Abstract:
A pulse width modulated brushless electric motor employs a conductive metal screw or similar connector to create a common potential between the stator core and the iron substrate on which the motor drive circuit is mounted. By approximately setting the common potential to, for example, the base line of the power source and motor drive circuit, the noise electromagnetically generated by the pulse width modulation is suppressed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a brushless motor which is used as a capstan motor for VCRs and the like. More specifically, the present invention relates to a brushless motor driven by a PWM (Pulse Width Modulation) method. 
     RELATED ART 
     As a capstan motor for VCR, a brushless motor comprising the following is adapted as shown in FIG.  1 . Rotor  5  which rotates together with rotational shaft  10  is held by bearing  20 . Stator  30  has stator core  32  with drive coil  31  wound therearound. A motor drive circuit  60  is formed on metal substrate  40  and which has a drive IC. Brushless motor  1  employs the direct PWM method in which electric flow to drive coil  31  is controlled by turning on or off a power transistor (a switching component) of motor drive circuit  60  and modulating the width of a switching pulse to the switching component. 
     According to the PWM method, the amount of electricity wasted as heat generated in a conventional motor drive circuit can be dramatically decreased. In addition, energy is efficiently saved while driving the motor. Further, few new parts are needed when the method is adapted. Therefore, the method provides high cost-performance. 
     FIGS.  2 (A), (B),  3  and  4  describe the PWM method. FIG.  2 (A) shows a motor drive circuit of a brushless motor employing the direct PWM method supplying electric power from a source to a motor drive coil. FIG.  2 (B) shows the same when regenerative current is caused due to counterelectromotive force generated in the drive coil as the supply of the electric power from the source to the drive coil is suspended. FIG. 3 shows waveforms of voltage and current applied to the drive coil during one phase when the control operations as described in FIGS.  2 (A) and (B) are performed. FIG. 4 shows waveforms of voltage and current for the following periods within period (a) in FIG.  3 : period (b) in which voltage is applied to the drive coil; and period (c) in which application of voltage to the drive coil is suspended. 
     As shown in FIGS.  2 (A) and  3 , when power transistor Q 4  and power transistor Q 1  are on, voltage VM is applied from source  66  to drive coil  31  such that current flows in drive coil  31 . This current flows to ground M-GND of motor source  66  via power transistor Q 4  (during period b in FIG.  4 ). The motor current gradually increases, corresponding to the time constant of drive coil  31 , as shown in FIG.  4 . 
     On the other hand, as shown in FIGS.  2 (B) and  3 , when power transistor Q 1  is turned off while power transistor Q 4  is still on, application of voltage VM from motor source  66  to drive coil  31  is interrupted. However, counterelectromotive forces E 1  and E 2  are generated in each drive coil  31 . Hence, regenerative current flows in drive coil  31  via diode  61  as motor current. The regenerative current gradually decreases corresponding to the time constant of drive coil  31 , as shown in FIG.  4 . Before the regenerative current reaches the minimum value, power transistor Q 1  is turned on such that current is supplied from motor source  66 . 
     As described above, a part of the motor current is supplied by the regenerative current in brushless motor  1 . Therefore, the amount of current (electricity) supplied from the outside can be reduced. Also, the power transistors through which the motor current flows are constantly saturated; hence the amount of electricity consumed in motor drive circuit  60  can be minimized. 
     ISSUES TO BE SOLVED 
     However, in brushless motor  1  employing the direct PWM method, the voltage applied to drive coil  31  fluctuates between drive source voltage VM and ground potential M-GND in a short period of time. As a result, voltage applied to wiring between motor drive circuit  60  and coil  31  and voltage applied to coil  31  shows rapid fluctuations, causing electromagnetic noise which has various negative effects on the operation of apparatus having the motor. This electromagnetic noise is alleviated by motor parts, which form a capacitive coupling with drive coil  31  and the wiring, e.g. stator core  32 , around which drive coil  31  is wound, or an iron plate sandwiching an insulating layer with a wiring on metal substrate  40  as a circuit substrate of the motor, to help the electromagnetic noise to diffuse. 
     Further, the current from motor source  66  is supplied during only period b in FIG.  4 . It is suspended during period c which follows period b. Therefore, pulse current, which can be turned on or off with a PWM carrier frequency, flows through the wiring of motor source  66  on metal substrate  40 . This pulse current generates the electromagnetic noise to which is propagated by the stator core  32  and metal substrate  40  The pulse current also causes undesirable results in the operation of the apparatus by generating ripples in motor source  66 . 
     Considering the above issues, the present invention intends to provide a brushless motor employing the PWM method which has a configuration to suppress generation of electromagnetic noise. 
