Abstract:
A device and method for driving a concentrated winding synchronous motor which, upon supply of a given electric current to a stator coil of the motor, generates an increased torque. Periodical corrections are executed to the waveform of the current to be supplied to the coils of each phase of the stator and corrections are executed to decrease the alternating current supplied to the coil of any tooth at which a portion of a magnetic field generated, with respect to the pole of the closest rotor, is in a direction reverse to a rotation of the rotor.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. HEI 10-243955 filed on Aug. 28, 1998 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a driving method and a driving device for a motor, and more particularly, to a driving method and a driving device for a concentrated winding synchronous motor. 
     2. Description of the Related Art 
     FIGS. 5A,  5 B and  5 C illustrate partially sectional views of a concentrated winding synchronous motor. Referring to the drawings, a rotor  32  shown in FIG. 5B has been rotated counterclockwise at 7.5 from the state of the rotor shown in FIG. 5A as a reference, and the rotor  32  shown in FIG. 5C has been rotated counterclockwise at  15  from the reference state shown in FIG.  5 A. 
     In the concentrated winding synchronous motor shown in the drawings, a stator  35  having a plurality of teeth  34   a  through  34   f  is fitted inside a cylindrical case  30 . In the case of the motor shown in the drawings, the coil of the stator  35  is of the concentrated winding type, and each of the teeth  34  is wound with a coil of either phase U, V, or W. 
     The rotor  32  is pivotally supported inside the stator  35 . Radially magnetized permanent magnets are attached to the outer surface of the rotor  32 . These permanent magnets function as magnetic poles  36   a  through  36   d  of the rotor  32 . In this type of motor, three phase alternating current is supplied to the coils of the teeth  34  to form a rotating magnetic field in the stator  35 , and the repeated cycle of attraction and repulsion between the respective magnetic poles  36  and the teeth  34  serves to drive the rotor  32  at a predetermined torque. 
     Adoption of such concentrated winding can simplify the manufacturing process of the synchronous motor owing to the easy installation of the coil compared with the distributed winding (wrapping) in which the coil is applied to wind about one or more teeth  34 . 
     However, applying the coil of concentrated winding type to the teeth  34  may fail to efficiently obtain the required torque. 
     FIG. 6 illustrates how the aforementioned problem arises. The upper side of FIG. 6A shows the portion interposed between the magnetic poles  36   b ,  36   c  and the teeth  34   b  through  34   e  opposite thereto in a rotating direction in FIG.  5 A. The lower side of FIG. 6A shows the magnetic field formed by the stator  35  at the position of the rotor  32 . FIGS. 6B and 6C likewise show each of the portions interposed between the magnetic poles  36   b ,  36   c  and the teeth  34   b  through  34   e  opposite thereto illustrated in FIGS. 5B and 5C, and the magnetic field formed by the stator  35  at the position of the rotor  32 . 
     As indicated in FIG. 6A, the S pole magnetic field (inward magnetic field) formed by the tooth  34   b  and the N pole magnetic field (outward magnetic field) formed by the tooth  34   c  coexist separately left and right on the surface facing the magnetic pole  36   b . Similarly the N pole magnetic field formed by the tooth  34   c  and the S pole magnetic field formed by the tooth  34   e  coexist left and right on the surface facing the magnetic pole  36   c . Therefore, when the rotor  32  is at the position illustrated in FIG. 6A or FIG. 5A, the magnetic poles  36   b  and  36   c  are attracted by the respective destination teeth  34   c  and  34   e  as well as being repelled by the respective teeth  34   b  and  34   c  behind thereof, thereby enabling the rotor  32  to generate torque efficiently. 
     In the state shown in FIG. 6C, the magnetic poles  36   b  and  36   c  are likewise attracted by the respective destination teeth  34   d  and  34   e  as well as being repelled by the respective teeth  34   b  and  34   d  behind thereof, thereby enabling the rotor  32  to generate the torque efficiently. 
     However, a state shown in FIG. 6B exists in the process proceeding from the state of FIG. 6A to that of FIG.  6 C. That is, since the current supplied to each phase has a sinusoidal wave, the generated N pole magnetic field opposing the magnetic pole  36   b  becomes relatively weak compared with the state shown in FIG. 6A, and the generated N magnetic field opposing the magnetic pole  36   c  becomes relatively strong as compared with the state shown in FIG.  6 A. In this state, a part of the magnetic fields formed on the surface facing the magnetic poles  36   b  and  36   c  may fail to contribute to generation of the torque, and act to reduce the torque generated. In the case of the magnetic pole  36   b , as shown by arrow A in FIG. 6B, a portion of N pole magnetic field generated by the tooth  34   c  exists behind a centerline  38   b  of the magnetic pole  36   b , that is, at the side reverse to the rotating direction. Therefore the magnetic pole  36   b  is pulled back in a direction reverse to the rotating direction. The aforementioned phenomenon applies to the magnetic pole  36   c . As indicated by arrow B in FIG. 6B, a portion of the N pole magnetic field generated by the tooth  34   d  exists at a forward side, in the rotating direction, of a centerline  38   c  of the magnetic pole  36   c . This may push back the magnetic pole  36   c  in a direction reverse to the rotating direction. 
     Accordingly, when adopting the concentrated winding for the synchronous motor coil, depending on the position of the rotating rotor  32 , a portion of each phase current supplied to the coil of the stator  35  of the synchronous motor may serve to impede generation of torque. As a result, the rotating torque is not efficiently obtained. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned drawbacks, and it is the object of the present invention to provide a motor driving method and a motor driving device capable of generating a larger torque upon the supply of the same electric current to the stator coil of the concentrated winding synchronous motor. 
     Therefore, in the present invention, a correction is executed by decreasing the alternating current supplied to the coil wound around the teeth if a portion of the magnetic field generated by the teeth of the stator produces magnetism in a direction opposite of a direction of rotation of a rotor with respect to the pole of the closest rotor. When using the electric current supplied to the coil so as to impede generation of the torque, the amount of current supplied is limited to improve torque generating efficiency. 
     The present invention also executes a correction by decreasing the alternating current supplied to the coil of a first tooth for a predetermined first period included in the time taken for a pole of the rotor to move from a position opposing the first tooth of the stator to a bridge portion between the first tooth and an adjacent second tooth. It may also execute a correction by decreasing the alternating current to be supplied to the coil of a second tooth for a predetermined second period included in the time taken for the pole of the rotor to move from the bridge portion between the first tooth and the adjacent second tooth of the stator to a position opposing the second tooth. In this way, the current is supplied to the coil of the first tooth for a predetermined period, thus impeding generation of the torque. By limiting the supply of such current, the torque generating efficiency can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating the structure of a motor driving device according to an embodiment of the present invention; 
     FIG. 2 shows the waveform of the respective phase current supplied to a concentrated winding synchronous motor of the motor driving device according to the embodiment of the present invention; 
     FIG. 3 is a flowchart representing an operation of the motor driving device according to the embodiment of the present invention; 
     FIG. 4 is a graphical representation showing a relationship of the degree of correction applied to the current waveform with respect to the torque ripple and torque increase amount, respectively; 
     FIG. 5 is a sectional view of the concentrated winding synchronous motor at the respective state; and 
     FIG. 6 illustrates each magnetic field generated at the stator of the concentrated winding synchronous motor shown in FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiment of the present invention will be described in detail with reference to the drawings. 
     FIG. 1 shows the structure of the motor driving device according to an embodiment of the present invention A motor drive device  14  shown in FIG. 1 is mounted on an electric vehicle, and a three phase alternating current with the corrected waveform in accordance with the present invention is supplied to a concentrated winding synchronous motor  20  from a current supply portion  18 . The concentrated winding synchronous motor  20  is provided with eight poles and twelve teeth, which is the same as the motor shown in FIGS. 5 and 6. Therefore, the same reference numerals as in FIGS. 5 and 6 will be used for the description hereinafter. Furthermore, the current supply portion  18  is formed of a DC power source, an inverter and a PWM (Pulse Width Modulation) control circuit (not shown). 
     The motor driving device  14  further includes a main control portion  16 . The main control portion  16  is formed of a CPU and a memory. Operational information including revolutions of the motor  20 , vehicle weight, accelerator operating degree is input from a vehicle ECU (Electronic Control Unit), and the rotational position of the rotor  32  is input from a rotor position detector  22  installed in the motor  20 . The current waveform supplied to the motor  20  is determined by the main control portion  16  in accordance with the aforementioned input information. Furthermore, each phase current value output from the current supply portion  18  may be fed back to the main control portion  16  for stable motor control. 
     FIG. 2 illustrates an example of each waveform supplied from the current supply portion  18  to the coil of the stator  35  of the motor  20  for each phase. Specifically, FIG. 2A shows a W-phase current waveform, FIG. 2B shows an U-phase current waveform, and FIG. 2C shows a V-phase current waveform. 
     In FIGS. 2A,  2 B and  2 C, the axis of abscissa represents the current phase, and the axis of ordinates represents the magnetic field generated in the stator  35  of the motor  20 . In the drawings, the N pole magnetic field (magnetic field in the direction of the rotor  32 ) is generated in the positive range of the axis of ordinates, and the S pole magnetic field is generated in the negative range. 
     In FIGS. 2A,  2 B and  2 C, relatively thick lines  10 U,  10 V and  10 W indicate the reference current waveforms. Meanwhile, relatively thin lines  12 U,  12 V and  12 W indicate the corrected current waveforms IU, IV and IW, which can be expressed by the following equations (1) through (3), respectively. 
     
