Double-stator rotating electric machine

A double-stator rotating electric machine includes a rotor, an outer stator having an outer multi-phase coil wound thereon, and an inner stator having an inner multi-phase coil wound thereon. Each corresponding pair of phase windings of the outer and inner multi-phase coils are formed of at least one common electric conductor wire. The electric conductor wire includes a bridging portion that bridges the corresponding pair of phase windings of the outer and inner multi-phase coils across the rotor. The bridging portion extends obliquely with respect to both radial and circumferential directions of the rotor so that radially outer and radially inner ends of the bridging portion, which are respectively connected to the corresponding pair of phase windings of the outer and inner multi-phase coils, are circumferentially offset from each other by an offset angle θ. The offset angle θ is greater than 0° and less than 180° in electrical angle.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority from Japanese Patent Application No. 2014-247850 filed on Dec. 8, 2014, the content of which is hereby incorporated by reference in its entirety into this application.

BACKGROUND

1. Technical Field

The present invention relates to double-stator rotating electric machines which include a rotor, an outer stator disposed radially outside the rotor, and an inner stator disposed radially inside the rotor.

2. Description of Related Art

Japanese Patent Application Publication No. JP2007261342A discloses an in-wheel motor which includes a rotor and a pair of outer and inner stators. The rotor is connected to a wheel shaft so as to rotate together with the wheel shaft. The outer stator is fixed to a housing so as to be positioned radially outside the rotor with an outer gap formed therebetween. The inner stator is fixed to the housing so as to be positioned radially inside the rotor with an inner gap formed therebetween. That is to say, the in-wheel motor is a double-gap and double-stator motor.

Moreover, in the in-wheel motor, the outer stator includes a plurality of iron cores each having a coil wound thereon. The inner stator includes an iron core having a plurality of protruding pieces; each of the protruding pieces has a coil wound thereon. The rotor includes an annular rotor core, a plurality of outer permanent magnets and a plurality of inner permanent magnets. The rotor core is formed by laminating a plurality of thin steel sheets. The rotor core has a plurality of fitting holes that are formed in a radially outer surface of the rotor core along a circumferential direction of the rotor core. Each of the outer permanent magnets is fitted in one of the fitting holes of the rotor core. Each of the inner permanent magnets is attached on a radially inner surface of the rotor core along the circumferential direction so as to be radially aligned with one of the outer permanent magnets.

With the above configuration, magnetomotive forces of the outer and inner stators are serially arranged with each other. In other words, the in-wheel motor has a serial arrangement of the magnetomotive forces of the outer and inner stators.

Moreover, since the coils of the outer stator are provided separately from the coils of the inner stator, the parts count of the in-wheel motor and thus the number of manufacturing steps of the in-wheel motor are large. In addition, the coil end height (i.e., the axial length of coil ends that protrude from corresponding axial end faces of the cores of the outer and inner stators) is also large.

On the other hand, the magnetomotive forces of the outer and inner stators may be arranged parallel to each other. In this case, for each radially-aligned pair of the coils of the outer and inner stators, electric currents respectively flowing in the pair of the coils are opposite in phase to each other; therefore, it is possible to use a bridging wire, which radially extends across the rotor, to bridge (or electrically connect) the pair of the coils. In other words, it is possible to integrally form the pair of the coils of the outer and inner stators and the bridging wire bridging them into one piece. Consequently, with the integral formation, it is possible to reduce the coil end height.

However, with the parallel arrangement of the magnetomotive forces of the outer and inner stators, the outer magnetic flux loops (i.e., the loops of magnetic flux flowing through the outer stator and a radially outer half of the rotor) are formed separately from the inner magnetic flux loops (i.e., the loops of magnetic flux flowing through the inner stator and a radially inner half of the rotor). Moreover, both the outer magnetic flux loops and the inner magnetic flux loops flow through the rotor in the same direction. Consequently, magnetic saturation of the rotor may occur.

In comparison, in the case of serially arranging the magnetomotive forces of the outer and inner stators, it is possible to prevent magnetic saturation of the rotor from occurring. However, in this case, it may be difficult to minimize the coil end height.

