Abstract:
An electric motor, which has a stator and a rotor including a permanent magnet, has a guide for guiding movably in an axial direction of the rotor and an actuator which is operable to move the rotor axially. In addition, a motor-driven compressor includes the electric motor for compressing and discharging gas in its compression chamber by compression of the compressor based upon rotation of its rotary shaft driven by the electric motor.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to an electric motor and a motor-driven compressor.  
         [0002]     The electric motor is operable under a condition where the sum of induced voltage and voltage dropped in the electric motor (which is due to current flowing in a coil of the electric motor) is the same as or below the output voltage from an inverter to the electric motor. The induced electromotive force (or induced voltage) of the electric motor is determined by the magnetic flux developed by permanent magnet provided in a rotor of the electric motor and angular velocity of the electric motor. That is, the induced voltage of the electric motor increases in proportion to an increase of the angular velocity of the electric motor. As the induced voltage becomes dominant, the electric current that can be supplied to the electric motor is reduced. Since the torque developed by the electric motor is increased in proportion to an increase of the electric current supplied to the motor, it is difficult for the motor to develop a high torque in a high-speed region of the electric motor where the induced voltage becomes dominant.  
         [0003]     To solve the above problem, some electric motors use a means for expanding the high-speed region of the electric motor by weak field control. According to this prior art, however, it is necessary to increase the electric current for the weak field control in accordance with the magnitude of the induced electromotive force which increases in proportion to the angular velocity of the electric motor and, therefore, the operating efficiency of the electric motor deteriorates in its high-speed region.  
         [0004]     An inner rotor type electric motor disclosed in Japanese unexamined patent publication No. 2002-262534 widens the high-speed range without using weak field control. This electric motor has a rotor including permanent magnets having different poles which are arranged alternately as seen in the rotational direction of the rotor. The rotor is axially divided into two halves and one of them is axially movable. In the high-speed range of the motor, the movable rotor half is spaced away from the other rotor half, so that the centers of magnetic poles of the permanent magnets of the two movable rotor halves are shifted out of alignment. By so doing, the quantity of effective magnetic flux from the permanent magnets is reduced.  
         [0005]     The above-described inner rotor type electric motor which is disclosed in the Japanese unexamined patent publication No. 2002-262534 can avoid a decrease in the efficiency of the electric motor in the high-speed range.  
         [0006]     The present invention is directed to providing an electric motor and a motor-driven compressor which widen the high-speed range without using the weak field control.  
       SUMMARY  
       [0007]     In accordance with the present invention, an electric motor having a stator and a rotor including a permanent magnet has a guide for guiding movably in an axial direction of the rotor and an actuator operable to move the rotor axially.  
         [0008]     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0010]      FIG. 1  is a longitudinal cross-sectional view of a motor-driven compressor according to a first preferred embodiment of the present invention;  
         [0011]      FIG. 2  is a cross-sectional view that is taken along the line I-I in  FIG. 1 ;  
         [0012]      FIG. 3  is a cross-sectional view that is taken along the line II-II in  FIG. 1 ;  
         [0013]      FIG. 4  is a partially enlarged cross-sectional view of the motor-driven compressor according to the first preferred embodiment of the present invention;  
         [0014]      FIG. 5  is a partially enlarged cross-sectional view of a motor-driven compressor according to a second preferred embodiment of the present invention; and  
         [0015]      FIG. 6  is a partially enlarged cross-sectional view of the motor-driven compressor according to an alternative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     A first preferred embodiment of an electric motor and a motor-driven compressor of a fixed displacement piston type according to the present invention will now be described with reference to  FIGS. 1 through 4 .  
         [0017]     As shown in  FIG. 1 , the compressor includes a cam housing  12  accommodating therein a swash plate  11  and connected at one end thereof to a cylinder block  13  and at the other end thereof to a center housing  14 . A rear housing  15  is connected to the cylinder block  13  and a motor housing  29  is connected to the center housing  14 . The cam housing  12 , the cylinder block  13 , the center housing  14 , the rear housing  15  and the motor housing  29  cooperate to form the housing of the motor-driven compressor  10 . The cam housing  12  and the cylinder block  13  rotatably support a rotary shaft  16  through radial bearings  17 ,  18 . The swash plate  11  is fixed on the rotary shaft  16  for rotation therewith in the cam housing  12 .  
