Patent Publication Number: US-2022235957-A1

Title: Outdoor unit, air conditioner, and operation control method for air conditioner

Description:
Cross Reference to Related Application 
     This application is a U.S. national stage application of International Patent Application No. PCT/JP 2019 / 019882  filed on May  20 ,  2019 , the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an outdoor unit, an air conditioner, and an operation control method for the air conditioner. 
     BACKGROUND 
     A motor is used as a drive source of a fan in an outdoor unit of an air conditioner. In recent years, it has been proposed to switch the connection state of coils of the motor between a Y- connection and a delta-connection (see, for example, Patent Reference 1). 
     PATENT REFERENCE 
     [PATENT REFERENCE  1 ] Japanese Patent Application Publication No.  2018 - 057114  (see  FIG. 1 ) 
     In the outdoor unit, the outside wind may cause the fan to rotate even when the fan is not driven. When the outside wind causes the fan to rotate, the motor acts as a generator, so that an induced voltage is generated. For this reason, the number of turns of the coils in the motor is restricted so that the induced voltage at this time does not exceed a withstand voltage of a drive circuit of the motor. Meanwhile, in order to improve the operation efficiency of the motor, it is necessary to increase the number of turns of the coils. Thus, if the number of turns of the coils is restricted as described above, it may hinder the improvement in the operation efficiency of the motor. 
     Further, when the outside temperature is low, there occurs a phenomenon called frost formation, in which moisture in the air adheres to fins of a heat exchanger provided in the outdoor unit and then freezes. When frost formation occurs, a ventilation resistance of the fins increases, and the operation capacity of the heat exchanger decreases. Therefore, when the frost formation occurs, it is necessary to increase a rotation speed of the motor to thereby increase a blowing air volume. 
     However, a motor voltage cannot exceed the maximum voltage defined by a voltage of a commercial power source. When the number of turns of coils is increased to improve the operation efficiency of the motor, the motor voltage is more likely to reach the maximum voltage, and thus it is difficult to increase the blowing air volume in case of frost formation. 
     SUMMARY 
     The present invention is intended to solve the above-described problem, and an object of the present invention is to improve the operation efficiency of a motor, to suppress an increase in an induced voltage due to the outside wind, and to enable an increase in the blowing air volume in case of frost formation. 
     An outdoor unit of the present invention includes a heat exchanger, a fan having a motor having coils, the fan blowing air to the heat exchanger, a connection switching unit to switch a connection state of the coils between a first connection state and a second connection state in which a line voltage is lower than a line voltage in the first connection state, and a temperature sensor to detect a temperature. When the motor is not driven, the connection switching unit sets the connection state of the coils to the second connection state. When the motor rotates, the connection switching unit sets the connection state of the coils to the first connection state if a detected temperature by the temperature sensor is higher than or equal to a threshold, and the connection switching unit sets the connection state of the coils to the second connection state if the detected temperature is lower than the threshold. 
     According to the present invention, since the connection state of the coils is the second connection state when the motor is not driven, an increase in the induced voltage when the outside wind causes the motor to rotate is suppressed. Thus, the number of turns of the coils can be increased, and the operation efficiency of the motor can be improved. Since the connection state of the coils is switched based on the comparison result between the detected temperature by the temperature sensor and the threshold, the blowing air volume can be increased while the increase in the induced voltage can be suppressed at a temperature at which frost formation is more likely to occur, whereas the operation efficiency of the motor can be improved at a temperature at which frost formation is less likely to occur. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view illustrating a motor of a first embodiment. 
         FIG. 2  is a schematic diagram illustrating an air conditioner of the first embodiment. 
         FIG. 3(A)  is a diagram illustrating the air conditioner of the first embodiment, and  FIG. 3(B)  is a sectional view illustrating an outdoor fan. 
         FIG. 4  is a schematic diagram illustrating a part of a heat exchanger of an outdoor unit of the first embodiment. 
         FIG. 5  is a block diagram illustrating the motor and an inverter of the first embodiment. 
         FIG. 6  is a block diagram illustrating a drive device that drives the motor of the first embodiment. 
         FIG. 7  is a block diagram illustrating the drive device that drives the motor of the first embodiment. 
         FIGS. 8(A) and 8(B)  are schematic diagrams illustrating connection states of coils of the first embodiment. 
         FIG. 9  is a flowchart illustrating an operation of the air conditioner of the first embodiment. 
         FIG. 10  is a block diagram illustrating a drive device that drives a motor of a comparative example. 
         FIG. 11  is a graph illustrating a relationship between a rotation speed of the motor of the comparative example due to the outside wind and an induced voltage. 
         FIG. 12  is a graph illustrating the relationship between the rotation speed of the motor of the first embodiment due to the outside wind and the induced voltage. 
         FIG. 13  is a flowchart illustrating an operation of an air conditioner of a modification of the first embodiment. 
         FIG. 14  is a schematic diagram illustrating an example of a display screen of a remote controller in an air conditioner of a second embodiment. 
         FIG. 15  is a schematic diagram illustrating an example of the display screen of the remote controller in the air conditioner of the second embodiment. 
         FIG. 16  is a schematic diagram illustrating an example of the display screen of the remote controller in the air conditioner of the second embodiment. 
         FIGS. 17(A) and 17(B)  are schematic diagrams illustrating a switching operation of the connection state of coils of a modification. 