SUMMARY

According to an exemplary embodiment, there is provided a double-stator rotating electric machine which includes a rotor, an outer stator and an inner stator. The outer stator is disposed radially outside the rotor with an outer gap formed therebetween. The outer stator has an outer multi-phase coil wound thereon. The inner stator is disposed radially inside the rotor with an inner gap formed therebetween. The inner stator has an inner multi-phase coil wound thereon. Each corresponding pair of phase windings of the outer and inner multi-phase coils are formed of at least one common electric conductor wire. The electric conductor wire includes a bridging portion that bridges the corresponding pair of phase windings of the outer and inner multi-phase coils across the rotor. The bridging portion extends obliquely with respect to both radial and circumferential directions of the rotor so that radially outer and radially inner ends of the bridging portion, which are respectively connected to the corresponding pair of phase windings of the outer and inner multi-phase coils, are circumferentially offset from each other by an offset angle θ. The offset angle θ is greater than 0° and less than 180° in electrical angle.

With the above configuration, the magnetomotive forces of the outer and inner stators are offset in phase from each other by the offset angle θ that is greater than 0° and less than 180° in electrical angle. That is, the magnetomotive forces of the outer and inner stators are arranged neither in parallel nor in series with each other. Consequently, compared to the case of arranging the magnetomotive forces of the outer and inner stators parallel to each other (i.e., θ=0° in electrical angle), it is more difficult for magnetic saturation of the rotor to occur and thus it is possible to improve the performance of the rotating electric machine. Moreover, compared to the case of serially arranging the magnetomotive forces of the outer and inner stators (i.e., θ=180° in electrical angle), it is possible to reduce the coil end height and thus the size of the rotating electric machine.

It is preferable that 60°≤θ≤150°. In this case, it is possible to considerably improve the performance of the rotating electric machine while considerably reducing the coil end height and thus the size of the rotating electric machine.

It is more preferable that θ=90°. In this case, it is possible to maximize the torque of the rotating electric machine while minimizing the coil end height and thus the size of the rotating electric machine.

DESCRIPTION OF EMBODIMENT

FIG. 1shows the overall configuration of a double-stator rotating electric machine10according to an exemplary embodiment.

In this embodiment, the rotating electric machine10is configured as a motor-generator that selectively functions either as an electric motor or as an electric generator.

As shown inFIG. 1, the rotating electric machine10includes a housing12, an outer stator13, an inner stator14, a rotor15, a disc16, a pair of bearings17and a rotating shaft18.

The housing12includes a main body12aand a cover12b.The main body12ais substantially cup-shaped to have an open end. The cover12bis disc-shaped and fixed to the main body12aso as to cover the open end of the main body12a.

Moreover, in the housing12, there are provided the pair of bearings17via which the rotating shaft18is rotatably supported by the housing12. In addition, the rotating shaft18may have any shape suitable for rotation.

The outer stator13is fixed to an outer circumferential wall of the housing12so as to be positioned radially outside the rotor15. The inner stator14is fixed to an inner circumferential wall of the housing12so as to be positioned radially inside the rotor15. In other words, the outer and inner stators13and14are radially opposed to each other with the rotor15interposed therebetween. In addition, the outer and inner stators13and14may be fixed to the housing12by any suitable fixing means.

The outer stator13has an outer multi-phase coil (e.g., outer three-phase coil)13awound thereon, while the inner stator14has an inner multi-phase coil (e.g., inner three-phase coil)14awound thereon. More specifically, the outer multi-phase coil13ais wound on a stator core of the outer stator13, while the inner multi-phase coil14ais wound on a stator core of the inner stator14. In addition, each of the stator cores of the outer and inner stators13and14may be formed of either a laminate of magnetic steel sheets or a single piece of a magnetic material.

The rotor15is fixed to the disc16, and the disc16is further fixed to the rotating shaft18. That is, the rotor15is fixed to the rotating shaft18via the disc16. In addition, the rotor15, the disc16and the rotating shaft18may be fixed together by any suitable fixing means.

The configuration of the rotor15will be described in detail later (seeFIG. 4). The disc16may have any shape suitable for connecting the rotor15and the rotating shaft18. In the present embodiment, the disc16has a hollow cylindrical boss portion formed at a radial center thereof and a flange portion extending radially outward from the boss portion. The rotating shaft18is fitted in the hollow space of the boss portion of the disc16. The rotor15is fixed to one surface (i.e., the left surface inFIG. 1) of the flange portion of the disc16.

