Patent Publication Number: US-10763773-B2

Title: Driving device, air conditioner, and method for driving motor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Patent Application No. PCT/JP2016/082208 filed on Oct. 31, 2016, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a driving device for driving a motor, an air conditioner including the motor, and a method for driving the motor. 
     BACKGROUND 
     In a motor for use in an air conditioner and the like, a connection state of coils of the motor is switched between a Y connection (star connection) and a delta connection (also referred to as a Δ connection) in order to enhance operation efficiency during low-speed rotation and during high-speed rotation (see, for example, Patent Reference 1). 
     Specifically, control is performed in such a manner that a rotation speed of the motor is compared to a threshold, and switching from the Y connection to the delta connection is performed when a state in which the rotation speed is higher than or lower than a threshold continues for a certain time period (see, for example, Patent Reference 2). 
     PATENT REFERENCE 
     
         
         
           
             Patent Reference 1: Japanese Patent Application Publication No. 2009-216324 
             Patent Reference 2: Japanese Patent Publication No. 4619826 
           
         
       
    
     In this case, however, it is difficult to sufficiently enhance efficiency of the motor simply by switching between the Y connection and the delta connection. 
     SUMMARY 
     The present invention is made to solve the problem described above, and an object of the present invention is to sufficiently enhance efficiency of a motor. 
     A driving device according to the present invention is a driving device to drive a motor having coils, and includes a converter to generate a bus voltage, an inverter to convert the bus voltage to an AC voltage and supply the AC voltage to the coils, and a connection switching unit to switch a connection state of the coils. The bus voltage generated by the converter is switched in accordance with the connection state of the coils. 
     According to the present invention, the bus voltage of the converter is switched in accordance with the connection state of the coils, and thus motor efficiency can be sufficiently enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration of a motor according to a first embodiment. 
         FIG. 2  is a cross-sectional view illustrating a configuration of a rotary compressor according to the first embodiment. 
         FIG. 3  is a block diagram illustrating a configuration of an air conditioner according to the first embodiment. 
         FIG. 4  is a conceptual diagram illustrating a basic configuration of a control system of the air conditioner according to the first embodiment. 
         FIG. 5(A)  is a block diagram illustrating the control system of the air conditioner according to the first embodiment, and  FIG. 5(B)  is a block diagram illustrating a section that controls the motor of the compressor based on a room temperature. 
         FIG. 6  is a block diagram illustrating a configuration of a driving device according to the first embodiment. 
         FIG. 7  is a block diagram illustrating a configuration of the driving device according to the first embodiment. 
         FIGS. 8(A) and 8(B)  are schematic diagrams illustrating a switching operation of a connection state of coils according to the first embodiment. 
         FIG. 9  is a schematic diagram illustrating the connection state of the coils according to the first embodiment. 
         FIG. 10  is a flowchart showing a basic operation of the air conditioner according to the first embodiment. 
         FIG. 11  is a flowchart showing a connection switching operation of the air conditioner according to the first embodiment. 
         FIG. 12  is a flowchart showing a connection switching operation of the air conditioner according to the first embodiment. 
         FIGS. 13(A) and 13(B)  are flowcharts showing other examples of the connection switching operation of the air conditioner according to the first embodiment. 
         FIG. 14  is a timing chart showing an example of an operation of the air conditioner according to the first embodiment. 
         FIG. 15  is a graph showing a relationship between a line voltage and a rotation speed of the motor in a case where coils are connected in a Y connection. 
         FIG. 16  is a graph showing a relationship between the line voltage and the rotation speed of the motor in a case where the coils are connected in the Y connection and field-weakening control is performed. 
         FIG. 17  is a graph showing a relationship between motor efficiency and a rotation speed in a case where the field-weakening control shown in  FIG. 16  is performed. 
         FIG. 18  is a graph showing a relationship between motor torque and the rotation speed in a case where the field-weakening control shown in  FIG. 16  is performed. 
         FIG. 19  is a graph showing relationships between the line voltage and the rotation speed in the case where the connection state of the coils is a Y connection and in the case where the connection state of the coils is a delta connection. 
         FIG. 20  is a graph showing a relationship between the line voltage and the rotation speed in a case where switching from the Y connection to the delta connection is performed. 
         FIG. 21  is a graph showing relationships between the motor efficiency and the rotation speed in the case where the connection state of the coils is the Y connection and in the case where the connection state of the coils is the delta connection. 
         FIG. 22  is a graph showing a relationship between the motor efficiency and the rotation speed in a case where the connection state of the coils is the Y connection, the number of turns is adjusted such that the line voltage reaches an inverter maximum output voltage at a rotation speed slightly lower than that in an intermediate heating condition, and switching from the Y connection to the delta connection is performed. 
         FIG. 23  is a graph showing relationships between the motor torque and the rotation speed in the case where the connection state of coils is the Y connection and in the case where the connection state of coils is the delta connection. 
         FIG. 24  is a graph showing a relationship between the motor torque and the rotation speed in the case where the connection state of the coils is the Y connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed slightly lower than that in the intermediate heating condition, and switching from the Y connection to the delta connection is performed. 
         FIG. 25  is a graph showing a relationship between the line voltage and the rotation speed in a case where a bus voltage is switched by a converter. 
         FIG. 26  is a graph showing a relationship between the line voltage and the rotation speed in a case where switching of the connection state of the coils and switching of the bus voltage of the converter are performed in the first embodiment. 
         FIG. 27  is a graph showing relationships between the motor efficiency and the rotation speed in the case where the connection state of the coils is the Y connection and in the case where the connection state of the coils is the delta connection. 
         FIG. 28  is a graph showing a relationship between the motor efficiency and the rotation speed in a case where switching of the connection state of the coils and switching of the bus voltage of the converter are performed in the first embodiment. 
         FIG. 29  is a graph showing relationships between the motor torque and the rotation speed in the case where the connection state of the coils is the Y connection and in the case where the connection state of the coils is the delta connection. 
         FIG. 30  is a graph showing a relationship between the motor efficiency and the rotation speed in the case where switching of the connection state of the coils and switching of the bus voltage of the converter are performed in the first embodiment. 
         FIGS. 31(A) and 31(B)  are graphs each showing a relationship between motor efficiency and a rotation speed in a first modification of the first embodiment. 
         FIG. 32  is a graph showing a relationship between a line voltage and a rotation speed in a second modification of the first embodiment. 
         FIGS. 33(A) and 33(B)  are schematic diagrams for describing a switching operation of the connection state of the coils in a third modification of the first embodiment. 
         FIGS. 34(A) and 34(B)  are schematic diagrams for describing another example of the switching operation of the connection state of the coils in the third modification of the first embodiment. 
         FIG. 35  is a flowchart showing a connection switching operation in a fourth modification of the first embodiment. 
         FIG. 36  is a flowchart showing a connection switching operation in a fifth modification of the first embodiment. 
         FIG. 37  is a block diagram illustrating a configuration of an air conditioner according to a second embodiment. 
         FIG. 38  is a block diagram illustrating a control system of the air conditioner according to the second embodiment. 
         FIG. 39  is a block diagram illustrating a control system of a driving device according to the second embodiment. 
         FIG. 40  is a flowchart showing a basic operation of the air conditioner according to the second embodiment. 
         FIG. 41  is a flowchart showing a basic operation of an air conditioner according to a modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     (Configuration of Motor) 
     A first embodiment of the present invention will be described.  FIG. 1  is a cross-sectional view illustrating a configuration of a motor  1  according to the first embodiment of the present invention. The motor  1  is a permanent magnet embedded type motor, and is used for a rotary compressor, for example. The motor  1  includes a stator  10  and a rotor  20  rotatably provided inside the stator  10 . An air gap of, for example, 0.3 mm to 1 mm is formed between the stator  10  and the rotor  20 .  FIG. 1  is a cross-sectional view taken along a plane perpendicular to a rotation axis of the rotor  20 . 
     Hereinafter, an axial direction (direction of the rotation axis) of the rotor  20  will be simply referred to as an “axial direction”. A direction along an outer periphery (circumference) of each of the stator  10  and the rotor  20  will be simply referred to as a “circumferential direction”. A radial direction of each of the stator  10  and the rotor  20  will be simply referred to as a “radial direction”. 
     The stator  10  includes a stator core  11  and coils  3  wound around the stator core  11 . The stator core  11  is made by stacking a plurality of electromagnetic steel sheets each having a thickness of 0.1 mm to 0.7 mm (0.35 mm in this example) in the direction of the rotation axis and fastening the sheets by crimping. 
     The stator core  11  includes a ring-shaped yoke  13  and a plurality of (nine in this example) tooth portions  12  extending inward in the radial direction from the yoke  13 . A slot is formed between adjacent ones of the tooth portions  12 . Each of the tooth portions  12  has a tooth end part at an end on an inner side in the radial direction, and the tooth end part has a wide width (dimension in the circumferential direction of the stator core  11 ). 
     The coil  3  as a stator winding is wound around each of the tooth portions  12  via an insulator  14 . The coil  3  is obtained by, for example, winding a magnet wire having a wire diameter (diameter) of 0.8 mm around each of the tooth portions  12  by concentrated winding in 110 turns. The number of turns and the wire diameter of each coil  3  are determined based on characteristics (rotation speed, torque or the like) required for the motor  1 , a supply voltage, or a cross-sectional area of the slot. 
     The coils  3  are constituted by three-phase windings of a U-phase, a V-phase, and a W-phase (hereinafter referred to as coils  3 U,  3 V, and  3 W). Both terminals of the coil  3  in each phase are open. That is, the coils  3  have six terminals in total. A connection state of the coils  3  is switchable between a Y connection and a delta connection, which will be described later. The insulator  14  is made of, for example, a film of polyethylene terephthalate (PET), and has a thickness of 0.1 mm to 0.2 mm. 
     The stator core  11  has a configuration in which a plurality of (nine in this example) blocks are coupled to each other via thin portions. The magnet wire is wound around each of the tooth portions  12  in a state where the stator core  11  is extended in a band shape, and then the stator core  11  is bent into a ring shape and both ends of the stator core  11  are welded. 
     A configuration in which the insulator  14  is made of a thin film and the stator core  11  has a divided structure in order to facilitate winding as above is effective for increasing the number of turns of the coil  3  in the slot. In this regard, the stator core  11  is not limited to the above described configuration in which the plurality of blocks (split cores) are coupled to each other. 
     The rotor  20  includes a rotor core  21  and a permanent magnet  25  attached to the rotor core  21 . The rotor core  21  is made by stacking a plurality of electromagnetic steel sheets each having a thickness of 0.1 mm to 0.7 mm (0.35 mm in this example) in the direction of the rotation axis and fastening the sheets by crimping. 
     The rotor core  21  has a cylindrical shape, and a shaft hole  27  (center hole) is formed at a center in the radial direction. A shaft serving as a rotation shaft of the rotor  20  (for example, a shaft  90  of a rotary compressor  8 ) is fixed to the shaft hole  27  by shrinkage fitting, press fitting, or the like. 
     A plurality of (six in this example) magnet insertion holes  22  in which the permanent magnets  25  are inserted are formed along an outer peripheral surface of the rotor core  21 . The magnet insertion holes  22  are openings, and one magnet insertion hole  22  corresponds to one magnetic pole. Since the six magnet insertion holes  22  are provided in this example, the rotor  20  has six poles in total. 
     The magnet insertion hole  22  has a V shape such that a center portion in the circumferential direction projects inward in the radial direction in this example. In this regard, the magnet insertion hole  22  is not limited to the V shape, but may have a straight shape, for example. 
     Two permanent magnets  25  are disposed in each magnet insertion hole  22 . That is, two permanent magnets  25  are disposed for one magnetic pole. In this example, since the rotor  20  has six poles as described above, twelve permanent magnets  25  are disposed in total. 
     The permanent magnet  25  is a flat-plate member elongated in the axial direction of the rotor core  21 , has a width in the circumferential direction of the rotor core  21 , and has a thickness in the radial direction of the rotor core  21 . The permanent magnet  25  is constituted by, for example, a rare earth magnet containing neodymium (Nd), iron (Fe), and boron (B) as main components. 
     The permanent magnets  25  are magnetized in the thickness direction. Two permanent magnets  25  disposed in one magnet insertion hole  22  are magnetized in such a manner that the same magnetic poles face the same side in the radial direction. 
     Flux barriers  26  are formed at both ends of the magnet insertion hole  22  in the circumferential direction. The flux barriers  26  are openings formed continuously with the magnet insertion hole  22 . The flux barriers  26  are provided for reducing leakage magnetic flux between adjacent magnetic poles (i.e., magnetic flux flowing through inter-pole parts). 
     In the rotor core  21 , a first magnet retention portion  23  that is a projection is formed at a center of each magnet insertion hole  22  in the circumferential direction. Further, in the rotor core  21 , second magnet retention portions  24  that are projections are formed at both ends of the magnet insertion hole  22  in the circumferential direction. The first magnet retention portion  23  and the second magnet retention portions  24  are provided for positioning and retaining the permanent magnets  25  in each magnet insertion hole  22 . 
     As described above, the number of slots of the stator  10  (i.e., the number of tooth portions  12 ) is nine, and the number of poles of the rotor  20  is six. That is, in the motor  1 , a ratio of the number of poles of the rotor  20  to the number of slots of the stator  10  is 2:3. 
     In the motor  1 , the connection state of the coils  3  is switched between a Y connection and a delta connection. In the case where the delta connection is used, a cyclic current may flow and may cause degradation of performance of the motor  1 . The cyclic current is caused by a third harmonic wave generated in an induced voltage in the winding of each phase. It is known that in the case of concentrated winding where the ratio of the number of poles to the number of slots is 2:3, no third harmonic wave is generated in an induced voltage on the assumption that there is no influence of magnetic saturation or the like, and therefore no performance degradation is caused by the cyclic current. 
     (Configuration of Rotary Compressor) 
     Next, the rotary compressor  8  using the motor  1  will be described.  FIG. 2  is a cross-sectional view illustrating a configuration of the rotary compressor  8 . The rotary compressor  8  includes a shell  80 , a compression mechanism  9  disposed in the shell  80 , and the motor  1  for driving the compression mechanism  9 . The rotary compressor  8  further includes a shaft  90  (crank shaft) coupling the motor  1  and the compression mechanism  9  to each other so that a driving force can be transferred. The shaft  90  is fitted in the shaft hole  27  ( FIG. 1 ) of the rotor  20  of the motor  1 . 