         FIGS. 18(A) and 18(B)  are schematic diagrams illustrating another example of the switching operation of the connection state of the coils. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described in detail below with reference to the figures. The present invention is not limited to these embodiments. 
     FIRST EMBODIMENT (Configuration of Motor) 
       FIG. 1  is a sectional view illustrating a motor  1  used in an outdoor unit of an air conditioner of a first embodiment. 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 to 1.0 mm, is famed between the stator  10  and the rotor  20 . 
     Hereinafter, an axis that defines a rotational axis of the rotor  20  is referred to as an axis Cl. A direction of the axis Cl is referred to as an “axial direction”. A circumferential direction about the axis Cl is referred to as a “circumferential direction”. A radial direction about the axis Cl is referred to as a “radial direction”.  FIG. 1  is a sectional view in a plane perpendicular to the axis Cl. 
     The stator  10  includes a stator core  11  and coils  3  wound around the stator core  11 . The stator core  11  is formed of a plurality of electromagnetic steel sheets, each having a thickness of 0.2 to 0.5 mm, which are stacked in the axial direction and fastened by crimping or the like. 
     The stator core  11  has an annular yoke  12  and a plurality of teeth  13  protruding inward in the radial direction from the yoke  12 . The number of teeth  13  is twelve in this example. The number of teeth  13  is not limited to twelve, and only needs to be two or more. Each tooth  13  has a tooth tip portion at its tip on the inner side in the radial direction, and the tooth tip portion is wide in the circumferential direction. 
     The coils  3  are wound around the teeth  13  via insulators (not shown). Each coil  3  is formed of a conductive wire made of aluminum or copper, for example, and is wound around the tooth  13  in concentrated winding. More specifically, the coil  3  is formed of a magnet wire having a wire diameter (diameter) of 0.2 to 0.5 mm. 
     The coils  3  are formed of a U-phase coil  3 U, a V-phase coil  3 V, and a W-phase coil  3 W. Both terminals of each of the coils  3 U,  3 V, and  3 W are open. In other words, the coils  3  have six terminals in total. The connection state of the coils  3  can be switched between a Y-connection and a delta-connection, as described below. 
     The rotor  20  includes a cylindrical rotor core  21 , a shaft  22  attached to a center of the rotor core  21 , and permanent magnets  23   a  and  23   b  attached to an outer circumference of the rotor core  21 . 
     The rotor core  21  is famed of a plurality of electromagnetic steel sheets, each having a thickness of 0.2 to    0 . 5   mm, which are stacked in the axial direction and fastened by crimping or the like. A shaft hole to which the shaft  22  is fixed is formed at the center of the rotor core  21  in the radial direction. The shaft  22  is fixed to the shaft hole by shrink-fitting, press-fitting, or the like. 
     The permanent magnets  23   a  and  23   b  are alternately arranged in the circumferential direction. Here, the permanent magnets  23   a  are N-poles, and the permanent magnets  23   b  are S-poles. There are five permanent magnets  23   a  and five permanent magnets  23   b , and therefore the number of poles is 10. In this regard, the number of poles is not limited to 10. Each of the permanent magnets  23   a  and  23   b  is made of a rare earth magnet that contains, for example, neodymium (Nd), iron (Fe), and boron (B). 
     The motor in which the permanent magnets  23   a  and  23   b  are fixed to the surface of the rotor core  21  is referred to as a Surface Permanent Magnet (SPM) type motor. However, the motor  1  is not limited to the SPM type, but may be an Inner Permanent Magnet (IPM) type motor in which permanent magnets  23  are embedded in magnet insertion holes of the rotor core  21 . 
     (Configuration of Air Conditioner) 
       FIG. 2  is a schematic diagram illustrating an air conditioner  5  of the first embodiment. The air conditioner  5  includes an outdoor unit  5 A installed outdoors and an indoor unit  5 B installed in a room to be air-conditioned. The outdoor unit  5 A and the indoor unit  5 B are connected by a refrigerant pipe  50 . 
     The air conditioner  5  includes a compressor  51  that compresses and discharges a refrigerant, an indoor heat exchanger  52  that exchanges heat between the refrigerant and indoor air, an expansion valve  53  as a decompressor that depressurizes the refrigerant, and an outdoor heat exchanger  6  as a heat exchanger that exchanges heat between the refrigerant and outdoor air. The compressor  51 , the indoor heat exchanger  52 , the expansion valve  53 , and the outdoor heat exchanger  6  are connected by the refrigerant pipe  50  to constitute a refrigerant circuit. 
     In a heating operation, the refrigerant discharged from the compressor  51  flows through the indoor heat exchanger  52 , the expansion valve  53 , and the outdoor heat exchanger  6  in this order. In a cooling operation, the flow path is switched to the opposite direction by a refrigerant flow path switching valve (four-way valve) not illustrated. 
     The outdoor unit  5 A includes the compressor  51  and the outdoor heat exchanger  6  which are described above, and an outdoor fan  4  as a fan. The outdoor fan  4  blows air to the outdoor heat exchanger  6 . The outdoor unit  5 A further includes a controller  110  that controls the operation of the air conditioner  5 . 
     The indoor unit  5 B includes the indoor heat exchanger  52  described above and an indoor fan  8 . The indoor fan  8  supplies air subjected to heat exchange in the indoor heat exchanger  52 , to the inside of the room. The indoor unit  5 B further includes a signal receiver  108  that receives a signal transmitted from a remote controller  9  as an operating device. 