Referring toFIG. 2, between the outer stator13and the rotor15, there is formed an annular outer gap G Similarly, between the inner stator14and the rotor15, there is formed an annular inner gap G

In addition, with decrease in the outer and inner gaps it becomes easier for magnetic flux to flow across the gaps thereby increasing the torque. However, at the same time, it also becomes easier for the rotor15to make contact with the outer and inner stators13and14upon application of a large external force or vibration to the rotating electric machine10. Therefore, the gaps G may be preferably set by taking into consideration both ease of the flow of magnetic flux and avoidance of contact between the rotor15and the outer and inner stators13and14. In addition, the outer gap G between the outer stator13and the rotor15may be set to the same value as or a different value from the inner gap G between the inner stator14and the rotor15.

As shown inFIGS. 2-3, the stator core of the outer stator13has a plurality of teeth13tand a plurality of slots13s.The teeth13teach radially extend and are circumferentially spaced at a predetermined pitch. Each of the slots13sis formed between one circumferentially-adjacent pair of the teeth13t.The outer multi-phase coil13ais wound on the teeth13tso as to be received in the slots13s.Similarly, the stator core of the inner stator14has a plurality of teeth14tand a plurality of slots14s.The teeth14teach radially extend and are circumferentially spaced at a predetermined pitch. Each of the slots14sis formed between one circumferentially-adjacent pair of the teeth14t.The inner multi-phase coil14ais wound on the teeth14tso as to be received in the slots14s.

In the present embodiment, each of phase windings of the outer multi-phase coil13ais formed integrally with a corresponding one of phase windings of the inner multi-phase coil14ausing at least one electric conductor wire11. In other words, each corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aare formed of at least one common electric conductor wire11. As shown inFIGS. 1-3, the electric conductor wire11includes a bridging portion11athat bridges (or electrically connects) the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aacross the rotor15on one axial side (i.e., the left side inFIG. 1) of the rotor15. The bridging portion11aand the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14abridged by the bridging portion11atogether form a substantially U-shape as shown inFIG. 3. In addition, a coil end height H (seeFIG. 3) of the outer and inner multi-phase coils13aand14ais represented by the axial length of those parts of the outer and inner multi-phase coils13aand14awhich protrude from the axial end faces of the stator cores of the outer and inner stators13and14.

Moreover, in the present embodiment, the bridging portion11aextends obliquely with respect to both radial and circumferential directions of the rotor15so that the slots13sand14sof the outer and inner stators13and14, in which the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14abridged by the bridging portion11aare respectively received, are circumferential offset from each other by an offset angle θ. That is, the bridging portion11ais provided between the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aso that radially outer and radially inner ends of the bridging portion11a are circumferentially offset from each other by the offset angle θ. The offset angle θ is equal to 0° in electrical angle when magnetomotive forces of the outer and inner stators13and14are arranged parallel to each other. The magnetomotive forces of the outer and inner stators13and14are generated upon supply of electric current to the outer and inner multi-phase coils13aand14a.More specifically, when electric current is supplied to the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14ain directions as indicated inFIG. 2, the magnetomotive forces of the outer and inner stators13and14are offset in phase from each other by the offset angle θ in electrical angle.

As shown inFIG. 2, in the present embodiment, the rotor15includes an annular (or hollow cylindrical) rotor core, a plurality of outer permanent magnets M1and a plurality of inner permanent magnets M2. The rotor core is formed of a soft-magnetic material. More specifically, the rotor core may be formed of either a laminate of soft-magnetic steel sheets or a single piece of a soft-magnetic material. The rotor core is comprised of an outer yoke part15afacing the outer stator13and an inner yoke part15bfacing the inner stator14. In the outer yoke part15a,there are formed a plurality of outer receiving portions15c,in each of which is received one of the outer permanent magnets M1. Similarly, in the inner yoke part15b,there are formed a plurality of inner receiving portions15d,in each of which is received one of the inner permanent magnets M2. In addition, the outer yoke part15aand the inner yoke part15bmay be either integrally formed as one piece or separately formed and then fixed together by any suitable fixing means.