         [0018]     The cylinder block  13  has formed therethrough a plurality of cylinder bores  131 . Each cylinder bore  131  accommodates therein a piston  19 . Torque of the swash plate  11  is transmitted to the pistons  19  through a pair of shoes  20  in a known manner. As the swash plate  11  is driven to rotate by the rotary shaft  16 , each piston  19  is moved reciprocally in its associated cylinder bore  131 . A compression chamber  132  is defined by the piston  19  and the cylinder bore  131 .  
         [0019]     The rear housing  15  has formed therein a suction chamber  151  and a discharge chamber  152 . As the piston  19  moves from the top dead center toward the bottom dead center (or leftward as seen in  FIG. 1 ), refrigerant gas in the suction chamber  151  is drawn into the compression chamber  132  through a suction port  21  while pushing open a suction valve  22 . As the piston  19  moves from the bottom dead center toward the top dead center (or rightward as seen in  FIG. 1 ), on the other hand, refrigerant gas is compressed in the compression chamber  132  and then discharged out thereof into the discharge chamber  152  through a discharge port  23  while pushing open a discharge valve  24 .  
         [0020]     The suction chamber  151  and the discharge chamber  152  are connected to an external refrigerant circuit  25 , respectively, as shown schematically in  FIG. 1 . The external refrigerant circuit  25  includes a condenser  26  for radiating heat from refrigerant gas thereby to condense the refrigerant, an expansion valve  27  and an evaporator  28  for transferring the ambient heat to the refrigerant. Refrigerant gas in the discharge chamber  152  flows out into the external refrigerant circuit  25  and returns to the suction chamber  151 .  
         [0021]     An electric motor M having an output shaft  30  is disposed in the motor housing. The output shaft  30  of the motor M is axially movably supported by radial bearings  31 ,  32  in the motor housing and the center housing  14 , respectively. The radial bearings correspond to a guide in this embodiment. One end of the output shaft  30  extends into the center housing  14  and has therein an internally splined hole  303 . One end of the rotary shaft  16  protrudes into the center housing  14  and has an externally splined protrusion  161 . As shown in  FIGS. 1 and 3 , the protrusion  161  of the rotary shaft  16  is fitted in the hole  303  of the output shaft  30  of the electric motor M by spline engagement. Thus, the output shaft  30  and the rotary shaft  16  are connected in the center housing  14  in such a way that the output shaft  30  of the electric motor M is axially movable while being rotated together with the rotary shaft  16 .  
         [0022]     As shown in  FIG. 2 , the electric motor M has a rotor  33  which is fixed on the output shaft  30  in the motor housing  29  and a plurality of stators  34  which are provided on the inner peripheral surface of the motor housing  29 . The rotor  33  includes a rotor core  331  fixed on the output shaft  30  and a plurality of permanent magnets  332  provided on the circumferential surface of the rotor core  331 . The permanent magnets  332  are disposed such that any two adjacent permanent magnets  332  have different magnetic poles on the side thereof adjacent to the stators  34 .  
         [0023]     Each stator  34  includes a stator core  341  and a coil wound around the stator core  341 . The rotor  33  and hence the output shaft  30  are rotated when electric current is supplied to the coil  342 . The rotary shaft  16  and the swash plate  11  rotate integrally with the output shaft  30 . Therefore, the speed of the compressor coincides with the speed of the electric motor M.  
         [0024]     As shown in  FIG. 1 , the output shaft  30  which is a part of the rotor  33  is axially movably supported by the radial bearings  31 ,  32 . The radial bearings  31 ,  32  serve as a guide means for guiding the rotor  33  moving in its axial direction.  
