CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on, and claims priority to, Japanese Patent Application No. 2012-185102, filed on Aug. 24, 2012, contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor driving system that employs either an estimated value of a magnetic pole position or a detected value of a magnetic pole position corresponding to a normal or abnormal condition in sensorless control of the motor driving system. The invention also relates to an electric vehicle provided with such a motor driving system. 
     2. Description of the Related Art 
     In order to reduce a size and a cost of a driving system for a synchronous motor, a drive control method so-called sensorless control method has been proposed that does not employ a magnetic pole position detecting device to detect the magnetic pole position of a rotor. 
     The sensorless control, as is well known, executes estimation operation of the magnetic pole position of a rotor from the information on the terminal voltage and the armature current of the motor, and performs torque control and speed control of the motor by controlling current based on the estimated magnetic pole position. 
     Known traditional technologies for a motor driving system employing the sensorless control method are disclosed in Japanese Unexamined Patent Application Publication No. 2001-112282 and Japanese Unexamined Patent Application Publication No. 2007-209105, for example. 
     Japanese Unexamined Patent Application Publication No. 2001-112282 (paragraphs [0007] and [0011], and FIG. 1, in particular) discloses a motor control device for driving a motor, comprising a magnetic pole position detecting sensor and a magnetic pole position estimating device that employs sensorless control. In normal operating conditions, the motor is driven based on the position value detected by the magnetic pole position detecting sensor, and if any failure has occurred in the magnetic pole position detecting sensor, the control is changed over to execution based on the position value estimated by the magnetic pole position estimating device. 
     Japanese Unexamined Patent Application Publication No. 2007-209105 (Paragraphs [0013] through [0020], and FIGS. 1 and 2, in particular) discloses an electric vehicle driving device that compares a detected value of a magnetic pole position from a magnetic pole position detector and an estimated value of the magnetic pole position by a sensorless controller. If the difference between the detected value and the estimated value exceeds a predetermined value, some failure in the magnetic pole position detector is determined. 
     In the conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-112282, although the information on the magnetic pole position is doubled improving reliability, the provision of the magnetic pole position detector, an abnormality detecting device, and the magnetic pole position estimating device enlarges the system and raises the costs. 
     In the conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2007-209105, the abnormality of the magnetic pole position detector is determined based on the assumption that the magnetic pole position value estimated by the sensorless controller is correct. As a consequence, if the sensorless control itself becomes unstable, the accuracy of magnetic pole position estimation deteriorates, which leads to erroneous determination of abnormality of the magnetic pole position detector, and even run-away of the motor may occur. 
     A motor driving system having a sensorless control algorithm and without a magnetic pole position detector should be smaller and cheaper than a traditional driving system using a magnetic pole position detector. Such a motor driving system without a magnetic pole position detector has an additional advantage that the failure rate of the overall system is reduced because of elimination of problems due to vibration, heating, or noise. 
     If the reliability of the sensorless control is not guaranteed, however, a driving system depending on the sensorless control cannot be employed without hesitation for such an application as electric vehicles that require absolute safety. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a motor driving system that can be guaranteed reliable and employs a sensorless control algorithm while ensuring safety. Another object of the invention is to provide an electric vehicle exhibiting high degree of safety by installing such a motor driving system that has been guaranteed reliable therein. 
     To accomplish the above object, an aspect of the present invention is a motor driving system for controlling a torque or a velocity of a synchronous motor by converting a DC power from a DC power supply into an AC power through a power converter and delivering the AC power to the motor, the motor driving system having a control device for controlling the power converter, the control device comprising a sensorless control algorithm that generates a magnetic pole position estimated value of the motor used for controlling the power converter in a normal state of the sensorless control algorithm, wherein the sensorless control algorithm comprises a first abnormality determining means and when the first abnormality determining means determines abnormality in the sensorless control algorithm, the control device controls the power converter using a magnetic pole position detected value from a magnetic pole position detecting means attached to the motor in place of using the magnetic pole position estimated value. 
     When the sensorless control operation becomes unstable due to any reason, a motor driving system of this aspect of the invention allows the motor operation being continued by changing-over to control based on normal magnetic pole position information provided by the magnetic pole position detecting means. Thus, a motor driving system of the invention prevents the motor from running out of control such as abrupt acceleration or deceleration and enables verification tests of the sensorless control under a sufficiently stable condition. 
     Preferably, the first abnormality determining means determines abnormality of the sensorless control algorithm based on a magnetic pole position error estimated value obtained by executing operation using at least armature current detected value of the motor. 
     Preferably in particular, the first abnormality determining means determines abnormality of the sensorless control algorithm according to the magnetic pole position error estimated value that has exceeded a predetermined angle. 
