Abstract:
A controller for electric vehicles comprising a vector control inverter for controlling motors, which drive wheels of the electric vehicles, by dividing the primary current of the motors into an exciting current component and a torque current component, and controlling respective of the current components based on a respectively designated command, characterized in further comprising a dectector for detecting a wheel velocity (including a rotor frequency of the motor proportional to the wheel velocity), a detector for detecting slipping and skidding of the wheel based on a differential value (a changing rate with time) of the detected wheel velocity, a dectector for detecting re-adhesion of the wheel based on the differential value and a twice differential value of the detected wheel velocity, and an adjuster for adjusting the designated command for the torque current component in response to the detectors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a controller for electric vehicles driven by motors with an inverter using vector control, particularly, a controller for electric vehicles performing torque control for re-adhering by detecting slipping or skidding generated between wheels and rails. 
     As a prior art, a method for controlling re-adhesion by detecting slipping or skidding of wheel of electric vehicles, and reducing generated torque of motors has been disclosed in JP-A-4-197004 (1992). A method for detecting re-adhesion of wheel was disclosed in JPA-4-69003 (1992). A technique for driving motors of electric vehicles of railway by vector control of inverters was disclosed in JP-A-5-83976 (1993). 
     In accordance with the conventional re-adhesion controlling method disclosed in the above JP-A-4-197004 (1992), the slipping of wheel is detected by a method which recognizes whether a changing rate with time (differential value) of the rotor frequency (proportional to the wheel velocity) of the induction motor exceeds a fixed detecting level or not, and a control to reduce the motor torque is performed only during a period when the slipping is detected. However, if the differential value becomes smaller than a designated value, the recognition of the slipping is canceled, and the motor torque is controlled to resume irrelevant to whether the wheel are practically re-adhered or not. Therefore, if the wheel is not re-adhered practically, slipping the wheel occurs instantaneously, and a problem occurs that the slipping phenomena are generated very often repeatedly. 
     A method for controlling the torque by detecting the re-adhesion is disclosed in the above JP-A-4-69003 (1992), and the method for detecting the re-adhesion is explained hereinafter referring to FIG.  10 . The re-adhesion is detected by recognizing that a twice differential value fr″ (an axis jerking value) of the rotor frequency fr (proportional to the wheel velocity) at the time t2 exceeds a designated value Le, after detecting the slipping at the time t1. 
     However, the following problem can be anticipated with the detection of slipping using the twice differential value fr″. First, if the wheel velocity behaves as shown in FIG. 11 when re-adhering, the twice differential value fr″ of the wheel velocity does not exceed the designated value Le, and the re-adhesion can not be detected nevertheless the re-adhesion occurred at the time t2. Consequently, the torque is maintained in a reduced condition continuously, and a problem that the reduced acceleration of the electric vehicle is generated. Furthermore, in a case if the slipping, which has been likely to converge on an end, is re-generated at the time t1a as shown in FIG. 12, the twice differential value fr″ exceeds the designated value Le at the time t1a, and a problem that the re-adhesion is erroneously detected and the slipping is continued is generated. 
     These kind of problems can be caused when the skidding is generated. As explained above, the conventional technology has a problem that the re-adhesion can not be detected, or the re-adhesion is detected erroneously depending on conditions of slipping or skidding. 
     Currently, an inverter with vector control such as disclosed in JP-A-5-83976 (1993) come to be used as a controller of induction motors for driving electric vehicles. However, any technology to utilize performance of the vector control for controlling the re-adhesion has not been disclosed. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a controller for electric vehicles, which can utilize the torque effectively to the physical limit of adhesion by utilizing a fast torque response of the vector control, and can make the velocities of acceleration and deceleration of the electric vehicles as high as possible even in a condition when the adhesion coefficient is low. 
     The present invention relates to a controller for electric vehicles comprising a vector control inverter, which controls motors for driving wheels of the electric vehicles, by dividing the primary current of the motor into an exciting current component and a vector current component, and controlling respective of the current components based on a respectively designated command; further comprises means for detecting a wheel velocity (including rotor frequency of the motors proportional to the wheel velocity), means for detecting slipping and skidding of the wheel based on a differential value (a changing rate with time) of the detected wheel velocity, means for detecting re-adhesion of the wheel based on the detected differential value and a twice differential value of the detected wheel velocity, and means for adjusting the designated command for the torque current component in response to the above two means for detection. 
