JPH0340487B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention mainly relates to a magnet for a rotor of a stepping motor, and more particularly to a cylindrical permanent magnet having surface multipolar anisotropy. In order to apply multipolar magnetization to the surface, it is necessary for the permanent magnet to have a relatively high coercive force and a reversible magnetic permeability of around 1, so barium ferrite magnets, barium ferrite magnets,
Ferrite magnets such as strontium ferrite magnets are used. The stoichiometry of these permanent magnets is MO.6Fe ₂ O ₃ (M is Ba, Sr, Pb or a mixture thereof, and may also include other divalent metals such as Ca). However, in reality, the MO is slightly excessive, MO・5～
6Fe ₂ O ₃ . Examples of cylindrical permanent magnets include isotropic ferrite magnets, ring anisotropic ferrite magnets, and plastic magnets made by dispersing ferrite magnetic powder in synthetic rubber, synthetic resin, or natural rubber. The reality is that isotropic ferrite magnets do not have sufficient magnetic properties. Furthermore, in the ring anisotropic magnet 11 shown in FIG. 1, the grain arrangement direction is radial as shown by arrow B, but after sintering, the magnetization is made up of a yoke 1 and a coil 2 as shown in FIG. Since the sintered body 3 is multipolarized by the magnetic yoke A, there are portions where the particle arrangement direction differs from the direction of the magnetic lines of force (arrow C). Therefore, the ring anisotropic magnet 11 does not have an effective particle arrangement. Furthermore, there is a plastic magnet 12 as an example in which the particles are aligned in the same direction as the multi-pole magnetization direction (direction of arrow C) shown in FIG. 3, but a sufficient amount of magnetic flux cannot be obtained because the residual magnetic flux density is low. . In order to improve the magnetic properties, it is conceivable to orient the ferrite particles in the same direction as the multipolar magnetization direction and increase the degree of orientation as described above. Therefore, the inventors of the present invention have considered improving the magnetic properties of sintered ferrite magnets by arranging ferrite particles in the same direction as the multipole magnetization direction, similar to the plastic magnet shown in Figure 3. When the number of magnetic poles is small, a cylindrical permanent magnet that can be put to practical use cannot be obtained. In addition, increasing the wall thickness may be considered in order to obtain higher magnetic properties, but there is a problem that increasing the wall thickness increases the inertial force. An object of the present invention is to provide a multipolar surface magnetized magnet having a high surface magnetic flux density. A further object of the present invention is to provide a permanent magnet that exhibits extremely high holding torque when used in a stepping motor. The permanent magnet of the present invention is MO・nFe ₂ O ₃ (where M is
One or more of Ba, Sr, Pb, n=5
~6) A sintered cylindrical permanent magnet having a magneto-plumbite type crystal structure, in which the cylindrical portion has surface multipolar anisotropy of 12 to 24 poles. In the present invention, surface multipolar anisotropy refers to magnetic poles of different polarity existing or can be created on the same surface of a permanent magnet body, for example, the outer peripheral surface of a cylinder, and a line (usually an arc) connecting the magnetic poles in the magnet body. ) means that the easy magnetization axes of magnetoplumbite crystals with magnetic anisotropy are substantially aligned. In addition, in a cylindrical permanent magnet having such surface multipolar anisotropy, if the degree of orientation of the ferrite particles is increased in order to improve the magnetic properties as described above, if the number of magnetic poles is small (for example, 4 poles). There is also the problem of cracking. The cause of this cracking is that an arc-shaped magnetic field is applied from outside the cylinder during molding, and the ferrite particles are formed between the magnetic poles.
