Rotor for reluctance motor

A rotor described herein includes a plurality of flux barriers that include at least one magnetic path formed between a plurality of slits. The flux barriers are arranged in a circumferential direction at a predetermined interval. Adjacent flux barriers are concatenated on an inner circumferential side by an annular connector provided on the inner circumferential side, and are separated on an outer circumferential side by openings provided on the outer circumferential side. The rotor also includes a permanent magnet at least partially embedded within the annular connector.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011154268, filed Jul. 12, 2011, entitled “Rotor for Reluctance Motor”, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND

The embodiments described herein relate generally to reluctance motors, and more specifically, to a reluctance motor configured for use in an electric vehicle. In response to availability issues and costs associated with rare earth metals, electric motors free of rare earth metals are being developed. An example of a type of motor free of rare earth metals that is being developed is a reluctance motor. However, reluctance motors typically develop less torque than surface-mounted permanent magnet (SPM) motors and interior permanent magnet (IPM) motors that include high-performance magnets, for example, neodymium magnets. When included within an electric vehicle, an electric motor that develops higher torque is desired. As referred to herein, an electric vehicle is a vehicle that derives at least a portion of its propulsive force from an electric motor. For example, electric vehicles include vehicles that rely solely on an energy storage device and electric motor for propulsion, hybrid vehicles that rely on an energy storage device and electric motor for propulsion and a fossil fuel based motor to aid propulsion and/or to charge the energy storage device, and/or any other type of vehicle that includes an electric motor.

The magnitude of reluctance torque in a reluctance motor is known to rely on a difference (|Ld−Lq|) between a d-axis inductance (Ld) and a q-axis inductance (Lq). The size and number of windings can be increased to raise the reluctance torque, but it is difficult to increase output to the desired level because of the greater d-axis inductance (Ld) and q-axis inductance (Lq). The reluctance torque can be effectively increased by reducing the magnetic resistance of the LqIq magnetic path, increasing the magnetic resistance of the LdId magnetic path, and increasing a saliency ratio (Ld/Lq).

FIG. 6is a front view of a known reluctance motor100. Reluctance motor100includes a stator core102that includes a plurality of teeth103and a plurality of slots104defined between adjacent teeth. Motor100also includes coils105wound and fitted into the slots104formed between the teeth103. Motor100also includes a rotor core107having a plurality of arc-shaped slits108and a plurality of arc-shaped magnetic paths109formed between each slit108. The plurality of arc-shaped slits108formed in the rotor core107function as flux barriers, which increase the magnetic resistance in magnetic path LqIq, reduce the q-axis inductance (Lq), and increase the saliency ratio.

However, when slits108are provided within the rotor core107to function as flux barriers, q-axis magnetic flux leakage occurs on an outer diameter side of the rotor core107which limits the amount the q-axis inductance (Lq) may be reduced. In order to suppress q-axis magnetic flux leakage and further increase the saliency ratio, a known reluctance motor includes a plurality of independent (e.g., segmented) flux barriers having a magnetic path formed between a plurality of slits to reduce the q-axis inductance (Lq).

However, the independently formed and segmented flux barriers may cause issues related to centrifugal force during high-speed rotation of the rotor and to rotor strength with respect to acceleration. The segmented flux barriers also increase manufacturing complexity (i.e., it is difficult to realize a workable structure).

SUMMARY

In one aspect, a rotor for a reluctance motor is provided. The rotor includes a plurality of flux barriers that includes at least one magnetic path formed between a plurality of slits. The flux barriers are arranged in a circumferential direction at a predetermined interval, wherein adjacent flux barriers are concatenated on an inner circumferential side by an annular connector provided on the inner circumferential side. Adjacent flux barriers are separated on an outer circumferential side by openings provided on the outer circumferential side. The rotor also includes a permanent magnet at least partially embedded within the annular connector.

In another aspect, a reluctance motor is provided. The reluctance motor includes a stator that includes a stator core having a plurality of stator teeth. The reluctance motor also includes a rotor configured to rotate with respect to the stator about a central axis that extends through a center of the rotor from a first end of the rotor to a second end of the rotor. The rotor includes a rotor core that includes an inner surface and an outer surface coaxially arranged about the central axis. The rotor also includes a plurality of flux barriers defined within the rotor core, the flux barriers extending axially through the rotor core from the first end to the second end. The rotor also includes an annular connector included within the rotor core, wherein adjacent flux barriers are concatenated on an inner circumferential side by the annular connector. The rotor also includes a plurality of openings defined within the rotor core along the outer surface of the rotor core, wherein adjacent flux barriers are separated by an opening of the plurality of openings.

