Abstract:
Disclosed herein are a device and method of driving a brushless DC (BLDC) motor. The device to drive the BLDC motor includes a rectifier which converts an AC voltage into a DC voltage; an inverter which converts the DC voltage into an AC voltage by switching a plurality of power switches and supplies the converted AC voltage to the BLDC motor; and a microcomputer which detects a position of a rotor before a terminal voltage of the BLDC motor exceeds 1/2 of an output voltage of the rectifier and performs phase switching, when a driving speed of the BLDC motor reaches a rated speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 2007-2627, filed on Jan. 9, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a device and method of driving a brushless DC (BLDC) motor, and more particularly, to a device and method of driving a BLDC motor that can increase the maximum driving speed of the BLDC motor. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a BLDC motor indicates a DC motor which uses a semiconductor device instead of a brush. The BLDC motor detects the position of a rotor using a magnetic or optical sensor. Alternatively, the BLDC motor may detect the position of the rotor using a sensorless method. 
         [0006]    The sensorless BLDC motor is disclosed in Korean Unexamined Patent Publication No. 1999-81167, in which the position of a rotor is detected by a sensorless detector and the controller of a driving device. 
         [0007]    That is, in the sensorless detector, a plurality of comparators compare voltages of phases of the BLDC motor with a reference voltage, as shown in  FIG. 3 . At this time, an output signal of each comparator is changed at a time point when a terminal voltage of an unexcited phase of the BLDC motor is ½ of an output voltage of a rectifier (zero-cross time point of  FIG. 3 ). Then, the controller detects the position of the rotor of the BLDC motor by detecting a variation in the output signal of the comparator. 
         [0008]    Meanwhile, the sensorless BLDC motor needs to operate at a rated speed or higher. At this time, the controller of the driving device allows the driving speed of the BLDC motor to exceed the rated speed by advancing a phase switching time point, which is a time point when a power switch of an inverter is turned on/off when the motor is driven at the rated speed. This is referred to as “weak flux control”. 
         [0009]    However, in the weak flux control used in the conventional device to drive the BLDC motor, the phase switching time point cannot be earlier than the zero-cross time point shown in  FIG. 3 . Accordingly, the weak flux control range is restricted, and thus the speed of the BLDC motor cannot significantly increase. 
       SUMMARY 
       [0010]    Therefore, it is an aspect to provide a device and method of driving a BLDC motor, which can increase the weak flux control range. 
         [0011]    Additional aspects and/or advantages of the embodiment will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
         [0012]    The foregoing and/or other aspects can be achieved by providing a device to drive a brushless DC (BLDC) motor, including: a rectifier which converts an AC voltage into a DC voltage; an inverter which converts the DC voltage into an AC voltage by switching a plurality of power switches and supplies the converted AC voltage to the BLDC motor; and a microcomputer which detects a position of a rotor before a terminal voltage of the BLDC motor exceeds ½ of an output voltage of the rectifier and performs phase switching, when a driving speed of the BLDC motor reaches a rated speed. 
         [0013]    The BLDC motor may be driven using a two-excitation sensorless driving method and the terminal voltage may be a terminal voltage of an unexcited phase of the three phases of the BLDC motor. 
         [0014]    The microcomputer may include a position detecting unit which detects the position of the rotor of the BLDC motor, and the position detecting unit may detect the position of the rotor by detecting any time point of a period when the terminal voltage of unexcited phase is in a range from 0 to ½ of the output voltage of the rectifier. 
         [0015]    The microcomputer may detect the position of the rotor by detecting a time point when the terminal voltage of the unexcited phase is ½ of the output voltage of the rectifier and perform the phase switching, when the driving speed of the BLDC motor is less than the rated speed. 
         [0016]    The the time point when the terminal voltage of the unexcited phase is ½ of the output voltage of the rectifier is a time point when a pole of a permanent magnet mounted in the rotor passes through the unexcited phase. 
         [0017]    The foregoing and/or other aspects are achieved by providing a device to drive a brushless DC (BLDC) motor, including: a rectifier which converts an AC voltage into a DC voltage; an inverter which converts the DC voltage into an AC voltage and supplies the AC voltage to the BLDC motor; and a microcomputer having a position detecting unit detecting a position of a rotor of the BLDC motor by detecting a time point when a terminal voltage is ½ of an output voltage of the rectifier when the driving speed of the BLDC motor is less than a rated speed, and by detecting any time point of a period when the terminal voltage is in a range from 0 to ½ of the output voltage of the rectifier when the driving speed of the BLDC motor is greater than or equal to the rated speed. 
         [0018]    The microcomputer may advance a position detection time point of the rotor and a phase switching time point to increase the weak flux control range when the driving speed of the BLDC motor is greater than or equal to the rated speed. 
         [0019]    The foregoing and/or other aspects are achieved by providing a device to drive a brushless DC (BLDC) motor, including: a rectifier which converts an AC voltage into a DC voltage; an inverter which converts the DC voltage into an AC voltage by switching a plurality of power switches and supplies the converted AC voltage to the BLDC motor; and a microcomputer which detects a position of a rotor and performs phase switching before a terminal voltage of the BLDC motor exceeds ½ of an output voltage of the rectifier when the BLDC motor is driven. 
