Patent Publication Number: US-2020295639-A1

Title: Induction motor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-002700, filed on Jan. 11, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments of the present invention relate to an induction motor. 
     BACKGROUND 
     A so-called squirrel-cage induction motor is one of known induction motors. The squirrel-cage induction motor is configured by a stator in which a stator coil is arranged around a substantially cylindrical stator core and a rotor provided radially inward of the stator to be rotatable with respect to the stator. 
     The rotor has a core fixed to a rotating shaft. A plurality of teeth extending in a radial direction are arranged on the core radially. A slot is formed between circumferentially adjacent teeth. A conductive bar is inserted in the slot. The conductive bars are mutually connected at ends in an axial direction of the rotor core by a shorting ring. 
     Generally, a gap is provided between the end of the core and the shorting ring in order to weld the conductive bar and the shorting ring to each other. However, the conductive bar is exposed in this gap and therefore, because of rotation of the rotor, wind noise in association with the rotation is generated around the conductive bar itself or in a space between adjacent conductive bars. This wind noise results in increase of noise of the induction motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a configuration example of an induction motor according to a first embodiment; 
         FIG. 2  is a perspective view illustrating a configuration example of a rotor; 
         FIG. 3  is a side view illustrating a configuration example of a rotor core; 
         FIG. 4  is a diagram illustrating a portion of a section of the rotor perpendicular to an axial direction; 
         FIG. 5  is a cross-sectional view illustrating a configuration example of a conductive bar  30  according to the first embodiment; 
         FIG. 6  is a perspective view illustrating a configuration example of one end of the conductive bar  30 ; 
         FIG. 7  is a graph illustrating a relation between an inclination angle of an inclined surface  31  and wind noise caused by rotation of the rotor  3 ; 
         FIG. 8  is a cross-sectional view illustrating a configuration example of the conductive bar  30  according to a second embodiment; 
         FIG. 9  is a perspective view illustrating a configuration example of one end of the conductive bar  30  according to the second embodiment; and 
         FIG. 10  is a cross-sectional view illustrating a configuration example of the conductive bar  30  and a slot S according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. These embodiments do not limit the present invention. The drawings are schematic or conceptual and the ratio of respective parts and the like are not necessarily the same as those of real products. In the specification and the drawings, constituent elements identical to those described with respect to the drawings that have been already described are denoted by like reference signs, and detailed explanations thereof are appropriately omitted. 
     An induction motor according to an embodiment comprises a stator for generating a magnetic field and a rotor driven by the magnetic field from the stator. The rotor includes a core provided on a rotating shaft. A plurality of conductive bars are inserted into a plurality of slots provided in the core. A first ring is connected to one ends of the conductive bars. A second ring is connected to the other ends of the conductive bars. Gaps are formed between the core and the first ring and between the core and the second ring. The conductive bars are connected to the first or second ring across the gaps from the slots. A section of each conductive bar perpendicular to a longitudinal direction thereof is a polygon having a first surface located on the outer circumference side of the rotor, a second surface located on the inner circumference side of the rotor, a plurality of side surfaces located between the first surface and the second surface, and inclined surfaces that are located between the first surface and the side surfaces at least in the gaps and are inclined with respect to the first surface and the side surfaces. The inclined surfaces are inclined with respect to the first surface at an angle of 10 degrees to 40 degrees. A width of each inclined surface in a direction parallel to the first surface is equal to or less than a width from a slot opening that is provided in an outer circumference side of the core and communicates with one of the slots to a corner of the slot in the direction parallel to the first surface. 
     First Embodiment 
       FIG. 1  is a cross-sectional view illustrating a configuration example of an induction motor  1  according to a first embodiment.  FIG. 1  only illustrates a configuration of one side half of the induction motor  1  along a center axis C. The induction motor  1  is, for example, a motor used for driving a railroad vehicle (not illustrated). 
     The induction motor  1  includes a stator  2 , a rotor  3  provided to be rotatable with respect to the stator  2 , and a casing  4  that supports the stator  2  and the rotor  3 . The stator  2  is fixed to the casing  4 . The rotor  3  is configured to be rotatable about the center axis C with respect to the stator  2 . In the induction motor  1 , a current is supplied to the stator (a primary side)  2 , and an induced current is generated in the rotor (a secondary conductor)  3  by a magnetic field generated by the stator  2 . The rotor obtains rotational torque by the magnetic field from the stator, thereby being driven to rotate. 
     In the following descriptions, a direction along the center axis C and a direction rotating around the center axis C are simply referred to as “axial direction” and “circumferential direction (a rotating direction)”, respectively, and a direction perpendicular to the axial direction and the circumferential direction is referred to as “radial direction (radiation direction)”. 
