Electric motor

An electric motor includes: a stator; and a rotor, in which the rotor includes: a rotor core having magnet insertion holes; and magnets in the magnet insertion holes. Further, the rotor core includes: a rotor core main body; rotor core outer peripheral portions positioned outward in the rotor radial direction with respect to the magnets and forming parts of inner surfaces of the magnet insertion holes outward in the rotor radial direction; and bridges, forming the respective magnet insertion holes by connecting the rotor core main body and the rotor core outer peripheral portions, the bridges being formed to be thinner than thickest portions of the rotor core outer peripheral portions, and the magnets are shaped to be a cross-section polyhedron having oblique sides which narrow as the oblique sides go from the inward toward the outward in the rotor radial direction.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2018-195331 filed in Japan on Oct. 16, 2018.

BACKGROUND

The present disclosure relates to an electric motor.

In Japanese Laid-open Patent Publication No. 2004-364369, there is disclosed an electric motor in which, in a plan view in which a rotor is viewed from the axial direction, a trapezoidal magnet having a pair of oblique sides inclined such that the distance therebetween narrows as it goes from the inner side toward the outer side in the rotor radial direction is embedded in a rotor core.

SUMMARY

There is a need for providing an electric motor capable of suppressing stress concentration on a rotor core due to centrifugal force applied to a magnet and reducing a leakage flux.

According to an embodiment, an electric motor includes: a stator; and a rotor rotatably provided with respect to the stator. Further, the rotor includes: a rotor core having a plurality of magnet insertion holes formed in a rotor circumferential direction; and a plurality of magnets respectively provided in the magnet insertion holes, the rotor core includes: a rotor core main body forming at least inner surfaces of the magnet insertion holes inward in a rotor radial direction; a plurality of rotor core outer peripheral portions positioned outward in the rotor radial direction with respect to the respective magnets and forming parts of inner surfaces of the magnet insertion holes outward in the rotor radial direction; and a plurality pairs of bridges, forming the respective magnet insertion holes together with the rotor core main body and the rotor core outer peripheral portions by connecting the rotor core main body and the rotor core outer peripheral portions, the bridges being formed to be thinner than thickest portions of the rotor core outer peripheral portions in a plan view in which the rotor is viewed from an axial direction, and the magnets are shaped to be a cross-section polyhedron having a pair of oblique sides inclined such that a distance between the oblique sides narrows as the oblique sides go from the inward toward the outward in the rotor radial direction in the plan view in which the rotor is viewed from the axial direction.

DETAILED DESCRIPTION

In the electric motor disclosed in Japanese Laid-open Patent Publication No. 2004-364369, there has been still room for consideration in reducing, among magnetic fluxes from the trapezoidal magnet, a leakage flux that is a flux not directed to a stator disposed outward in the rotor radial direction, and in stress concentration on the rotor core due to centrifugal force applied to the magnet when the rotor rotates.

Hereinafter, an embodiment of an electric motor according to the present disclosure will be described. Note that the present disclosure is not limited by the present embodiment.

FIG. 1is a cross-sectional view of an electric motor1according to the embodiment.FIG. 2is a view of a rotor3as viewed from the axial direction. As illustrated inFIG. 1, the electric motor1according to the embodiment includes a shaft2, the rotor3, a stator4and the like.

The shaft2is a metallic rotary shaft member elongated in the axial direction. Note that the term “axial direction” in the descriptions is defined to be the axial direction (longitudinal direction) of the shaft2. The rotor3includes a rotor core31, a magnet32and the like. The rotor core31is formed in a cylindrical shape by laminating a plurality of electromagnetic steel sheets in the axial direction of the shaft2. In the rotor core31, there is a gap between the electromagnetic steel sheets in the axial direction, whereby the magnetoresistance in the axial direction is greater than the magnetoresistance in the circumferential direction and the radial direction that is a direction orthogonal to the axial direction of the rotor core31. Therefore, in the rotor core31, a magnetic flux hardly flows in the axial direction, and the magnetic flux easily flows in the radial direction and the circumferential direction. The magnet32is embedded in the rotor core31, and extends in the axial direction of the rotor core31. Both axial end surfaces of the magnet32are substantially flush with both axial end surfaces of the rotor core31.

