Flux-Modulated Magnetic Gear

Abstract

In a flux modulation type magnetic gear using a pole shoe (4), improving the demagnetization resistance and suppressing the reduction of torque are issues. Therefore, in a structure in which a first rotor (1) is concentrically arranged inside the pole shoe (4) so as to be rotatable relative to the center of the pole shoe (4) about the rotation axis, and a second rotor (3) is concentrically arranged outside the pole shoe (4) so as to be rotatable relative to the center of the pole shoe (4) about the rotation axis, a first hollow portion (104) is provided. The first hollow portion houses a first permanent magnet (102) provided in the first rotor (2) at a predetermined angle with respect to a line segment representing the magnetic pole center when viewed from the rotation axis inside the first rotor (2), and is configured to increase the usage amount of the permanent magnets used as much as possible.

Description

Technical Field

[0001] The present invention relates to a flux modulation type magnetic gear. Background Art

[0002] A general magnetic gear has a structure in which the teeth of a mechanical gear are simply replaced with permanent magnets. Therefore, the magnetic gear can increase or decrease speed in a non-contact manner, has small vibration and noise, and an improvement in maintainability can be expected. However, in the transmission of torque based on permanent magnets, only the magnets facing each other contribute to torque transmission, so the torque is smaller than that of a mechanical gear. On the other hand, in order to increase the torque, a flux modulation type magnetic gear composed of the following components is used: an inner rotor having permanent magnets adhesively attached to its outer peripheral surface with polarities alternating in the circumferential direction, an outer rotor having permanent magnets adhesively attached to its inner peripheral surface with polarities alternating in the circumferential direction, and a plurality of magnetic pole pieces called pole pieces arranged at equal intervals in the circumferential direction between the two inner and outer rotors.

[0003] Further, in order to reduce the eddy current generated in the magnets and efficiently transmit torque, Patent Document 1 proposes a structure in which the permanent magnets constituting the inner rotor and the outer rotor are embedded inside a magnetic material.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent No. 5526281 Gazette Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] However, the pole pieces are arranged at equal intervals in the circumferential direction using a non-conductive material such as resin, and there are magnetic voids between the pole pieces. Therefore, the magnetic resistance in the magnetic flux passage path of the permanent magnets of the inner rotor and the outer rotor facing each other in the radial direction of the pole pieces increases due to the voids, and there is a problem that the operating point of the permanent magnets decreases and demagnetization easily occurs.

[0009] Especially when the flux modulation type magnetic gear is used as a flux modulation type magnetic gear in a vehicle drive system, thermal demagnetization due to high-speed rotation of 10,000 r / min or more and a high-temperature environment due to heat conduction from the engine can be expected. Therefore, it is necessary to improve the demagnetization resistance. In the flux modulation type magnetic gear shown in Patent Document 1, since the permanent magnets are embedded in the magnetic material of the rotor, the permanent magnets are far from the pole pieces, so there is a problem that the magnetic bonding force decreases and the torque that can be transmitted decreases.

[0010] This application is completed to solve the above problems. The purpose is to simultaneously achieve an improvement in demagnetization resistance and suppression of a decrease in transmitted torque, and to improve the operating performance under high-speed rotation and high-temperature environments.

[0011] Means for Solving the Problem

[0012] The flux modulation type magnetic gear disclosed in this application includes: an annular member formed by arranging pole shoes in an annular shape; a first rotor concentrically arranged inside the annular member with respect to the annular member, and first permanent magnets are provided on a plurality of magnetic poles respectively. The first rotor can rotate relative to the pole shoes with the center of the annular member as the rotation axis; and a second rotor concentrically arranged outside the annular member with respect to the annular member, and second permanent magnets are provided on a plurality of magnetic poles respectively. The second rotor can rotate relative to the pole shoes with the center of the annular member as the rotation axis. The flux modulation type magnetic gear is characterized in that it includes a first hollow portion that houses the first permanent magnet inside the first rotor at a predetermined angle inclined with respect to a line segment representing the center of the magnetic pole when viewed from the rotation axis.

