Patent Publication Number: US-2023145142-A1

Title: Motor driver and heat pump

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a U.S. National Stage Application of International Application No. PCT/JP2020/023633 filed on Jun. 16, 2020, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a motor driver that drive a motor, and a heat pump. 
     BACKGROUND 
     Conventionally, in a heat pump, a fan is used for the purpose of blowing air to a heat exchanger. Further, in the heat pump, a highly efficient permanent magnet synchronous motor is widely used to drive the fan. As a means for inexpensively driving a motor, position sensorless control technology for estimating a rotor position of the motor from a current of the motor without using a position sensor is widely known. For example, Patent Literature 1 discloses a technique of causing a direct current to flow through a motor at a start of the motor and drawing a rotor position of the motor to a desired position, in position sensorless control. 
     PATENT LITERATURE 
     
         
         Patent Literature 1: Japanese Patent Application Laid-open No. 2017-221001 
       
    
     A method described in Patent Literature 1 is a method in which currents of two phases are detected by a current sensor, and a current of the remaining one phase is calculated using of a three-phase equilibrium condition. Patent Literature 1 does not clearly describe what kind of current sensor is used, but direct current transformer (DCCT) is used for two phases since alternating current current transformer (ACCT) cannot detect a DC amount. However, in general, there has been a problem that the DCCT is more expensive and costs more than the ACCT. 
     SUMMARY 
     The present disclosure has been made in view of the above, and an object is to obtain a motor driver capable of performing overcurrent protection in control of causing a direct current to flow, with an inexpensive circuit configuration. 
     In order to solve the above-described problem and achieve the object, a motor driver according to the present disclosure drives a motor including three-phase windings. The motor driver includes an inverter that applies a desired voltage to the motor, and an inverter controller that controls an operation of the inverter. The inverter includes: a direct-current detector that detects a direct current in a first connecting line among three-phase connecting lines connecting the respective three-phase windings and the inverter; and an alternating-current detector that detects an alternating current in a second connecting line among the three-phase connecting lines. In a first control mode for positioning a rotor of the motor, the motor driver applies a maximum direct current to the first connecting line. 
     The motor driver according to the present disclosure has an effect of being able to perform overcurrent protection in control of causing a direct current to flow, with an inexpensive circuit configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a configuration example of a heat pump according to a first embodiment. 
         FIG.  2    is a diagram illustrating a configuration example of an inverter according to the first embodiment. 
         FIG.  3    is a flowchart illustrating an operation of an inverter controller included in a motor driver according to the first embodiment. 
         FIG.  4    is a flowchart illustrating a detailed operation of a positioning control mode, in the inverter controller included in the motor driver according to the first embodiment. 
         FIG.  5    is a first diagram illustrating an example of an equivalent circuit indicating an energization state of the heat pump when the inverter controller according to the first embodiment is operating in the positioning control mode. 
         FIG.  6    is a first diagram illustrating an example of a magnetic flux vector generated by a motor of the heat pump according to the first embodiment. 
         FIG.  7    is a second diagram illustrating an example of a magnetic flux vector generated by the motor of the heat pump according to the first embodiment. 
         FIG.  8    is a second diagram illustrating an example of an equivalent circuit indicating an energization state of the heat pump when the inverter controller according to the first embodiment is operating in the positioning control mode. 
         FIG.  9    is a third diagram illustrating an example of a magnetic flux vector generated by the motor of the heat pump according to the first embodiment. 
         FIG.  10    is a diagram illustrating an example of a hardware configuration that implements the inverter controller included in the heat pump according to the first embodiment. 
         FIG.  11    is a first flowchart illustrating an operation of determining establishment of a transition condition from a voltage/frequency (V/F) control mode to a position sensorless control mode in an inverter controller included in a motor driver according to a second embodiment. 
         FIG.  12    is a first view illustrating an operation state of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller included in the motor driver according to the second embodiment. 
         FIG.  13    is a second flowchart illustrating an operation of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller included in the motor driver according to the second embodiment. 
         FIG.  14    is a second view illustrating an operation state of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller included in the motor driver according to the second embodiment. 