         [0025]     In the center housing  14 , a disc-shaped guide plate  35  is fixed on the rotary shaft  16 . The guide plate  35  is formed at the outer periphery thereof with an integral cylindrical portion  351 . A disc-shaped guide plate  36  is fixed to the distal end surface of the cylindrical portion  351 . The guide plate  35  is in parallel relation to the guide plate  36 . A plurality of movable bodies  37  (four such bodies in the illustrated embodiment, each being fan-shaped, as shown in  FIG. 3 ) is accommodated between the guide plates  35 ,  36 . The movable bodies  37  are arranged equidistantly around the axis  301  of the output shaft  30 . Each body  37  is movable radially of the rotor  33 . One end surface  371  of the movable body  37  slides on the guide plate  35 , while the other end surface  372  of the movable body  37  slides on the guide plate  36 .  
         [0026]     The cylindrical portion  351  has therein an annular elastic member  38  which is made of rubber and urges the movable bodies  37  radially inward of the output shaft  30 .  
         [0027]     The guide plate  36  is formed at its center with a shaft hole  361 . One end of the output shaft  30  passes through the shaft hole  361  and extends into the space between the guide plates  35 ,  36 . Four planar inclined surfaces  302  are formed on one end of the output shaft  30  between the guide plates  35 ,  36 , while four planar cam surfaces  373  are formed on the movable bodies  37  so as to be contactable in area with the inclined surfaces  302 . Referring to  FIGS. 1 and 3 , the one end of the output shaft  30  is formed with four planar surfaces  302  each of which is inclined at an angle with respect to the axis  301  of the output shaft  30  so that a quadrilateral pyramid formed by the one end, while each movable body  37  has a planar cam surface  373  formed so as to be contactable with each one of the inclined surfaces  302  of the output shaft  30 , as shown specifically in  FIG. 3 .  
         [0028]     A race  39  and a compression spring  40  are interposed between the end wall  291  of the motor housing  29  and the end surface of the output shaft  30 . The compression spring  40  urges the output shaft  30  in its axial direction through the race  39  so that the inclined surfaces  302  of the output shaft  30  and the cam surfaces  373  of the movable bodies  37  are pressed against each other by the urging force of the compression spring  40 .  
         [0029]      FIG. 1  shows a state of the compressor  10  where the electric motor M is at a stop. In this state, the elastic member  38  is elastically deformed and the movable bodies  37  are pressed against the circumferential surface of the protrusion  161  of the rotary shaft  16  by the elastic force of the deformed elastic member  38 . That is, the elastic member  38  provides the movable bodies  37  with a preload of predetermined magnitude which causes the movable bodies  37  to be in contact with the protrusion  161  of the rotary shaft  16 . The compression spring  40  is compressed, producing an urging force to press the inclined surfaces  302  of the output shaft  30  against the cam surfaces  373  of the movable bodies  37 . That is, the urging force of the compression spring  40  acts on the movable bodies  37  through the engagement between the inclined surfaces  302  and the cam surfaces  373  thereby to urge the movable bodies  37  radially outward.  
         [0030]     When the electric motor M is running, the four movable bodies  37  are subjected to centrifugal force resulting from the rotation of the output shaft  30  (the rotor  33 ) and acting in radially outward direction. When the sum of the centrifugal force acting on the movable bodies  37  and the above radial urging force of the compression spring  40  acting on the movable bodies  37  exceeds the aforementioned preload by the elastic member  38 , the movable bodies  37  are moved radially outwardly. Then, the output shaft  30  is moved in axial direction thereof with the rotor  33  mounted thereof rightward as seen in  FIG. 1  by the urging force of the compression spring  40 .  
         [0031]      FIG. 4  shows a state of the compressor  10  where the electric motor M is running at a high speed. In the state shown in  FIG. 4 , the movable bodies  37  are spaced apart from the protrusion  161  of the rotary shaft  16  by the centrifugal force resulting from the high-speed rotation of the rotor  33 . As apparent from  FIG. 4 , the elastic member  38  is then deformed further than the state of  FIG. 1 . The distal end of the protrusion  161  contacts the bottom of the hole  303 , and the movable bodies  37  are spaced farthest away from the peripheral surface of the protrusion  161 , accordingly.  
