Patent Publication Number: US-7911177-B2

Title: AC motor drive controller

Description:
TECHNICAL FIELD 
     The present invention relates to an alternating current (AC) motor drive controller, and more particularly to an AC motor drive controller which performs overmodulation PWM control. 
     BACKGROUND ART 
     In order to drive an alternating current (AC) motor by means of a direct current power source, a driving method in which an inverter is employed is being adopted. Switching of an inverter is controlled by an inverter driver circuit, so that, based on the result of comparison between a voltage instruction and a triangular wave carrier, a pulse width modulation (PWM) voltage which is output from the inverter and applied to the AC motor is controlled, for example. 
     Overmodulation PWM control has been proposed so as to increase the amplitude of a fundamental wave component of the pulse width modulation voltage to be applied to the AC motor (see JP 7-194130A, JP 7-79570 A, and so on). In the overmodulation PWM control, the pulse width modulation voltage to be applied to the AC motor is controlled based on the result of comparison between a voltage instruction in which an amplitude exceeds a peak value of a triangular wave carrier, and the triangular wave carrier. 
     In overmodulation PWM control in which a phase voltage instruction amplitude exceeds a peak value of a triangular wave carrier, the amplitude of a fundamental wave component of the pulse width modulation voltage output from the inverter does not increase linearly with respect to a linear increase in the phase voltage instruction amplitude and is smaller than the phase voltage instruction amplitude. To address this disadvantage, in JP 7-194130 A, a voltage instruction E is calculated from the following formula (1) to perform linear compensation of the voltage instruction E. In formula (1), A represents a percent modulation and E d  represents a direct current (DC) voltage to be input to the inverter. 
     
       
         
           
             
               
                 
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     However, according to formula (1), a waveform (continuous values) which is obtained by clamping a phase voltage instruction value having a sinusoidal wave with half the direct current voltage to be input to the inverter is obtained. Accordingly, when the number of triangular wave carriers per one cycle of a voltage instruction is sufficiently large, a pulse which is output by PWM is substantially equal to the continuous values of waveform described above and therefore formula (1) is substantially satisfied, whereas when the number of triangular wave carriers per one cycle of a phase voltage instruction is reduced, formula (1) cannot be satisfied. Consequently, the fundamental wave amplitude corresponding to the phase voltage instruction amplitude cannot be obtained, resulting in a reduction of controllability of overmodulation PWM control. 
     DISCLOSURE OF THE INVENTION 
     Therefore, the present invention provides an AC motor drive controller which is capable of stably controlling a fundamental wave amplitude of a pulse width modulation voltage to be applied to an AC motor to conform to a phase voltage instruction amplitude. 
     According to an aspect of the invention, there is provided an AC motor drive controller, including an AC motor; an inverter; and an overmodulation PWM controller which controls a pulse width modulation voltage which is output from the inverter and which is to be applied to the AC motor, based on a result of comparison between a phase voltage instruction in which an amplitude exceeds a peak value of a triangular wave carrier and the triangular wave carrier, wherein the overmodulation PWM controller includes: a voltage instruction calculation unit which calculates a d-axis voltage instruction and a q-axis voltage instruction such that a d-axis current and a q-axis current which are detected correspond to the d-axis current instruction and the q-axis current instruction; a voltage amplitude calculator which calculates a voltage instruction amplitude from the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit; a voltage instruction correction unit which corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit, such that a pulse width modulation voltage has a fundamental wave amplitude corresponding to the voltage instruction amplitude calculated by the voltage amplitude calculator; a voltage instruction conversion unit which converts the d-axis voltage instruction and the q-axis voltage instruction which are corrected into a phase voltage instruction and outputs the phase voltage instruction; and a synchronization PWM controller which controls a phase of the phase voltage instruction from the voltage instruction conversion unit and the triangular wave carrier, and wherein the voltage instruction correction unit corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit in accordance with the synchronization number, which is the number of the triangular wave carriers per one cycle of the phase voltage instruction. 
     In one aspect of the present invention, preferably, the voltage instruction correction unit corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit by using amplitude characteristics having different relationships between the voltage instruction amplitude and the fundamental wave amplitude of the pulse width modulation voltage depending on the synchronization number. 
     In another aspect of the present invention, preferably, the synchronization PWM controller determines the synchronization number based on the rotational speed of the AC motor. 
     In accordance with a further aspect of the invention, there is provided an AC motor drive controller, including an AC motor; an inverter; and an overmodulation PWM controller which controls a pulse width modulation voltage which is output from the inverter and which is to be applied to the AC motor, based on a result of comparison between a phase voltage instruction in which an amplitude exceeds a peak value of a triangular wave carrier and the triangular wave carrier, wherein the overmodulation PWM controller includes: a voltage instruction calculation unit which calculates a d-axis voltage instruction and a q-axis voltage instruction such that a d-axis current and a q-axis current which are detected correspond to the d-axis current instruction and the q-axis current instruction; a voltage amplitude calculator which calculates a voltage instruction amplitude from the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit; a voltage instruction correction unit which corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit, such that a pulse width modulation voltage has a fundamental wave amplitude corresponding to the voltage instruction amplitude calculated by the voltage amplitude calculator; a voltage instruction conversion unit which converts the d-axis voltage instruction and the q-axis voltage instruction which are corrected into a phase voltage instruction and outputs the phase voltage instruction; and a synchronization PWM controller which controls a phase of the phase voltage instruction from the voltage instruction conversion unit and the triangular wave carrier, and wherein the phase voltage instruction which is compared with the triangular wave carrier is updated for each predetermined cycle, and the voltage instruction correction unit corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit in accordance with update timing of the phase voltage instruction. 
     In one aspect of the present invention, preferably, the voltage instruction correction unit corrects the d-axis voltage instruction and the q-axis voltage instruction calculated by the voltage instruction calculation unit by using amplitude characteristics having different relationships between the voltage instruction amplitude and the fundamental wave amplitude of the pulse width modulation voltage depending on the update timing of the phase voltage instruction. 