         FIG.  15    is a flowchart illustrating an operation of the inverter controller included in the motor driver according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a motor driver and a heat pump according to an embodiment of the present disclosure will be described in detail with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a diagram illustrating a configuration example of a heat pump  100  according to a first embodiment. The heat pump  100  is included in, for example, an air conditioner, a refrigerator, and the like. The heat pump  100  includes a refrigeration cycle in which a compressor  1 , a four-way valve  2 , a heat exchanger  3 , an expansion mechanism  4 , and a heat exchanger  5  are sequentially connected via a refrigerant pipe  6 . The heat exchangers  3  and  5  perform heat exchange of a refrigerant. The compressor  1  includes a compression mechanism  7  that compresses a refrigerant, and a motor  8  that is for the compressor  1  and operates the compression mechanism  7 . Further, the heat pump  100  includes a fan  9  that is for sending air to the heat exchanger  3 , a motor  10  that is for driving the fan  9 , a fan  11  that is for sending air to the heat exchanger  5 , and a motor  12  that is for driving the fan  11 . The motors  8 ,  10 , and  12  are three-phase motors including three-phase windings of a U phase, a V phase, and a W phase (not illustrated). The motors  8 ,  10 , and  12  are, for example, permanent magnet synchronous motors. 
     Further, the heat pump  100  includes an inverter  13  that applies a desired voltage to the motor  10  to drive, and an inverter controller  14  that controls an operation of the inverter  13 . The inverter  13  is electrically connected to the motor  10 . The inverter  13 : uses, as an input power supply, a bus voltage Vdc which is a DC voltage; applies a voltage Vu to the U-phase winding of the motor  10 ; applies a voltage Vv to the V-phase winding of the motor  10 ; and applies a voltage Vw to the W-phase winding of the motor  10 . The inverter controller  14  is electrically connected to the inverter  13 . The inverter controller  14 : generates a pulse width modulation (PWM) signal, which is a drive signal for driving the inverter  13 , by using motor current information, which is information on a current flowing between the inverter  13  and the motor  10 ; and outputs the PWM signal to the inverter  13 . As control modes for control of the operation of the inverter  13 , the inverter controller  14  includes: a positioning control mode; a V/F control mode; and a position sensorless control mode. 
     In the heat pump  100 , the inverter  13  and the inverter controller  14  constitute a motor driver  50 . The motor driver  50  drives the motor  10 . Note that, although not illustrated in  FIG.  1   , the heat pump  100  includes: an inverter that applies a voltage to the motor  8  to drive; and an inverter controller that controls an operation of the inverter that drives the motor  8 . Similarly, the heat pump  100  includes: an inverter that applies a voltage to the motor  12  to drive; and an inverter controller that controls an operation of the inverter that drives the motor  12 . The heat pump  100  individually drives the motors  8 ,  10 , and  12  by including the inverter and the inverter controller, that is, a motor driver, for each of the motors  8 ,  10 , and  12 . 
       FIG.  2    is a diagram illustrating a configuration example of the inverter  13  according to the first embodiment. The inverter  13  includes a drive circuitry  18  that uses the bus voltage Vdc as an input power supply, and outputs voltages Vu, Vv, and Vw for three phases. The drive circuitry  18  includes six switching elements  18   a  to  18   f , and has a configuration in which three series connection units are connected in parallel, which are: a series connection unit of the switching elements  18   a  and  18   b ; a series connection unit of the switching elements  18   c  and  18   d ; and a series connection unit of the switching elements  18   e  and  18   f . The inverter  13  drives the switching elements  18   a  to  18   f  of the drive circuitry  18  corresponding to each PWM signal in accordance with PWM signals UP, UN, VP, VN, WP, and WN outputted from the inverter controller  14 . In the example of  FIG.  2   : the switching element  18   a  is driven according to the PWM signal UP; the switching element  18   b  is driven according to the PWM signal UN; the switching element  18   c  is driven according to the PWM signal VP; the switching element  18   d  is driven according to the PWM signal VN; the switching element  18   e  is driven according to the PWM signal WP; and the switching element  18   f  is driven according to the PWM signal WN. The inverter  13 : generate the voltages Vu, Vv, and Vw for three phases by driving the switching elements  18   a  to  18   f  of the drive circuitry  18 ; and applies voltages to individual windings of U phase, V phase, and W phase of the motor  10 . 