     The shell  80  is a closed container made of, for example, a steel sheet, and covers the motor  1  and the compression mechanism  9 . The shell  80  includes an upper shell  80   a  and a lower shell  80   b . The upper shell  80   a  is provided with glass terminals  81  serving as a terminal portion for supplying electric power from outside of the rotary compressor  8  to the motor  1 , and a discharge pipe  85  for discharging refrigerant compressed in the rotary compressor  8  to outside. Here, six lead wires in total corresponding to two portions for each of the U-phase, the V-phase, and the W-phase of the coils  3  of the motor  1  ( FIG. 1 ) are drawn out from the glass terminals  81 . The lower shell  80   b  houses the motor  1  and the compression mechanism  9 . 
     The compression mechanism  9  has an annular first cylinder  91  and an annular second cylinder  92  along the shaft  90 . The first cylinder  91  and the second cylinder  92  are fixed to an inner peripheral portion of the shell  80  (the lower shell  80   b ). An annular first piston  93  is disposed on an inner peripheral side of the first cylinder  91 , and an annular second piston  94  is disposed on an inner peripheral side of the second cylinder  92 . The first piston  93  and the second piston  94  are rotary pistons that rotate together with the shaft  90 . 
     A partition plate  97  is provided between the first cylinder  91  and the second cylinder  92 . The partition plate  97  is a disk-shaped member having a through hole at a center thereof. Vanes (not shown) are provided in cylinder chambers of the first cylinder  91  and the second cylinder  92  to divide each of the cylinder chambers into a suction side and a compression side. The first cylinder  91 , the second cylinder  92 , and the partition plate  97  are integrally fixed using bolts  98 . 
     An upper frame  95  is disposed above the first cylinder  91  so as to close an upper side of the cylinder chamber of the first cylinder  91 . A lower frame  96  is disposed below the second cylinder  92  so as to close a lower side of the cylinder chamber of the second cylinder  92 . The upper frame  95  and the lower frame  96  rotatably support the shaft  90 . 
     Refrigerating machine oil (not shown) for lubricating sliding portions of the compression mechanism  9  is stored at a bottom portion of the lower shell  80   b  of the shell  80 . The refrigerating machine oil flows upward through a hole  90   a  formed in the axial direction in the shaft  90  and is supplied to the sliding portions from oil supply holes  90   b  formed at a plurality of positions of the shaft  90 . 
     The stator  10  of the motor  1  is attached to an inner side of the shell  80  by shrinkage fitting. Electric power is supplied to the coils  3  of the stator  10  from the glass terminals  81  attached to the upper shell  80   a . The shaft  90  is fixed to the shaft hole  27  ( FIG. 1 ) of the rotor  20 . 
     An accumulator  87  for storing refrigerant gas is attached to the shell  80 . The accumulator  87  is held by, for example, a holding portion  80   c  provided on an outer side the lower shell  80   b . A pair of suction pipes  88  and  89  are attached to the shell  80 , and refrigerant gas is supplied from the accumulator  87  to the cylinders  91  and  92  through the suction pipes  88  and  89 . 
     As the refrigerant, R410A, R407C, or R22, for example, may be used. It is preferable to use low global warming potential (GWP) refrigerant from the viewpoint of prevention of global warming. As the low GWP refrigerant, for example, the following refrigerants can be used. 
     (1) First, a halogenated hydrocarbon having a double bond of carbon in its composition, such as hydro-fluoro-orefin (HFO)-1234yf (CF3CF=CH2) can be used. The GWP of HFO-1234yf is 4. 
     (2) Further, a hydrocarbon having a double bond of carbon in its composition, such as R1270 (propylene), may be used. The GWP of R1270 is 3, which is lower than that of HFO-1234yf, but flammability of R1270 is higher than that of HFO-1234yf. 
     (3) Further, a mixture containing at least one of a halogenated hydrocarbon having a double bond of carbon in its composition or a hydrocarbon having a double bond of carbon in its composition, such as a mixture of HFO-1234yf and R32, may be used. Since the above described HFO-1234yf is a low-pressure refrigerant and tends to cause an increase in pressure loss, its use may cause degradation of performance of a refrigeration cycle (especially an evaporator). Thus, it is practically preferable to use a mixture with R32 or R41 which is a higher pressure refrigerant than HFO-1234yf. 
     A basic operation of the rotary compressor  8  is as follows. Refrigerant gas supplied from the accumulator  87  is supplied to the cylinder chambers of the first cylinder  91  and the second cylinder  92  through the suction pipes  88  and  89 . When the motor  1  is driven and the rotor  20  rotates, the shaft  90  rotates together with the rotor  20 . Then, the first piston  93  and the second piston  94  fitted to the shaft  90  rotate eccentrically in the cylinder chambers and compress the refrigerant in the cylinder chambers. The compressed refrigerant flows upward in the shell  80  through holes (not shown) provided in the rotor  20  of the motor  1  and is discharged to outside through the discharge pipe  85 . 
     (Configuration of Air Conditioner) 
     Next, the air conditioner  5  including the driving device according to the first embodiment will be described.  FIG. 3  is a block diagram illustrating a configuration of the air conditioner  5 . The air conditioner  5  includes an indoor unit  5 A placed in a room (air conditioning target space) and an outdoor unit  5 B placed outdoors. The indoor unit  5 A and the outdoor unit  5 B are connected by connecting pipes  40   a  and  40   b  through which refrigerant flows. Liquid refrigerant passing through a condenser flows through the connection pipe  40   a . Gas refrigerant passing through an evaporator flows through the connection pipe  40   b.    
     The outdoor unit  5 B includes a compressor  41  that compresses and discharges refrigerant, a four-way valve (refrigerant channel switching valve)  42  that switches a flow direction of the refrigerant, an outdoor heat exchanger  43  that exchanges heat between outside air and the refrigerant, and an expansion valve (pressure reducing device)  44  that depressurizes high-pressure refrigerant to a low pressure. The compressor  41  is constituted by the rotary compressor  8  described above ( FIG. 2 ). The indoor unit  5 A includes an indoor heat exchanger  45  that performs heat exchange between indoor air and the refrigerant. 
     The compressor  41 , the four-way valve  42 , the outdoor heat exchanger  43 , the expansion valve  44 , and the indoor heat exchanger  45  are connected by a pipe  40  including the above described connection pipes  40   a  and  40   b  to constitute a refrigerant circuit. These components constitute a compression type refrigeration cycle (compression type heat pump cycle) in which refrigerant is circulated by the compressor  41 . 
     In order to control an operation of the air conditioner  5 , an indoor controller  50   a  is disposed in the indoor unit  5 A, and an outdoor controller  50   b  is disposed in the outdoor unit  5 B. Each of the indoor controller  50   a  and the outdoor controller  50   b  has a control board on which various circuits for controlling the air conditioner  5  are formed. The indoor controller  50   a  and the outdoor controller  50   b  are connected to each other by a communication cable  50   c . The communication cable  50   c  is bundled together with the connecting pipes  40   a  and  40   b  described above. 
     In the outdoor unit  5 B, an outdoor fan  46  that is a fan is disposed so as to face the outdoor heat exchanger  43 . The outdoor fan  46  generates an air flow passing through the outdoor heat exchanger  43  by rotation. The outdoor fan  46  is constituted by, for example, a propeller fan. 
     The four-way valve  42  is controlled by the outdoor controller  50   b  and switches the direction of flow of refrigerant. When the four-way valve  42  is in the position indicated by the solid line in  FIG. 3 , gas refrigerant discharged from the compressor  41  is sent to the outdoor heat exchanger  43  (condenser). When the four-way valve  42  is in the position indicated by the broken line in  FIG. 3 , gas refrigerant flowing from the outdoor heat exchanger  43  (evaporator) is sent to the compressor  41 . The expansion valve  44  is controlled by the outdoor controller  50   b , and changes its opening degree to reduce the pressure of high-pressure refrigerant to a low pressure. 
     In the indoor unit  5 A, an indoor fan  47  that is a fan is disposed so as to face the indoor heat exchanger  45 . The indoor fan  47  rotates to generate an air flow passing through the indoor heat exchanger  45 . The indoor fan  47  is constituted by, for example, a crossflow fan. 
     In the indoor unit  5 A, a room temperature sensor  54  as a temperature sensor is provided. The room temperature sensor  54  measures a room temperature Ta which is an air temperature in the room (air conditioning target space), and sends the measured temperature information (information signal) to the indoor controller  50   a . The room temperature sensor  54  may be constituted by a temperature sensor used in a general air conditioner. Alternatively, a radiant temperature sensor detecting a surface temperature of, for example, a wall or a floor in the room may be used. 
     In the indoor unit  5 A, a signal receiving unit  56  that receives an instruction signal (operation instruction signal) transmitted from a remote controller  55  (remote operation device) operated by a user is also provided. The remote controller  55  is used by a user to give an instruction of an operation input (start and stop of operation) or operation content (set temperature, wind speed, or the like) to the air conditioner  5 . 
     The compressor  41  is configured to change an operating rotation speed in a range from 20 rps to 130 rps during a normal operation. As the rotation speed of the compressor  41  increases, a circulation amount of refrigerant in the refrigerant circuit increases. The rotation speed of the compressor  41  is controlled by the controller  50  (more specifically, the outdoor controller  50   b ) in accordance with a temperature difference ΔT between the current room temperature Ta obtained by the room temperature sensor  54  and a set temperature Ts set by the user with the remote controller  55 . As the temperature difference ΔT increases, the compressor  41  rotates at a higher rotation speed, and the circulation amount of refrigerant is increased. 
     Rotation of the indoor fan  47  is controlled by the indoor controller  50   a . The rotation speed of the indoor fan  47  can be switched to a plurality of stages. In this example, the rotation speed can be switched to, for example, three stages of strong wind, middle wind, and soft wind. When the wind speed setting is set to an automatic mode with the remote controller  55 , the rotation speed of the indoor fan  47  is switched in accordance with the temperature difference ΔT between the measured room temperature Ta and the set temperature Ts. 
     Rotation of the outdoor fan  46  is controlled by the outdoor controller  50   b . The rotation speed of the outdoor fan  46  can be switched to a plurality of stages. In this example, the rotation speed of the outdoor fan  46  is switched in accordance with the temperature difference ΔT between the measured room temperature Ta and the set temperature Ts. 
     The indoor unit  5 A further includes a lateral wind direction plate  48  and a vertical wind direction plate  49 . The lateral wind direction plate  48  and the vertical wind direction plate  49  change a blowing direction when conditioned air subjected to heat exchange in the indoor heat exchanger  45  is blown into the room by the indoor fan  47 . The lateral wind direction plate  48  changes the blowing direction laterally, and the vertical wind direction plate  49  changes the blowing direction vertically. An angle of each of the lateral wind direction plate  48  and the vertical wind direction plate  49 , that is, a wind direction of the blown air is controlled by the indoor controller  50   a  based on the setting of the remote controller  55 . 
     A basic operation of the air conditioner  5  is as follows. In a cooling operation, the four-way valve  42  is switched to a position indicated by the solid line, and high-temperature and high-pressure gas refrigerant discharged from the compressor  41  flows into the outdoor heat exchanger  43 . In this case, the outdoor heat exchanger  43  operates as a condenser. When air passes through the outdoor heat exchanger  43  by rotation of the outdoor fan  46 , heat of condensation of the refrigerant is taken by the air due to heat exchange. The refrigerant is condensed into high-pressure and low-temperature liquid refrigerant, and is adiabatically expanded by the expansion valve  44  to become low-pressure and low-temperature two-phase refrigerant. 
     The refrigerant passing through the expansion valve  44  flows into the indoor heat exchanger  45  of the indoor unit  5 A. The indoor heat exchanger  45  operates as an evaporator. When air passes through the indoor heat exchanger  45  by rotation of the indoor fan  47 , heat of vaporization of the air is taken by the refrigerant due to heat exchange, and the cooled air is supplied to the room. The refrigerant evaporates to become low-temperature and low-pressure gas refrigerant, and is compressed again into high-temperature and high-pressure refrigerant by the compressor  41 . 
     In a heating operation, the four-way valve  42  is switched to a position indicated by the dotted line, and high-temperature and high-pressure gas refrigerant discharged from the compressor  41  flows into the indoor heat exchanger  45 . In this case, the indoor heat exchanger  45  operates as a condenser. When air passes through the indoor heat exchanger  45  by rotation of the indoor fan  47 , heat of condensation is taken from the refrigerant due to heat exchange, and the heated air is supplied to the room. The refrigerant is condensed into high-pressure and low-temperature liquid refrigerant, and is adiabatically expanded by the expansion valve  44  to become low-pressure and low-temperature two-phase refrigerant. 
     The refrigerant passing through the expansion valve  44  flows into the outdoor heat exchanger  43  of the outdoor unit  5 B. The outdoor heat exchanger  43  operates as an evaporator. When air passes through the outdoor heat exchanger  43  by rotation of the outdoor fan  46 , heat of vaporization of the air is taken by the refrigerant due to heat exchange. The refrigerant evaporates to become low-temperature and low-pressure gas refrigerant, and is compressed again into high-temperature and high-pressure refrigerant by the compressor  41 . 
       FIG. 4  is a conceptual diagram showing a basic configuration of a control system of the air conditioner  5 . The indoor controller  50   a  and the outdoor controller  50   b  described above exchange information with each other through the communication cable  50   c  to control the air conditioner  5 . In this example, the indoor controller  50   a  and the outdoor controller  50   b  are collectively referred to as a controller  50 . 
       FIG. 5(A)  is a block diagram showing a control system of the air conditioner  5 . The controller  50  is constituted by, for example, a microcomputer. The controller  50  incorporates an input circuit  51 , an arithmetic circuit  52 , and an output circuit  53 . 
     The input circuit  51  receives an instruction signal received by the signal receiving unit  56  from the remote controller  55 . The instruction signal includes, for example, signals for setting an operation input, an operation mode, a set temperature, an airflow amount, or a wind direction. The input circuit  51  also receives temperature information indicating a room temperature detected by the room temperature sensor  54 . The input circuit  51  outputs the received information to the arithmetic circuit  52 . 
     The arithmetic circuit  52  includes a central processing unit (CPU)  57  and a memory  58 . The CPU  57  performs calculation processing and determination processing. The memory  58  stores various set values and programs for use in controlling the air conditioner  5 . The arithmetic circuit  52  performs calculation and determination based on the information received from the input circuit  51 , and outputs the result to the output circuit  53 . 
     Based on the information input from the arithmetic circuit  52 , the output circuit  53  outputs control signals to the compressor  41 , a connection switching unit  60  (described later), a converter  102 , an inverter  103 , the compressor  41 , the four-way valve  42 , the expansion valve  44 , the outdoor fan  46 , the indoor fan  47 , the lateral wind direction plate  48 , and the vertical wind direction plate  49 . 
