Polar anisotropic magnet, method for manufacturing same, and permanent magnet synchronous motor comprising said polar anisotropic magnet

Abstract

The polar anisotropic magnet according to the present disclosure is composed of a cylindrical sintered magnet body having an inner peripheral surface of an inner diameter D1 [unit: mm] and an outer peripheral surface of an outer diameter D2 [unit: mm], has P (P is an even number of 8 or more) magnetic poles arranged in the circumferential direction on the inner peripheral surface, and satisfies the relationship 2 mm < (D2 - D1) / 2 ≤ (0.6 × D1 × π) / P, the peak value of the surface magnetic flux density Bg on the inner peripheral surface being 0.5 T or more.

Description

Polar anisotropic magnet, manufacturing method thereof, and permanent magnet synchronous motor equipped with said polar anisotropic magnet

[0001] The present application relates to a polar-anisotropic magnet, a manufacturing method thereof, and a permanent magnet synchronous motor including the polar-anisotropic magnet.

[0002] Permanent magnet synchronous motors use permanent magnets of various shapes in their rotors. These permanent magnets can be broadly categorized into "segment type," in which multiple permanent magnet segments are incorporated into the rotor body, and "ring magnet type," in which a single cylindrical magnet is used. Segment type permanent magnets require the same number of permanent magnet segments as the number of rotor poles to be incorporated into or on the rotor body. In contrast, ring magnet type permanent magnets require only one cylindrical magnet, regardless of the number of rotor poles, making them easier to assemble.

[0003] Cylindrical magnets (ring magnets) are produced by sintering a powder compact made by compacting permanent magnet alloy powder into a cylindrical shape. By controlling the direction of the oriented magnetic field applied during the production of the cylindrical powder compact and the direction of the magnetic field applied during the magnetization process, radially anisotropic or polar anisotropic magnets can be achieved. Radially anisotropic magnets are magnets in which the magnet's easy direction of magnetization is distributed radially from the center of the ring cross section, while polar anisotropic magnets are magnets in which the easy direction of magnetization is controlled to a special non-radial distribution so that a magnetic path corresponding to the number of poles in the product is formed inside the magnet. As described below, in radially anisotropic magnets, the surface magnetic flux density Bg varies in a rectangular wave pattern along the circumferential direction, while in polar anisotropic magnets, the surface magnetic flux density Bg varies in a sinusoidal wave pattern along the circumferential direction. Therefore, polar anisotropic magnets have the advantage of increasing the induced voltage of motors and reducing torque ripple.

[0004] Patent Document 1 discloses a "ring magnet type" polar anisotropic magnet in which magnetic poles are arranged on the outer periphery, and a method for manufacturing the same.

[0005] Patent Document 2 discloses a segmented polar anisotropic magnet with magnetic poles aligned on the inner circumferential surface, and a method for manufacturing the same. This polar anisotropic magnet is produced by arranging multiple permanent magnet segments in a ring shape and fixing them in a resin mold.

[0006] JP 2006-237067 A JP 2018-019081 A

[0007] A polar-anisotropic magnet with magnetic poles aligned on the outer circumferential surface (Patent Document 1) is used in the inner rotor of a permanent magnet synchronous motor, with the magnetic poles facing an outer stator located outside the inner rotor. On the other hand, a polar-anisotropic magnet with magnetic poles aligned on the inner circumferential surface (Patent Document 2) is used in the outer rotor of a permanent magnet synchronous motor, with the magnetic poles facing an inner stator located inside the outer rotor.

[0008] Comparing outer rotors and inner rotors, the former has the advantage of providing higher torque than the latter for permanent magnet synchronous motors of the same diameter. For this reason, it is preferable to use outer rotors, which are lightweight and can achieve high torque, for motors used in small unmanned aerial vehicles such as drones. However, the "segment type" described in Patent Document 2 has the problem of being difficult to assemble after magnetization.

[0009] The present disclosure provides a polar-anisotropic magnet that is not a segment type but a ring magnet type with magnetic poles aligned on the inner circumferential surface, a method for manufacturing the same, and a permanent magnet synchronous motor equipped with the polar-anisotropic magnet.

[0010] This application discloses the inventions described in the following items.