         [0032]     As the movable bodies  37  are moved radially inward by the elastic force of the elastic member  38 , the inclined surfaces  302  of the output shaft  30  are pressed by the cam surfaces  373 , and the output shaft  30  and hence the rotor  33  are moved axially so as to increase the facing area between the rotor  33  and the stator  34 .  
         [0033]     The elastic member  38  functions as an elastic urging means for urging the movable bodies  37  radially inward. The elastic member  38  functions also as a preloading means for preloading the movable bodies  37 . The compression spring  40  functions as an urging means for urging the rotor  33  axially. The preloading means, the urging means, the inclined surfaces  302  and the cam surfaces  373  cooperate to form an interlocking means for moving the rotor  33  axially in conjunction with the movement of the movable bodies  37 . Then, the movable bodies  37  and the interlocking means cooperate to form an actuator which is operable to move the rotor axially using the centrifugal force.  
         [0034]     According to the first preferred embodiment, the following advantages are obtained.  
         [0035]     (1-1) When the electric motor M is running at a higher speed, the movable bodies  37  are spaced radially farther away from the axis  301  of the rotor  33  and the rotor  33  is moved axially, accordingly. This movement of the rotor  33  reduces the facing area between the rotor  33  and the stator  34 . This reduction of the facing area reduces the magnitude of induced electromotive force (induced voltage) during the high-speed operation of the electric motor M. That is, a decrease in the efficiency of the electric motor M in the high-speed range is prevented and the high-speed range of the electric motor M is widened.  
         [0036]     (1-2) As the centrifugal force acting on the movable bodies  37  exceeds the preload, the movable bodies  37  are moved radially outward and the rotor  33  is moved axially, accordingly. The speed of the electric motor M (in terms of rpm) at which the magnitude of induced electromotive force begins to be controlled for reduction may be set as desired by determining the magnitude of preload appropriately. Such determination of the preload is preferable for appropriately reducing the magnitude of induced electromotive force in connection with the speed of the electric motor M.  
         [0000]     (1-3) The rubber elastic member  38  which requires only a small space for installation is a suitable elastic urging means for setting the preload.  
         [0000]     (1-4) The structure which allows the cam surfaces  373  to slide in contact with the inclined surfaces  302  for moving the rotor  33  axially in conjunction with the radial movement of the movable bodies  37  is advantageously simple.  
         [0000]     (1-5) The provision of plural movable bodies  37  at equiangular positions around the axis  301  of the rotor  33  permits the rotor  33  to move axially smoothly in conjunction with the radial movement of the movable bodies  37 .  
         [0037]     (1-6) When the fixed-displacement motor-driven compressor  10  is operating at a high speed, the discharge pressure of refrigerant gas is high and the load torque on the compressor  10  is large, accordingly. For a compressor which operates in the high-speed range and on which the large load torque is applied, the electric motor M which is operable at the high torque is suitable as a drive source of the compressor.  
         [0038]     (1-7) The output shaft  30  of the rotor  33  is in spline engagement with the rotary shaft  16  of the compressor. The spline engagement is a suitable structure for the output shaft  30  of the rotor  33  to be movable axially relative to the rotary shaft  16  and for transmitting the rotation of the output shaft  30  of the rotor  33  to the rotary shaft  16 .  
         [0039]     The following will describe a second preferred embodiment of the invention with reference to  FIG. 5 . The same reference numerals denote the substantially similar components or elements to those of the first preferred embodiment.  
         [0040]     The rotary shaft  16  is in spline engagement with the output shaft  30 . An annular base plate  41  is fixedly mounted on the rotary shaft  16 . A pair of support brackets  42  is secured to one end face of the base plate  41  and Levers  43  are supported pivotally about respective shafts  44  by the respective support brackets  42 . A spring  45  is interposed between each lever  43  and the base plate  41  and one end of the lever  43  is pressed against the rear end of the output shaft  30 . A weight  46  is secured to the other end of each lever  43 . The base plate  41 , the levers  43  and the weights  46  are rotatable integrally with the output shaft  30  and the rotary shaft  16 .  