In yet another aspect, a rotor core having an inner circumferential side and an outer circumferential side is provided. The rotor core includes a plurality of flux barriers arranged in a circumferential direction around the rotor core at a predetermined interval. The flux barriers that are adjacent to each other in the circumferential direction are concatenated on the inner circumferential side by an annular connector. Adjacent flux barriers are separated by openings in the rotor core that extend to the outer circumferential side.

DETAILED DESCRIPTION

The embodiments described herein relate generally to electric motors, and more specifically, to reluctance motors. More specifically, a rotor described herein facilitates improving torque characteristics of a reluctance motor while maintaining rotor strength and manufacturing ease.

For example, in the exemplary embodiment, the rotor includes a plurality of flux barriers (for example, flux barrier20in the embodiment described below) with a magnetic path (for example, magnetic path13in the embodiment described below) formed between a plurality of slits (for example, slit12in the embodiment described below) arranged in the circumferential direction at a predetermined interval. In this rotor, adjacent flux barriers are concatenated on the inner circumferential side by an annular connector (for example, annular connector14in the embodiment described below) provided on the inner circumferential side and separated on the outer circumferential side by openings (for example, opening16in the embodiment described below) provided on the outer circumferential side, and permanent magnets (for example, permanent magnet15in the embodiment described below) are embedded in the annular connector. In at least some embodiments, the permanent magnets are magnetized in the radial direction of the rotor.

Adjacent flux barriers are concatenated by an annular connector that includes a permanent magnet. The permanent magnet causes magnetic saturation of the annular connector, which increases the magnetic resistance, and reduces the q-axis inductance (Lq). Reducing the q-axis inductance (Lq) while maintaining a d-axis inductance (Ld) increases reluctance torque and, thus, improves torque properties. Because this can increase the magnetic resistance of the annular connector, a configuration can be realized having a greater saliency difference. Because the permanent magnets only have enough magnetic force to magnetically saturate the annular connector, they do not experience the demagnetizing field effect that commonly occurs in permanent magnets. Also, in order to use them in a high permeance environment, a high retention force material is not required. Thus, dysprosium (Dy)-free magnets and inexpensive ferrite magnets can be used. Because the flux barriers are integrally concatenated by an annular connector, a conventional structural design can be used for the rotor, the mechanical strength of the rotor is greater, and the rotor is highly reliable in operating environments requiring high-speed rotation and both rapid acceleration and deceleration.

Because the permanent magnets in the second aspect of the present invention are magnetized in the radial direction of the rotor and embedded in the annular connector, the magnetic force of the permanent magnets does not affect the stator. As a result, drag loss does not occur, and measures such as field weakening are not required even at high rotational speeds.

Referring now to the figures,FIG. 1is a front view of an exemplary reluctance motor1,FIG. 2is an explanatory diagram showing the flow of magnetic flux within the reluctance motor1, andFIG. 3is an enlarged view showing a magnetization direction of an exemplary permanent magnet included within reluctance motor1. As shown inFIG. 1andFIG. 2, the reluctance motor1includes a stator2, and a rotor10opposing an inner circumferential portion of the stator2via an air gap (g). Stator2includes a stator core3. An outer circumferential portion of the stator core3is fixed to a frame or the like (not shown in the drawings). Rotor10includes a rotor core11. Rotor10is supported rotatably by a shaft (not shown in the drawings) positioned along a central axis18of the rotor core11.

In the exemplary embodiment, the stator core3and the rotor core11are formed by punching magnetic steel sheets into a predefined shape and stacking a plurality of these sheets in an axial direction to form a laminated stator core3and/or rotor core11. Alternatively, stator core3and/or rotor core11may be solid rather than laminated, for example, but not limited to, composed of a soft magnetic material using a sintering process.

In the exemplary embodiment, stator core3includes a plurality of teeth4and a corresponding plurality of slots5defined between adjacent teeth4. Teeth4extend from an inner circumferential portion of the stator core3. Stator2also includes a plurality of coils6that are wound around teeth4and disposed within slots5. Although illustrated as including forty-eight teeth4and forty-eight slots5, stator2may include any suitable number of teeth and/or slots that allow reluctance motor1to function as described herein.

In the exemplary embodiment, rotor core11includes a plurality of flux barriers20arranged in a circumferential direction around rotor core11at a predetermined interval. For example, rotor core11may include eight flux barriers20spaced evenly around a circumference of rotor core11. In the exemplary embodiment, each flux barrier20includes a plurality of arc-shaped slits12defined within rotor core11. For example, in the illustrated embodiment, each flux barrier20includes three arc-shaped slits12defined within rotor core11. Although described herein as arc-shaped, slits12may alternatively be formed in a substantially v-shape and/or any other suitable shape that allows reluctance motor1to function as described herein. A protruding side of each slit12faces the axial center18of rotor core11, and the slits12are spaced in a radial direction from the axial center18of rotor core11. Each slit12constitutes a barrier to magnetic flux. In the exemplary embodiment, rotor core11also includes a plurality of arc-shaped magnetic paths13defined between the slits12.

