Abstract:
An inverter includes: a converter circuitry to convert an alternating-current voltage to a direct-current voltage; an inverter circuitry to convert a direct-current voltage obtained by conversion by the converter circuitry to an alternating-current voltage and to supply the alternating-current voltage obtained by conversion to a load; a current detector to detect an output current of the inverter circuitry; a calculator to calculate an output frequency based on the output current detected by the current detector, and a pulse-signal outputter to output a pulse signal based on the output frequency calculated by the calculator. The inverter circuitry converts the direct-current voltage obtained by conversion by the converter circuitry to the alternating-current voltage based on the pulse signal output from the pulse-signal outputter, so as to obtain a frequency corresponding to the magnitude of the load.

Description:
FIELD 
       [0001]    The present invention relates to an inverter device that converts a direct-current (DC) voltage to an alternating-current (AC) voltage. 
       BACKGROUND 
       [0002]    An inverter device controls an elevating speed of an elevator, a crane, a multistory parking lot, and a printing machine by adjusting a voltage and a frequency. 
         [0003]    Patent Literature 1 discloses an inverter device that suppresses an over-torque state of an induction motor for winding up or rewinding a cable of a winch. When a torque current detected by a torque-current detecting circuit exceeds a set level and the induction motor becomes in the over-torque state, the inverter device described in Patent Literature 1 determines a frequency and a voltage of an AC voltage to be generated by an inverter based on a speed of the induction motor output from a speed detector. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Patent Application Laid-open No. H10-191689 
       SUMMARY 
     Technical Problem 
       [0005]    However, because the inverter device described in Patent Literature 1 determines the frequency based on the speed of the induction motor when being in the over-torque state, there is a possibility that a large change of a load occurs and it takes much time to reach a frequency corresponding to the magnitude of a load. 
         [0006]    The present invention has been achieved in view of the above problems, and an object of the present invention is to provide an inverter device that can perform adjustment to obtain a frequency corresponding to the magnitude of a load without being affected by a load change. 
       Solution to Problem 
       [0007]    To solve the above problems and achieve the object an inverter device according to the present invention includes: a converter circuit unit to convert an alternating-current voltage to a direct-current voltage; an inverter circuit unit to convert a direct-current voltage obtained by conversion by the converter circuit unit to an alternating-current voltage and to supply the alternating-current voltage obtained by conversion to a load; a current detecting unit to detect an output current of the inverter circuit unit; a calculating unit to calculate an output frequency based on the output current detected by the current detecting unit; and a pulse-signal output unit to output a pulse signal based on the output frequency calculated by the calculating unit. The inverter circuit unit converts the direct-current voltage obtained by conversion by the converter circuit unit to an alternating-current voltage with a frequency corresponding to the load based on the pulse signal output by the pulse-signal output unit. 
       Advantageous Effects of Invention 
       [0008]    The inverter device according to the present invention can perform adjustment to obtain a frequency corresponding to the magnitude of a load without being affected by a load change. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a configuration diagram of an inverter device according to a first embodiment. 
           [0010]      FIG. 2  is an explanatory diagram of a procedure of calculating a torque-current limit value performed by a torque-current limit-value calculating unit according to the first embodiment. 
           [0011]      FIG. 3  is a flowchart for explaining an operation of a calculating unit according to the first embodiment. 
           [0012]      FIG. 4  is an explanatory diagram of an operation of an output-frequency calculating unit according to the first embodiment. 
           [0013]      FIG. 5  is an explanatory diagram of a relation between an output frequency and a torque current according to the first embodiment. 
           [0014]      FIG. 6  is a configuration diagram of an inverter device according to a second embodiment. 
           [0015]      FIG. 7  is an explanatory diagram of a procedure of calculating a correction frequency performed by a correction-frequency calculating unit according to the second embodiment. 
           [0016]      FIG. 8  is a flowchart for explaining an operation of a calculating unit according to the second embodiment. 
           [0017]      FIG. 9  is a block diagram illustrating a configuration of the calculating unit according to the second embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Exemplary embodiments of an inverter device according to the present invention will be explained below in detail with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
       First Embodiment 
       [0019]      FIG. 1  is a diagram illustrating a configuration of an inverter device  1  according to a first embodiment.  FIG. 2  is a diagram illustrating a relation between a torque-current limit value and an output frequency according to the first embodiment.  FIG. 3  is a flowchart for explaining an operation of a calculating unit according to the first embodiment.  FIG. 4  is an explanatory diagram of an operation of an output-frequency calculating unit according to the first embodiment.  FIG. 5  is an explanatory diagram of a relation between an output frequency and a torque current according to the first embodiment. 
         [0020]    The inverter device  1  converts an AC voltage supplied from an AC power source  2  to a DC voltage, converts the DC voltage obtained by the conversion to an AC voltage again, and supplies the AC voltage obtained by the conversion to a motor  3 . The inverter device  1  includes a converter circuit unit  11  that converts an AC voltage to a DC voltage, an inverter circuit unit  12  that converts a DC voltage to an AC voltage, a current detecting unit  13  that detects an output current of the inverter circuit unit  12 , a calculating unit  14  that calculates an output frequency, and a pulse-signal output unit  15  that outputs a pulse signal. A smoothing capacitor  16  that smooths the DC voltage obtained by conversion by the converter circuit unit  11  is inserted between the converter circuit unit  11  and the inverter circuit unit  12 . The inverter device  1  also includes a memory  17 . The memory  17  holds therein a torque-current limit-reference value and a motor rated frequency that is a rated frequency of the motor  3 , and details thereof will be described later. 
         [0021]    The inverter circuit unit  12  converts the DC voltage obtained by conversion by the converter circuit unit  11  to an AC voltage with a frequency corresponding to the motor  3  being a load based on the pulse signal output from the pulse-signal output unit  15 , and supplies the AC voltage after conversion to the motor  3 . Specifically, the inverter circuit unit  12  variably controls a frequency and a voltage of the AC voltage under VVVF (Variable Voltage Variable Frequency) control. 
         [0022]    The current detecting unit  13  detects phase currents of a U-phase, a V-phase, and a W-phase output from the inverter circuit unit  12 . These phase currents are collectively referred to as “output current”. 
         [0023]    The calculating unit  14  calculates an output frequency based on the output current detected by the current detecting unit  13 . Detailed configurations of the calculating unit  14  will be described later. 
         [0024]    The pulse-signal output unit  15  outputs a pulse signal to the inverter circuit unit  12  based on the output frequency calculated by the calculating unit  14 . The pulse signal is a signal subjected to pulse width modulation. The output frequency is also referred to as “frequency instruction value”. 
         [0025]    The configuration of the calculating unit  14  is described below. The calculating unit  14  includes: a torque-current detecting unit  21  that detects a torque current; a torque-current limit-value calculating unit  22  that calculates a torque-current limit value; a determining unit  23  that determines whether to suspend or resume acceleration of a load; and an output-frequency calculating unit  24  that calculates an output frequency. 
         [0026]    The torque-current detecting unit  21  detects the torque current based on the output current detected by the current detecting unit  13 . Specifically, the torque-current detecting unit  21  performs d-q coordinate transformation based on the output current detected by the current detecting unit  13  to detect the torque current. 
         [0027]    The torque-current limit-value calculating unit  22  calculates the torque-current limit value based on the output frequency. A procedure of calculating the torque-current limit value performed by the torque-current limit-value calculating unit  22  is described below. The torque-current limit value decreases as the output frequency increases, as illustrated in  FIG. 2 . 
         [0028]    The torque-current limit-value calculating unit  22  substitutes the torque-current limit-reference value and the motor rated frequency read from the memory  17  and the output frequency input from the output-frequency calculating unit  24  into expression (1) to calculate the torque-current limit value. The torque-current limit value is a value decreasing with reduction of a magnetic flux of the motor  3 . 