         [0020]    The foregoing and/or other aspects are achieved by providing a method of driving a brushless DC (BLDC) motor, including: checking the driving speed of the BLDC motor; and detecting a position of a rotor and performing phase switching before a terminal voltage of the BLDC motor exceeds ½ of an output voltage of a rectifier, when the driving speed reaches a rated speed. 
         [0021]    The method may further include detecting the position of the rotor by detecting a time point when the terminal voltage is ½ of the output voltage of the rectifier and performing the phase switching, when the driving speed is less than the rated speed. 
         [0022]    The method may further include determining whether the driving speed has reached a desired driving speed, and when it is determined that the driving speed has reached the desired driving speed, determining whether the driving speed is continuously increasing, and when it is determined that the driving speed has not reached the desired driving speed, driving the BLDC motor while maintaining the applied voltages of phases of the motor, a position detection time point of the rotor and a phase switching time point. 
         [0023]    When the determination is made as to whether the driving speed is continuously increasing, when the driving speed is determined to be continuously increasing, it is determined that the driving speed has not reached the rated speed, and when the driving speed is determined to not be continuously increasing, it is determined that the driving speed has reached the rated speed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0025]      FIG. 1  is a block diagram showing a device to drive a BLDC motor according to an embodiment; 
           [0026]      FIG. 2  is a flowchart illustrating a method of driving the BLDC motor according to the embodiment; and 
           [0027]      FIG. 3  is a view showing a time point when the position of a rotor is detected in the method of driving the BLDC motor illustrated in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0028]    Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
         [0029]    In the present embodiment, a two-excitation sensorless BLDC motor will be described. 
         [0030]    As shown in  FIG. 1 , the device to drive the BLDC motor according to the embodiment includes a rectifier  12  that converts an AC voltage of an external AC power supply source  16  into a DC voltage, an inverter  11  which converts the DC voltage outputted from the rectifier  12  into an AC voltage by switching a plurality of power switches and which transmits the AC voltage to each phase of the BLDC motor  10 , a smoothing capacitor  13 , a division resistor  14 , a filter  15  to amplify a voltage detected by the division resistor  14 , and a microcomputer  20 . 
         [0031]    The microcomputer  20  includes an A/D converting port  22  which receives the terminal voltages (that is, the voltages of phases) of the BLDC motor  10  and the output voltage of the rectifier  12  and performs A/D conversion, and a position detecting unit  21  which compares the terminal voltages and the output voltage of the rectifier  12  and detects the position of a rotor of the BLDC motor. The position detecting unit  21  has a comparison equation to perform the same function as the comparators of a conventional sensorless detector. When a reference voltage compared with each terminal voltage is changed and substituted for the comparison equation, it is possible to detect the position of the rotor at a desired time point. 
         [0032]    For example, if a time point when the terminal voltage of an unexcited phase is ½ of the output voltage of the rectifier is detected such that the position of the rotor is detected, ½ Vdc (direct current voltage) is substituted as the reference voltage and, if a time point when the terminal voltage of the unexcited phase is ¼ of the output voltage of the rectifier is detected such that the position of the rotor is detected, ¼ Vdc is substituted as the reference voltage. Thus, the result of the comparison equation varies depending on the time point when the terminal voltage of the unexcited phase is ½ Vdc or ¼ Vdc such that the microcomputer  20  can detect the position of the rotor. 
         [0033]    The microcomputer  20  controls the switching of the power switches of the inverter  11  and detects driving current of the BLDC motor  10  using an input voltage of the filter  15 . 
         [0034]    Hereinafter, a method of driving the BLDC motor according to the embodiment will be described with reference to  FIGS. 2 and 3 . When a main microcomputer (not shown) of an electronic apparatus having the BLDC motor  10  transmits an operation command, the microcomputer  20  applies a voltage to the BLDC motor  10  to turn on the BLDC motor  10  and increases the applied voltage, using PWM control, to increase the driving speed of the BLDC motor  10  (operation  30 ). 
         [0035]    The position detecting unit  21  detects a time point when the voltage of the unexcited phase, among the terminal voltages inputted to the A/D converting port is ½ of the output voltage of the rectifier to detect the position of the rotor (operation  32 ). This time point is a point B shown in  FIG. 3 , and the three trapezoidal waveforms shown in  FIG. 3  indicate applied voltages or counter-electromotive voltages of the phases of the BLDC motor  10 . A rectangular waveform included in each trapezoidal waveform indicates current applied to each phase of the BLDC motor  10 . The time point when the terminal voltage of the unexcited phase is ½ Vdc is a time point when a pole of a permanent magnet mounted in the rotor passes through the unexcited phase. 
         [0036]    When the position of the rotor is detected, the microcomputer  20  performs phase switching, which is illustrated by point C shown in  FIG. 3 . 
         [0037]    The phase switching indicates an operation to turn on/off the power switches of the inverter  11  in order to allow the phases to be excited or unexcited. A period from the position detection time point of the rotor to the phase switching time point is called a phase switching phase. In  FIG. 3 , the phase switching phase of operation  32  is, for example, 30°, which is the phase difference between the point B and the point C. 
