Abstract:
A starting apparatus for a direct current (DC) brushless motor and a method thereof are provided. The DC brushless motor comprises a plurality of windings presenting a joint connection via a common connection. The starting apparatus provides current to two of the three windings and rotates the DC brushless motor to obtain a Back Electro-Motive Force (BEMF) from the floating winding. Then, the starting apparatus provides a current to another two windings to operate the motor according to the variation of BEMF induced by the swing of the motor when it rotates to a static equilibrium point.

Description:
[0001]    This application claims the benefit from the priority of Taiwan Patent Application No. 097100686 filed on Jan. 8, 2008 and Taiwan Patent Application No. 097145344 filed on Nov. 24, 2008, the disclosures of which are incorporated by the later reference herein in their entirety. 
       CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to a starting apparatus for a direct current (DC) brushless motor and a method thereof. More particularly, this invention relates to a starting apparatus and a method thereof that can start a DC brushless motor without need of a sensor. 
         [0005]    2. Descriptions of the Related Art 
         [0006]    To detect the correct position of a rotor in a DC brushless motor during the start-up period, the conventional technology is to place a sensor (e.g., a Hall sensor) within the motor. The sensor is configured to sense the variations of a magnetic field between the rotor and the sensor when the motor is running to obtain information of the rotor position. However, the Hall sensor has to be located inside the motor module and placed at the proper position, which appears to increase difficulties in assembly and add production costs to small motors. 
         [0007]    DC brushless motors that do not use sensors have been widely adopted in various products requiring a drive force. Generally, for most motors, the speed thereof can be well controlled when running at a medium or high rotational speed. However, when stationary, it is difficult to determine the rotor position, and a particular starting procedure must be implemented to ensure the successful start-up of the motor before entering into the normal driving mode. 
         [0008]    Conventional technologies aimed to start a DC brushless motor without the need of a sensor have also been proposed, for example, in U.S. Pat. No. 5,343,127 and U.S. Pat. No. 7,202,623. According to both U.S. patents, a back electromotive force (BEMF) generated across the rotor winding in response to the rotational movement thereof is detected as reference information for determining the rotor position to start the motor. Unfortunately, these technologies require complex operations to start the motor, causing increased difficulties in controlling the motor. 
         [0009]    Accordingly, it is important to provide a control method and a circuit thereof that eliminates the need of a sensor while still properly starting a DC brushless motor. 
       SUMMARY OF THE INVENTION 
       [0010]    One objective of this invention is to provide a method for starting a direct current (DC) brushless motor. The DC brushless motor comprises a plurality of windings jointly connected to each other through a joint juncture. The method comprises the following steps: exciting a first phase by supplying a current to a first winding and a second winding of the windings; measuring a first back electromotive force (BEMF) of a third winding which the current does not flow therethrough; switching to a second phase by switching the current to flow sequentially through the second winding and the third winding according to a start-time period or the determination that the first BEMF exceeds a reference value during the start-time period when a second BEMF of the first winding does not have a current that flows therethrough and crosses a negative zero-crossing point during the second phase. The apparatus then switches to a third phase by switching the current to flow sequentially through the second winding and the first winding. 
         [0011]    Another objective of this invention is to provide a starting apparatus of a DC brushless motor. The DC brushless motor comprises a plurality of windings. By supplying a current to two of the windings, the DC brushless motor is rotated to excite a BEMF in the other winding. Then, according to the BEMF variation induced by the swing of the motor when the motor rotates to a static equilibrium point, the current is switched to another two-winding combination, thereby ensuring successful running of the motor. 
         [0012]    To achieve the aforementioned objective, this invention provides a starting apparatus of a DC brushless motor. The starting apparatus comprises a control circuit and a detection circuit. The control circuit is configured to excite a first phase by supplying a current to a first winding and a second winding of the windings, and to switch the current to other two-winding combinations in a specified order according to a start-time period or according to the determination that a BEMF of the winding does not have a current that flows therethrough exceeds a reference value during the start-time period to start the DC brushless motor. The detection circuit is configured to measure the BEMF of the winding which the current does not flow therethrough. 