     In another aspect of the present invention, preferably, there are further provided a sinusoidal wave PWM controller which controls a pulse width modulation voltage which is output from the inverter and which is to be applied to the AC motor, based on a result of comparison between a phase voltage instruction in which an amplitude does not exceed a peak value of a triangular wave carrier and the triangular wave carrier, and a control switching unit which selectively switches between the sinusoidal wave PWM controller and the overmodulation PWM controller serving as a controller which controls the voltage to be applied to the AC motor. 
     In a further aspect of the present invention, preferably, there are further provided a sinusoidal wave PWM controller which controls a pulse width modulation voltage which is output from the inverter and which is to be applied to the AC motor, based on a result of comparison between a phase voltage instruction in which an amplitude does not exceed a peak value of a triangular wave carrier and the triangular wave carrier, a rectangular wave voltage controller which controls a phase of a rectangular wave voltage in which one pulse is output per one cycle of the phase voltage instruction, to control a rectangular wave voltage which is output from the inverter and which is to be applied to the AC motor, and a control switching unit which selectively switches among the sinusoidal wave PWM controller, the overmodulation PWM controller, and the rectangular wave voltage controller, serving as a controller which controls the voltage to be applied to the AC motor. 
     According to the present invention, in overmodulation PWM control, a fundamental wave amplitude of a pulse width modulation voltage to be applied to an AC motor can be stably controlled to conform to a phase voltage instruction amplitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects of the invention will be explained in the below description by reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram schematically illustrating a structure of an AC motor drive controller according to an embodiment of the present invention; 
         FIG. 2  is a block diagram schematically illustrating a structure of an overmodulation PWM controller for use in the AC motor drive controller according to the embodiment of the present invention; 
         FIG. 3  is a view for explaining modulation processing in a sinusoidal wave PWM controller; 
         FIG. 4  is a view for explaining modulation processing in an overmodulation PWM controller; 
         FIG. 5  is a chart illustrating an examination result of the fundamental wave amplitude of a pulse width modulation voltage obtained by changing the synchronization number while the voltage instruction amplitude is fixed; 
         FIG. 6  is a chart illustrating an examination result of the fundamental wave amplitude of a pulse width modulation voltage obtained by changing the phase of a phase voltage instruction and a triangular wave carrier while the voltage instruction amplitude is fixed; 
         FIG. 7  is a chart illustrating examples of the phases of a phase voltage instruction and a triangular wave carrier; 
         FIG. 8  is a chart illustrating an examination result of the fundamental wave amplitude of a pulse width modulation voltage obtained by changing the update timing of the phase voltage instruction while the voltage instruction amplitude is fixed; 
         FIG. 9  shows example voltage-amplitude characteristic maps representing a relationship between the voltage instruction amplitude and the fundamental wave amplitude of a pulse width modulation voltage; 
         FIG. 10  shows further example voltage-amplitude characteristic maps representing a relationship between the voltage instruction amplitude and the fundamental wave amplitude of a pulse width modulation voltage; 
         FIG. 11  is a chart illustrating an example of the phase of a phase voltage instruction and a triangular wave carrier when the synchronization number is an odd number; 
         FIG. 12  is a chart illustrating an example of the phase of a phase voltage instruction and a triangular wave carrier when the synchronization number is an even number; 
         FIG. 13  is a chart illustrating an example of the ideal phase of a phase voltage instruction and a triangular wave carrier when the synchronization number is an even number; 
         FIG. 14  is a view illustrating examples of the phase of the phase voltage instruction and the triangular wave carrier; 
         FIG. 15  is a graph illustrating an example of a relationship between the triangular wave carrier frequency and the synchronization number with respect to the rotational speed of an AC motor; 
         FIG. 16  is a graph illustrating an example of a relationship between the triangular wave carrier frequency and the synchronization number with respect to the rotational speed of an AC motor; 
         FIG. 17  is a graph illustrating an example of a relationship between the triangular wave carrier frequency and the synchronization number with respect to the rotational speed of an AC motor; 
         FIG. 18  is a flowchart for explaining example processing for selectively switching the control mode of an AC motor; 
         FIG. 19  is a graph illustrating an example of a relationship between the rotational speed of the AC motor and the voltage amplitude, and the drive control mode; 
         FIG. 20  is a Lissajous figure illustrating a change of the d-axis current value and the q-axis current value; and 
         FIG. 21  is a graph illustrating an example of a relationship between the rotational speed of the AC motor and the voltage amplitude, and the drive control mode. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A preferred embodiment of the present invention will be described with reference to the drawings. 
       FIGS. 1 and 2  are block diagrams schematically illustrating a structure of an AC motor drive controller  100  according to an embodiment of the present invention. Specifically,  FIG. 1  schematically illustrates the overall structure of the drive controller  100  and  FIG. 2  schematically illustrates the structure of an overmodulation PWM controller  104 . The drive controller  100  according to the present embodiment includes a sinusoidal wave PWM controller  102 , the overmodulation PWM controller  104 , a rectangular wave voltage controller  106 , a control mode decision unit  108 , a switching unit  109 , and a current instruction creator  110 . 
     The current instruction creator  110 , based on a torque instruction T* of an AC motor  300 , a d-axis current instruction id*, and a q-axis current instruction iq*. The sinusoidal wave PWM controller  102  calculates a d-axis voltage instruction and a q-axis voltage instruction such that a d-axis current value id and a q-axis current value iq which are detected correspond to the d-axis current instruction id* and the q-axis current instruction iq*, respectively, and generates a phase voltage instruction (sinusoidal wave modulation wave) by means of coordinate conversion. The sinusoidal wave PWM controller  102  then compares this phase voltage instruction with a triangular wave carrier as illustrated in  FIG. 3(   a ) to generate a switching instruction signal (a pulse width modulation signal) having a pulse width corresponding to the voltage instruction amplitude as illustrated in  FIG. 3(   b ). The sinusoidal wave PWM controller  102  generates a phase voltage instruction having an amplitude which does not exceed a peak value of the triangular wave carrier as illustrated in  FIG. 3(   a ). The switching instruction signal which is based on the result of comparison between this phase voltage instruction and the triangular wave carrier is supplied via the switching unit  109  to an inverter  200 , so that sinusoidal wave PWM control is performed to control the voltage (pulse width modulation voltage) which is output from the inverter and applied to the AC motor  300 . 