     To accomplish the above purpose, the present invention provides a brushless motor employing the direct PWM method which controls an electric flow to a drive coil by modulating the width of a switching pulse to a switching component wherein at least one of a metal plate forming a metal substrate and a stator core is short-circuited to a fixed electric potential. Also, an insulating resistance between a mounting portion of the motor to be connected to a chassis of a main body and the metal plate and an insulating resistance between the mounting portion and the stator core are established to be higher than 1K ohm. 
     According to the present invention, when a brushless motor is driven by the direct PWM method, both voltage applied to the wiring between a motor drive circuit and a drive coil and voltage applied to the drive coil itself show rapid fluctuations. The electric potentials of a metal plate as a base of a metal substrate and a stator core, which form capacitive coupling with the above parts are fixed. Hence, propagation of electromagnetic noise by those parts can be prevented. In the present invention, the fixed electric potential can be a ground potential of the motor drive circuit and the source potential. 
     It is preferable that a capacitor with a capacitance of 0.1 micro fared or higher, is electrically connected to the motor source in parallel at a position close to the motor drive circuit according to the present invention. In this configuration, even when pulse current flows through the wiring of the motor source, ripples in the motor source can be absorbed by the capacitor. This ensures that the apparatus can function optimally. 
     According to the present invention, a mounting portion is a bearing holder which is made of a conductive resin and which holds a bearing, for example. In the case that a rotor comprises a pulley, electrostatic potential may be generated due to the movement of the pulley and a connecting belt. Since the bearing holder is formed of a conductive resin, the electrostatic potential built up in the rotor including the pulley can be removed. Also, even when the stator core is held at the ground electric potential, no negative effects are seen in the operation of the apparatus due to the fact that the bearing holder is made of a conductive resin. Even though the bearing holder is fixed to a chassis of a main body of the apparatus, the chassis will not be short-circuited with ground electric potential M-GND. 
     In the present invention, the mounting portion can be a metal bearing holder holding a bearing. In this case, it is preferable that insulators are placed between the bearing holder and the metal plate and between the bearing holder and the stator core. 
     In the above case, the bearing holder has a hole, into which a member with a screw hole, made of a conductive resin, is press-fitted. It is preferable that the bearing holder and the chassis of the main body are fixed with a screw screwed in the hole. In this configuration, the bearing holder is made of a metal, and [the motor] is connected with the chassis via the member with a screw hole made of a conductive resin. Therefore, even when a pulley is formed on the rotor, electrostatic potential pooled in the rotor can be removed. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a side view with a partial longitudinal section of a brushless motor employing a direct PWM method according to the present invention. 
     FIG.  2 (A) shows a motor drive circuit of the brushless motor in FIG. 1 when electric power is supplied from a motor source to a coil. FIG.  2 (B) shows the same when a regenerative current is caused due to counterelectromotive force generated in the coil as the supply of the electric power from the motor source to the drive coil is suspended. 
     FIG. 3 illustrates waveforms of voltage and current applied to the coil during one phase when the control operations as described in FIGS.  2 (A) and (B) are performed in the brushless motor in FIG.  1 . 
     FIG. 4 shows waveforms of voltage and current for the periods within period (a) of FIG.  3 . In FIG. 4, period (b) is when voltage is applied to the coil and period (c) is when application of voltage to the coil is suspended. 
     FIG. 5 is a cross section of a brushless motor employing the direct PWM method according to Embodiment 1 of the present invention indicating formation of a short circuit with an iron substrate and a core. 
     FIG. 6 is a cross section of a brushless motor employing the direct PWM method according to Embodiment 2, indicating formation of a short circuit with the iron substrate. 
     FIG. 7 is a cross section of a brushless motor employing the direct PWM method according to Embodiment 3, indicating formation of a short circuit with the iron substrate and the core. 
     FIG. 8 is a cross section of a brushless motor employing the direct PWM method according to Embodiment 4, indicating formation of a short circuit with the iron substrate and core and a configuration of a bearing holder fixed to a chassis. 
     FIG.  9 (A) is a circuit diagram of a motor drive circuit in the brushless motor according to Embodiment 5, indicating a configuration to prevent trouble cased by ripples which are generated when the motor is driven by the direct PWM method. FIG.  9 (B) is a modified diagram of the above circuit diagram showing a preferred embodiment incorporating a capacitor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes embodiments of the present invention in reference to drawings. In the embodiments described below, basic configurations are common to the one of a conventional motor. Therefore, identical symbols are used for those common parts. 
     FIG. 1 is a side view with a partial longitudinal section of a brushless motor according to the present invention. 