         [0000]      Torque-current limit value=torque-current limit-reference value×motor rated frequency/output frequency   (1)
 
         [0029]    The determining unit  23  determines whether to suspend or resume acceleration of the load based on the torque current and the torque-current limit value. 
         [0030]    Specifically, the determining unit  23  outputs a signal for suspending acceleration to the output-frequency calculating unit  24  when the torque current is equal to or larger than the torque-current limit value, and outputs a signal for resuming acceleration to the output-frequency calculating unit  24  when the torque current is smaller than the torque-current limit value. While it is assumed in the following descriptions that the signal for suspending acceleration is “0” and the signal for resuming acceleration is “1”, the signals may be any signals as long as results of determination can be distinguished from each other and output to the output-frequency calculating unit  24 , and the signals are not limited to “0” and “1”. 
         [0031]    The output-frequency calculating unit  24  calculates the output frequency based on the determination by the determining unit  23 . The output-frequency calculating unit  24  outputs the calculated output frequency to the pulse-signal output unit  15 . 
         [0032]    A specific operation of the calculating unit  14  is described below with reference to the flowchart illustrated in  FIG. 3 . 
         [0033]    At Step ST 1 , the torque-current detecting unit  21  detects a torque current based on an output current detected by the current detecting unit  13 . 
         [0034]    At Step ST 2 , the torque-current limit-value calculating unit  22  calculates a torque-current limit value based on an output frequency. 
         [0035]    At Step ST 3 , the determining unit  23  determines whether the torque current is equal to or larger than the torque-current limit value. When determining that the torque current is equal to or larger than the torque-current limit value (YES), the process proceeds to Step ST 4 . When determining that the torque current is not equal to or larger than the torque-current limit value, that is, the torque current is smaller than the torque-current limit value (NO), the process proceeds to Step ST 5 . 
         [0036]    At Step ST 4 , the determining unit  23  outputs the signal for suspending acceleration to the output-frequency calculating unit  24 . 
         [0037]    At Step ST 5 , the determining unit  23  outputs the signal for resuming acceleration to the output-frequency calculating unit  24 . 
         [0038]    At Step ST 6 , the output-frequency calculating unit  24  calculates an output frequency based on the determination by the determining unit  23 . 
         [0039]    A specific operation of the output-frequency calculating unit  24  is described below. In the following descriptions, it is assumed that the signal for suspending acceleration is input at a time t 1  and the signal for resuming acceleration is input at a time t 2 . 
         [0040]    The output-frequency calculating unit  24  outputs an output frequency that is increased at a constant rate at constant time intervals to the pulse-signal output unit  15  until the output frequency reaches a target output frequency fm as illustrated in  FIG. 4 , when no signal from the determining unit  23  is input or when the signal for resuming acceleration is input. “A” in  FIG. 4  represents a waveform of the output frequency in a case where the output frequency is increased to the target output frequency fm without suspending acceleration. B in  FIG. 4  represents a waveform of the output frequency in a case where the output frequency is increased to the target output frequency fm while acceleration is suspended and resumed. 
         [0041]    When the signal for suspending acceleration is input from the determining unit  23  at the time t 1 , the output-frequency calculating unit  24  does not increase the output frequency and outputs the output frequency with the same value to the pulse-signal output unit  15 . When the signal for resuming acceleration is input from the determining unit  23  at the time t 2 , the output-frequency calculating unit  24  outputs the output frequency that is increased at the constant rate to the pulse-signal output unit  15 . 
         [0042]    A relation between the output frequency and the torque current is described below. The output frequency increases until the torque current reaches a torque-current limit value or a higher value as illustrated in  FIG. 5( a ) . The torque current is obtained by adding a load torque current and an acceleration torque current as illustrated in  FIG. 5( b ) . 
         [0043]    The determining unit  23  suspends acceleration when the torque current becomes the torque-current limit value or a higher value. The torque current decreases by the acceleration torque current because of suspension of acceleration. 
         [0044]    When the torque current falls below the torque-current limit value because of decrease of the torque current, the determining unit  23  resumes acceleration. The output frequency rises because of resumption of acceleration. 