[0011] [Item 1] A polar anisotropic magnet comprising a cylindrical sintered magnet body having an inner peripheral surface with an inner diameter D1 (unit: mm) and an outer peripheral surface with an outer diameter D2 (unit: mm), with P magnetic poles (P is an even number equal to or greater than 8) arranged circumferentially on the inner peripheral surface, and satisfying the following formula (1): 2 mm < (D2 - D1) / 2 ≦ 1.2 × 1 / 2 × (D1 × π) / P (1) The peak value of the surface magnetic flux density Bg on the inner peripheral surface is 0.5 T or greater. [Item 2] The polar anisotropic magnet according to item 1, in which the surface magnetic flux density Bg on the inner peripheral surface varies sinusoidally along the circumferential direction. [Item 3] The polar anisotropic magnet according to item 1 or 2, in which the crystal grains in the cylindrical sintered magnet body are oriented radially at each magnetic pole and circumferentially at the centers of adjacent magnetic poles. [Item 4] The polar anisotropic magnet according to any one of items 1 to 3, wherein the peak value of the surface magnetic flux density Bg on the outer peripheral surface is 0.1 T or less. [Item 5] The polar anisotropic magnet according to any one of items 1 to 4, wherein the inner diameter D1 is 50 mm or more and 100 mm or less. [Item 6] The polar anisotropic magnet according to any one of items 1 to 5, wherein the inner diameter D1 is within ±3% of the average inner diameter along the circumferential direction. [Item 7] A permanent magnet synchronous motor comprising: an outer rotor equipped with a cylindrical polar-anisotropic magnet; and an inner stator located inside the outer rotor, wherein the polar-anisotropic magnet is made of a cylindrical sintered magnet body having an inner circumferential surface with an inner diameter D1 (unit: mm) and an outer circumferential surface with an outer diameter D2 (unit: mm), and has P magnetic poles (P is an even number equal to or greater than 8) arranged circumferentially on the inner circumferential surface, and satisfies the following formula (1): 2 mm<(D2-D1) / 2≦1.2×1 / 2×(D1×π) / P (1) The peak value of the surface magnetic flux density Bg on the inner circumferential surface is 0.5 T or more.[Item 8] A method for manufacturing a polar anisotropic magnet, comprising the steps of: preparing an alloy powder for a sintered magnet; filling the cavity of a magnetic powder press having a cavity with the alloy powder; producing a powder compact by pressing the powder filled in the cavity of the magnetic powder press by applying an aligning magnetic field generated by a plurality of coils; and sintering the powder compact to produce a sintered magnet body, wherein the cavity has a cylindrical molding space defined by an inner circumferential surface and an outer circumferential surface, the shape of the inner circumferential surface varying in a wavy manner along the circumferential direction, the plurality of coils are disposed on the inner circumferential side of the cavity, and polar anisotropy is oriented by the aligning magnetic field. [Item 9] A method for manufacturing a polar anisotropic magnet according to item 8, wherein the aligning magnetic field is a pulsed magnetic field. [Item 10] A method for manufacturing a polar anisotropic magnet according to item 8 or 9, wherein the powder compact is cylindrical with an inner circumferential surface whose shape varies in a wavy manner along the circumferential direction. [Item 11] The method for manufacturing a polar anisotropic magnet according to any one of Items 8 to 10, wherein the diameter of the inner circumferential surface of the cavity has a minimum value at each of P positions (P is an even number equal to or greater than 8) where the strength of the oriented magnetic field generated by the multiple coils reaches a peak.

[0012] According to the present disclosure, there are provided a polar anisotropic magnet in which the magnetic poles are arranged on the inner circumferential surface of a ring magnet type rather than a segment type, a method for manufacturing the same, and a permanent magnet synchronous motor equipped with the polar anisotropic magnet.

[0013] FIG. 1A is a perspective view showing an example of a polar anisotropic magnet according to an embodiment of the present disclosure. FIG. 1B is a top view showing an example of a polar anisotropic magnet according to an embodiment of the present disclosure. FIG. 2 is a diagram schematically showing eight magnetic poles formed on the inner circumferential surface of a polar anisotropic magnet according to an embodiment of the present disclosure and the orientation of magnetic flux lines inside the polar anisotropic magnet. FIG. 3 is a diagram schematically showing 28 magnetic poles formed on the inner circumferential surface of a polar anisotropic magnet according to an embodiment of the present disclosure and the orientation of magnetic flux lines inside the polar anisotropic magnet. FIG. 4 is a graph showing an example of the circumferential distribution of surface magnetic flux density Bg in an example of a polar anisotropic magnet 100 having 28 magnetic poles on its inner circumferential surface. FIG. 5 is a diagram schematically showing magnetic poles formed on the inner and outer circumferential surfaces of a radially aligned magnet and the orientation of magnetic flux lines inside the radially aligned magnet. FIG. 6 is a graph showing an example of the circumferential distribution of surface magnetic flux density Bg on the inner circumferential surface of the radially aligned magnet of FIG. 5. FIG. 7 is a cross-sectional view schematically illustrating an example configuration of a permanent magnet synchronous motor according to an embodiment of the present disclosure. FIG. 8 is a flowchart showing the main steps of a method for manufacturing a polar-anisotropic magnet according to an embodiment of the present disclosure. FIG. 9 is a cross-sectional view schematically illustrating an example configuration of a magnetic field powder press (powder molding apparatus) that can be used to manufacture a polar-anisotropic magnet according to an embodiment of the present disclosure. FIG. 10A is a diagram illustrating an enlarged view of a portion of a cavity in the powder molding apparatus of FIG. 9 , showing the pattern of an oriented magnetic field. FIG. 10B is a diagram illustrating an enlarged view of a portion of a cavity in a modified example of the powder molding apparatus of FIG. 9 , showing the pattern of an oriented magnetic field. FIG. 11 is a graph showing the relationship between the oriented magnetic field strength during molding and the peak value of the surface magnetic flux density Bg on the inner circumferential surface for this example and a comparative example. FIG. 12 is a graph showing the relationship between the peak value of the surface magnetic flux density Bg on the inner circumferential surface and the no-load induced voltage of the motor (three-phase average value, unit: [V / krpm]) for the example and a radially oriented magnet.

[0014] <Polar Anisotropic Magnet> Figure 1A is a perspective view showing an example of a polar anisotropic magnet 100 according to an embodiment of the present disclosure, and Figure 1B is a top view thereof. As shown in the example, polar anisotropic magnet 100 according to this embodiment is composed of a cylindrical (ring-shaped) sintered magnet body 10 having an inner circumferential surface 10A with an inner diameter D1 (unit: mm) and an outer circumferential surface 10B with an outer diameter D2 (unit: mm).

