Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2011-259777, filed on Nov. 29, 2011, the entirety of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique of detecting improper wiring in a parallel inverter system that drives a load such as a motor with a plurality of inverters connected in parallel. 
     2. Background Art 
     Cables for supplying electric power from an inverter to a motor are connected to the inverter and the motor through a connection unit such as a terminal block. This, in the work of laying the cables, sometimes causes improper cable connection to the terminal block, improper cable connection to the motor with an incorrect phase sequence or an erroneous omission of cable connection (hereinafter generically referred to as improper wiring). The driving of a motor without correcting such improper wiring results in an excessive current flowing in the motor, for example, to cause breakdowns of devices or a failure of the system. Hence, an operator must carry out checking beforehand so that there is no such improper wiring. 
     Here, as a system of related art detecting an unfinished connection between an inverter and a motor as a kind of improper wiring, the system shown in  FIG. 4  and  FIG. 5  is known, for example. 
     The system of the related art is that described in JP-A-7-20190 (paragraphs [0007] and [0008] and FIG. 1 and FIG. 2 etc.).  FIG. 4  is a circuit diagram showing a motor driving system by an inverter in the related art. The motor driving system shown in  FIG. 4  includes a rectifier circuit  101  converting a three-phase AC voltage to a DC voltage, a smoothing capacitor  102 , an inverter section  103  including semiconductor switching devices Q 1  to Q 6 , an AC motor M to which a three-phase AC voltage is supplied from the inverter section  103 , current sensors  104  detecting their respective output currents in at least two phases of the inverter section  103 , a current detector  105 , a CPU (Central Processing Unit)  106  carrying out an operation of an output voltage instruction on the basis of the values of the detected currents to output the output voltage instruction, and a base driving circuit  107  producing driving signals for the switching devices Q 1  to Q 6  according to the output voltage instruction to output the driving signals. 
       FIG. 5  is a flow chart showing the operation of detecting a state of an unfinished connection between the inverter and the motor M in the motor driving system in the related art shown in  FIG. 4 . 
     First, by operating the rectifier circuit  101 , the smoothing capacitor  102  is charged to bring the voltage across the smoothing capacitor  102  as the main circuit voltage of the inverter to the specified voltage (S 101 ). In the next, by turning-on a switching device in each of two phases of the three phases in the inverter section  103 , a closed circuit ranging from the DC circuit of the inverter to the motor M is formed. For example, by turning-on the switching devices Q 1  and Q 4 , a closed circuit is formed for ascertaining whether or not a current flows from the U-phase to the V-phase in the inverter section  103  or, by turning-on the switching devices Q 3  and Q 2 , a closed circuit is formed for ascertaining whether or not a current flows from the V-phase to the U-phase in the inverter section  103  (S 102 ). 
     Similar operations are carried out with respect to all of the combinations of the two rest phases (S 103  and S 104 ). When it is ascertained that currents flow with respect to all of the combinations of two phases, a decision is made that the motor is normally connected to the output side of each of the phases of the inverter and the processing is made completed (“Yes” at S 105 ). Moreover, when it is ascertained that no current flows with respect to any one of the combinations of two phases, a decision is made that the connection at the connection point in the corresponding phase is unfinished (“No” at S 105 , and S 106 ). 
     Furthermore, in Japanese Patent No. 2,797,882 (paragraphs [0018] to [0021] and FIG. 1 and FIG. 2 etc.), a control system of a servomotor is described which the system detects a current at turning-on of switching devices in two-phases of an inverter to make a decision as to whether the servomotor is connected to the inverter or not. 
     In addition, in JP-A-2010-213557 (paragraphs [0007] to [0012] and FIG. 1 and FIG. 2 etc.), a control system of a three-phase synchronous motor is described. In the control system, output currents in at least two phases of an inverter driving the three-phase synchronous motor are detected to be subjected to orthogonal biaxial transformation, by which a q-axis current feedback signal, a fed back speed signal and a q-axis current instruction are obtained so that the improper wiring in two phases or in three phases of the motor is detected on the basis of thus obtained signals and instruction. 
     Patent Document 1: JP-A-7-20190 (paragraphs [0007] and [0008] and FIG. 1 and FIG. 2 etc.) 