     The inverter  13  includes a voltage detector  19  for detection of the bus voltage Vdc on an input side of the drive circuitry  18 , that is, a side on which the bus voltage Vdc is supplied to the drive circuitry  18 . The voltage detector  19  outputs a detected voltage value, that is, the bus voltage Vdc to the inverter controller  14 . In order to detect a current flowing from the drive circuitry  18  to the motor  10 , the inverter  13  includes a current detector  20  that detects a direct current flowing between the motor  10  and the inverter  13 , in a first connecting line  22   a  among three-phase connecting lines connecting the respective three-phase windings of the motor  10  and the inverter  13 . The current detector  20  outputs a detected current value, that is, a U-phase current Iu to the inverter controller  14 . Further, in order to detect a current flowing from the drive circuitry  18  to the motor  10 , the inverter  13  includes a current detector  21  that detects an alternating current flowing between the motor  10  and the inverter  13 , in a second connecting line  22   b  among the three-phase connecting lines. The current detector  21  outputs a detected current value, that is, a W-phase current Iw to the inverter controller  14 . Here, in the first embodiment, in the inverter  13 , DCCT is used for the current detector  20  which is a direct-current detector, and ACCT is used for the current detector  21  which is an alternating-current detector. Note that, in  FIG.  2   , the DCCT is attached to the U-phase first connecting line  22   a , and the ACCT is attached to the W-phase second connecting line  22   b . However, this is an example, and a relationship between each current detector and a phase to be attached is not limited. 
     The switching elements  18   a  to  18   f  constituting the drive circuitry  18  of the inverter  13  are semiconductor switching elements. The semiconductor switching element is, for example, an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), or the like. The semiconductor switching element may have a configuration in which a reflux diode (not illustrated) is connected in parallel for the purpose of preventing a surge voltage due to switching. The reflux diode may be a parasitic diode of the semiconductor switching element. However, in a case of a MOSFET, it is also possible to realize a similar function by causing an ON state at a timing of reflux. In addition, a material included in the semiconductor switching element is not limited to silicon Si, and it is possible to realize low loss and high-speed switching by using silicon carbide SiC, gallium nitride GaN, gallium oxide Ga2O3, diamond, or the like, which is a wide bandgap semiconductor. 
     Next, an operation of the motor driver  50  will be described.  FIG.  3    is a flowchart illustrating an operation of the inverter controller  14  included in the motor driver  50  according to the first embodiment. The inverter controller  14  determines whether or not there is a drive command to the motor  10  from a configuration in a preceding stage (not illustrated) (step S 101 ). If there is no drive command (step S 101 : No), the inverter controller  14  waits until there is a drive command. If there is a drive command (step S 101 : Yes), the inverter controller  14  operates in the positioning control mode (step S 102 ). The positioning control mode is a first control mode for controlling an operation of the inverter  13  to cause a direct current to flow from the inverter  13  to the motor  10 , for the inverter controller  14  to draw a rotor position of the motor  10  to a desired position at a start of the motor  10 . A detailed operation in the positioning control mode in the inverter controller  14  will be described later. 
     The inverter controller  14  determines whether or not a prescribed first time period has elapsed from a start of the operation in the positioning control mode (step S 103 ). The first time period is a time period longer than a time period taken until the rotor position of the motor  10  is drawn to a desired position by causing a direct current to flow from the inverter  13  to the motor  10 . The first time period may be changed by a current value of the direct current to flow to the motor  10 . If the first time period has not elapsed (step S 103 : No), the inverter controller  14  continues the operation in the positioning control mode (step S 102 ). If the first time period has elapsed (step S 103 : Yes), the inverter controller  14  transitions from the positioning control mode to the operation of the V/F control mode (step S 104 ). The V/F control mode is generally known, and is a second control mode in which the inverter controller  14  drives the motor  10  by controlling the operation of the inverter  13  to increase an amplitude and a frequency of an output voltage from the inverter  13  in proportion to a speed command for the motor  10 . The V/F control mode is a control mode in which the inverter controller  14  does not use current values acquired from the current detectors  20  and  21  as feedback. 