     In the heating operation, a high-temperature and high-pressure gas refrigerant discharged from the compressor  51  flows through the refrigerant pipe  50  and flows into the indoor heat exchanger  52  of the indoor unit  5 B. The indoor heat exchanger  52  operates as a condenser. When the air blown by the indoor fan  8  passes through the indoor heat exchanger  52 , condensation heat is transmitted from the refrigerant to the air by heat exchange, and the heated air is supplied into the inside of the room. The refrigerant is condensed into a high-pressure and low-temperature liquid refrigerant, and flows into the expansion valve  53  of the outdoor unit  5 A through the refrigerant pipe  50 . The refrigerant expands adiabatically in the expansion valve  53  to be a low-pressure and low-temperature, two-phase refrigerant. 
     The refrigerant passing through the expansion valve  53  flows into the outdoor heat exchanger  6 . The outdoor heat exchanger  6  operates as an evaporator. When the air blown by the outdoor fan  4  passes through the outdoor heat exchanger  6 , evaporative heat is transmitted from the air to the refrigerant by heat exchange. The refrigerant evaporates to be a low-temperature and low-pressure gas refrigerant, and is then compressed by the compressor  51  again into a high-temperature and high-pressure refrigerant. 
       FIG. 3(A)  is a schematic diagram illustrating components of the outdoor unit  5 A and the indoor unit  5 B. The outdoor fan  4  of the outdoor unit  5 A includes an impeller  41  and the motor  1  that rotates the impeller  41 . The motor  1  has the configuration described with reference to  FIG. 1 . The impeller  41  is fixed to the shaft  22  of the motor  1  by a hub  42 . 
       FIG. 3(B)  is a sectional view illustrating the outdoor unit  5 A. The outdoor heat exchanger  6  is disposed to face the outdoor fan  4  in the axial direction of the motor  1 . When the outdoor fan  4  blows air, an airflow passing through the outdoor heat exchanger  6  is generated. The outdoor heat exchanger  6  does not necessarily face the outdoor fan  4 , but only needs to be in an air passage of the outdoor fan  4 . 
     The outdoor unit  5 A also has a fin temperature sensor  65  serving as a temperature sensor for measuring a surface temperature of fins  61  ( FIG. 4 ) of the outdoor heat exchanger  6 . The fin temperature sensor  65  is formed of a thermistor, for example. However, other sensors than the thermistor may be used. 
     The indoor fan  8  of the indoor unit  5 B includes an impeller  82  and a motor  81  that rotates the impeller  82 . The impeller  82  is composed of, for example, a cross flow fan. The motor  81  may have the same configuration as the motor  1  ( FIG. 1 ) of the outdoor fan  4 , but is not limited thereto. When the indoor fan  8  blows air, an airflow passing through the indoor heat exchanger  52  ( FIG. 2 ) is generated. 
       FIG. 4  is an enlarged schematic diagram illustrating a part of the outdoor heat exchanger  6 . The outdoor heat exchanger  6  is, for example, a fin-and-tube heat exchanger. Specifically, the outdoor heat exchanger  6  has a plurality of fins  61  that are elongated in the vertical direction and are arranged at equal intervals in the horizontal direction in  FIG. 4 . Each fin  61  has a flat-plate shape and is famed of a metal such as aluminum. 
     Each fin  61  has a plurality of holes famed at equal intervals in the longitudinal direction, and heat transfer tubes  62  are inserted into the holes. The heat transfer tubes  62  are elongated in the direction perpendicular to the longitudinal direction of the fins  61  and are arranged at equal intervals in the direction perpendicular to the arrangement direction of the fins  61 . Each heat transfer tube  62  is a tube in which a flow path  62   a  for a refrigerant is formed. The heat transfer tube  62  is formed of a metal such as aluminum. The fins  61  and the heat transfer tubes  62  are desirably arranged in a plane perpendicular to the axial direction of the motor  1 . 
     Heat exchange is carried out between the air flowing through between adjacent fins  61  and the refrigerant flowing in the heat transfer tubes  62 . The numbers of fins  61  and heat transfer tubes  62  are determined according to the capacity required for the outdoor heat exchanger  6 . The configuration of the fins  61  illustrated in  FIG. 4  is merely an example, and any fins having other configurations may be used. 
     (Configuration of Drive Device) 
       FIG. 5  is a block diagram illustrating the motor  1  and an inverter  103  that drives the motor  1 . The inverter  103  converts an input voltage V, which is a DC voltage, into an AC voltage, and outputs the AC voltage to the coils  3 U,  3 V, and  3 W of the motor  1 . 
     The inverter  103  has an upper arm  131  and a lower arm  132 . Each of the upper arm  131  and the lower arm  132  has U-phase, V- phase, and W-phase switching elements. These U-, V-, and W-phase switching elements are subjected to Pulse Width Modulation (PWM) control by a control signal from the controller  110  ( FIG. 6 ). 
       FIG. 6  is a block diagram illustrating a drive device  100  including the above-described inverter  103 . The drive device  100  includes a converter  102  that rectifies an output from a power source  101 , the inverter  103  that outputs an AC voltage to the coils  3  of the motor  1 , a connection switching unit  7  that switches the connection state of the coils  3 , and the controller  110 . 