Moreover, as shown inFIG. 4, in the present embodiment, each of the outer permanent magnets M1is comprised of a pair of permanent magnet segments that are separated from each other by a substantially T-shaped bridge15fformed in the outer yoke part15aof the rotor core. The bridge15fis formed so as to make the outer circumferential surface of the rotor core smooth. On the other hand, each of the inner permanent magnets M2is formed as a single piece. There is no substantially T-shaped bridge formed in the inner yoke part15bof the rotor core.

In the present embodiment, the rotor core (i.e., the outer yoke part15aplus the inner yoke part15b) includes a main body15g,a plurality of outer protrusions15eand a plurality of inner protrusions15h.The main body15gis annular-shaped to form a magnetic path via which magnetic flux ϕ flows mainly in the circumferential direction of the rotor15. Each of the outer protrusions15eprotrudes from the main body15gradially outward to form a magnetic path via which magnetic flux ϕ flows between the outer stator13and the rotor15mainly in a radial direction of the rotor15. Each of the inner protrusions15hprotrudes from the main body15gradially inward to form a magnetic path via which magnetic flux ϕ flows between the inner stator14and the rotor15mainly in a radial direction of the rotor15.

Moreover, in the present embodiment, each of the outer protrusions15eis circumferentially offset from a corresponding one of the inner protrusions15h.The amount of the circumferential offset between each corresponding pair of the outer and inner protrusions15eand15hmay be set to any suitable value. In particular, it is preferable to set the amount of the circumferential offset according to the above-described offset angle θ.

Furthermore, in the present embodiment, the outer protrusions15eand the inner protrusions15hhave a circumferential width W1, while the main body15gof the rotor core has a radial width W2. The circumferential width W1and the radial width W2may be set to any suitable values according to the type and rating of the rotating electric machine10. To allow magnetic flux ϕ to smoothly flow through the rotor15without causing excessive magnetic saturation of the rotor15, it is preferable to set the circumferential width W1and the radial width W2so as to satisfy the following relationship: 0.5≤W2/W1≤1.

In addition, it should be noted that the circumferential width of the outer protrusions15emay be set to be different from the circumferential width of the inner protrusions15h.

As illustrated inFIG. 4, magnetic flux ϕ flowing between the outer and inner stators13and14has both a circumferential component Δc and a radial component Δr when passing through the rotor15. In addition, the magnitudes of the circumferential and radial components Δc and Δr vary depending on the position where the magnetic flux ϕ flows.

FIG. 5illustrates the changes in the torque T and the coil end height H with the offset angle θ in the rotating electric machine10. Specifically, inFIG. 5, the change in the torque T of the rotating electric machine10with the offset angle θ is indicated by a characteristic line (one-dot chain line) L2; the change in the coil end height H with the offset angle θ is indicated by a characteristic line (two-dot chain line) L3; and the change in the ratio (T/H) of the torque T to the coil end height H with the offset angle θ is indicated by a characteristic line (continuous line) L1.

As indicated by the characteristic line L2, the toque T increases rapidly as the offset angle θ increases from 0° to the vicinity of 60°. After the offset angle θ exceeds 60°, the torque T then increases slowly with the offset angle θ. This change in the torque T with the offset angle θ indicates that the performance of the rotating electric machine10is improved with the shift of the arrangement of the magnetomotive forces of the outer and inner stators13and14from the parallel arrangement to the serial arrangement. On the other hand, as indicated by the characteristic line L3, the coil end height H almost remains unchanged as the offset angle θ increases from 0° to the vicinity of 100°. After the offset angle θ exceeds 100°, the coil end height H then increases rapidly with the offset angle θ. This change in the coil end height H with the offset angle θ indicates that the bridging portions11aare crowded (or densely arranged) with the shift of the arrangement of the magnetomotive forces of the outer and inner stators13and14from the parallel arrangement to the serial arrangement. Moreover, as indicated by the characteristic line L1, the ratio (T/H) increases with the offset angle θ until reaching its peak at the offset angle θ of 90°. Then, the ratio (T/H) decreases with increase in the offset angle θ.