       
           IU=I×α×βU×sin(θ)   (1)  
       
     
     
       
           IV=I×α×βV×sin(θ+π /3)  (2)  
       
     
     
       
           IW=I×α×βW×sin(θ+ 2π/3)  (3)  
       
     
     In the above equations, the corrected current waveforms IU, IV and IW are determined such that the sum of each instantaneous value thereof becomes 0. In the equations, I denotes the amplitude of the reference current waveform, and βU, βV and βW denote correction coefficients of each phase, α denotes the amplitude of the correction coefficient βU, βV, and βW, i.e., thecorrectedamplitude. Each cycle of the correction coefficients βU, βV and β W is 6 times that of the reference current waveform as shown in the drawings. A table is stored in the memory (not shown) of the main control portion  16 . 
     Using the corrected current waveforms IU, IV and IW shown in the drawings allows the motor  20  to improve the torque generation efficiency. For example, the timings at which the rotor  32  of the motor  20  comes to the position shown in FIGS. 5B and 6B correspond to the timings indicated by the arrows  40 U,  40 V and  40 W shown in FIG.  2 . In these cases, especially as indicated by the arrow  40 V, a current less than the reference current is supplied to the coil wound around the tooth  34   c  (V-phase) as a corrected current. This reduces the degree to which the magnetic field generated by the tooth  34   c  impedes the generation of torque, thus improving the torque generation efficiency. At this timing, the current value of the W-phase indicated by the arrow  40 W is likewise corrected to a decreased level, which weakens the influence of the tooth  34   d  on impeding generation of the torque. Meanwhile, the current value of the U-phase indicated by the arrow  40 U is corrected to an increased level such that the sum of the instantaneous values of the respective phase currents becomes  0 . 
     FIG. 3 is a flow chart representing the operation of the motor driving device  14  according to the embodiment of the present invention. Referring to the drawing, in the motor drive device  14 , the vehicle ECU of the main control portion  16  receives the operational information including the revolutions of the motor  20 , vehicle speed, vehicle weight, accelerator opening degree or the like (S 101 ). Then, the main control portion  16  calculates the required torque and the permissible torque ripple in accordance with the input information (S 102 ), and the correction amplitude a is determined in accordance with the required torque and the permissible torque ripple (S 103 ). 
     That is, the torque ripple will increase upon supply of the current with the waveform shown in FIG. 2 to the motor  20 . Leaving the torque ripple increasing, however, may deteriorate the driving feel of the electric vehicle. FIG. 4 is a graphical representation of the relationship of the value of the correction amplitude α with respect to the torque ripple and the torque increase amount. As shown in the figure, the torque can be increased for the same input by increasing the correction coefficient α. At the same time, however, this may increase the torque ripple. For this reason, the motor driving device  14  of the present invention is designed to determine as to what extent the deterioration of the driving feel is permissible based on the vehicle weight and vehicle speed, and calculate the correction coefficient α in accordance with the permissible value. 
     Next, in the motor driving device  14 , the main control portion  16  calculates the current waveforms IU, IV and IW of each phase as shown in FIG. 2 to generate a current command value map (S 104 ). Further, the main control portion  16  determines the current phase to be supplied to the motor  20  based on the output from the rotor position detector  22  installed in the motor  20  (S 105 ). Based on the results of the aforementioned processing, control signals are sent to the current supply portion  18  to supply current to the motor  20  (S 106 ). 
     According to the motor driving device  14  described above, the degree of contribution of a coil of a phase of the stator  35  to the torque is focused. In the case where the relative position of the rotor  32  and the stator  35  may cause an adverse effect on torque generation, the current amount of the phase is limited. On the other hand, in the case where the relative position may be advantageous for torque generation, the current amount of the phase is increased. This may prevent the increase in the amount of the heat generated by the inverter employed in the current supply portion  18  by maintaining supply of the average current to the motor  20 , yet improving the torque.