     The power source  101  is an AC power source of, for example,  200  V (effective voltage), and specifically is, for example, a commercial power source. The converter  102  is a rectifier circuit that converts the AC voltage supplied from the power source  101  into a DC voltage. The voltage output from the converter  102  is also referred to as a bus voltage. 
     The inverter  103  has the configuration described with reference to  FIG. 5 . 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 . The inverter  103  is connected to wirings  104 ,  105 , and  106  that are connected to the coils  3 U,  3 V, and  3 W, respectively. 
     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 wiring  104  is connected to the terminal  31 U of the coil  3 U. The wiring  105  is connected to the terminal  31 V of the coil  3 V. The wiring  106  is connected to the terminal  31 W of the coil  3 W. 
     The connection switching unit  7  has switching elements  71 ,  72 , and  73 . The switching element  71  connects the terminal  32 U of the coil  3 U to either the wiring  105  or a neutral point  33 . The switching element  72  connects the terminal  32 V of the coil  3 V to either the wiring  106  or the neutral point  33 . The switching element  73  connects the terminal  32 W of the coil  3 W to either the wiring  104  or the neutral point  33 . Each of the switching elements  71 ,  72 , and  73  are configured as a mechanical switch, i.e., a relay contact in this example. However, each of the switching elements  71 ,  72 , and  73  may also be configured as a semiconductor switch. 
     The controller  110  controls the converter  102 , the inverter  103 , and the connection switching unit  7 . An operation instruction signal from the remote controller  9  received by the signal receiver  108  of the indoor unit  5 B, information about a surface temperature of the fins  61  detected by the fin temperature sensor  65 , and an indoor temperature detected by an indoor temperature sensor are input to the controller  110 . 
     The remote controller  9  includes a display  91  and an operation section  92 . The display  91  is, for example, a liquid crystal display, and displays an operation state of the air conditioner  5  or a menu screen. The operation section  92  is, for example, keys or the like, with which start and stop of the air conditioner  5 , switching between operation modes, and setting of operation contents are performed. The operation modes include, for example, the heating operation and the cooling operation. The operation contents include, for example, a set temperature and a wind speed. 
     Based on these input information, the controller  110  outputs an inverter drive signal to the inverter  103  and a connection switching signal to the connection switching unit  7 . 
     In the state illustrated in  FIG. 6 , the switching element  71  connects the terminal  32 U of the coil  3 U to the neutral point  33 , the switching element  72  connects the terminal  32 V of the coil  3 V to the neutral point  33 , and the switching element  73  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 , while 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 of the drive device  100  in which the switching elements  71 ,  72 , and  73  of the connection switching unit  7  are switched. In the state illustrated in  FIG. 7 , the switching element  71  connects the terminal  32 U of the coil  3 U to the wiring  105 , the switching element  72  connects the terminal  32 V of the coil  3 V to the wiring  106 , and the switching element  73  connects the terminal  32 W of the coil  3 W to the wiring  104 . 
       FIG. 8(A)  is a schematic diagram illustrating the connection state of the coils  3 U,  3 V, and  3 W when the switching elements  71 ,  72 , and  73  are in the state illustrated 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. Therefore, 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 the connection state of the coils  3 U,  3 V, and  3 W when the switching elements  71 ,  72 , and  73  are in the state illustrated 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 wiring  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 wiring  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 wiring  104  ( FIG. 7 ). Therefore, the connection state of the coils  3 U,  3 V, and  3 W is the delta- connection (triangle connection). 
     The line voltage generated when the connection state of the coils  3  is the Y-connection is higher than the line voltage generated when the connection state of the coils  3  is the delta- connection, on the assumption that the rotation speed of the motor  1  is the same. The Y-connection corresponds to a first connection state, while the delta-connection corresponds to a second connection state. The connection switching unit  7  switches the connection state of the coils  3 U,  3 V, and  3 W of the motor  1  between the Y-connection as the first connection state and the delta- connection as the second connection state by switching the switching elements  71 ,  72 , and  73 . 
     As illustrated in  FIG. 1 , the motor  1  has twelve teeth  13 , and each of the coils  3 U,  3 V, and  3 W is wound around four teeth  13 . That is, the coil  3 U illustrated in  FIGS. 8(A) and 8(B)  is famed of U-phase coil portions wound around four teeth  13  and connected in series. Similarly, the coil  3 V is famed of V-phase coil portions wound around four teeth  13  and connected in series. The coil  3 W is formed of W-phase coil portions wound around four teeth  13  and connected in series. 
     (Operation of Air Conditioner) 
       FIG. 9  is a flowchart illustrating the operation of the air conditioner  5  with a focus on the operation of the outdoor unit  5 A. The controller  110  of the air conditioner  5  starts the operation when the controller  110  receives a start signal from the remote controller  9  through the signal receiver  108  (step S 101 ). At this stage, the connection state of the coils  3  of the motor  1  in the outdoor fan  4  is the delta-connection as described below. 
     The controller  110  acquires a surface temperature of the fins  61  (hereinafter referred to as a detected temperature Tf) detected by the fin temperature sensor  65  of the outdoor unit  5 A, and determines whether the detected temperature Tf is higher than or equal to a threshold T (step S 102 ). The threshold T is a temperature at which frost formation of the fins  61  may occur, and is, for example, 0° C. 