[0015] The cylindrical sintered magnet body 10 has a cylindrical shape with a thickness t defined as half the difference between the outer diameter D2 and the inner diameter D1, i.e., (D2 - D1) / 2. Here, the size of the cylindrical sintered magnet body 10 in a direction parallel to the central axis of the cylinder is referred to as its height, and this dimension is expressed as H (unit: mm). The lower limit of the height H is, for example, 5 mm, and the upper limit can be, for example, approximately the same as the outer diameter D2.

[0016] As shown in FIG. 2 , the polar-anisotropic magnet 100 of this embodiment has P magnetic poles (P is an even number equal to or greater than eight) arranged circumferentially on the inner circumferential surface 10A of the cylindrical sintered magnet body 10. FIG. 2 schematically shows the N and S magnetic poles formed on the inner circumferential surface 10A and the orientation of magnetic flux lines inside the cylindrical sintered magnet body 10. While the number of magnetic poles (number of magnetic poles) P in the example of FIG. 2 is 8, the number of magnetic poles P of the polar-anisotropic magnet of the present disclosure is not limited to 8 as long as it is 8 or greater. FIG. 3 is a top view schematically showing an example of a polar-anisotropic magnet 100 with 28 magnetic poles P. FIG. 3 also schematically shows the N and S magnetic poles formed on the inner circumferential surface 10A and the orientation of magnetic flux lines inside the cylindrical sintered magnet body 10.

[0017] The polar-anisotropic magnet in this disclosure is not an assembled component in which permanent magnet segments, the number of which corresponds to the number of magnetic poles or magnetic pole pairs, are arranged in a ring shape, but rather is a one-piece body in which each magnetic pole is formed by magnetizing a cylindrical member.

[0018] The upper limit of the number of magnetic poles P is determined by the size of the cylindrical sintered magnet body 10, specifically the inner diameter D1. If the spacing between adjacent magnetic poles on the inner circumferential surface 10A (=(D1×π) / P) is too short, it becomes difficult to achieve the desired orientation. The polar anisotropic magnet 100 of the present disclosure satisfies the following formula (1): 2 mm<(D2−D1) / 2≦1.2×1 / 2×(D1×π) / P (1)

[0019] The above formula (1) stipulates that the thickness (= (D2 - D1) / 2) of the cylindrical sintered magnet body 10 must be 2 mm or greater, and also stipulates the minimum distance (= (D1 × π) / P) between adjacent magnetic poles on the inner circumferential surface 10A of the cylindrical sintered magnet body 10, based on the thickness (= (D2 - D1) / 2) of the cylindrical sintered magnet body 10. The inner circumferential surface of the cylindrical sintered magnet body 10 does not need to be a mathematically strict cylindrical surface. When the polar-anisotropic magnet 100 of the present disclosure is used in the rotor of a permanent magnet synchronous motor, the inner diameter D1 need only be within ±3% of the average inner diameter along the circumferential direction. The above formula (1) may also be written as follows: 2 mm < (D2 - D1) / 2 ≦ (0.6 × D1 × π) / P (1). Here, the "average inner diameter" refers to the value obtained by measuring the inner diameter n times (n is a natural number greater than or equal to 2) at equal intervals around the circumference and dividing the sum by n.

[0020] When the polar-anisotropic magnet 100 is used in the outer rotor of a magnet synchronous motor mounted on a mobile object such as a drone, the inner diameter D1 is, for example, 50 mm or more and 100 mm or less. The outer diameter D2 can be set to an appropriate value within a range that satisfies the above formula (1). If the outer diameter D2 of the rotor is large, for example, exceeding 100 mm, it may be easier to fabricate the rotor by assembling permanent magnet segments in a number equal to the number of poles, rather than using the polar-anisotropic magnet 100.

[0021] Figure 4 is a graph showing an example of the circumferential distribution of surface magnetic flux density Bg in an embodiment of a polar-anisotropic magnet 100 having 28 magnetic poles on its inner circumferential surface. In Figure 4, solid curve 30A shows the relationship between surface magnetic flux density Bg and circumferential angular position on inner circumferential surface 10A, while dashed curve 30B shows the relationship between surface magnetic flux density Bg and circumferential angular position on outer circumferential surface 10B. Figure 4 shows the distribution of surface magnetic flux density Bg over half the circumference (circumferential angular positions: 0° to 180°) rather than the entire circumference. The distribution of surface magnetic flux density Bg in the circumferential angular position range from 180° to 360° is also represented by a curve similar to that shown in Figure 4.

[0022] 4, the surface magnetic flux density Bg on inner circumferential surface 10A of polar-anisotropic magnet 100 varies sinusoidally with circumferential angular position. On the other hand, the surface magnetic flux density Bg on outer circumferential surface 10B is significantly smaller than the surface magnetic flux density Bg on inner circumferential surface 10A (for example, the peak value of the surface magnetic flux density Bg on outer circumferential surface 10B is 0.1 T or less), indicating that leakage magnetic field to the outside is sufficiently suppressed.

[0023] In the polar-anisotropic magnet 100 of the present disclosure, the peak value of the surface magnetic flux density Bg on the inner circumferential surface 10A is 0.5 T or higher. The peak value of the surface magnetic flux density Bg on the inner circumferential surface is preferably 0.6 T or higher, and more preferably 0.75 T or higher. As will be described later, the peak value of the surface magnetic flux density Bg on the inner circumferential surface 10A of the polar-anisotropic magnet 100 can be increased by increasing the strength of the aligning magnetic field applied when the sintered magnet alloy powder is compression-molded. Experiments by the inventors have shown that if the strength of the aligning magnetic field applied in pulses is 1.0 T or higher, the peak value of the surface magnetic flux density Bg on the inner circumferential surface 10A of the polar-anisotropic magnet 100 can be increased to 0.75 T or higher.