         [0045]    When the acceleration torque current no longer flows and the torque current is not decreased even by suspension of acceleration, the output frequency no longer rises. That is, a frequency at which the output frequency becomes constant is the target output frequency fm. When the output frequency reaches the target output frequency fm, the output-frequency calculating unit  24  continues to output the constant output frequency to the pulse-signal output unit  15  so that that output frequency fm is maintained. 
         [0046]    When a torque current becomes equal to or larger than a torque-current limit value, the inverter device  1  suspends acceleration to cause an output frequency to be constant so that an acceleration torque is reduced and the torque current is decreased. When the torque current falls below the torque-current limit value, the inverter device  1  resumes acceleration to raise the output frequency. By doing these operations, the inverter device  1  finally sets the torque current to be equal to the torque-current limit value. 
         [0047]    Therefore, the inverter device  1  can perform adjustment to obtain an output frequency corresponding to a load without being affected by a load change during acceleration. 
       Second Embodiment 
       [0048]    A configuration and an operation of the inverter device  1  after an output frequency reaches the target output frequency fm are described next. The inverter device  1  according to a second embodiment is obtained by adding a correction-frequency calculating unit  25  to the configuration of the first embodiment. The configuration other than the correction-frequency calculating unit  25  is the same as that described in the first embodiment, and therefore like reference signs are given to omit the descriptions. The memory  17  holds therein the torque-current limit-reference value, the motor rated frequency, and a correction coefficient, and details thereof will be described later. 
         [0049]      FIG. 6  is a diagram illustrating a configuration of the inverter device  1  according to the second embodiment.  FIG. 7  is an explanatory diagram of a procedure of calculating a correction frequency performed by the correction-frequency calculating unit  25  according to the second embodiment.  FIG. 8  is a flowchart for explaining an operation of the calculating unit  14  according to the second embodiment.  FIG. 9  is a block diagram illustrating a configuration of the calculating unit  14  according to the second embodiment. 
         [0050]    The calculating unit  14  further includes the correction-frequency calculating unit  25  that calculates a correction frequency based on a torque current and a torque-current limit value, as illustrated in  FIG. 6 . 
         [0051]    The torque-current detecting unit  21  detects a torque current based on an output current detected by the current detecting unit  13 , and outputs the detected torque current to the correction-frequency calculating unit  25 . 
         [0052]    The torque-current limit-value calculating unit  22  calculates a torque-current limit value based on an output frequency, and outputs the calculated torque-current limit value to the correction-frequency calculating unit  25 . 
         [0053]    The determining unit  23  outputs a trigger signal to the correction-frequency calculating unit  25  when the torque current is no longer decreased even by suspension of acceleration of the motor  3  being a load, that is, when the determining unit  23  determines that the torque current is not equal to or larger than the torque-current limit value. 
         [0054]    The correction-frequency calculating unit  25  outputs a correction frequency calculated based on the torque current and the torque-current limit value to the output-frequency calculating unit  24  when the trigger signal is input from the determining unit  23 . 
         [0055]    The output-frequency calculating unit  24  calculates an output frequency based on the correction frequency and outputs the calculated output frequency to the pulse-signal output unit  15 . 
         [0056]    A procedure of calculating a correction frequency performed by the correction-frequency calculating unit  25  is described below. In the following descriptions, it is assumed that a trigger signal for starting correction is input from the determining unit  23  at a time t 3  and a trigger signal for ending correction is input from the determining unit  23  at a time t 4 , as illustrated in  FIG. 7 . C in  FIG. 7  represents a waveform of the output frequency in a case where no trigger signal is input. D in  FIG. 7  represents a waveform of the output frequency in a case where the trigger signal is input. 
         [0057]    The correction-frequency calculating unit  25  corrects the output frequency to suppress rise of the torque current when a load becomes large during an operation of the motor  3  at a constant speed after suspension of acceleration. Specifically, the correction-frequency calculating unit  25  reads out the torque-current limit-reference value, the motor rated frequency, and the correction coefficient from the memory  17 . The correction-frequency calculating unit  25  substitutes the torque-current limit-reference value, the motor rated frequency, the correction coefficient, the torque current input from the torque-current detecting unit  21 , and the torque-current limit value input from the torque-current limit-value calculating unit  22  into expression (2) to calculate the correction frequency. 