     If the detected temperature Tf is higher than or equal to the threshold T, the connection state of the coils  3  of the motor  1  is switched from the delta-connection to the Y-connection (step S 103 ). If the detected temperature Tf is lower than a threshold T, the connection state of the coils  3  of the motor  1  is maintained as the delta-connection. 
     Thereafter, the controller  110  drives the motor  1  to cause the outdoor fan  4  to start blowing air (step S 104 ). Simultaneously, the controller  110  drives the motor  81  of the indoor fan  8  and a motor of the compressor  51 . 
     Thus, as described with reference to  FIG. 2 , the refrigerant is discharged from the compressor  51  and then flows through the indoor heat exchanger  52 , the expansion valve  53 , and the outdoor heat exchanger  6  in this order. The outdoor heat exchanger  6  is supplied with the air blown from the outdoor fan  4 , while the indoor heat exchanger  52  is supplied with the air blown from the indoor fan  8 . 
     Each of the motor  1  of the outdoor fan  4  and the motor  81  of the indoor fan  8  rotates at the rotation speed previously set. The controller  110  controls the rotation speed of the motor of the compressor  51  according to a temperature difference between the indoor temperature and the set temperature, but the description thereof is omitted. 
     When the signal receiver  108  receives a stop signal from the remote controller  9  (step S 105 ), the controller  110  outputs a stop signal to the inverter  103  (step S 106 ). 
     When the current flowing through the coils  3  of the motor  1  decreases to zero, the motor  1  stops or is brought into a free-run state. When the controller  110  detects that the current is zero (step S 107 ), the controller  110  determines whether the connection state of the coils  3  of the motor  1  is the Y-connection or the delta-connection (step S 108 ). 
     When the connection state of the coils  3  of the motor  1  is the Y-connection, the controller  110  switches the connection state of the coils  3  to the delta-connection by the connection switching unit  7  (step S 109 ). When the connection state of the coils  3  of the motor  1  is the delta-connection, the controller  110  maintains the connection state of the coils  3  as the delta-connection. Thus, in a state where the motor  1  completely stops, the connection state of the coils  3  is the delta-connection. 
     When the controller  110  stops the motor  1  in step  5106 , the controller  110  also stops the motor  81  of the indoor fan  8  and the motor of the compressor  51 . Thus, the circulation of the refrigerant in the refrigerant pipe  50  and the blowing of air to the outdoor heat exchanger  6  and the indoor heat exchanger  52  are stopped. 
     (Action of Air Conditioner) 
     Next, the action of the air conditioner according to the first embodiment will be described. In general, as the number of turns of the coils  3  of the stator  10  is increased, less current is needed to obtain the required torque. Consequently, the loss due to the energization of the inverter  103  is reduced, and the operation efficiency of the motor  1  is improved. Furthermore, as the number of turns of the coils  3  is increased, the inductance is improved and the iron loss due to carrier harmonics is reduced, so that the motor efficiency is improved. 
     In the case of the motor  1  used for the outdoor fan  4 , if the outside wind causes the impeller  41  to rotate when the motor  1  is not driven, the motor  1  serves as a generator to thereby generate an induced voltage proportional to the rotation speed and the number of turns of the coils  3 . Thus, the number of turns of the coils  3  is restricted so that the induced voltage generated by the outside wind does not exceed a withstand voltage of the drive circuit such as the inverter  103 . Therefore, an issue is to achieve both the above-described improvement in the operation efficiency of the motor  1  and the suppression of the induced voltage due to the outside wind. 
     In an environment where the outside temperature is low, there occurs a phenomenon called frost formation, in which moisture in air adheres to the fins  61  of the outdoor heat exchanger  6  and then freezes. When frost formation occurs, the ventilation resistance of the fins  61  increases, and the operation capacity of the outdoor heat exchanger  6  decreases. In order to compensate for the decrease in the operation capacity, it is necessary to increase the rotation speed of the motor  1 . 
     However, if the rotation speed of the motor  1  is increased, the induced voltage increases in proportion to the rotation speed. This increases the motor voltage (line voltage) which is dominated by the induced voltage. The motor voltage cannot exceed the input voltage V ( FIG. 5 ) input to the inverter  103 . The input voltage V is determined by the voltage of the power source  101  (the commercial power source), and thus the maximum rotation speed of the motor  1  is restricted. Since the motor voltage is proportional to the number of turns of the coils  3 , the maximum rotation speed of the motor  1  decreases as the number of turns of the coils  3  increases. Therefore, an issue is to achieve both the above-described improvement in the operation efficiency of the motor  1  and the suppression of decrease in the operation efficiency of the outdoor heat exchanger  6  due to frost formation. 
     Here, a drive device  100 A of the motor  1  in an outdoor fan of a comparative example will be described.  FIG. 10  is a block diagram illustrating the drive device  100 A of the motor  1  in the outdoor fan of the comparative example. 
     The drive device  100 A of the motor  1  in the outdoor fan of the comparative example includes the converter  102 , the inverter  103 , the signal receiver  108 , and the controller  110 . The drive device  100 A of the comparative example does not includes the connection switching unit  7  ( FIG. 6 ), and the connection state of the coils  3  of the motor  1  is fixed to the Y-connection. 