     In these aspects of the invention, determining abnormality can be performed without using the information from the magnetic pole position detecting means but executing operation of magnetic pole position error estimated value based on magnetic pole position information extracted from the armature current for use in sensorless control. Thus, abnormality in the sensorless control itself, if any, can be detected. The abnormality of the sensorless control algorithm can be determined separately from the abnormality of the magnetic pole position detecting means. Consequently, uncontrolled running of the motor due to erroneous determination as in Japanese Unexamined Patent Application Publication No. 2007-209105 is avoided and safety is assured by changing-over to the control based on normal information on the magnetic pole position provided by the magnetic pole position detecting means. 
     Preferably, the control device comprises a first alarm generating means that generates an alarm signal when the first abnormality determining means determines abnormality in the sensorless control algorithm. 
     This aspect of the invention allows the operator to recognize the abnormality of the sensorless control algorithm. As a consequence, the operator can control by handling according to the operator&#39;s own intension based on the magnetic pole position detected value, thereby stopping the motor driving system with safety. 
     Preferably, the control device comprises a data storage means that stores information of the sensorless control algorithm during a predetermined period of time before and after occurrence of abnormality in the sensorless control algorithm. The information of the sensorless control algorithm preferably includes, input data and output data into and out of the sensorless control algorithm. The data can specifically be armature current detected values of the motor, voltage command values, and magnetic pole position estimated value. 
     These data can be used for analysis of reason for abnormality in the sensorless control algorithm, contributing improvement of reliability of the sensorless control. 
     Preferably, the control device comprises a second abnormality determining means for determining abnormality in the magnetic pole position detecting means and, if the second abnormality determining means determines abnormality in the magnetic pole position detecting means during control operation of the power converter according to the magnetic pole position estimated value generated by the sensorless control algorithm, the control operation of the power converter is continued according to the magnetic pole position estimated value. This aspect of the invention allows the motor driving system to continue its operation even though any abnormality occurs in the magnetic pole position detecting means during testing of the sensorless control algorithm, thereby avoiding abrupt stop of the motor and enhancing safety. 
     Preferably, the control device comprises a second alarm generating device for generating an alarm signal using an output of the second abnormality determining means. This aspect of the invention gives a warning to the operator about abnormality in the magnetic position detecting means. 
     Preferably, a motor driving system without a magnetic pole position detecting means can be constructed, wherein reliability of the sensorless control algorithm has been guaranteed through normality determination by means of the motor driving system. This aspect of the invention provides a motor driving system with a small overall size, a low manufacturing cost, and a reduced failure rate. These characteristics are best fits to application to electric vehicles. 
     A motor driving system of the present invention certifies reliability of the sensorless control algorithm with the first abnormality determining means provided in the control device. A motor driving system equipped with a sensorless control algorithm that has been guaranteed reliable performs a smaller size, a lower price, and a reduced failure rate than a driving system having a magnetic pole position detecting means. A motor driving system of the invention also contributes to improving safety of an electric vehicle provided with such a driving system. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  defines d- and q-axes as well as γ- and δ-axes; 
         FIG. 2  shows an example of structure of a motor driving system according to a first embodiment of the present invention; 
         FIG. 3  shows a first example of a sensorless control algorithm in the motor control system of  FIG. 2 ; 
         FIG. 4  shows a second example of a sensorless control algorithm in the motor control system of  FIG. 2 ; 
         FIG. 5  shows an example of structure of a motor driving system according to a second embodiment of the present invention; 
         FIG. 6  shows an example of a structure of a motor driving system according to a third embodiment of the present invention; 
         FIG. 7  shows an example of a structure of the data storage means in the motor driving system of  FIG. 6 ; 
         FIG. 8  is a time chart showing operation of the data storage means of  FIG. 7 ; 
         FIG. 9  shows an example of a structure of a motor driving system according to a fourth embodiment of the present invention; 
         FIG. 10  shows an example of a structure of a motor driving system according to a fifth embodiment of the present invention; and 
         FIG. 11  shows an example of a structure of a motor driving system according to a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following describes some preferred embodiments of the present invention with reference to the accompanying drawings. The embodiments described in the following are motor driving systems that are applied to control the torque of a permanent magnet synchronous motor. 
     First described is sensorless control of a permanent magnet synchronous motor. 
     So-called sensorless control, being unable to directly identify a magnetic pole position of a rotor on d-axis and q-axis coordinates, uses an estimating rotating γ-axis and δ-axis coordinates instead, to control the torque and speed of the synchronous motor.  FIG. 1  defines the d-axis and q axis as well as the γ-axis and δ-axis. The d-axis is the axis in the direction of N-pole of the rotor of the permanent magnet synchronous motor; the q-axis is the axis in advance by 90 degrees from the d-axis; the γ-axis is an estimated axis corresponding to the d-axis; and the δ-axis is the axis in advance by 90 degrees from the γ-axis. 
     In  FIG. 1 , ω r  designates an electric angular velocity of the d-axis and the q axis; ω 1  is an electric angular velocity of the γ-axis and the δ-axis, which is an estimated velocity; and θ err  designates the angle of the γ-axis with respect to the d-axis and, at the same time, the angle of the δ-axis with respect to the q-axis. The θ err  is represented by Mathematical Formula 1 below.
 