         [0000]      Correction frequency=((torque current−torque-current limit value)/(torque-current limit-reference value)×motor rated frequency×correction coefficient  (2)
 
         [0058]    A specific operation of the calculating unit  14  is described next with reference to the flowchart illustrated in  FIG. 8 . 
         [0059]    At Step ST 11 , the torque-current detecting unit  21  detects a torque current based on an output current detected by the current detecting unit  13 . 
         [0060]    At Step ST 12 , the torque-current limit-value calculating unit  22  calculates a torque-current limit value based on an output frequency. 
         [0061]    At Step ST 13 , the determining unit  23  performs a process of reading a state of acceleration based on the torque current and the torque-current limit value. 
         [0062]    At Step ST 14 , the determining unit  23  determines whether to suspend acceleration based on the process of reading the state of acceleration. When acceleration is to be suspended (YES), the process proceeds to Step ST 15 . When acceleration is not to be suspended (NO), a series of processes is ended. 
         [0063]    At Step ST 15 , the determining unit  23  determines whether the torque current is equal to or larger than the torque-current limit value. When the torque current is determined as being equal to or larger than the torque-current limit value (YES), the process proceeds to Step ST 16 . When it is determined that the torque current is not equal to or larger than the torque-current limit value, that is, the torque current is smaller than the torque-current limit value (NO), a series of processes is ended. The determining unit  23  outputs the trigger signal to the correction-frequency calculating unit  25  when determining that the torque current is equal to or larger than the torque-current limit value at this step. 
         [0064]    At Step ST 16 , the correction-frequency calculating unit  25  calculates a correction frequency based on the torque current and the torque-current limit value and outputs the correction frequency to the output-frequency calculating unit  24 , when the trigger signal is input from the determining unit  23 . 
         [0065]    At Step ST 17 , the output-frequency calculating unit  24  calculates an output frequency based on the correction frequency, and outputs the calculated output frequency to the pulse-signal output unit  15 . 
         [0066]    The inverter device  1  calculates an output frequency based on a correction frequency, while the motor  3  is operated at a constant speed after a torque current becomes equal to a torque-current limit value and acceleration is suspended. Therefore, it is possible to suppress a load change and prevent the motor  3  from stopping because of an overload. 
         [0067]    The calculating unit  14  according to the first and second embodiments described above may be configured by a CPU  101  that performs computing, a ROM  102  that has programs to be read by the CPU  101  stored therein, a RAM  103  in which the programs stored in the ROM  102  are to be developed, and an interface  104  that performs input and output of signals, as illustrated in  FIG. 9 . Each constituent element of the calculating unit  14  is programmed and is stored in the ROM  102 . The interface  104  receives input of an output signal from the current detecting unit  13 , and outputs an output frequency to the pulse-signal output unit  15 . 
         [0068]    The CPU  101  reads the programs stored in the ROM  102 , develops the read programs in the RAM  103 , and performs computing based on an output signal input from the current detecting unit  13  and data held in the memory  17  to obtain an output frequency described above. The output frequency calculated by the CPU  101  is output to the pulse-signal output unit  15  via the interface  104 . 
         [0069]    The configurations described in the above embodiments are only examples of the contents of the present invention and can be combined with other known techniques. A part of the configurations can be omitted or modified without departing from the scope of the present invention. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  inverter device 
               2  AC power source 
               3  motor 
               11  converter circuit unit 
               12  inverter circuit unit 
               13  current detecting unit 
               14  calculating unit 
               15  pulse-signal output unit 
               16  smoothing capacitor 
               17  memory 
               21  torque-current detecting unit 
               22  torque-current limit-value calculating unit 
               23  determining unit 
               24  output-frequency calculating unit 
               25  correction-frequency calculating unit