     The overmodulation PWM controller  104  calculates a d-axis voltage instruction and a q-axis voltage instruction such that the d-axis current value id and the q-axis current value iq which are detected correspond to the d-axis current instruction id* and the q-axis current instruction iq*, respectively, and generates a phase voltage instruction (sinusoidal wave modulation wave) by means of coordinate conversion. The overmodulation PWM controller  104  then compares this phase voltage instruction with a triangular wave carrier as illustrated in  FIG. 4(   a ) to generate a switching instruction signal (a pulse width modulation signal) having a pulse width corresponding to the voltage instruction amplitude as illustrated in  FIG. 4(   b ). The overmodulation PWM controller  104  generates a phase voltage instruction having an amplitude which exceeds a peak value of the triangular wave carrier as illustrated in  FIG. 4(   a ), and then generates a switching instruction signal having a pulse width which is longer than the cycle of a triangular wave carrier as illustrated in  FIG. 4(   b ). The switching instruction signal which is generated in the overmodulation PWM controller  104  is supplied via the switching unit  109  to the inverter  200 , so that overmodulation PWM control is performed to control the voltage (pulse width modulation voltage) which is output from the inverter and applied to the AC motor  300 . 
     The rectangular wave voltage controller  106  generates a voltage phase φv such that an output torque or an estimated torque of the AC motor  300  corresponds to the torque instruction T*, and, based on this voltage phase φv, controls the phase of a rectangular wave voltage, in which one pulse is output per one cycle of the phase voltage instruction. The rectangular wave voltage which is output from the rectangular wave voltage controller  106  is supplied via the switching unit  109  to the inverter  200 , so that rectangular wave voltage control is performed to control the voltage (rectangular voltage) which is output from the inverter  200  and applied to the AC motor  300 . 
     The switching unit  109  selectively supplies to the inverter  200  one of the switching instruction signal from the sinusoidal wave PWM controller  102 , the switching instruction signal from the overmodulation PWM controller  104 , and the rectangular wave voltage from the rectangular wave voltage controller  106 . The control mode decision unit  108  performs switching control of the switching unit  109 , to thereby selectively switch among the sinusoidal wave PMW controller  102 , the overmodulation PWM controller  104 , and the rectangular wave voltage controller  106 , for selecting a controller which controls the voltage which is output from the inverter  200  and applied to the AC motor  300 . In other words, the control mode decision unit  108  selectively switches the control mode for the AC motor  300  among the sinusoidal wave PWM control, the overmodulation PWM control, and the rectangular wave voltage control. A specific example operation of the control mode decision unit  108  for selectively switching the control mode for the AC motor  300  will be described below. 
     The inverter  200  includes a voltage-type inverter circuit. The inverter  200 , upon receiving the switching instruction signal from the sinusoidal wave PWM controller  102 , the switching instruction signal from the overmodulation PWM controller  104 , or the rectangular wave voltage from the rectangular wave voltage controller  106 , generates a three-phase pseudo sinusoidal wave voltage, which is applied to the AC motor  300 . The AC motor  300  is a synchronous motor such as a permanent magnet synchronous (PM) motor, for example. The AC motor  300 , upon receiving the three-phase pseudo sinusoidal wave voltage from the inverter  200 , rotates a rotor. 
     Current sensors  202  are provided on the power supply line from the inverter  200  to the AC motor  300 . The current values detected by these current sensors  202  in real time (two-phase current value iu and iw of the three phases) are input to a three-phase/dq axis converter  204 . The three-phase/dq axis converter  204  converts the current values iu and iw into the d-axis current value id and the q-axis current value iq for output. 
     Further, a resolver  302  is attached to the AC motor  300  for detecting a rotational position (rotation angle) θe of the rotor. The resolver  302  detects the rotational position θe of the rotor and outputs the rotational position θe to a rotational frequency calculator  304 . The rotational frequency calculator  304 , using the rotational position θe supplied from the resolver  302 , calculates and outputs the rotational frequency (the rotational speed) we of the rotor of the AC motor  300 . 