         [0013]    The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic view of connections between a starting apparatus of this invention and internal windings of a DC brushless motor; 
           [0015]      FIGS. 2A and 2B  are schematic graphs of magnetic torque waveforms and BEMF waveforms according to an embodiment of this invention; and 
           [0016]      FIGS. 3A and 3B  are a flowchart of a second embodiment of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    In the following description, this invention will be explained with reference to embodiments thereof. This invention relates to a starting apparatus for a direct current (DC) brushless motor and a method thereof in which, in response to the BEMF variation induced by the swing of the motor when the rotor rotates to a static equilibrium point, current is supplied to another two-winding combination to ensure the successful running of the motor. However, these embodiments are not intended to limit this invention to any specific context, application or particular implementation described in these embodiments. Therefore, these embodiments are described only for purposes of illustration but not limitation. In the following embodiments and attached drawings, elements unrelated to this invention are omitted from depiction; and, dimensional scales among the individual elements are exaggerated for ease of understanding. 
         [0018]    The preferred embodiment of this invention is depicted in  FIG. 1 , which schematically illustrates a starting apparatus  10  and connections between the starting apparatus  10  and internal windings of a DC brushless motor. In this embodiment, the DC brushless motor is a three-phase motor comprising a winding U, a winding V and a winding W with a central tap CT. The number of windings in the motor is not intended to limit this invention; rather, this invention is applicable to DC brushless motors with three or more windings. The starting apparatus  10  comprises a control circuit  11  and a detection circuit  12 . In this embodiment, the control circuit  11  is configured to generate a digital output signal  101 , which controls switch elements  121 ,  122  and  123  disposed between the windings U, V, W and the power supply to regulate the power supplied to these windings. 
         [0019]    Furthermore, the control circuit  11  receives an output signal  102  from the detection circuit  12 , which represents a BEMF generated by a winding which a current does not flow therethrough when the DC brushless motor is running. The detection circuit  12  is configured to measure the BEMF of the winding which a current does not flow therethrough. According to the output signal  102  and a start-time period, the control circuit  11  supplies a current to the windings in a specified order to start the DC brushless motor. In more detail, the control circuit  11  supplies a current flowing sequentially through the first winding and the second winding to excite a first phase, and then according to the start-time period or the determination that the BEMF of the winding does not have a current that flows therethrough (i.e., the third winding) and exceeds a reference value during the start-time period, the starting apparatus  10  switches the current to other two-winding combinations in a specified order shown in  FIGS. 2A and 2B  to start the DC brushless motor. That is, according to both the first BEMF of the third winding which a current does not flow therethrough and the start-time period, the control circuit  11  switches the current to the second winding and the third winding to switch to a second phase. 
         [0020]    For example, the windings U, V and W are connected to the power supply terminal  111 , an input terminal  112  of the detection circuit  12  and a ground terminal  113  respectively through switching on the switch elements  121 ,  122  and  123  by the control circuit  11 . The digital output signal  101  is adapted to control the connection relationships between the windings U, W and V and the power supply terminal  111 , an input terminal  112  of the detection circuit  12  and a ground terminal  113 . For example, when the winding U is connected to the power supply terminal  111  and the winding V is connected to the ground terminal  113 , the winding W will be connected to the input terminal  112 , in which case the BEMF generated across the winding W is just the input signal of the detection circuit  12 . 
         [0021]    The control circuit  11  further comprises a delay circuit (not shown) configured to generate a delay time. The length of the delay time is adapted to prevent the control circuit  11  from determining that a pseudo BEMF crosses either a positive zero-crossing or a negative zero-crossing. 
         [0022]    In more detail, the control circuit  11  determines whether the first BEMF crosses the positive zero-crossing point during the start-time period. If the first BEMF crosses the positive zero-crossing point, the current is switched to flow sequentially through the second winding and the third winding to switch to a second phase. Otherwise, after the elapse of the start-time period, the current is switched to flow sequentially through the second winding and the third winding to switch to the second phase. 
