Patent Publication Number: US-2022239168-A1

Title: Stator, motor, compressor, and air conditioner

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. National Stage Application of International Patent Application No. PCT/JP2019/028036 filed on Jul. 17, 2019, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a stator of a motor. 
     BACKGROUND 
     As a refrigerant of a compressor, a refrigerant including 1,1,2-trifluoroethylene is generally used (see, for example, Patent Reference 1). 
     PATENT REFERENCE 
     Patent Reference 1: International Patent Publication No. 2015/136977 
     In conventional techniques, however, depending on structures of a motor in a compressor, a refrigerant might expand to cause a failure in a cylinder in the compressor. As a result, a failure might occur in the compressor. 
     SUMMARY 
     It is therefore an object of the present invention to solve the problem described above and reduce occurrence of a failure in a compressor. 
     A stator according to an aspect of the present invention is a stator to be disposed outside a rotor of a motor disposed in a compressor used with a refrigerant containing a substance having a property of causing disproportionation, and the stator includes a stator core including a plurality of sheets laminated in an axial direction of the rotor, wherein the stator core includes: a yoke part; and N tooth parts, wherein each of the N tooth parts includes a tooth end surface to face the rotor, and each of the plurality of sheets satisfies 0.75≤(θ1×N)/360≤0.97, where θ1 (degrees) is an angle formed by two lines passing through both ends of the tooth end surface and a rotation center of the rotor in a plane perpendicular to the axial direction. 
     A motor according to another aspect of the present invention includes: the stator; and the rotor disposed inside the stator. 
     A compressor according to another aspect of the present invention includes: a closed container; a compression device disposed in the closed container; and the motor to drive the compression device. 
     An air conditioner according to another aspect of the present invention includes: the compressor; and a heat exchanger. 
     According to the present invention, a failure in a compressor is less likely to occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically illustrating an internal structure of a motor including a stator according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a configuration of a driving device. 
         FIG. 3  is a perspective view schematically illustrating a structure of a divided iron core. 
         FIG. 4  is a plan view schematically illustrating a structure of a stator core. 
         FIG. 5  is a cross-sectional view schematically illustrating a structure of the divided iron core. 
         FIG. 6  is a plan view schematically illustrating a structure of an iron core part. 
         FIG. 7  is a perspective view schematically illustrating a structure of the iron core part. 
         FIG. 8  is a cross-sectional view schematically illustrating a structure of a rotor. 
         FIG. 9  is a diagram illustrating a structure of a tooth part. 
         FIG. 10  is a cross-sectional view schematically illustrating another example of the motor. 
         FIG. 11  is a cross-sectional view schematically illustrating still another example of the motor. 
         FIG. 12  is a plan view schematically illustrating an example of a metal member. 
         FIG. 13  is a diagram illustrating another example of the rotor. 
         FIG. 14  is a graph showing a relationship between a rotation angle of the rotor and an internal pressure of a cylinder. 
         FIG. 15  is a graph showing a relationship between an aperture angle proportion [%] and a torque ripple ratio [%] in a case where the motor is driven with a torque less than or equal to a rated torque. 
         FIG. 16  is a diagram illustrating a magnetic flux density in a stator core in the case where the motor is driven with a torque less than or equal to the rated torque. 
         FIG. 17  is a diagram illustrating a magnetic flux density in a stator core in a case where the motor is driven with a torque larger than the rated torque. 
         FIG. 18  is a graph showing a relationship between an aperture angle proportion [%] and a torque ripple ratio [%] in the case where a motor is driven with a torque larger than the rated torque. 
         FIG. 19  is a cross-sectional view schematically illustrating a structure of a compressor according to a second embodiment of the present invention. 
         FIG. 20  is a diagram schematically illustrating a configuration of a refrigeration air conditioning apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     In an xyz orthogonal coordinate system shown in each drawing, a z-axis direction (z axis) represents a direction parallel to an axis A 1  of a motor  1 , an x-axis direction (x axis) represents a direction perpendicular to the z-axis direction (z axis), and a y-axis direction (y axis) represents a direction perpendicular to both the z-axis direction and the x-axis direction. The axis A 1  is a rotation center of a rotor  3 . The axis A 1  also represents a center of a stator  2 . The direction parallel to the axis A 1  is also referred to as an “axial direction of the motor  1 ,” “axial direction of the rotor  3 ,” or simply as an “axial direction.” A radial direction refers to a radial direction of the rotor  3  or the stator  2 , and is a direction perpendicular to the axis A 1 . An xy plane is a plane perpendicular to the axial direction. An arrow D 1  represents a circumferential direction about the axis A 1 . A circumferential direction of the rotor  3  or the stator  2  will also be referred to as a “circumferential direction.” 
       FIG. 1  is a cross-sectional view schematically illustrating an internal structure of the motor  1  including the stator  2  according to a first embodiment of the present invention. 
     The motor  1  includes the stator  2  and the rotor  3 . The motor  1  is, for example, an interior permanent magnet motor. 
     The motor  1  is a motor disposed in a compressor to be used with a refrigerant containing a substance having a property of causing disproportionation. 
     For example, the refrigerant described above only needs to contain 1 wt % or more of a substance having a property of causing disproportionation. The refrigerant may be a refrigerant composed only of a substance having a property of causing disproportionation. That is, the proportion of the substance having a property of causing disproportionation in the refrigerant described above only needs to be 1 wt % to 100 wt %. 
     The substance having a property of causing disproportionation is, for example, 1,1,2-trifluoroethylene or 1,2-difluoroethylene. 