θ err =θ 1 −θ r ,  [Mathematical Formula 1]
 
where θ 1  and θ r  are angles of γ-axis and d-axis, respectively,
 
       FIG. 2  shows a structure of a motor driving system according to a first embodiment of the present invention. The motor driving system of  FIG. 2  is composed of a main circuit and a control device  4 A. The main circuit comprises a permanent magnet synchronous motor  1 , a power converter  2  such as an inverter, a DC power supply  3 , and a magnetic pole position detector  30  (a magnetic pole position detecting means) that is additionally provided to guarantee reliability of sensorless control. The control device  4 A comprises a sensorless control algorithm device  20 , which is a feature of the present invention. 
     The following describes the structure and operation of the control device  4 A. 
     In the control device  4 A in  FIG. 2 , a current command operator  12  executes operation of γ-axis and δ-axis current command values i γ * and i δ * for controlling an output torque of the motor  1  to a torque command value τ*. A u-phase current detector  5   u  and a w-phase current detector  5   w  deliver a u-phase current detected value i u  and a w-phase current detected value i w , which then are given to a current coordinate transformation device  6 . The current coordinate transformation device  6  executes coordinate transformation from the detected current values i u  and i w  to γ-axis and δ-axis detected current values i γ  and i δ  using an estimated value of a magnetic pole position θ 1  or a detected value of a magnetic pole position θ r . 
     The deviation of the γ-axis current command value i γ * delivered by the current command operator  12  from the γ-axis current detected value i γ  is obtained in a subtractor  11   a . This deviation is given to a γ-axis current regulator  10   a , which amplifies the deviation and executes operation to give a γ-axis voltage command value v γ *. In the same way, the deviation of the δ-axis current command value i δ * from the δ-axis current detected value i δ  is obtained in a subtractor  11   b . This deviation is given to a δ-axis current regulator  10   b , which amplifies the deviation and executes operation to give a δ-axis voltage command value v δ *. The γ-axis and δ-axis voltage command values v γ * and v δ * are transformed to phase voltage command values v u *, v v *, and v w * in a voltage coordinate transformation device  9  using the magnetic pole position estimated value θ 1  or the magnetic pole position detected value θ r . 
     A voltage detecting circuit  7  detects a DC voltage E dc  supplied by the DC power supply  3  to the power converter  2 . A PWM circuit  8  generates gate signals to control the output voltage of the power converter  2  to be the phase voltage command values v u *, v v *, and v w * according to the phase voltage command values v u *, v v *, and v w *, and the detected DC voltage E dc . The power converter  2  controls operation of semiconductor elements such as IGBTs provided in the power converter  2  according to the gate signals to obtain the terminal voltages of the motor  1  that equal the phase voltage command values v u *, v v *, and v w * thereby achieving an output torque of the motor  1  that equals the torque command value τ*. 
     The output signal of the magnetic pole position detector  30  is delivered to a magnetic pole position operator  31 . The magnetic pole position operator  31  executes operation of a magnetic pole position detected value θ r , which is given to an input terminal of a change-over switching means  32 . The other input terminal of the change-over switching means  32  receives a magnetic pole position estimated value θ 1  that is generated in the sensorless control algorithm device  20 . 
     The change-over switching means  32  selects either the magnetic pole position estimated value θ 1  or the magnetic pole position detected value θ r  corresponding to a flag flg SLerr  delivered by the sensorless control algorithm device  20  and delivers the selected magnetic pole position value to the current coordinate transformation device  6  and the voltage coordinate transformation device  9 . 
     The sensorless control algorithm device  20  having a structure described below generates the magnetic pole position estimated value θ 1  and the flag flg SLerr  based on the γ-axis and δ-axis voltage command values v γ * and v δ * and the γ-axis and δ-axis current detected values i γ  and i 6 . 
       FIG. 3  shows a first example of the sensorless control algorithm device  20  that uses extended induced voltage in sensorless control. The specific example of sensorless control algorithm of  FIG. 3  is designated by a symbol  20 A. 
     Referring to  FIG. 3 , the sensorless control algorithm  20 A comprises an extended induced voltage operator  21 , a position estimation error operator  22 , a velocity estimating device  23 , a magnetic pole position estimating device  24 , and an abnormality determining device  25 . 
     The extended induced voltage operator  21  executes operation of extended induced voltages θ exγ  and θ exδ  as shown by the Mathematical Formula 2 below based on the γ-axis and δ-axis voltage command values v γ * and v δ *, the γ-axis and δ-axis current detected values i γ  and i δ , an angular velocity estimated value ω 1 , and motor parameters. 
     