An outer circumferential surface of the rotor core11beyond the slits12positioned on an outermost circumferential side of rotor core11in the radial direction is formed so as to decline towards the axial center18of rotor core11, and magnetic paths13are formed beyond the slits12positioned on the outermost circumferential side in the radial direction. In other words, a plurality of indentations are defined within an outer surface of rotor core11. The indentations extend axially along the outer surface of rotor core11and are centered at a center of the slits12(i.e., are aligned with a centerline of flux barriers20(seeFIG. 2)). Therefore, in the exemplary embodiment, a radial distance from the axial center18to the outer surface of the rotor core at a center of the slits12is less than a maximum radial distance from the axial center18to the outer surface of the rotor core11.

In the exemplary embodiment, rotor core11includes an annular connector14provided on an inner circumferential side of the rotor core11. Adjacent flux barriers20are concatenated on the inner circumferential side by annular connector14. Rotor core11also includes a plurality of openings16defined therein. More specifically, openings16open into the outer circumference of the rotor core11between adjacent flux barriers20and extend in a radial direction from the outer circumference of rotor core11toward the annular connector14. Each of openings16constitutes a barrier to magnetic flux. In other words, flux barriers20adjacent to each other in the circumferential direction are separated on the outer circumferential side of rotor core11by openings16.

As shown inFIG. 3, rotor10also includes a plurality of permanent magnets15at least partially embedded within rotor core11. For example, rotor10includes permanent magnets15embedded within a plurality of magnet openings defined within rotor core11, and more specifically, within a plurality of magnet openings defined within the annular connector14portion of rotor core11. Permanent magnets15may include ferrite magnets and/or any other suitable type of magnet that allows reluctance motor1to function as described herein. In the exemplary embodiment, magnets15are magnetized in the radial direction of the rotor10and are embedded within annular connectors14. The permanent magnet15inFIG. 3is arranged so that a north (N) pole is positioned on the outside in the radial direction and a south (S) pole is positioned inward in the radial direction. Alternatively, the S pole may be positioned on the outside in the radial direction and the N pole positioned inward in the radial direction. Also, the orientation of the magnetic poles of all of the permanent magnets15(e.g., eight in the illustrated embodiment) may be aligned, or the orientation of the magnetic poles of the permanent magnets15may alternate.

In the exemplary embodiment, flux barriers20are configured such that magnetic flux flows between adjacent flux barriers20with ease in a first direction (d-axis direction) and flows between adjacent flux barriers20with difficulty in a second direction (q-axis direction). In other words, the magnetic flux readily flows in the d-axis direction because arc-shaped magnetic paths13are formed between slits12, and passage of the magnetic flux is difficult in the q-axis direction because slits12functioning as flux barriers intervene at nearly a right angle to the magnet flux. Because the magnetic poles of a typical motor point in the d-axis direction, a centerline between adjacent flux barriers20is on a d-axis centerline of the magnetic poles (position of N and S), and the centerline of the flux barriers20is on the q-axis centerline between magnetic poles.

Also, the reluctance torque (Tq) of reluctance motor1is known to be represented by Equation 1. Therefore, to effectively generate greater reluctance torque (Tq), the d-axis inductance (Ld) can be increased or the q-axis inductance (Lq) decreased.
Tq∝|Ld−Lq|×Id·Iq(Equation 1)
Tq is the reluctance torque, Ld is the d-axis inductance, Lq is the q-axis inductance, Id is the d-axis current, and Iq is the q-axis current.

FIG. 5AandFIG. 5Bare diagrams showing the flow of d-axis and q-axis magnetic flux in a rotor in which the annular connector14is not provided with permanent magnets15(shown inFIG. 3). As shown inFIG. 5A, magnetic paths13made of a magnetic steel plate are provided between slits12in the magnetic passages on the d-axis. As a result, magnetic resistance is reduced, and the magnetic flux of the coil6easily passes along the magnetic paths13in the direction of arrow A.

Meanwhile, as shown inFIG. 5B, the plurality of arc-shaped slits12functioning as flux barriers intervene with the magnetic passages on the q-axis so as to be perpendicular to the magnetic passages. As a result, the magnetic resistance increases, and the magnetic flux of the coils6has difficulty passing through. This creates a difference between the d-axis inductance (Ld) and the q-axis inductance (Lq), and generates reluctance torque Tq. However, some of the magnetic flux of the coil6flows through a portion of the rotor core having a low magnetic resistance, that is, through an outer circumferential portion of the rotor10and the annular connector14(in the direction of arrow B). This sufficiently reduces the q-axis inductance (Lq), and impedes the generation of large reluctance torque (Tq).