         [0041]     When the electric motor M is at a stop, the levers  43  and the weights  46  are at the position indicated by the dotted line in  FIG. 5  by the urging force of the spring  45 . With the levers  43  and the weights  46  thus positioned, the weights  46  are in contact with the outer periphery of the base plate  41 . Focusing on the upward lever  43 , shaft  44 , spring  45  and weight  46  in  FIG. 5 , the lever  43  is prevented from pivoting clockwise about the shaft  44 . The lever  43  and the weight  46  are preloaded by the urging force of the spring  45  so as to pivot clockwise about the shaft  44 . The urging force of the compression spring  40  acts on the lever  43  through the output shaft  30 . Thus, the lever  43  is loaded by the urging force of the compression spring  40  so as to pivot counterclockwise about the shaft  44 . The clockwise moment Mo due to the preload which acts clockwise about the shaft  44  is set greater than the counterclockwise moment M 1  due to the load which acts counterclockwise about the shaft  44  by the urging force of the compression spring  40 .  
         [0042]     When the electric motor M is running, the lever  43  and the weight  46  are urged counterclockwise around the shaft  44  by the centrifugal force due to the rotation of the output shaft  30  (the rotor  33 ). When the sum of the counterclockwise moment M 2  due to the load which acts counterclockwise around the shaft  44  and the moment M 1  exceeds the moment Mo because of an increase of the above urging force, the lever  43  and the weight  46  are pivoted counterclockwise about the shaft  44 . Accordingly, the output shaft  30  and the rotor  33  are moved axially from the motor housing  29  toward the center housing  14  by the urging force of the compression spring  40 .  
         [0043]     In  FIG. 5 , when the levers  43  are at the position indicated by the solid line, the electric motor M is running at a high speed. In this state, the weights  46  are positioned away from the outer periphery of the base plate  41  by the centrifugal force due to the high speed rotation of the rotor  33  and the distal end of the protrusion  161  is placed in contact with the bottom of the hole  303 . Therefore, the weights  46  are placed farthest away from the outer periphery of the base plate  41  by the contact between the distal end of the protrusion  161  and the bottom of the hole  303 .  
         [0044]     The springs  45  function as an elastic urging means for urging their associated weights  46 , or the movable bodies, radially outward. The springs  45  also function as a preloading means for preloading the weighs  46 . The compression spring  40  functions as an urging means for urging the rotor axially. The preloading means, the urging means and the levers  43  cooperate to form an interlocking means for moving the rotor  33  axially in conjunction with the movement of the weights  46 . Then, the weights  46  and the interlocking means cooperate to form an actuator which is operable to move the rotor  33  axially using the centrifugal force.  
         [0045]     According to the second preferred embodiment, the same advantages as the above-mentioned (1-1), (1-2), (1-6) and (1-7) of the first preferred embodiment are obtained.  
         [0046]     The present invention is not limited to the embodiments described above but may be modified into various alternative embodiments as exemplified below.  
         [0000]     (1) In an alternative embodiment to the first preferred embodiment, instead of the rubber elastic member  38 , a coil-shaped compression spring is used for each movable body  37 .  
         [0000]     (2) In an alternative embodiment as shown in  FIG. 6 , the rotor is moved axially by an electric actuator  401  which is operable in response to a command such as electrical signal.  
         [0047]     (3) In the first and second preferred embodiments, the electric motor is of an inner rotor type which has the stator arranged around the rotor having permanent magnets. In an alternative embodiment, however, the present invention is applicable to an outer rotor type electric motor which has a rotor having permanent magnets arranged around a stator for rotation therearound.  
         [0000]     (4) In an alternative embodiment, the present invention is applicable to a scroll type compressor.  
         [0000]     (5) In an alternative embodiment, the present invention is applicable to a variable displacement motor-driven compressor.  
         [0048]     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims.