A motor may be referred to as a multi-function part for changing electric energy to mechanical energy by a combination of a shaft, a bearing, a stator, etc. made by high-precision machining of various of kinds of materials such as steel, nonferrous metal, polymer and the like. In recent years, a so-call permanent magnet type motor has been put in the mainstream, which uses a magnet having the ability to attract or repel other magnetic materials and the ability to permanently generate a static magnetic field without using external energy. A physical difference of the magnet from other magnetic material is that, for the magnet, effective magnetization M is left even after removal of an external magnetic field, and magnetization inversion (demagnetization) is initially induced under application of heat or a relatively large reversal magnetic field, therefore magnetization M is decreased. Energy Density (BH) max is one of important characteristic values of such a magnet. This means magnet's latent energy per unit volume.
The magnet's strong attractive and repulsive ability does not necessarily give high performance to the magnet depending on the kind of magnet. For example, it is disclosed in Non-Patent Document 1 that increase of energy density (BH) max of a magnet leads to increase of torque density in a radial airgap type motor, which is a subject matter of the present invention, from a relationship between residual magnetic flux density Br, which is one of basic characteristics of the magnet, and a motor constant KJ (KJ being a ratio of power torque KT to square root of Ohmic loss √R), which is an indicator of motor performance, with motor diameter, rotator diameter, giagap, soft magnetic material, magnet dimension and the like fixed.
However, while the increase of energy density (BH) max of the magnet leads to the increase of torque density in the motor, which is the subject matter of the present invention, since a rotator iron core of the motor has slots in which a coil is accommodated, and teeth forming a part of a magnetic circuit, a permeance is varied with rotation of the motor. Accordingly, the increase of energy density (BH) max of the magnet leads to increase of torque pulsation, i.e., cogging torque. The increase of cogging torque disturbs smooth rotation of the motor, which results in increase of vibration and noise of the motor, thereby causing a bad effect such as deterioration of rotation controllability.
To avoid such a bad effect, many studies for reduction of cogging torque have conventionally been made to approach an airgap magnetic flux density distribution between a rotator and a stator iron core of a motor to a sinusoidal wave-shaped distribution.
First, regarding magnetic poles having a certain thickness in a magnetization direction, thickness-deviation of a magnet will be described. For example, as shown in FIG. 11A of Non-Patent Document 2, when a motor has 12 magnetic poles/18 slots, with the magnetic poles having thickness-deviation with residual magnetization Br=1.2 T, the maximum thickness of 3 mm in their center, the minimum thickness of 1.5 mm in their both ends, cogging torque of the motor can be minimized. The motor shown in FIG. 11A of this document includes magnetic poles 1 having thickness-deviation, stator iron core 2, stator iron core slots 3 and stator iron core teeth 4. Although magnetic poles 1 are segmented in a side of their outer diameter in this example, it is noted that magnetic poles 1 may be segmented from the reverse side, that is, from the side of their inner diameter for reduction of cogging torque.
In addition, as shown in FIG. 11A of Non-Patent Document 2, it is required to be the magnetic poles having such thickness-deviation that the maximum thickness in their center is equal to about ½ of the minimum thickness in their both ends in order to minimize cogging torque. Thus, when the thickness of magnetic poles 1, that is, direction (thickness) of magnetization M, becomes small, it is difficult to sufficiently reduce cogging torque through the thickness-deviation of magnetic poles 1. Moreover, in general, it is difficult to treat the magnetic poles because of their fragile mechanical property.
In the meantime, with regard to magnetic poles having small thickness in a magnetization direction, there has been known a method of skewing magnetic poles as shown in FIG. 11B of Non-Patent Document 3 or a method of continuously deleting a magnetic pole area between magnetic poles as shown in FIG. 11C of Non-Patent Document 4.
Summarizing the above conventional techniques, magnetic pole ends of thick magnetic poles are thinned to ½ of their original thickness to widen an airgap between the magnetic poles and a stator iron core or reduce a magnetic pole area between thin magnetic poles. Thus, the amount of static magnetic field Ms generated from the magnetic poles and flowing into the stator iron core as a magnetic flux φ is suppressed. As a result, the above-mentioned methods typically lower torque density by 10 to 15% due to the reduction of cogging torque. Accordingly, the cogging torque reduction method in the conventional techniques shown in FIGS. 11A, 11B and 11C is in contradiction to the method of increasing torque density of a motor with increase of energy density (BH) max of a magnet.