The stator4is disposed at a predetermined interval radially outward of the rotor, and includes an annular stator core41in which a plurality of slots43is formed in the circumferential direction, and a stator coil42inserted into each slot43and wound around the stator core41. The stator core41is formed by laminating a plurality of electromagnetic steel sheets in the axial direction.

The rotor core31has a plurality of magnet insertion holes33disposed at regular intervals along the rotor circumferential direction, which is formed in a trapezoidal shape with a direction orthogonal to the rotor radial direction serving as a longitudinal direction when the rotor3is viewed from the axial direction, and is formed to axially penetrate. Note that the rotor3ofFIG. 2has four magnet insertion holes33provided at every 90 degrees in the rotor circumferential direction.

The rotor core31includes a rotor core main body31aforming at least an inner surface33aof the magnet insertion hole33inward in the rotor radial direction, a rotor core outer peripheral portion31bpositioned outward in the rotor radial direction with respect to the magnet32and forming a part of an inner surface33bof the magnet insertion hole33outward in the rotor radial direction, and a bridge36forming the magnet insertion hole33together with the rotor core main body31aand the rotor core outer peripheral portion31bby connecting the rotor core main body31aand the rotor core outer peripheral portion31b.

In addition, the rotor core31has a plurality of gaps35disposed in the rotor circumferential direction, which is formed in a substantially U-shape opened outward in the rotor radial direction when the rotor3is viewed in the axial direction, and axially penetrates. The gap35and the bridge36are adjacent to each other in the rotor circumferential direction.

The magnet32is, for example, a neodymium magnet. In the present embodiment, the magnet32is shaped to be a cross-section polyhedron having a pair of oblique sides inclined such that the distance therebetween narrows as it goes from the inner side toward the outer side in the rotor radial direction in a plan view in which the rotor3is viewed from the axial direction. Specifically, the cross-section polyhedron is a symmetric trapezoid surrounded by four sides including a short side321and a long side323orthogonal to the rotor radial direction, and a pair of oblique sides322inclined such that the distance therebetween narrows as it goes from the inner side toward the outer side in the rotor radial direction. In the plan view in which the rotor3is viewed from the axial direction as described above, the trapezoidal magnet32is inserted into the trapezoidal magnet insertion hole33. When the magnet32is inserted into the magnet insertion hole33, there is a slight gap between the outer periphery of the magnet32and the inner periphery of the magnet insertion hole33, and the gap is filled with resin to be hardened so that the magnet32is fixed in the magnet insertion hole33. Alternatively, the magnet32may be press-fitted and fixed in the magnet insertion hole33.

In this manner, a plurality of magnets32is embedded in the rotor core31in the rotor circumferential direction, and the rotor3is what is called an interior permanent magnet (IPM) rotor (magnet-embedded rotor). Further, in the rotor3illustrated inFIG. 2, a portion in which the magnet32is disposed to be sandwiched between the pair of gaps35in the rotor circumferential direction is set to be a magnetic pole portion34A. Furthermore, in the rotor3illustrated inFIG. 2, a portion adjacent to the magnetic pole portion34A with the gap35interposed therebetween in the rotor circumferential direction is set to be a pseudo magnetic pole portion34B in which no magnet32is disposed. Each of the magnets32is set to the same magnetic pole (e.g., N pole), and the magnetism of the magnet32causes the pseudo magnetic pole portion34B to function as a magnetic pole different from that of the magnetic pole portion34A (e.g., S pole). Accordingly, in the rotor3ofFIG. 2, the magnetic pole portion34A and the pseudo magnetic pole portion34B having mutually different magnetic poles are alternately arranged in the rotor circumferential direction. In this manner, the rotor3ofFIG. 2is what is called a consequent-pole rotor.

The bridge36is formed to be thinner than the thickest portion of the rotor core outer peripheral portion31bin the plan view in which the rotor3is viewed in the axial direction. Specifically, it is formed such that a side surface361on the side of the gap35of the bridge36is recessed toward the side of the magnet32in the rotor circumferential direction. Meanwhile, in the rotor circumferential direction, an inner surface33c(side surface on the side of the magnet32of the bridge36) of the magnet insertion hole33formed by the bridge36in the rotor circumferential direction is formed along the oblique side322of the magnet32facing the inner surface33c. Accordingly, the width of the bridge36in the rotor circumferential direction can be made narrower than in the case where the side surface361on the side of the gap35of the bridge36is not formed to be recessed toward the side of the magnet32. As a result, since the width of the bridge36can be narrowed, among magnetic fluxes MF from the magnet32ofFIG. 3, a leakage flux LF that is a magnetic flux passing through the bridge36without being directed toward the side of the stator4can be reduced.

Here, the shape of the magnet32is a trapezoid in the plan view in which the rotor3is viewed from the axial direction, whereby the rotor core31is pressed by the pair of oblique sides322and the short side321of the magnet32due to centrifugal force F in the radial direction generated by the rotor3being rotated. At this time, the force with which the magnet32presses the rotor core31due to the centrifugal force F is dispersed to the short side321and the pair of oblique sides322, whereby stress concentration on the rotor core31due to the centrifugal force F applied to the magnet32can be suppressed.

Further, on the oblique side322of the magnet32, a component of the centrifugal force F is divided into a component force Fa in a direction orthogonal to the oblique side322and a component force Fb in a direction parallel to the oblique side322. Accordingly, the force applied from the oblique side322of the magnet32to the bridge36due to the centrifugal force F becomes the component force Fa smaller than the centrifugal force F. Therefore, bending stress hardly acts on the bridge36even if the width of the bridge36is narrowed as described above, whereby deformation of the bridge36can be suppressed.

Furthermore, in the present embodiment, there are provided core clearance shape portions331and332that are axially penetrating through holes connected to the magnet insertion hole33at positions of the rotor core31facing the corners of the magnet32in the plan view in which the rotor3is viewed from the axial direction. Specifically, the core clearance shape portion331is provided at a portion facing the corner formed by the oblique side322and the long side323of the magnet32in the rotor core31. Furthermore, the core clearance shape portion332is provided at a portion facing the corner formed by the oblique side322and the short side321of the magnet32in the rotor core31in the plan view in which the rotor3is viewed from the axial direction. In this manner, the core clearance shape portions331and332are formed at the portions facing the corners of the magnet32in the rotor core31, whereby the stress concentration from each corner of the magnet32to the rotor core31can be suppressed.

Furthermore, in the plan view in which the rotor3is viewed from the axial direction, the core clearance shape portion331is positioned at the corner formed by the inner surface33cof the magnet insertion hole33formed by the bridge36in the rotor circumferential direction and the inner surface33aof the magnet insertion hole33formed by the rotor core main body31ainward in the rotor radial direction. In addition, as illustrated inFIG. 4, the core clearance shape portion331is formed closer to the side of the magnet32than a straight line A passing on the inner surface33cof the magnet insertion hole33in the rotor circumferential direction in the plan view in which the rotor3is viewed from the axial direction.

For example, as illustrated inFIG. 5, when the core clearance shape portion331is formed only on the side opposite to the side of the magnet32with respect to the straight line A in the plan view in which the rotor3is viewed from the axial direction, stress is concentrated in the direction in which the bridge36opens (direction away from the magnet32) due to the centrifugal force, which may lead to deformation of the bridge36. In view of the above, as illustrated inFIG. 4, the core clearance shape portion331is formed on the side closer to the magnet32than the straight line A, whereby the stress concentration in the direction in which the bridge36opens (direction away from the magnet32) due to the centrifugal force can be suppressed.

In the present embodiment, the shape of the core clearance shape portion331, which is for suppressing the stress concentration in the direction in which the bridge36opens (direction away from the oblique side322of the magnet32) due to the centrifugal force, is not limited to the one formed only on the side of the magnet32with respect to the straight line A. That is, as illustrated inFIG. 6, the core clearance shape portion331only needs to be formed such that an area (1) on the side of the magnet32with respect to the straight line A is greater than an area (2) on the side opposite to the side of the magnet32with respect to the straight line A.