     An example structure of the overmodulation PWM controller  104  will be now described. The d-axis current value id and the q-axis current value iq from the three-phase/dq axis converter  204 , as well as the d-axis current instruction id* and the q-axis current instruction iq* from the current instruction creator  110 , are input to the current controller  20 . The current controller  20  calculates and outputs a d-axis voltage instruction vd B  and a q-axis voltage instruction vq B  before compensation, such that the d-axis current value id and the q-axis current value iq approach the d-axis current instruction id* and the q-axis current instruction iq*, respectively (ideally, such that the d-axis current value id and the q-axis current value iq correspond to the d-axis current instruction id* and the q-axis current instruction iq*, respectively). The current controller  20  calculates the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  having voltage amplitudes and phases in accordance with the torque instruction T*, with the voltage amplitude exceeding the peak value of the triangular wave carrier. A voltage amplitude calculator  21 , based on the d-axis voltage instruction vd B  and a q-axis voltage instruction vq B  before compensation from the current controller  20 , calculates a voltage instruction amplitude before compensation, E B =(vd B   2 +vq B   2 ) 0.5  and the phase. A voltage amplitude linear compensation unit  22 , upon receiving the d-axis voltage instruction vd B  and a q-axis voltage instruction vq B  from the current controller  20  and the voltage instruction amplitude E B  from the voltage amplitude calculator  21 , calculates and outputs a d-axis voltage instruction vd and a q-axis voltage instruction vq obtained by linear compensation of the d-axis voltage instruction vd B  and a q-axis voltage instruction vq B . The details of the linear compensation which is performed here will be described below. A dq axis/three-phase converter  24 , using the rotational position θe supplied from the resolver  302  as a reference, converts the d-axis voltage instruction vd and the q-axis voltage instruction vq after linear compensation supplied from the voltage amplitude linear compensation unit  22  into three-phase (u-phase, v-phase, and w-phase) voltage instructions vu, vv, and vw and outputs the voltage instructions vu, vv, and vw to a PWM generator  26 . The PWM generator  26  compares the three-phase voltage instructions vu, vv, and vw with the triangular wave carrier to generate and output switching instructions having a pulse width corresponding to the amplitudes of the three-phase voltage instructions vu, vv, and vw. Here, the three-phase voltage instructions vu, vv, and vw to be compared with the triangular wave carrier are updated in a predetermined cycle (e.g. a cycle which is synchronized with the triangular wave carrier). A synchronous PWM controller  28 , based on the rotational frequency (the rotational speed) ωe of the AC motor  300  calculated by the rotational frequency calculator  304 , determines the frequency fc of the triangular wave carrier and the number K of the triangular wave carriers per one cycle of the phase voltage instruction (triangular wave carrier frequency fc/phase voltage instruction frequency fm, which will be referred to as the synchronization number), to thereby control the phase of the three-phase voltage instructions vu, vv, and vw and the triangular wave carrier. The details of the phase control performed here will be described below. Here, the processing by the voltage amplitude calculator  21 , the voltage amplitude linear compensation unit  22 , the dq axis/three-phase converter  24 , the PWM generator  26 , and the synchronous PWM controller  28  can be performed in synchronization with the triangular wave carrier cycle, for example. On the other hand, the processing by the current instruction creator  110 , the current controller  20 , and the three-phase/dq axis converter  204  can be performed in a fixed cycle, for example, and need not necessarily be performed in synchronization with the triangular wave carrier. 
     Here, because the sinusoidal wave PWM controller  102  and the rectangular wave voltage controller  106  can be implemented by using a known technology, the details of the specific structures thereof will not be described. For example, the sinusoidal wave PWM controller  102  can have a structure in which the voltage amplitude linear compensation unit  22  and the voltage amplitude characteristic storage unit  29  are omitted from the example structure of the overmodulation PWM controller  104  illustrated in  FIG. 2 . 
     In the overmodulation PWM control in which the amplitude of the phase voltage instruction (three-phase voltage instructions vu, vv, vw) exceeds the peak value of the triangular wave carrier, the amplitude of the fundamental wave component of the pulse width modulation voltage output from the inverter  200  does not increase linearly with respect to the linear increase in the amplitude of the three-phase voltage instructions vu, vv, and vw, and the amplitude of the fundamental wave component of the pulse width modulation voltage is smaller than the amplitude of the three-phase voltage instructions vu, vv, and vw. Accordingly, the voltage amplitude linear compensation unit  22  corrects the d-axis voltage instruction vd B  and a q-axis voltage instruction vq B  to thereby correct the voltage instruction amplitude E B , such that the fundamental wave amplitude of the output voltage (pulse width modulation voltage) of the inverter  200  corresponds to the voltage instruction amplitude E B  calculated by the voltage amplitude calculator  21 . Here, a voltage amplitude characteristic map representing a relationship between the voltage instruction amplitude and the fundamental wave amplitude of the pulse width modulation voltage is prestored in the voltage amplitude characteristic storage unit  29 . Then, the voltage amplitude linear compensation unit  22 , using the voltage amplitude characteristic map read from the voltage amplitude characteristic storage unit  29 , calculates a voltage instruction amplitude which is necessary for making the fundamental wave amplitude of the pulse width modulation voltage conform to the voltage instruction amplitude E B , and, based on the voltage instruction amplitude thus calculated, calculates the d-axis voltage instruction vd and the q-axis voltage instruction vq after correction. In this manner, the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  can be linearly compensated, so that the fundamental wave amplitude of the pulse width modulation voltage can be linearly increased with respect to the linear increase in the voltage instruction amplitude E B . 
     However, when the synchronization number (the number of triangular wave carriers per one cycle of the phase voltage instruction) K changes, the fundamental wave amplitude of the output voltage (pulse width modulation voltage) of the inverter  200  changes, even if the voltage instruction amplitude remains unchanged. Here,  FIG. 5  illustrates a result of examination of the fundamental wave amplitude of the pulse width modulation voltage obtained when the synchronization number K is changed in a state in which the voltage instruction amplitude is fixed. In the result illustrated in  FIG. 5 , a relative phase of the phase voltage instruction and the triangular wave carrier is determined such that a zero cross point from positive to negative of the phase voltage instruction and an intermediate point of transition from the valley to peak of the triangular wave carrier are in synchronism with each other (a state illustrated in  FIG. 7(   a ), for example), and the voltage instruction amplitude is made twice the amplitude of the triangular wave carrier and the update timing of the phase voltage instruction with respect to the triangular wave carrier is made continuous. As illustrated in  FIG. 5 , even when the voltage instruction amplitude is fixed, the fundamental wave amplitude of the pulse width modulation voltage changes with respect to a change of the synchronization number K. 