       
         
               
             
           
               
                   
               
             
             
               
                 Mathematical Formula 2 
               
               
                 
                   
                     
                       
                         
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                                           1 
                                         
                                       
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                                         q 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       
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                 e exγ : γ-axis extended induced voltage 
               
               
                 e exδ : δ-axis extended induced voltage 
               
               
                 v γ *: γ-axis voltage command value 
               
               
                 v δ *: δ-axis voltage command value 
               
               
                 i γ : γ-axis current detected value 
               
               
                 i δ : δ-axis current detected value 
               
               
                 L d : d-axis inductance 
               
               
                 L q : q-axis inductance 
               
               
                 ω 1 : angular velocity estimated value 
               
               
                 r a : armature resistsance 
               
               
                 p: differentiation operator 
               
             
          
         
       
     
     The position estimation error operator  22  executes operation of a magnetic pole position error estimated value (hereinafter referred to simply as position error estimated value) δ eex  from the γ-axis extended induced voltage θ exγ  and the δ-axis extended induced voltage θ exδ  according to Mathematical Formula 3 below.
 
δ eex =tan −1 (− e   exγ   /e   exδ )  [Mathematical Formula 3]
 
     The angular velocity estimating device  23  is composed of a PI regulator and executes operation of an angular velocity estimated value ω1 by amplifying the position error estimated value δ eex  according to Mathematical Formula 4 below.
 
ω1=( K   P   +K   I   /s )δ eex   [Mathematical Formula 4]
 
where K p  is a proportional gain, K I  is an integral gain, and s is a Laplace operator.
 
     The magnetic pole position estimating device  24  executes operation of the magnetic pole position estimated value θ 1  by integrating the angular velocity estimated value ω 1  according to Mathematical Formula 5 below.
 
θ 1 =∫ω 1   dt   [Mathematical Formula 5]
 
     The abnormality determining device  25 , as shown by Mathematical Formula 6 below, sets a flag flg SLerr  to “1” when the sensorless control algorithm  20 A becomes abnormal due to a certain event, and sets a flag flg SLerr  to “0” when the sensorless control algorithm  20 A is in a normal state.
 
flg SLerr =0 in a normal state of the sensorless control
 
flg SLerr =1 in a abnormal state of the sensorless control  [Mathematical Formula 6]
 