     Patent Document 2: Japanese Patent No. 2,797,882 (paragraphs [0018] to [0021] and FIG. 1 and FIG. 2 etc.) 
     Patent Document 3: JP-A-2010-213557 (paragraphs [0007] to [0012] and FIG. 1 and FIG. 2 etc.) 
     Any of the systems of related art described in JP-A-7-20190, Japanese Patent No. 2,797,882 and JP-A-2010-213557 is a system for the case of driving a motor with a single inverter. 
     Compared with this, in a parallel inverter system driving a single motor with a plurality of inverters connected in parallel, cables are present by the number of the inverters connected in parallel even in the same phase. Thus, there is a problem in that it is unknown which inverter has improper wiring to the motor. In particular, in a parallel inverter system, when inductance components among inverters are small, the inverters brought into operation with improper wiring being included therein may cause a short circuit by some way of energization to result in an excessive current flowing in switching devices in the inverters, which might damage the system. 
     In addition, there is also a problem in that although in the system described in Japanese Patent No. 2,797,882, detection of an incorrect phase sequence in the wiring connected to the motor is possible (paragraph [0030] etc.), in each of the systems described in JP-A-7-20190 and JP-A-2010-213557, detection of an incorrect phase sequence is impossible. 
     Accordingly, it is an object of the invention to provide an improper wiring detecting system which is capable of surely detecting improper wiring such as an unfinished connection to a terminal block and incorrect connection in a parallel inverter system including a plurality of inverters. Moreover, it is another object of the invention to provide an improper wiring detecting system which makes detection of incorrect phase sequences possible as required. 
     SUMMARY OF THE INVENTION 
     For solving the problem, a first aspect of the invention is an improper wiring detecting system of a parallel inverter system formed of a plurality of inverters connected in parallel, each outputting polyphase AC electric power of variable voltage, with the output sides of the respective inverters connected to a single load, the improper wiring detecting system including: 
     a voltage detecting means for detecting an output voltage of each of the phases of each of the inverters; 
     a controlling means for controlling turning-on and -off of semiconductor devices forming each of the inverters; and 
     a wiring condition deciding means for operating the controlling means to turn-on specified switching devices in one inverter to form a closed circuit between arbitrary two phases of the one inverter, carrying out comparisons among values of output voltages of a plurality of the inverters including the one inverter, each of the output voltages being an output voltage of each of the phases of each inverter and being detected by the voltage detecting means, and making a decision as to whether wiring is correct or not on the basis of the results of the comparisons. 
     A second aspect of the invention is that, in the improper wiring detecting system of a parallel inverter system according to the first aspect of the invention, a means of mutually transmitting the values of the output voltages detected by the voltage detecting means among a plurality of the inverters and sharing the values is provided in each of a plurality of the inverters. 
     A third aspect of the invention is that, in the improper wiring detecting system of a parallel inverter system according to the first or second aspect of the invention, a phase sequence deciding means is further included for starting a polyphase AC motor as the load, connected to the inverter with wiring decided to be normal by the wiring condition deciding means, without the use of information of a rotated position to make a comparison between the direction of rotation based on a speed instruction value of the motor and the direction of actual rotation for making a decision as to whether the phase sequence of the motor is correct or not. 
     A fourth aspect of the invention is that, in the improper wiring detecting system of a parallel inverter system according to any one of the first to third aspects of the invention, a plurality of the inverters are allowed to carry out normal operations only at the completion of the decision processing by the wiring condition deciding means or the phase sequence deciding means. 
     A fifth aspect of the invention is that, in the improper wiring detecting system of a parallel inverter system according to any one of the first to fifth aspects of the invention, the wiring condition deciding means or the phase sequence deciding means is actualized by a processing unit in the controlling means. 