     The stator  2  includes a substantially cylindrical stator core  5 . The stator core  5  is a stack of a plurality of electromagnetic steel sheets  6  stacked along the axial direction. The electromagnetic steel sheets  6  are thin steel sheets manufactured by adding silicon to iron, for example. 
     A plurality of stator teeth  7  are provided on an inner circumferential surface of the stator core  5  to project toward the center axis C. The stator teeth  7  are arranged in the circumferential direction substantially equidistantly. A stator slot  8  is provided between the circumferentially adjacent stator teeth  7 . A stator coil  9  is wound around each stator tooth  7  via the corresponding stator slots  8 . The stator coil  9  is provided to overhang from both ends in the axial direction of the stator core  5  toward outside in the axial direction. Direct-current power, for example, supplied from an overhead wire via a pantograph (both not illustrated) is supplied to this stator coil  9  after being converted to alternating-current power. 
     Stator core clampers  10  are provided at both the ends in the axial direction of the stator core  5 . The stator core clampers  10  clamp and hold the electromagnetic steel sheets  6  that are stacked to configure the stator core  5  from both ends in the axial direction to prevent the electromagnetic steel sheets  6  from being separated from each other. The stator core clampers  10  are formed of metal, such as iron, to be substantially annular, and the outer diameter thereof is set to be larger than the outer diameter of the stator core  5 . Further, the inner diameter of the stator core clampers  10  is set to prevent the stator core clampers  10  and the stator coil  9  from coming into contact with each other. The stator core  5  and the stator core clampers  10  are integrated with each other by welding or the like. 
     The casing  4  is configured by a pair of tubular mirror lids  11  and  12  arranged on both sides in the axial direction of the stator  2  and a pair of bearing brackets  13  and  14  respectively integrated with the mirror lids  11  and  12 . The mirror lids  11  and  12  are arranged in such a manner that their openings  11   a  and  12   a  face the stator core  5 . Further, outer flange portions  15  and  16  are formed on outer peripheral edges of the openings  11   a  and  12   a  of the mirror lids  11  and  12 , respectively. 
     The outer diameter of the outer flange portions  15  and  16  is set to be substantially the same as the outer diameter of the stator core clampers  10 . Accordingly, the stator core clampers  10  and the outer flange portions  15  and  16  of the mirror lids  11  and  12  overlap each other in the axial direction. Each stator core clamper  10  and a corresponding one of the outer flange portions  15  and  16  of the mirror lids  11  and  12  are fastened and fixed to each other by a bolt and a nut (not illustrated). Accordingly, the stator  2  is supported by the mirror lids  11  and  12 . 
     Openings  11   c  and  12   c  are formed in center portions in the radial direction of bottoms  11   b  and  12   b  of the mirror lids  11  and  12 , respectively. To close the openings  11   c  and  12   c,  the corresponding bearing brackets  13  and  14  are provided. The bearing brackets  13  and  14  are integrated with the corresponding mirror lids  11  and  12  with each other, respectively. 
     Each of the bearing brackets  13  and  14  is formed to be substantially frustoconical and is arranged to project toward the stator  2 . Insertion holes  13   a  and  14   a  that allow a rotating shaft  21  to be inserted therethrough penetrate through center portions in the radial direction of the bearing brackets  13  and  14  along the axial direction. Further, bearing accommodating portions  13   b  and  14   b  are provided to be concave in the center portions in the radial direction of the bearing brackets  13  and  14  in outer portions in the axial direction, respectively. Bearings  17  and  18  are provided in the respective bearing accommodating portions  13   b  and  14   b.  The rotating shaft  21  is supported by the bearing brackets  13  and  14  via the bearings  17  and  18  to be rotatable. The casing  4  is fixed under the floor of a railroad vehicle (both not illustrated), for example. 
     The rotor  3  has the rotating shaft  21  that is supported to be rotatable about the center axis C. A rotor core  22  (hereinafter, also simply “core  22 ”) that is substantially cylindrical is externally fitted and fixed onto the rotating shaft  21 . The outer diameter of the core  22  is set to allow a small gap to be formed between an outer circumferential surface  22   a  of the core  22  and the stator teeth  7  of the stator  2 . 
     The core  22  is also formed by stacking a plurality of electromagnetic steel sheets  23  in the axial direction. A through hole  24  is provided at the center in the radial direction of the core  22 . The rotating shaft  21  penetrates through the through hole  24  over the axial direction and rotates about the center axis C together with the core  22  as one unit. In a case where the rotating shaft  21  is inserted into the core  22 , the core  22  and the rotating shaft  21  are integrated with each other by press fitting, shrink fitting, or the like. 
     Rotor core clampers  25  (hereinafter, also simply “core clampers  25 ”) that are substantially disk-shaped are provided at both ends in the axial direction of the core  22 . The core clamper  25  is also formed of metal, such as iron, and has a through hole  25   a  formed at its center in the radial direction. The rotating shaft  21  penetrates through the through hole  25   a  over the axial direction and rotates about the center axis C together with the core  22  as one unit. The core clampers  25  have a role of holding the electromagnetic steel sheets  23  that are stacked to configure the core  22  to prevent the electromagnetic steel sheets  23  from being separated from each other and being displaced in the axial direction with respect to the rotating shaft  21 . 
       FIG. 2  is a perspective view illustrating a configuration example of the rotor  3 .  FIG. 3  is a side view illustrating a configuration example of the core  22 .  FIG. 4  is a diagram illustrating a portion of a section of the rotor  3  perpendicular to the axial direction. 
     The rotor  3  includes the rotating shaft  21 , the core  22 , conductive bars  30 , a first shorting ring  41 , a second shorting ring  42 , and the core clampers  25 . 
     The rotating shaft  21  has an elongate shape along the center axis C as illustrated in  FIG. 2 , and can rotate about the center axis C. The core  22  is fixed to the rotating shaft  21  and has a plurality of teeth T projecting radially outward. As illustrated in  FIG. 4 , a slot S is provided between the teeth T that are adjacent to each other in the circumferential direction Dr. The teeth T are aligned in a circumferential direction substantially equidistantly. Circumferential widths between the teeth T are substantially equal to each other. Therefore, in association with the alignment of the teeth T, the slots S are also aligned in the circumferential direction substantially equidistantly, and their circumferential widths are also substantially equal to each other. As illustrated in  FIGS. 2 and 3 , each slot S is a space that extends in the axial direction between the teeth T and allows the conductive bar  30  to be inserted thereinto. Further, as illustrated in  FIG. 4 , each slot S communicates with a slot opening OP provided on the outer circumference of the core  22 . The width in the circumferential direction Dr of the slot opening OP is narrower than the circumferential width of the slot S, so that the conductive bar  30  is prevented from falling out of the slot opening OP. As illustrated in  FIGS. 2 and 3 , the core clampers  25  are provided at both ends in the axial direction of the core  22  to prevent the electromagnetic steel sheets  23  stacked to configure the core  22  from being separated from each other. 
     The conductive bar  30  is inserted in the slot S. The conductive bar  30  has an elongate shape along the axial direction similarly to the slot S, and is longer than the slot S. Therefore, the conductive bar  30  projects from both the ends in the axial direction of the core  22 . One projecting end of the conductive bar  30  is connected to the first shorting ring  41 , and the other projecting end is connected to the second shorting ring  42 . Accordingly, the conductive bars  30  are coupled and electrically connected to each other by the first and second shorting rings  41  and  42 . A material that is electrically conductive and non-magnetic, for example, copper or aluminum is used for the conductive bars  30 . 
     As illustrated in  FIGS. 2 and 3 , the first shorting ring  41  is connected to one ends of the conductive bars  30 , for example, by welding. The second shorting ring  42  is connected to the other ends of the conductive bars  30 , for example, by welding. A material that is electrically conductive and non-magnetic, for example, copper or aluminum is used for the first and second shorting rings  41  and  42 . 
     Gaps (cavities)  50  are provided between the core  22  (the teeth T) and the first shorting ring  41  and between the core  22  and the second shorting ring  42 , as illustrated in  FIG. 3 , in order to connect the first and second shorting rings  41  and  42  and the conductive bars  30  to each other by welding or the like. The cavities  50  are provided at both ends in the axial direction of the conductive bars  30  between the conductive bars  30  adjacent to each other in the circumferential direction. The conductive bars  30  are connected to the first or second shorting ring  41  or  42  from the slots S across the cavities  50 . The cavities  50  are necessary in manufacturing of the rotor  3 , but cause wind noise during a normal operation as described above. This wind noise results in increase of noise of an induction motor. 
       FIG. 5  is a cross-sectional view illustrating a configuration example of the conductive bar  30  according to the first embodiment.  FIG. 5  is an enlarged view of a section surrounded by a broken line frame B in  FIG. 4 . The slot S that extends in the axial direction (the direction perpendicular to the drawing of  FIG. 5 ) is provided in the core  22 . The conductive bar  30  that also extends in the axial direction is inserted in the slot S. 