On the other hand, D. Howe et al. has reported a motor cogging torque reduction method as a method of using a Nd2Fe14B-based rare-earth sintered magnet having small thickness of 1.2 mm in a magnetization direction and having high energy density with residual magnetization Mr of 1 T so that the thickness in the magnetization direction or the magnetic pole area as shown in FIGS. 11A, 11B and 11C may not be reduced. In this report, a so-called Halbach Cylinder has been proposed in which each of magnetic poles segmented into 2 to 5 pieces and magnetization direction (direction of magnetic anisotropy) of each of the pieces is adjusted step by step as shown in FIGS. 12A to 12D. In FIGS. 12A to 12D, subscripts (2) to (5) of magnetic poles 1 designate the number (2 to 5) of pieces of each of magnetic poles 1. An arrow in each of the pieces indicates direction of magnetization vector M along an oriented magnetization easy axis (C axis), that is, direction of anisotropy.
FIG. 13 shows a relationship between the number of magnetic pole pieces of segmented magnetic poles and a cogging torque for a motor having 12 poles/18 slots using the above-configured magnetic poles. It can be seen from the figure that the number (N) of magnetic pole pieces of segmented magnetic poles and the cogging torque (Tcog) have a relationship of exponential approximation of Tcog.=61.753 exp(−0.1451N). In addition, FIG. 13 implies that it is ideal that magnetization vector (M) for any mechanical angle (φ), and Mθ/φp (Mθ is an angle with respect to a circumferential tangent line of magnetic poles are continuously varied minutely in, particularly, a specified direction between different magnetic poles. However, for the Nd2Fe14B-based rare-earth sintered magnet having the thickness of 1.2 mm and the high energy density with residual magnetization (Mr) of 1 T, it is difficult to regularly and minutely arrange magnetic pole pieces having different directions of anisotropy and configure the magnetic poles with high dimensional precision. Thus, it is very difficult to manufacture a multi-pole rotator provided with an integral multiple of magnetic poles or a radial airgap type magnet motor using the multi-pole rotator. In addition, it may be guessed without difficulty that motors employing the above-mentioned Halbach Cylinder are uneconomical.
An object of the present invention is to provide a permanent magnet rotator without reduction of a magnetic pole volume or area and with additional reduction of cogging torque for anisotropic magnetic poles having a shape difficult to be segmented to, for example, small thickness of 1.5 mm, and high energy density.
The gist of the present invention lies in a permanent magnet rotator which is capable of providing continuous direction control for anisotropy with modification of magnetic poles so that the average absolute value of differences between Mθ and 90×sin [φ{2π/(360/p)}] (where Mθ is a direction of anisotropy with respect to a radial tangent line of a magnetic pole plane, φ is a mechanical angle, and p is the number of pole pairs) is set to be 3° or less. That is, the gist of the present invention lies in a permanent magnet rotator with the direction (Mθ) of anisotropy distributed in a range of 0 to 90° with respect to the mechanical angle (φ) as a sinusoidal wave-shaped distribution. Permanent magnet rotators with such continuous direction control for anisotropy have not been yet known in the art.
The permanent magnet rotator related to the present invention has magnetic poles modified to provide continuous direction control for anisotropy. More particularly, assuming that an angle between a uniform oriented magnetic field (Hex) and a radial tangent line of inner and outer circumferences of magnetic poles is Hθ, a circumference length at an airgap side of magnetic poles before modified is Lo, and a circumference length at an airgap side of magnetic poles after modified is L, the magnetic poles are modified in the radial direction with a specified range of Lo/L=1.06 to 1.14. For the modification, first, a relationship of Hθ≈Mθ is set in circumferential magnetic pole ends and a circumferential magnetic pole center. Subsequently, in portions except the circumferential magnetic pole ends and the circumferential magnetic pole center, continuous direction control is provided for anisotropy by means of action of shear stress (τ) toward the circumferential center at the air gap side of magnetic poles.
The permanent magnet rotator related to the present invention has a high-precise relationship of Mθ and 90×sin [φ{2π/(360/p)}] for magnetic anisotropic magnetic poles, so that reduction of cogging torque and increase of torque density, which are contradict from each other, can be compatible.