     As shown in FIG. 1, brushless motor  1  is comprised of: rotor  5  having pulley  55  which rotates together with rotational shaft  10 , stator  30  having stator core  32  around which drive coil  31  is wound; iron substrate as a metal substrate and as a circuit substrate; motor drive circuit  60  having a circuit pattern and a drive IC and being placed on the bottom surface of iron substrate. Iron substrate  40  has a circuit pattern which connects motor drive circuit  60  and drive coil  31  wound around stator core  32 . 
     Cylindrical bearing holder  25 , mounted to iron substrate  40 , has a mounting portion for fixing the motor to a chassis of the main body of the apparatus with a screw. Brushless motor  1  can be mounted to the apparatus by fixing the mounting portion of bearing holder  25  to the chassis of the apparatus with a screw. 
     Bearing holder  25  is formed of a conductive resin herein; however, it can be also formed of a metal as described in Embodiment 4, later. In the case of bearing holder  25  formed of a conductive resin, a given conductivity can be provided by mixing conductive carbon into engineering plastic. 
     Magnetic sensor  7  is placed on iron substrate  40  via sensor holder  70  such that FG outputs can be obtained from a magnetic pattern formed on outer circumference  51  of rotor  5 . 
     A pair of bearings  20 , such as sintered bearings, are held on top of each other inside bearing holder  25 . Also, rotational shaft  10  passes through bearings  20 . Stator core  32  is held on the outer surface of bearing holder  25  by using a step formed on the outer surface of bearing holder  25 . Core  32  is covered by core holder  33  which is further covered by iron substrate. Iron substrate  40 , core holder  33 , and stator core  32  are fixed with screw  35 . Drive coil  31  is wound around stator core  32 , and a drive magnet (not shown in the figure) is mounted to the inner surface of rotor  5  facing the outer surface of stator core  32 , forming a motor. 
     In brushless motor  1  having a configuration described above, a part of motor current is substituted by regenerative current by switching conditions of motor drive circuit  60  from that power transistor Q 4  is on while power transistor Q 1  is on to that power transistor Q 4  is on while power transistor Q 1  is off. 
     However, in brushless motor  1  employing the direct PWM method as described above, the voltage applied to coil  31  fluctuates between the voltage VM from the drive source and ground potential M-GND in a short period of time. Therefore, rapid fluctuation of the voltage applied to the wiring on iron substrate  40  and the voltage applied to coil  31  occurs such that electromagnetic noise tend to result. The configuration of the above embodiment prevents propagation of the electromagnetic noise. 
     FIG. 5 is a cross section of a brushless motor employing the direct PWM method as described above in which an iron substrate and a stator core are short-circuited. 
     In FIG. 5, metal tap screw  35  provides a conductive metal connecting member which at flange portion  251  joins bearing holder  25 , formed of a conductive resin, and iron substrate  40 . The tap screw  35  fixes mechanically the flange portion  251 , stator core  32 , core holder  33  and iron substrate  40  together. The flange portion  251  and core holder  33  are insulators. Therefore, only the screw  35 , stator core  32 , and iron substrate  40  are connected electrically together, and the screw  35  is connected to the wiring pattern  43 , then connected to the ground potential M-GND. 
     Herein, the upper surface of iron plate portion  41  of iron substrate  40 , on which metal tap screw  35  is fixed, is exposed. Also, the lower surface of iron plate portion  41  has insulating layer  42 . Wound wiring pattern  43 , to which ground potential M-GND is applied, is formed at a position of insulating layer  42  where tap screw  35  is screwed. Since tap screw  35  has threads of a given size, iron plate portion  41  of iron substrate  40  is short-circuited to ground potential M-GND by connecting to wiring pattern  43  via tap screw  35 . Additionally, stator core  32  is connected to wiring pattern  43  via tap screw  5  to be short-circuited to ground potential M-GND. 
     When brushless motor  1  is driven by the direct PWM method, rapid fluctuation is observed in the voltage applied to the wiring and the circuit pattern between motor drive circuit  60  and drive coil wound around stator core  32  and the voltage applied to coil. However, the potential of iron plate portion  41  as iron substrate  40 , which forms a capacitive coupling with the wiring circuit pattern and coil  31  separately, or the potential of stator core  32  is short-circuited or fixed to ground potential M-GND via tap screw  35 . Therefore, the above motor parts can prevent propagation of electromagnetic noise. In the above embodiment, tap screw  35  is used as a conductive connecting member which forms a short circuit between iron plate portion  41  or stator core  32  to ground potential M-GND. However, one is not limited to a screw for the connecting member; any member capable of forming an electric connection, can be adapted. 