[0024] FIG. 5 is a schematic diagram showing the magnetic poles formed on the inner and outer peripheral surfaces of a radially aligned magnet 100P as a reference example, and the orientation of the magnetic flux lines within the radially aligned magnet 100P. The magnetic flux lines extend along the radial direction, connecting a pole (e.g., a south pole) on the inner peripheral surface with a pole (e.g., a north pole) on the outer peripheral surface. FIG. 6 is a graph showing an example (curve 32) of the circumferential distribution of the surface magnetic flux density Bg on the inner peripheral surface of the radially aligned magnet 100P of FIG. 5. As can be seen from FIG. 6, the surface magnetic flux density Bg varies in a rectangular wave pattern depending on the circumferential angular position. Note that the circumferential distribution of the surface magnetic flux density Bg on the outer peripheral surface is shown by a curve similar to curve 32 in FIG. 6. As such, with radial alignment, magnetic poles are formed on both the inner and outer peripheral surfaces, creating unnecessary leakage magnetic fields. As a result, it is difficult to increase the surface magnetic flux density Bg on one surface, resulting in lower motor torque compared to polar anisotropy. Furthermore, because the rectangular wave-shaped surface magnetic flux density Bg contains harmonic components, a synchronous motor using radially aligned magnets generates larger torque ripples than one using polar anisotropic magnet 100. This causes vibration and noise during motor operation. From these facts, it can be seen that polar anisotropic magnets that exhibit a surface magnetic flux density Bg distribution such as that shown in Figure 4 have excellent characteristics as permanent magnets used in the rotors of permanent magnet motors.

[0025] <Permanent Magnet Synchronous Motor> FIG. 7 is a cross-sectional view that schematically illustrates an example configuration of a permanent magnet synchronous motor 200 according to an embodiment of the present disclosure.

[0026] This permanent magnet synchronous motor 200 includes an outer rotor OR equipped with a cylindrical polar-anisotropic magnet 100, and an inner stator IS located inside the outer rotor OR. The polar-anisotropic magnet 100 has the configuration described above. That is, the permanent magnet equipped in the outer rotor OR is not an assembled part in which multiple permanent magnet segments are arranged in a ring shape, but rather is a one-piece ring-shaped member with multiple magnetic poles on the inner circumferential surface of the cylindrical member.

[0027] In the example of Figure 7, the outer rotor OR has a reinforcing ring 120 that covers the outer peripheral surface of the polar-anisotropic magnet 100. The reinforcing ring 120 can be made of a non-magnetic material such as aluminum. In this example, the inner stator IS has a stator core 20, a plurality of teeth 22 extending radially outward from the stator core 20, and copper wire (windings) 24 wound around each tooth 22. A three-phase AC current, for example, flows through the windings 24 from an inverter circuit (not shown), generating magnetic flux (stator magnetic flux) from the tips of the teeth 22 that acts on the magnetic flux of the outer rotor OR. The stator magnetic flux forms a rotating magnetic field, allowing the outer rotor OR to rotate with the desired torque and speed.

[0028] The permanent magnet synchronous motor 200 uses a polar anisotropic magnet in which the surface magnetic flux density Bg on the inner surface of the outer rotor OR is distributed sinusoidally along the circumferential direction. This allows for high-torque rotation with less torque ripple and less vibration and noise than a radial anisotropic magnet with the same diameter. Furthermore, the outer rotor OR is ring-shaped rather than segment-shaped, and the reluctance of the outer rotor OR is uniform along the circumferential direction, thereby suppressing the generation of cogging torque. Furthermore, because it is a ring magnet, only one cylindrical magnet is required regardless of the number of rotor poles, making it easier to assemble. Furthermore, because the permanent magnet synchronous motor 200 uses an outer rotor OR, it also has the advantage of being able to achieve higher torque with the same outer diameter compared to a permanent magnet synchronous motor with an inner rotor.

[0029] In this way, the polar anisotropic magnet according to the present disclosure is suitable for use in permanent magnet synchronous motors used in small unmanned aerial vehicles such as drones, and contributes to achieving lightweight and high torque.

[0030] <Method for manufacturing a polar-anisotropic magnet> As shown in FIG. 8 , the method for manufacturing a polar-anisotropic magnet in an embodiment of the present disclosure includes the following steps: a step (S10) of preparing alloy powder for a sintered magnet; a step (S20) of filling the alloy powder into the cavity of a magnetic powder press (powder molding device) having a cavity; a step (S30) of producing a powder compact by applying an aligning magnetic field generated by multiple coils to the powder filled into the cavity of the magnetic powder press, thereby pressing; and a step (S40) of producing a sintered magnet body by sintering the powder compact.

[0031] The cavity of the magnetic press used in this manufacturing method has a cylindrical molding space defined by an inner peripheral surface and an outer peripheral surface. The cylindrical shape may also include a roughly cylindrical shape (approximately cylindrical), such as a shape in which the inner peripheral surface of the cavity changes in a wavy shape along the circumferential direction, as described below. The magnetic press also includes multiple coils arranged on the inner peripheral side of the cavity that perform polar anisotropic orientation using an aligning magnetic field. The aligning magnetic field is preferably a pulsed magnetic field.

