Patent Publication Number: US-2015061694-A1

Title: Method of detecting state of power cable in inverter system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2013-00104839, filed on Sep. 2, 2013, the contents of which are all hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     The present disclosure relates to an inverter system, and more particularly, to a method of detecting the state of a power cable in an inverter system that may detect the disconnection of a high-voltage cable connecting an inverter to a motor. 
     An inverter system that is a motor controller used for an environment-friendly vehicle is an electric/electronic sub assembly (ESA) or electric/electronic component that plays a role of converting high-voltage direct current (DC) power into alternating current (AC) or DC power for controlling a motor. Thus, the inverter system is an important component that belongs to the electric motor of a vehicle. 
     As such, a permanent magnet type motor is applied to the environment-friendly vehicle as a driving unit. The motor applied to the environment-friendly vehicle as the driving unit is driven by a phase current that is transmitted through a first high-voltage power cable from an inverter that converts a DC voltage into a three-phase voltage by a pulse width modulation (PWM) signal of a controller. 
     Also, the inverter converts a DC link voltage transmitted through a second high-voltage power cable into a three-phase voltage by the opening/closing of a main relay. 
     Thus, if any one of the first power cable connecting the inverter to the motor and the second power cable connecting the high-voltage battery to the inverter is separated, the motor does not smoothly operate and a high voltage/current is introduced into a system, so a vital limitation that damages the entire inverter system occurs. 
       FIG. 1  represents a device for detecting separation of a power cable in an inverter system according to a related art. 
     Referring to  FIG. 1 , the device for detecting the separation of the power cable according to the related art includes a power cable  10 , a connector  20 , and a sensor  30  that is formed between the power cable  10  and the connector  20  and transmits a signal according to whether the power cable  10  is separated from the connector  20 . 
     The sensor  30  is connected to (a contact portion) between the power cable  10  and the connector  20 , and transmits a digital signal to a controller according to whether the power cable  10  is connected to the connector  20 . 
     That is, a sensor that checks whether the power cable  10  is separated is typically installed on the power cable  10  or the connector  20  as separate hardware, and whether the power cable  10  is separated is checked in real time by using the digital signal output from the sensor. 
     However, since the device for detecting the separation of the power cable as described above detects by using hardware whether the power cable is separated, there are constraints of money and space. 
     Also, the device for detecting the separation of the power cable as described above is more likely to perform malfunction due to an external factor such as vibration and this works as a factor that threatens driver&#39;s safety. 
     Recently, a method of detecting the disconnection of the power cable by using software is being provided. 
       FIG. 2  represents a change in current when a general power cable has disconnection. 
     Referring to  FIG. 2 , when a power cable is disconnected, a current flow varies, in which case, when two or more phases are disconnected, three phase currents all become zeroes, and when only one phase is disconnected, only the current of a disconnected phase (v phase in  FIG. 2 ) becomes zero. Thus, disconnection is determined according to whether there is a big difference between the magnitude of a current for a certain time and an instruction value or the magnitude of the current is zero. 
     However, since the above-described method detects only the magnitude of a current, there is a chance of malfunction by a motor&#39;s speed and a sampling period. 
     SUMMARY 
     Embodiments provide a method of detecting the state of a power cable in an inverter system that may detect the state of the power cable by using the size of the space vector of a current in addition to the magnitude of the current. 
     Technical tasks to be achieved by presented embodiments are not limited to the above-mentioned technical tasks and other technical tasks not mentioned will be able to be clearly understood by a person skilled in the art from the following descriptions. 
     In one embodiment, a method of detecting the states of power cables in an inverter system supplying power generated from an inverter to a motor by using three phase power cables includes: calculating the location of a current space vector for a first period when the first period arrives; using the calculated location of the current space vector for the first period to calculate the predicted location of the current space vector for a second period; calculating the actual location of the current space vector for the second period when the second period arrives; comparing the calculated predicted location with the actual location; and detecting the states of the three phase power cables according to a comparison result. 