     The inverter controller  14  determines whether or not a prescribed transition condition is established during the operation in the V/F control mode (step S 105 ). Details of the transition condition in the inverter controller  14  will be described in the second embodiment. If the transition condition is not established (step S 105 : No), the inverter controller  14  continues the operation in the V/F control mode (step S 104 ). If the transition condition is established (step S 105 : Yes), the inverter controller  14  transitions from the V/F control mode to the operation of the position sensorless control mode (step S 106 ). The position sensorless control mode is generally known, and is a third control mode by vector control capable of highly efficient driving in a case where the inverter controller  14  controls the operation of the inverter  13  to drive the motor  10 . The position sensorless control mode is a control mode in which the inverter controller  14  performs estimation of a position of the rotor of the motor  10 , current control, and the like by using the current values acquired from the current detectors  20  and  21  as feedback. 
     The inverter controller  14  determines whether or not there is a stop command to the motor  10  from a configuration in a preceding stage (not illustrated) (step S 107 ). If there is no stop command (step S 107 : No), the inverter controller  14  continues the operation in the position sensorless control mode (step S 106 ). If there is a stop command (step S 107 : Yes), the inverter controller  14  performs control to stop the motor  10  (step S 108 ). 
     Here, a detailed operation of the positioning control mode in step S 102  illustrated in the flowchart of  FIG.  3    will be described.  FIG.  4    is a flowchart illustrating a detailed operation in the positioning control mode, in the inverter controller  14  included in the motor driver  50  according to the first embodiment. 
     The inverter controller  14  sets an energization phase in which a direct current is caused to flow in the three-phase connecting lines, and sets Duty of a PWM signal for the switching elements  18   a  to  18   f  of the drive circuitry  18  corresponding to individual phases (step S 201 ). In the first embodiment, in the positioning control mode, the inverter controller  14  sets, as a phase in which a maximum current flows, the U phase of the first connecting line  22   a  connected with the current detector  20  which is a DCCT. The maximum current is a current having a largest value among currents flowing through the three-phase connecting lines. That is, the inverter controller  14  applies a maximum direct current to the first connecting line  22   a  in the positioning control mode. The inverter controller  14  performs positioning control of the rotor of the motor  10  by causing a direct current to flow in accordance with the flowchart illustrated in  FIG.  4   . In order to control a current flowing through each of the U phase, the V phase, and the W phase, the inverter controller  14  performs PWM-control on the switching elements  18   a  to  18   f  of the drive circuitry  18  with a PWM signal, for example, and sets a ratio of the Duty of the switching elements  18   a  to  18   f  corresponding individually to the U phase, the V phase, and the W phase to U phase=1:V phase=0.5:W phase=0.5. 
     An energization state of the heat pump  100  at this time can be expressed by an equivalent circuit as illustrated in  FIG.  5   .  FIG.  5    is a first diagram illustrating an example of an equivalent circuit indicating an energization state of the heat pump  100  when the inverter controller  14  according to the first embodiment is operating in the positioning control mode. Here, resistance values of: a U-phase resistance  31  indicating a resistance of the U-phase winding, wiring, and the like; a V-phase resistance  32  indicating a resistance of the V-phase winding, wiring, and the like; and a W-phase resistance  33  indicating a resistance of the W-phase winding, wiring, and the like, are assumed to be equal. In addition, in the equivalent circuit illustrated in  FIG.  5   , it is assumed that a direct current flows in a direction of an arrow.  FIG.  5    illustrates that, in the heat pump  100 , a maximum current flows in the U phase of the motor  10 , and ½ of the maximum current of the U phase flows in the other V phase and W phase. In the inverter controller  14 , a ratio of absolute values of a U-phase direct current flowing through the first connecting line  22   a , a W-phase direct current flowing through the second connecting line  22   b , and a V-phase direct current flowing through a third connecting line  22   c  among the three-phase connecting lines is assumed to be 1:0.5:0.5. That is, a current ratio of the U-phase current Iu flowing in the U phase:a V-phase current Iv flowing in the V phase:the W-phase current Iw flowing in the W phase=1:0.5:0.5. 
     A magnetic flux vector generated in the motor  10  in such an energization state is as illustrated in  FIG.  6   .  FIG.  6    is a first diagram illustrating an example of a magnetic flux vector generated by the motor  10  of the heat pump  100  according to the first embodiment. A resultant magnetic flux vector  44  of three-phase currents obtained by combining a magnetic flux vector  41  by the U-phase current Iu, a magnetic flux vector  42  by the V-phase current Iv, and a magnetic flux vector  43  by the W-phase current Iw has a direction on the U-phase axis, that is, a direction of the phase=0°, as illustrated in  FIG.  6   . Since the resultant magnetic flux vector  44  has a direction on the U-phase axis, the heat pump  100  can draw the rotor position of the motor  10  on the U-phase axis. 