[0013] Effects of the Invention

[0014] It is possible to achieve an improvement in the demagnetization resistance of the permanent magnets constituting the flux modulation type magnetic gear, and at the same time, it is possible to suppress a decrease in the transmitted torque. Description of the Drawings

[0015] Figure 1 It is a cross-sectional view along the rotation axis of the flux modulation type magnetic gear of Embodiment 1.

[0016] Figure 2 It is a cross-sectional view of the flux modulation type magnetic gear of Embodiment 1.

[0017] Figure 3 It is a partial cross-sectional view of the flux modulation type magnetic gear of Embodiment 1.

[0018] Figure 4 It is a partial cross-sectional view of the first rotor of Embodiment 1.

[0019] Figure 5 It is a partial cross-sectional view of the flux modulation type magnetic gear of Embodiment 2.

[0020] Figure 6 It is a partial cross-sectional view of the flux modulation type magnetic gear of Embodiment 3.

[0021] Figure 7 It is a partial cross-sectional view of the flux modulation type magnetic gear of Embodiment 4.

[0022] Figure 8It is a partial cross-sectional view of the flux modulation type magnetic gear according to Embodiment 5.

[0023] Figure 9 It is a partial cross-sectional view of the flux modulation type magnetic gear according to Embodiment 6. Detailed Embodiment

[0024] Embodiment 1

[0025] Figure 1 It is a cross-sectional view along the rotation axis of the flux modulation type magnetic gear according to Embodiment 1. In addition, the same reference numerals in the following figures respectively denote the same or corresponding parts.

[0026] Figure 1 It shows a schematic structure of the flux modulation type magnetic gear which is the object of the present application. The flux modulation type magnetic gear shown here is a radial type flux modulation type magnetic gear.

[0027] For the radial type flux modulation type magnetic gear, a first rotor 1, a second rotor 3, a magnetic body called a pole shoe (hereinafter referred to as a pole shoe 4), and a stator 5 which is an annular member for arranging a plurality of pole shoes 4 in a circular shape are concentrically arranged with respect to the rotation center of the first rotation axis 2 of the first rotor 1. The first rotor 1 is arranged on the inner side, i.e., the inner peripheral side, of the stator 5, and the second rotor 3 is arranged on the outer side, i.e., the outer peripheral side, of the stator 5. The first rotor 1 is mounted on the first rotation axis 2, and the second rotor 3 is mounted on the second rotation axis 6. A first bearing 7 is provided between the first rotor 1 and the stator 5, and a second bearing 8 is provided between the stator 5 and the second rotor 3. The first rotor 1 and the second rotor 3 are configured to be able to rotate independently.

[0028] Figure 2 It shows Figure 1 the structure of the magnetic coupling portion 9 shown in Figure 1 and is a sectional view taken along line A-A in

[0029] The stator 5 has 24 pole shoes 4 made of a magnetic material arranged at equal intervals in a circular shape and a fixing portion 10 made of a non-conductive material for filling the gaps between the pole shoes 4 to fix the pole shoes 4.

[0030] The first rotor 1 arranged on the inner diameter side of the stator 5 is arranged with a first magnetic gap portion 11 interposed between the circumferential inner diameter surface of the pole shoe 4. The first rotor 1 becomes a multi-pole rotor formed by embedding 32 flat first permanent magnets 102 in a first magnetic material 101.

[0031] That is, one magnetic pole is formed by two permanent magnets. The second rotor 3 disposed on the outer diameter side of the stator 5 is disposed with respect to the circumferential outer diameter surface of the pole shoe 4 with a second magnetic gap portion 12 therebetween. The second rotor 3 is a multi-pole rotor formed by embedding 32 flat plate-shaped second permanent magnets 302 in a second magnetic material 301. That is, one magnetic pole is formed by one permanent magnet.