     As illustrated in  FIG. 5 , the shape of a section of the slot perpendicular to its longitudinal direction is substantially similar to the shape of a section of the conductive bar  30  perpendicular to its longitudinal direction and slightly larger than the shape of a section of the conductive bar  30  perpendicular to its longitudinal direction. Accordingly, the conductive bar  30  can be inserted into the slot S. In a state where the conductive bar  30  is inserted in the slot S, only a small clearance is formed in the slot S. Therefore, the circumferential width of the conductive bar  30  is substantially the same as or slightly smaller than the circumferential width of the slot S. 
     The conductive bar  30  inserted in the slot S is fixed in the slot S, for example, by swaging, crimping, or using an adhesive via the slot opening OP. 
     The conductive bar  30  is chamfered at an end on the outer circumference side of the core  22  to have inclined surfaces  31 . The inclined surface  31  is provided in an exposed portion in the cavity  50  in the present embodiment, but is not provided in a portion inserted in the slot S. 
       FIG. 6  is a perspective view illustrating a configuration example of one end of the conductive bar  30 . The conductive bar  30  has a conductive portion  30   a  exposed from the cavity  50  and a conductive portion  30   b  inserted in the slot S. A section of the conductive bar  30  perpendicular to its longitudinal direction (the axial direction) includes a first surface F 1  located on the outer circumference side of the rotor  3 , a second surface F 2  located on the inner circumference side of the rotor  3 , and a plurality of side surfaces (third and fourth surfaces) F 3  and F 4  located between the first surface F 1  and the second surface F 2 . Further, in the conductive portion  30   a  exposed by the cavity  50 , the conductive bar  30  has two inclined surfaces  31  between the first surface F 1  and the side surfaces F 3  and F 4 . Each inclined surface  31  is inclined with respect to the first surface F 1  and the corresponding side surface F 3  or F 4 . In other words, the inclined surface  31  is inclined with respect to the circumferential direction (the rotating direction) and the radial direction. Accordingly, the section of the conductive bar  30  perpendicular to the axial direction is substantially hexagonal. In the conductive portion  30   b  hiding in the slot S, the section of the conductive bar  30  perpendicular to the axial direction has a substantially rectangular shape formed by the first surface F 1 , the second surface F 2 , and the side surfaces F 3  and F 4 . The inclined surfaces  31  may be flat or curved. Further, the inclined surfaces  31  may have convex portions and concave portions to a certain degree. 
     By chamfering both ends of the first surface F 1  that is on the outer circumferential side of the conductive bar  30 , which are exposed in the cavities  50 , to form the inclined surfaces  31 , wind noise of the conductive bar  30  and/or the cavity  50  caused by rotation of the rotor  3  is reduced. Accordingly, noise of the induction motor  1  can be suppressed. 
       FIG. 7  is a graph illustrating a relation between an inclination angle of the inclined surface  31  and wind noise caused by rotation of the rotor  3 . The inclination angle of the inclined surface  31  represents an inclination angle of the inclined surface  31  with respect to the first surface F 1  or the rotating direction. That is, the inclination angle of the inclined surface  31  is an angle represented with θ in  FIG. 6 . The graph illustrates the result of calculating noise generated in the cavity  50  by thermal fluid analysis. 
     When an inclination angle θ is too small, the inclined surface  31  is substantially flush with the first surface F 1  (the circumferential direction or the rotating direction) and therefore wind noise (a sound pressure level) is not reduced so much. On the other hand, when the inclination angle θ is too large, the inclined surface  31  comes close to the side surface F 3  or F 4  (the radial direction) and therefore wind noise (the sound pressure level) is not reduced either. 
     In order to reduce the sound pressure level to 115 dB or less, for example, it is preferable that the inclination angle θ is from approximately 10 degrees to approximately 40 degrees. In particular, when the inclination angle θ is around 30 degrees, the sound pressure level is less than 110 dB. Accordingly, it is more preferable that the inclination angle θ is around 30 degrees. Setting the inclination angle of the inclined surface  31  in this manner further reduces wind noise of the conductive bar  30  and/or the cavity  50  caused by rotation of the rotor  3 . 
     Further, it is assumed that the width of the inclined surface  31  in a direction parallel to the first surface F 1  is W 31  as illustrated in  FIG. 5 or 6  and the width from an end of the slot opening OP to a corner of the slot in the direction parallel to the first surface F 1  is Ws as illustrated in  FIG. 5 . In this case, it is preferable that the width W 31  is equal to or less than the width Ws. This is because this setting enables the conductive bar  30  to be surely fixed in the slot S when the conductive bar  30  is swaged. A fillet with a radius of approximately 0.5 millimeter is usually formed at an end of the conductive bar  30 . Therefore, the width W 31  of the inclined surface  31  is 0.5 millimeter or more. 