     For example, the refrigerant described above only needs to contain 1 wt % or more of 1,1,2-trifluoroethylene. The refrigerant may be a refrigerant composed only of 1,1,2-trifluoroethylene. That is, the refrigerant only needs to contain 1 wt % to 100 wt % of 1,1,2-trifluoroethylene. 
     For example, the refrigerant described above only needs to contain 1 wt % or more of 1,2-difluoroethylene. The refrigerant may be a refrigerant composed only of 1,2-difluoroethylene. That is, the refrigerant only needs to contain 1 wt % to 100 wt % of 1,2-difluoroethylene. 
     The refrigerant described above may be a mixture of 1,1,2-trifluoroethylene and difluoromethane (also referred to as R32). For example, a mixture containing 40 wt % of 1,1,2-trifluoroethylene and 60 wt % of R32 may be used as a refrigerant. In this mixture, R32 may be replaced by another substance. For example, a mixture of 1,1,2-trifluoroethylene and another ethylene-based fluorocarbon may be used as a refrigerant. Examples of another ethylene-based fluorocarbon include fluoroethylene (also referred to as HFO-1141), 1,1-difluoroethylene (also referred to as HFO-1132a), trans-1,2-difluoroethylene (also referred to as “HFO-1132(E)”), and cis-1,2-difluoroethylene (also referred to as “HFO-1132(Z)”). 
     R32 may be replaced by any one of 2,3,3,3-tetrafluoropropene (also referred to as R1234yf), trans-1,3,3,3-tetrafluoropropene (also referred to as “R1234ze(E)”), cis-1,3,3,3-tetrafluoropropene (also referred to as “R1234ze(Z)”), 1,1,1,2-tetrafluoroethane (also referred to as R134a), or 1,1,1,2,2-pentafluoroethane (also referred to as R125). R32 may be replaced by a mixture of at least two of R32, R1234yf, R1234ze(E), R1234ze(Z), R134a, and R125. 
     The stator  2  includes an annular stator core  2   a  and coils  27  wound around the stator core  2   a.  The stator  2  is formed in an annular shape in a circumferential direction about the axis A 1  (i.e., the rotation center of the rotor  3 ). 
     The stator  2  is disposed outside the rotor  3 . The rotor  3  is rotatably provided inside the stator  2 . An air gap of 0.3 mm to 1 mm is provided between the inner surface of the stator  2  and the outer surface of the rotor  3 . When a current is supplied to an inverter to the coils  27  of the stator  2 , the rotor  3  rotates. The current supplied to the coils  27  is a current having a frequency in synchronization with an instructed rotation speed. 
     The stator  2  includes a plurality of divided iron cores  25   a.  In the example illustrated in  FIG. 1 , the plurality of divided iron cores  25   a  are arranged in an annular shape in the circumferential direction about the axis A 1  to thereby form the stator  2 . 
     Next, a driving device  101  will be described. 
       FIG. 2  is a block diagram illustrating a configuration of the driving device  101 . 
     The motor  1  may include the driving device  101  illustrated in  FIG. 2 . The driving device  101  includes a converter  102  that rectifies an output of a power supply, an inverter  103  that supplies electric power to the stator  2  (specifically, the coils  27 ) of the motor  1 , and a control device  50 . 
     In the example illustrated in  FIG. 2 , the coils  27  are three-phase coils having a U phase, a V phase, and a W phase. 
     The converter  102  is supplied with electric power from a power supply that is an alternating current power supply. The converter  102  applies a voltage to the inverter  103 . A voltage applied from the converter  102  to the inverter  103  will also be referred to as a “converter voltage.” A bus voltage of the converter  102  is supplied to the control device  50 . 
     The inverter  103  operates by a pulse width modulation control method (also referred to as a PWM control method). 
     An inverter voltage for driving the motor  1 , that is, a voltage applied to the coils  27  of the motor  1 , is generated by a PWM control method. As described above, the coils  27  of the motor  1  are, for example, three-phase coils. In this case, the inverter  103  includes at least one inverter switch corresponding to each phase, and each inverter switch includes a pair of switching devices (two switching devices in this embodiment). 
     In the PWM control method, a waveform of an inverter voltage is generated by controlling proportions of on and off times of the inverter switch corresponding to each phase. In this manner, a desired output waveform from the inverter  103  can be obtained. Specifically, in the inverter  103 , when the inverter is on, a voltage is supplied from the inverter  103  to the coils  27 , and an inverter voltage increases. When the inverter switch is off, a voltage supply from the inverter  103  to the coils  27  is shut off, and the inverter voltage decreases. A difference between the inverter voltage and an induced voltage is supplied to the coils  27 , a motor current is generated, and a rotary force of the motor  1  occurs. The proportions of on and off times of the inverter switch is controlled to match with a target motor current value and consequently a desired output waveform from the inverter  103  can be thereby obtained. 
     An on/off timing of each inverter switch is determined based on a carrier wave. The carrier wave is constituted by a triangular wave having a constant amplitude. A pulse width modulation cycle in the PWM control method is determined by a carrier frequency that is a frequency of a carrier wave. In this embodiment, the control device  50  stores a predetermined pattern of a carrier wave or a predetermined carrier frequency. The control device  50  controls the carrier frequency, and controls on and off of each inverter switch. In this manner, the control device  50  controls an output from the inverter  103  to be supplied to the coils  27 . 
     A carrier frequency that is a frequency of a carrier wave will also be referred to as a “carrier frequency of the inverter  103 .” That is, the carrier frequency of the inverter  103  is a control frequency of a voltage to be applied to the coils  27 , the control device  50  controls the carrier frequency of the inverter  103 . 
     In this embodiment, the inverter  103  includes three inverter switches (i.e., six switching devices), and control on one of the three inverter switches, that is, one inverter switch corresponding the U phase, the V phase, or the W phase, will be described. The control on one inverter switch is also applicable to control on the other two inverter switches. 
     The control device  50  compares a voltage value of a carrier wave with an inverter output voltage instruction value. The inverter output voltage instruction value is calculated based on a target motor current value in the control device  50 , for example. The inverter output voltage instruction value is set based on, for example, a driving instruction signal input to the control device  50  from a remote controller of a refrigeration air conditioning apparatus such as an air conditioner. 
     If a voltage value of a carrier wave is smaller than the inverter output voltage instruction value, the control device  50  turns a PWM control signal so that an inverter switch is turned on. If a voltage value of a carrier wave is greater than or equal to the inverter output voltage instruction value, the control device  50  turns the PWM control signal off so that the inverter switch is turned off. Accordingly, the inverter voltage approaches a target value. 
     As described above, the control device  50  generates a PWM control signal based on a difference between the inverter output voltage instruction value and a voltage value of a carrier wave. 
     The control device  50  outputs a control signal such as an inverter driving signal based on a PWM control signal to the inverter  103 , and performs on/off control of the inverter switch. The inverter driving signal may be the same as the PWM control signal, or may be different from the PWM control signal. 
     While the inverter switch is on, an inverter voltage is output from the inverter  103 . The inverter voltage is supplied to the coils  27 , a motor current (specifically, a U-phase current, a V-phase current, and a W-phase current) is generated in the motor  1 . Accordingly, an inverter voltage is converted to a rotary force of the motor  1  (specifically, the rotor  3 ). The motor current is measured by a measuring instrument such as a current sensor, and a measurement result (e.g., a signal indicating a current value) is transmitted to the control device  50 . 
     The control device  50  is composed of, for example, a processor and a memory. For example, the control device  50  is a microcomputer. The control device  50  may be composed of a processing circuit as dedicated hardware such as a single circuit or a composite circuit. 
     A structure of the divided iron cores  25   a  will now be described. 
       FIG. 3  is a perspective view schematically illustrating a structure of the divided iron core  25   a.    
     In this embodiment, the stator  2  is composed of the plurality of divided iron cores  25   a.  Each of the divided iron cores  25   a  includes an iron core part  21  as a divided iron core, first insulators  24   a,  a second insulator  24   b,  and a coil  27 . The example illustrated in  FIG. 3  does not show the coil  27 . 
     The first insulators  24   a  are combined with the stator core  2   a  (specifically, the iron core part  21 ). In this embodiment, the first insulators  24   a  are provided at both end of the stator core  2   a  in the axial direction. The first insulator  24   a  may be provided at one end of the stator core  2   a  in the axial direction. In this embodiment, the first insulator  24   a  is an insulating resin. 
     The second insulator  24   b  is, for example, a thin polyethylene terephthalate (PET) film. The PET film has a thickness of, for example, 0.15 mm. The second insulator  24   b  covers a side surface of a tooth part (a tooth part  22   a  described later) of the stator core  2   a.    
       FIG. 4  is a plan view schematically illustrating a structure of the stator core  2   a.    
     The stator core  2   a  includes at least one yoke part  21   a  and at least two tooth parts  22   a.  The stator core  2   a  is composed of a plurality of iron core parts  21 . Thus, each of the iron core parts  21  includes the yoke part  21   a  and the tooth parts  22   a.    
     In the example illustrated in  FIG. 4 , the stator core  2   a  is composed of nine iron core parts  21 . 
     The stator core  2   a  may not be divided into the plurality of iron core parts  21 . In this case, the stator core  2   a  may be composed of the plurality of iron core parts  21  integrated as one member. For example, the stator core  2   a  may be formed by laminating a plurality of annular materials (e.g., electromagnetic steel sheets). 
     A region surrounded by two yoke parts  21   a  and two tooth parts  22   a  is a slot part  26 . In the stator core  2   a,  a plurality of slot parts  26  are arranged at regular intervals in the circumferential direction. In the example illustrated in  FIG. 4 , the stator core  2   a  has nine slot parts  26 . 
     As illustrated in  FIG. 4 , the stator core  2   a  includes the plurality of tooth parts  22   a,  and each of the tooth parts  22   a  is adjacent to another tooth part  22   a  across the slot part  26 . Accordingly, the plurality of tooth parts  22   a  and the plurality of slot parts  26  are alternately arranged in the circumferential direction. The pitch of the plurality of tooth parts  22   a  arranged in the circumferential direction (i.e., the width of the slot parts  26  in the circumferential direction) is uniform. That is, the plurality of tooth parts  22   a  are radially positioned. 
     In this embodiment, the stator  2  includes N divided iron cores  25   a  (where N is a natural number of two or more). Thus, the stator  2  includes N tooth parts  22   a.  In the example illustrated in  FIG. 1 , the stator  2  includes nine divided iron cores  25   a.  Accordingly, in the example illustrated in  FIG. 1 , the stator  2  includes nine tooth parts  22   a.    
       FIG. 5  is a cross-sectional view schematically illustrating a structure of the divided iron core  25   a.    