     Also, the above embodiment has a configuration in which a ground wiring pattern of iron substrate  40  is positioned at a position on stator core  32  and iron plate portion  41  where a screw is placed such that stator core  32  and iron plate portion  41  are electrically connected to the ground wiring pattern. Also, bearing holder  25  is formed of a conductive resin. When bearing holder  25  is fixed to the chassis of the main body of the apparatus (not shown in the figure) via a mounting portion of bearing holder  25 , stator core  32  or iron plate portion  41 , that is the ground wiring, is insulated from the chassis by an insulating resistance higher than 1K ohm and lower than 100K ohm. Therefore, when the chassis functions as a ground for the entire apparatus, the chassis is not short-circuited to ground potential M-GND of the motor. As a result, the operation of the apparatus is not disturbed. 
     It is preferable that bearing holder  25  formed of a conductive resin has an insulating resistance of higher than 1K ohm and lower than 100K ohm to the chassis. However, an insulating resistance of 1M ohm or lower may not disturb the operation of the apparatus. Therefore, the range of the insulating resistance can be determined accordingly. It is suggested to determine such a range by considering whether electrostatic potential caused by rotation of pulley  55  with a belt can escape to the chassis via bearing holder  25  or whether the electrostatic potential of the motor may negatively affect the electronic parts of the motor. From these considerations, it is preferable to establish bearing holder  25  as an insulating resistance of 100K ohm or lower to the chassis. 
     To briefly, recapitulate, the pulse current generates the electromagnetic noise, which is propagated by means of the stator core  32  and the metal substrate  40 . To prevent this undesirable result, the screw  35 , stator core  32 , and iron substrate  40  are connected electrically together, and the screw  35  is connected to the wiring pattern  43 , then connected to the ground potential M-GND. 
     Second Embodiment 
     With iron substrate  40  as a metal substrate, iron plate portion  41  as a metal plate can be short-circuited to ground potential M-GND by using sensor holder  70  fixing magnetic sensor  7  to iron substrate  40  as a connecting member, as shown in FIG.  6 . 
     FIG. 6 is a cross section of brushless motor  1  of this embodiment in which iron plate portion  41  of iron substrate  40  is short-circuited to ground potential M-GND by using sensor holder  70 . 
     As shown in FIG. 6, sensor holder  70  is formed of a conductive metal plate, comprised of sensor holding portion  71 , through hole  44  formed on iron substrate  40 , and fixing hook  72  which is inserted into through hole  44 . After passing through hole, fixing hook  72  is bent towards iron plate portion  41  at the opposite side of the substrate such that it contacts the exposed surface of iron plate portion  41 . As a result, fixing hook  72  fixes sensor holder  70  onto iron substrate  40  by sandwiching iron substrate  40 . Therefore, metal sensor holder  70  is in contact with wiring pattern  43  of iron substrate via solder  77  as well as iron plate portion  41 . 
     Consequently, when brushless motor  1  is driven by the direct PWM method, iron plate portion  41  of iron substrate  40  is constantly held at ground potential M-GND via wiring pattern  43 . Therefore, iron plate portion  41  does not propagate electromagnetic noise. 
     As shown in FIG. 6, solder  77  connects wiring pattern  43  and sensor holder  70 . In addition, by placing solder  77  into through hole  44  of iron substrate  40  and bringing solder  77  in contact with iron plate portion  41 , iron plate portion  41  can be further firmly fixed to ground potential M-GND. 
     Embodiment 3 
     FIG. 7 (A), shows a further embodiment in which iron plate portion  41  of iron substrate  40  and stator core  32  are short-circuited to ground potential MGND. Herein, stator core  32 , core holder  33  and iron substrate  40  are fixed all together with tap screw  35  as an example of a connecting member formed of a conductive metal. Further, as shown in FIG. 7 (B), landing portion  47  of ground potential M-GND is formed at the end of the circuit pattern on iron substrate  40 . Then, iron plate portion  41  can be short-circuited to ground potential M-GND by placing metal screw  48  as another connecting member of a conductive metal in a hole formed at the center of landing portion  47 . 
     In such a configuration, iron plate portion  41  of iron substrate  40  and stator core  32  can be short-circuited to ground potential M-GND via tap screw  35 . Moreover, iron plate portion  41  of iron substrate  40  is short-circuited to ground potential M-GND by bringing [iron plate portion  41 ] in contact with landing portion  47  via screw  48  to further ensure the formation of the short circuit. Additionally, stator core  32  can be definitely short-circuited to ground potential M-GND. 