     According to the invention, in a parallel inverter system in which a plurality of inverters are connected in parallel, improper wiring including an unfinished connection, an incorrect connection to a terminal block and further an incorrect phase sequence can be surely and easily detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a circuit configuration of a parallel inverter system as an embodiment according to the invention; 
         FIG. 2  is a flow chart showing an example of the whole of an improper wiring detecting operation in the embodiment according to the invention; 
         FIG. 3  is a flow chart showing another example of the improper wiring detecting operation in the embodiment according to the invention; 
         FIG. 4  is a circuit diagram showing an example of a motor driving system with an inverter in related art; and 
         FIG. 5  is a flow chart showing the operation of detecting a state of an unfinished connection between the inverter and the motor in the motor driving system in the related art shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following, an embodiment of the invention will be explained with reference to the attached drawings. 
     First,  FIG. 1  is a circuit diagram showing a circuit configuration of a parallel inverter system as the embodiment according to the invention. The circuit is that in the case in which two inverters are connected in parallel to combine their output for driving a motor as a load. 
     In  FIG. 1 , reference numeral  1  denotes a three-phase AC motor and reference numerals  2   a  and  2   b  denote inverters. Here, reference numeral  2   a  is to denote a master inverter and reference numeral  2   b  is to denote a slave inverter. To the motor  1 , a rotated position detector  3  such as a pulse encoder is connected so that the information of the rotated position (rotation angle and rotation speed) of the motor  1  can be obtained. 
     The inverters  2   a  and  2   b  are provided with electric power converters  4   a  and  4   b , respectively, to each of which a DC voltage is inputted. The electric power converter  4   a  is formed of combinations of capacitors C 1a  and C 2a  and semiconductor switching devices such as IGBTs Q 1a  and Q 2a , Q 3a  and Q 4a , and Q 5a  and Q 6a , each of the combinations being connected in series between DC bus wires, and an unillustrated gate driving circuit. The electric power converter  4   b  is formed of combinations of capacitors C 1b  and C 2b  and semiconductor switching devices such as IGBTs Q 1b  and Q 2b , Q 3b  and Q 4b , and Q 5b  and Q 6b , each of the combinations being connected in series between DC bus wires, and an unillustrated gate driving circuit. 
     The switching devices Q 1a  to Q 6a  and Q 1b  to Q 6b  are driven by the control units  5   a  and  5   b , respectively, each of which includes a processor such as a microcomputer (CPU) and various kinds of electronic circuits. The unillustrated gate driving circuits described before can be contained in the control units  5   a  and  5   b , respectively. 
     The output voltages and the output currents of the inverters  2   a  and  2   b  are detected by voltage detectors  6   a  and  6   b  and current detectors  7   a  and  7   b  to be inputted to the control units  5   a  and  5   b , respectively. Along with this, the information of the rotated position from the rotated position detector  3  is also inputted to the control units  5   a  and  5   b . In the control units  5   a  and  5   b , gate signals for the switching devices Q 1a  to Q 6a  and Q 1b  to Q 6b  are produced, respectively, on the basis of the inputted information. The AC currents, outputted from the electric power converters  4   a  and  4   b  by the on-off operations of the switching devices Q 1a  to Q 6a  and Q 1b  to Q 6b  and transmitted by cables through terminal blocks  8   a  and  8   b , respectively, to the connection point to the motor  1 , are combined at the connection point to be thereafter supplied to the motor  1 . 
     In addition, to the control units  5   a  and  5   b , operation readouts  10   a  and  10   b  are connected, respectively, as interfaces with an operator. The control units  5   a  and  5   b  mutually transmit items of internal information by using transmitting means  9   a  and  9   b , respectively, to share the items of the information by storing the items of the information in their respective memories. 
     In the next, the operation of the parallel inverter system as the embodiment will be explained. First, the output currents of the inverters  2   a  and  2   b , the number of the parallel arrangement of which is determined so as to be commensurate with the rated current and the operating conditions of the motor  1 , are combined as was explained before to be supplied to the motor  1 . 
     Then, in an operation under a normal condition, the control unit  5   a  in the master inverter  2   a  produces speed instruction values with which the motor  1  is started into motion from a standstill, then accelerated to reach a target speed and operated until being decelerated to be stopped depending on conditions. From the speed instruction values and actual rotation speed values detected by the rotated position detector  3 , current instruction values corresponding to the generated torques of the motor  1  are calculated out so that the motor  1  follows the speed instruction values. Along with this, from the output current values detected by the current detector  7   a  and the current instruction values, voltage instruction values are calculated out. 