     As described above, since the indoor controller  50   a  and the outdoor controller  50   b  ( FIG. 4 ) exchange information with each other through the communication cable  50   c  and control the various devices of the indoor unit  5 A and the outdoor unit  5 B. Thus, in this example, the indoor controller  50   a  and the outdoor controller  50   b  are collectively referred to as the controller  50 . Practically, each of the indoor controller  50   a  and the outdoor controller  50   b  is constituted by a microcomputer. It is also possible that a controller is provided in only one of the indoor unit  5 A and the outdoor unit  5 B and controls various devices of the indoor unit  5 A and the outdoor unit  5 B. 
       FIG. 5(B)  is a block diagram showing a section of the controller  50  for controlling the motor  1  of the compressor  41  based on the room temperature Ta. The arithmetic circuit  52  of the controller  50  includes a received content analysis unit  52   a , a room temperature acquiring unit  52   b , a temperature difference calculation unit  52   c , and a compressor control unit  52   d . These components are included in, for example, the CPU  57  of the arithmetic circuit  52 . 
     The received content analysis unit  52   a  analyzes an instruction signal input from the remote controller  55  via the signal reception unit  56  and the input circuit  51 . Based on the analysis result, the received content analysis unit  52   a  outputs, for example, the operation mode and the set temperature Ts to the temperature difference calculation unit  52   c . The room temperature acquiring unit  52   b  acquires the room temperature Ta input from the room temperature sensor  54  via the input circuit  51 , and outputs the acquired room temperature Ta to the temperature difference calculation unit  52   c.    
     The temperature difference calculation unit  52   c  calculates a temperature difference ΔT between the room temperature Ta input from the room temperature acquiring unit  52   b  and the set temperature Ts input from the received content analysis unit  52   a . If the operation mode input from the received content analysis unit  52   a  is the heating operation, a temperature difference ΔT=Ts-Ta is calculated. If the operation mode is the cooling operation, a temperature difference ΔT=Ta−Ts is calculated. The temperature difference calculation unit  52   c  outputs the calculated temperature difference ΔT to the compressor control unit  52   d.    
     Based on the temperature difference ΔT input from the temperature difference calculation unit  52   c , the compressor control unit  52   d  controls the driving device  100  to thereby control the rotation speed of the motor  1  (i.e., the rotation speed of the compressor  41 ). 
     (Configuration of Driving Device) 
     Next, the driving device  100  for driving the motor  1  will be described.  FIG. 6  is a block diagram illustrating a configuration of the driving device  100 . The driving device  100  includes the converter  102  for rectifying an output of a power source  101 , the inverter  103  for outputting an alternating-current voltage to the coils  3  of the motor  1 , the connection switching unit  60  for switching the connection state of the coils  3 , and the controller  50 . The converter  102  is supplied with electric power from the power source  101  that is an alternating-current (AC) power source. 
     The power source  101  is, for example, an AC power source of 200 V (effective voltage). The converter  102  is a rectifier circuit, and outputs a direct-current (DC) voltage of, for example, 280 V. The voltage output from the converter  102  is referred to as a bus voltage. The inverter  103  is supplied with a bus voltage from the converter  102 , and outputs a line voltage (also referred to as a motor voltage) to the coils  3  of the motor  1 . Wires  104 ,  105 , and  106  connected to the coils  3 U,  3 V, and  3 W, respectively, are connected to the inverter  103 . 
     The coil  3 U has terminals  31 U and  32 U. The coil  3 V has terminals  31 V and  32 V. The coil  3 W has terminals  31 W and  32 W. The wire  104  is connected to the terminal  31 U of the coil  3 U. The wire  105  is connected to the terminal  31 V of the coil  3 V. The wire  106  is connected to the terminal  31 W of the coil  3 W. 
     The connection switching unit  60  has switches  61 ,  62 , and  63 . The switch  61  connects the terminal  32 U of the coil  3 U to either the wire  105  or a neutral point  33 . The switch  62  connects the terminal  32 V of the coil  3 V to either the wire  106  or the neutral point  33 . The switch  63  connects the terminal  32 W of the coil  3 W to either the wire  104  or the neutral point  33 . The switches  61 ,  62 , and  63  of the connection switching unit  60  are constituted by relay contacts in this example. In this regard, the switches  61 ,  62 , and  63  may be constituted by semiconductor switches. 
     The controller  50  controls the converter  102 , the inverter  103 , and the connection switching unit  60 . The configuration of the controller  50  is as described with reference to  FIG. 5 . The controller  50  receives the operation instruction signal from the remote controller  55  received by the signal receiving unit  56  and also receives the room temperature detected by the room temperature sensor  54 . Based on the received information, the controller  50  outputs a voltage switching signal to the converter  102 , outputs an inverter driving signal to the inverter  103 , and outputs a connection switching signal to the connection switching unit  60 . 
     In a state shown in  FIG. 6 , the switch  61  connects the terminal  32 U of the coil  3 U to the neutral point  33 , the switch  62  connects the terminal  32 V of the coil  3 V to the neutral point  33 , and the switch  63  connects the terminal  32 W of the coil  3 W to the neutral point  33 . That is, the terminals  31 U,  31 V, and  31 W of the coils  3 U,  3 V, and  3 W are connected to the inverter  103 , and the terminals  32 U,  32 V, and  32 W are connected to the neutral point  33 . 
       FIG. 7  is a block diagram illustrating a state where the switches  61 ,  62 , and  63  of the connection switching unit  60  in the driving device  100  are switched. In the state illustrated in  FIG. 7 , the switch  61  connects the terminal  32 U of the coil  3 U to the wire  105 , the switch  62  connects the terminal  32 V of the coil  3 V to the wire  106 , and the switch  63  connects the terminal  32 W of the coil  3 W to the wire  104 . 
       FIG. 8(A)  is a schematic diagram illustrating a connection state of the coils  3 U,  3 V, and  3 W when the switches  61 ,  62 , and  63  are in the state shown in  FIG. 6 . The coils  3 U,  3 V, and  3 W are connected to the neutral point  33  at the terminals  32 U,  32 V, and  32 W, respectively. Thus, the connection state of the coils  3 U,  3 V, and  3 W is the Y connection (star connection). 
       FIG. 8(B)  is a schematic diagram illustrating a connection state of the coils  3 U,  3 V, and  3 W when the switches  61 ,  62 , and  63  are in the state shown in  FIG. 7 . The terminal  32 U of the coil  3 U is connected to the terminal  31 V of the coil  3 V via the wire  105  ( FIG. 7 ). The terminal  32 V of the coil  3 V is connected to the terminal  31 W of the coil  3 W via the wire  106  ( FIG. 7 ). The terminal  32 W of the coil  3 W is connected to the terminal  31 U of the coil  3 U via the wire  104  ( FIG. 7 ). Thus, the connection state of the coils  3 U,  3 V, and  3 W is the delta connection. 
     In this manner, the connection switching unit  60  is capable of switching the connection state of the coils  3 U,  3 V, and  3 W of the motor  1  between the Y connection (first connection state) and the delta connection (second connection state) by switching the switches  61 ,  62 , and  63 . 
       FIG. 9  is a schematic diagram illustrating coil portions of the coils  3 U,  3 V, and  3 W. As described above, the motor  1  has nine tooth portions  12  ( FIG. 1 ), and each of the coils  3 U,  3 V, and  3 W is wound around three tooth portions  12 . That is, the coil  3 U is obtained by connecting, in series, U-phase coil portions Ua, Ub, and Uc wound around the three tooth portions  12 . Similarly, the coil  3 V is obtained by connecting, in series, V-phase coil portions Va, Vb, and Vc wound around the three tooth portions  12 . The coil  3 W is obtained by connecting, in series, W-phase coil portions Wa, Wb, and We wound around the three tooth portions  12 . 
     (Operation of Air Conditioner) 
       FIGS. 10 through 12  are flowcharts showing a basic operation of the air conditioner  5 . The controller  50  of the air conditioner  5  starts an operation when the signal reception unit  56  receives a start signal from the remote controller  55  (step S 101 ). In this example, the CPU  57  of the controller  50  is activated. As will be described later, since the connection state of the coils  3  is switched to the delta connection when the preceding operation of the air conditioner  5  is terminated, the connection state of the coils  3  is the delta connection when the operation is started (at start-up). 
     Next, the controller  50  performs a start process of the air conditioner  5  (step S 102 ). Specifically, fan motors of the indoor fan  47  and the outdoor fan  46  are driven, for example. 
     Then, the controller  50  outputs a voltage switching signal to the converter  102  to raise a bus voltage of the converter  102  to a bus voltage (for example, 390 V) corresponding to the delta connection (step S 103 ). The bus voltage of the converter  102  is the maximum voltage applied from the inverter  103  to the motor  1 . 
     Then, the controller  50  starts the motor  1  (step S 104 ). Thus, the motor  1  is started while the connection state of the coils  3  is set to the delta connection. Further, the controller  50  controls the output voltage of the inverter  103  to control the rotation speed of the motor  1 . 
     Specifically, the controller  50  increases the rotation speed of the motor  1  stepwise at a preset speed in accordance with the temperature difference ΔT. An allowable maximum rotation speed of the rotation speed of the motor  1  is, for example, 130 rps. As a result, the amount of refrigerant circulated by the compressor  41  is increased, and a cooling capacity is increased in the case of the cooling operation whereas a heating capacity is increased in the case of the heating operation. 
     Further, when the room temperature Ta approaches the set temperature Ts due to air conditioning effect and the temperature difference ΔT shows a decreasing tendency, the controller  50  reduces the rotation speed of the motor  1  in accordance with the temperature difference ΔT. When the temperature difference ΔT decreases to a preset temperature near zero (but larger than zero), the controller  50  operates the motor  1  at an allowable minimum rotation speed (for example, 20 rps). 
     Further, when the temperature Ta reaches the set temperature Ts (i.e., when the temperature difference ΔT is zero or less), the controller  50  stops rotation of the motor  1  in order to prevent overcooling (or overheating). Accordingly, the compressor  41  is stopped. Then, when the temperature difference ΔT becomes larger than zero again, the controller  50  restarts rotation of the motor  1 . The controller  50  restricts restart of rotation of the motor  1  in a short time period so as not to repeat rotation and stop of the motor  1  in a short time period. 
     When the rotation speed of the motor  1  reaches a preset rotation speed, the inverter  103  starts field-weakening control. The field-weakening control will be described later with reference to  FIGS. 15 through 30 . 
     The controller  50  determines whether or not an operation stop signal (signal to stop an operation of the air conditioner  5 ) is received from the remote controller  55  through the signal receiving unit  56  (step S 105 ). If the operation stop signal is not received, the controller  50  proceeds to step S 106 . If the operation stop signal is received, the controller  50  proceeds to step S 109 . 
     The controller  50  acquires the temperature difference ΔT between the room temperature Ta detected by the room temperature sensor  54  and the set temperature Ts set by the remote controller (step S 106 ). Based on the temperature difference ΔT, the controller  50  determines whether switching of the connection state of the coils  3  from the delta connection to the Y connection is necessary or not. That is, it is determined whether or not the connection state of the coils  3  is the delta connection and an absolute value of the temperature difference ΔT is less than or equal to a threshold ΔTr (step S 107 ). The threshold ΔTr is a temperature difference corresponding to an air-conditioning load (also referred to simply as “load”) that is small enough to perform switching to the Y connection. 
     As described above, ΔT is represented as ΔT=Ts−Ta when the operation mode is the heating operation and is represented as ΔT=Ta−Ts when the operation is the cooling operation. Thus, in this example, the absolute value of ΔT and the threshold ΔTr are compared to each other to determine whether switching to the Y connection is necessary or not. 
     If the result of the comparison in step S 107  indicates that the connection state of the coils  3  is the delta connection and the absolute value of the temperature difference ΔT is less than or equal to the threshold ΔTr, the process proceeds to step S 121  ( FIG. 11 ). 
     As shown in  FIG. 11 , in step S 121 , the controller  50  outputs a stop signal to the inverter  103  to stop rotation of the motor  1 . Thereafter, the controller  50  outputs the connection switching signal to the connection switching unit  60 , and switches the connection state of the coils  3  from the delta connection to the Y connection (step S 122 ). Subsequently, the controller  50  outputs a voltage switching signal to the converter  102  to lower the bus voltage of the converter  102  to a voltage (280 V) corresponding to the Y connection (step S 123 ), and restarts rotation of the motor  1  (step S 124 ). Thereafter, the process returns to step S 105  described above ( FIG. 10 ). 
     If the result of the comparison in step S 107  indicates that the connection state of the coils  3  is not the delta connection (if it is the Y connection), or that the absolute value of the temperature difference ΔT is larger than the threshold ΔTr (i.e., if switching to the Y connection is unnecessary), the process proceeds to step S 108 . 
     In step S 108 , it is determined whether switching from the Y connection to the delta connection is necessary or not. That is, it is determined whether or not the connection state of the coils  3  is the Y connection and the absolute value of the temperature difference ΔT described above is larger than the threshold ΔTr. 
     If the result of the comparison in step S 108  indicates that the connection state of the coils  3  is the Y connection and the absolute value of the temperature difference ΔT is larger than the threshold ΔTr, the process proceeds to step S 131  ( FIG. 12 ). 
     As shown in  FIG. 12 , in step S 131 , the controller  50  stops rotation of the motor  1 . Thereafter, the controller  50  outputs the connection switching signal to the connection switching unit  60 , and switches the connection state of the coils  3  from the Y connection to the delta connection (step S 132 ). Subsequently, the controller  50  outputs the voltage switching signal to the converter  102  to raise the bus voltage of the converter  102  to the voltage (390 V) corresponding to the delta connection (step S 133 ), and restarts rotation of the motor  1  (S 134 ). 
     In the case of the delta connection, the motor  1  can be driven to a higher rotation speed as compared to the case of the Y connection, and thus it is possible to respond to a larger load. It is therefore possible to converge the temperature difference ΔT between the room temperature and the set temperature in a short time period. Thereafter, the process returns to step S 105  described above ( FIG. 10 ). 
     If the result of the comparison in step S 108  indicates that the connection state of the coils  3  is not the Y connection (i.e., if it is the delta connection), or that the absolute value of the temperature difference ΔT is less than or equal to the threshold ΔTr (i.e., if switching to the delta connection is unnecessary), the process returns to step S 105 . 
