition, the means for heat discharge is a heat radiator of aluminum or an aluminum alloy to which the field effect transistors are bonded via silicon oil, and is provided in the vicinity of the rotating range of the fan located at the first case and is exposed to the outside from the first case. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the invention and the concomitant advantages will be better understood and appreciated by persons skilled in the field to which the invention pertains in view of the following description given in conjunction with the accompanying drawings, which illustrate a preferred embodiment. In the drawings: 
     FIG. 1 is a cross section of the brushless motor in an embodiment of the present invention; 
     FIG. 2 is a schematic block diagram of the excitation circuit of the brushless motor in the embodiment of the present invention; and 
     FIG. 3 shows an example of the protection mechanism in the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following is an explanation of the preferred embodiment of the present invention in reference to the drawings. A brushless motor  1  in FIG. 1 may be employed, for instance, in an air blower in an air conditioning system for vehicles. This motor comprises a rotating shaft  2  to which a sirocco type fan is secured, a rotor  5  which is secured to the rotating shaft  2 , a stator  12  that generates a rotating magnetic field for the rotor  5 , a control circuit board  19  including an excitation circuit that supplies an exciting current to the stator  12 , and a case housing  45  that stores the control circuit board  19 . 
     The case housing  45  includes an upper case member  46 , a lower case member  50 , and a lid body  47 . The control circuit board  19  is located within case housing  45 . Provided at the upper case member  46  is a mounting portion  52  having a screw hole  51  for mounting of the brushless motor  1 , and an opening portion  57  through which a heat radiator  56  to which a plurality of field effect transistors (FETs)  55  are secured. These FETs  55  constitute switch devices or means for switching the direction of a current. A first case includes the upper case member  46  and the lid body  47 , and a second case is the lower case member  50 . 
     The rotating shaft  2  is rotatably held by bearings  16  and  17 . The bearings  16  and  17  are secured to upper and lower bearing holders  14  and  15  via a bearing stopper  33  together with a felt  34  that contains lubricating oil and is in contact with the bearings  16  and  17 . The bearing holders  14  and  15  are mounted within a through hole  13  of the stator. In addition, a sensor magnet  18  that indicates the positions of permanent magnets  11  of the rotor  5  is press-fitted to the lower end of the rotating shaft  2 . With the sensor magnet  18  held in the axial direction of the rotating shaft  2  by a push-nut  79 , the distance between the sensor magnet  18  and a plurality of Hall elements  20  mounted to the control circuit board  19  is maintained constant. 
     The sensor magnet  18  is mounted to the rotating shaft  2  by passing the sensor magnet  18  through an opening  21  formed in the control circuit board  19 . A flange portion  22 , which is formed at an end of the sensor magnet  18 , extends in a radial direction under the control circuit board  19 . The plurality of the Hall elements  20  are provided at the rear side of the control circuit board  19  and face the flange-portion. These Hall elements  20  accurately detect the positions of the permanent magnets  11  by detecting the magnetism of the sensor magnet  18 . And, a rotating magnetic field is generated at the stator  12  by means of the excitation circuit provided on the control circuit board  19  based upon he results of the detection. Provided between the sensor magnet  18  and the bearing  17 , is a washer group  23  comprising a plurality of washers to reduce the sling resistance between the sensor magnet  18  and the bearing. 
     The rotating shaft  2  is provided at its upper end with an interlocking end portion  3  for securing the fan  4 , and the rotor  5  is secured to the shaft below the interlocking end portion  3 . The rotor  5  includes a hub portion  7  which is press-fitted and secured to the rotating shaft  2 , an umbrella portion  9  that expands from the hub portion  7  and is provided with a plurality of ventilation holes  8 , a cylindrical portion  10  which extends downwardly from the outermost circumferential edge of the umbrella portion  9 , and a plurality of magnets  11  that are provided at the internal circumferential side surface of the cylindrical portion  10 . 
     The sliding resistance at a dust stopper  6 , which holds the rotor  5  in the axial direction, is reduced with a nylon-based washer  24  and a washer  25  of NBR rubber that are provided between the thrust stopper  6  and the bearing  16 . The thrust stopper  6  includes a cylindrical portion  27  having a through hold  26  through which the rotating shaft  2  is inserted, and a circumferential wall  29  that extends from the upper end of the cylindrical portion  27  in the radial direction along the umbrella portion  9  and extends downwardly outwardly from the circumferential edge of the cylindrical portion  27  over a specific width. Furthermore, at the lower level end surface of the cylindrical portion  27  an oil guide  30  is formed which gradually slopes downwardly and radially outwardly from the sliding contact surface between the cylindrical portion  27  and the washer  25 . 
     An upper end portion  32  of the bearing holder  14  is positioned in a space formed between the oil guide  30  and the circumferential wall  29  so that the lubricating oil dripping from oil guide  30  is reliably returned to the felt  34 , and so that dust particles from the outside are prevented from adhering to the bearing  16 . 