     On the basis of the voltage instruction values, gate signals for the switching devices Q 1a  to Q 6a  in the electric power converter  4   a  are produced. With the use of the gate signals, the switching devices Q 1a  to Q 6a  are made to be turned-on and -off, by which a PWM controlled voltage is outputted. 
     In the slave inverter  2   b , voltage instruction values operated by the master inverter  2   a  are captured in the control units  5   b  through the transmitting means  9   a  and  9   b , by which the electric power converter  4   b  operates similarly to the electric power converters  4   a  in the master inverter  2   a . Moreover, an output current detected by the current detector  7   b  is also transmitted to the control unit  5   a  in the master inverter  2   a  through the transmitting means  9   a  and  9   b.    
     Between the inverters  2   a  and  2   b , a current referred to as a cross current or a circulating current can be caused to flow with a value according to the degree of a voltage error caused by coupling the outputs of the inverters  2   a  and  2   b . Against this, cross current control is also carried out in some cases in which the control unit  5   a  in the master side and the control unit  5   b  in the slave side correct their respective voltage instruction values so that the respective output current values from the inverters  2   a  and  2   b  become equal. 
     Next, explanations will be made with respect to an improper wiring detecting operation as a principal part of the invention. 
       FIG. 2  is a flow chart showing an example of the whole of an improper wiring detecting operation in the embodiment. First, wiring condition decision processing SA is carried out with phase sequence decision processing SB carried out next. Although an error in a phase sequence (a discrepancy between the phase sequence on the output side of the inverters and the phase sequence on the input side of the motor) is due to a kind of improper wiring, the phase sequence decision processing SB here will be explained as being distinguished from the wiring condition decision processing SA for the sake of convenience. 
     In the wiring condition decision processing SA, in order that a current is made to flow from one phase (the U-phase, for example) in the master inverter  2   a  to another phase (the V-phase, for example), a closed circuit is formed between the two phases (step S 1 ). Specifically, a specified current instruction value is prepared by the control unit  5   a  and current instruction values (V ur  and V vr ) are then obtained so that the value of a current I u1  detected by the current detector  7   a  follows the current instruction value to turn-on switching devices in the two phases (the U-phase and the V-phase, for example) in the electric power converter  4   a . Here, in the electric power converter  4   a , no switching devices in the rest phase (the W-phase, for example) are made to be turned-on. Then, the values of the output voltages V u1 , V v1  and V w1  in all of the phases are detected by the voltage detector  6   a  to be captured in the control unit  5   a  together with the value of the currents I u1  and I v1  detected by the current detector  7   a.    
     In the slave inverter  2   b , with no switching devices in all of the phases being turned-on by the electric power converter  4   b , the values of the output voltages V u2 , V v2  and V w2  are detected to be captured in the control unit  5   b . Along with this, the control unit  5   b  mutually shares information with the control unit  5   a  on the side of the master inverter  2   a  through the transmitting means  9   a  and  9   b  (the control units  5   a  and  5   b  store and keep the values of the output voltages V u1 , V v1 , V w1 , V u2 , V v2  and V w2  in their respective internal memories together). 
     Following this, the presence or absence of the disconnection between the U-phase and the V-phase in the master inverter  2   a  is ascertained. Here, the disconnection includes a failure such as an unfinished connection to the terminal block in addition to a so-called disconnection caused by breaking of wire. 
     The formation of a closed circuit without disconnection allows the current in the U-phase and the current in the V-phase to flow with the polarities thereof being opposite to each other. Therefore, it is ascertained whether the relation of I u1 ≈−I v1  holds or not (S 2 ). When the relation of I u1 ≈−I v1  holds, the processing shifts to step S 3 . When the relation does not hold, the disconnection is regarded as a kind of improper wiring and the processing jumps to step S 11 . 
     When both of the master inverter  2   a  and the slave inverter  2   b  have no improper wiring, the impedance of the wiring can be sufficiently smaller than the impedance of the motor  1  that the output voltages in the respective corresponding phases are brought to be approximately equal to each other. Therefore, subsequent to step S 2 , the detected values of the voltages in the respective phases shared by both of the inverters  2   a  and  2   b  are compared to ascertain whether or not the values of the detected voltages become as V u1 ≈V u2 , V v1 ≈V v2 , and V w1 ≈V w2 , by which the presence or absence of improper wiring is decided (S 3  to S 5 , and S 11 ). 