[0032] Figure 3 Is Figure 2 An enlarged view of two magnetic poles of the low-pole rotor, i.e., the first rotor 1, four magnetic poles of the multi-pole rotor, i.e., the second rotor 3, and the stator 5 of the flux modulation type magnetic gear shown.

[0033] The first permanent magnet 102 of the first rotor 1 is formed by two adjacent permanent magnets to form one magnetic pole, magnetized in the direction in which the magnetic flux faces the first magnetic gap portion 11, and magnetically configured to reverse at each adjacent magnetic pole.

[0034] The first permanent magnet 102 has a flat plate shape and is symmetrically disposed with respect to a line segment ( Figure 3 dashed line C) representing the magnetic pole center when viewed from the center of the first rotating shaft 2. And, if the angle formed by the orientation of the first permanent magnet 102 and the line segment ( Figure 3 dashed line C) representing the magnetic pole center when viewed from the center of the first rotating shaft 2 is set as θ, an angle satisfying the relationship of θ < 90° is preset.

[0035] The second permanent magnet 302 of the second rotor 3 is formed by one permanent magnet to form one magnetic pole, magnetized in the direction in which the magnetic flux faces the second magnetic gap portion 12, and the magnetization direction is configured to reverse at each adjacent magnetic pole.

[0036] For the first permanent magnet 102, the permanent magnet is disposed in a direction away from the magnetic gap portion compared to the arc X. The arc X passes through the point B closest to the first magnetic gap portion 11 of the first permanent magnet 102 forming one magnetic pole of the first rotor 1 and has the center of rotation of the first rotor 1 (corresponding to the center of the first rotating shaft 2) as the origin. That is, the first permanent magnet 102 is embedded from the outer peripheral surface of the first rotor 1 in a direction toward the first rotating shaft 2 compared to the position of the point B.

[0037] In addition, a first hollow portion 103 for preventing magnetic flux short circuit is provided at the end in the longitudinal direction of the first permanent magnet 102 of the first rotor 1, and a second hollow portion 303 for preventing magnetic flux short circuit is provided at the end of the second permanent magnet 302 of the second rotor 3.

[0038] In other words, the first rotor 1 is provided with a first hollow portion 104 for accommodating the first permanent magnet 102, and a space for the first hollow portion 103 is left at the end of the first permanent magnet 102 accommodated in the first hollow portion 104. Similarly, the second rotor 3 also has a second hollow portion 304, in which the second permanent magnet 302 is accommodated, and a space for the second hollow portion 303 is left at the end of the second permanent magnet 302.

[0039] Figure 4 Enlarged view Figure 3 of the structure of one magnetic pole of the first rotor 1. The interior of the first rotor 1 contains a cavity due to the first hollow portion 104. That is, a bridge portion 105 having a thickness from the outer peripheral surface of the first rotor 1 to the inner wall surface of the first hollow portion 104 is formed. As Figure 4 shown, the first hollow portion 104 is provided inside the arc X of the dimension Lm. Here, the dimension Lm is the minimum value for relieving stress concentration corresponding to the centrifugal force applied to the first permanent magnet 102. Therefore, as the dimension of the radial bridge portion 105, there is no problem as long as it is larger than the minimum dimension Lm. In this Figure 4 case, a situation where the dimension Lm and the dimension L1 are equal is shown.

[0040] The formation of the radial bridge portion 105 indicates that the first permanent magnet 102 of the first rotor 1 is embedded in a direction away from the first magnetic gap portion 11. There is also a radial bridge portion in the relationship between the second permanent magnet 302 and the second magnetic gap portion 12 in the second rotor 3. However, since the centrifugal force applied to the second permanent magnet 302 acts outward from the second magnetic gap portion 12, the thickness of the radial bridge portion in the second rotor 3 can be set to be small.

