Abstract:
A brushless direct current motor. The brushless direct current motor comprises a rotor, a stator, and a driver. The rotor comprises magnetic poles. The stator is enclosed by or enclosing the rotor. The stator comprises salient poles and at least one permanent magnetic element. The salient poles correspond to the magnetic poles, and the permanent magnetic element is disposed on one of the salient poles to facilitate the rotation of the rotor. The driver is coupled to the stator and produces a primary magnetic field on the salient poles. The rotor is rotated by a secondary salient pole induced by the permanent magnetic element and the primary magnetic field alternately.

Description:
BACKGROUND  
       [0001]     The present invention relates in general to a brushless direct current motor and in particular to a brushless direct current motor having permanent magnetic elements disposed on a stator and located at an inner side of the rotor.  
         [0002]      FIG. 1  shows a conventional brushless Direct current motor disclosed in U.S. Pat. No. 6,013,966. A stator of the brushless direct current motor has a first stator yoke  10 , a second stator yoke  20  (under the first stator yoke  10 ) and a coil around an axis therebetween, which is an axial stator. When a current is applied on the coil, salient poles  1  generate induced magnetic force to rotate a rotor  2 .  
         [0003]     The conventional brushless Direct current motor further includes two permanent magnets  3 , disposed outside the rotor  2  to control a starting position of the rotor  2  and provide a starting torque.  
         [0004]     To provide sufficient starting torque, the permanent magnets  3  must be fixed and maintained at an angle θ to the stator. The permanent magnets  3  are, however, fixed outside the rotor  2 , hence, the rotor  2  and the permanent magnets  3  must be enclosed by a non-magnetically permeable cover, for example, a plastic cover, to prevent the magnetic field between the cover and the permanent magnets  3  from decreasing the positioning accuracy of the rotor  2 .  
         [0005]     When the rotor  2  is enclosed by a non-magnetically permeable cover, however, instead of a magnetically permeable cover, the torque of the rotor  2  and the magnetic force between the rotor  2  and the stator  1  is decreased.  
       SUMMARY  
       [0006]     A brushless direct current motor comprises a rotor, a stator, and a driver. The rotor comprises magnetic poles. The stator is enclosed by or encloses the rotor. The stator comprises salient poles and at least one permanent magnetic element. The salient poles correspond to the magnetic poles, and the permanent magnetic element is disposed on at least one of the salient poles to facilitate the rotation of the rotor. The driver is coupled to the stator and produces a primary magnetic field on the salient poles. The rotor is rotated by a secondary salient pole induced by the permanent magnetic element and the primary magnetic field alternately.  
         [0007]     The permanent magnetic element is disposed on the stator and located at an inner side of the rotor. Thus, the rotor can be enclosed by a magnetically permeable cover. Additionally, the driver stops the primary magnetic field automatically when the rotor is blocked.  
         [0008]     The invention further relates to a driver for a brushless direct current motor which comprises a primary magnetic field and an auxiliary magnetic field. The driver comprises a first coil, a start-up device and a control device. The first coil is around the stator, wherein an induced signal is produced on the first coil from a rotation of the rotor. The start-up device provides a start-up signal when the driver receives a power. The control device is coupled to the first coil and the start-up device for receiving the start-up signal and the induced signal, wherein the control device determines whether to produce the primary magnetic field according to the induced signal and the start-up signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention will become more fully understood from the following detailed description and the accompanying drawings, given by the way of illustration only and thus not intended to limit the disclosure.  
         [0010]      FIG. 1  shows a conventional brushless direct current motor;  
         [0011]      FIG. 2A  shows the first embodiment of the brushless direct current motor;  
         [0012]      FIG. 2B  shows a variation of the first embodiment of the brushless direct current motor;  
         [0013]      FIG. 3  shows an embodiment of the salient pole;  
         [0014]      FIGS. 4   a - 4   c  show the stators and the auxiliary poles of variations of the first embodiment;  
         [0015]      FIG. 5  shows the second embodiment of the brushless direct current motor;  
         [0016]      FIGS. 6A-6F  show variations of the second embodiment;  
         [0017]      FIG. 7  shows a driver of the brushless direct current motor;  
         [0018]      FIG. 8  shows the rotation data produced by the brushless direct current motor.  
     
    
     DETAILED DESCRIPTION  
       [0019]     Stator structures will be described in greater detail in the following.  