     If the operation stop signal is received in step S 105  described above, rotation of the motor  1  is stopped (step S 109 ). Thereafter, the controller  50  switches the connection state of the coils  3  from the Y connection to the delta connection (step S 110 ). If the connection state of the coils  3  is already the delta connection, the connection state is unchanged. In this regard, although not shown in  FIG. 10 , if the operation stop signal is received in steps S 106  through S 108 , the process proceeds to step S 109  and rotation of the motor  1  is stopped. 
     Thereafter, the controller  50  performs a stop process of the air conditioner  5  (step S 111 ). Specifically, the fan motors of the indoor fan  47  and the outdoor fan  46  are stopped. Thereafter, the CPU  57  of the controller  50  is stopped, and the operation of the air conditioner  5  is terminated. 
     As described above, if the absolute value of the temperature difference ΔT between the room temperature Ta and the set temperature Ts is relatively small (i.e., if the absolute value is less than or equal to the threshold ΔTr), the motor  1  is operated using the Y connection achieving high efficiency. If it is necessary to respond to a larger load, that is, if the absolute value of the temperature difference ΔT is larger than the threshold ΔTr, the motor  1  is operated using the delta connection enabling responding to a larger load. Accordingly, the operation efficiency of the air conditioner  5  can be enhanced. 
     In this regard, in the switching operation from the Y connection to the delta connection ( FIG. 12 ), as shown in FIG.  13 (A), it is also possible to detect the rotation speed of the motor  1  (step S 135 ) and to determine whether or not the detected rotation speed is greater than or equal to a threshold (reference value of rotation speed) (step S 136 ), before step  131  in which the rotation of the motor  1  is stopped. The rotation speed of the motor  1  is detected as a frequency of an output current of the inverter  103 . 
     In step S 136 , a rotation speed of 60 rps is used as a threshold of the rotation speed of the motor  1 . The rotation speed of 60 rps is intermediate between a rotation speed of 35 rps corresponding to an intermediate heating condition described later and a rotation speed of 85 rps corresponding to a rated heating condition. If the rotation speed of the motor  1  is greater than or equal to the threshold, rotation of the motor  1  is stopped and switching to the delta connection is performed, and the bus voltage of the converter  102  is raised (steps S 131 , S 132 , and S 133 ). If the rotation speed of the motor  1  is less than the threshold, the process returns to step S 105  in  FIG. 10 . 
     As above, in addition to the determination on whether the connection switching is necessary or not based on the temperature difference ΔT (step S 108 ), the determination on whether the connection switching is necessary or not is performed based on the rotation speed of the motor  1 . Thus, frequent repetition of connection switching can be sufficiently suppressed. 
     Further, as shown in  FIG. 13(B) , it is also possible to detect an output voltage of the inverter  103  (step S 137 ), and to determine whether or not the detected output voltage is greater than or equal to a threshold (reference value of the output voltage) (step S 138 ), before step S 131  in which rotation of the motor  1  is stopped. 
       FIGS. 13(A) and 13(B)  show the switching operations from the Y connection to the delta connection. However, determination based on the rotation speed of the motor  1  or the output voltage of the inverter  103  may be performed when the switching from the delta connection to the Y connection is performed. 
       FIG. 14  is a timing chart showing an example of an operation of the air conditioner  5 .  FIG. 14  shows an operation state of the air conditioner  5  and driving states of the outdoor fan  46  and the motor  1  (compressor  41 ). The outdoor fan  46  is shown as an example of components other than the motor  1  of the air conditioner  5 . 
     When the signal receiving unit  56  receives an operation start signal (ON command) from the remote controller  55 , the CPU  57  is activated, and the air conditioner  5  is brought into an active state (ON state). The fan motor of the outdoor fan  46  starts rotating when a time t0 elapses after the air conditioner  5  is brought into the active state. The time t0 is a delay time due to communication between the indoor unit  5 A and the outdoor unit  5 B. 
     Rotation of the motor  1  using the delta connection is started when a time t1 elapses after the air conditioner  5  is brought into the active state. The time t1 is a waiting time until rotation of the fan motor of the outdoor fan  46  is stabilized. By rotating the outdoor fan  46  before rotation of the motor  1  starts, a temperature of the refrigeration cycle can be prevented from rising higher than necessary. 
     In the example of  FIG. 14 , switching from the delta connection to the Y connection is performed, then switching from the Y connection to the delta connection is performed, and subsequently the operation stop signal (OFF instruction) is received from the remote controller  55 . A time t2 necessary for switching the connection is a waiting time necessary for restarting the motor  1  and is set at a time necessary until a refrigerant pressure in the refrigeration cycle becomes substantially uniform. 
     When the operation stop signal is received from the remote control  55 , rotation of the motor  1  is stopped. Then, rotation of the fan motor of the outdoor fan  46  is stopped when a time t3 elapses after the motor  1  is stopped. The time t3 is a waiting time necessary for the temperature of the refrigeration cycle to sufficiently decrease. Then, the CPU  57  stops when a time t4 elapses, and the air conditioner  5  is brought into an operation stop state (OFF state). The time t4 is a preset waiting time. 
     (Connection Switching Based on Temperature Detection) 
     In the foregoing operation of the air conditioner  5 , determination on whether switching of the connection state of the coils  3  is necessary or not (steps S 107  and S 108 ) may be performed based on, for example, the rotation speed of the motor  1  or the inverter output voltage. However, since the rotation speed of the motor  1  may fluctuate in a short time period, it is necessary to determine whether a state where the rotation speed is less than or equal to a threshold (or greater than or equal to the threshold) continues for a certain time period. The same applies to the inverter output voltage. 
     In particular, in a case where the set temperature by the remote controller  55  is greatly changed, or in a case where the load of the air conditioner  5  rapidly changes due to opening of a window of the room or the like, there may be a delay until the operation state of the compressor  41  responds to the load change if it takes time to determine whether switching of the connection state is necessary or not. As a result, comfort provided by the air conditioner  5  may deteriorate. 
     Meanwhile, in this embodiment, the temperature difference ΔT (absolute value) between the room temperature Ta detected by the room temperature sensor  54  and the set temperature Ts is compared to the threshold. Since fluctuation in temperature is small in a short time period, it is not necessary to continue the detection of the temperature difference ΔT and the comparison to the threshold, and thus it is possible to determine whether switching of the connection is necessary or not in a short time period. Accordingly, the operation state of the compressor  41  can quickly respond to the load change, and comfort provided by the air conditioner  5  can be enhanced. 
     In the operation of the air conditioner  5  described above, determination on whether switching from the delta connection to the Y connection is necessary or not (step S 107 ) and determination on whether switching from the Y connection to the delta connection is necessary or not (step S 108 ) are consecutively performed. However, switching from the delta connection to the Y connection is performed when the air-conditioning load is decreasing (when the room temperature is approaching the set temperature), and the air-conditioning load is less likely to rapidly increase after that. Thus, it is unlikely that the connection switching is frequently performed. 
     Further, in the operation of the air conditioner  5  described above, switching of the connection state of the coils  3  (steps S 122  and S 132 ) is performed in a state where rotation of the motor is stopped (i.e., a state where the inverter  103  stops outputting). Although switching of the connection state of the coils  3  may be performed in a state where electric power is continuously supplied to the motor  1 , it is preferable to perform switching in a state where the power supply to the motor  1  is stopped, from the viewpoint of reliability of relay contacts constituting the switches  61 ,  62 , and  63  ( FIG. 6 ) of the connection switching unit  60 . 
     In this regard, it is also possible to perform switching of the connection state of the coils  3  in a state where the rotation speed of the motor  1  is sufficiently reduced, and then to return the rotation speed to the original rotation speed. 
     Further, the switches  61 ,  62  and  63  of the connection switching unit  60  are constituted by relay contacts in this example. If the switches are constituted by semiconductor switches, however, it is unnecessary to stop rotation of the motor  1  when the connection state of the coils  3  is switched. 
     Further, the connection state of the coils  3  may be switched when a state in which the temperature difference ΔT (absolute value) between the room temperature Ta and the set temperature Ts is less than or equal to the threshold ΔTr is repeated a plurality of times (a preset number of times). This suppresses repetition of connection switching due to small temperature changes. 
     In this regard, when the temperature difference ΔT between the room temperature and the set temperature becomes zero or less (ΔT≤0), the controller  50  stops rotation of the motor  1  in order to prevent overcooling (overheating) as described above. The connection state of the coils  3  may be switched from the delta connection to the Y connection at this timing. Specifically, whether or not the temperature difference ΔT is less than or equal to zero may be determined at step S 107  described above, and if the temperature difference ΔT is less than or equal to zero, rotation of the motor  1  may be stopped and the connection state of the coils  3  may be switched to the Y connection. 
     Further, in the operation of the air conditioner  5  described above, since the bus voltage of the converter  102  is raised when switching from the Y connection to the delta connection is performed, high torque can be generated by the motor  1 . Thus, the difference ΔT between the room temperature and the set temperature can be converged in a shorter time period. The raising of the bus voltage of the converter  102  will be described later. 
     (Regarding Connection State at Start-Up) 
     As described above, when the air conditioner  5  according to the first embodiment receives the operation start signal and starts the motor  1 , the air conditioner  5  starts control while the connection state of the coils  3  is set to the delta connection. Further, when the operation of the air conditioner  5  is terminated, the connection state of the coils  3  is switched to the delta connection. 
     It is difficult to accurately detect an air-conditioning load when the air conditioner  5  starts operation. In particular, when the air conditioner  5  starts operation, the difference between the room temperature and the set temperature is generally large and the air-conditioning load is generally large. Thus, in the first embodiment, the motor  1  is started while the connection state of the coils  3  is set to the delta connection capable of responding to a larger load (i.e., capable of rotating to a higher rotation speed). Accordingly, it is possible to converge the difference ΔT between the room temperature Ta and the set temperature Ts in a shorter time period when the air conditioner  5  starts operation. 
     Further, even in a case where the air conditioner  5  stops for a long time period, and an abnormality (for example, inoperability of relays of the switches  61  through  63  due to sticking, or the like) occurs in the connection switching unit  60  during the stop, the motor  1  can be started with the delta connection since switching from the Y connection to the delta connection is performed before termination of the operation of the air conditioner  5 . Accordingly, degradation of performance of the air conditioner  5  can be prevented, and comfort is not impaired. 
     In this regard, in a case where the motor  1  is started while the connection state of the coils  3  is set to the delta connection and switching to the Y connection is not performed, it is possible to obtain motor efficiency equal to that of a general motor in which the connection state of coils is fixed to the delta connection (i.e., having no connection switching function). 
     (Motor Efficiency and Motor Torque) 
     Next, improvements of motor efficiency and motor torque will be described. In general, household air conditioners are subject to Energy Conservation Act, and it is mandatory to reduce CO 2  emissions from the viewpoint of global environments. With the advance of technology, compression efficiency of compressors, operation efficiency of motors of the compressors, heat transfer coefficient of heat exchangers and the like have been improved, a coefficient of performance (COP) of energy consumption efficiency of the air conditioners has been increased year by year, and running costs (power consumption) and CO 2  emissions of the air conditioners have also been reduced. 
     The COP is used for evaluating performance in the case of operation under a certain temperature condition, and an operating condition of the air conditioner for each season is not taken into consideration. However, when the air conditioner is actually used, capacity and power consumption necessary for cooling or heating change with a change in outdoor air temperature. Thus, in order to perform evaluation in a state close to actual use, an annual performance factor (APF) is used as an index of energy saving. The APF is efficiency obtained by determining a certain model case, and calculating a total load and a total electric power consumption throughout the year. 
     In particular, in the inverter motor, which is a current mainstream, the capacity varies depending on the rotation speed of the compressor, and thus there is a problem in performing evaluation close to actual use only under the rated condition. 
     The APF of a household air conditioner is obtained by calculating a power consumption amount in accordance with annual total load at five evaluation points: a rated cooling condition, an intermediate cooling condition, a rated heating condition, an intermediate heating condition, and a low heating temperature. As the calculated amount is larger, energy saving performance is evaluated to be higher. 
     As a breakdown of the annual total load, the ratio of the intermediate heating condition is very large (50%), and the ratio of the rated heating condition is the next largest (25%). Thus, it is effective in enhancing energy saving performance of air conditioners to increase motor efficiency under the intermediate heating condition and the rated heating condition. 
     The rotation speed of a motor of a compressor under evaluation load conditions of the APF varies depending on a capacity of an air conditioner and performance of a heat exchanger. For example, in a household air conditioner having a refrigeration capacity of 6.3 kW, a rotation speed N1 (first rotation speed) under the intermediate heating condition is 35 rps, and a rotation speed N2 (second rotation speed) under the rated heating condition is 85 rps. 
     The motor  1  according to this embodiment is intended to obtain high motor efficiency and high motor torque at the rotation speed N1 corresponding to the intermediate heating condition and the rotation speed N2 corresponding to the rated heating condition. That is, out of the two load conditions for which performance is to be improved, the rotation speed at a low-speed side is N1 and the rotation speed at a high-speed side is N2. 
     In the motor  1  in which the permanent magnets  25  are mounted on the rotor  20 , when the rotor  20  rotates, the magnetic fluxes of the permanent magnets  25  interlink with the coils  3  of the stator  10 , and an induced voltage is generated in the coils  3 . The induced voltage is proportional to the rotation speed (rotation velocity) of the rotor  20  and is also proportional to the number of turns of each coil  3 . As the rotation speed of the motor  1  increases and the number of turns of the coil  3  increases, the induced voltage increases. 
     The line voltage (motor voltage) output from the inverter  103  is equal to a sum of the induced voltage and a voltage generated by a resistance and an inductance of the coils  3 . The resistance and the inductance of the coils  3  are negligibly small as compared to the induced voltage, and thus the line voltage is practically dominated by the induced voltage. A magnet torque of the motor  1  is proportional to the product of the induced voltage and a current flowing through the coils  3 . 
     As the number of turns of the coil  3  increases, the induced voltage increases. Thus, as the number of turns of the coil  3  increases, a current for generating a necessary magnet torque decreases. Consequently, a conduction loss of the inverter  103  can be reduced, and operation efficiency of the motor  1  can be enhanced. Meanwhile, since the induced voltage increases, the line voltage dominated by the induced voltage reaches an inverter maximum output voltage (i.e., a bus voltage supplied from the converter  102  to the inverter  103 ) at a lower rotation speed, and the rotation speed cannot be increased higher than that. 
     Further, when the number of turns of the coil  3  is reduced, the induced voltage decreases and the line voltage dominated by the induced voltage does not reach the inverter maximum output voltage even at a higher rotation speed, and high-speed rotation is made possible. However, since the induced voltage decreases, the current for generating the necessary magnet torque increases, and thus the conduction loss of the inverter  103  increases, so that operation efficiency of the motor  1  decreases. 