     Further, even when the voltage instruction amplitude remains unchanged, the fundamental wave amplitude of the pulse width modulation voltage also changes when a relative phase of the phase voltage instruction and the triangular wave carrier changes. Here,  FIG. 6  illustrates a result of examination of the fundamental wave amplitude of the pulse width modulation voltage obtained when the relative phase of the phase voltage instruction and the triangular wave carrier is changed in a state in which the voltage instruction amplitude is fixed. In the result illustrated in  FIG. 6 , the synchronization number K is 9, the voltage instruction amplitude is twice the amplitude of the triangular wave carrier, and the update timing of the phase voltage instruction is continuous. Further, in  FIG. 6 , the phase 90° represents a state in which a zero cross point from positive to negative of the phase voltage instruction and an intermediate point of transition from the valley to peak of the triangular wave carrier are in synchronism with each other (a state illustrated in  FIG. 7(   a ) in which the phase of the phase voltage instruction and the triangular wave carrier is 90°, for example), the phase 180° represents a state in which a zero cross point from positive to negative of the phase voltage instruction and a peak (an apex) of the triangular wave carrier are in synchronism with each other (a state illustrated in  FIG. 7(   b ) in which the phase of the phase voltage instruction and the triangular wave carrier is 180°, for example), and the phase 270° represents a state in which a zero cross point from positive to negative of the phase voltage instruction and an intermediate point of transition from the peak to valley of the triangular wave carrier are in synchronism with each other (a state illustrated in  FIG. 7(   c ) in which the phase of the phase voltage instruction and the triangular wave carrier is 270°, for example). As illustrated in  FIG. 6 , even when the voltage instruction amplitude is fixed, the fundamental wave amplitude of the pulse width modulation voltage changes with respect to a change in the relative phase of the phase voltage instruction and the triangular wave carrier. 
     In addition, even when the voltage instruction amplitude remains unchanged, the fundamental wave amplitude of the pulse width modulation voltage also changes when the update timing of the phase voltage instruction changes. Here,  FIG. 8  illustrates a result of examination of the fundamental wave amplitude of the pulse width modulation voltage obtained when the update timing of the phase voltage instruction with respect to the triangular wave carrier is changed in a state in which the voltage instruction amplitude is fixed. In the result illustrated in  FIG. 8 , the synchronization number K is 9, the voltage instruction amplitude is twice the amplitude of the triangular wave carrier, and the relative phase of the phase voltage instruction and the triangular wave carrier is determined such that a zero cross point from positive to negative of the phase voltage instruction and an intermediate point of transition from the valley to peak of the triangular wave carrier are in synchronism with each other (the state illustrated in  FIG. 7(   a ), for example). Further, in  FIG. 8 , “continuous” represents a case in which the phase voltage instruction is updated continuously (in an analog manner), “whole cycle” represents a case in which the phase voltage instruction is updated for each cycle of the triangular wave carrier (for each valley of the triangular wave carrier), and “half cycle” represents a case in which the phase voltage instruction is updated for every half cycle of the triangular wave carrier (for each of the peak and the valley of the triangular wave carrier). As illustrated in  FIG. 8 , even when the voltage instruction amplitude is fixed, the fundamental wave amplitude of the pulse width modulation voltage changes with respect to a change of the update timing of the phase voltage instruction. 
     As described above, even when the voltage instruction amplitude remains unchanged, the fundamental wave amplitude of the output voltage (pulse width modulation voltage) of the inverter  200  changes in accordance with a change of at least one of the synchronization number K, the relative phase of the phase voltage instruction and the triangular wave carrier, and the update timing of the phase voltage instruction. Such a change of the fundamental wave amplitude makes it impossible to obtain the fundamental wave amplitude corresponding to the voltage instruction amplitude, which results in a reduction in controllability of the overmodulation PWM control. 
     Accordingly, in the present embodiment, the voltage amplitude linear compensation unit  22  corrects the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  in accordance with the synchronization number K supplied from the synchronous PWM controller  28  to correct the voltage instruction amplitude E B . Here, the voltage amplitude characteristic storage unit  29  stores, in association with the synchronization number K, a plurality of voltage amplitude characteristic maps having different relationships between the voltage instruction amplitude and the fundamental wave amplitude of the pulse width modulation voltage in accordance with different synchronization numbers K. Then, the voltage amplitude linear compensation unit  22  reads out the voltage amplitude characteristic map corresponding to the synchronization number K from the voltage amplitude characteristic storage unit  29 , and uses the thus-read voltage amplitude characteristic map to correct the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  (the voltage instruction amplitude E B ). 
       FIGS. 9 and 10  illustrate examples of these voltage amplitude characteristic maps. Specifically,  FIG. 9(   a ) is a voltage amplitude characteristic map corresponding to the synchronization number K=6;  FIG. 9(   b ) is a voltage amplitude characteristic map corresponding to the synchronization number K=9;  FIG. 9(   c ) is a voltage amplitude characteristic map corresponding to the synchronization number K=12;  FIG. 9(   d ) is a voltage amplitude characteristic map corresponding to the synchronization number K=15;  FIG. 9(   e ) is a voltage amplitude characteristic map corresponding to the synchronization number K=18; and  FIG. 9(   f ) is a voltage amplitude characteristic map corresponding to the synchronization number K=21. Further,  FIG. 10(   a ) is a voltage amplitude characteristic map corresponding to the synchronization number K=24, and  FIG. 10(   b ) is a voltage amplitude characteristic map corresponding to the synchronization number K=27. Here, in the voltage amplitude characteristic maps illustrated in  FIGS. 9 and 10 , the values of the fundamental wave amplitude and the voltage instruction amplitude are values obtained by dividing the fundamental wave amplitude and the voltage instruction amplitude by the amplitude of the triangular wave carrier. For example, in the case of the synchronization number K=6 and the voltage instruction amplitude E B =1.2 (1.2 times the triangular wave carrier amplitude), in order to make the fundamental wave of the pulse width modulation voltage 1.2 (1.2 times the triangular wave carrier amplitude), the value of the voltage instruction amplitude is corrected to 2.2 (2.2 times the triangular wave carrier amplitude) by using the voltage amplitude characteristic map illustrated in  FIG. 9(   a ). On the other hand, in the case of the synchronization number K=9 and the voltage instruction amplitude E B =1.2 (1.2 times the triangular wave carrier amplitude), in order to make the fundamental wave of the pulse width modulation voltage 1.2 (1.2 times the triangular wave carrier amplitude), the value of the voltage instruction amplitude is corrected to 1.9 (1.9 times the triangular wave carrier amplitude) by using the voltage amplitude characteristic map illustrated in  FIG. 9(   b ). Thus, by correcting the voltage instruction amplitude by using a different voltage amplitude characteristic map in accordance with the synchronization number K, it is possible to suppress a change of the fundamental wave amplitude of the pulse width modulation voltage caused by the change of the synchronization number K. 