         [0020]     In an exemplary embodiment of a stator structure, a permanent magnet is disposed on a stator and inside a rotor to drive the rotor to rotate, thus eliminating the need for a permanent magnet to be located at a precise position.  
         [0021]      FIG. 2A  shows the structure of an embodiment of a brushless direct current (DC) motor. The brushless DC motor comprises a stator  150  and a rotor  50 . The rotor  50  is an annular magnet disposed around the stator  150  and coaxial with the stator  150 . The stator  150  is an axial stator structure comprising an upper yoke  80  and an under yoke  90  disposed at an upper layer  60  and an under layer  70  thereof respectively. A permanent magnet  18  is symmetrically disposed between two salient poles  100  of the upper layer  60  of the stator  150 . The outer layer, magnetically N-pole, of the permanent magnet  18  is an auxiliary magnetic polar layer for driving the rotor  50  to rotate.  
         [0022]      FIG. 2B  shows the structure of an embodiment of a brushless direct current (DC) motor. In this embodiment, an additional permanent magnet  19  is disposed between two salient poles  100  of the under layer  70  of the stator  150 . The outer layer, magnetic S-pole, of the permanent magnet  18  is an auxiliary magnetic polar layer for driving the rotor  50  to rotate.  
         [0023]      FIG. 3  shows the structure of an embodiment of a salient pole. Each salient pole, or magnetic pole, comprises a plurality of magnetic conductive layers  101 . The permanent magnet  18  provides an auxiliary magnetic polar layer for the stator  150 . Each permanent magnet  18  can be selectively disposed above the magnetic conductive layers  101 , below the magnetic conductive layers  101 , or between two magnetic conductive layers  101 .  
         [0024]     FIGS.  4 A˜ 4 C show methods for disposing an auxiliary magnetic polar layer of an embodiment of a stator structure. In  FIGS. 4A and 4B , the two permanent magnets  18  and  19  are parallel and corresponding, and disposed at the upper layer  60  and the under layer  70  respectively. The outer layers of the two permanent magnets  18  and  19  are magnetically identical. For example, in  FIG. 4A , the permanent magnet  18  is disposed above the salient pole  100  of the upper layer  60 , and the permanent magnet  19  is disposed between the two salient poles  100  of the under layer  70 . The outer layers of the two permanent magnets  18  and  19  are magnetically identical, such as N-pole or S-pole. In  FIG. 4C , the two permanent magnets  18  and  19  are interlaced and disposed at the upper layer  60  and the under layer  70  respectively. The outer layers of the two permanent magnets  18  and  19  are magnetically opposite. For example, in  FIG. 4C , the permanent magnet  18  is disposed between the two salient poles  100  of the upper layer  60 , and the permanent magnet  19  is disposed between the two salient poles  100  of the under layer  70 . The outer layers of the two permanent magnets  18  and  19  are magnetically N-pole and S-pole respectively.  
         [0025]      FIG. 5  shows the structure of an embodiment of a brushless direct current (DC) motor. The brushless DC motor comprises a stator comprising a yoke  180 , a plurality of salient poles A, B, C, and D, and a plurality of permanent magnets  28 . The stator is a radial stator structure. At least one of the permanent magnets  28  is disposed on at least one of the salient poles. For example, the permanent magnet  28  can be disposed on the salient poles C and D. The brushless DC motor further comprises a rotor  50 . The rotor  50  is an annular magnet coaxially with and outside the stator, wherein poles Sa and Sb are magnetically S-pole, and poles Na and Nb are magnetically N-pole. When necessary, the rotor  50  can be disposed inside the stator.  
         [0026]     FIGS.  6 A˜ 6 F show methods for disposing an auxiliary magnetic polar layer of an embodiment of a stator structure. Outer layers of two permanent magnets on two opposite salient poles are magnetically identical, and outer layers of two permanent magnets on two adjacent salient poles are magnetically opposite. For example, in  FIG. 6A , if the outer layer of the permanent magnet  28  on the salient pole A is magnetically N-pole, the outer layer of the permanent magnet  28  on the opposite salient pole B is magnetically N-pole, and the outer layers of the permanent magnet  29  on the adjacent salient poles C and D are both magnetically S-pole. In FIGS.  6 A˜ 6 F, locations  27  corresponding to the permanent magnets  28  and  29  are provided with silicon steel, ferromagnetic material, permanent magnets, soft magnetic material, plastic magnets, rubber magnets, magnet-cored plastics, or non-magnetic conductive material such as plastics. If the material at location  27  is magnetic, the material and the corresponding permanent magnet  28  or  29  are magnetically opposite. Alternatively, the corresponding locations  27  can be holes.  