         [0023]    Subsequent to switching to the second phase, the detection circuit  12  detects the second BEMF of the first winding in which a current does not flow therethrough, and the control circuit  11  switches the current to the second winding and the first winding when the second BEMF crosses a negative zero-crossing point to switch to a third phase. Thus, the starting process of the DC brushless motor is accomplished. 
         [0024]      FIG. 2A  illustrates a waveform diagram of magnetic torque and BEMF is illustrated therein to more clearly explain how the starting apparatus  10  starts the DC brushless motor. The waveform diagram includes the magnetic torque waveforms and the BEMF waveforms, and defines the forward direction of rotation. For example, with the windings U (i.e., the aforementioned first winding) and V (i.e., the aforementioned second winding), the switch elements  121 ,  122  are coupled to the power supply terminal  111  and the ground terminal  113  respectively, and the central tap CT is coupled to the detection circuit  12  to complete a circuit, so that the control circuit  11  supplies a current to the windings U, V via the power terminal  111  to excite the U-V phase  201  (i.e., the aforementioned first phase). The magnetic torque of the U-V phase  201  is denoted as a curve  211 . The switch element  123  is coupled to the input terminal  112  so that no current flows through the winding W presently. In other words, the first BEMF (i.e., a curve  221 ) will be generated across the winding W. It should be noted that if the current is supplied to the windings U and V continuously, the static equilibrium point  204  can be observed on the magnetic-torque curve  211 . This is a characteristic of the DC brushless motors; that is, once the rotor rotates to the static equilibrium point  204 , it will come to a standstill and cease to rotate at the static equilibrium point  204 . This invention just drives a DC brushless motor by virtue of this characteristic. 
         [0025]    Furthermore, the detection unit  12  detects the variations of the first BEMF continuously. When the rotor rotates the static equilibrium point  204 , it is rotating in the forward direction, and due to the inertia, the rotor will rotate towards the forward direction a little further before rotating in the reverse direction. At this point, the detection unit  12  will detect the first BEMF of the reverse direction (i.e., the curve  224 ). Because the BEMF varies on a continuous basis, the BEMF detected by the detection unit  12  at this point will abruptly jump from the curve  221  to the curve  224 , thereby giving rise to the positive zero-crossing point  225 . Hence, according to the output signal  102  from the detection circuit  12 , the control circuit  11  switches the current to flow sequentially through the windings V (i.e., the second winding) and W (i.e., the third winding), i.e., to switch to the V-W phase  202  (i.e., the aforementioned second phase). Then, the detection circuit  12  can detect the second BEMF of the winding U (i.e., the curve  222 ). Arrows before and after the positive zero-crossing point  225  in  FIG. 2A  are illustrated to assist in the further understanding of the aforementioned variations of the BEMF. 
         [0026]    After the current is switched to the V-W phase  202 , the control circuit  11  determines whether the BEMF curve  222  crosses a negative zero-crossing  226  according to the output signal  102 . If the BEMF curve  222  crosses a negative zero-crossing  226 , the current is switched to flow sequentially through the windings V and U, i.e., switched to the V-U phase  203  (i.e., the aforementioned third phase) so that the motor can enter the normal driving mode after starting the DC brushless motor. 
         [0027]    Furthermore, it is also possible that when being started, the rotor of the DC brushless motor rotates in the reverse direction according to the magnetic torque of the U-V phase  201  (i.e., the curve  211 ). In referring to  FIG. 1  and  FIG. 2B  together, when subjected to the action of the magnetic torque shown between the points  304  and  305  on the curve  211 , the rotor rotates in the reverse direction, in which case the detection unit  12  detects the first BEMF in the reverse direction (i.e., the curve  224 ) of the winding W which a current does not flow therethrough. If the current is supplied to the windings U and V continuously, the first BEMF of the winding W will cross a positive zero-crossing point  324  when the rotor rotates beyond the point  304 , in which case the control circuit  11  switches the current to flow sequentially through the windings V and W (i.e., the third winding) according to the output signal  102  from the detection circuit  12  to switch the current to the V-W phase  202 . Then, the detection circuit  12  can detect the second BEMF of the winding U (i.e., the curve  222 ). It can be seen from the BEMF waveforms shown in  FIG. 2B  that a negative zero-crossing  325  occurs when the BEMF changes from the curve  224  to the curve  222 . Then, according to the output signal  102 , the control circuit  11  determines that the negative zero-crossing has occurred and then switches the current to flow sequentially through the windings V and U, i.e., to the V-U phase  203 , so that the motor enters the normal driving mode once started as described above. 