     In accordance with the above method of the present invention, if the differential value of the wheel velocity exceeds a designated value when the electric vehicle is accelerated, it can be regarded as slipping occurs, and the control to reduce the torque current is performed. As the result, when the slipping velocity is decreased and re-adhesion occurs, the acceleration of the wheel is resumed. The re-adhesion point can be detected as a point when the differential value of the wheel velocity becomes negative and the twice differential value becomes positive. According to the above point, it can be determined that the slipping of the wheel is certainly converging on an end, the wheel is re-adhered, and the acceleration of the wheel is resumed. The torque current is maintained in a reduced condition until the re-adhesion is detected. Therefore, even if the torque current is resumed fast after the re-adhesion occurred, a possibility to cause re-slipping can be made low by confirming the re-adhesion. Consequently, the torque can be increased fast as much as the torque current is resumed fast, and the acceleration of the electric vehicle can be increased. 
     Even if the slipping occurs and before the re-adhesion is detected, the slipping is converging on an end when the differential value of the wheel velocity is decreased. Consequently, the decrease of the torque can be reduced by reducing the decrease of the torque current, and the acceleration can be increased as much as reducing the decrease of the torque. If skidding occurs when the electric vehicle is decelerated, the theory is quite same except the sign of detecting level is opposite. 
     As explained above, if the motor for driving the wheel is vector controlled, the torque current component in the primary current of the motor can be controlled independently. The torque current control influences only to leakage impedance of the motor, and it has a feature that time constant is small and control response is fast. Accordingly, if the re-adhesion control of the present invention is performed with the vector control, the re-adhesion performance having a fast response can be naturally obtained, and the torque can be utilized effectively to the physical adhesion limit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the controller indicating an embodiment of the present invention. 
     FIG. 2 is an illustration indicating a detailed composition of a slipping-skidding detector  2  shown in FIG.  1 . 
     FIG. 3 is an illustration indicating a detailed composition of a re-adhesion detector  3  shown in FIG.  1 . 
     FIG. 4 is an illustration indicating a detailed composition of a torque current controller  6  shown in FIG.  1 . 
     FIG. 5 is an illustration indicating a detailed composition of a differentiator  4  shown in FIG.  1 . 
     FIG.  6 -FIG. 8 are figures for explaining operations of the present invention. 
     FIG. 9 is a block diagram of the controller indicating the second embodiment of the present invention. 
     FIG.  10 -FIG. 12 are figures for explaining operations of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention is explained referring to figures. FIG. 1 is a block diagram indicating an outline of a control composition in a controller of electric vehicles, wherein induction motors are driven by converting a direct current to an alternate current with a vector control inverter. Although each of the blocks in FIG. 1 is indicated by a name of the apparatus for facilitating the explanation of the present invention, the block can be a software for microcomputer which treats the functions depending on necessity. 
     In accordance with FIG. 1, a driving command P, or a braking command B output from the operating device  54 , and a signal  8  on a rotor frequency fr obtained by a rotation velocity detector  7  connected to the induction motor  60  are input into a current command calculator  56 ; and an exciting current command Id and a current pattern Iqp are generated. A torque current command Iq is calculated by a subtracter  116  from a difference between the Iqp and a torque current control value ΔIqp obtained from a slipping-skidding controller  1 . The Iq, the rotor frequency fr, and motor current detected values, iu, iv, and iw obtained by current detectors  61 ,  62 , and  63  are input into a vector control calculator  57 ; and a voltage command for output voltage of the inverter is generated. In accordance with a PWM signal calculator  58 , a PWM signal is generated by comparison of the voltage command with a carrier of chopping wave which is not shown in the figure, and the PWM signal is output as a gate signal. A PWM inverter  59  operates switching elements composing a main circuit with the gate signal; a direct current obtained from a direct current power source  52  via a filter condenser  53  is converted to three phase alternate current electric power; and the electric power is supplied to the induction motor  60 . 
     The wheels of the electric vehicle (not shown in the figure) are driven by the above induction motor, and the wheel velocity and the rotation velocity of the induction motor are in a proportional relationship. Details of composition and operation of the above calculators  56 ,  57 ,  58  and the PWM inverter  59  are disclosed in JP-A-5-83976 (1993), and are not described herewith. The present invention is based on a premise that the electric vehicles driven by the vector control PWM inverter having a composition described above is controlled. 