         [0027]     For example, in  FIG. 6A , the stator  51  comprises magnetic poles A, B, C, and D. Each magnetic pole comprises five magnetic sub-poles. The sub-pole having the permanent magnet  28  and the sub-pole at the corresponding location  27  constitute a first auxiliary magnetic polar layer. The sub-pole having the permanent magnet  29  and the sub-pole at the corresponding location  27  constitute a second auxiliary magnetic polar layer. The middle three sub-poles of magnetic poles A, B, C, and D constitute three magnetic conductive layers. Thus, the first auxiliary magnetic polar layer is above the three magnetic conductive layers, and the second auxiliary magnetic polar layer is below the three magnetic conductive layers. Each auxiliary magnetic polar layer contains a portion of magnetic poles A, B, C, and D. Each magnetic conductive layer contains a portion of magnetic poles A, B, C, and D. Thus, the number of magnetic poles relating to the magnetic conductive layer is equal to the number of magnetic poles relating to each magnetic conductive layer.  
         [0028]     In FIGS.  6 D˜ 6 F, the permanent magnet  28  or  29  is located at a middle sub-pole. Thus, the auxiliary magnetic polar layer is disposed between two magnetic conductive layers. The permanent magnet  28  or  29  comprises permanent magnetic material, such as a permanent magnet, a plastic magnet, a rubber magnet, or a magnet-cored plastic. The salient pole, or magnetic pole, comprises magnetic conductive material, such as ferromagnetic material or soft magnetic material.  
         [0029]      FIG. 7  shows a driver of an embodiment of a brushless DC motor. The driver  700  comprises a power coil L 1 , a conduction coil L 2 , a start-up device  710 , a control device  720 , and a voltage detection device  730 . The driver  700  is described as below in reference to the brushless DC motor in  FIG. 5 . The power coil L 1  in  FIG. 5  and the power coil L 1  in  FIG. 7  are the same. The conduction coil L 2  in  FIG. 5  and the conduction coil L 2  in  FIG. 7  are the same. A diode D 2  is added at a DC current input end (Vdc) to prevent reverse current. Resistors R, R 1 , R 2 , and R 3  are added in the driver  700  to prevent overflow current. A Zener diode ZD is added in the control device  720  to stabilize voltage.  
         [0030]     If the DC current Vdc is  12 V, the transistor Q 1  is a PNP transistor, the transistor Q 2  is a NPN transistor, and the permanent magnet  28  is magnetically N-pole. When the start-up device is coupled to the DC current Vdc, the transistor Q 1  is turned on due to a reverse base-emitter voltage (12V) greater than a reverse junction voltage (0.7V). When the transistor Q 1  is turned on, the DC current Vdc charges the capacitor C through the current limiting resistor R 1  and the transistor Q 1 . A start-up voltage is output from a collector of the transistor Q 1 . The capacitor C can be replaced by a storage circuit.  
         [0031]     When the control device  720  receives the start-up voltage, the transistor  2  is turned on because a base-emitter forward bias is greater than a junction voltage (0.7V). Thus, a current from the start-up device  710  flows into the control device  720  through the power coil L 1 .  
         [0032]     According to the right-hand principle, the direction of a current on a coil determines magnetic pole of a conducted magnetic field. Thus, the salient poles A and B of the stator are conducted to be N-pole, and the poles C and D of the stator are conducted to be S-pole. The pole Sa of the rotor  50  is attracted by the salient pole A and rejected by the salient pole D, the pole Sb thereof is attracted by the salient pole B and rejected by the salient pole C, thereby driving the rotor  50  to rotate.  
         [0033]     When the control device  720  is continuously coupled to the DC current Vdc, the control device  720  determine whether the start-up device should stop output of a start-up signal according to electric power stored in the capacitor C.  