     Further, from the viewpoint of the switching frequency of the inverter  103 , a harmonic component caused by an ON/OFF duty of switching of the inverter  103  decreases as the line voltage is closer to the inverter maximum output voltage, and thus an iron loss caused by the high harmonic component of the current can be reduced. 
       FIGS. 15 and 16  are graphs each showing a relationship between the line voltage and the rotation speed in the motor  1 . The connection state of the coils  3  is the Y connection. The line voltage is proportional to the product of a field magnetic field and a rotation speed. If the field magnetic field is constant, the line voltage and the rotation speed are proportional as shown in  FIG. 15 . In this regard, in  FIG. 15 , the rotation speed N1 corresponds to the intermediate heating condition, and the rotation speed N2 corresponds to the rated heating condition. 
     The line voltage increases as the rotation speed increases. However, as shown in  FIG. 16 , when the line voltage reaches the inverter maximum output voltage, the line voltage cannot be increased higher than that, and thus field-weakening control by the inverter  103  is started. In this example, it is assumed that the field-weakening control is started at a rotation speed between the rotation speeds N1 and N2. 
     In the field-weakening control, the induced voltage is weakened by causing a current having a d-axis phase (in a direction of canceling magnetic fluxes of the permanent magnets  25 ) to flow in the coils  3 . This current will be referred to as a weakening current. Since the weakening current is needed to flow in addition to a usual current for generating motor torque, a copper loss due to the resistance of the coils  3  increases, and the conduction loss of the inverter  103  also increases. 
       FIG. 17  is a graph showing a relationship between the motor efficiency and the rotation speed in a case where the field-weakening control shown in  FIG. 16  is performed. As shown in  FIG. 17 , the motor efficiency increases as the rotation speed increases, and immediately after the field-weakening control starts, the motor efficiency reaches its peak as indicated by an arrow P. 
     When the rotation speed further increases, the weakening current flowing in the coils  3  also increases, and thus the copper loss increases accordingly so that the motor efficiency decreases. In the overall efficiency that is the product of the motor efficiency and the inverter efficiency, a change represented by a curve similar to that in  FIG. 17  is observed. 
       FIG. 18  is a graph showing a relationship between the maximum torque and the rotation speed of the motor in a case where the field-weakening control shown in  FIG. 16  is performed. Before the field-weakening control is started, the maximum torque of the motor is constant (due to, for example, restriction by a current threshold). When the field-weakening control is started, the maximum torque of the motor  1  decreases as the rotation speed increases. The maximum torque of the motor  1  is set to be larger than a load (necessary load) actually generated by the motor  1  when a product is used. Hereinafter, the maximum torque of the motor will be referred to as motor torque, for convenience of description. 
       FIG. 19  is a graph showing relationships between the line voltage and the rotation speed for the Y connection and the delta connection. In a case where the connection state of the coils  3  is the delta connection, a phase impedance of the coils  3  is 1/√3 times as large as a phase impedance in a case where the connection state of the coils  3  is the Y connection, suppose that the number of turns is the same in each case. Thus, the line voltage (chain line) in the case where the connection state of the coils  3  is the delta connection is 1/√3 times as high as the line voltage (solid line) in the case where the connection state of the coils  3  is the Y connection, suppose that the rotation speed is the same in each case. 
     That is, when the coils  3  are connected in the delta connection, if the number of turns is made √3 times as large as the number of turns in the case of the Y connection, the line voltage (motor voltage) is equivalent to that in the case of the Y connection for the same rotation speed N. Thus, an output current of the inverter  103  is also equivalent to that in the case of the Y connection. 
     In motors in which the number of turns around each tooth is several tens or more, the Y connection is more often used than the delta connection for the following reasons. One reason is that the number of turns of each coil in the delta connection is larger than that in the Y connection, and thus the time necessary for winding the coils is longer in a manufacturing process. Another reason is that there is a possibility that a circulating current may occur in the case of the delta connection. 
     In general, in a motor employing the Y connection, the number of turns of the coil is adjusted such that the line voltage (motor voltage) reaches the inverter maximum output voltage at the rotation speed N2 (i.e., the rotation speed at the high-speed side of the rotation speeds for which performance is to be enhanced). In this case, however, the motor is operated with the line voltage lower than the inverter maximum output voltage at the rotation speed N1 (i.e., the rotation speed at the low-speed side of the rotation speeds for which performance is to be enhanced), and thus it is difficult to obtain high motor efficiency. 
     Thus, the connection state of the coils is set to the Y-connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at a rotation speed slightly lower than the rotation speed N1, and control is performed to switch the connection state of the coils to the delta connection before the motor reaches the rotation speed N2. 
       FIG. 20  is a graph showing a relationship between the line voltage and the rotation speed in the case where switching from the Y connection to the delta connection is performed. In the example shown in  FIG. 20 , when the motor reaches a rotation speed (hereinafter referred to as a rotation speed N11) slightly lower than the rotation speed N1 (intermediate heating condition), the field-weakening control described above is started. When the rotation speed N further increases and reaches a rotation speed NO, switching from the Y connection to the delta connection is performed. Here, the rotation speed N11 is 5% lower than the rotation speed N1 (i.e., N11=N1×0.95). 
     By switching to the delta connection, the line voltage decreases to be 1/√3 times as high as that in the Y connection, and thus the degree of field-weakening can be reduced (i.e., the weakening current can be reduced). Accordingly, a copper loss due to the weakening current can be reduced, and decreases in motor efficiency and motor torque can be suppressed. 
       FIG. 21  is a graph showing relationships between the motor efficiency and the rotation speed for the Y connection and the delta connection. As described above, the connection state of the coils  3  is the Y connection and the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11 slightly lower than the rotation speed N1. Thus, as indicated by the solid line in  FIG. 21 , high motor efficiency can be obtained at the rotation speed N1. 
     Meanwhile, in the case of the delta connection, motor efficiency higher than that in the Y connection can be obtained at the rotation speed N2 as indicated by the chain line in  FIG. 21 , suppose that the number of turns of the coil  3  is the same. Thus, by switching from the Y connection to the delta connection at an intersection of the solid line and the chain line shown in  FIG. 21 , high motor efficiency can be obtained at both of the rotation speed N1 (intermediate heating condition) and the rotation speed N2 (rated heating condition). 
     Thus, as described with reference to  FIG. 20 , the connection state of the coils  3  is set to the Y connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11 (rotation speed slightly lower than the rotation speed N1), and control is performed to switch from the Y connection to the delta connection at the rotation speed NO higher than the rotation speed N1. 
     However, motor efficiency cannot be sufficiently enhanced by simply switching the connection state of the coils  3  from the Y connection to the delta connection. This will be described below. 
       FIG. 22  is a graph showing a relationship between the motor efficiency and the rotation speed in a case (solid line) where the connection state of the coils  3  is the Y connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11, and switching from the Y connection to the delta connection is performed at the rotation speed NO. The broken line shows a relationship between the motor efficiency and the rotation speed in a case where field-weakening control is performed while the connection state of the coils  3  is set to the Y-connection as shown in  FIG. 17 . 
     The line voltage is proportional to the rotation speed. For example, in a household air conditioner having a refrigeration capacity of 6.3 kW, the rotation speed N1 (intermediate heating condition) is 35 rps and the rotation speed N2 (rated heating condition) is 85 rps, and thus the line voltage in the rated heating condition is 2.4 times (=85/35) as high as the line voltage in the intermediate heating condition. 
     The line voltage in the rated heating condition (rotation speed N2) after the connection state of the coils  3  is switched to the delta connection is 1.4 times (=85/35/√3) as high as the inverter maximum output voltage. Since the line voltage cannot be made larger than the inverter maximum output voltage, the field-weakening control is started. 
     In the field-weakening control, the weakening current necessary for weakening the field flows in the coils  3 , and thus the copper loss increases and the motor efficiency and the motor torque decrease. Consequently, as indicated by the solid line in  FIG. 22 , the motor efficiency in the rated heating condition (rotation speed N2) cannot be improved. 
     In order to reduce the degree of field weakening in the rated heating condition (rotation speed N2) (i.e., to reduce a weakening current), it is necessary to reduce the line voltage by reducing the number of turns of the coils  3 . In this case, the line voltage in the intermediate heating condition (rotation speed N1) also decreases, and the improving effect of the motor efficiency by switching the connection decreases. 
     That is, if there are two load conditions for which performance is to be improved and the rotation speed N1 at the low-speed side and the rotation speed N2 at the high-speed side satisfy (N2/N1)&gt;√3, the field-weakening control is necessary even when switching from the Y connection to the delta connection is performed. Thus, sufficient improving effect of the motor efficiency cannot be obtained by simply performing switching from the Y connection to the delta connection. 
       FIG. 23  is a graph showing relationships between the motor torque and the rotation speed for the Y connection and the delta connection. In the case of the Y connection, as described with reference to  FIG. 18 , the motor torque is constant with respect to an increase in the rotation speed N, but when the field-weakening control is started, the motor torque decreases as the rotation speed N increases. In the case of the delta connection, the field-weakening control is started at a higher rotation speed than that in the case of the Y connection (N11), but when the field-weakening control is started, the motor torque decreases as the rotation speed N increases. 
       FIG. 24  is a graph showing a relationship between the motor torque and the rotation speed in a case where the connection state of the coils  3  is the Y connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11 (rotation speed slightly lower than the rotation speed N1), and switching from the Y connection to the delta connection is performed at the rotation speed NO higher than the rotation speed N1. As shown in  FIG. 24 , when the rotation speed reaches the rotation speed N11 and the field-weakening control is started, the motor torque decreases as the rotation speed N increases. 
     When the rotation speed further increases to reach the rotation speed NO and switching from the Y connection to the delta connection is performed, the field-weakening control temporarily stops, and thus the motor torque increases. However, when the rotation speed N further increases and the field-weakening control is started, the motor torque decreases as the rotation speed N increases. In this manner, simply switching from the Y connection to the delta connection is not enough to suppress a decrease in the motor torque especially in a high rotation speed range. 
     Thus, the driving device  100  according to the first embodiment switches the bus voltage by the converter  102 , in addition to switching of the connection state of the coils  3  by the connection switching unit  60 . The converter  102  is supplied with a power supply voltage (200 V) from the power source  101  and supplies the bus voltage to the inverter  103 . The converter  102  is preferably constituted by an element exhibiting a small loss due to an increase in voltage (voltage rising), such as a SiC element or a GaN element. 
     Specifically, a bus voltage V1 (first bus voltage) when the connection state of the coils  3  is the Y connection is set to 280 V (DC). Meanwhile, a bus voltage V2 (second bus voltage) when the connection state of the coils  3  is the delta connection is set to 390 V (DC). That is, the bus voltage V2 in the case of the delta connection is set to 1.4 times as high as the bus voltage V1 in the case of the Y connection. In this regard, it is sufficient that the bus voltage V2 satisfies V2≥(V1/√3)×N2/N1 in relation to the bus voltage V1. The inverter  103  supplied with the bus voltage from the converter  102  supplies the line voltage to the coils  3 . The inverter maximum output voltage is 1/√ 2  of the bus voltage. 
       FIG. 25  is a graph showing relationships between the line voltage and the rotation speed in a case where the bus voltage is switched by the converter  102  for the Y connection and the delta connection. As shown in  FIG. 25 , the line voltage (solid line) in the case where the connection state of the coils  3  is the Y connection is 1/√2 (i.e., V1×1/√2) of the bus voltage V1 at maximum. The line voltage (chain line) in the case where the connection state of the coils  3  is the delta connection is 1/√ 2  (i.e., V2×1/√2) of the bus voltage V2 at maximum. 
       FIG. 26  is a graph showing a relationship between the line voltage and the rotation speed in a case where the connection state is switched by the connection switching unit  60  and the bus voltage is switched by the converter  102 . As shown in  FIG. 26 , in a rotation speed range including the rotation speed N1 (intermediate heating condition), the connection state of the coils  3  is the Y connection. As the rotation speed increases, the line voltage increases, and the line voltage reaches the inverter maximum output (V1×1/√2) at the rotation speed N11 slightly lower than the rotation speed N1. Thus, the field-weakening control is started. 
     When the rotation speed further increases to reach the rotation speed NO, the connection switching unit  60  switches the connection state of the coils  3  from the Y connection to the delta connection. At the same time, the converter  102  raises the bus voltage from V1 to V2. As the bus voltage is raised, the inverter maximum output becomes V2×1/√2. At this point of time, the line voltage is lower than the inverter maximum output, and thus the field-weakening control is not performed. 
     Thereafter, the line voltage increases as the rotation speed N increases, the line voltage reaches the inverter maximum output (V2×1/√2) at a rotation speed N21 slightly lower than the rotation speed N2 (rated heating condition), and the field-weakening control is started. In this regard, the rotation speed N21 is 5% lower than the rotation speed N2 (i.e., N21=N2×0.95). 
     In the first embodiment, the connection state of the coils  3  is switched based on the result of comparison between the temperature difference ΔT between the room temperature Ta and the set temperature Ts and the threshold ΔTr, as described above. Switching from the Y connection to the delta connection at the rotation speed NO corresponds to the switching from the Y connection to the delta connection shown in step S 108  in  FIG. 10  and steps S 131  through S 134  in  FIG. 12 . 
     The improving effect of the motor efficiency in this case will be described.  FIG. 27  is a graph showing relationships between the motor efficiency and the rotation speed for the Y connection and the delta connection. In  FIG. 27 , the motor efficiency (solid line) in the case where the connection state of the coils  3  is the Y connection is similar to the motor efficiency in the case of the Y connection shown in  FIG. 21 . Meanwhile, the motor efficiency (chain line) in the case where the connection state of the coils  3  is the delta connection is higher than the motor efficiency in the delta connection shown in  FIG. 21  because of an increase in the bus voltage of the converter  102 . 
       FIG. 28  is a graph showing a relationship between the motor efficiency and the rotation speed in a case where the connection state is switched by the connection switching unit  60  and the bus voltage is switched by the converter  102 . Since the connection state of the coils  3  is the Y connection and the number of turns is set such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11 (rotation speed slightly lower than the rotation speed N1), high motor efficiency can be obtained in a rotation speed range including the rotation speed N1. 
     When the rotation speed reaches the rotation speed N11, the field-weakening control is started. When the rotation speed then reaches the rotation speed NO, the connection state of the coils  3  is switched from the Y connection to the delta connection, and the bus voltage is increased by the converter  102 . 