     Next, referring to FIG. 1, the composition of the slipping-skidding controller  1 , which is the present invention, is explained. Its details are explained referring to FIG.  2 -FIG.  5 . The controller  1  comprises a differentiator  4 , a slipping-skidding detector  2 , a re-adhesion detector  3 , and a torque current controller  6 . The differentiator  4  calculates a differential value fr′, which is a changing rate with time of the signal  8  of the rotor frequency fr obtained by the rotation velocity detector  7 , and a twice differential value fr″ which is a changing rate with time of the fr′. The slipping-skidding detector  2  detects the slipping or skidding from the driving command P and braking command B, which are output from the operating device  54 , based on the signal  24  of the differential value fr′, and outputs the detecting signal  21 . The re-adhesion detector  3  detects the re-adhesion of wheel and rail based on the slipping-skidding detecting signal  21 , P and B command signals  23  from the operating device  54 , and the fr′ and fr″ signals  24 ,  25  from the differentiator  4 ; and the re-adhesion detecting signal  22  is output. The torque current controller  6  calculates the torque current control value ΔIqp based on the slipping-skidding detecting signal  21 , the re-adhesion detecting signal  22 , a signal  27  on the torque current pattern Iqp from the current command calculator  56 , and the fr′ signal  24 . 
     FIG. 2 indicates an example of practical composition of the slipping-skidding detector  2 . The detector comprises comparators  68 ,  69  and a switch  82 . The signal  24  of the differential value fr′ is input into respective of the comparators  68 ,  69 . If the differential value fr′ is larger than a designated value, the comparator  68  outputs “1”, and if the differential value fr′ is smaller than a designated value, the comparator  69  outputs “1”. The switch  82  is switched to P side when the driving command P is output in the command signal  23  from the operating device  54 , the output from the comparator  68  is output as the slipping-skidding detecting signal  21 ; the switch  82  is switched to B side when the braking command B is output, and the output from the comparator  69  is output as the slipping-skidding detecting signal  21 . Generally, the detecting level of the comparator  68  is set at approximately 1.5-2 times of the maximum acceleration (a positive value), and the detecting level of the comparator  69  is set at approximately 1.5-2 times of the maximum deceleration (a negative value). In accordance with the above composition, slipping or skidding can be detected, because when the slipping is generated, the differential value fr′ of the rotor frequency fr is increased, and the differential value fr′ of the rotor frequency fr is decreased when the skidding is generated. 
     FIG. 3 indicates an example of practical composition of the re-adhesion detector  3 . The detector  3  comprises comparators  64 - 67 , AND-circuits  101 ,  102 , a switch  81 , an OR-circuit  111 , and a timer  80 . First, as the detection of re-adhesion after slipping, a signal  24  on the differential value fr′ is input into the comparator  64 , and a signal  25  on the twice differential value fr″ is input into the comparator  65 . The comparators are set so that the comparator  64  outputs “1” when the fr′ signal  24  is smaller than a designated value, and the comparator  65  outputs “1” when the fr″ signal  25  is larger than a designated value. The AND-circuit  101  takes a logical product from the both comparators, and outputs a signal whether the re-adhesion occurs or not after the slipping based on the logic product. That is, because when the re-adhesion occurs after slipping has been once generated, a condition is achieved that the differential value fr′ of the rotor frequency fr becomes negative, and the twice differential value fr″ becomes positive. 
     On the other hand, the re-adhesion after skidding is detected by inputting a signal  24  on the differential value fr′ into the comparator  66 , and a signal  25  on the twice differential value fr″ into the comparator  67 . The comparators are set so that the comparator  66  outputs “1” when the fr′ signal  24  is larger than a designated value, and the comparator  67  outputs “1” when the fr″ signal  25  is smaller than a designated value. The AND-circuit  102  takes a logical product from the both comparators, and outputs a signal whether the re-adhesion occurs or not after the skidding based on the logic product. That is, because when the re-adhesion occurs after skidding has been once generated, a condition is achieved that the differential value fr′ of the rotor frequency fr becomes positive, and the twice differential value fr″ becomes negative. 
     The switch  81  is switched to P side when the driving command P is in the command signal  23  from the operating device  54 , and is switched to B side when the braking commandB is in the command signal  23 . During the driving, the output from the AND-circuit  101  is inputs into the OR-circuit  111 , and during the braking, the output from the AND-circuit  102  is inputs into the OR-circuit  111 , and the re-adhesion detecting signal  22  after slipping, or skidding, respectively, is output. 