         [0028]    In reference to  FIG. 2A , it is also possible that the rotor of the DC brushless motor already stays at the static equilibrium point  204  in the stationary state, in which case exciting the U-V phase  201  will fail to rotate the rotor. Therefore, if the first BEMF does not cross the positive zero-crossing point during the start-time period, the control circuit  12  will switch the current to the windings V and W, i.e., to the V-W phase  202 , and then proceed with the aforementioned operations. 
         [0029]    With the above arrangement of this invention, by supplying a current to two of the windings of the DC brushless motor, the DC brushless motor is rotated in the forward direction to excite a BEMF in the other winding. Then, in response to the variation of the BEMF induced by the swing of the motor when the motor rotates to the static equilibrium point, the current is switched to another two-winding combination to ensure successful running of the motor. In this way, a complex operational procedure is not needed to start the motor. 
         [0030]    The second preferred embodiment of this invention is depicted in  FIGS. 3A and 3B , which jointly depict the flow diagram of a method for starting a DC brushless motor. The DC brushless motor comprises a plurality of windings jointly connected to each other through a joint juncture. This method comprises the following steps. Initially, in reference to  FIG. 3A , step  400  is executed to excite a first phase by supplying a current to a first winding and a second winding of the windings. Then, step  401  is executed to wait a delay time, in which the length of the delay time is adapted to avoid that a pseudo BEMF crosses either a positive zero-crossing point or a negative zero-crossing point. This is because the erroneous noise signals that are possibly generated when the DC brushless motor is started might cause a pseudo positive or pseudo negative zero-crossing of the BEMF, so a delay time is necessary to prevent this phenomenon from interfering with the starting process of the motor. 
         [0031]    Next, step  402  is executed to measure the first BEMF of a third winding which the current does not flow therethrough. Then, step  403  is executed to determine whether the positive zero-crossing occurs during the start-time period, i.e., whether the first BEMF exceeds a reference value. If the positive zero-crossing occurs during the start-time period, step  405  is executed to switch to a second phase by switching the current to flow sequentially through the second winding and the third winding; otherwise, if the positive zero-crossing does not occur during the start-time period, step  404  is executed to determine whether the start-time period has elapsed. If the start-time period has elapsed, then step  405  is executed; otherwise, step  403  is repeated. 
         [0032]    Next, step  406  is executed to measure a second BEMF of the first winding, and step  407  is executed to switch to a third phase by switching the current to flow sequentially through the second winding and the first winding when the second BEMF of the first winding does not have a current that flows therethrough and crosses the negative zero-crossing point. Now, the DC brushless motor has been started successfully to enter the normal driving mode. The normal driving mode will be understood by those skilled in the art upon reviewing  FIGS. 2A and 2B  and thus will not be further described herein. 
         [0033]    In addition to the steps depicted in  FIGS. 3A and 3B , the second preferred embodiment may also execute all the operations and functionalities of the first preferred embodiment. Those of ordinary skill in the art may readily understand how the second preferred embodiment executes these operations and functionalities based on the descriptions of the first preferred embodiment. Thus, this will not be further described herein. 
         [0034]    Accordingly, according to the variation of BEMF induced by the swing of the motor when the motor rotates to a static equilibrium point, this invention supplies a current to another two-winding combination to ensure the successful running of the motor. This reduces the cost by eliminating the disposition of the Hall sensors and ensuring a proper and fast start of the DC brushless motor. 
         [0035]    The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.