         [0034]     In  FIG. 7 , when a voltage level of the capacitor C increases, the reverse base-emitter voltage of the transistor Q 1  decreases. When the reverse base-emitter voltage thereof is below the junction voltage (0.7V), the transistor Q 1  is turned off, thereby stopping output of the start-up voltage. Thus, the transistor Q 2  is turned off, and no current flows through the power coil L 1 . The conducted magnetic field of the stator disappears, and the rotor  50  rotates by a particular angle, which is 90 degree counterclockwise in this example.  
         [0035]     In first state, the permanent magnets  28  on the salient poles C and D attract poles Sa and Sb of the rotor  50  respectively to drive the rotor  50  to continue rotating forward.  
         [0036]     In second state, when the permanent magnet  28  attracts the rotor  50  to drive the rotor  50  to rotate, the conduction coil L 2  generates a induced signal, such as a conduction voltage. When the control device  720  receives the induced signal, the transistor Q 2  is turned on. The DC current Vdc flows through the power coil L 1 . The outer layers of the salient poles A and B of the stator are conducted to be N-pole again, and the poles C and D of the stator are conducted to be S-pole again. Due to the magnetic force of the poles C and D being greater than that of the permanent magnet  28 , the rotor  50  is driven by an attraction force between the poles C and D and the poles Sa and Sb to continue rotating forward in the same direction.  
         [0037]     In third state, when the salient poles C and D attract the rotor  50  to drive the rotor  50  to rotate, the salient poles C and D and the permanent magnet  28  are magnetically opposite, and thus the conduction coil L 2  generates a reverse induced signal, such as a reverse conduction voltage. Therefore, the reverse base-emitter voltage of the transistor Q 2  is below the junction voltage, so the transistor Q 2  is turned off.  
         [0038]     When the transistor Q 2  is turned off, no current flows through the power coil L 1 . The conducted magnetic field of the stator disappears, and the rotor  50  continues rotating forward in the same direction. Thus, return to the first state.  
         [0039]     The torque of the rotor  50  is provided half by the conducted magnetic field generated by the power coil L 1  and half by the permanent magnet  28 .  
         [0040]     Similar operations can be derived for the driver  700  used in the brushless DC motor in  FIG. 2 .  
         [0041]     The voltage detection device  730  detects the induced signal. When the rotor  50  rotates, the brushless DC motor operates in the first, the second, and the third state alternately. The conduction coil L 2  generates the conduction voltage and the reverse conduction voltage alternately, so the transistor Q 3  is turn on and off alternately. Thus, a high-low signal is generated, for example a square wave pulse signal. After calculation, the rotational speed of the rotor  50  can be obtained. The high-low signal can be a voltage signal or a current signal. An extra DC current Vcc can be added in the voltage detection device  730  to control a high-low rate of an output voltage.  
         [0042]      FIG. 8  is an output voltage to time graph when a brushless DC motor rotates. The horizontal axis represents time t, and the vertical axis represents output voltage Vo. The wave corresponding to T 1  is the output wave when the rotational speed of the rotor  50  becomes slow due to dust or other objects. The wave corresponding to T 2  is the output wave when the rotor  50  operates normally. The wave corresponding to T 3  is the output wave when the rotor  50  stops rotating.  
         [0043]     When the rotor  50  stops rotating, the conduction coil L 2  stops generating the conduction voltage, the transistors Q 1 , Q 2 , and Q 3  are all turned off. Thus, no undesired current flows into the power coil L 1 , the transistors Q 1 , Q 2 , and Q 3 , and the conduction coil L 2 .  
         [0044]     In some embodiments of a brushless DC motor, when the rotor  50  stops rotating, no undesired current flows into any active component or coil of the driver, preventing overheating or burn-out. Any malfunctions can be easily eliminated by coupling the brushless DC motor to the DC current Vdc again, to restore operation.  
         [0045]     Thus, the disclosed driving device  700  can potentially stabilize the brushless DC motor.  
         [0046]     The start-up device  710  further comprises a releaser comprising a diode D 1  and a resistor R 2 . When the start-up device  710  is disconnected from the DC current Vdc, the releaser releases electric power stored in the capacitor C by discharging the capacitor C through the diode D 1  and the resistor R 2 . Thus, the capacitor C is re-charged when the start-up device  710  is again coupled to the DC current Vdc.  
         [0047]     An embodiment of the stator structure is appropriate for a motor or a fan with coils axially or radially wound thereon.  
         [0048]     While the invention has been described by way of example and in terms of several embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.