     Further, in the heat pump  100 , since the maximum current flows in the U phase even in a case where a winding resistance value of each phase varies, the rotor of the motor  10  can be positioned while overcurrent protection is appropriately performed, by monitoring the U-phase current Iu detected by the current detector  20 . For example, in  FIG.  5   , a case is assumed in which there is no variation in the U-phase resistance  31 , a variation in the V-phase resistance  32  is +5%, and a variation in the W-phase resistance  33  is −5%. Even in this case, the maximum current flows in the U phase of the motor  10 , but the current ratio is to be the U-phase current Iu flowing in the U phase:the V-phase current Iv flowing in the V phase:the W-phase current Iw flowing in the W phase≈1:0.48:0.52. 
       FIG.  7    is a second diagram illustrating an example of a magnetic flux vector generated by the motor  10  of the heat pump  100  according to the first embodiment. In the resultant magnetic flux vector  44  of three-phase currents obtained by combining the magnetic flux vector  41  by the U-phase current Iu, the magnetic flux vector  42  by the V-phase current Iv, and the magnetic flux vector  43  by the W-phase current Iw, a direction is shifted from the U-phase axis, that is, shifted from the phase=0°, as illustrated in  FIG.  7   . Even in this case, the heat pump  100  can draw the rotor position of the motor  10  in the direction of the resultant magnetic flux vector  44  illustrated in  FIG.  7   . 
     The description returns to  FIG.  4   . The inverter controller  14  acquires the U-phase current Iu from the current detector  20  (step S 202 ). The inverter controller  14  compares the U-phase current Iu with a threshold value defined for overcurrent protection (step S 203 ). If the U-phase current Iu is equal to or larger than the threshold value (step S 203 : No), the inverter controller  14  stops energization from the inverter  13  to the motor  10 , for overcurrent protection (step S 204 ). That is, in the positioning control mode, the inverter controller  14  stops energization to the motor  10  when a current value of the current detector  20  becomes equal to or larger than a prescribed threshold value. In this case, the inverter controller  14  also ends the operation of the flowchart illustrated in  FIG.  3   . If the U-phase current Iu is less than the threshold value (step S 203 : Yes), the inverter controller  14  performs current control on the U-phase current Iu (step S 205 ). The current control for the U-phase current Iu is, for example, control by proportional integral (PI) control. As described above, in a case where the first time period has not elapsed (step S 103 : No), the inverter controller  14  continues the operation in the positioning control mode in step S 102 , that is, steps S 201  to S 205 . 
     Note that the inverter controller  14  only needs to cause the maximum current to flow through the first connecting line  22   a  connected with the current detector  20 , that is, the U phase. Therefore, for example, the inverter controller  14  may control Duty of each phase such that a current does not flow in the third connecting line  22   c , that is, the V phase, and currents of the U phase of the first connecting line  22   a  and the W phase of the second connecting line  22   b  have an equal value. 
     An energization state of the heat pump  100  at this time can be expressed by an equivalent circuit as illustrated in  FIG.  8   .  FIG.  8    is a second diagram illustrating an example of an equivalent circuit indicating an energization state of the heat pump  100  when the inverter controller  14  according to the first embodiment is operating in the positioning control mode. Here, a resistance value of the U-phase resistance  31  indicating a resistance of the U phase and a resistance value of the W-phase resistance  33  indicating a resistance of the W phase are equal. In addition, in the equivalent circuit illustrated in  FIG.  8   , it is assumed that a direct current flows in a direction of an arrow.  FIG.  8    indicates that, in the heat pump  100 , since the U phase and the W phase constitute a series circuit, equal currents, that is, the maximum currents flow through the U phase and the W phase of the motor  10 , and no current flows through the other V phase. In the inverter controller  14 , a ratio of absolute values of the U-phase direct current flowing through the first connecting line  22   a , and the direct current flowing through the second connecting line  22   b  or the third connecting line  22   c  is set to 1:1. That is, a current ratio is to be the U-phase current Iu flowing in the U phase:the V-phase current Iv flowing in the V phase:the W-phase current Iw flowing in the W phase=1:0:1. 