     Since the inverter maximum output voltage increases with an increase in the bus voltage, the line voltage becomes lower than the inverter maximum output voltage, and accordingly the field-weakening control is stopped. As the field-weakening control is stopped, a copper loss caused by the weakening current is reduced, and thus the motor efficiency increases. 
     Thereafter, when the rotation speed N reaches the rotation speed N21 slightly lower than the rotation speed N2 (rated heating condition), the line voltage reaches the inverter maximum output voltage, and the field-weakening control is started. Although the copper loss increases as a result of start of the field-weakening control, high motor efficiency can be obtained since the bus voltage has been increased by the converter  102 . 
     That is, as indicated by the solid line in  FIG. 28 , high motor efficiency can be obtained at both of the rotation speed N1 (intermediate heating condition) and the rotation speed N2 (rated heating condition). 
     Next, the improving effect of the motor torque will be described.  FIG. 29  is a graph showing relationships between the motor torque and the rotation speed in the case where the connection state of the coils  3  is the Y connection and the case where the connection state of the coils  3  is the delta connection. The motor torque (solid line) in the case of the Y connection is similar to that in  FIG. 18 . When the field-weakening control is started at the rotation speed N21 slightly lower than the rotation speed N2 (rated heating condition), the motor torque (chain line) in the case of the delta connection decreases as the rotation speed N increases. 
       FIG. 30  is a graph showing a relationship between the motor torque and the rotation speed in a case where the connection state of the coils  3  is the Y connection, the number of turns is adjusted such that the line voltage reaches the inverter maximum output voltage at the rotation speed N11, the connection state is switched from the Y connection to the delta connection at the rotation speed NO (&gt;N1), and the bus voltage is further raised. As shown in  FIG. 30 , when the field-weakening control is started at the rotation speed N11 slightly lower than the rotation speed N1 (intermediate heating condition), the motor torque decreases as the rotation speed N increases. 
     When the rotation speed N further increases to reach the rotation speed NO, the connection state of the coils  3  is switched from the Y connection to the delta connection, and the bus voltage is raised. With the switching to the delta connection and the raising of the bus voltage, the line voltage becomes lower than the inverter maximum output voltage, and thus the field-weakening control is stopped. Accordingly, the motor torque increases. Thereafter, when the field-weakening control is started at the rotation speed N21 slightly lower than the rotation speed N2 (rated heating condition), the motor torque decreases as the rotation speed N increases. 
     In this manner, since the field-weakening control is not performed until the rotation speed N reaches the rotation speed N21 (rotation speed slightly lower than the rotation speed N2) after the switching to the delta connection, a decrease in motor torque can be suppressed especially in a rotation speed range including the rotation speed N2 (rated heating condition). 
     Specifically, as indicated by the solid line in  FIG. 30 , high motor torque can be obtained at both of the rotation speed N1 (intermediate heating condition) and the rotation speed N2 (rated heating condition). That is, high performance (motor efficiency and motor torque) can be obtained in both of the intermediate heating condition and the rated heating condition of the air conditioner  5 . 
     In this regard, when the voltage of the converter  102  is raised, a loss due to the raising of the voltage occurs. Thus, in the connection state under the intermediate heating condition (i.e., Y connection) where a contribution ratio to the motor efficiency is the highest, it is preferable to use the power supply voltage without raising. The power supply voltage of the power source  101  is 200 V (effective value), and the maximum value is 280 V (=200V×√2). Accordingly, it can be said that the bus voltage (280 V) of the converter  102  in the case of the Y connection is the same as the maximum value of the power supply voltage. 
     Further, switching of the bus voltage supplied to the inverter  103  may be performed by raising or lowering the power supply voltage. 
     Further, in the operation control of the air conditioner  5  described above, the Y connection is set to the rotation speed N1 (intermediate heating condition) and the delta connection is set to the rotation speed N2 (rated heating condition). However, if no specific load condition is determined, the voltage level may be adjusted by setting the rotation speed N1 as the maximum rotation speed during the operation in the Y connection state and setting the rotation speed N2 as the maximum rotation speed during the operation in the delta connection state. With such control, the efficiency of the motor  1  can be enhanced. 
     As described above, in the household air conditioner  5 , the efficiency of the motor  1  can be enhanced by setting the rotation speed N1 at the rotation speed in the intermediate heating condition and the rotation speed N2 at the rotation speed in the rated heating condition. 
     (Advantages of First Embodiment) 
     As described above, in the first embodiment, since the converter  102  changes a level of the bus voltage in accordance with switching of the connection state of the coils  3  by the connection switching unit  60 , high motor efficiency and high motor torque can be obtained before and after the switching of the connection state. 
     Further, since the connection state of the coils  3  is switched between the Y connection (first connection state) and the delta connection (second connection state) in which the line voltage is lower than in the first connection state, the connection state suitable for the operation state of the motor  1  can be selected. 
     Further, in the first embodiment, the bus voltage of the converter  102  is set to the first bus voltage V1 when the connection state of the coils  3  is the first connection state, and the bus voltage of the converter  102  is set to the second bus voltage V2 higher than the first bus voltage V1 when the connection state of the coils  3  is the second connection state. Thus, when the rotation speed of the motor  1  increases, the connection state is switched to the second connection state in which the line voltage is low, and the bus voltage is increased, so that motor efficiency and motor torque can be enhanced. 
     Further, in the case where the first connection state is the Y connection, the second connection state is the delta connection, and the first rotation speed N1 and the second rotation speed N2 of the motor  1  satisfy N2/N2&gt;√3, the connection state of the coils  3  is set to the Y connection when the rotation speed of the motor  1  is the first rotation speed N1, and the connection state of the coils  3  is set to the delta connection when the rotation speed of the motor  1  is the second rotation speed N2. Thus, motor efficiency and motor torque can be enhanced at both of the rotation speeds N1 and N2. 
     Further, the first bus voltage V1, the second bus voltage V2, the first rotation speed N1 and the second rotation speed N2 satisfy V2≥(V1/√ 3 )×N2/N1, and thus high motor efficiency and high motor torque can be obtained at the rotation speeds N1 and N2. 
     Further, since the first rotation speed N1 corresponds to the intermediate heating condition, and the second rotation speed N2 corresponds to the rated heating condition, high motor efficiency and high motor torque can be obtained at the intermediate heating condition and the rated heating condition for which performance is to be improved. 
     That is, since the first rotation speed N1 corresponds to a condition that yields the highest ratio in the annual performance factor (APF), and the second rotation speed N2 corresponds to a condition that yields the second highest ratio in the annual performance factor, a large enhancing effect of energy consumption efficiency is obtained. 
     Further, since the first bus voltage V1 is the same as √2 times an effective value of the power supply voltage, the converter  102  can use the power supply voltage without raising it when the coils  3  is in the first connection state, and thus energy efficiency can be enhanced. 
     Further, since the converter  102  is composed of a SiC element or a GaN element, loss due to an increase in voltage is small, and thus energy efficiency can be further enhanced. 
     Further, since the field-weakening control is performed in accordance with the rotation speed of the motor  1  in each of the cases of the Y connection (first connection state) and the delta connection (second connection state), the rotation speed of the motor  1  can be increased even when the line voltage reaches the inverter maximum output voltage. 
     Further, in a case where the controller  50  receives the operation stop signal from the remote controller  55  through the signal receiving unit  56 , the connection state of the coils  3  is switched from the Y connection to the delta connection, and then the controller  50  terminates the operation of the air conditioner  5 . In a case where the connection state of the coils  3  is already the delta connection, this connection state is unchanged. Accordingly, at the start of the operation (start-up) of the air conditioner  5 , the operation of the air conditioner  5  can be started in a state where the connection state of the coils  3  is the delta connection. Accordingly, even in a case where the temperature difference ΔT between the room temperature Ta and the set temperature Ts is large, the operation of the air conditioner  5  can be started in the delta connection state, and the room temperature Ta can be quickly brought close to the set temperature Ts. 
     First Modification 
     Next, a first modification of the first embodiment will be described. In the first embodiment described above, the rotation speed NO at which the connection state of the coils is switched from the Y connection to the delta connection (i.e., the rotation speed when the temperature difference ΔT becomes equal to the threshold ΔTr) is equal to the rotation speed NO (temperature difference) at which the connection state is switched from the delta connection to the Y connection, but these rotation speeds may be different from each other. 
       FIGS. 31(A) and 31(B)  are graphs each showing a relationship between the motor efficiency and the rotation speed in a case where the connection state is switched by the connection switching unit  60  and the bus voltage is switched by the converter  102 . As shown in  FIGS. 31(A) and 31(B) , a rotation speed N4 at which the connection state of the coils  3  is switched from the Y connection to the delta connection and a rotation speed N5 at which the connection state of the coils  3  is switched from the delta connection to the Y connection are different from each other. 
     Further, switching of the bus voltage by the converter  102  is performed at the same time as switching of the connection state of the coils  3 . That is, the bus voltage is raised when the rotation speed is N4 at which the Y connection is switched to the delta connection. Meanwhile, the bus voltage is lowered when the rotation speed is N5 at which the delta connection is switched to the Y connection. 
     Such control can be performed by, for example, setting the threshold ΔTr in step S 107  and the threshold ΔTr in step S 108  in  FIG. 10  to different values. In the examples shown in  FIGS. 31(A) and 31(B) , the rotation speed N4 at which the Y connection is switched to the delta connection is higher than the rotation speed N5 at which the delta connection is switched to the Y connection, but this relationship may be reversed. The other operations and configurations in the first modification are similar to those in the first embodiment. 
     In this first modification, the connection state of the coils  3  is switched based on the room temperature Ta, and thus the operation state of the compressor  41  can quickly respond to a rapid load change of the air conditioner  5 . In addition, high motor efficiency can be obtained by switching the bus voltage of the converter  102  in accordance with the switching of the connection state of the coils  3 . 
     Second Modification 
     Next, a second modification of the first embodiment will be described. In the first embodiment described above, the bus voltage of the converter  102  is switched to two stages (V1/V2), but the bus voltage may be switched to three stages as shown in  FIG. 32 . 
       FIG. 32  is a graph showing a relationship between a line voltage and a rotation speed in a case where switching of the connection state and switching of the bus voltage of the converter  102  are performed in the second modification. In the example of  FIG. 32 , the bus voltage of the converter  102  is set to V1 when the rotation speed is N1 (Y connection) corresponding to the intermediate heating condition, and the connection state is switched from the Y connection to the delta connection when the rotation speed is N6 between the rotation speed N1 and the rotation speed N2 (rated heating condition), and the bus voltage is raised to V2 at the same as the switching of the connection state. 
     In addition, when the rotation speed is N7 higher than the rotation speed N2, the bus voltage of the converter  102  is raised to V3 while the connection state is unchanged. During a period from the rotation speed N7 to a maximum rotation speed N8, the bus voltage of the converter  102  is V3. The other operations and configurations in the second modification are similar to those in the first embodiment. 
     As described above, in the second modification, since the bus voltage of the converter  102  is switched to three stages of V1, V2, and V3, high motor efficiency and high motor torque can be obtained especially in a high rotation speed range. 
     In this regard, the switching of the bus voltage is not limited to the two stages or the three stages, and may be performed in four or more stages. Further, in the first modification ( FIG. 31 ), the bus voltage of the converter  102  may be switched to three or more stages. 
     Third Modification 
     Next, a third modification of the first embodiment will be described. In the first embodiment described above, the connection state of the coils  3  is switched between the Y connection and the delta connection. However, the connection state of the coils  3  may be switched between a series connection and a parallel connection. 
       FIGS. 33(A) and 33(B)  are schematic diagrams for describing switching of the connection state of the coils  3  according to the third modification. In  FIG. 33(A) , the three-phase coils  3 U,  3 V, and  3 W are connected in the Y connection. Further, the coil portions Ua, Ub, and Uc of the coil  3 U are connected in series, the coil portions Va, Vb, and Vc of the coil  3 V are connected in series, and the coil portions Wa, Wb, and Wc of the coil  3 W are connected in series. That is, the coil portions of each phase of the coils  3  are connected in series. 
     In contrast, in  FIG. 33(B) , the three-phase coils  3 U,  3 V, and  3 W are connected in the Y connection, but the coil portions Ua, Ub, and Uc of the coil  3 U are connected in parallel, the coil portion Va, Vb, and Vc of the coil  3 V are connected in parallel, and the coil portions Wa, Wb, and Wc of the coil  3 W are connected in parallel. That is, the coil portions of each phase of the coils  3  are connected in parallel. Switching of the connection state of the coils  3  as shown in  FIGS. 33(A) and 33(B)  can be achieved by, for example, providing each of the coil portions of the coils  3 U,  3 V, and  3 W with a selector switch. 
     Suppose that the number of coil portions (i.e., the number of rows) connected in parallel in each phase is n, the line voltage decreases to 1/n times by switching from the series connection ( FIG. 33(A) ) to the parallel connection ( FIG. 33(B) ). Thus, by switching the connection state of the coils  3  from the series connection to the parallel connection when the line voltage approaches the inverter maximum output voltage, the degree of field-weakening can be reduced (i.e., weakening current can be reduced). 
     In a case where there are two load conditions for which performance is to be improved and the rotation speed N1 at the low-speed side and the rotation speed N2 at the high-speed side satisfy (N2/N1)&gt;n, the line voltage becomes larger than the inverter maximum output voltage only by switching the connection state of the coils  3  from the series connection to the parallel connection, and thus the field-weakening control is necessary. Thus, as described in the first embodiment, the bus voltage of the converter  102  is raised at the same time as when the connection state of the coils  3  is switched from the series connection to the parallel connection. Accordingly, high motor efficiency and high motor torque can be obtained in both of the rotation speed range including the rotation speed N1 and the rotation speed range including the rotation speed N2. 
       FIGS. 34(A) and 34(B)  are schematic diagrams for describing another configuration example of the third modification. In  FIG. 34(A) , the three-phase coils  3 U,  3 V, and  3 W are connected in the delta connection. Further, the coil portions Ua, Ub, and Uc of the coil  3 U are connected in series, the coil portions Va, Vb, and Vc of the coil  3 V are connected in series, and the coil portions Wa, Wb, and Wc of the coil  3 W are connected in series. That is, the coil portions of each phase of the coils  3  are connected in series. 
     In contrast, in  FIG. 34(B) , the three-phase coils  3 U,  3 V, and  3 W are connected in the delta connection, but the coil portions Ua, Ub, and Uc of the coil  3 U are connected in parallel, the coil portion Va, Vb, and Vc of the coil  3 V are connected in parallel, and the coil portions Wa, Wb, and Wc of the coil  3 W are connected in parallel. That is, the coil portions of each phase of the coils  3  are connected in parallel. 