     Here, the conditions of the relative phase of the phase voltage instruction and the triangular wave carrier at the time of creating the voltage amplitude characteristic maps illustrated in  FIGS. 9 and 10  are that the zero cross point of the phase voltage instruction from positive to negative is in synchronization with the intermediate point of transition from the valley to peak of the triangular wave carrier (the condition illustrated in  FIG. 11 , for example) when the synchronization number K is an odd number, and that the zero cross point of the phase voltage instruction is in synchronization with the peak of the triangular wave carrier (the condition illustrated in  FIG. 12 , for example) when the synchronization number K is an even number. Accordingly, the synchronization PWM controller  28  controls the relative phase of the phase voltage instruction and the triangular wave carrier to make the zero cross point of the phase voltage instruction from positive to negative synchronized with the intermediate point of transition from the valley to the peak of the triangular wave carrier when the synchronization number K is an odd number, and controls the relative phase of the phase voltage instruction and the triangular wave carrier to make the zero cross point of the phase voltage instruction synchronized with the peak of the triangular wave carrier when the synchronization number K is an even number. Thus, when the voltage instruction amplitude is corrected by using the voltage amplitude characteristic map in accordance with the synchronization number K, the synchronization PWM controller  28  controls the relative phase of the phase voltage instruction and the triangular wave carrier to conform to the phase at the time of creation of the corresponding voltage amplitude characteristic map. Consequently, the conditions of the relative phase of the phase voltage instruction and the triangular wave carrier are the same for a given same synchronization number, so that a change of the fundamental wave amplitude of the pulse width modulation voltage with the change of the relative phase of the phase voltage instruction and the triangular wave carrier can be suppressed. Here, because the technology of controlling the phases of the phase voltage instruction and the triangular wave carrier itself is well known, detailed description thereof will be omitted (see JP 2607488 B and JP 2001845 B, for example). Further, the conditions of the update timing of the phase voltage instruction at the time of creating the voltage amplitude characteristic maps illustrated in  FIGS. 9 and 10  are that the update timing of the phase voltage instruction is the peak and the valley of the triangular wave carrier (a half cycle of the triangular wave carrier). 
     As illustrated in  FIG. 11 , when the zero cross point of the phase voltage instruction from positive to negative is synchronized with the intermediate point of transition from the valley to peak of the triangular wave carrier in the case of the synchronization number K being an odd number, the comparison result of the phase voltage instruction and the triangular wave carrier (the output pulse) has identical shapes in the positive region and the negative region of the phase voltage instruction, which is symmetrical about the maximum value (the minimum value) of the phase voltage instruction. Then, at the maximum value of the phase voltage instruction, a switching element on the upper side of the inverter arm is placed in an ON state. Under this condition, the fundamental wave amplitude of the pulse width modulation voltage can be controlled from the fundamental wave amplitude maximum value in the sinusoidal wave PWM control to the fundamental wave amplitude value in the rectangular wave voltage control. 
     When the synchronization number K is an even number, on the other hand, preferably, the zero cross point of an absolute value of the phase voltage instruction is synchronized with the peak of the triangular wave carrier and the comparison results between the absolute value of the phase voltage instruction and the triangular wave carrier are inverted in the negative region of the phase voltage instruction (which is a region indicated by an arrow in  FIG. 13 ), thereby generating the output pulses, as illustrated in  FIG. 13 . However, when the overmodulation PWM controller  104  is configured by a microcomputer, it is difficult to instantaneously switch between inversion and non-inversion of the comparison results. Accordingly, in the present embodiment, when the synchronization number K is an even number, the zero cross point of the phase voltage instruction is synchronized with the peak of the triangular wave carrier (the relative phase of the phase voltage instruction and the triangular wave carrier is controlled to be 180°, as illustrated in  FIG. 12 . 
     For example, as illustrated in  FIG. 14(   a ), when the relative phase of the phase voltage instruction and the triangular wave carrier is controlled to be 90° in the case of the synchronization number K being an even number, the phase voltage instruction and the triangular wave carrier cross each other without fail when the value of the phase voltage instruction is 0, and consequently there are cases in which the comparison result (output pulse) between the phase voltage instruction and the triangular wave carrier does not result in one pulse per one cycle of the phase voltage instruction even if the voltage instruction amplitude is increased. On the other hand, by controlling the relative phase of the phase voltage instruction and the triangular wave carrier to be 180° as illustrated in  FIG. 14(   b ), it is possible to obtain the comparison result (output pulse) between the phase voltage instruction and the triangular wave carrier such that one pulse is generated per one cycle of the phase voltage instruction when the voltage instruction amplitude is increased. As a result, it is possible to control the fundamental wave amplitude of the pulse width modulation voltage from the maximum value of the fundamental wave amplitude in the sinusoidal wave PWM control to the fundamental wave amplitude value in the rectangular wave voltage control. Here,  FIGS. 14(   a ) and  14 ( b ) illustrate examples in which the synchronization number K is 6 and the update timing of the phase voltage instruction is the peak and the valley of the triangular wave carrier (a half cycle of the triangular wave carrier). 