     The stator  12  comprises a stator core  35  which is formed by laminating silicon steel plates. Upper and lower insulating covers  36  and  37  are mounted to clamp the stator core  35  from above and below, and exciting coils  38  are wound around the stator core  35 . The stator core is insulated by upper and lower insulating covers  36  and  37 . The stator core  35  is provided with the through hole  13  at its center, within which the bearing holders  14  and  15  are mounted. Coil winding portions  40  extend in six directions from a circumferential wall  39  of the through hole  13 . Arch-shaped magnetic pole portions  41  that face the permanent magnets  11  of the rotor  5  are formed at the front end of the coil winding portions  40 . In this embodiment, the exciting coils  38  ( 38   a ˜ 38   c ) are delta-connected as shown in FIG. 2 . 
     Additionally, the lower insulating cover  37  is provided with leg portions  42  extending in six directions, the front ends of which are clamped by elastic members  43  and  44  and secured between the upper case member  46  and the lid body  47 . Thus, the stator  12  is secured to the case housing  45 , with the shaft  2  rotatable relative there. The lid body  47  is positioned by a pin  48  and is secured to the upper case member  46  with a screw  49 . 
     The excitation circuit provided on the control circuit board  19  in the brushless motor as described above may have the arrangement as shown in FIG.  2 . This excitation circuit comprises a Hall detection circuit  110  that detects the positions of the permanent magnets  11  at the rotor  5  as detected by the Hall elements  20  ( 20   a ,  20   b  and  20   c ). The circuit also comprises a three-phase logic circuit  120  that determines the direction of the rotating magnetic field relative to the position of the Hall detection circuit  110 , and selects a current supply pattern that will achieve the desired direction of the rotatating magnetic field. Also included is an output signal circuit  130  that outputs gate signals to the individuals FETs  55  ( 55   a ˜ 55   f ), i.e. switch devices, and uses the signal output from the three-phase logic circuit  120  along with an output pulse signal set by a rotation rate setting signal and the FETs  55  ( 55   a ˜ 55   f ). Accordingly, the FETs  55   a ˜ 55   f  at specific positions are sequentially turned on or off and the direction of the current supplied to the exciting coils  33   a ˜ 38   c  is switched to generate a rotating magnetic field at the stator  12 , thereby causing the rotor  5  to rotate together with the rotating magnetic field. Between a source-side wiring pattern  62  connected to the source supply line  90 , and a FET-side wiring pattern  61  connected to the FETs  55   a ˜ 55   f , a protection mechanism or device  60  is provided. R 1 , R 2  and R 3  indicate voltage dividing resistors at the Hall elements  20   a ˜ 20   c.    
     This protection mechanism  60  is mounted in the vicinity of the FETs  55  to the control circuit board  19  and extends toward the lower case member  50 . As shown in FIG. 3, this protection mechanism  60  comprises a supporting portion  64  that is inserted through a hole  70  in the control circuit board  19 . It further comprises a springup-side terminal portion  67  that contacts the FET-side wiring pattern  61  connected to the FETs  55  ( 55   ˜a ˜ 55   f ). Also included is a fixed-side terminal portion  66  that contacts the source-side wiring pattern  62  connected to the source supply line, and an elastic deforming portion  63  that connects the springup-side terminal portion  67  to the fixed-side terminal portion  66 . A retaining portion  65  is formed at the supporting portion  64 . In this protection mechanism  60 , the springup-side terminal portion  67  and the FET-side wiring pattern  61  press against the elastic deforming portion  63  to cause it to become elastically deformed, with the springup-side terminal portion  67  soldered to the FET-side wiring pattern  61 . Likewise, the fixed-side terminal portion  66  is soldered to the source-side wiring pattern  62 . Reference numbers  68  and  69  indicate solder which melts at a temperature in the range of, for instance, 160° C. through 180° C. 
     With the protection mechanism  60  if the FETs  55  generate heat at or exceeding a specific level, this heat is communicated through the FET-side wiring pattern  61  and melts the solder  69 . Because of this melting, the springup-side terminal portion  67  springs up due to the restoring force of the elastic deforming portion  63 , and thereby isolates the source-side wiring pattern  62  from the FET-side wiring pattern  61 . Thus, since the circuit extending to the exciting coils  38  via the FETs  55  is cut off from the source supply line  90 , the drive of the brushless motor  1  stops, achieving protection of portions operating normally. In addition, since the protection mechanism  60  is provided inside the case housing  45 , the solder melts when the temperature inside the case housing  45  reaches a level equal to or exceeding a specific level, thereby making it possible to achieve protection against a high temperature equal to or exceeding the specific level occurring due to some other cause. 
     As has been explained, since the means for protection is capable of shutting down the circuit depending upon the temperature, protection is achieved if the circuit portion is heated to a level equal to or exceeding a specific level and/or if the mechanical portion is heated to a level equal to or exceeding a specific level, thereby protecting both the mechanical portion and the circuit portion. 
     In addition, since the means for protection is provided on the control circuit board and extends toward the second case, maintenance can be performed with ease by simply removing the second case, thereby facilitating repair work required after eliminating the cause of an abnormality.