FromFIG. 5, it has been made clear that specifying 0°<θ<180°, it is possible to increase the torque T (or improve the performance) of the rotating electric machine10while reducing the coil end height H and thus the size of the rotating electric machine10in comparison with the prior art. Moreover, it also has been made clear that specifying 60°≤θ≤150°, it is possible to considerably increase the torque T of the rotating electric machine10while considerably reducing the coil end height H and thus the size of the rotating electric machine10in comparison with the prior art. Furthermore, it also has been made clear that specifying θ=90°, it is possible to maximize the torque T of the rotating electric machine10while minimizing the coil end height H and thus the size of the rotating electric machine10.

To further investigate the effect of the offset angle θ on the performance of the rotating electric machine10, analysis was carried out for five models of the rotating electric machine10.

Specifically, in each of the five models, the outer diameter of the outer stator13was 120 mm; the outer diameter of the inner stator14was 76 mm; the outer diameter of the rotor15was 99.6 mm; the axial length (or lamination thickness) of the rotor15was 66 mm; both the number of magnetic poles formed in the outer stator13and the number of magnetic poles formed in the inner stator14were12; the phase current supplied to the outer multi-phase coil13aand the inner multi-phase coil14awas 150 Arms; the relative permeability of the outer permanent magnets M1and inner permanent magnets M2was 1.05; and the coercivity of the outer permanent magnets M1and inner permanent magnets M2was 953092 A/m.

FIG. 6illustrates the configuration of the first model, in which the offset angle θ was 30° in electrical angle.FIG. 7illustrates the flow of magnetic flux ϕ in the first model.FIG. 8illustrates the configuration of the second model, in which the offset angle θ was 60° in electrical angle.FIG. 9illustrates the flow of magnetic flux ϕ in the second model.FIG. 10illustrates the configuration of the third model, in which the offset angle θ was 90° in electrical angle.FIG. 11illustrates the flow of magnetic flux ϕ in the third model.FIG. 12illustrates the configuration of the fourth model, in which the offset angle θ was 120° in electrical angle.FIG. 13illustrates the flow of magnetic flux ϕ in the fourth model.FIG. 14illustrates the configuration of the fifth model, in which the offset angle74was 150° in electrical angle.FIG. 15illustrates the flow of magnetic flux ϕ in the fifth model. In addition, inFIGS. 7, 9, 11, 13 and 15, the flow of magnetic flux ϕ is illustrated in the form of contour lines.

As can be seen fromFIGS. 7, 9, 11, 13 and 15, the magnetic flux flowing through the rotor15had both a circumferential component Δc and a radial component Δr (see alsoFIG. 4). Moreover, the length of the path of the magnetic flux ϕ flowing through the rotor15decreased with increase in the offset angle θ.

The analysis results are as follows. Taking the torque T obtained with the parallel arrangement (i.e., θ=0°) as a reference (i.e., 100%), the torque T obtained with the first model (i.e., θ=30°) was 107%; the torque T obtained with the second model (i.e., θ=60°) was 115%; the torque T obtained with the third model (i.e., θ=90°) was 120%; the torque T obtained with the fourth model (i.e., θ=120°) was 121%; and the torque T obtained with the fifth model (i.e., θ=150°) was 120%.

From the above results, it has been made clear that to suppress increase in the size of the rotating electric machine10, it is preferable to set the offset angle θ to be less than or equal to 90°. Moreover, it also has been made clear that to increase the torque T (or improve the performance) of the rotating electric machine10, it is preferable to set the offset angle θ to be greater than or equal to 60°.

In addition, in setting the offset angle θ according to the type and rating of the rotating electric machine10, the outer yoke part15aand the inner yoke part15bof the rotor core may be formed separately from each other, then arranged so as to be circumferentially offset from each other by the desired the offset angle θ, and finally fixed together by any suitable fixing means.

Next, advantages of the double-stator rotating electric machine10according to the present embodiment will be described.

In the present embodiment, the double-stator rotating electric machine10includes the outer stator13, the inner stator14and the rotor15. The outer stator13is disposed radially outside the rotor15with the annular outer gap G formed therebetween. The outer stator13has the outer multi-phase coil13awound thereon. The inner stator14is disposed radially inside the rotor15with the annular inner gap G formed therebetween. The inner stator14has the inner multi-phase coil14awound thereon. Each corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aare formed of at least one common electric conductor wire11. The electric conductor wire11includes the bridging portion11athat bridges the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aacross the rotor15. The bridging portion11aextends obliquely with respect to both radial and circumferential directions of the rotor15so that the radially outer and radially inner ends of the bridging portion11a,which are respectively connected to the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14a,are circumferentially offset from each other by the offset angle θ. The offset angle θ is greater than 0° and less than 180° in electrical angle.