     A screw is used as a connecting member for forming a short-circuit in the above embodiment. However, a pin can be adapted as long as fixing and connection can be ensured. 
     Embodiment 4 
     The above embodiment was an example of a brushless motor employing bearing holder  25  formed of a conductive resin. The following describes an example using metal bearing holder  25  in reference to FIG.  8 . 
     In FIG. 8, first cylinder  331  for insulation between iron substrate  40  and the outer surface of bearing holder  25  and cylinder  332  for insulation between the inner surface of stator core  32  and bearing holder  25  are used as core holder  33 . Core holder  33  assures insulation in the two sections. 
     Also, iron plate portion  41  of iron substrate  40  and stator core  32  are short-circuited to ground potential M-GND wherein stator core  32 , core holder  33  and iron substrate  40  are fixed together by metal tap screw  35 . First insulating spacer  255  made of a resin is placed between bearing holder  25  and tap screw  35  while second insulating spacer  256  made of a resin is placed between bearing holder  25  and stator core  32 . As a result, even when iron plate portion  41  and stator core  32  are short-circuited to ground potential M-GND, the areas between bearing holder  25  and iron plate portion  41  and between bearing holder  25  and stator core  32  maintain insulating resistance higher than 1K ohm. Therefore, when metal bearing holder  25  is directly fixed to chassis  90 , insulating resistance higher than 1K ohm is ensured between chassis  90  and iron plate portion  41  of iron substrate  40  and between chassis  90  and stator core  32 . 
     Further, cap  91  with a screw hole made of a conductive resin is adapted in this embodiment such that bearing holder  25  and chassis  90  are fixed with a given level of insulating resistance. In other words, bearing holder  25  has hold  259  in which cap  91  with a screw hole made of conductive resin is press-fitted. Fixing screw  95  to pierce chassis  90  is fitted in cap  91  with a screw hole in hole  259 . 
     In this embodiment, bearing holder  25  has insulating resistance higher than 1K ohm in relation to chassis  90 . Therefore, even though chassis  90  functions as a ground for the entire apparatus, chassis  90  does not form a short circuit with ground potential M-GND of the motor, guaranteeing the apparatus is not negatively affected. 
     Additionally, it is preferable that bearing holder  25  has insulating resistance lower than 100K ohm in relation to chassis  90 , considering whether electrostatic potential causes negative effects on electronic parts of the motor. Accordingly, electrostatic potential caused by rotation of pulley  55  with a belt can escape to the chassis via bearing holder  25 . 
     FIG.  9 (A) is a circuit diagram of motor drive circuit  60  in brushless motor  1  according to the above embodiments 1 through 4 provide a configuration to prevent trouble cased by ripples which are generated when the motor is driven by the direct PWM method. FIG.  9 (B) is a modified diagram of the above circuit diagram. 
     As shown in FIG.  9 (A), capacitor  68  with a capacitance larger than 0.1ƒ{circumflex over (Ε)}F is connected to motor source  66  in parallel with motor drive circuit  60  of brushless motor  1  according to this embodiment. Therefore, when ripples are generated in the wiring between motor source  66  and coil  31 , these ripples escape to the wiring for ground potential M-GND via capacitor  68 . As a result, since the ripples do not appear in motor source  66 , the operation of the apparatus, such as a VCR, is not disturbed. In addition, the direct PWM method has a moment when all power transistors are turned off. In this case, regenerative current tends to return to motor source  66 ; however, capacitor  68  can absorb such a pulse. Hence, motor source  66  is protected [from the ripples] while motor drive circuit  60  is protected from voltage higher than its maximum voltage. 
     As shown FIG.  9 (B), resistance RS for detecting motor current may be placed on the wiring between motor source  66  and coil  31  in motor drive circuit  60  of brushless motor  1 . In this case, it is preferable to effectively remove ripples generated in motor drive circuit  60  by electrically connecting capacitor  68  for removing ripples, described in reference to FIG.  9 (A), to motor source  66  in parallel at a position closer to motor drive circuit  60  than resistance RS for detecting motor current. 
     In the above embodiment, iron plate portion  41  and stator core  32  are fixed at ground potential M-GND. However, as long as it is a fixed potential, iron plate portion  41  and stator core  32  can be fixed at motor source voltage VM. 
     As described above, in a brushless motor employing the direct PWM method according to the present invention, the potential of a metal plate forming a metal substrate which forms a capacitive coupling with the two sections or a stator core is fixed. Therefore, even when rapid fluctuation in voltage applied to the wiring between a motor drive circuit and a coil and voltage applied to the coil occurs, propagation of electromagnetic noise is suppressed.