     The calculating of the location of the current space vector for the first period or the calculating of the actual location of the current space vector for the second period may include: obtaining three phase current values supplied to the motor for a corresponding period; using obtained three phase current values to calculate the d-axis current and q-axis current of a stator&#39;s coordinate system; and using the ratio of a calculated q-axis current to a calculated d-axis current and an arctangent function to calculate the location of the current space vector. 
     The calculating of the predicted location of the current space vector for the second period may include: obtaining the rotating speed of the motor; using the rotating speed of the motor to calculate the rotating speed of the current space vector; and using the sampling time between the first period and the second period and the calculated rotating speed of the current space vector to calculate the predicted location of the current space vector for the second period. 
     The comparing of the calculated predicted location with the actual location may include determining whether the difference between the predicted location and the actual location is larger than a preset reference value. 
     The detecting of the states of the three phase power cables may include: checking three phase current values obtained for the second period when the difference between the predicted location and the actual location is larger than the preset reference value; and detecting that two or more of the three phase power cables are disconnected when checked three phase current values are all zeroes. 
     The detecting of the states of the three phase power cables may include: checking the actual location of the current space vector for the second period when the difference between the predicted location and the actual location is larger than the preset reference value; and determining which of the three-phase power cables is disconnected when checked three phase current values are all zeroes. 
     The determining of which of the three-phase power cables is disconnected may include: detecting that the u-phase power cable of the three-phase power cables is disconnected when the actual location of the current space vector for the second period is 90° or −90°; and detecting that the v-phase power cable of the three-phase power cables is disconnected when the actual location of the current space vector for the second period is −30° or 150°; and detecting that the w-phase power cable of the three-phase power cables is disconnected when the actual location of the current space vector for the second period is 30° or −150°. 
     According to an embodiment, by detecting the state of the power cables connected to a motor by using the size of the space vector of the current in stead of the magnitude of the current, it is possible to remarkably decrease an detection error probability caused by detecting sizes for repetitive sampling operations, and by quickly detecting whether the power cables are disconnected, it is possible to prevent further serious accidents 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a device for detecting separation of a power cable in an inverter system according to a related art. 
         FIG. 2  represents a change in current when a general power cable has disconnection. 
         FIG. 3  is a schematic diagram of an inverter system according to an embodiment. 
         FIG. 4  represents the space vector of three-phase currents according to an embodiment. 
         FIG. 5  represents the space vector of three-phase currents varying when a u-phase cable of power cables has disconnection according to an embodiment. 
         FIG. 6  represents the space vector of three-phase currents varying when a v-phase cable of power cables has disconnection according to an embodiment. 
         FIG. 7  represents the space vector of three-phase currents varying when a w-phase cable of power cables has disconnection according to an embodiment. 
         FIG. 8  is a flow chart of a method of detecting the state of a power cable in an inverter system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The followings only illustrate the principle of the present invention. Therefore, a person skilled in the art may invent various devices that implement the principle of the present invention and are included in the concepts and scope of the present invention, although being not clearly shown or described in the specification. Also, all conditional terms and embodiments enumerated in the specification are, in principle, intended only for the purpose of understanding the concepts of the present invention and thus it should be understood that the present invention is not limited to embodiments and state to be particularly enumerated. 