     Each of the divided iron cores  25   a  includes the yoke part  21   a,  the tooth part  22   a  located at the inner side of the yoke part  21   a  in the radial direction, the coil  27 , the first insulator  24   a  insulating the stator core  2   a,  and the second insulator  24   b  insulating the stator core  2   a.  In this embodiment, the tooth part  22   a  is integrated with the yoke part  21   a  as one member, but a tooth part  22   a  formed as a separate member from the yoke part  21   a  may be attached to the yoke part  21   a.    
     The coil  27  is wound around the stator core  2   a  with the first insulator  24   a  and the second insulator  24   b  interposed therebetween. Specifically, the coil  27  is wound around the tooth part  22   a.  When a current flows through the coil  27 , a rotating magnetic field is generated from the coil  27 . 
     The coil  27  is, for example, a magnet wire. For example, the stator  2  has three phases, and connection of the coil  27  is, for example, Y connection (also referred to as star connection) or delta connection. The number of turns and the wire diameter of each coil  27  are determined depending on the rotation speed of the motor  1 , torque, voltage specifications, and the cross-sectional area of the slot parts  26 , for example. The wire diameter of the coil  27  is, for example, 1.0 mm. The coil  27  is wound around each tooth part  22   a  of the stator core  2   a  in, for example,  80  turns. The wire diameter and the number of turns of the coil  27  are not limited to these examples. 
     The winding method of the coils  27  is, for example, concentrated winding. For example, the coils  27  can be wound around the iron core parts  21  in a state before the iron core parts  21  are arranged in the annular shape (e.g., a state where the iron core parts  21  are arranged linearly). The iron core parts  21  (i.e., the divided iron cores  25   a ) around which the coils  27  are wound are folded in an annular shape and fixed by, for example, welding. 
     The coils  27  may be attached to the tooth parts  22   a  of the stator core  2   a  by distributed winding, instead of concentrated winding. 
       FIG. 6  is a plan view schematically illustrating a structure of the iron core part  21 . 
       FIG. 7  is a perspective view schematically illustrating a structure of the iron core part  21 . 
     The yoke part  21   a  extends in the circumferential direction, and the tooth part  22   a  extends inward (in the -y direction in  FIG. 6 ) in the radial direction of the stator core  2   a.  In other words, the tooth part  22   a  projects from the yoke part  21   a  toward the axis A 1 . 
     As illustrated in  FIGS. 6 and 7 , each tooth part  22   a  includes a body  221   a,  a tooth tip  222   a,  and a tooth end surface  223   a.  The tooth tip  222   a  is disposed at the tip of the tooth part  22   a  (specifically, an end of the body  221   a ) in the radial direction. In the example illustrated in  FIGS. 6 and 7 , the body  221   a  has a uniform width in the radial direction. The tooth tip  222   a  extends in the circumferential direction, and is formed to expand in the circumferential direction. 
     The tooth end surface  223   a  faces the rotor  3  in the motor  1 . Specifically, the tooth end surface  223   a  is a surface of the tooth tip  222   a  facing the rotor  3  in the motor  1 . 
     As illustrated in  FIGS. 5 through 7 , the iron core part  21  (e.g., the yoke part  21   a ) has a fixing hole  24   c  for fixing the first insulator  24   a.    
     As illustrated in  FIG. 7 , the iron core part  21  is constituted by at least one sheet  28  (also referred to as a plate). In this embodiment, the iron core part  21  is formed by laminating a plurality of sheets  28  in the axial direction (i.e., in the z-axis direction). 
     The sheets  28  are formed in a predetermined shape by press work (specifically, punching). The sheets  28  are, for example, electromagnetic steel sheets. In the case of using electromagnetic steel sheets as the sheets  28 , each of the sheets  28  has a thickness of, for example, 0.01 mm through 0.7 mm. In this embodiment, the thickness of each sheet  28  is 0.35 mm. Each of the sheets  28  is fixed to its adjacent sheet  28  by caulked parts  24   d.    
     A structure of the rotor  3  will now be described. 
       FIG. 8  is a cross-sectional view schematically illustrating the structure of the rotor  3 . 
     The rotor  3  includes a rotor core  31 , a shaft  32 , at least one permanent magnet  33 , at least one magnet insertion hole  34 , at least one flux barrier  35 , at least one air opening  36 , and at least one slit  38 . The rotor  3  is rotatable about the axis A 1 . The rotor  3  is rotatably disposed inside the stator  2 . The axis A 1  is the rotation center of the rotor  3 , and is the axis of the shaft  32 . 
     In this embodiment, the rotor  3  is an interior permanent magnet rotor. The rotor core  31  has a plurality of magnet insertion holes  34  arranged in the circumferential direction of the rotor  3 . The magnet insertion holes  34  are space in which the permanent magnets  33  are arranged. One permanent magnet  33  is disposed in each of the magnet insertion holes  34 . A plurality of permanent magnets  33  may be disposed in each of the magnet insertion holes  34 . The permanent magnets  33  disposed in the magnet insertion holes  34  are magnetized in the radial direction of the rotor  3  (i.e., the direction perpendicular to the axis A 1 ). The number of magnet insertion holes  34  corresponds to the number of magnetic poles of the rotor  3 . The positional relationships of the magnetic poles are the same. In this embodiment, the number of magnetic poles of the rotor  3  is six. The number of magnetic poles of the rotor  3  only needs to be two or more. 
     Rare earth magnets containing neodymium (Nd), iron (Fe), and boron (B) (hereinafter referred to as “Nd—Fe—B permanent magnets”), for example, are applied to the permanent magnets  33 . 