     An output from the timer  80  is input into the OR-circuit  111  as another input. When the vehicle passes on a junction of rails or a point of switch, slipping or skidding can be detected erroneously, and sometimes the re-adhesion can not be detected. The above composition is a countermeasure for responding the above case. That is, if the re-adhesion can not be detected, the torque of the induction motor is reduced continuously notwithstanding slipping or skidding is not generated. Therefore, the slipping-skidding detecting signal  21  and the re-adhesion detecting signal  22  are input into the timer  80 , and if the re-adhesion is not detected during a designated time after the slipping or skidding is detected, the re-adhesion is regarded as being generated, and the re-adhesion signal is output from the timer  80  via the OR-circuit  111 . 
     FIG. 4 in dictates a practical composition and functions of the torque current controller  6 . At a flip-flop  44 , once the slipping-skidding detecting signal  21  becomes “1”, the slipping-skidding detecting signal  21  is kept at “1” until the re-adhesion signal  22  becomes “1”. During t he slipping-skidding detecting signal  21  is kept at “1”, that is, during slipping or skidding, switches  83 ,  84  are switched to “1” side, and a function generator  40  outputs a designated value depending on the signal  24  of the differential value fr′ of the rotor frequency to a subtracter  113 . At that time, since the switch  84  is switched to the “1” side, a difference input value  105  of the subtracter  113  is “0”. Accordingly, the output from the function generator  40  is input into an integrator with limiter  43  without any change. At the integrator with limiter, the output from the function generator  40  is added to the output from a holder  47  for obtaining an integrated value. The integrated value is limited to a value less than the torque current pattern Iqp and larger than “0” by the limiter  42 , and is output as an amount of the torque current control ΔIqp. That means, during a period from the slipping-skidding detecting signal  21  becomes “1” to the re-adhesion signal detecting signal  22  becomes “1”, ΔIqp is increased, and the torque of the induction motor is decreased. Then, when the re-adhesion detecting signal  22  becomes “1”, the slipping-skidding detecting signal  21  becomes “0”; the switches  83 ,  84  are switched to the “0” side; a torque current return calculator  41  outputs a designated value depending on the amount of the torque current control ΔIqp, which is input into the subtracter  113  as the difference input value  105 . At that time, since the switch  83  is switched to the “0” side, a sum input value  104  of the subtracter  113  is “0”. Accordingly, the output from the torque current return calculator  41  is input into an integrator with limiter  43  as a negative value. As a result, an adder  112  reduces the integrated value by subtracting the output from the torque current return calculator  41  from a previous value obtained by the holder  47 , and an amount of the torque current controlΔIqp, which is limited to a value larger than “0”, is output. The function generator  40  can generate a constant value irrelevant to the signal  24  of fr′. 
     FIG. 5 indicates a practical composition of the differentiator  4 . The subtracter  114  calculates a difference between the rotor frequency fr at the time and the rotor frequency fr before a time T1 second held by the holder  50 ; converts its output to a changing amount of rotor frequency per second by multiplying with 1/T1 by a multiplier 99; and outputs as a signal  24  of the differential value fr′ (equivalent to a changing rate with time of the rotor frequency) of the rotor frequency. Furthermore, a difference between the signal  24  of fr′ and the fr′ before a time T2 second held by the holder  51  is calculated by the subtracter  115 ; converts the difference to a changing amount of fr′ per second by multiplying with 1/T2 by a multiplier 98; and outputs as a signal  25  of the twice differential value fr″ (equivalent to a changing rate of a changing rate with time of the rotor frequency) of the rotor frequency. 