     A magnetic flux vector generated in the motor  10  in such an energization state is as illustrated in  FIG.  9   .  FIG.  9    is a third diagram illustrating an example of a magnetic flux vector generated by the motor  10  of the heat pump  100  according to the first embodiment. The resultant magnetic flux vector  44  of two-phase currents obtained by combining the magnetic flux vector  41  by the U-phase current Iu and the magnetic flux vector  43  by the W-phase current Iw is, as illustrated in  FIG.  9   , a vector connecting a start point of the magnetic flux vector  41  by the U-phase current Iu and an end point of the magnetic flux vector  43  by the W-phase current Iw. Even in this case, the heat pump  100  can draw the rotor position of the motor  10  in the direction of the resultant magnetic flux vector  44  illustrated in  FIG.  9    while performing overcurrent protection. Even when there is a variation in resistance of each phase in the equivalent circuit illustrated in  FIG.  8   , the heat pump  100  can control a current value of each phase to a desired value by adjusting Duty of a switching element corresponding to the U phase or the W phase. 
     Next, a hardware configuration of the heat pump  100  will be described.  FIG.  10    is a diagram illustrating an example of a hardware configuration that implements the inverter controller  14  included in the heat pump  100  according to the first embodiment. The inverter controller  14  is implemented by a processor  91  and a memory  92 . 
     The processor  91  is a central processing unit (CPU) (may also be referred to as a central processing device, a processing unit, an arithmetic unit, a microprocessor, a microcomputer, a processor, or a digital signal processor (DSP)) or a system large scale integration (LSI). The memory  92  can be exemplified by a nonvolatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable read only memory (EPROM), or an electrically erasable programmable read only memory (EEPROM) (registered trademark). In addition, the memory  92  is not limited thereto, and may be a magnetic disk, an optical disk, a compact disk, a mini disk, or a digital versatile disc (DVD). Note that the inverter controller  14  may include an electric circuit element such as an analog circuit or a digital circuit. 
     As described above, according to the first embodiment, in the heat pump  100 , the motor driver  50  includes, between the inverter  13  and the motor  10 , the current  20  which is DCCT and the current detector  21  which is ACCT. The inverter controller  14  uses a current value detected by the current detector  20  to perform an operation in the positioning control mode for causing a direct current to flow to draw the rotor of the motor  10  to a desired position. Thus, by adopting a circuit configuration in which two current detectors  20  and  21  are configured by combining the DCCT and the ACCT, the motor driver  50  can stably perform overcurrent protection in control of causing a direct current to flow, while realizing an inexpensive circuit configuration. 
     Second Embodiment 
     In a second embodiment, a transition condition from the V/F control mode to the operation of the position sensorless control mode in step S 105  of the flowchart illustrated in  FIG.  3    of the first embodiment will be described. 
     A configuration of the heat pump  100  according to the second embodiment is similar to the configuration of the heat pump  100  according to the first embodiment illustrated in  FIG.  1   , and a configuration of the inverter  13  is similar to the configuration of the inverter  13  of the first embodiment illustrated in  FIG.  2   . In the inverter  13 , since the current detector  21  is ACCT as described above, it is not possible to accurately detect a current in a low frequency region. In addition, the current detector  21  has individual differences, that is, variations in a frequency at which the current can be accurately detected. Therefore, when the inverter controller  14  transitions from the V/F control mode to the operation of the position sensorless control mode, it is necessary to increase a speed of the motor  10  to a frequency at which current detection by the current detector  21  which is ACCT can be accurately performed. 
     Therefore, in the second embodiment, during the operation in the V/F control mode, the inverter controller  14  compares a current value detected by the current detector  20 , which is DCCT, with a current value detected by the current detector  21  which is ACCT, to monitor whether or not the current detector  21  which is ACCT is in a state of being able to accurately detect a current. The inverter controller  14  transitions from the V/F control mode to the operation of the position sensorless control mode when the current detector  21  which is an ACCT is brought into a state of being able to accurately detect the current. 