       FIGS. 15 to 17  illustrate example relationships of the triangular wave carrier frequency fc and the synchronization number K with respect to the rotational speed we of the AC motor  300 . Here, it is preferable to make the frequency of the triangular wave carrier fc and the frequency of the phase voltage instruction fm multiples of 3 so as to avoid the beat phenomenon. Specifically,  FIG. 15  illustrates an example in which multiples of 3 (including both odd numbers and even numbers) are used as the synchronization number K (fc/fm),  FIG. 16  illustrates an example in which only multiples of 3 which are odd numbers are used as the synchronization number K, and  FIG. 17  illustrates an example in which multiples of 3 which are odd numbers and 6 are used as the synchronization number K. In the examples illustrated in  FIGS. 15 to 17 , the synchronization number K is decreased with an increase of the rotational speed we of the AC motor  300 . Within the range of the rotational speed we in which the synchronization number K remains unchanged, the triangular wave carrier frequency f increases with the rotational speed ωe. Further, while the amount of change of the triangular wave carrier frequency fc is increased when only the multiples of 3 which are odd numbers are used as the synchronization number K as illustrated in  FIG. 16 , the amount of change of the triangular wave carrier frequency fc can be decreased by using multiples of 3 which are odd numbers and those that are even numbers as the synchronization numbers K as illustrated in  FIGS. 15 and 17 . 
     According to the present embodiment described above, in the overmodulation PWM control, by correcting the voltage instruction amplitude by using a different voltage amplitude characteristic map in accordance with the synchronization number K, it is possible to suppress a change of the fundamental wave amplitude of the pulse width modulation voltage to be applied to the AC motor  300  caused by a change of the synchronization number K. Accordingly, in the overmodulation PWM control, even when the number of triangular wave carriers per one cycle of the phase voltage instruction is small (when the synchronization number K is small), it is possible to stably control the fundamental wave amplitude of the pulse width modulation voltage to conform to the voltage instruction amplitude. As a result, controllability of the overmodulation PWM control can be increased, and in particular, controllability can be increased in the high voltage region in which the number of triangular wave carriers per one cycle of the phase voltage instruction is small and is close to that of a rectangular wave (one pulse per one cycle of the phase voltage instruction). 
     In the description of the embodiment described above, when the synchronization number K is an odd number, a voltage amplitude characteristic map is created under conditions that the zero cross point of the phase voltage instruction from positive to negative is synchronized with the intermediate point of transition from the valley to peak of the triangular wave carrier, to thereby control the relative phase between the phase voltage instruction and the triangular wave carrier, and when the synchronization number K is an even number, a voltage amplitude characteristic map is created under conditions that the zero cross point of the phase voltage instruction is synchronized with the peak of the triangular wave carrier, to thereby control the relative phase between the phase voltage instruction and the triangular wave carrier. However, in the present embodiment, the relative phase of the phase voltage instruction and the triangular wave carrier can be set as desired. In order to change the relative phase of the phase voltage instruction and the triangular wave carrier, a voltage amplitude characteristic map in accordance with the phase is newly created. Similarly, the update timing of the phase voltage instruction can also be set as desired. In order to change the update timing of the phase voltage instruction and the triangular wave carrier, a voltage amplitude characteristic map in accordance with the update timing is newly created. 
     Further, in the present embodiment, when the update timing of the phase voltage instruction with respect to the triangular wave carrier is changed, the voltage amplitude linear compensation unit  22  can also correct the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  in accordance with the update timing of the phase voltage instruction to thereby correct the voltage instruction amplitude E B . In this case, the voltage amplitude characteristic storage unit  29  stores, in association with the update timing of the phase voltage instruction, a plurality of voltage amplitude characteristic maps in which the relationship between the voltage instruction amplitude and the fundamental wave amplitude of the pulse width modulation voltage varies depending on the update timing of the phase voltage instruction. For example, the voltage amplitude characteristic storage unit  29  is capable of storing a voltage amplitude characteristic map corresponding to a case in which the phase voltage instruction is updated for each half cycle of the triangular wave carrier (for each peak and valley of the triangular wave carrier, for example) and a voltage amplitude characteristic map corresponding to a case in which the phase voltage instruction is updated for each whole cycle of the triangular wave carrier (for each peak or each valley of the triangular wave carrier, for example). The voltage amplitude characteristic storage unit  29  is capable of further storing a voltage amplitude characteristic map corresponding to a case in which the phase voltage instruction is updated substantially continuously for each cycle which is sufficiently shorter than the triangular wave carrier. The voltage amplitude linear compensation unit  22  then reads from the voltage amplitude characteristic storage unit  29  a different voltage amplitude characteristic map in accordance with the update timing of the phase voltage instruction, and uses the thus-read voltage amplitude characteristic map to correct the d-axis voltage instruction vd B  and the q-axis voltage instruction vq B  (voltage instruction amplitude E B ). Thus, in the overmodulation PWM control, a change of the fundamental wave amplitude of the pulse width modulation voltage caused by a change of the update timing of the phase voltage instruction can be suppressed. Consequently, in the overmodulation PWM control, it is possible to stably control the fundamental wave amplitude of the pulse width modulation voltage to conform to the voltage instruction amplitude. 
     A preferred example of a case in which the control mode decision unit  108  selectively switches the control mode for the AC motor  300  will be now described. 
     The control mode decision unit  108  is capable of switching the control mode of the drive controller  100  in accordance with the flowchart illustrated in  FIG. 18 , for example. Once the control is started, the current control mode is determined in step S 10 . When the sinusoidal wave PWM controller  102  is selected, processing proceeds to step S 12 , and when the overmodulation PWM controller  104  is selected, processing proceeds to step S 14 . Further, when the rectangular wave voltage controller  106  is selected, processing proceeds to step S 16 . 
     In step S 12 , determination for switching the control mode is performed in accordance with an absolute value |V R | of a required voltage amplitude, using an absolute value |Vc P | of the peak value of the triangular wave carrier voltage as a reference. The required voltage amplitude V R  can be calculated based on the following formula:
 
[Formula 2]
 
Required voltage amplitude  V   R =power  P /(current amplitude | I |×power factor φ)  (2)
 
     Here, the absolute value of current amplitude |I| can be calculated from the following formula (3), and the power P and the power factor φ can be calculated from the following formulas (4) and (5), respectively: 
     
       
         
           
             
               
                 
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     When sinusoidal wave modulation in which the third harmonic components are not included is performed in the sinusoidal wave PWM controller  102 , the threshold voltage for control |V 1 | is set to the absolute value |Vc P | of the peak value of the triangular wave carrier voltage Vc. When the absolute value |V R | of the required voltage amplitude V R  is less than the threshold voltage |V 1 |, processing proceeds to step S 20 , and the AC motor  300  continues to be controlled by the sinusoidal wave PWM controller  102 . On the other hand, when the rotational speed of the AC motor  300  increases to make the absolute value |V R | of the required voltage amplitude V R  equal to or greater than the threshold voltage |V 1 | as indicated by a point X in  FIG. 19 , processing proceeds to step S 22 , where control of the AC motor  300  shifts from the sinusoidal wave PWM controller  102  to the overmodulation PWM controller  104 . Here, in  FIG. 19 , the horizontal axis indicates the rotational speed and the vertical axis indicates the voltage amplitude. 