In contrast, the rotor10of the exemplary embodiment, as shown inFIG. 2andFIG. 3, includes permanent magnets15embedded within the annular connector14to magnetically saturate the annular connector14. The magnetic saturation increases the magnetic resistance of the magnetic paths on the q-axis, reduces the q-axis inductance (Lq), and increases inductance torque (Tq).

FIG. 4is a graph showing a relationship between inductance values (Lq, Ld) associated with the reluctance motor shown inFIG. 6and inductance values associated with the reluctance motor shown inFIG. 1. The inductance values of the rotor shown inFIG. 6(i.e., the comparative example in which permanent magnets15are not embedded in the annular connector14) are indicated by the dotted lines, and the inductance values of the rotor10(shown inFIG. 1) are indicated by the solid lines. As shown inFIG. 4, the embedding of permanent magnets15in the annular connector14increases the magnetic resistance in the magnetic passages of the annular connector14on the q-axis. The d-axis inductance (Ld) is substantially unchanged, and the q-axis inductance (Lq) is effectively reduced. This increases the difference (|Ld−Lq|) between the d-axis inductance (Ld) and the q-axis inductance (Lq), and therefore increases the reluctance torque (Tq) of the reluctance motor1.

Because the permanent magnets15only need enough magnetic force to magnetically saturate the annular connector14, high-performance magnets with strong magnetic force such as neodymium magnets are not required, and dysprosium (Dy)-free magnets and inexpensive ferrite magnets can be used.

Also, because the permanent magnets15are embedded within the annular connector14, the magnetic force of the permanent magnets15does not affect the stator2(shown inFIG. 1). As a result, an occurrence of drag loss does not affect the efficiency of the reluctance motor1.

In the rotor10for reluctance motor1in this embodiment, as explained above, flux barriers20, which include magnetic paths13formed between slits12, are arranged in the circumferential direction at a predetermined interval, and adjacent flux barriers20are concatenated on the inner circumferential side by annular connector14provided on the inner circumferential side and separated on the outer circumferential side by openings16provided on the outer circumferential side. Also, permanent magnets15are embedded within the annular connector14. The annular connector14in which adjacent flux barriers20are concatenated is magnetically saturated by the embedded permanent magnets15, and the magnetic resistance of the annular connector14is increased. The increased magnetic resistance lowers the q-axis inductance (Lq), increases the reluctance torque, and improves the torque characteristics of reluctance motor1. In other words, magnetic saturation of the annular connector14(i.e., a portion of the rotor yoke) is the main purpose for including permanent magnets15within the annular connector14. Because this increases the magnetic resistance of the annular connector14, a configuration can be realized having a greater saliency difference.

Because the permanent magnets15only have enough magnetic force to magnetically saturate the annular connector14, they do not experience the demagnetizing field effect that commonly occurs in permanent magnets15. Also, in order to use them in a high permeance environment, a high retention force material is not required. Thus, dysprosium (Dy)-free magnets and inexpensive ferrite magnets can be used.

Because the flux barriers20are integrally concatenated by annular connector14, a conventional structural design can be used for the rotor10, the mechanical strength of the rotor10is greater, and the rotor10is highly reliable in operating environments requiring high-speed rotation and both rapid acceleration and deceleration.

Because the permanent magnets15are magnetized in the radial direction of the rotor10and embedded within the annular connector14, the magnetic force of the permanent magnets15acting on the stator2does not cause drag loss. As a result, measures such as field weakening are not required even at high rotational speeds.

Although described herein as magnetized in the radial direction of rotor10, in an alternative embodiment permanent magnets15may be magnetized in the circumferential direction of the rotor10. In this way, the annular connector14may also be magnetically saturated by the embedded permanent magnets15, and the magnetic resistance of the annular connector14increased.

The systems and apparatus described herein are not limited to the embodiments described above. Suitable modifications and improvements are certainly possible. For example, if necessary, a highly magnetic-resistant material may be included within the slits12to further increase the magnetic resistance and reduce the q-axis inductance (Lq). Also, the number of slits12is not limited to three. Any number of slits is possible within the range of acceptable strength for the rotor10.

Moreover, the systems and apparatus described herein facilitate operation of a reluctance motor. More specifically, the rotor described herein facilitates improving torque characteristics of a reluctance motor while maintaining rotor strength and manufacturing ease. A plurality of flux barriers20that include at least one magnetic path13formed between a plurality of slits12are arranged in the circumferential direction at a predetermined interval. Adjacent flux barriers20are concatenated on the inner circumferential side by a portion of rotor core11referred to herein as the annular connector14and separated on the outer circumferential side by openings16provided on the outer circumferential side. Moreover, in some embodiments, permanent magnets15are embedded within the annular connector14.