       FIG. 11  is a graph illustrating a relationship between the rotation speed of the motor  1  rotated by the outside wind and the induced voltage generated in the coils  3  of the motor  1  in the outdoor fan of the comparative example. On the assumption that the number of turns of the coils  3  is the same, the induced voltage increases in proportion to the rotation speed of the motor  1 , i.e., the wind speed of the outside wind. Therefore, it is necessary to restrict the number of turns of the coils  3  so that the induced voltage does not reach the withstand voltage for the expected wind speed. 
       FIG. 12  is a graph illustrating a relationship between the rotation speed of the motor  1  rotated by the outside wind and the induced voltage generated in the coils  3  of the motor  1  in the outdoor fan of the first embodiment. As described above, in the first embodiment, the connection switching unit  7  switches the connection state of the coils  3  of the motor  1  between the Y- connection and the delta-connection. 
     In a case where the connection state of the coils  3  is the delta-connection, the induced voltage generated by the outside wind is  1 /√ 3  times that in a case where the connection state of the coils  3  is the Y-connection, on the assumption that the wind speed of the outside wind is the same and the number of turns of the coils  3  is the same. Thus, by setting the connection state of the coils  3  to the delta-connection when the motor  1  is not driven, a wind speed V 2  of the outside wind at which the induced voltage reaches the withstand voltage is √ 3  times a wind speed V 1  in the case where the connection state of the coils  3  is the Y-connection. 
     That is, by setting the connection state of the coils  3  to the delta-connection when the motor  1  is not driven, it is possible to rotate the motor  1  at a higher speed while ensuring that the induced voltage does not exceed the withstand voltage. Therefore, the number of turns of the coils  3  can be made greater than that of the motor  1  of the comparative example. 
     Thus, the number of turns of the coils  3  can be increased as described above, and the loss due to the energization of the inverter  103  can be reduced, so that the operation efficiency of the motor  1  can be improved. Furthermore, by increasing the number of turns of the coils  3 , the iron loss due to carrier harmonics can be reduced, and thus the motor efficiency can be improved. 
     Further, when the motor  1  is not driven, the connection state of the coils  3  is set to the delta-connection. Before the motor  1  starts rotating, the detected temperature Tf by the fin temperature sensor  65  is compared with the threshold T. If the detected temperature Tf is higher than or equal to the threshold T, the connection state of the coils  3  is switched to the Y-connection. If the detected temperature Tf is lower than the threshold T, the delta-connection is maintained. 
     In the case where the connection state of the coils  3  is the delta-connection, the induced voltage generated by the rotation of the motor  1  is  1 /√ 3  times that in the case where the connection state is the Y-connection, on the assumption that the rotation speed of the motor is the same and the number of turns of the coils  3  is the same. Thus, when the surface temperature of the fins  61  is the temperature at which frost formation occurs, the motor  1  is rotated with the delta-connection. This enables the motor  1  to rotate at a high rotation speed to thereby compensate for the decrease in the operation capacity of the outdoor heat exchanger  6  due to frost formation, while suppressing an increase in the induced voltage. 
     When the surface temperature of the fins  61  is not the temperature at which frost formation occurs, the motor  1  is rotated with the Y-connection with high efficiency, and thus the operation efficiency of the motor  1  can be improved. 
     Thus, in the first embodiment, the increase in the induced voltage due to the outside wind can be suppressed, the operation efficiency of the motor  1  can be improved by increasing the number of turns of the coils  3 , and the decrease in the operation capacity of the outdoor heat exchanger  6  due to frost formation can be suppressed. 
     Here, the connection state of the coils  3  is switched based on the surface temperature of the fins  61  (the detected temperature Tf). However, the connection state of the coils  3  may be switched based not only on the surface temperature of the fins  61  but also on any temperature that affects the state of frost formation on the fins  61 . For example, the connection state of the coils  3  may be switched based on the temperature of the heat transfer tube  62  or the ambient temperature around the outdoor unit  5 A. 
     (Effects of First Embodiment) 
     As described above, the outdoor unit  5 A of the first embodiment includes the outdoor heat exchanger  6 , the outdoor fan  4  that blows air to the outdoor heat exchanger  6 , the connection switching unit  7  that switches the connection state of the coils  3 , and the fin temperature sensor  65 . When the motor  1  is not driven, the connection switching unit  7  sets the connection state of the coils  3  to the delta-connection. When the motor  1  rotates, the connection switching unit  7  sets the connection state of the coils  3  to the Y- connection if the detected temperature Tf by the fin temperature sensor  65  is higher than or equal to the threshold T, and the connection switching unit  7  sets the connection state of the coils  3  to the delta-connection if the detected temperature Tf is lower than the threshold T. Accordingly, the increase in the induced voltage due to the outside wind when the motor  1  is not driven can be suppressed, the operation efficiency of the motor  1  can be improved by increasing the number of turns of the coils  3 , and the decrease in the operation capacity of the outdoor heat exchanger  6  due to frost formation can be suppressed. 
     Since the fin temperature sensor  65  detects the surface temperature of the fins  61  of the outdoor heat exchanger  6 , the connection state of the coils  3  can be switched based on likelihood of the occurrence of frost formation on the fins  61 . 
     The detected temperature Tf is compared with the threshold T before the motor  1  starts rotating, and the connection state of the coils  3  is switched to the Y-connection if the detected temperature Tf is higher than or equal to the threshold T, whereas the connection state of the coils  3  is maintained as the delta- connection if the detected temperature Tf is lower than the threshold T. Thus, the rotation of the motor  1  can be started with the connection state appropriate to the likelihood of the occurrence of frost formation on the fins  61 . 