     When processing proceeds to step S 14 , determination for switching the control mode is performed in accordance with the absolute value |V R | of the required voltage amplitude V R , using the absolute value |Vc P | of the peak value of the triangular wave carrier voltage as a reference. Here, the threshold voltage |V 2 | which is used as a reference for determination is set to 1.27 times (4/π times) the absolute value |Vc P | of the peak value of the triangular wave carrier voltage Vc. 
     Then, processing proceeds to step S 18  when the absolute value |V R | of the required voltage amplitude V R  is less than the threshold voltage |V 2 |, and processing proceeds to step S 24  when the absolute value |V R | of the required voltage amplitude V R  is equal to or greater than the threshold voltage |V 2 |. In step S 18 , processing proceeds to step S 20  when the absolute value |V R | of the required voltage amplitude V R  is less than the threshold voltage |V 1 −α 1 |, and processing proceeds to step S 22  when the absolute value |V R | of the required voltage amplitude V R  is equal to or greater than the threshold voltage |V 1 −α 1 |. Here, α 1  is an offset voltage value having a positive value, and is used to make the threshold voltages different between the case of shifting from the sinusoidal wave PWM control mode to the overmodulation PWM control mode and the case of shifting from the overmodulation PWM control mode to the sinusoidal wave PWM control mode, so as to prevent chattering in the control of the AC motor  300  between the sinusoidal wave PWM control mode and the overmodulation PWM control mode with the threshold voltage |V 1 | serving as a boundary therebetween. For example, it is preferable to set α 1  to a value which is 1% to 10% the threshold voltage |V 1 |. 
     When processing proceeds to step S 22 , the AC motor  300  continues to be controlled by the overmodulation PWM controller  104 . When the absolute value |V R | of the required voltage amplitude V R  is equal to or greater than the threshold voltage |V 2 | as indicated by a appoint Y in  FIG. 19  and processing proceeds to step S 24 , the control of the AC motor  300  shifts from the overmodulation PWM controller  104  to the rectangular wave voltage controller  106 . On the other hand, when the absolute value |V R | of the required voltage amplitude V R  is less than the threshold voltage |V 1 −α 1 | and processing proceeds to step S 20 , the control of the AC motor  300  is returned to the sinusoidal wave PWM controller  102  from the overmodulation PWM controller  104 . 
     When processing proceeds to step S 16 , processing proceeds to step S 24  when the absolute value |V R | of the required voltage amplitude V R  is equal to or greater than the threshold voltage |V 2 −α 2 |, and processing proceeds to step S 22  when the absolute value |V R | of the required voltage amplitude V R  is less than the threshold voltage |V 2 −α 2 |. Here, α 2  is an offset voltage value having a positive value, and is used to make the threshold voltages different between the case of shifting from the overmodulation PWM control mode to the rectangular wave voltage control mode and the case of shifting from the rectangular wave voltage control mode to the overmodulation PWM control mode, so as to prevent chattering in the control of the AC motor  300  between the overmodulation PWM control mode and the rectangular wave voltage control mode, with the threshold voltage |V 2 | serving as a boundary therebetween. For example, it is preferable to set α 2  to a value which is 1% to 10% the threshold voltage |V 2 |. 
     When processing proceeds to step S 24 , the AC motor  300  continues to be controlled by the rectangular wave voltage controller  106 . On the other hand, when the absolute value |V R | of the required voltage amplitude V R  is smaller than the threshold voltage |V 2 −α 2 | and processing proceeds to step S 22 , the control of the AC motor  300  is returned from the rectangular wave voltage controller  106  to the overmodulation PWM controller  104 . 
     By performing switching of the control mode of the AC motor  300  by using the required voltage amplitude V R  as described above, delay in switching of the control from a control state by the rectangular wave voltage controller  106  (point Z in  FIGS. 19 and 20 ) to a control state by the overmodulation PWM controller  104  (between points X and Y in  FIGS. 19 and 20 ) can be eliminated. This can reduce the amount of shift of the current phase of the current to be supplied to the AC motor  300  from the optimal current phase, so that hunting (oscillation) can be suppressed. Consequently, the drive control of the AC motor  300  can be stabilized. 
     Here, when modulation in which the third harmonic components are included is performed in the sinusoidal wave PWM controller  102 , the threshold value |V 1 | for control is preferably set to 1.15 times the absolute value |Vc P | of the peak value of the triangular wave carrier voltage Vc. In this case, switching of the drive control mode of the AC motor  300  in accordance with the flowchart of  FIG. 18  is as shown  FIG. 21 . In  FIG. 21 , the horizontal axis indicates the rotational speed and the vertical axis indicates the voltage amplitude. 
     In the description of the embodiment described above, a controller which controls the voltage to be applied to the AC motor  300  is selectively switched among the sinusoidal wave PWM controller  102 , the overmodulation PWM controller  104 , and the rectangular wave voltage controller  106 . However, in the present embodiment, it is possible to omit the rectangular wave voltage controller  106  and selectively switch between the sinusoidal wave PWM controller  102  and the overmodulation PWM controller  104  as a controller which controls the voltage to be applied to the AC motor  300 . 
     Although embodiments for implementing the present invention have been described above, the present invention is not limited to these embodiments and can be implemented in various manners within the scope of the invention.