[0032] FIG. 9 is a cross-sectional view schematically illustrating an example of the configuration of a magnetic field powder pressing machine that can be used in the manufacturing method described above. The magnetic field pressing machine 300 illustrated in FIG. 9 includes a cylindrical cavity 50 defined by an inner peripheral surface 50A and an outer peripheral surface 50B, and multiple coils 52 disposed inside the cavity 50 to generate an aligning magnetic field for polar anisotropy. The magnetic field pressing machine 300 further includes a core 54 that defines the inner peripheral surface 50A of the cavity 50 and a die 56 that defines the outer peripheral surface of the cavity 50. The multiple coils 52 are disposed inside the core 54. Upper and lower punches (not shown) sandwich the alloy powder in the cavity 50 and compress it from above and below (directions perpendicular to the plane of FIG. 9 ).

[0033] FIG. 10A is an enlarged view of a portion of the cavity 50 in the magnetic field press 300 of FIG. 9 , showing the pattern of the oriented magnetic field. The cavity 50 is filled with alloy powder 64. When the upper and lower punches compress the alloy powder 64, a pulse current flows through the coil 52, generating a pulsed oriented magnetic field. For example, the current may be set so that the oriented magnetic field is between 0.32 T and 1.9 T. FIG. 10A includes a bidirectional arrow 62 that schematically represents the direction of the oriented magnetic field within the cavity 50. The powder particles constituting the alloy powder 64 are oriented so as to coincide with the direction of the oriented magnetic field at the position of the powder particle (the direction of the bidirectional arrow 62). Also, in FIG. 10A , the direction of the oriented magnetic field generated by the coil 52 at the position where the strength of the oriented magnetic field is greatest is schematically indicated by thick arrows 58A and 58B.

[0034] In a preferred embodiment of the present disclosure, the powder particles constituting the alloy powder filled in the cavity 50 are substantially single crystals, and form crystal grains after sintering. By applying an aligning magnetic field, the powder particles in the cavity 50 are oriented so that their respective easy axes of magnetization are aligned with the direction of the aligning magnetic field. This orientation is maintained even after sintering. In other words, the orientation direction of the crystal grains after sintering refers to the direction of the easy axes of magnetization of the crystals in each crystal grain. Therefore, by observing the easy axes of magnetization of the crystals in the sintered crystal grains, it is possible to identify the direction of the aligning magnetic field applied by the magnetic field press.

[0035] In the example of Figure 10A, the strongest magnetic field is formed at the center position between adjacent coils 52, and the orientation of the powder particles in the region extending radially from that position is aligned radially. Coil 52 conceptually represents the cross section of the flow path through which the current that forms the magnetic field flows, but it may be a single coil or a bundle of multiple coils. The magnetization process described below forms a magnetic pole (north or south pole) in the region where the orientation of the powder particles is aligned radially. In the polar anisotropic magnet formed in this way, the orientation of the crystal grains is radial at each pole and circumferential at the center of adjacent poles.

[0036] The spacing between the magnetic poles on the inner circumferential surface of the polar anisotropic magnet ((D1×π) / P) is determined by the spacing between the coils 52 that form the aligning magnetic field. From the perspective of manufacturing the mold (core 54), it is desirable that the spacing between the coils 52 be 4 mm or more.

[0037] The cylindrical powder compact removed from cavity 50 through the pressing step (S30 in FIG. 8) is transformed into a cylindrical sintered magnet body through the sintering step (S40 in FIG. 8). The powder compact shrinks and hardens through sintering. For example, the sintering may be performed at a holding temperature of 1000°C or higher and 1150°C or lower for a holding time of 60 minutes or higher and 2160 minutes or lower.

[0038] The resulting sintered magnet body is machined into a cylindrical shape with the desired dimensions through a grinding process. The sintered magnet body is then magnetized through a magnetizing process to obtain a polar-anisotropic magnet with the desired magnetic poles arranged along the inner circumferential surface. The magnetization is performed to match the orientation of the magnets in the pressing process.

[0039] According to the inventors' research, the higher the strength of the oriented magnetic field, the higher the peak value of the surface magnetic flux density Bg on the inner peripheral surface of the final polar-anisotropic magnet can be. However, it has been found that cracks may occur on the inner peripheral surface depending on the type (composition), particle size distribution, compaction pressure, sintering conditions, etc. of the alloy powder. It is believed that such cracks arise because the rate of shrinkage due to sintering differs between the direction perpendicular to the orientation direction and the direction parallel to it. Considering the ranges of the type (composition), particle size distribution, compaction pressure, sintering conditions, etc. of alloy powder currently commonly used to produce polar-anisotropic magnets, it is preferable to produce a compact that will produce a magnet that satisfies formula (1).

[0040] The inventors have found that changing the shape of the cavity 50 to match the pattern of the oriented magnetic field is an effective way to prevent such cracks from occurring. Below, with reference to Figure 10B, we will explain an example of the configuration of a magnetic field press that is less likely to cause cracks even when the strength of the oriented magnetic field is increased.

[0041] FIG. 10B is an enlarged view of a portion of the cavity 50 in a modified example 300X of the magnetic field press 300, illustrating the pattern of the oriented magnetic field. In this modified example 300X, the shape of the inner circumferential surface 50A of the cavity 50 varies in a wavy manner along the circumferential direction. In other words, the inner diameter of the inner circumferential surface 50A of the cavity 50 varies periodically along the circumferential direction. More specifically, the inner diameter of the inner circumferential surface 50A of the cavity 50 has a minimum value at each of P positions 54A (P is an even number greater than or equal to 8) where the intensity of the oriented magnetic field generated by the multiple coils 52 reaches its peak value. Furthermore, the inner diameter of the inner circumferential surface 50A of the cavity 50 has a maximum value at each of P positions 54B where the intensity of the oriented magnetic field reaches its bottom value. By employing a cavity 50 having such a shape, the resulting powder compact has a cylindrical shape with an inner circumferential surface whose shape varies in a wavy manner along the circumferential direction (an inner circumferential surface whose inner diameter varies periodically along the circumferential direction). The inner diameter defined by the inner peripheral surface of the powder compact thus produced has a minimum value and a maximum value that are outside the range of ±5% of the average inner diameter along the circumferential direction, for example.