     Moreover, when a closed circuit is formed between the U-phase and the V-phase, the relation of V u1 =−V v1  holds, and the output voltage in an unenergized phase (the W-phase, for example) can be caused to become zero. This is also ascertained at the same time (S 4 , S 5  and S 11 ). 
     With this ascertainment, there is the possibility that the wiring of the unenergized phase (in the example, the W-phase) is not connected (e.g., unfinished connection to the terminal block or broken). Therefore, with the combination of the two phases, between which a closed circuit is formed by turning-on the switching devices, being changed (in the example shown in  FIG. 2 , from the combination of the U-phase and the V-phase to the combination of the V-phase and the W-phase), processing similar to that from step S 1  to step S 5  is carried out (S 6  to S 10 , S 11 ). 
     By the processing in the foregoing, the wiring condition decision processing SA is completed. When decision is made in the processing that there is improper wiring (S 11 ), the operation readouts  10   a  and  10   b  display that there is improper wiring to give warning to operators. 
     Incidentally, execution of the wiring condition decision processing SA permits the ascertainment of the presence or absence of improper wiring in each of the phases in the master inverter  2   a  and the slave inverter  2   b . The execution, however, cannot ascertain whether or not the three phase outputs of each of the inverters  2   a  and  2   b  are connected to the input side of the motor  1  with a correct phase sequence through cables. 
     Thus, by the phase sequence decision processing SB, a decision is made as to whether the phase sequence is correct or not from the direction of rotation when the motor  1  is rotated. 
     When the rotation of the motor  1  is controlled by making use of the rotation angle and the rotation speed obtained from the rotated position detector  3  shown in  FIG. 1 , an incorrect phase sequence might cause an excessive current to flow. Thus, at the beginning, the motor  1  is started without using a detector such as the rotated position detector  3  (S 12 ). For specific driving control systems in this case, control systems such as a constant V/f (voltage/frequency) control system and a sensorless vector control system are possibilities, for example. 
     Next, a comparison is made between the direction of rotation based on the speed instruction value when driving the motor  1  and the actual direction of rotation of the motor  1  obtained from the rotated position detector  3  for making a decision as to whether both of the directions of rotation are the same or not (S 13 ). Here, when the phase sequence is correct, the direction of rotation based on the speed instruction value is the same as the actual direction of rotation. Thus, the processing is brought to normal completion (“Yes” at S 13 ). When the phase sequence is incorrect, the motor  1  rotates in the direction opposite to the direction of rotation based on the speed instruction value. Then, a decision is made that there is discrepancy in the phase sequences and the operation readouts  10   a  and  10   b  display that there is an error in the phase sequence to give warning to operators (“No” at S 13 , and S 14 ). 
       FIG. 3  is a flow chart showing another example of the improper wiring detecting operation in the embodiment according to the invention. 
     In  FIG. 3 , first, the execution of the wiring condition decision processing SA and the phase sequence decision processing SB shown in  FIG. 2  is ascertained (step S 21 ). Next, a decision is made as to whether the items of the processing have been normally completed or not (S 22 ). When the items of the processing have been normally completed, the processing permitting the normal operation explained in the foregoing (the operation of the motor  1  by the inverters  2   a  and  2   b ) is executed (“Yes” at S 22 , and S 23 ). When the items of the processing have not been normally completed, the processing not permitting the normal operation is executed (“No” at S 22 , and S 24 ). 
     The improper wiring detecting operation shown in  FIG. 3  is based on the assumption that the execution of the wiring condition decision processing SA and the execution of the phase sequence decision processing SB have been completed when the parallel inverter system is operated. Thus, according to the operation shown in  FIG. 3 , a normal operation is to be permitted only when no wiring problem is found as a result of both of the decision processing SA and the decision processing SB. Therefore, troubles occurring after the inverter system is brought into practical operation can be prevented before the troubles occur. 
     While the present invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention.

Technology Category: h