In addition, a configuration of the rotor3included in the electric motor1according to the embodiment is not limited to the consequent-pole rotor ofFIG. 2. For example, the magnet32may also be disposed in the pseudo magnetic pole portion34B of the rotor3illustrated inFIG. 2, and the rotor3may be configured such that the magnetic pole portions34A are adjacently arranged with the gap35interposed therebetween in the rotor circumferential direction ofFIG. 7.

Furthermore, the rotor3included in the electric motor1according to the present embodiment may have a configuration in which the rotor core outer peripheral portion31b(outer periphery of the rotor core31) is annular such that the outer peripheries of the magnetic pole portion34A and the pseudo magnetic pole portion34B adjacent to each other are connected by a bridge37ofFIG. 8in the rotor3illustrated inFIG. 2, or the outer peripheries of the adjacent magnetic pole portions34A are connected by the bridge37ofFIG. 9in the rotor3illustrated inFIG. 7. Accordingly, wind noise that may be generated when the rotor rotates can be reduced. The rotor3ofFIGS. 2 and 8is of the consequent-pole type, whereby the number of the magnets32provided in the rotor core31can be reduced compared with the rotor3ofFIGS. 7 and 9. Since the number of the magnets32can be reduced, portions of the rotor core31pressed by the plurality of magnets32due to the centrifugal force is reduced, whereby durability of the rotor core31can be improved.

Moreover, in the present embodiment, at least one corner of the magnet32may be rounded (R-shaped). For example, as illustrated inFIG. 10, four corners of a trapezoidal magnet32A may be rounded (R-shaped) in the plan view in which the rotor3is viewed from the axial direction. With this arrangement, stress concentration in the portion facing each corner of the magnet32in the rotor core31can be suppressed.

Moreover, in the present embodiment, a shape of the magnet32that is a cross-section polyhedron in the plan view in which the rotor3is viewed from the axial direction may be a hexagonal magnet32B ofFIG. 11, an octagonal magnet32C ofFIG. 12, a triangular magnet32D ofFIG. 13, or may be a pentagonal magnet32E ofFIG. 14. Although each corner of the magnet32having the hexagonal shape ofFIG. 11or the octagonal shape ofFIG. 12is sharp (angular shape), it may be rounded (R-shaped). In addition, although each corner of the magnet32having the triangular shape ofFIG. 13or the pentagonal shape ofFIG. 14is rounded (R-shaped), it may be sharp (angular shape). As described above, in any of the shapes of the magnets32A to32E ofFIGS. 11 to 14, it has a pair of oblique sides inclined such that the distance therebetween narrows as it goes from the inward toward the outward in the rotor radial direction in the plan view in which the rotor3is viewed from the axial direction, whereby the stress concentration on the rotor core31due to the centrifugal force F can be suppressed. The shape of the magnet insertion hole33into which the magnets32A to32E ofFIGS. 11 to 14are inserted may be appropriately set according to the shapes of the magnets32A to32E.

In the electric motor according to the present disclosure, the force with which the magnet presses the rotor core due to the centrifugal force is dispersed to at least a pair of oblique sides, whereby the stress concentration on the rotor core due to the centrifugal force applied to the magnet can be suppressed, and a leakage flux can be reduced as a width of the bridge can be narrowed.

According to an embodiment, stress concentration in the portion of the rotor core facing the corner of the magnet can be suppressed.

According to an embodiment, stress concentration in the direction in which the bridge opens due to centrifugal force can be suppressed.

According to an embodiment, wind noise that may be generated when the rotor rotates can be reduced.

According to an embodiment, pressing force from the magnet due to the centrifugal force is applied to the rotor core at the short side and the oblique sides of the trapezoid, whereby the stress concentration can be suppressed.

According to an embodiment, stress concentration in the portion of the rotor core facing the corner of the magnet can be suppressed.

According to an embodiment, the number of the magnets provided in the rotor core can be reduced so that portions of the rotor core pressed by the magnets due to the centrifugal force is reduced, whereby durability of the rotor core can be improved.