       FIG.  11    is a first flowchart illustrating an operation of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller  14  included in the motor driver  50  according to the second embodiment. The flowchart illustrated in  FIG.  11    is an excerpt of a portion from step S 104  to step S 106  of the flowchart illustrated in  FIG.  3   .  FIG.  12    is a first diagram illustrating an operation state of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller  14  included in the motor driver  50  according to the second embodiment. 
     The inverter controller  14  acquires the U-phase current Iu from the current detector  20  (step S 301 ). The inverter controller  14  compares the U-phase current Iu acquired from the current detector  20  for one cycle of a current, and acquires a maximum value Iu_max of the U-phase current Iu during one cycle of a current as illustrated in  FIG.  12    (step S 302 ). The inverter controller  14  acquires, from the current detector  21 , the W-phase current Iw at a timing when the maximum value Iu_max of the U-phase current Iu is obtained (step S 303 ). Here, if a driving frequency of the motor  10  is increased to a state in which the current detector  21  which is ACCT can accurately detect the current, a relationship between the maximum value Iu_max of the U-phase current Iu and the W-phase current Iw is expressed by Equation (1) as illustrated in  FIG.  12   , since the motor  10  has a three-phase equilibrium relationship. 
       | Iw|=|Iu _max|/2  (1)
 
     Therefore, the inverter controller  14  can determine that the current detector  21 , which is ACCT, can accurately detect the current when Equation (1) is established, and a transition can be made from the V/F control mode to the operation of the position sensorless control mode. In the flowchart illustrated in  FIG.  11   , the inverter controller  14  determines whether or not an absolute value of the W-phase current Iw is equal to ½ of an absolute value of the maximum value Iu_max of the U-phase current Iu (step S 304 ). If the absolute value of the W-phase current Iw is not equal to ½ of the absolute value of the maximum value Iu_max of the U-phase current Iu (step S 304 : No), the inverter controller  14  determines that the transition condition is not established and continues the operation in the V/F control mode (step S 104 ). If the absolute value of the W-phase current Iw is equal to ½ of the absolute value of the maximum value Iu_max of the U-phase current Iu (step S 304 : Yes), the inverter controller  14  determines that the transition condition is established, and transitions from the V/F control mode to the operation of the position sensorless control mode (step S 106 ). That is, in a case where half of the absolute value of the maximum value Iu_max in one cycle of the U-phase current Iu detected by the current detector  20  is equal to the absolute value of the W-phase current Iw, which is the current value detected by the current detector  21  when the maximum value Iu_max is obtained by the current detector  20 , the inverter controller  14  transitions from the V/F control mode to the operation of the position sensorless control mode. 
     Note that a method for determining whether the transition condition from the V/F control mode to the position sensorless control mode is established in the inverter controller  14  is not limited to the method illustrated in  FIGS.  11  and  12   .  FIG.  13    is a second flowchart illustrating an operation of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller  14  included in the motor driver  50  according to the second embodiment. The flowchart illustrated in  FIG.  13    is an excerpt of a portion from step S 104  to step S 106  of the flowchart illustrated in  FIG.  3   .  FIG.  14    is a second diagram illustrating an operation state of determining establishment of a transition condition from the V/F control mode to the position sensorless control mode, in the inverter controller  14  included in the motor driver  50  according to the second embodiment. 
     The inverter controller  14  acquires the U-phase current Iu in a U-phase current phase θu from the current detector  20  (step S 401 ). The inverter controller  14  estimates, that is, calculates a W-phase current Iw* in the U-phase current phase θu by using Equations (2) and (3) (step S 402 ). 