     Returning back to  FIG. 2 , the change-over switching means  32  selects the magnetic pole position estimated value θ 1  delivered by the magnetic pole position estimating device  24  in the sensorless algorithm  20 A when the flag flg SLerr  is “0”, and selects the magnetic pole position detected value θ r  delivered by the magnetic pole position operator  31  when the flag flg SLerr  is “1”. 
     This selection means that in an abnormal state of the sensorless control algorithm  20 A, the magnetic pole position for use in the current coordinate transformation device  6  and the voltage coordinate transformation device  9  is changed over from the magnetic pole position estimated value θ 1  to the magnetic pole position detected value θ r . Thus, the torque control of the motor  1  continues without interruption. 
       FIG. 4  shows a second example of the sensorless control algorithm. This specific example of sensorless control algorithm of  FIG. 4  is designated by a reference symbol  20 B. 
     The sensorless control algorithm  20 B carries out abnormality determination in the abnormality determining device  25  using the Mathematical Formulas 2 and 3, which give position error estimated value δ eex  based on the γ-axis current detected value i γ  and the δ-axis current detected value i δ . The sensorless control algorithm  20 B carries out abnormality determination according to this position error estimated value δ eex  and sets the flag flg SLerr . 
     The position error estimated value δ eex  is an estimated angular difference between the d-axis of the motor and the estimated γ-axis. Thus, the determination of abnormality in sensorless control is performed solely based on the input information to the sensorless control algorithm  20 B without using the information from the magnetic pole position detector  30 . 
     A specific example of abnormality determination in the abnormality determination device  25  is as follows. As shown by Mathematical Formula 7 below, the sensorless control algorithm  20 B determines abnormality and sets the flg SLerr  to the value “1” when the absolute value of the position error estimated value δeex exceeds a predetermined angle θ errmax .
 
flg SLerr =0 for |δ eex |≦θ errmax .
 
flg SLerr =1 for |δ eex |&gt;θ errmax .  [Mathematical Formula 7]
 
       FIG. 5  shows an example of structure of a motor driving system according to a second embodiment of the present invention. 
     A control device  4 B of the motor driving system according to the second embodiment has an alarm generator  40  added to the control device  4 A shown in  FIG. 2 . The alarm generator  40  generates a first alarm alarm1 when the sensorless control algorithm device  20  becomes abnormal setting the flg SLerr  to the value “1”. The alarm1 can be a warning sound or a warning light giving a caution to the operator. 
     Noticing the alarm, the operator recognizes that the sensorless control algorithm has become abnormal and that the magnetic pole position for use in the current coordinate transformation device  6  and the voltage coordinate transformation device  9  has changed over from the magnetic pole position estimated value θ 1  to the magnetic pole position detected value θ r . Thus, the operator can safely manually stop the motor driving system. 
       FIG. 6  shows an example of structure of a motor driving system according to a third embodiment of the invention. 
     The control device  4 C of the motor driving system according to the third embodiment has a data storage means  50  that is essentially composed of a memory. The date storage means  50  stores the input and output data of the sensorless control algorithm device  20 , the data including θ 1 , v γ *, v δ *, i γ , and i δ  in the example of  FIG. 6 , during a specified period of time around the occurrence of abnormality in the sensorless control algorithm corresponding to the value of the flag flg SLerr . 
     The data storage means  50 , as shown in  FIG. 7 , comprises a ring buffer  52  that is composed using a memory  51  in a random access memory (RAM) or static RAM (SRAM) within a CPU and is capable of storing data during a certain period of time. The data storage means  50  also has a delay element  53 . 
     In a normal state of the sensorless control algorithm device  20 , in which flg SLerr =“0”, data are written in real time sequentially from the top address of the ring buffer  52 . When all the addresses of the ring buffer  52  are filled with written data, the address retunes to the top address and the next data is overwritten on the address. If some abnormality occurs in the sensorless control algorithm device  20 , in which flg SLerr =“1”, the delay element  53  delays the flag flg SLerr  by a predetermined time to generate a signal flg delay . When the signal flg delay  becomes “1” after the predetermined time, the date storage means  50  stops data writing into the ring buffer  52 . 
     Thus, the data storage means  50  stores data in the predetermined period of time before and after the occurrence of the abnormality in the sensorless control algorithm device  20  as illustrated in  FIG. 8 . The data in  FIG. 8  is one of the input and output data θ 1 , v γ *, v δ *, i γ , and i δ  mentioned above in conceptual representation. 
       FIG. 9  shows an example of structure of a motor driving system according to a fourth embodiment of the present invention. 
     A control device  4 D of the motor driving system according to the fourth embodiment is provided, in addition to the control device  4 A of  FIG. 2 , with a second abnormality determining device  33  for determining abnormality in the magnetic pole position detector  30 , The second abnormality determining device  33  detects occurrence of abnormality, including breaking of a wire, in the magnetic pole position detector  30  through the magnetic pole position operator  31  and delivers a flag flg Serr  as indicated in Mathematical Formula 8 below. The abnormality determining device  33  can take in directly the output signal of the magnetic pole position detector  30 .
 