With the above configuration, the magnetomotive forces of the outer and inner stators13and14are offset in phase from each other by the offset angle θ that is greater than 0° and less than 180° in electrical angle. That is, the magnetomotive forces of the outer and inner stators13and14are arranged neither in parallel nor in series with each other. Consequently, compared to the case of arranging the magnetomotive forces of the outer and inner stators13aand14aparallel to each other (i.e., θ=0° in electrical angle), it is more difficult for magnetic saturation of the rotor15to occur and thus it is possible to improve the performance of the rotating electric machine10. Moreover, compared to the case of serially arranging the magnetomotive forces of the outer and inner stators13aand14a(i.e., θ=180° in electrical angle), it is possible to reduce the coil end height H and thus the size of the rotating electric machine10.

Moreover, specifying 60°≤θ≤150°, it is possible to considerably improve the performance of the rotating electric machine10while considerably reducing the coil end height H and thus the size of the rotating electric machine10.

Furthermore, specifying θ=90°, it is possible to maximize the torque T of the rotating electric machine10while minimizing the coil end height H and thus the size of the rotating electric machine10. In addition, in this case, the outer multi-phase coil13aand the inner multi-phase coil14aare electrically orthogonal to each other and thus there is less mutual interference between them; consequently, it is possible to further improve the performance of the rotating electric machine10.

In the present embodiment, the rotor15includes the annular rotor core that is formed of a soft-magnetic material and shaped so as to allow magnetic flux ϕ flux having both a circumferential component Δc and a radial component Δr to flow through the rotor15.

With the above configuration, it is possible to suppress magnetic saturation of the rotor15and increase the reluctance torque that is generated due to the anisotropy in magnetic reluctance of the rotor15. Moreover, it is also possible to increase the permeance of a magnetic circuit formed in the rotor15, thereby making it difficult for the rotor15to be demagnetized.

In the present embodiment, the rotor core includes the annular main body15gand the outer and inner protrusions15eand15heach of which radially protrudes from the main body15g. Moreover, the following relationship is satisfied: 0.5≤W2/W1≤1, where W1is the circumferential width of the outer and inner protrusions15eand15h,and W2is the radial width of the main body15g.

With the above configuration, it is possible to secure the flow of magnetic flux ϕ between the rotor15and the outer and inner stators13and14as well as the flow of magnetic flux ϕ through the main body15g,thereby making it difficult for magnetic saturation of the rotor15to occur.

In the present embodiment, the rotor15has the outer permanent magnets M1provided in the outer yoke part15afacing the outer stator13and the inner permanent magnets M2provided in the inner yoke part15bfacing the inner stator14. Each of the outer permanent magnets M1is circumferentially offset from a corresponding one of the inner permanent magnets M2based on the offset angle θ of the bridging portions11a(seeFIG. 4).

With the above configuration, the opposing angle relationship between the outer permanent magnets M1and the outer stator13(or the relative circumferential position of the outer permanent magnets M1to the outer stator13) is identical to the opposing angle relationship between the inner permanent magnets M2and the inner stator14(or the relative circumferential position of the inner permanent magnets M2to the inner stator14). Therefore, it is possible to maximize the torque T of the rotating electric machine10. Moreover, the optimal phase of electric current supply to the outer stator13relative to the rotational position of the rotor15is identical to the optimal phase of electric current supply to the inner stator14relative to the rotational position of the rotor15. Therefore, it is possible to directly connect the outer multi-phase coil13aand the inner multi-phase coil14a.Consequently, it is possible to obtain a compact coil end at low cost. Furthermore, magnet torque can be obtained without impeding the flow of magnetic flux ϕ between the outer stator13and the inner stator14, thereby improving the performance of the entire rotating electric machine10.