[0041] In addition, as Figure 3 and Figure 4 shown, a circumferential bridge portion is formed of a magnetic material sandwiched by the space portions (the first hollow portion 103 and the second hollow portion 303) that prevent the above-mentioned magnetic flux short circuit and are adjacent in the circumferential direction. Moreover, the dimension W1 of the circumferential bridge portion of the first rotor 1 is set larger than the dimension W2 of the circumferential bridge portion of the second rotor 3. Thereby, the effect of preventing the permanent magnet from flying off due to the centrifugal force can be strengthened.

[0042] According to the first embodiment, by deepening the embedding position of the first permanent magnet 102, the demagnetization resistance can be improved, and by accommodating the first permanent magnet 102 in the interior of the first rotor while being inclined by a predetermined angle with respect to the line segment representing the magnetic pole center as viewed from the first rotation axis 2, the reduction of torque can be reduced. That is, by increasing the amount of the permanent magnet used for one magnetic pole constituting the first rotor 1, it helps to suppress the reduction of torque.

[0043] Further, by using a flat-shaped permanent magnet and making the relationship between the orientation of the flat magnet and the angle θ formed by the line segment representing the center of the magnetic pole as viewed from the center of the rotation axis satisfy the relationship of θ < 90°, it is possible to use a flat-shaped magnet with a low manufacturing cost while increasing the amount of magnets used, and it is possible to expect to suppress the manufacturing cost.

[0044] Embodiment 2

[0045] Figure 5 FIG. is a partial cross-sectional view showing the structure of the flux modulation type magnetic gear according to Embodiment 2. In this Embodiment 2, the arrangement of the second permanent magnets 302 of the second rotor 3 in Embodiment 1 is changed.

[0046] As shown in this Figure 5 figure, the second permanent magnets 302 are embedded in such a manner as to achieve circumferential magnetization. Other structures are the same as those in Embodiment 1. By arranging a plurality of second permanent magnets 302 such that the magnetization directions are in the circumferential direction of the second rotor 3, a magnetic circuit is formed in which the magnetic flux is concentrated in the second magnetic gap portion 12. Thereby, the magnetic flux utilization rate is improved, and the distance between adjacent permanent magnets is increased, so that a permanent magnet with a large volume can be used. Therefore, the increase in the amount of magnets used can be freely set, and the degree of freedom in design can be improved.

[0047] Embodiment 3

[0048] Figure 6 FIG. is a partial cross-sectional view showing the structure of the flux modulation type magnetic gear according to Embodiment 3. In this Embodiment 3, the second permanent magnets 302 of the second rotor 3 in Embodiment 1 are changed. The second permanent magnets 302 of the second rotor 3 are configured such that two permanent magnets are used for each magnetic pole. The two second permanent magnets 302 constituting one magnetic pole are magnetized in the direction of the second magnetic gap portion 12, and the magnetization directions of adjacent magnetic poles are reversed.

[0049] Other structures are the same as those in Embodiment 1. It is possible to increase the amount of magnets used for the second permanent magnets 302 of the second rotor 3 in the same manner as the first rotor 1. In addition, the demagnetization resistance of the second permanent magnets 302 of the second rotor 3 is also improved.

[0050] Embodiment 4

[0051] Figure 7FIG. 0 is a partial cross-sectional view showing the structure of the flux modulation type magnetic gear according to Embodiment 4. The first permanent magnet 102 of the first rotor 1 has the following structure: each magnetic pole is composed of one magnet, and near the center of the magnetic pole, there is a bent portion 106 in the direction away from the first magnetic gap portion 11. With this structure of the bent portion 106, the same effect as that of Embodiment 1 can be obtained while suppressing the number of permanent magnets used. As the permanent magnet material of the first rotor 1, bonded magnets with a high degree of freedom in shape design can be used. Other structures are the same as those in Embodiment 1. In addition, by combining the structure of this first rotor 1 with the structure of the second rotor 3 in Embodiment 2 or Embodiment 3, a synergistic effect can be expected.