     In addition, since the connection switching unit  7  switches the connection state of the coils  3  from the Y-connection to the delta-connection before the motor  1  stops, the connection state of the coils  3  can be surely set to the delta-connection when the motor  1  is not driven, and thus the increase in the induced voltage due to the outside wind can be suppressed. 
     The coils  3  are famed of the three-phase coils, and the connection switching is performed between the Y-connection and the delta-connection. Thus, the delta-connection in which the induced voltage is less likely to increase can be selected in a situation where frost formation is more likely to occur. In other situations, the highly efficient Y-connection can be selected. 
     According to the operation control method of the first embodiment, the motor  1  is rotated while the connection state of the coils  3  is set to the Y-connection if the detected temperature Tf by the fin temperature sensor  65  is higher than or equal to the threshold T. The motor  1  is rotated while the connection state of the coils  3  is set to the delta-connection if the detected temperature Tf is lower than the threshold T. Before the motor  1  stops, the connection state of the coils  3  is set to the delta- connection. Thus, the increase in the induced voltage due to the outside wind when the motor  1  is not driven can be suppressed, the operation efficiency of the motor  1  can be improved by increasing the number of turns of the coils  3 , and the decrease in the operation capacity of the outdoor heat exchanger  6  due to frost formation can be suppressed. 
     Modification 
     Next, a modification of the first embodiment will be described. In the above-described first embodiment, the detected temperature Tf by the fin temperature sensor  65  is compared with the threshold T when the motor  1  starts rotating, and the connection state of the coils  3  is selected based on the comparison result. 
     Meanwhile, after the motor  1  starts rotating, there is a case where the outside temperature increases and the temperature of the fins  61  increases, so that the frost formation is eliminated. Therefore, in this modification, even after the motor  1  starts rotating, the detected temperature Tf by the fin temperature sensor  65  is compared with the threshold T, and the connection state of the coils  3  is selected based on the comparison result. 
       FIG. 13  is a flowchart for explaining the operation of an air conditioner of the modification of the first embodiment. The processes up to the start of rotation of the motor  1  (steps  5101  to S 104 ) are the same as those of the first embodiment. 
     When an elapsed time from the start of rotation of the motor reaches the specified time (step S 201 ), the controller  110  acquires the surface temperature of the fins  60 , i.e., the detected temperature Tf, by the fin temperature sensor  65  and then determines whether the detected temperature Tf is higher than or equal to the threshold T (step S 202 ). The specified time is, for example, one hour, but the specified time is not limited to one hour. 
     If the detected temperature Tf is higher than or equal to the threshold T, the controller  110  checks the connection state of the coils  3  of the motor  1  (step S 203 ). If the connection state of the coils  3  is the delta-connection, the controller  110  transmits the stop signal to the inverter  103  to thereby stop the rotation of the motor  1  (step S 204 ), and thereafter performs switching to the Y- connection (step S 205 ). After the switching is completed, the rotation of the motor  1  is restarted (step S 206 ). In contrast, if the connection state of the coils  3  is the Y-connection, the connection state is maintained as the Y-connection. The subsequent processes (steps  5105  to  5109 ) are as described in the first embodiment. 
     The reason why the rotation of the motor  1  is temporarily stopped in step  5204  is to ensure the reliability of the relay contacts that constitute the switching elements  71 ,  72 , and  73  (FIG.  6 ) of the connection switching unit  7 . If the switching elements  71 ,  72 , and  73  of the connection switching unit  7  are formed of semiconductor switches, it is possible to perform the connection switching while the motor  1  is rotated. 
     According to this modification, the connection state of the coils  3  is switched to the highly efficient Y-connection when the outside temperature increases and the frost formation is eliminated after the motor  1  starts rotating, and thus the operation efficiency of the motor  1  can be further improved. 
     Second Embodiment 
     Next, a second embodiment will be described. The second embodiment relates to a display screen of the remote controller  9 .  FIG. 14  is a schematic diagram of the remote controller  9 . The remote controller  9  is an operating device to be used by a user for operating the air conditioner  5  as described in the first embodiment. 
     The remote controller  9  includes a display  91  and an operation section  92 . The operation section  92  has, for example, an on/off switch  93  and setting buttons  94 . The display  91  is, for example, a liquid crystal display, and displays an operation state of the air conditioner  5  or a menu screen. The on/off switch  93  is operated when the air conditioner  5  is started and stopped. The setting buttons  94  are portions to change and determine the settings of the operation contents shown on the menu screen of the display  91 . 
     In the above-described first embodiment, the detected temperature Tf of the fins  61  is compared with the threshold T when the motor  1  starts rotating, and then the Y-connection or the delta- connection is selected based on the comparison result. However, the user who starts the air conditioner  5  using the remote controller  9  cannot recognize the connection state of the coils  3  of the motor  1 . 
     Therefore, in the second embodiment, when the connection switching unit  7  switches the connection state of the coils  3  of the motor  1  from the delta-connection to the Y-connection (step S 103  in  FIG. 9 ), the controller  110  displays a message indicating that the switching to the Y-connection is performed, on a display screen  95  of the display  91  of the remote controller  9 . 