     Although the inclined surfaces  31  are formed at both ends of the first surface F 1  that is on the outer circumferential side of the conductive bar  30 , they may be formed at both ends of the second surface F 2  that is on the inner circumferential side of the conductive bar  30 . 
     Second Embodiment 
       FIG. 8  is a cross-sectional view illustrating a configuration example of the conductive bar  30  according to a second embodiment. In the second embodiment, the conductive bar  30  is entirely chamfered at an end on the outer circumference side of the core  22  to have the inclined surfaces  31 . That is, in the second embodiment, the inclined surfaces  31  are provided in both the conductive portion  30   a  exposed in the cavity  50  and the conductive portion  30   b  inserted in the slot S. 
       FIG. 9  is a perspective view illustrating a configuration example of one end of the conductive bar  30  according to the second embodiment. With reference to  FIG. 9 , it is apparent that the inclined surfaces  31  are provided in both the conductive portion  30   a  and the conductive portion  30   b.  It suffices that the shape and the size of the inclined surfaces  31  are identical to those of the inclined surfaces  31  in the first embodiment. It suffices that other configurations in the second embodiment are identical to the corresponding configurations in the first embodiment. 
     The inclined surfaces  31  may be entirely provided along an extending direction of the conductive bar  30  in this manner. The second embodiment can obtain identical advantageous effects to those in the first embodiment. Further, because the inclined surfaces  31  are provided in the entire conductive bar  30 , it is possible to process the conductive bar  30  relatively easily. That is, in manufacturing of the conductive bar  30 , it is unnecessary to process the conductive portion  30   a  and the conductive portion  30   b  in different manners from each other. It suffices to perform the same process. Accordingly, the manufacturing cost is reduced. For example, the conductive bar  30  can be chamfered by extrusion or the like in the longitudinal direction in a manufacturing stage. 
     Third Embodiment 
       FIG. 10  is a cross-sectional view illustrating a configuration example of the conductive bar  30  and the slot S according to a third embodiment. The conductive bar  30  according to the second embodiment is used in the third embodiment. Further, in the third embodiment, the shape of a section of the slot S perpendicular to its longitudinal direction is substantially similar to the shape of a section of the conductive bar  30  perpendicular to its longitudinal direction and slightly larger than the shape of a section of the conductive bar  30  perpendicular to its longitudinal direction. 
     For example, the section of the slot S perpendicular to the longitudinal direction (the axial direction) includes a first inner surface FS 1  located on the outer circumference side of the rotor  3 , a second inner surface FS 2  located on the inner circumference side of the rotor  3 , and a plurality of inner side surfaces FS 3  and FS 4  located between the first inner surface FS 1  and the second inner surface FS 2 . Further, the slot S has inner inclined surfaces  33  between the first inner surface FS 1  and the inner side surfaces FS 3  and FS 4 . Each inner inclined surface  33  is inclined with respect to the first inner surface FS 1  and the corresponding inner side surface FS 3  or FS 4 . In other words, the inner inclined surface  33  is inclined with respect to the circumferential direction (the rotating direction) and the radial direction. Accordingly, the section of the slot S perpendicular to the axial direction is substantially hexagonal. The inner inclined surfaces  33  may be flat or curved. Further, the inner inclined surfaces  33  may have convex portions and concave portions to a certain degree. 
     The inner inclined surfaces  33  of the slot S are inclined with respect to the first inner surface FS 1  at substantially the same angle as the inclination angle θ of the inclined surfaces  31  of the conductive bar  30 . That is, it is preferable that the inner inclined surfaces  33  of the slot S are inclined with respect to the first inner surface FS 1  (the circumferential direction or the rotating direction) at an angle of approximately 10 degrees to approximately 40 degrees. In particular, when the inclination angle θ of the inclined surfaces  31  is around 30 degrees, it is preferable that the inclination angle of the inner inclined surfaces  33  of the slot S is also around 30 degrees. Accordingly, it is possible to easily insert the conductive bar  30  into the slot S, and an unnecessary gap is not formed between the conductive bar  30  and the slot S. Therefore, the conductive bar  30  can be easily fixed in the slot S. 
     Other configurations of the third embodiment may be identical to the corresponding configurations of the second embodiment. Therefore, the third embodiment can obtain identical advantageous effects to those in the second embodiment. 
     Although several embodiments of the present invention have been described above, these embodiments are presented for purposes of illustration only and are not intended to limit the scope of the invention. These embodiments can also be carried out in other various modes, and various types of omissions, replacements, and modifications can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the spirit and scope of the invention, and are also included in the invention described in the appended claims and equivalents thereof.