     Also, it should be understood that all detailed descriptions enumerating specific embodiments as well as the principle, view and embodiments of the present invention are intended to include their structural and functional equivalents. Also, such equivalents should be understood as including currently known equivalents as well as equivalents to be developed in future, namely, all elements invented to perform the same function irrespective of their structures. 
       FIG. 3  is a schematic diagram of an inverter system according to an embodiment. 
     Referring to  FIG. 3 , an inverter system includes an inverter  110 , three-phase power cables  120  supplying, to a motor, power output through the inverter  110 , a sensor obtaining information on the operating state of the motor, and a control unit  140  controlling the operation of the inverter  110 , detecting the disconnection of the three-phase cables  120  and stopping the operation of the inverter  110 . 
     The inverter  110  is arranged in an electric vehicle and thus converts direct current (DC) power generated from a battery (not shown) arranged in the electric vehicle, into three-phase alternating current (AC) power. 
     In this case, the battery is a high-voltage battery and may be formed as a set of a plurality of unit cells. 
     In order to maintain a constant voltage, the plurality of unit cells may be managed by a battery management system (not shown) and the battery may emit a constant voltage by the control of the battery management system. 
     Also, power output by the discharge of the battery is transmitted to a capacitor in the inverter  110 . 
     In this case, a relay is formed between the battery and the inverter  110 , and the power supplied to the inverter  110  may be controlled by the operation of the relay. 
     That is, when the relay performs an ON operation, the power from the battery may be supplied to the inverter  110 , and when the relay performs an OFF operation, a power supply to the inverter  110  may be cut off 
     The inverter  110  converts DC power supplied to the battery into AC power and supplies the AC power to the motor. 
     In this case, the AC power converted by the inverter  110  may be three-phase power. 
     The inverter  110  includes the above-described capacitor and a plurality of insulated gate bipolar transistors (IGBTs) which perform pulse width modulation (PWM) switching according to a control signal applied from the control unit  140  to be described below, phase-convert power supplied from the battery and supply phase-converted power to the motor. 
     The motor may include a stator that does not rotate and is fixed, and a rotor that rotates. The motor receives AC power supplied through the inverter  110 . 
     The motor may be e.g., a three-phase motor, and when voltage-variable/frequency-variable AC power having each phase is applied to the coil of a stator having each phase, the rotating speed of the rotor varies depending on an applied frequency. 
     The motor may include an induction motor, a blushless DC (BLDC) motor, or a reluctance motor. 
     A driving gear (not shown) may be arranged on one side of the motor. The driving gear converts the rotational energy of the motor according to a gear ratio. The rotational energy output from the driving gear is transmitted to a front wheel and/or a rear wheel to enable an electric vehicle to move. 
     The power cables  120  are arranged between the inverter  110  and the motor. The power cables may be three-phase power cables and thus includes a u-phase cable, a v-phase cable, and a w-phase cable. 
     The sensor  130  obtains information on the driving state of the motor. In this case,  FIG. 3  shows that the sensor  130  is a speed sensor. That is, the sensor  130  is arranged on one side of the motor and detects a rotating speed when the motor rotates. 
     In addition, when the rotating speed of the motor is detected, the sensor  130  transmits a detected rotating speed to the control unit  140 . 
     Also, the sensor  130  may include a current sensor. 
     That is, the sensor  130  may include a current sensor that is arranged on each output line of the three-phase power cables  120  arranged between the inverter  110  and the motor and obtains three-phase currents. 
     Thus, the sensor  130  detects three phase current values (a u-phase current value, a v-phase current value, and a w-phase current value) supplied to the motor, and the rotating speed of the motor and transmits detected values to the control unit  140 . 
     The control unit  140  controls the overall operations of the inverter  110 . 
     For example, the control unit  140  uses the currents (three phase currents) supplied to the motor to calculate a value to operate the motor, and generates a switching signal for the control of the inverter (e.g., the switching control of the IGBT configuring the inverter) according to a calculated value. 
     Thus, the inverter  110  selectively performs ON/OFF operation according to a switching signal generated through the control unit  140  and converts DC power supplied from the battery into AC power. 
     The control unit  140  uses three phase values transmitted through the sensor  130  and a rotating speed to detect the states of the power cables  120 . 
     In addition, when the power cables  120  have a problem (e.g., disconnection, separation, or failure in connection), the control unit  140  may significantly affect the running of the electric vehicle because AC power converted through the inverter  110  is not supplied to the motor. 
     Thus, the control unit  140  detects whether the power cables  120  are disconnected, and when it is detected that the power cables  120  are disconnected, the control unit  140  cuts off an AC power supply to the motor. 
     The operation of detecting the disconnection of the power cables  120  performed by the control unit  140  is described below in detail. 
       FIG. 4  represents the space vector of three-phase currents according to an embodiment,  FIG. 5  represents the space vector of three-phase currents varying when a u-phase cable of power cables has disconnection according to an embodiment,  FIG. 6  represents the space vector of three-phase currents varying when a v-phase cable of power cables has disconnection according to an embodiment, and  FIG. 7  represents the space vector of three-phase currents varying when a w-phase cable of power cables has disconnection according to an embodiment. 
     The operation of detecting the disconnection of the power cables  120  performed by the control unit  140  is described with reference to  FIGS. 4 to 7 . 
     Firstly, the relationship between a motor&#39;s speed and the speed of the space vector of a current is described. 
     When three phase currents are supplied to the motor through the power cables  120 , the motor has torque and thus rotates. 
     In this case, when the motor is a synchronous motor, the rotating speed of the space vector of a current is the same as that of the motor, and when the motor is an asynchronous motor, the rotating speed of the space vector of the current is rather different from that of the motor. 
     Thus, when the rotating speed of the motor is known, it is possible to obtain the speed of the space vector of the current as well. 
     In this example, the space vector means a current vector in a 3D coordinate system. 
     That is, referring to  FIG. 4 , three phase windings that have a mechanical difference of 120° from one another are arranged at the motor, and three phase currents that have an electrical phase difference of 120° from one another flow on the three phase windings. Then, a magnetic field is formed by the three phase currents flowing, and the magnetic field is referred to as a space vector. 
     In this case, when normal three phase currents continue to flow, the space vector of the currents rotates. 
     However, when the three phase currents abnormally flow, the space vector of the currents does not rotate but varies to alternately appear on specific locations (that may be referred to as angles). 
     In other words, when it is assumed that the location of a current space vector obtained in Nth sampling is as shown in  FIG. 4 , the location of a N+1th current space vector when normal three phase currents flow rotates in an arrow direction in  FIG. 4 . In this case, the rotating speed of the space vector is affected by the rotating speed of the motor. For example, when the location of an Nth current space vector is 20° and the rotating speed of the motor is A, the location of the N+1th period current space vector rotates in the arrow direction by reflecting the time difference between the Nth period and the N+1th period and the rotating speed of the motor. 
     However, when the three phase currents abnormally flow (i.e., the power cables are disconnected), the rotation of the current space vector corresponding to a reflected angle is not performed. Thus, in a normal case, the location of the current space vector for the current period and the location of the current space vector for the next period have a certain gap according to the speed of the motor and a sampling time, but in an abnormal case, there is no association between the location of the current space vector for the current period and the location of the current space vector for the next period. 
     Thus, the control unit  140  calculates the location of the current space vector for the current period, and predicts where the current space vector for the next period is located, according to the location of the current space vector for a calculated current period. A prediction method may performed by using a sampling time and the speed of the motor. 
     Related descriptions are provided below in detail. 
     Firstly, the control unit  140  uses three phase values obtained through the sensor  130  to calculate the d-axis current and q-axis current of a stator coordinate system. 
     A method of calculating the d-axis current id and the q-axis current iq is as follows. 
     In order to calculate the d-axis current and the q-axis current, a vector idq is first found. 
     The vector idq may be calculated by Equation 1 below: 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       dq 
                     
                     = 
                     
                       
                         
                           i 
                           d 
                         
                         + 
                         
                           ji 
                           q 
                         
                       
                       = 
                       
                         
                           2 
                           3 
                         
                          
                         
                           ( 
                           
                             
                               i 
                               a 
                             
                             + 
                             
                               a 
                               · 
                               
                                 i 
                                 b 
                               
                             
                             + 
                             
                               
                                 a 
                                 2 
                               
                               · 
                               
                                 i 
                                 c 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     a 
                     = 
                     
                       
                         1 
                          
                         ∠120° 
                       
                       = 
                       
                         
                           - 
                           
                             1 
                             2 
                           
                         
                         + 
                         
                           j 
                            
                           
                             
                               
                                 3 
                               
                               2 
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   〉 
                 
               
             
           
         
       
     
     Thus, the d-axis current id and the q-axis current iq may be found from the vector idq by Equation 2 below: 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       d 
                     
                     = 
                     
                       Re 
                        
                       
                         [ 
                         
                           
                             2 
                             3 
                           
                            
                           
                             ( 
                             
                               
                                 i 
                                 a 
                               
                               + 
                               
                                 a 
                                 · 
                                 
                                   i 
                                   b 
                                 
                               
                               + 
                               
                                 
                                   a 
                                   2 
                                 
                                 · 
                                 
                                   i 
                                   c 
                                 
                               
                             
                             ) 
                           
                         
                         ] 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       i 
                       q 
                     
                     = 
                     
                       
                         Im 
                          
                         
                           [ 
                           
                             
                               2 
                               3 
                             
                              
                             
                               ( 
                               
                                 
                                   i 
                                   a 
                                 
                                 + 
                                 
                                   a 
                                   · 
                                   
                                     i 
                                     b 
                                   
                                 
                                 + 
                                 
                                   
                                     a 
                                     2 
                                   
                                   · 
                                   
                                     i 
                                     c 
                                   
                                 
                               
                               ) 
                             
                           
                           ] 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   〉 
                 
               
             
           
         
       
     
     That is, by Equations 1 and 2, it is possible to calculate each of the d-axis current and q-axis current of the stator coordinate system from obtained three phase currents Ia, Ib and Ic. 
     Also, when the d-axis current and the q-axis current are calculated, it is possible to calculate the location of the current space vector by using the calculated d-axis current and q-axis current. 
     In other words, the location of the current space vector may be calculated by using the ratio of the q-axis current value of the stator coordinate system to its d-axis current value and an arctangent function. 
     The location of the current space vector may be calculated by Equation 3 below: 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           
                             i 
                             q 
                           
                           
                             i 
                             d 
                           
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   〉 
                 
               
             
           
         
       
     
     Based on Equations 1 to 3 above, the control unit  140  calculates the location of the current space vector every certain period N, N+1, or N+2 
     In this example, the location of the current space vector for the Nth period may be as follows: 
     
       
         
           
             θ 
             = 
             
               
                 
                   tan 
                   
                     - 
                     1 
                   
                 
                  
                 
                   ( 
                   
                     
                       
                         i 
                         q 
                       
                        
                       
                         [ 
                         N 
                         ] 
                       
                     
                     
                       
                         i 
                         d 
                       
                        
                       
                         [ 
                         N 
                         ] 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     Also, the location of the current space vector for the N+1th period may be as follows: 
     
       
         
           
             θ 
             = 
             
               
                 
                   tan 
                   
                     - 
                     1 
                   
                 
                  
                 
                   ( 
                   
                     
                       
                         i 
                         q 
                       
                        
                       
                         [ 
                         
                           N 
                           + 
                           1 
                         
                         ] 
                       
                     
                     
                       
                         i 
                         d 
                       
                        
                       
                         [ 
                         
                           N 
                           + 
                           1 
                         
                         ] 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     In this case, the location of the current space vector varies through rotation according to the speed of the motor. 
     Thus, when the location of the current space vector for the current period is known, it is possible to predict the location of the current space vector for the next period based on a sampling time (the time difference between the N+1th period and the Nth period). 
     That is, the control unit  140  may calculate the rotating speed of the current space vector according to the speed of the motor. The rotating speed of the current space vector may be calculated according to the type of the motor, and as described above, when the motor is the synchronous motor, the rotating speed of the current space vector is the same as the speed of the motor, and when the motor is the asynchronous motor, the rotating speed of the current space vector is rather different from the speed of the motor. 
     Thus, the control unit  140  may use the type of the motor and the speed of the motor to calculate the rotating speed of the current space vector. 
     Accordingly, the control unit  140  may use the location of the current space vector for the current period N, and the rotating speed of the current space vector for the current period N obtained from a rotor&#39;s speed (speed of the motor) to predict the location of the current space vector for the next period N+1. 
     The location of the current space vector for the next period may be predicted by Equation 4 below: 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                        
                       
                         ( 
                         
                           
                             
                               i 
                               q 
                             
                              
                             
                               [ 
                               N 
                               ] 
                             
                           
                           
                             
                               i 
                               d 
                             
                              
                             
                               [ 
                               N 
                               ] 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         ω 
                         m 
                       
                       · 
                       
                         T 
                         s 
                       
                     
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   〉 
                 
               
             
           
         
       
     
     where ω m  is the rotating speed of the current space vector and T s  is a sampling time. 
     Then, when the next period N+1 arrives, the control unit  140  calculates the location of the current space vector according to a corresponding period. 
     The location of the current space vector for the corresponding period N+1 may be calculated by Equations 1 to 3 above. 
     Also, when the actual location of the current space vector for the next period is calculated, the control unit  140  compares the predicted location of the current space vector for the next period with the calculated actual location of the current space vector for the next period and checks whether there is a difference between the predicted location and the actual location. 
     In this case, since what the predicted location is the same as the actual location represents that three phase currents normally flow, the control unit  140  will be able to confirm that the power cables  120  are normally connected. 
     However, when there is a difference between the predicted location and the actual location, the control unit  140  checks whether the difference is within or outside an error bound. The error bound may be determined by calculating differences in location that may appear through various experiments, and may be designated as a reference value. For example, the error bound may be set to 10°. 
     Then, when the difference between the predicted location and the actual location is within the error bound, the control unit  140  determines that the power cables  120  are normally connected. 
     However, when the difference between the predicted location and the actual location is outside the error bound, the control unit  140  determines that the power cables  120  are abnormally connected (e.g., disconnected). 
     Then, the control unit  140  informs that the power cables  120  have an error, and cuts off a power supply to or from the inverter  110  correspondingly. 
     In this case, the control unit  140  uses the calculated actual location to check which of the power cables  120  has an error. 
     To this end, the control unit  140  checks three phase current values obtained through the sensor  130 . In addition, when all the three phase current values checked are zeroes, the control unit  140  determines that two or more of the three phase cables have errors. That is, when two or more of the three phase cables are in a disconnected state, all the three phase current values become zeroes, in which case the control unit  140  may use the three phase current values to check whether two or more cables have errors. 
     Also, when all the three phase current values checked are not zeroes, the control unit  140  checks according to the change state of the actual location of the current space vector whether any one of the three phase power cables has an error. 
     In this case, when any one of the power cables is disconnected, only a current value corresponding to a disconnected phase becomes zero, in which case, there is a regular change in the location of the current space vector. 
     That is, in a state where three phase currents normally flow, the current space vector rotates to correspond to the rotating speed of the motor as shown in  FIG. 4 . 
     However, when any one of the three phase currents does not flow, the current space vector does not rotate and varies to alternately appear on two locations. 
     In other words, when the u-phase cable of the power cables is disconnected as shown in  FIG. 5 , the current space vector alternately appears on 90° and −90°. 
     Thus, the control unit  140  checks the location of the current space vector, and when the location of the current space vector varies to alternately appear on 90° and −90° as shown in  FIG. 5 , the control unit  140  determines that the u-phase cable of the power cables is disconnected. 
     Also, when the v-phase cable of the power cables is disconnected as shown in  FIG. 6 , the current space vector varies to alternately appear on −30° and 150°. 
     Thus, the control unit  140  checks the location of the current space vector, and when the location of the current space vector varies to alternately appear on −30° and 150° as shown in  FIG. 6 , the control unit  140  determines that the v-phase cable of the power cables is disconnected. 
     Also, when the w-phase cable of the power cables is disconnected as shown in  FIG. 7 , the current space vector varies to alternately appear on −150° and 30°. 
     Thus, the control unit  140  checks the location of the current space vector, and when the location of the current space vector varies to alternately appear on −150° and 30° as shown in  FIG. 7 , the control unit  140  determines that the w-phase cable of the power cables is disconnected. 
     As described above, according to an embodiment, by detecting the state of the power cables connected to a motor by using the size of the space vector of the current in stead of the magnitude of the current, it is possible to remarkably decrease an detection error probability caused by detecting sizes for repetitive sampling operations, and by quickly detecting whether the power cables are disconnected, it is possible to prevent further serious accidents. 
       FIG. 8  is a flow chart of a method of detecting the states of power cables in an inverter system according to an embodiment. 
     Referring to  FIG. 8 , the control unit  140  first calculates the location of the current space vector for the current period (Nth period, hereinafter referred to as a ‘first period’) in step S 101 . 
     The current space vector may be calculated by Equations 1 to 3 above. 
     If the location of the current space vector for the first period is calculated, the control unit  140  calculates the predicted location of the current space vector for the next period (N+1th period, hereinafter referred to as a ‘second period’) in step S 102 . 
     That is, the control unit uses the speed of a motor to calculate the rotating speed of the current space vector, and uses a sampling time (the time difference between the first period and the second period) and the rotating speed of the current space vector to calculate the predicted location of the current space vector that will appear for the second period. 
     Then, when the second period arrives, the control unit  140  calculates the actual location of the current space vector for the second period in step S 103 . 
     When the actual location of the current space vector for the second period is calculated, the control unit  140  determines whether the difference between the predicted location and the actual location is larger than a preset reference value in step S 104 . That is, the control unit  140  determines whether the difference between the predicted location and the actual location is outside an error bound that is obtained through various experiments. 
     When the difference between the predicted location and the actual location is smaller than the preset reference value as a determination result in step S 104 , the control unit  140  considers that the power cables are normally connected, and returns to step S 102 . 
     However, when the difference between the predicted location and the actual location is larger than the preset reference value as a determination result in step S 104 , the control unit  140  senses that the power cables have errors, and checks which of the three phase power cables has an error in steps S 105  to S 112 . 
     To this end, the control unit  140  first determines whether the three phase current values obtained to calculate the location of the current space vector are all zeroes in step S 105 . 
     Then, when all the three phase current values are zeroes as a determination result in step S 105 , it is determined that at least two the power cables are disconnected in step S 106 . 
     However, when all the three phase current values are not zeroes, the control unit  140  determines whether the location of the current space vector for the second period is 90° or −90° in step S 107 . 
     Then, when the location of the current space vector for the second period is 90° or −90° as a determination result in step S 107 , the control unit  140  detects that the u-phase cable of the power cables is disconnected in step S 108 . 
     Also, when the location of the current space vector for the second period is not 90° or −90° as a determination result in step S 107 , it is determined whether the location of the current space vector for the second period is −30° or 150° in step S 109 . 
     Then, when the location of the current space vector for the second period is −30° or 150° as a determination result in step S 109 , the control unit  140  detects that the v-phase cable of the power cables is disconnected in step S 110 . 
     Also, when the location of the current space vector for the second period is not −30° or 150° as a determination result in step S 109 , it is determined whether the location of the current space vector for the second period is 30° or −150° in step S 111 . 
     Then, when the location of the current space vector for the second period is 30° or −150° as a determination result in step S 111 , the control unit  140  detects that the w-phase cable of the power cables is disconnected in step S 112 . 
     According to an embodiment, by detecting the states of the power cables connected to a motor by using the size of the space vector of the current in stead of the magnitude of the current, it is possible to remarkably decrease an detection error probability caused by detecting sizes for repetitive sampling operations, and by quickly detecting whether the power cables are disconnected, it is possible to prevent further serious accidents. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.