     In this case, similarly to the examples shown in  FIGS. 33(A) and 33(B) , when the low-speed side rotation speed N1 and the high-speed side rotation speed N2 of the two load conditions for which performance is to be improved satisfy (N2/N1)&gt;n, the connection state of the coils  3  is switched from the series connection ( FIG. 33(A) ) to the parallel connection ( FIG. 33(B) ), and at the same time, the bus voltage of the converter  102  is raised. The other operations and configurations in the third modification are similar to those in the first embodiment. It is sufficient that the raised bus voltage V2 satisfies V2 (V1/n)×N2/N1 in relation to the bus voltage V1 before raising. 
     As described above, in the third modification, the connection state of the converter  102  is switched between the series connection and the parallel connection, and therefore the degree of field-weakening can be reduced and the motor efficiency can be increased. Further, the bus voltages V1 and V2 and the rotation speeds N1 and N2 satisfy V2 (V1/n)×N2/N1, and therefore high motor efficiency and high motor torque can be obtained at the rotation speeds N1 and N2. 
     In the first and second modifications, switching between the series connection (first connection state) and the parallel connection (second connection state) may be performed. 
     Fourth Modification 
     In the first embodiment described above, the absolute value of the difference ΔT between the room temperature Ta detected by the room temperature sensor  54  and the set temperature Ts is compared to the threshold ΔTr, and the connection state of the coils  3  and the bus voltage of the converter  102  are switched. However, an air-conditioning load may be calculated based on the room temperature Ta, and the connection state of the coils  3  and the bus voltage of the converter  102  may be switched based on the air-conditioning load. 
       FIG. 35  is a flowchart showing a basic operation of an air conditioner according to a fourth modification. Steps S 101  through S 105  are the same as those in the first embodiment. If the operation stop signal is not received after the motor  1  is started in step S 104  (step S 105 ), the controller  50  acquires a temperature difference ΔT between a room temperature Ta detected by the room temperature sensor  54  and a set temperature Ts set by the remote controller  55  (step S 201 ), and calculates an air-conditioning load based on the temperature difference ΔT (step S 202 ). 
     Next, based on the calculated air-conditioning load, it is determined whether switching of the coils  3  from the delta connection to the Y connection is necessary or not. Specifically, it is determined whether or not the connection state of the coils  3  is the delta connection and the air-conditioning load calculated in step S 202  is less than or equal to a threshold (reference value of the air-conditioning load) (step S 203 ). 
     If the result of the comparison in step S 203  indicates that the connection state of the coils  3  is the delta connection and the air-conditioning load is less than or equal to the threshold, processes in steps S 121  through S 124  shown in  FIG. 11  are performed. As described in the first embodiment, in steps S 121  through S 124  in  FIG. 11 , the switching from the delta connection to the Y connection and the raising of the bus voltage by the converter  102  are performed. 
     If the result of the comparison in step S 203  indicates that the connection state of the coils  3  is not the delta connection (if it is the Y connection) or that the air-conditioning load is larger than the threshold (i.e., if switching to the Y connection is unnecessary), the process proceeds to step S 204 . 
     In step S 204 , it is determined whether switching from the Y connection to the delta connection is necessary or not. Specifically, it is determined whether or not the connection state of the coils  3  is the Y connection and the air-conditioning load calculated in step S 202  is larger than the threshold. 
     If the result of the comparison in step S 204  indicates that the connection state of the coils  3  is the Y connection and the air-conditioning load is larger than the threshold, processes in steps S 131  through S 134  shown in  FIG. 12  are performed. As described in the first embodiment, in steps S 131  through S 134  in  FIG. 12 , the switching from the Y connection to the delta connection and the lowering of the bus voltage by the converter  102  are performed. 
     If the result of the comparison in step S 204  indicates that the connection state of the coils  3  is not the Y connection (if it is the delta connection) or that the air-conditioning load is larger than the threshold (i.e., if switching to the delta connection is unnecessary), the process returns to step S 105 . Processes in the case where the operation stop signal is received (steps S 109  through S 111 ) are similar to those in the first embodiment. The other operations and configurations in the fourth modification are similar to those in the first embodiment. 
     As described above, in the fourth modification, the air-conditioning load is calculated based on the room temperature Ta and the connection state of the coils  3  and the bus voltage of the converter  102  are switched based on the calculated air-conditioning load, and therefore the operation state of the compressor  41  can quickly respond to a load change of the air conditioner  5 . As a result, comfort can be enhanced. 
     In the first, second, and third modifications, the connection state of the coils  3  and the bus voltage of the converter  102  may be switched based on the air-conditioning load. 
     Fifth Modification 
     In the first embodiment described above, the connection state of the coils  3  and the bus voltage of the converter  102  are switched based on the temperature difference ΔT between the room temperature Ta detected by the room temperature sensor  54  and the set temperature Ts. However, the connection state of the coils  3  and the bus voltage of the converter  102  may be switched based on the rotation speed of the motor  1 . 
       FIG. 36  is a flowchart showing a basic operation of an air conditioner according to a fifth modification. Steps S 101  through S 105  are the same as those in the first embodiment. If the operation stop signal is not received after the motor  1  is started in step S 104  (step S 105 ), the controller  50  acquires a rotation speed of the motor  1  (step S 301 ). The rotation speed of the motor  1  is a frequency of an output current of the inverter  103 , and can be detected by using a current sensor or the like mounted on the motor  1 . 
     Next, it is determined whether switching of the coils  3  from the delta connection to the Y connection is necessary or not, based on this rotation speed of the motor  1 . That is, it is determined whether or not the connection state of the coils  3  is the delta connection and the rotation speed of the motor  1  is less than or equal to a threshold (reference value of the rotation speed) (step S 302 ). 
     In the case of the heating operation, the threshold used in step S 302  is preferably a value (more preferably an intermediate value) between the rotation speed N1 corresponding to the intermediate heating condition and the rotation speed N2 corresponding to the rated heating condition. In the case of the cooling operation, the threshold used in step S 302  is preferably a value (more preferably an intermediate value) between the rotation speed N1 corresponding to the intermediate cooling condition and the rotation speed N2 corresponding to the rated cooling condition. 
     For example, in the case of the household air conditioner having a refrigeration capacity of 6.3 kW, the rotation speed N1 corresponding to the intermediate heating condition is 35 rps and the rotation speed N2 corresponding to the rated heating condition is 85 rps, and therefore the threshold used in step S 302  is preferably 60 rps, which is an intermediate value between the rotation speed N1 and the rotation speed N2. 
     However, the rotation speed of the motor  1  may fluctuate. Thus, in this step S 302 , it is determined whether or not a state where the rotation speed of the motor  1  is greater than or equal to the threshold continues for a preset time. 
     If the result of the comparison in step S 302  indicates that the connection state of the coils  3  is the delta connection and the rotation speed of the motor  1  is less than or equal to the threshold, processes in steps S 121  through S 124  shown in  FIG. 11  are performed. As described in the first embodiment, in steps S 121  through S 124  in  FIG. 11 , the switching from the delta connection to the Y connection and the raising of the bus voltage of the converter  102  are performed. 
     If the result of the comparison in step S 302  indicates that that the connection state of the coils  3  is not the delta connection (if it is, is the Y connection) or that the rotation speed of the motor  1  is greater than the threshold (i.e., switching to the Y connection is unnecessary), the process proceeds to step S 303 . 
     In step S 303 , it is determined whether switching from the Y connection to the delta connection is necessary or not. Specifically, it is determined whether or not the connection state of the coils  3  is the Y connection and the rotation speed of the motor  1  is greater than the threshold. 
     If the result of the comparison in step S 303  indicates that the connection state of the coils  3  is the Y connection and the rotation speed of the motor  1  is greater than the threshold, processes in steps S 131  through S 134  shown in  FIG. 12  are performed. As described in the first embodiment, in steps S 131  through S 134  in  FIG. 12 , the switching from the Y connection to the delta connection and the lowering of the bus voltage of the converter  102  are performed. 
     If the result of the comparison in step S 303  indicates that the connection state of the coils  3  is not the Y connection (if it is the delta connection) or that the rotation speed of the motor  1  is greater than the threshold (i.e., if switching to the delta connection is unnecessary), the process returns to step S 105 . Processes in the case where the operation stop signal is received (steps S 109  through S 111 ) are similar to those in the first embodiment. The other operations and configurations in the fifth modification are similar to those in the first embodiment. 
     As described above, in the fifth modification, the connection state of the coils  3  and the bus voltage of the converter  102  are switched based on the rotation speed of the motor  1 , and therefore high motor efficiency and high motor torque can be obtained. 
     In this regard, the connection state of the coils  3  and the bus voltage of the converter  102  may be switched based on the rotation speed of the motor  1  in the first, second, and third modifications. 
     In this regard, although the rotary compressor  8  has been described as an example of the compressor, the motor of the embodiment may be applied to a compressor other than the rotary compressor  8 . 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. 
     (Configuration of Air Conditioner) 
       FIG. 37  is a block diagram illustrating a configuration of an air conditioner  500  according to the second embodiment.  FIG. 38  is a block diagram illustrating a control system of the air conditioner  500  according to the second embodiment.  FIG. 39  is a block diagram illustrating a control system of a driving device  100   a  according to the second embodiment. The air conditioner  500  according to the second embodiment further includes a compressor temperature sensor  71  as a compressor state detection unit. The compressor temperature sensor  71  is a temperature sensor for detecting a compressor temperature T C  indicating a state of the rotary compressor  8 . In this regard, the compressor state detection unit may be a detector capable of detecting the state of the rotary compressor  8 , and is not limited to the temperature sensor. 
     Except for the compressor temperature sensor  71 , configurations of the air conditioner  500  and the driving device  100   a  according to the second embodiment are the same as those of the air conditioner  5  and the driving device  100  according to the first embodiment, respectively. 
     In the example illustrated in  FIG. 39 , the driving device  100   a  includes a converter  102  for rectifying an output of a power source  101 , an inverter  103  for outputting an AC voltage to coils  3  of a motor  1 , a connection switching unit  60  for switching the connection state of the coils  3 , a controller  50 , and the compressor temperature sensor  71 . The converter  102  is supplied with electric power from the power source  101  as an alternating current (AC) power source. 
     The configuration of the driving device  100   a  according to the second embodiment is similar to that of the driving device  100  according to the first embodiment, except for the compressor temperature sensor  71 . In this regard, the compressor temperature sensor  71  may not be a component of the driving device  100   a . The driving device  100   a  is used together with the rotary compressor  8 , and drives the motor  1 . 
     A neodymium rare earth magnet containing neodymium-iron-boron (Nd—Fe—B) as a main component is used in a permanent magnet type motor, and has characteristics such that a coercive force decreases with temperature. When the motor using a neodymium rare earth magnet is used in a high-temperature atmosphere of 140° C. as in a compressor, the coercive force of the magnet decreases with temperature (−0.5 to −0.6%/ΔK), and therefore it is necessary to add a dysprosium (Dy) element to enhance the coercive force. 
     When the Dy element is added to the magnet, the coercive force is enhanced, but there is a disadvantage such that a residual flux density decreases. When the residual flux density decreases, a magnet torque of the motor decreases and a supply current increases, and accordingly a copper loss increases. Thus, there is a strong demand for reducing an adding amount of Dy in terms of efficiency. 
     For example, when the maximum temperature of the compressor while the compressor is driven is reduced, the maximum temperature of the magnets can be reduced, and demagnetization of the magnets can be alleviated. Thus, it is effective to control the compressor (for example, the rotation speed of the motor) based on a compressor temperature threshold as a threshold for limiting the temperature of the compressor. 
     However, if the compressor temperature threshold is set to be low, an instruction to reduce the rotation speed of the motor or an instruction to stop the motor may be issued in a state where the load (air-conditioning load) is low, depending on the set value. In this case, the maximum operation range of the motor is narrowed, and operation of the motor is restricted irrespective of the situation in the room (for example, the temperature difference ΔT described above) in which the air conditioner is provided. 
     Thus, in the second embodiment, the controller  50  issues an instruction to change a method for driving the motor  1  based on different thresholds (compressor temperature thresholds) depending on the connection state of the coils  3 . Specifically, if it is determined that the compressor temperature T C  detected by the compressor temperature sensor  71  is higher than the compressor temperature threshold, the controller  50  issues an instruction to change the driving method of the motor  1 . Accordingly, the temperature of the rotary compressor  8  is reduced, and the rotary compressor  8  is protected. 
     The compressor temperature sensor  71  detects a compressor temperature T C  indicating the state of the rotary compressor  8 . In this embodiment, the compressor temperature sensor  71  is fixed to a discharge pipe  85  of the rotary compressor  8 . However, the position to which the compressor temperature sensor  71  is fixed is not limited to the discharge pipe  85 . 
     The compressor temperature T C  is a temperature of at least one of a shell  80  of the rotary compressor  8 , the discharge pipe  85  (for example, an upper portion of the discharge pipe  85 ) of the rotary compressor  8 , refrigerant in the rotary compressor  8  (for example, refrigerant flowing in the discharge pipe  85 ), and the motor  1  disposed in the rotary compressor  8 . The compressor temperature T C  may be a temperature of an element other than these elements. 
     The compressor temperature T C  is, for example, a maximum temperature measured within a preset time period. A correlation between temperature data in the rotary compressor  8  measured in advance and the compressor temperature T C  may be stored in a memory in the controller  50  for each measurement target of the compressor temperature T C . The temperature data in the rotary compressor  8  measured in advance is data indicating a temperature (maximum temperature) in the rotary compressor  8  that varies depending on a circulation amount of refrigerant, a heat generation temperature of the motor  1 , and the like. In this case, the compressor temperature T C  detected by the compressor temperature sensor  71  may be used as a first detection value or a second detection value described later. The temperature data calculated based on the correlation with the compressor temperature T C  may be used as a first detection value or a second detection value described later. 
     When the connection state of the coils  3  is the first connection state (for example, the Y connection), the controller  50  controls the motor  1  based on the first detection value detected by the compressor temperature sensor  71  and a threshold T Y  (first threshold) as the compressor temperature threshold. The threshold T Y  is, for example, 90° C. In the case where a detector other than the temperature sensor is used as the compressor state detection unit, a value other than the temperature may be set as the threshold. 
     Specifically, if the first detection value is larger than the threshold T Y , the controller  50  controls the motor  1  such that at least one temperature detected by the compressor temperature sensor  71  (compressor temperature T C ) decreases. For example, the controller  50  issues an instruction to change the rotation speed of the motor  1  to thereby reduce the rotation speed of the motor  1 , or stops driving (rotation) of the motor  1 . Accordingly, the compressor temperature T C  can be reduced. 