[0052] Embodiment 5

[0053] Figure 8 FIG. 7 is a partial cross-sectional view showing the structure of the flux modulation type magnetic gear according to Embodiment 5.

[0054] As shown in the figure, each magnetic pole of the first permanent magnet 102 of the first rotor 1 is composed of three flat-shaped magnets, two of which are symmetrically embedded at an angle less than 90° with respect to the magnetic pole center, and the remaining one is symmetrically embedded perpendicular to the magnetic pole center.

[0055] The first permanent magnets 102 of the first rotor 1 are magnetized in the short side direction of the flat-shaped magnets, and the adjacent magnetization directions are reversed. If the angle formed by the orientation of each flat magnet and the line segment representing the magnetic pole center (dashed line C in the figure) observed from the center of the rotation axis is set as θ, then for two of them, θ < 90°, and for the remaining one, θ = 90° and it is embedded in a symmetric manner with respect to the line segment representing the magnetic pole center. According to this structure, the magnetic material region between the first permanent magnet 102 of the first rotor 1 and the first magnetic gap portion 11 can be further enlarged, achieving a further improvement in the demagnetization resistance and a reduction in the loss caused by eddy currents. Other structures are the same as those in Embodiment 1. In addition, by combining the structure of this first rotor 1 with the structure of the second rotor 3 in Embodiment 2 or Embodiment 3, a synergistic effect can be expected.

[0056] Embodiment 6

[0057] Figure 9 FIG. 20 is a partial cross-sectional view showing the structure of the flux modulation type magnetic gear according to Embodiment 6.

[0058] In this Embodiment 6, the pole shoe 4 is a structure that rotates by some external force, and the second rotor 3 is fixed. Since the second rotor 3 does not rotate, it is herein renamed as the outer magnetic pole structure 31.

[0059] By fixing the outer magnetic pole structure 31 in this way, a cooling mechanism can be easily added, and the risk of thermal demagnetization of the magnet can be reduced.

[0060] In addition, the permanent magnet constituting the first rotor has 6 poles, the outer magnetic pole structure 31 has 22 poles, and there are 14 pole shoes 4. Other structures are the same as those in the first embodiment.

[0061] When the outer magnetic pole structure 31 is fixed in this way, according to the principle of the flux modulation type magnetic gear, (reduction ratio) = (the number of pole shoes 4) / (the number of poles of the first rotor 1). That is, the reduction ratio of this embodiment is 14 / 3 = 4.67, and a value larger than the reduction ratio of 22 / 6 = 3.67 in the case where the outer magnetic pole structure 31 is rotated and the pole shoes 4 are fixed can be obtained.

[0062] Moreover, the ratio of the number of poles of the first rotor 1 to the number of poles of the outer magnetic pole structure 31 is not an integer but is represented by a reduced fraction. At this time, according to the principle of the magnetic gear, the order of the cogging torque caused by the interaction of the magnetomotive forces between the first rotor 1 and the outer magnetic pole structure 31 is represented by the least common multiple of the number of poles of the first rotor 1 and the number of poles of the outer magnetic pole structure 31.

[0063] Therefore, in the case where the above ratio of the number of poles is represented by a reduced fraction, the number of poles of the first rotor 1 and the number of poles of the outer magnetic pole structure 31 do not have a common divisor greater than 2. Thus, compared with the case of a combination of the number of poles represented by an integer near the above ratio of the number of poles (for example, when the number of poles of the outer magnetic pole structure 31 is 24 and the number of poles of the first rotor 1 is 6, 24 / 6 = 4), the order of the cogging torque becomes larger (for example, in this embodiment, the greatest common divisor GCD(6, 22) of the number of poles is 2, and the order of the cogging torque is the least common multiple LCM(6, 22) = 66. In contrast, in the above example, the greatest common divisor GCD(6, 24) of the number of poles is 6, and the order of the cogging torque is LCM(6, 24) = 24). Since the cogging torque generally has a smaller amplitude as the order becomes larger, by selecting a number represented by a reduced fraction such that the ratio of the number of poles of the first rotor 1 to the number of poles of the outer magnetic pole structure 31 is not an integer as in this embodiment, the vibration and noise caused by the cogging torque can be reduced.