In particular, the magnetic poles of the permanent magnet rotator have a macro structure in which Nd2Fe14B-based rare-earth magnet particles having energy density (BH) max≧150 kJ/m3 and size of less than 150 μm are isolated from each other in a matrix (continuous phase) including Sm2Fe17N3-based rare-earth magnet powders having average particle diameter of 3 to 5 μm and a coupling agent, with a volume fraction of magnetic material having energy density (BH) max≧270 kJ/m3 in magnetic anisotropic magnetic poles set to be more than 80 vol. %, with a magnetizing field (Hm) set in parallel to an oriented magnetic field (Hex), and with the magnetizing field (Hm) set to be more than 2.4 MA/m.
A magnetically-isotropic magnet can be freely magnetized in any directions depending on a direction of a magnetizing field and a strength distribution of the magnetizing field. Thus, a magnetization pattern as indicated by arc-like arrows of magnetic poles 1 in FIG. 14 can be obtained by a shape of a magnetizing yoke and optimization of a magnetomotive force. This allows an airgap magnetic flux density distribution between the magnetic poles and a stator iron core to be easily adjusted to a sinusoidal wave shape. Thus, it is very ease to reduce cogging torque of the motor as compared when thin magnetic poles are made of magnetically-anisotropic magnetic material.
It is believed that the research on the above-mentioned isotropic rare-earth magnetic material originates from R. W. Lee et al. suggested that an isotropic Nd2Fe14B-based bonded magnet having energy density (BH) max=72 kJ/m3 is obtained when a rapid-solidified ribbon having energy density (BH) max=111 kJ/m3 is fixed with a resin (see Non-Patent Document 6).
Since then, from the late 1980's to the present, studies on isotropic rare-earth magnetic materials with rapid-solidification of rare earth-iron molten alloy have been actively made. For example, in addition to isotropic magnetic materials with various micro-controlled alloy structures including nano composite magnetic materials using exchange coupling based on micro-structures of Nd2Fe14B family, Sm2Fe17N3 family, or exchange coupling based on micro-structures of these families and αFe, FeB and Fe3B families, other isotropic magnetic materials having different shapes of powder have been available in the industries (For example, see Non-Patent Documents 7 to 10).
In addition, H. A. Davis et al. have reported isotropic magnetic materials having energy density (BH) max of 220 kJ/m3 (see Non-Patent Document 11). However, the energy density (BH) max of isotropic magnetic materials available in the industries is no more than 134 kJ/m3, and the energy density (BH) max of general Nd2Fe14B bonded magnets applied to small motors having power capacity of 50 W or less is about 80 kJ/m3 or less. In other words, since R. W. Lee et al. have made at 1985 the isotropic Nd2Fe14B-based bonded magnet having energy density (BH) max=72 kJ/m3 using the ribbon having energy density (BH) max=111 kJ/m3, the amount of increase in energy density (BH) max is below 10 kJ/m3 even at the present after the elapse of more than 20 years. Accordingly, it can not be anticipated when sufficient high motor torque density by a permanent magnet rotator, which is the subject matter of the present invention, can be achieved with increase of energy density along the advance of isotropic magnetic materials.
On the other hand, since change from an isotropic magnet to an anisotropic magnet is typically followed by increase of energy density (BH) max, a motor equipped with a permanent magnet rotator, which is the subject matter of the present invention, has higher torque density while increasing in cogging torque.
In addition, N. Takahashi et al. have proposed a method of controlling a direction of anisotropy by rearranging a magnetic material in a nonmagnetic forming mold and changing a magnetic flux (φ) of a cavity portion from a fixed direction to any different directions in manufacturing an arc-like anisotropic magnet used for a motor (see Non-Patent Document 12).
However, it is difficult to finely control the direction of magnetic flux (φ) of the cavity portion. Thus, it is difficult to provide precise continuous direction control for anisotropy so that the average absolute value of differences between Mθ and 90×sin [φ{2π/(360/p)})] (where Mθ is a direction of anisotropy with respect to a radial tangent line of a magnetic pole plane, φ is a mechanical angle, and p is the number of pole pairs) is set to be 3° or less, as in the present invention.
As described above, although the conventional techniques increase torque density of a motor with increase of energy density (BH) max, these techniques have the problem of deterioration of quiescence and controllability with increase of cogging torque.
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