     Specifically, the message “Switching to Energy Saving Mode Has Been Performed” is displayed on the display screen  95  of the display  91  of the remote controller  9 , as illustrated in  FIG. 14 . This allows the user to recognize that the connection state of the coils  3  of the motor  1  is switched from the delta-connection to the highly efficient Y-connection. 
     When the motor  1  starts rotating, the connection state of the coils  3  of the motor  1  is maintained as the delta-connection if the detected temperature Tf of the fins  61  is lower than the threshold T, in other words, if the detected temperature Tf is the temperature at which the frost formation occurs. At this time, the message “Switching to High Power Mode Has Been Performed Due to Frost Formation” is displayed on the display screen  95  of the display  91  of the remote controller  9 , as illustrated in  FIG. 15 . This allows the user to recognize that the motor  1  starts rotating while the connection state of the coils  3  is maintained as the delta- connection. 
     In the modification of the first embodiment, when the rotation of the motor  1  is temporarily stopped for switching the connection state of the coils  3  (step  5204  of  FIG. 13 ), the message “Operation Mode Will Be Switched. Temporary Step Will Be Made.” is displayed on the display screen  95  of the display  91  of the remote controller  9 , as illustrated in  FIG. 16 . 
     The display examples of the display screen  95  illustrated in  FIGS. 14 to 16  are illustrative only and can be changed appropriately. 
     As described above, in the second embodiment, when the connection switching unit  7  switches the connection state of the coils  3 , the message indicating the connection switching is displayed on the display screen  95  of the display  91  of the remote controller  9  which is the operating device. Thus, the user can recognize that the connection state of the coils  3  is switched. 
     Modification 
     Next, a modification of the first and second embodiments will be described. In the first and second embodiments 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. 17(A) and 17(B)  are schematic diagrams for explaining the switching of the connection state of the coils  3  in the modification. 
     In  FIG. 17(A) , the coils  3 U,  3 V, and  3 W are connected in the Y-connection. Coil portions Ua, Ub, Uc, and Ud of the coil  3 U are connected in series. Similarly, coil portions Va, Vb, Vc, and Vd of the coil  3 V are connected in series, and coil portions Wa, Wb, Wc, and Wd of the coil  3 W are connected in series. This connection state corresponds to the first connection state. 
     In contrast, in  FIG. 17(B) , the coils  3 U,  3 V, and  3 W are connected in the Y-connection, but the coil portions Ua, Ub, Uc, and Ud of the coil  3 U are connected in parallel, the coil portions Va, Vb, Vc, and Vd of the coil  3 V are connected in parallel, and the coil portions Wa, Wb, Wc, and Wd of the coil  3 W are connected in parallel. In other words, the coil portions of the coil  3  of each phase are connected in parallel. This connection state corresponds to the second connection state. 
     When the number of coil portions (i.e., the number of rows) of the coil of each phase which are connected in parallel in  FIG. 17(B)  is represented by “n”, the switching from the series connection ( FIG. 17(A) ) to the parallel connection ( FIG. 17(B) ) reduces the line voltage by a factor of  1 /n. Therefore, when the motor  1  is not driven, an increase in the induced voltage due to the outside wind can be suppressed by setting the connection state of the coils  3  to the connection state illustrated in  FIG. 17(B) . Meanwhile, “n” is  4  in this example, but only needs to be 2 or more. 
     The switching of the connection state of the coils  3  illustrated in  FIGS. 17(A) and 17(B)  can be implemented, for example, by installing selector switches on the coil portions of the coils  3 U,  3 V, and  3 W. 
       FIGS. 18(A) and 18(B)  are schematic diagrams for explaining another configuration example of the modification. In  FIG. 18(A) , the three-phase coils  3 U,  3 V, and  3 W are connected in the delta- connection. Further, the coil portions Ua, Ub, Uc, and Ud of the coil  3 U are connected in series, the coil portions Va, Vb, Vc, and Vd of the coil  3 V are connected in series, and the coil portions Wa, Wb, Wc, and Wd of the coil  3 W are connected in series. That is, the coil portions of the coil  3  of each phase are connected in series. This connection state corresponds to the first connection state. 
     In contrast, in  FIG. 18(B) , the three-phase coils  3 U,  3 V, and  3 W are connected in the delta-connection, but the coil portions Ua, Ub, Uc, and Ud of the coil  3 U are connected in parallel, the coil portions Va, Vb, Vc, and Vd of the coil  3 V are connected in parallel, and the coil portions Wa, Wb, Wc, and Wd of the coil  3 W are connected in parallel. That is, the coil portions of the coil  3  of each phase are connected in parallel. This connection state corresponds to the second connection state. 
     Also in this case, as described with reference to  FIGS. 17(A) and 17(B) , the switching from the series connection ( FIG. 18(A) ) to the parallel connection ( FIG. 18(B) ) reduces the line voltage by a factor of 1/n. Therefore, when the motor  1  is not driven, an increase in the induced voltage due to the outside wind can be suppressed by setting the connection state of the coils  3  to the connection state illustrated in  FIG. 18(B) . 
     The switching of the connection state illustrated in  FIGS. 17(A) and 17(B) , and the switching of the connection state illustrated in  FIGS. 18(A) and 18(B)  can provide the same effects as those of the switching between the Y-connection and the delta- connection. 
     Although the desirable 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 changes can be made to those embodiments without departing from the scope of the present invention.