     When the connection state of the coils  3  is the second connection state (for example, the delta connection), the controller  50  controls the motor  1  based on the second detection value detected by the compressor temperature sensor  71  and a threshold T Δ  (second threshold) as the compressor temperature threshold. 
     Specifically, if the second detection value is larger than the threshold T Δ , the controller  50  controls the motor  1  such that at least one temperature detected by the compressor temperature sensor  71  (compressor temperature T C ) decreases. For example, the controller  50  issues an instruction to change the rotation speed of the motor  1  to thereby reduce the rotation speed of the motor  1 , or stops driving (rotation) of the motor  1 . Accordingly, the compressor temperature T C  can be reduced. 
     The motor  1  is designed so that the magnets are not demagnetized at the maximum temperature (compressor temperature threshold) the magnets can reach, taking into consideration a temperature change due to heat generation of the motor  1 , a cooling effect by refrigerant, and the like. For example, in this embodiment, the permanent magnets  25  of the motor  1  are designed so that the magnets are not demagnetized at near 140° C., which is the maximum magnet temperature. In this case, the threshold TA is set to 140° C. 
     The compressor temperature threshold is set to be higher in the connection state in which the line voltage is lower, out of the connection states of the coils  3  switchable by the connection switching unit  60 . In this embodiment, the line voltage of the inverter  103  in the delta connection is lower than the line voltage of the inverter  103  in the Y connection. Thus, the threshold T Δ  is set to be larger than the threshold T Y . Accordingly, it is possible to prevent the maximum operation range of the motor  1  (especially the maximum rotation speed of the motor  1  in the delta connection) from being narrowed. 
     (Operation of Air Conditioner) 
     Next, a basic operation of the air conditioner  500  according to the second embodiment (method for controlling the motor  1 , the rotary compressor  8 , and the air conditioner  500 ) will be described. 
       FIG. 40  is a flowchart showing a basic operation of the air conditioner  500  according to the second embodiment. 
     Steps S 101  through S 105  are similar to those in the first embodiment ( FIG. 10 ). In a case where the operation stop signal is not received in step S 105 , the process proceeds to step S 401 . 
     In accordance with the temperature difference ΔT, the rotation speed of the motor  1  or the like, the connection switching unit  60  switches the connection state of the coils  3  between the delta connection (second connection state in this embodiment) and the Y connection (first connection state in this embodiment). 
     The compressor temperature sensor  71  detects the state of the rotary compressor  8  (step S 401 ). In this embodiment, the compressor temperature sensor  71  detects the compressor temperature T C  (for example, the temperature of the discharge pipe  85 ) indicating the state of the rotary compressor  8 . 
     In step S 401 , when the connection state of the coils  3  is the Y connection, the compressor temperature T C  is detected as a first detection value. In contrast, when the connection state of the coils  3  is the delta connection, the compressor temperature T C  is detected as a second detection value. 
     Further, the controller  50  determines whether or not the connection state of the coils  3  is the Y connection and the compressor temperature T C  is greater than the threshold T Y  (step S 402 ). 
     If the result of the comparison in S 402  indicates that the connection state of the coils  3  is the Y connection and the compressor temperature T C  is greater than the threshold T Y , the process proceeds to step S 404 . 
     If the result of the comparison in step S 402  indicates that the connection state of the coils  3  is not the Y connection (if it is the delta connection), or that the compressor temperature T C  is less than or equal to the threshold T Y , the process returns to step S 403 . 
     In step S 403 , the controller  50  determines whether or not the connection state of the coils  3  is the delta connection and the compressor temperature T C  is greater than the threshold T Δ . 
     If the result of the comparison in S 403  indicates that the connection state of the coils  3  is the delta connection and the compressor temperature T C  is greater than the threshold T Δ , the process proceeds to step S 404 . 
     If the result of the comparison in step S 403  indicates that the connection state of the coils  3  is not the delta connection (if it is the Y connection), or that the compressor temperature T C  is less than or equal to the threshold T Δ , the process returns to step S 105 . 
     In step S 404 , the controller  50  reduces the rotation speed of the motor  1 . In this regard, instead of reducing the rotation speed of the motor  1 , the motor  1  may be stopped. In the case where the motor  1  is stopped in step S 404 , the motor  1  is stopped without changing the connection state of the coils  3 . In the case where the motor  1  is stopped in step S 404 , the motor  1  is started after a lapse of a preset time and then the process returns to step S 105 , for example. 
     That is, in steps S 401  through S 404 , when the connection state of the coils  3  is the Y connection, the motor  1  is controlled based on the first detection value and the first threshold (threshold T Y ), whereas when the connection state of the coils  3  is the delta connection, the motor  1  is controlled based on the second detection value and the second threshold (threshold T Δ ). Accordingly, the rotary compressor  8  can be controlled such that the compressor temperature T C  is lower than the threshold T Y  or the threshold T Δ . 
     If the operation stop signal is received in step S 105  described above, the controller  50  stops rotation of the motor  1  (step S 109 ). In this regard, if the operation stop signal is received in a state where the motor  1  is stopped in step S 404 , the process proceeds to step S 110  while the motor  1  is stopped. In this regard, although not shown in  FIG. 40 , if the operation stop signal is received during a period between steps S 401  through S 404 , the process proceeds to step S 109  and rotation of the motor  1  is stopped. 
     Thereafter, the controller  50  performs a process of stopping the air conditioner  500  (step S 110 ). Specifically, the fan motors of the indoor fan  47  and the outdoor fan  46  are stopped. Thereafter, the CPU  57  of the controller  50  is stopped, and operation of the air conditioner  500  is terminated. 
     In the case where the process of stopping the air conditioner  500  is performed in step S 110 , the connection state of the coils  3  is preferably the delta connection. For example, in step S 110 , if the connection state of the coils  3  is the Y connection, the controller  50  outputs the connection switching signal to the connection switching unit  60  to switch the connection state of the coils  3  from the Y connection to the delta connection. 
     (Advantages of Second Embodiment) 
     According to the second embodiment, the motor  1  is controlled by using the compressor temperature threshold in consideration of the connection state of the coils  3 . For example, if the detected value detected by the compressor temperature sensor  71  is greater than the compressor temperature threshold, the motor  1  is controlled such that the compressor temperature T C  (i.e., the temperature in the rotary compressor  8 ) decreases. As a result, demagnetization in the motor  1  can be prevented, and the motor  1  can be appropriately controlled in accordance with the state of the rotary compressor  8 . 
     As described in the first embodiment, in the driving device that operates while switching the connection state of the coils  3  between the Y connection and the delta connection, a conventional high-speed operation is performed with the delta connection, whereas a low-speed operation for a small air-conditioning load is performed with the Y connection. Thus, by switching the connection state of the coils  3  from the delta connection to the Y connection, the maximum temperature of the rotary compressor  8  (the maximum value of the compressor temperature T C ) in performing a normal load operation can be set in such a manner that the maximum temperature of the rotary compressor  8  during the operation in the Y connection state is lower than that during the operation in the delta connection state. 
     For example, in a case where the motor  1  is controlled based on one preset compressor temperature threshold (for example, the same value as the threshold T Y ) without taking the connection state of the coil  3  into consideration, there is a case where the maximum operating range of the motor  1  (especially, the maximum rotation speed of the motor  1  in the delta connection state) may be narrowed. Thus, in the second embodiment, the motor  1  is controlled by using a plurality of compressor temperature thresholds by taking the connection state of the coils  3  into consideration. 
     Specifically, the motor  1  is controlled based on different compressor temperature thresholds (for example, threshold T Y  and threshold T Δ ) depending on the connection states of the coils  3 . Thus, even if the compressor temperature threshold is set lower during the operation in the Y connection state than during the operation in the delta connection state, the maximum operation range of the motor  1  (especially, the maximum rotation speed of the motor  1  in the delta connection) can be prevented from being narrowed. 
     For example, in the configuration in which the connection state of the coils  3  is switched as described in the first embodiment, when the connection state of the coils  3  is the Y connection and the rotation speed of the motor  1  is low (intermediate heating condition), the line voltage (motor voltage) is approximately equal to the inverter maximum output voltage, so that the efficiency is improved. In this case, there is a case where it is desired to rotate the motor  1  at a rotation speed as high as possible in order to reduce the number of times of connection switching. For this reason, operation is performed using field-weakening. However, the weakening current increases, and demagnetization deteriorates. 
     As the temperature is lower, the coercive force of the permanent magnet  25  is higher and the permanent magnet  25  is less likely to be demagnetized even when the current is increased. Thus, according to the second embodiment, the compressor temperature threshold T Y  when the connection state of the coils  3  is the Y connection is set to be lower than the compressor temperature threshold T Δ  in the delta connection. Thus, the maximum temperature of the rotary compressor  8  during the operation in the Y connection state can be lower than that during the operation in the delta connection state. Accordingly, a configuration in which demagnetization does not occur even when the weakening current increases is achieved, and it becomes possible to perform driving in the Y connection state at a higher rotation speed. Thus, there is an advantage such that flexibility in switching the connection can be increased. In other words, the motor  1  can be driven in a state where the coercive force of the magnets of the motor  1  is high, and demagnetization is less likely to occur even when a larger current flows in the motor  1 . Furthermore, when the connection state of the coils  3  is the Y connection, the motor  1  can be driven at a higher rotation speed using field-weakening. 
     Further, in the case where the connection of the coils is switched to the delta connection from the Y connection in which the winding number (number of turns) is close to the number of turns of conventional coils that are not subjected to connection switching, field-weakening at a high rotation speed can be suppressed, and a configuration resistant to demagnetization in the delta connection as compared to a conventional configuration can be obtained. 
     Furthermore, in the Y connection, since the compressor temperature threshold can be set lower than that of the conventional configuration, demagnetization characteristics can be enhanced in both of the Y connection and the delta connection, and the magnet to which no dysprosium (Dy) is added can be used. 
     For example, as the permanent magnet  25 , a rare earth magnet containing neodymium (Nd), iron (Fe), and boron (B) as main components can be used, and this permanent magnet  25  does not contain dysprosium (Dy) as an additive for increasing the coercive force. In this case, the permanent magnet  25  has the residual magnetic flux density of 1.36 T to 1.42 T, the coercive force of 1671 kA/m to 1989 kA/m, and the maximum energy product of 354 kJ/m 3  to 398 kJ/m 3 . 
     Modification of Second Embodiment 
     Next, a modification of the second embodiment of the present invention will be described. The second embodiment can be combined with the first embodiment (including the modifications thereof). Thus, in the modification of the second embodiment, another example of operation of the air conditioner described in the second embodiment (method for controlling the motor  1 , the rotary compressor  8 , and the air conditioner  500 ) will be described. A configuration of an air conditioner according to the modification of the second embodiment is the same as that of the air conditioner  500  of the second embodiment. Thus, the air conditioner of the modification of the second embodiment will be referred to as the air conditioner  500 . 
       FIG. 41  is a flowchart showing a basic operation of the air conditioner  500  according to the modification of the second embodiment. 
     Steps S 101  through S 106  are similar to those in the first embodiment ( FIG. 10 ). 
     In step S 107 , the controller  50  determines whether switching of the connection state of the coils  3  from the delta connection to the Y connection is necessary or not based on the temperature difference ΔT between the room temperature Ta detected by the room temperature sensor  54  and the set temperature Ts set by the remote controller  55 . That is, it is determined whether or not the connection state of the coils  3  is the delta connection and the absolute value of the temperature difference ΔT is less than or equal to the threshold ΔTr (step S 107 ). 
     If the result of the comparison in step S 107  indicates that the connection state of the coils  3  is the delta connection and the absolute value of the temperature difference ΔT is less than or equal to the threshold ΔTr, the process proceeds to step S 121  ( FIG. 11 ). 
     If the result of the comparison in step S 107  indicates that the connection state of the coils  3  is not the delta connection (if it is the Y connection), or that the absolute value of the temperature difference ΔT is larger than the threshold ΔTr (i.e., switching to the Y connection is unnecessary), the process proceeds to step S 108 . 
     In step S 108 , it is determined whether switching from the Y connection to the delta connection of the coils  3  is necessary or not. For example, similarly to the first embodiment (step S 108 ), the controller  50  determines whether or not the connection state of the coils  3  is the Y connection and the absolute value of the temperature difference ΔT described above is larger than the threshold ΔTr. 
     If the result of the comparison in step S 108  indicates that the connection state of the coils  3  is the Y connection and the absolute value of the temperature difference ΔT is larger than the threshold ΔTr, the process proceeds to step S 131  ( FIG. 12 ). In the modification of the second embodiment, the processes in steps S 131  through S 134  shown in  FIG. 12  may be replaced by the processes (steps S 135 , S 136 , and S 131  through S 134 ) shown in  FIG. 13(A)  or the processes (steps S 137 , S 138 , and S 131  through S 134 ) shown in  FIG. 13(B) . 
     The processes in steps S 106  through S 108  shown in  FIG. 41  may be replaced by the processes in the modifications of the first embodiment (e.g., steps S 201  through S 204  shown in  FIG. 35  or steps S 301  through S 303  shown in  FIG. 36 ). 
     If the result of the comparison in step S 108  indicates that the connection state of the coils  3  is not the Y connection (if it is the delta connection), or that the absolute value of the temperature difference ΔT is less than or equal to the threshold ΔTr (i.e., if switching to the delta connection is unnecessary), the process proceeds to step S 401 . 
     Steps S 401  through S 404  are similar to those in the second embodiment ( FIG. 40 ). 
     If the operation stop signal is received in step S 105  described above, the controller  50  stops rotation of the motor  1  (step S 109 ). If the operation stop signal is received in a state where the motor  1  stopped in step S 404 , the process proceeds to step S 110  in a state where the motor  1  is stopped. Although not shown in  FIG. 41 , if the operation stop signal is received in the steps S 105  through S 108  or the steps S 401  through S 404 , the process proceeds to step S 109  and rotation of the motor  1  is stopped. 
     Thereafter, the controller  50  (specifically, the connection switching unit  60 ) switches the connection state of the coils  3  from the Y connection to the delta connection (step S 110 ). If the connection state of the coils  3  is already the delta connection, the connection state is unchanged. 
     Step S 111  is similar to that in the first embodiment ( FIG. 10 ). 
     The modification of the second embodiment has the same advantages as those described in the first embodiment (including the modifications thereof) and the second embodiment. 
     Features of the embodiments and the modifications described above can be combined as appropriate. 
     Although the preferred embodiments of the present invention have been specifically described above, the present invention is not limited to the above described embodiments, and various modifications or variations can be made without departing from the gist of the present invention.