     Next, an operation in the embodiment of the present invention indicated in FIG. 1 is explained referring to FIG.  6 -FIG. 8 on a case when slipping is generated. FIG. 6 indicates an example of operation in a case when the function generator  40  indicated in FIG. 4 output a fixed value irrelevant to the differential value fr′ of the rotor frequency. In accordance with FIG. 6, when the rotor frequency is increased rapidly by generating slipping at the time T1, the differential value of the rotor frequency fr′ is increased rapidly. When the fr′ exceeds a detecting level  35 , the slipping-skidding detecting signal  21  becomes “1”; the slipping-skidding signal  29  is held at “1”, and if the slipping-skidding detecting signal  21  becomes “0” at the time T2, the slipping-skidding signal  29  is kept at “1”. During a period when the slipping-skidding signal  29  is kept at “1”, the function generator  40  indicated in FIG. 4 outputs a designated value, and a sum input value  104  of the subtracter  104  becomes a fixed value ΔIqa 1 . Therefore, the amount of torque current control ΔIqp is increased with a fixed gradient so that the wheel and rail come to be re-adhered by reducing the torque of the induction motor. When the wheel and the rail are re-adhered at the time T4, the signal  24  of fr′ changes from negative to positive, and the signal  25  of fr″ becomes a positive value. Therefore, at that time, a condition that fr′ is negative and fr″ becomes positive is achieved; the re-adhesion detecting signal  22  becomes “1”, and the slipping-skidding signal  29  becomes “0”. When the slipping-skidding signal  29  becomes “0”, the torque current return calculator  41  generates an output corresponding to the amount of the torque current control ΔIqp. In accordance with FIG. 6, it is the case when the output from the torque current return calculator  41  is changed from ΔIqb 1  to ΔIqb 2 , the amount of the torque current control ΔIqp is changed by two steps to resume the torque of the induction motor. Because the torque of the induction motor can be resumed rapidly by detecting the re-adhesion as explained above, the torque of the induction motor can be utilized effectively. 
     FIG. 7 indicates an example of operation in a case when the function generator  40  shown in FIG. 4 outputs a value corresponding to the differential value fr′ of the rotor frequency. During the time from T1 to T3, when the fr′ is positive, ΔIqa 1  is output, and during the time from T3 to T4, when the fr′ is negative, ΔIqa 1  is output (ΔIqa 1 &gt;ΔIqa 2 ). As the result, increase of the amount of torque current control ΔIqp is suppressed after the time T3. However, at the time T3, fr′ changes from positive to negative. Because it indicates that the slipping is converging to an end, even if the increase rate of the amount of torque current control ΔIqp is suppressed in comparison with the initial period of the slipping, the wheel and the rail are going to re-adhere, and are adhered at the time T4. As explained above, the amount of the torque current control ΔIqp is not increased more than its necessity when the slipping is started to converge to an end by changing the amount of the torque current control corresponding to fr′. Accordingly, the torque utilization fraction of the induction motor can be increased more than the case of FIG. 6, because the integrated value of the ΔIqp can be suppressed at the minimum. If the apparatus is set so as to obtain the operation pattern indicated in FIG. 7, fluctuation of the torque of the induction motor can be reduced, because the maximum value of the ΔIqp can be smaller than that in FIG. 6, and a riding quality can be improved. 
     FIG. 8 indicates an example of operation in a case that, after slipping has started once to converge to an end, the slipping is resumed to expand again (this case corresponds to the slipping condition shown in FIG. 12 of the prior art). In accordance with FIG. 8, when the slipping is resumed to expand again at the time T2a after the slipping has started once to converge to an end, the fr″ becomes positive. However, as previously explained relating to FIG. 3, the present invention detects the re-adhesion when the slipping is generated by detecting the condition that fr′ is negative and fr″ becomes positive. Because the fr′ is positive at the time T2, the re-adhesion can not be detected erroneously. In accordance with the present invention, the re-adhesion can be detected exactly at the time T4 as well as the case shown in FIG. 6, even if the slipping of the above case is generated. 
     Although it is not shown in FIG.  6 -FIG. 8, if the slipping or skidding is generated again, and the slipping-skidding signal  21  becomes “1” during a process when the re-adhesion detecting signal  22  becomes “1”, and the torque of the induction motor is resumed by reducing the amount of the torque current control A Iqp; the amount of the torque current control ΔIqp at the time is taken as the initial value, and the amount of the torque current control ΔIqp is increased to operate the apparatus to come to re-adhere. 
     In accordance with the embodiment of the present invention indicated in FIG. 1, a case when a motor is driven by an inverter has been indicated. However, in another case, plural motors are provided at respective wheel axes in an electric vehicle, and the plural motors are driven by an inverter. FIG. 9 indicates the second embodiment of the present invention, which is an example wherein the present invention is applied to a controller which controls plural induction motors. Rotor frequencies fr 1 -fr n  (n is the number of controlling induction motors) obtained by rotation speed detectors connected to each of the induction motors are differentiated respectively by differentiators  31 - 33  to calculate the differential values of the rotor frequencies fr 1 ′-fr n ′. The selector  77  selects a representative value of the (fr 1 ′-fr n ′), and defines it as fr′. If, for instance, the maximum value is selected as the representative value during the driving operation, and the minimum value is selected as the representative value during the braking operation, it becomes possible to detect slipping or skidding of only one axis. 