flg Serr =0 for normal state of the magnetic pole position detector
 
flg Serr =1 for abnormal state of the magnetic pole position detector including wire breaking, for example.  [Mathematical Formula 8]
 
     Even though the magnetic pole position detector  30  is in an abnormal state, in which flag flg Serr =“1”, the motor driving system of this fourth embodiment continues driving the motor  1  according to the flag flg SLerr =“0” from the sensorless control algorithm device  20  using the magnetic pole position estimated value θ 1  based on the sensorless control. Thus, despite occurrence of any abnormality in the magnetic poles position detector  30  in operation of the motor driving system, the motor  1  does not stop abruptly, and instead, can be manually stopped safely according to the operator&#39;s intention. 
       FIG. 10  shows an example of structure of a motor driving system according to a fifth embodiment of the present invention. 
     A control device  4 E of the motor driving system according to the fifth embodiment is provided, in addition to the control device  4 D shown in  FIG. 9 , with a second alarm generator  41  at the next stage of the abnormality determining device  33 . The second alarm generator  41  generates a second alarm alarm2 when the magnetic pole position detector  30  becomes abnormal setting the flg Serr  to the value “1”. The second alarm alarm2 can be a warning sound or a warning light giving a caution to the operator. Thus, the operator can recognize occurrence of some abnormality in the magnetic pole position detector  30  and manually stop the motor driving system safely. 
       FIG. 11  shows an example of structure of a motor driving system according to a sixth embodiment of the present invention. 
     In this sixth embodiment, at first, reliability of the sensorless control algorithm device  20  is guaranteed by means of one of the first through fifth embodiments. After notifying the flag flg SLerr =“0” and guaranteeing the normal state of the sensorless control algorithm device  20 , a control device  4 F as shown in  FIG. 11  is constructed that does not have any one of: the magnetic pole position detector  30 , the magnetic pole position operator  31 , the change-over switching means  32 , the alarm generators  40  and  41 , the date storage means  50 , the abnormality determining device  33 , which are included in the motor driving systems according to the first through fifth embodiments. In  FIG. 11 , the same reference symbols to the components of the main circuit and the control device  4 F are given as those in the first through fifth embodiments. 
     Because the reliability of the sensorless control algorithm has already been guaranteed in the motor driving system of the sixth embodiment that lacks the magnetic pole position detector  30  and related components, the motor  1  can be driven without reliance on the magnetic pole position detector  30 , the system as a whole has a small size and can be manufactured at a low cost. Moreover, the failure rate is reduced because any failure of a magnetic pole position detector due to problems of such as vibration, heat and noise can be excluded from consideration. 
     This motor driving system according to the fifth embodiment is preferably installed in electric vehicles. In that case, the DC power supply  3  can be the onboard battery, and the torque command value τ* can be given to the current command operator  12  based on the stroke of the accelerator pedal or the brake pedal. 
     The motor driving system of the present invention can be applied to transportation including electric vehicles and a wide variety of other industrial devices and equipment. 
     Although the invention has been described with regard to specific embodiments, it may be practiced otherwise than as specifically described herein.
A motor driving system is disclosed having a control device 4 A for controlling a synchronous motor 1, the control device 4 A comprising a sensorless control algorithm device 20 that includes an abnormality determining device 25 for determining abnormality of the algorithm based on a magnetic pole position error estimated value of the motor 1. When the abnormality determining device 25 has determined abnormality of the algorithm, the control device 4 A controls a power converter 2 using a magnetic pole position detected value detected by a magnetic pole position detector 30 attached to the motor, in place of using a magnetic pole position estimated value. This motor driving system can guarantee reliability of the sensorless control algorithm device 20 while assuring safety. Safety of electric vehicles is enhanced by installing the motor driving system that has been guaranteed reliable.