[0042] When a cylindrical powder compact having an inner peripheral surface whose shape changes circumferentially in a wavy manner is sintered, the occurrence of cracks described above can be suppressed. This is presumably because the distribution of internal residual stresses caused by shrinkage during the sintering process is made uniform as a result of adjusting the shape of the approximately cylindrical powder compact as described above. The magnitude (amplitude) of the waveforms to be formed on the inner peripheral surface of the powder compact is thought to depend on the type (composition) and particle size distribution of the alloy powder used in the powder compact.

[0043] It is preferable that the inner peripheral surface of the final polar-anisotropic magnet has a shape that will not come into contact with the inner stator when it rotates as the outer rotor of a permanent magnet synchronous motor. Such a cylindrical inner peripheral surface can be obtained by mechanically grinding the sintered body.

[0044] The polar-anisotropic magnet obtained by the manufacturing method described above has a structure exhibiting a polar-anisotropic orientation angle, which can result in a peak value of the surface magnetic flux density Bg on the inner circumferential surface of 0.5 T or more when magnetized using a typical magnetization method. The orientation angle of such a structure can be confirmed by, for example, EBSD (Electron Backscatter Diffraction) analysis of a cross section of the polar-anisotropic magnet before magnetization. The orientation angle corresponds to the direction of the bidirectional arrow 62 shown in Figures 10A and 10B , for example.

[0045] In this example, a polar anisotropic R-T-B based sintered magnet (R is at least one rare earth element and always includes Nd, and T is at least one transition metal element and always includes Fe) was produced.

[0046] Generally, the preferred composition of an RTB based sintered magnet is as follows:

[0047] R is a rare earth element and must include at least one selected from the group consisting of Nd, Pr, La, and Ce. Preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, or Nd-Pr-Dy-Tb is used.

[0048] Among R, Dy and Tb are particularly ｃＪ It is effective in improving the properties. In addition to the above elements, other rare earth elements such as La may be contained, and misch metal or didymium may also be used. Furthermore, R does not have to be a pure element, and may contain impurities unavoidable in the manufacturing process within the industrially available range. The content of R is, for example, 27% by mass or more and 35% by mass or less. Preferably, it is 27% by mass or more and 31% by mass or less, and more preferably, it is 28% by mass or more and 31% by mass or less.

[0049] T contains iron (including cases where T consists essentially of iron), and up to 50% by mass of iron may be substituted with cobalt (Co) (including cases where T consists essentially of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance, and may be contained in an amount of up to 10% by mass. The content of T may account for the remainder of R and B, or R, B, and M, which will be described later.

[0050] The content of B may be a known content, and for example, a preferred range is 0.8 mass % or more and 1.1 mass % or less. If it is less than 0.8 mass %, high H ｃＪ If it exceeds 1.1 mass%, B may not be obtained. ｒ A part of B can be substituted with C (carbon). The content is more preferably 0.8% by mass or more and 1.0% by mass or less, and even more preferably 0.8% by mass or more and 0.96% by mass or less.

[0051] In addition to the above elements, H ｃＪ An M element can be added to improve the properties. The M element is one or more elements selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, and W. The amount of M element added is preferably 5.0 mass% or less when one M element is used, and preferably 5.0 mass% or less in total when multiple M elements are used. If the amount exceeds 5.0 mass%, B ｒ In addition, unavoidable impurities can be tolerated.

[0052] The N (nitrogen) content in the R-T-B based sintered magnet is preferably 50 ppm to 800 ppm by mass. This is because it is possible to suppress the deterioration of magnetic properties due to nitriding. The nitrogen content is more preferably 50 ppm to 400 ppm, and most preferably 50 ppm to 300 ppm. Furthermore, the C (carbon) content in the R-T-B based sintered magnet is preferably 400 ppm to 1500 ppm by mass, and more preferably 500 ppm to 1300 ppm.

[0053] The main phase of the RTB based sintered magnet in this example is R. ２ T １４ The average grain size of the B phase is 2.0 μm or more and 8.0 μm or less. The average grain size can be determined by averaging the equivalent circle diameters of 5,000 or more grains evaluated by EBSD. Similarly, EBSD mapping of a cylindrical cross section perpendicular to the inner peripheral surface confirmed that the grains had an orientation angle toward the magnetic pole.

[0054] In this example, raw material alloys for producing RTB based sintered magnets having the compositions shown in Table 1 below were prepared.

[0055]

[0056] Raw material alloys adjusted to produce R-T-B based sintered magnets with the compositions 1 to 3 (excluding O, N, and C) were melted and strip-cast to obtain flake-shaped quenched alloys. Each quenched alloy was hydrogen-pulverized to reduce the size of the hydrogen-pulverized powder (coarsely pulverized powder) to 2.0 mm or less, and the coarsely pulverized powder was then pulverized in a jet mill to obtain alloy powders for sintered magnets corresponding to compositions 1 to 3. The average particle size (d50) can be measured using an airflow dispersion laser diffraction method (in accordance with JIS Z 8825: 2013 revised edition). In this specification, the average particle size refers to the particle size (median diameter) at which the cumulative particle size distribution (volume-based) from the smallest particle size side reaches 50%. More specifically, the average particle size (d50) indicates the d50 measured using a particle size distribution analyzer "HELOS & RODOS" manufactured by Sympatec under the following conditions: dispersion pressure: 4 bar, measurement range: R2, calculation mode: HRLD.