         Iu _max= Iu /Sin(θ u )  (2)
 
         Iw*=Iu _maxSin(θ u+ 2π/3)  (3)
 
     Note that the inverter controller  14  may obtain the maximum value Iu_max of the U-phase current Iu obtained by Equation (2) by the method of step S 302  of the flowchart illustrated in  FIG.  11    described above. The inverter controller  14  acquires the W-phase current Iw at a timing when the U-phase current Iu in the U-phase current phase θu is acquired from the current detector  21  (step S 403 ). The inverter controller  14  determines whether or not the W-phase current Iw acquired from the current detector  21  is equal to the calculated W-phase current Iw* (step S 404 ). If the acquired W-phase current Iw is not equal to the calculated W-phase current Iw* (step S 404 : No), the inverter controller  14  determines that the transition condition is not established, and continues the operation in the V/F control mode (step S 104 ). If the acquired W-phase current Iw is equal to the calculated W-phase current Iw* (step S 404 : Yes), the inverter controller  14  determines that the transition condition is established, and transitions from the V/F control mode to the operation of the position sensorless control mode (step S 106 ). That is, the inverter controller  14 : estimates a value of a current flowing through the second connecting line  22   b  when the U-phase current Iu, which is a first current value, is detected by the current detector  20 ; and transitions from the V/F control mode to the operation of the position sensorless control mode in a case where the estimated value of the current is equal to the W-phase current Iw, which is a second current value, detected by the current detector  21  when the first current value is detected by the current detector  20 . 
     As described above, by comparing a U-phase voltage command, that is, phase information of the voltage Vu, a zero-cross point of the U-phase current Iu obtained from the current detector  20 , and the like during the V/F control mode as illustrated in  FIG.  14   , the inverter controller  14  can obtain a phase difference A between the voltage Vu and the U-phase current Iu, and can estimate the U-phase current phase θu, which is a phase of the U-phase current Iu. If the U-phase current phase θu is known, since the remaining currents of the other phases have a phase difference of 120 degrees, the inverter controller  14  can calculate, from a trigonometric function, a V-phase current Iv* and the W-phase current Iw* that are ideal, that is, that are obtained in a case of being three-phase equilibrium with respect to the U-phase current Iu. Therefore, the inverter controller  14  can determine that the current detector  21  is in a state of being able to accurately detect the current, as long as the W-phase current Iw′, which is an estimated value of the W-phase current Iw in an identical phase obtained from an instantaneous value of the U-phase current Iu and the U-phase current phase θu, coincides with the W-phase current Iw obtained from the current detector  21 . 
     Note that, when comparing the W-phase current Iw obtained from the current detector  21 , which is ACCT, with the calculated W-phase current Iw′, the inverter controller  14  may provide a margin of about several percent to a dozen percent for the calculated W-phase current Iw′, in consideration of an influence of variation in accuracy of the current detector  21 , noise, and the like. That is, the inverter controller  14  may determine that the W-phase current Iw coincides with the W-phase current Iw* in a case where the W-phase current Iw obtained from the current detector  21  is within a range of the margin set for the calculated W-phase current Iw*. 
     As described above, according to the second embodiment, in the heat pump  100 , the inverter controller  14  of the motor driver  50 : determines whether or not a relationship between a current value acquired from the current detector  20  which is DCCT and a current value acquired from the current detector  21  which is ACCT is three-phase equilibrium; and transitions to the operation of the current feedback control such as the position sensorless control mode from the non-current feedback control such as the V/F control mode, when individual current values become the three-phase equilibrium state. As described above, the motor driver  50  can stably transition from the V/F control mode to the position sensorless control mode even in a circuit configuration in which the two current detectors  20  and  21  are configured by combining DCCT and ACCT. 
     Note that, in a case of providing, as a driving frequency at a start of the motor  10 , a frequency sufficient for ensuring current detection accuracy of the current detector  21  which is ACCT, the heat pump  100  may transition from the positioning control mode to the operation of the position sensorless control mode directly without executing the V/F control mode.  FIG.  15    is a flowchart illustrating an operation of the inverter controller  14  included in the motor driver  50  according to the second embodiment. Instead of step S 103  of the flowchart illustrated in  FIG.  3    of the first embodiment, the inverter controller  14  determines whether or not a prescribed second time period has elapsed from a start of the operation in the positioning control mode (step S 501 ). The second time period is assumed to be a time period longer than a time period taken to ensure the current detection accuracy of the current detector  21 . The second time period may be changed by a current value of the direct current to flow to the motor  10 . If the second time period has not elapsed (step S 501 : No), the inverter controller  14  continues the operation in the positioning control mode (step S 102 ). If the second time period has elapsed (step S 501 : Yes), the inverter controller  14  transitions from the positioning control mode to the operation of the position sensorless control mode (step S 106 ). 
     The configuration illustrated in the above embodiment illustrates one example and can be combined with another known technique, and it is also possible to combine embodiments with each other and omit and change a part of the configuration without departing from the subject matter of the present invention.