     A coercive force of the Nd—Fe—B permanent magnets has a property of decreasing depending on the temperature. For example, in the case of using a motor employing Nd rare earth magnets at a high temperature atmosphere of 100° C. or more, such as the case of a compressor, a coercive force of the magnets decreases by about −0.5 to −0.6%/ΔK depending on the temperature, and thus, a dysprosium (Dy) element needs to be added in order to increase the coercive force. The coercive force increases substantially in proportion to the content of the Dy element. In a general compressor, the upper limit of an ambient temperature of a motor is about 150° C., and the motor is used in a temperature rise range of about 130° C. with respect to 20° C. For example, the coercive force decreases by 65% at a temperature coefficient of −0.5%/ΔK. 
     To prevent demagnetization with a maximum load of a compressor, a coercive force of about 1100 to 1500 A/m is needed. To assure a coercive force in an ambient temperature of 150° C., a coercive force at room temperature needs to be designed at about 1800 to 2300 A/m. 
     In a state where no Dy element is added to Nd—Fe—B permanent magnets, the coercive force at room temperature is about 1800 A/m. To obtain a coercive force of about 2300 kA/m, about 2 wt % of the Dy element needs to be added. However, when the Dy element is added, a coercive force property is enhanced, but a remaining magnetic flux density property decreases. When the remaining magnetic flux density decreases, a magnet torque of the motor decreases and a supply current increases. Accordingly, a copper loss increases. Thus, in consideration of motor efficiency, it is desired to reduce the amount of Dy addition. 
     The rotor core  31  is formed by laminating a plurality of electromagnetic steel sheets. Each of the electromagnetic steel sheets of the rotor core  31  has a thickness of, for example, 0.1 mm to 0.7 mm. In this embodiment, the thickness of each electromagnetic steel sheet of the rotor core  31  is 0.35 mm. Each of the electromagnetic steel sheets of the rotor core  31  is fixed to its adjacent electromagnetic steel sheet by caulking. 
     At least one slit  38  is formed outside each magnet insertion hole  34  in the radial direction of the rotor  3 . In this embodiment, a plurality of slits  38  are formed outside each magnet insertion hole  34  in the radial direction of the rotor  3 . Each of the slits  38  is elongated in the radial direction. 
     The shaft  32  is coupled to the rotor core  31 . For example, the shaft  32  is fixed to a shaft hole  37  formed in the rotor core  31  by a fixing method such as shrink fitting or press fitting. In this manner, rotational energy generated by rotation of the rotor core  31  is transferred to the shaft  32 . 
     The flux barrier  35  is formed at a position adjacent to the magnet insertion hole  34  in the circumferential direction of the rotor  3 . In other words, each flux barrier  35  is adjacent to an end portion of a corresponding one of the magnet insertion holes  34  in the longitudinal direction of the magnet insertion hole  34 . The flux barrier  35  reduces leakage flux. To prevent a short circuit of magnetic flux between adjacent magnetic poles, the width of a thin portion between the flux barrier  35  and the outer peripheral surface of the rotor core  31  is preferably small. The width of the thin portion between the flux barrier  35  and the outer peripheral surface of the rotor core  31  is, for example, 0.35 mm. The air opening  36  is a through hole. For example, in a case where the motor  1  is used in a compressor, a refrigerant is allowed to pass through the air opening  36 . 
     A structure of the tooth part  22   a  will be specifically described. 
       FIG. 9  is a diagram illustrating a structure of the tooth part  22   a.    
     In the xy plane, the stator  2  satisfies 0.75≤(θ1×N)/360≤0.97, where θ1 (degrees) is an angle formed by two lines L 1  passing through both ends P 1  of the tooth end surface  223   a  and the rotation center of the rotor  3 . 
     In this embodiment, N=9. It should be noted that N only needs to be a natural number of two or more. 
     That is, in the xy plane, a proportion α [%] of the tooth end surface  223   a  in the circumference of a circle passing through the tooth end surface  223   a  is 75% or more and 97% or less. This proportion α will be hereinafter referred to as an aperture angle proportion α. The circle passing through the tooth end surface  223   a  is, for example, a circle indicated by the broken line R 1  in  FIG. 4 . 
     First Variation 
       FIG. 10  is a cross-sectional view schematically illustrating another example of the motor  1 . 
     In the axial direction of the rotor  3 , the rotor  3  is longer than the stator  2 . In this case, the rotor  3  only needs to be longer than the stator core  2   a.    
     Second Variation 
       FIG. 11  is a cross-sectional view schematically illustrating still another example of the motor  1 . 
       FIG. 12  is a plan view schematically illustrating an example of the metal member  39 . 
     In the second variation, the rotor  3  includes at least one metal member  39 . The metal member  39  is fixed to an end of the rotor core  31  in the axial direction of the rotor  3 . 
     In the example illustrated in  FIGS. 11 and 12 , the rotor  3  includes two metal members  39 , and the metal members  39  are fixed to both ends of the rotor core  31 . Each of the metal members  39  is preferably a single structure. Accordingly, costs for the metal members  39  can be reduced. 
     In the xy plane, a surface area of each metal member  39  is larger than a surface area of the rotor core  31  (specifically, the surface of the rotor core  39  facing the metal member  39 ). 
     Third Variation 
       FIG. 13  is a diagram illustrating another example of the rotor  3 . 