[0057] The powder of alloys for R-T-B based sintered magnets is active and easily oxidized, so the gas used in the jet mill was dry (high-purity) nitrogen with a dew point of -60°C or below, in order to avoid the risk of heat generation and fire and to reduce the oxygen content as an impurity, thereby improving the performance of the magnets.

[0058] When producing a polar anisotropic R-T-B based sintered magnet, the average particle size of the alloy powder used is preferably 2.0 μm or more and 6.0 μm or less. If it is less than 2.0 μm, the crushed particle size of the fine powder may be too small to prevent a decrease in crushing efficiency during jet mill crushing, while if it exceeds 6.0 μm, it may be difficult to obtain high magnetic properties. The crushed particle size of the fine powder is more preferably 2.0 μm or more and 5.0 μm or less. By reducing the average particle size, it becomes possible to improve the magnetic properties. In this example, the average particle size (d50) was 4.5 μm.

[0059] The above alloy powder was filled into the cavity of the magnetic field powder press (modified example 300X in Figure 10B), and then pressed at 98 MPa to produce a cylindrical powder compact. The cavity had an inner diameter (average) of 91 mm, an outer diameter of 101 mm, and a height of 15 mm. The inner diameter of the cavity had a minimum value (90.2 mm) at each position where the strength of the aligning magnetic field reached its peak value, and a maximum value (91.6 mm) at each position where the strength of the aligning magnetic field reached its bottom value. When pressing the alloy powder filled into the cavity, aligning magnetic fields set to 0.32 T, 0.60 T, 0.78 T, and 0.94 T were applied multiple times with a pulse time of 1 millisecond. In this example, the number of magnetic poles was 28. To suppress oxidation of the alloy powder, the powder compact was produced in an inert gas atmosphere. In addition, using another magnetic field powder press having a similar configuration to the above-mentioned magnetic field powder press (modified example 300X in FIG. 10B), a powder compact having a similar shape and size was produced by applying an aligning magnetic field set to 1.9 T multiple times with a pulse time of 1 millisecond. In the latter magnetic field powder press, the number of coils was changed, making it possible to apply a stronger aligning magnetic field.

[0060] Next, each powder compact was sintered to obtain a cylindrical rare earth sintered magnet body (sintered body). The cylindrical sintered body had an inner diameter of about 73 mm, an outer diameter of about 81 mm, and a height of about 13 mm. The sintering was carried out under a pressure of 0.13 Pa (10 －３ The heating was carried out at a pressure of 1080° C. or less (1000 Torr) and a temperature of 1080° C. for 180 minutes.

[0061] The inner circumferential surface, outer circumferential surface, upper end surface, and lower end surface of the cylindrical sintered magnet body thus obtained were ground to obtain a cylindrical sintered magnet body with the final shape and dimensions. A magnetization process was then carried out to produce the final polar-anisotropic magnet. The final polar-anisotropic magnet (processed cylindrical sintered magnet body) had an inner diameter D1 of 74 mm, an outer diameter D2 of 80 mm, and a height of 12 mm. The composition of the resulting R-T-B based sintered magnet was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES), and the results are shown in Table 1. The oxygen content (O) was measured using a gas fusion-infrared absorption method, the nitrogen content (N) was measured using a gas fusion-thermal conduction method, and the carbon content (C) was measured using a gas analyzer using a combustion-infrared absorption method. The results are shown in Table 1.

[0062] Figure 11 is a graph showing the relationship between the alignment field strength during molding and the peak value of the surface magnetic flux density Bg of the inner circumferential surface for the present example and the comparative example. The horizontal axis of the graph represents alignment field strength (unit: T), and the vertical axis represents the peak value of the surface magnetic flux density Bg (unit: T). In the graph, "◯" represents the measured value for the polar-anisotropic magnet of the example with composition 1, and "●" represents the measured value for the polar-anisotropic magnet of the example with composition 2. In contrast, "♦" represents only a portion of the measured data obtained for the segment-type magnet of the comparative example with composition 1. Figure 11 shows that, as long as the polar-anisotropic magnet is a segment-type magnet or a ring-type magnet, the same surface Bg can be obtained for the same alignment field strength. Note that for the radial magnets with compositions 1 and 3, even when the alignment field strength during molding was 1.9 T, the peak values ​​of the surface magnetic flux density Bg were 0.53 T and 0.52 T, respectively.

[0063] As can be seen from Figure 11, in the example, the peak value of the surface magnetic flux density Bg increases as the aligning magnetic field strength increases. According to this example, the peak value of the surface magnetic flux density Bg on the inner peripheral surface can be increased to 0.5 T or more, and a value of 0.7 T or more, which was not possible with the segment mold, was also achieved. By increasing the aligning magnetic field strength during molding to 1.0 T or more, the peak value of the surface magnetic flux density Bg can be further increased beyond 0.75 T. According to this example, in both the case of composition 1 and composition 2, the peak value of the surface magnetic flux density Bg can be increased to 0.8 T or more.