     The selected fr′ signal  24  is input into the slipping detector  9  and the skidding detector  10 , respectively, and slipping and skidding are detected. Hereinafter, the operation when slipping is generated during the driving operation is explained. If fr′ signal  24  exceeds a designated value at the slipping detector  9 , the comparator  70  detects the slipping, the slipping detecting signal  106  becomes “1”, and the slipping signal  26 , which an output from the flip-flop  78 , is maintained by “1”. At that time, the output from the switch  85  becomes “1”, because the switch  85  is at the P side, and the switches  83 ,  84  select the “1” side, respectively. Since the switch  86  has been selecting the P side, the output from the function generator  38  corresponding to the fr′ signal  24  is input into the subtracter  113  via the switch  86 —switch  83 . Since then, the input is integrated by the integrator with limiter  43  as same as the case shown in FIG. 4, and output as the amount of torque current control ΔIqp. 
     When the wheel and the rail are re-adhered, a condition, wherein fr′ is negative and fr″ is positive, is achieved. Then, the comparator  71  outputs “1” when fr′ is negative by setting the detecting level of the comparator  71  at approximately zero. The twice differential value fr″, which can be obtained by differentiating fr′ by the differentiator  34 , is input into the comparator  72 . When the fr″ is positive, the comparator  72  outputs “1” by setting the detecting level of the comparator  72  at approximately zero. Therefore, when re-adhered, the output of the AND circuit  94  becomes “1”, and the output of the AND circuit  91  becomes “1”, because the output of the comparator  70  is “0” at the time. Then, the output from the OR circuit  96  becomes “1”, and re-adhesion detecting signal  108  becomes “1”. Therefore, slipping signal  26  becomes “0”, the switches  83 ,  84  select the “0” side, respectively, and the output from the torque current return calculator  36  becomes negative at the subtracter  113 . The negative value is input into the integrator with limiter  43 , and the amount of torque current control ΔIqp is reduced to resume the torque of the induction motor. If slipping is generated again during the torque resuming process, the above operations are repeated to operate so as to re-adhere the wheel and the rail. When the vehicle passes on a junction of rails, or a point of switch, a case wherein fr′  24  becomes a large value instantaneously, the output from the comparator  70  becomes “1” momentarily, and the slipping signal  26  is maintained at “1” can be assumed. However, when the slipping is erroneously detected as the above case, the condition of the re-adhesion, that is, fr′ is negative and fr″ is positive can not be sometimes achieved. In a case of such an erroneous detection, the output from the comparator  70  becomes “0” immediately, the output from the AND circuit  90  becomes “1” at the moment, and the on-delay  88  starts counting and, after a designated time passes, outputs a signal “1” to make the re-adhesion detecting signal  108  “1”. As explained above, the setting time of the on-delay  88  can be short as approximately hundreds milliseconds by making the on-delay  88  start operation at the time when the output from the comparator becomes “0”. Therefore, because the re-adhesion can be detected immediately even if the slipping is detected erroneously, lowering the torque utilization rate can be suppressed at the minimum. When skidding is generated during braking period, a skidding detector  10  detects the skidding, and calculates the amount of torque current control ΔIqp as same as the case of the above slipping. The embodiment indicated in FIG. 9 differs from the first embodiment in its composition relating to points that plural control devices are used for controlling induction motors, the function generators and the torque current return calculators are used separately for slipping and skidding, respectively, and others. However, its operation is as same as indicated in FIG.  6 -FIG.  8 . 
     In accordance with the present invention, the re-adhesion point can be certainly detected when a wheel generates slipping or skidding, and the torque current component of motor can be resumed rapidly by utilizing the fast response of vector control. Accordingly, since the torque can be utilized effectively to a physical adhesion limit even in a condition where the adhesion coefficient is low such as a rainy day, acceleration and deceleration of the electric vehicle can be increased as much as possible. 
     Therefore, the present invention is suitable for utilizing in controlling electric vehicles of railway, wherein slipping and skidding are generated frequently. Furthermore, the present invention is suitable for electric automobiles, if the applying range of the present invention is extended broadly.