     The rotor  3  may include a rotor core  31   a  illustrated in  FIG. 13 , instead of the rotor core  31 . The rotor core  31   a  illustrated in  FIG. 13  has a plurality of different radiuses in the xy plane. Specifically, the radius of the rotor core  31   a  is at maximum in magnetic pole center parts of the rotor  3  and at minimum in inter-pole parts of the rotor  3 . In the example illustrated in  FIG. 13 , the outer diameter of the rotor core  31   a  is at maximum in magnetic pole center parts of the rotor  3  and at minimum in inter-pole parts of the rotor  3 . In the xy plane illustrated in  FIG. 13 , each of the magnetic pole center parts of the rotor  3  is located on a line passing through the center of the corresponding permanent magnet  33  and the axis A 1 . In the xy plane illustrated in  FIG. 13 , each of the inter-pole parts of the rotor  3  is located on a line passing through a point between adjacent permanent magnets  33  and the axis A 1 . 
     Advantages of the stator  2  according to the first embodiment will now be described below. 
       FIG. 14  is a graph showing a relationship between a rotation angle of the rotor and an internal pressure of a cylinder. In the example illustrated in  FIG. 14 , the solid line Bl corresponds to a motor having a large torque ripple, and the broken line B 2  corresponds to a motor having a small torque ripple. 
     In general, in a compressor, when compression of a refrigerant starts, the internal pressure of a cylinder increases. When the internal pressure reaches a target discharge pressure satisfying a required capacity, the refrigerant pushes away a valve, and the valve is opened. Accordingly, the cylinder communicates with a discharging muffler, and the refrigerant is discharged from a discharge pipe under a target discharge pressure. 
     However, in a case where a time lag occurs from when the internal pressure of the cylinder reaches the target discharge pressure to when the valve is completely opened, the internal pressure of the cylinder might exceed the target discharge pressure. This phenomenon is referred to as a “pressure overshoot” in the present application. 
     When the pressure overshoot occurs, a refrigerant containing a substance having a property of causing disproportionation, such as 1,1,2-trifluoroethylene or 1,2-difluoroethylene, rapidly causes volume expansion due to a chain of disproportionation reactions, and a failure is likely to occur in the cylinder in the compressor. Thus, occurrence of a pressure overshoot is preferably suppressed as much as possible. 
     In general, in a compressor, as the rotation speed of a motor increases, a period in which a refrigerant is compressed becomes shorter, and an influence of a delay of valve opening increases. That is, as the rotation speed of the motor increases, a pressure overshoot is more likely to occur. 
     In general, a torque ripple occurs during driving of a motor, the rotation speed of the motor fluctuates during driving of the motor. As an instantaneous rotation speed increases, an instantaneous internal pressure of a cylinder increases, and a failure is more likely to occur in the cylinder. 
       FIG. 15  is a graph showing a relationship between an aperture angle proportion α [%] and a torque ripple ratio [%] in a case where a motor is driven with a torque less than or equal to a rated torque. 
       FIG. 16  is a diagram illustrating a magnetic flux density in the stator core  2   a  in the case where a motor is driven with a torque less than or equal to the rated torque. 
       FIG. 17  is a diagram illustrating a magnetic flux density in the stator core  2   a  in a case where a motor is driven with a torque larger than the rated torque. 
     As shown in  FIG. 15 , in the case where the motor is driven with a torque less than or equal to the rated torque, as the angle θ 1  shown in  FIG. 9  increases, the torque ripple ratio during driving of the motor decreases. The torque ripple ratio is a ratio of a difference between a maximum torque and a minimum torque to a time average torque. As the torque ripple ratio decreases, fluctuation of the rotation speed of the motor during driving of the motor decreases, and a pressure overshoot is less likely to occur. As illustrated in  FIG. 16 , when a torque load is small, influence of magnetic saturation in each tooth part is small. On the other hand, as illustrated in  FIG. 17 , when a torque load is large, magnetic saturation in each tooth part increases. For example, when a torque load is large in the motor in the compressor, the internal pressure of the cylinder increases. Accordingly, a failure in the compressor due to disproportionation of a refrigerant is likely to occur. 
     Specifically, the angle  01  affects a magnetic attraction force generated between the stator and the rotor. Consequently, the angle θ 1  affects the torque ripple ratio. 
       FIG. 18  is a graph showing a relationship between an aperture angle proportion α [%] and a torque ripple ratio [%] in a case where the motor  1  is driven with a torque larger than the rated torque. 
     As shown in  FIG. 18 , at an aperture angle proportion α of 75% or more, the torque ripple ratio can be effectively reduced. That is, in the range of 0.75≤(θ1×N)/360, the torque ripple ratio can be effectively reduced. 
     At an aperture angle proportion α of 84% or more, the torque ripple ratio can be more effectively reduced. That is, in the range of 0.84≤(θ1×N)/360, the torque ripple ratio can be more effectively reduced. 
     At an aperture angle proportion α exceeding 97%, the torque ripple ratio rapidly increases. That is, in the range of (θ1×N)/360&gt;0.97, the torque ripple ratio rapidly increases. 
     Thus, the aperture angle proportion α is preferably 75% or more and 97% or less. That is, the stator  2  preferably satisfies 0.75≤(θ1×N)/360≤0.97. In this manner, the torque ripple ratio can be effectively reduced. As a result, a failure in the compressor is less likely to occur. 
     The aperture angle proportion α is more preferably 84% or more and 97% or less. That is, the stator  2  more preferably satisfies 0.84≤(θ1×N)/360≤0.97. In this manner, the torque ripple ratio can be more effectively reduced. As a result, a failure in the compressor can be much less likely to occur. 