[0064] FIG. 12 is a graph showing the relationship between the peak value of the surface magnetic flux density Bg on the inner circumferential surface of the magnet and the motor no-load induced voltage (three-phase average value, unit: [V / krpm]). The motor no-load induced voltage refers to the magnitude of the back electromotive force (back-EMF) generated inside the motor when the motor is rotated with zero mechanical load (shaft torque = 0 N·m) and is proportional to the magnetic flux Φ generated by the permanent magnet. In the graph of FIG. 11, "X" represents data for radially aligned magnets, "◯" represents data for the polar anisotropic magnet of the example with composition 1, and "●" represents data for the polar anisotropic magnet of the example with composition 2. As shown in FIG. 11, the example of the present invention achieves a motor no-load induced voltage exceeding 15 [V / krpm] due to the high peak value of the surface magnetic flux density Bg. Motors exhibiting such a high motor no-load induced voltage are suitable for applications requiring high torque at relatively low rotation speeds, such as large drones, in-wheel motors for automobiles, and electric bicycles. The motor used to measure the no-load induced voltage of the motor was an outer rotor permanent magnet synchronous motor with a 28-pole (14 pole pairs) / 24-slot configuration. The stator had 24 teeth, each with a concentrated winding consisting of 14 turns of 0.7 mm diameter copper wire. The windings were three-phase Y-connected, with each phase having four sub-coils connected in parallel.

[0065] The polar-anisotropic magnet of the present disclosure is suitable for use in the outer rotor of a permanent magnet synchronous motor, and the permanent magnet synchronous motor of the present disclosure can be used for a variety of purposes, such as driving the rotors of unmanned aerial vehicles such as drones.

[0066] DESCRIPTION OF SYMBOLS 10: Cylindrical sintered magnet body, 10A: Inner peripheral surface, outer peripheral surface 10B, 100: Polar anisotropic magnet, 120: Reinforcing ring, 20: Stator core, 22: Teeth, 24: Winding, 50: Cavity, 50A: Inner peripheral surface, 50B: Outer peripheral surface, 52: Coil, 54: Core, 56: Die, 200: Permanent magnet synchronous motor, 300: Press machine in magnetic field, OR: Outer rotor, IS: Inner stator

Claims

1. A polar-anisotropic magnet consisting of a cylindrical sintered magnet body having an inner surface with an inner diameter D1 (unit: mm) and an outer surface with an outer diameter D2 (unit: mm), with P magnetic poles (P is an even number of 8 or greater) arranged circumferentially on said inner surface, and satisfying the following formula (1): 2 mm < (D2 - D1) / 2 ≦ (0.6 × D1 × π) / P ... (1) The peak value of the surface magnetic flux density Bg on said inner surface is 0.5 T or greater.

2. A polar anisotropic magnet as set forth in claim 1, wherein the surface magnetic flux density Bg on the inner peripheral surface varies sinusoidally along the circumferential direction.

3. A polar-anisotropic magnet as set forth in claim 1, wherein the crystal grains in the cylindrical sintered magnet body are oriented radially at each magnetic pole and circumferentially at the center of adjacent magnetic poles.

4. A polar anisotropic magnet as set forth in claim 1, wherein the peak value of the surface magnetic flux density Bg on the outer peripheral surface is 0.1 T or less.

5. A polar anisotropic magnet according to claim 1, wherein the inner diameter D1 is 50 mm or more and 100 mm or less.

6. A polar anisotropic magnet as set forth in claim 5, wherein the inner diameter D1 is within a range of ±3% of the average inner diameter along the circumferential direction.

7. A permanent magnet synchronous motor comprising: an outer rotor equipped with a cylindrical polar-anisotropic magnet; and an inner stator located inside the outer rotor, wherein the polar-anisotropic magnet is composed of a cylindrical sintered magnet body having an inner circumferential surface with an inner diameter D1 (unit: mm) and an outer circumferential surface with an outer diameter D2 (unit: mm), and has P magnetic poles (P is an even number greater than or equal to 8) arranged circumferentially on the inner circumferential surface, and wherein the following formula (1) is satisfied: 2 mm < (D2 - D1) / 2 ≦ (0.6 × D1 × π) / P ... (1) and the peak value of the surface magnetic flux density Bg on the inner circumferential surface is 0.5 T or more.

8. A method for producing a polar-anisotropic magnet according to any one of claims 1 to 7, comprising the steps of: preparing an alloy powder for a sintered magnet; filling the alloy powder into the cavity of a magnetic powder press having a cavity; producing a powder compact by applying an aligning magnetic field generated by a plurality of coils to the powder filled into the cavity of the magnetic powder press, thereby pressing; and producing a sintered magnet body by sintering the powder compact, wherein the cavity has a cylindrical molding space defined by an inner circumferential surface and an outer circumferential surface, the shape of the inner circumferential surface varying in a wavy pattern along the circumferential direction, and the plurality of coils are arranged on the inner circumferential side of the cavity, and polar-anisotropic orientation is achieved by the aligning magnetic field.

9. The method for producing a polar anisotropic magnet according to claim 8, wherein the aligning magnetic field is a pulsed magnetic field.

10. The method for producing a polar anisotropic magnet according to claim 9, wherein the powder compact is cylindrical in shape with an inner circumferential surface that changes in a wavy shape along the circumferential direction.

11. A method for manufacturing a polar anisotropic magnet as described in claim 10, wherein the diameter of the inner surface of the cavity has a minimum value at each of P positions (P is an even number greater than or equal to 8) where the strength of the oriented magnetic field generated by the multiple coils reaches a peak.

Images