[0064] That is, when the greatest common divisor of the number of poles of the first rotor and the number of poles of the second rotor is 2, the cogging torque can be reduced.

[0065] In addition, a large order of the cogging torque means a large order of the variation of the magnetic energy, that is, it means that the skin depth of the high - order harmonic magnetic flux penetrating into the iron - core material due to the skin effect becomes smaller. As a result, the high - order harmonic magnetic flux passing through the magnet via the iron core can be reduced, and the demagnetization resistance can be improved by the synergistic effect of embedding the permanent magnet in the iron core.

[0066] In addition, in the above-described Embodiments 1 to 5, the number of magnetic poles of the first rotor 1 is shown as 16 poles, the number of magnetic poles of the second rotor 3 is shown as 32 poles, and the number of pole shoes 4 is shown as 24. However, even when the combination of the number of magnetic poles and the number of pole shoes 4 is different from this, the same effect can be obtained.

[0067] Similarly, in Embodiment 6, the number of magnetic poles of the first rotor 1 is shown as 6 poles, the number of magnetic poles of the second rotor 3 is shown as 22 poles, and the number of pole shoes 4 is shown as 14. However, the ratio of the number of magnetic poles of the first rotor 1 to the number of magnetic poles of the outer magnetic pole structure 31 is not an integer but is represented by a reduced fraction, and even when the combination of the number of magnetic poles and the number of pole shoes 4 is different from this, the same effect can be obtained.

[0068] In addition, in the above-described embodiments, the number of magnets per magnetic pole is shown as 1, 2, and 3. However, even when the number is 4 or more, the same effect can be obtained.

[0069] In addition, in the above-described Embodiments 1 to 5, a ring member formed by the pole shoes 4 is shown as the stator, and with respect to this stator, a low-pole rotor as the first rotor 1 is arranged on the inner diameter side, and a multi-pole rotor as the second rotor 3 is arranged on the outer diameter side. However, even when the low-pole rotor is arranged on the outer diameter side and the multi-pole rotor is arranged on the inner diameter side, the same effect can be obtained.

[0070] In addition, in the above-described Embodiments 1 to 5, a low-pole rotor and a multi-pole rotor having permanent magnets are shown as freely rotating, and the pole shoes 4 are shown as fixed. However, by adopting a structure in which the pole shoes 4 are rotated by some external force in the same manner as in Embodiment 6, and by setting the rotational speed relationship of the inner and outer two rotors and the ring member of the pole shoes, a mechanical planetary gear relationship can be achieved. Also, the low-pole rotor or the multi-pole rotor can be fixed.

[0071] In Embodiment 3, an example in which one magnetic pole of the second rotor 3 is constituted by two permanent magnets arranged in the radial direction is described. However, it is also possible to use a combination of two or more permanent magnets and axial arrangement. In the case of using two or more permanent magnets, for example, the first empty hole portion for embedding the magnets is designed so that permanent magnets having a flat plate shape with a certain standard size can be combined in the radial and axial directions to form one magnetic pole. Thus, in addition to the above-described effects, an effect of suppressing the manufacturing cost can be obtained. This effect becomes particularly significant in the case of constituting a multi-pole or high reduction ratio magnetic gear, for example, in the case of using a magnetic gear in a reduction mechanism of an automotive main motor.

[0072] In addition, in the above-described embodiments, only the case where the magnetic structure of each pole is centrosymmetric with respect to the center of the magnetic pole is shown, but a magnetic structure that is non-centrosymmetric with respect to the center of the magnetic pole may also be used, and the same effect can be obtained for structures having different characteristics in the rotational direction.