     Furthermore, the aperture angle proportion α is more preferably 87.5% or more and 92.5% or less. That is, the stator  2  more preferably satisfies 0.875≤(θ1×N)/360≤0.925. In this manner, the torque ripple ratio can be more effectively reduced. As a result, a failure in the compressor can be much less likely to occur. 
     At an aperture angle proportion α of 90%, the torque ripple ratio is at minimum. Thus, when the stator  2  satisfies (θ1×N)/360=0.9, the torque ripple ratio is at minimum. In this case, a failure in the compressor is much less likely to occur. 
     The aperture angle proportion α may be 87.5% or more and 97% or less. That is, when the stator  2  satisfies 0.875≤(θ1×N)/360≤0.97, the torque ripple ratio can be effectively reduced. As a result, a failure in the compressor is less likely to occur. 
     The aperture angle proportion α may be 87.5% or more and 92.5% or less. That is, when stator  2  satisfies 0.875≤(θ1×N)/360≤0.925, the torque ripple ratio can be effectively reduced. As a result, a failure in the compressor is less likely to occur. 
     The aperture angle proportion α may be 84% or more and 92.5% or less. That is, when the stator  2  satisfies 0.84≤(θ1×N)/360≤0.925, the torque ripple ratio can be effectively reduced. As a result, a failure in the compressor is less likely to occur. 
     In a case where the coils  27  are attached to the tooth parts  22   a  of the stator core  2   a  by distributed winding, magnetic flux from the coils  27  is widely dispersed in the stator  2 , as compared to concentrated winding. Accordingly, variations of magnetic attraction forces generated between the rotor  3  and the stator  2  during rotation of the rotor  3  becomes gentle, and the torque ripple ratio can be reduced. 
     In a case where the motor  1  includes an inverter that operates by a PWM control method, waveform of an inverter can be finely adjusted. Accordingly, torque waveform of the motor  1  due to an inverter voltage can be controlled, and the torque ripple ratio can be reduced. 
     In a case where the rotor  3  is longer than the stator  2  in the axial direction of the rotor  3 , the moment of inertia of the rotor  3  can be increased. Accordingly, occurrence of a pressure overshoot can be suppressed. In a case where the stator  2  is shorter than the rotor  3  in the axial direction of the rotor  3 , the size of the motor  1  can be reduced. Thus, the size of the compressor including the motor  1  can also be reduced. 
     In a case where the rotor  3  includes at least one metal member  39 , the moment of inertia of the rotor  3  can be increased. Accordingly, occurrence of a pressure overshoot can be suppressed. 
     In the case where the rotor  3  includes at least one metal member  39 , in the xy plane, a surface area of the metal member  39  is preferably larger than a surface area of the rotor core  31  (specifically, the surface of the rotor core  31  facing the metal member  39 ). Accordingly, the moment of inertia of the rotor  3  can be further increased. As a result, occurrence of a pressure overshoot can be effectively reduced. 
     In the case where the radius of the rotor core  31   a  is at maximum in magnetic pole center parts of the rotor  3  and at minimum in inter-pole parts of the rotor  3 , the magnetic flux density at the outer peripheral surface of the rotor  3  is at maximum in magnetic pole center parts and at minimum in inter-pole parts at the outer peripheral surface. That is, as the distance from the magnetic pole center parts toward the inter-pole part decreases, the magnetic flux density at the outer peripheral surface of the rotor  3  decreases. Accordingly, the waveform of an induced voltage in the motor  1  approaches a sine wave, and the torque ripple ratio can be reduced. As a result, occurrence of a pressure overshoot can be suppressed. 
     Second Embodiment 
     A compressor  6  according to a second embodiment of the present invention will be described. 
       FIG. 19  is a cross-sectional view schematically illustrating a structure of the compressor  6  according to the second embodiment. 
     The compressor  6  includes a motor  1  serving as an electric element, a closed container  61  serving as a housing, and a compression mechanism  62  serving as a compression element (also referred to as a compression device). The compressor  6  is used with the refrigerant described in the first embodiment, that is, a refrigerant containing a substance having a property of causing disproportionation. This refrigerant may be previously provided in the compressor  6 . In this embodiment, the compressor  6  is a rotary compressor. The compressor  6  is not limited to a rotary compressor. 
     The motor  1  in the compressor  6  is the motor  1  described in the first embodiment. The motor  1  drives the compression mechanism  62 . In this embodiment, although the motor  1  is an interior permanent magnet motor, but the present invention is not limited to this motor. 
     The closed container  61  covers the motor  1  and the compression mechanism  62 . Refrigerating machine oil for lubricating a sliding part of the compression mechanism  62  is stored in a bottom portion of the closed container  61 . 
     The compressor  6  also includes a glass terminal  63  fixed to the closed container  61 , an accumulator  64 , a suction pipe  65  for sucking a refrigerant, and a discharge pipe  66  for discharging a refrigerant. 
     The suction pipe  65  and the discharge pipe  66  are fixed to the closed container  61 . 
     The compression mechanism  62  is disposed inside the closed container  61 . In this embodiment, the compression mechanism  62  is disposed in a lower portion of the closed container  61 . 
     The compression mechanism  62  includes a cylinder  62   a,  a piston  62   b,  an upper frame  62   c  (first frame), a lower frame  62   d  (second frame), and a plurality of mufflers  62   e  individually attached to the upper frame  62   c  and the lower frame  62   d.  The compression mechanism  62  also includes a vane that divides the inside of the cylinder  62   a  into a suction side and a compression side. 