         [0038]    When operation  32  is performed, the driving speed of the BLDC motor  10  is detected. It is determined whether the driving speed of the BLDC motor  10  reaches a desired driving speed (operation  34 ). The desired driving speed indicates the driving speed of the BLDC motor  10  which is instructed by the main microcomputer. 
         [0039]    If the driving speed of the BLDC motor  10  reaches the desired driving speed, the microcomputer  20  continuously drives the BLDC  10  while the applied voltages of the BLDC motor, the position detection time point of the rotor and the phase switching time point are maintained (operation  40 ). However, if the driving speed of the BLDC motor  10  does not reach the desired driving speed, it is determined whether the driving speed of the BLDC motor  10  is continuously increasing (operation  36 ). 
         [0040]    If the driving speed of the BLDC motor  10  continuously increases, the method returns to operation  32  because it is determined that the driving speed of the BLDC motor  10  has not reached a rated speed. In contrast, if the driving speed of the BLDC motor  10  no longer increases, the microcomputer  20  determines that the driving speed of the BLDC motor  10  reached the rated speed and performs operation  38 . 
         [0041]    When the voltages applied to the phases of the BLDC motor  10  increase at a speed less than the rated speed, the driving speed of the BLDC motor  10  gradually increases. However, at the rated speed, the voltage applied to each phase become equal to the counter-electromotive voltage and thus the driving speed no longer increases. Accordingly, if the driving speed of the BLDC  10  no longer increases despite the increasing of the applied voltages, it is determined that the BLDC motor  10  has reached the rated speed. 
         [0042]    In operation  38 , the position detecting unit  21  detects any time point (0&lt; the terminal voltage of the unexcited phase&lt;½ Vdc) before the time point when the terminal voltage of the unexcited phase among the terminal voltages inputted to the A/D converting port  22  is ½ of the output voltage of the rectifier to detect the position of the rotor. That is, the position detection time point of the rotor can be selected in a range of 0&lt; the voltage terminal of the unexcited phase&lt;½ Vdc, if necessary (for example, point A shown in  FIG. 3 ). The time point before the terminal voltage of the unexcited phase becomes ½ Vdc indicates a time point before the pole of the permanent magnet mounted in the rotor passes through the unexcited phase. 
         [0043]    When the position of the rotor is detected, the microcomputer  20  performs the phase switching (operation  38 ). If the microcomputer  20  detects the position of the rotor at a time point when the terminal voltage of the unexcited phase is 0 (e.g., point A shown in  FIG. 3 ) and the phase switching is performed at point C shown in  FIG. 3 , the phase switching phase being 60°. The phase switching phase may be adjusted, if necessary. In particular, when the phase switching phase is reduced, the phase switching time point is advanced and the direction of the magnetic field generated in the excited phase is changed to offset the magnetic field of the permanent magnet of the rotor to cause the counter-electromotive voltage. Accordingly, the counter-electromotive voltage of the BLDC motor  10  is reduced, and thus the driving speed of the BLDC motor  10  increases. 
         [0044]    When the phase switching phase is reduced to 0°, the position of the rotor is detected, and the phase switching is performed at point A. Thus, the counter-electromotive voltage of the BLDC motor  10  significantly decreases. Accordingly, it is possible to increase the adjustment range of the phase switching phase up to twice that of the conventional device which cannot allow the phase switching time point to become earlier than the point B. Thus, the decreased width of the counter-electromotive voltage is higher than that of the conventional device and thus the driving speed of the BLDC motor  10  significantly increases. 
         [0045]    As a result, when the position detection time point of the rotor becomes earlier than that of the conventional device, i.e., the time point when the terminal voltage of the unexcited phase is ½ Vdc, the phase switching time point becomes earlier than that of the conventional device. Accordingly, the weak flux control range of the BLDC motor  10  increases. 
         [0046]    When operation  38  is performed, the method returns to operation  34 . If the driving speed of the BLDC motor  10  reaches the desired driving speed, the BLDC motor  10  is driven while maintaining the applied voltages of the phases, the position detection time point of the rotor and the phase switching time point (operation  40 ). However, if the driving speed of the BLDC motor  10  does not reach the desired driving speed, operations  36  and  38  are performed again. 
         [0047]    Although, in the present embodiment, the case where the driving speed of the BLDC motor  10  is less than the rated speed and the case where the driving speed of the BLDC motor  10  is greater than or equal to the rated speed are different from each other in the position detection time point of the rotor, the position of the rotor can be detected in the range of 0&lt; the terminal voltage of the unexcited phase&lt;½ Vdc, even when the driving speed of the BLDC motor  10  is less than the rated speed. 
         [0048]    As described above, according to the present embodiment, it is possible to increase the weak flux control range by allowing the position detection time point of the rotor and the phase switching time point to become earlier than those of the conventional device. 
         [0049]    According to the present embodiment, it is possible to significantly increase a maximum driving speed of the BLDC motor by significantly decreasing the counter-electromotive voltage of the BLDC motor compared with the conventional device. 
         [0050]    Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.