In the present embodiment, the rotor15has the outer yoke part15afacing the outer stator13and the inner yoke part15bfacing the inner stator14. The outer yoke part15aand the inner yoke part15bare circumferentially offset from each other based on the offset angle θ of the bridging portions11a.

With the above configuration, the opposing angle relationship between the outer yoke part15aand the outer stator13(or the relative circumferential position of the outer yoke part15ato the outer stator13) is identical to the opposing angle relationship between the inner yoke part15band the inner stator14(or the relative circumferential position of the inner yoke part15bto the inner stator14). Therefore, it is possible to maximize the torque T of the rotating electric machine10. Moreover, the optimal phase of electric current supply to the outer stator13relative to the rotational position of the rotor15is identical to the optimal phase of electric current supply to the inner stator14relative to the rotational position of the rotor15. Therefore, it is possible to directly connect the outer multi-phase coil13aand the inner multi-phase coil14a.Consequently, it is possible to obtain a compact coil end at low cost. Furthermore, the flow of magnetic flux ϕ can be reliably secured between the outer stator13and the inner stator14, thereby improving the performance of the entire rotating electric machine10.

While the above particular embodiment has been shown and described, it will be understood by those skilled in the art that various modifications, changes, and improvements may be made without departing from the spirit of the present invention.

(1) In the above-described embodiment, in the outer yoke part15aof the rotor core, there are formed the bridges15feach of which prevents a corresponding one of the outer permanent magnets M1from protruding radially outward. On the other hand, in the inner yoke part15bof the rotor core, there are formed no bridges (seeFIG. 4).

However, as shown inFIG. 16, it is also possible to form, in the inner yoke part15bof the rotor core, a plurality of bridges15ieach of which prevents a corresponding one of the inner permanent magnets M2from protruding radially inward.

(2) In the above-described embodiment, the bridging portion11aand the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aare integrally formed into one piece (seeFIG. 3).

However, as shown inFIG. 17, it is also possible to form the bridging portion11aseparately from the corresponding pair of phase windings of the outer and inner multi-phase coils13aand14aand then join the bridging portion11ato the corresponding pair of phase windings.

Alternatively, as shown inFIG. 18, it is also possible to form the bridging portion11aintegrally with only one of the phase windings of the corresponding pair (e.g., with only the corresponding phase winding of the outer multi-phase coil13ainFIG. 18) and then join the bridging portion11ato the other of the phase windings of the corresponding pair (e.g., to the corresponding phase winding of the inner multi-phase coil14ainFIG. 18).

(3) In the above-described embodiment, the present invention is directed to the double-stator rotating electric machine10that is configured as a motor-generator. However, the present invention can also be applied to other types of double-stator rotating electric machines, such as a double-stator electric motor or a double-stator electric generator.

(4) In the above-described embodiment, each of the outer permanent magnets M1is comprised of a pair of permanent magnet segments; and each of the inner permanent magnets M2is formed as a single piece (seeFIGS. 2 and 4).

However, each of the outer permanent magnets M1may also be comprised of three or more permanent magnet segments. Each of the inner permanent magnets M2may also be comprised of two or more permanent magnet segments.

In addition, in the case where each of the inner permanent magnets M2is also comprised of a plurality of permanent magnet segments, it is preferable to set the number of permanent magnet segments per inner permanent magnet M2to be less than the number of permanent magnet segments per outer permanent magnet M1.

(5) In the above-described embodiment, the rotor15is configured to be rotated by the magnet torque that is generated by the outer and inner permanent magnets M1and M2(seeFIGS. 2 and 4).

However, the outer and inner permanent magnets M1and M2may be omitted from the rotor15. In this case, the rotor15may be configured to be rotated by the reluctance torque that is generated due to the anisotropy in magnetic reluctance of the rotor15. In addition, in this case, the rotor15may or may not include the outer protrusions15eand the inner protrusions15h.

(6) In the above-described embodiment, the housing12is comprised of the cup-shaped main body12aand the disc-shaped cover12bthat is fixed to the main body12aso as to cover the open end of the main body12a(seeFIG. 1).

However, the main body12aand the cover12bmay have any other shapes such that the outer stator13, the inner stator14, the rotor15and the rotating shaft18can be received in the housing12. Moreover, the cover12bmay be detachably attached to the main body12a.