     The compression mechanism  62  is driven by the motor  1 . The compression mechanism  62  compresses a refrigerant. 
     The motor  1  is disposed in an upper portion of the closed container  61 . Specifically, the motor  1  is located between the discharge pipe  66  and the compression mechanism  62 . That is, the motor  1  is disposed above the compression mechanism  62 . 
     The stator  2  of the motor  1  is fixed in the closed container  61  by a fixing method such as press fitting or shrink fitting. The stator  2  may be attached directly to the closed container  61  by welding instead of press fitting or shrink fitting. 
     The coils (e.g., the coils  27  illustrated in  FIG. 1 ) of the stator  2  of the motor  1  are supplied with electric power through the glass terminal  63 . 
     A rotor (specifically, a shaft  32  of a rotor  3 ) of the motor  1  is rotatably held by the upper frame  62   c  and the lower frame  62   d  with interposition of bearing parts individually included in the upper frame  62   c  and the lower frame  62   d.    
     The shaft  32  is inserted in the piston  62   b.  The shaft  32  is rotatably inserted in the upper frame  62   c  and the lower frame  62   d.  The upper frame  62   c  is provided with a valve for preventing a backflow of a refrigerant. Specifically, this valve is located between the upper frame  62   c  and the muffler  62   e.  The upper frame  62   c  and the lower frame  62   d  close an end face of the cylinder  62   a.  The accumulator  64  supplies a refrigerant to the cylinder  62   a  through the suction pipe  65 . 
     Next, an operation of the compressor  6  will be described. The refrigerant supplied from the accumulator  64  enters the cylinder  62   a  through the suction pipe  65  fixed to the closed container  61 . When the motor  1  rotates, the piston  62   b  fitted in the shaft  32  rotates in the cylinder  62   a.  Accordingly, the refrigerant is compressed in the cylinder  62   a.    
     The refrigerant flows through the mufflers  62   e  and moves upward in the closed container  61 . When the refrigerant is compressed in the cylinder  62   a  and the internal pressure of the cylinder  62   a  reaches a given level or more, the valve provided on the upper frame  62   c  is opened and thus the compressed refrigerant is discharged from the discharge pipe  66 . In this manner, the compressed refrigerant is supplied toward a high-pressure side of a refrigeration cycle through the discharge pipe  66 . When the internal pressure of the cylinder  62   a  becomes less than the given level, the valve is closed and thus a flow of the refrigerant is shut off. 
     Since the compressor  6  according to the second embodiment includes the motor  1  described in the first embodiment, a failure in the compressor  6  is less likely to occur. 
     Third Embodiment 
     A refrigeration air conditioning apparatus  7  serving as an air conditioner and including the compressor  6  according to the second embodiment of the present invention will be described. 
       FIG. 20  is a diagram schematically illustrating a configuration of the refrigerating air conditioning device  7  according to a third embodiment. 
     The refrigeration air conditioning apparatus  7  is capable of performing cooling and heating operations, for example. A refrigerant circuit diagram illustrated in  FIG. 20  is an example of a refrigerant circuit diagram of an air conditioner capable of performing a cooling operation. 
     The refrigeration air conditioning apparatus  7  according to the third embodiment includes an outdoor unit  71 , an indoor unit  72 , and a refrigerant pipe  73  connecting the outdoor unit  71  and the indoor unit  72  to each other. 
     The outdoor unit  71  includes a compressor  6 , a condenser  74  serving as a heat exchanger, a throttling device  75 , and an outdoor air blower  76  (first air blower). The condenser  74  condenses a refrigerant compressed by the compressor  6 . The throttling device  75  decompresses the refrigerant condensed by the condenser  74  to thereby adjust a flow rate of the refrigerant. The throttling device  75  will also be referred to as a decompression device. 
     The indoor unit  72  includes an evaporator  77  serving as a heat exchanger, and an indoor air blower  78  (second air blower). The evaporator  77  evaporates the refrigerant decompressed by the throttling device  75  to thereby cool indoor air. 
     A basic operation of a cooling operation in the refrigeration air conditioning apparatus  7  will now be described below. In the cooling operation, a refrigerant is compressed by the compressor  6  and the compressed refrigerant flows into the condenser  74 . The condenser  74  condenses the refrigerant, and the condensed refrigerant flows into the throttling device  75 . The throttling device  75  decompresses the refrigerant, and the decompressed refrigerant flows into the evaporator  77 . In the evaporator  77 , the refrigerant evaporates, and the refrigerant (specifically a refrigerant gas) flows into the compressor  6  of the outdoor unit  71  again. When the air is sent to the condenser  74  by the outdoor air blower  76 , heat moves between the refrigerant and the air. Similarly, when the air is sent to the evaporator  77  by the indoor air blower  78 , heat moves between the refrigerant and the air. 
     The configuration and operation of the refrigeration air conditioning apparatus  7  described above are examples, and the present invention is not limited to the examples described above. 
     The refrigerating air conditioning device  7  according to the third embodiment has the advantages described in the first and second embodiments. 
     Since the refrigeration air conditioning apparatus  7  according to the third embodiment includes the compressor  6 , a failure in the refrigeration air conditioning apparatus  7  is less likely to occur. 
     As described above, preferred embodiments have been specifically described. However, it is obvious that those skilled in the art would take various modified variations based on the basic technical idea and teaching of the present invention. 
     Features of the embodiments and features of the variations described above can be combined as appropriate.