[0073] In addition, in the above-described embodiments, the case of the flux modulation type magnetic gear in the radial type where the magnetic gap portion is parallel to the rotation axis is shown, but the same effect can be obtained even in the axial type where the magnetic gap portion is perpendicular to the rotation axis.

[0074] Although the present application describes various exemplary embodiments and examples, the various features, modes, and functions described in one or more embodiments are not limited to the application of a specific embodiment, but can be applied alone or in various combinations to the embodiments.

[0075] Therefore, within the scope of the technology disclosed in the present application specification, countless variations that are not illustrated can be conceived. For example, it includes cases where at least one component is deformed, added, or omitted, and also includes cases where at least one component is extracted and combined with the components of other embodiments.

[0076] Description of Reference Numerals

[0077] 1 First rotor, 2 First rotating shaft, 3 Second rotor, 4 Pole shoe, 5 Stator, 6 Second rotating shaft, 7 First bearing, 8 Second bearing, 9 Magnetic coupling portion, 10 Fixing portion, 11 First magnetic gap portion, 12 Second magnetic gap portion, 31 Outer magnetic pole structure, 101 First magnetic material, 102 First permanent magnet, 103 First cavity portion, 104 First hole portion, 105 Bridge portion, 106 Bending portion, 301 Second magnetic material, 302 Second permanent magnet, 303 Second cavity portion, 304 Second hole portion.

Claims

1. A flux modulation type magnetic gear, comprising: an annular member formed by arranging pole shoes in an annular shape; a first rotor concentrically arranged inside the annular member with respect to the annular member, and having first permanent magnets provided on a plurality of magnetic poles respectively, the first rotor being capable of relatively rotating with respect to the pole shoes about the center of the annular member as a rotation axis; and a second rotor concentrically arranged outside the annular member with respect to the annular member, and having second permanent magnets provided on a plurality of magnetic poles respectively, the second rotor being capable of relatively rotating with respect to the pole shoes about the center of the annular member as a rotation axis. The flux modulation type magnetic gear is characterized in that it includes a first hollow portion that houses the first permanent magnet at a predetermined angle inclined with respect to a line segment representing the center of the magnetic pole when viewed from the rotation axis inside the first rotor, and a thickness dimension of a circumferential bridge between the first hollow portion of the first rotor and an adjacent hollow portion is larger than a thickness dimension of a circumferential bridge between a second hollow portion that houses the second permanent magnet of the second rotor and an adjacent hollow portion.

2. The flux modulation type magnetic gear according to claim 1, characterized in that the first hollow portion has a cavity portion at an end of the first permanent magnet.

3. The flux modulation type magnetic gear according to claim 1 or 2, characterized in that a plurality of the first permanent magnets form magnetic poles whose central portions are arranged in a V shape in the direction of the rotation center of the first rotor.

4. The flux modulation type magnetic gear according to claim 1, characterized in that one magnetic pole of the second rotor is formed by a plurality of the second permanent magnets.

5. The flux modulation type magnetic gear according to claim 2, characterized in that the first permanent magnets are respectively housed in a plurality of the first hollow portions provided at a predetermined interval corresponding to one magnetic pole of the first rotor.

6. The flux modulation type magnetic gear according to claim 1, characterized in that the first permanent magnet housed in the first hollow portion is in a flat plate shape and is symmetrically arranged at an angle less than 90° with respect to a line segment representing the center of the magnetic pole when viewed from the rotation axis.

7. The flux modulation type magnetic gear according to claim 1 or 2, characterized in that the second rotor is fixed and the pole shoes rotate freely.

8. The flux modulation type magnetic gear according to claim 1 or 2, characterized in that the greatest common divisor of the number of magnetic poles of the first rotor and the number of magnetic poles of the second rotor is 2.

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