Because there is a large difference between the coefficient of thermal expansion in the direction of the axis (the axis of easy magnetization of magneto-plumbite crystal) and the coefficient of thermal expansion in the direction perpendicular to the C-axis, and the difference in the shrinkage rate due to sintering, the degree of orientation due to the magnetic field during molding is It is thought that the larger the size, the more likely it is that cracks will occur. The present invention also takes this point into consideration,
In a sintered cylindrical permanent magnet body having a composition of MO・nFe ₂ O ₃ (M is one or more of Ba, Sr, and Pb, n = 5 to 6), the cylinder of this magnet body The surface has multipolar anisotropy so that 12 to 24 magnetic poles are arranged in the circumferential direction of the part, and the anisotropy is sufficient to overcome the internal stress caused by this anisotropy. It is a cylindrical permanent magnet having a sufficient number of magnetic poles to have a non-oriented portion in the cylindrical cross section. That is, when the number of magnetic poles is small, most of the cross section of the cylindrical magnet has surface multipolar anisotropy (polar anisotropy), which causes cracks during sintering. However, as the number of magnetic poles increases, the spacing between the magnetic poles becomes smaller, so the anisotropic part between them does not spread over the entire thickness of the cylinder, and the magnetic permeability is approximately 1, as in ferrite magnets. In this case, the depth of approximately half of the distance between the magnetic poles is completely anisotropic, and there remains a part that is not completely anisotropic, and this part is inside the anisotropic part. Relieve internal stress. The present invention is effective when 12 to 24 magnetic poles are attached to a cylindrical body as described above, and when the outer diameter is 40 mm or less and 12 to 24 magnetic poles are attached to the cylindrical body at approximately equal intervals like magnets for stepping motors. This is particularly effective when ~24 magnetic poles are attached. The reason why the number of magnetic poles is set to 12 to 24 in the present invention is that when the number of magnetic poles is 10 or less, for example, the conventional 8-pole type is insufficient in terms of motor performance when used in a rotor such as a stepping motor. However, the step angle cannot be reduced. This is because if the number of poles exceeds 24, the variation in magnetic flux density becomes large, and there is a possibility that it cannot be used practically. Furthermore, in the present invention, particles are arranged before sintering in order to obtain an effective amount of magnetic flux, imparting surface multipolar anisotropy, and optimal dimensions are obtained to obtain high magnetic properties and low inertial force. The present invention also provides a cylindrical permanent magnet body and a method for manufacturing the same. Generally, a cylindrical multipolar permanent magnet for a rotor is required to have high magnetic properties and low inertia. However, the fourth
As shown in the figure, as the wall thickness t of the magnet increases, the surface magnetic flux density Bo increases, but a low inertial force I cannot be obtained, and as the wall thickness t decreases, the surface magnetic flux density Bo
This results in a decrease in Therefore, it is necessary that the magnet body has an optimum size so that a low inertial force can be obtained without reducing the surface magnetic flux density Bo. Therefore, the optimum inner/outer diameter ratio T of the magnet body when carrying out the present invention is expressed by the following equation. T=D ₁ /D ₂ =1−K ₁ π/P (K ₁ ≧1.76) (D ₁ : Magnet inner diameter, D ₂ : Magnet outer diameter, P: Number of magnetized poles,
_K1 : constant) In the above formula, the inner/outer diameter ratio T is 0<T<1.0
As it approaches 1.0, the wall thickness t becomes thinner and the surface magnetic flux density Bo becomes smaller, and as it approaches 0, the wall thickness t
becomes thicker and approaches a cylindrical shape, resulting in a disadvantage that the inertial force I becomes large. Further, from the right-hand term of the above equation, when the number P of magnetized poles increases, the inner/outer diameter ratio T increases and the wall thickness t becomes thinner, and when the number P of magnetized poles decreases, the wall thickness t increases. Therefore, in relation to the number of magnetized poles P, the optimum inner and outer diameter dimensions, that is, the inner and outer diameter ratio T, can be obtained without degrading the magnetic properties. Next, the reason for introducing the above formula will be explained. When the outer periphery of a cylindrical magnet is surface magnetized with a number of poles P, the distance between each magnetic pole along the outer periphery (the distance between the centers of adjacent magnetic poles of different polarity) is πD ₂ /P. And the magnetic flux is, if the magnetic pole spacing is the approximate diameter,
It is distributed from the magnet surface toward the inside of the magnet to a depth of K ₁ times half the magnetic pole spacing [(πD ₂ /2P)×(K ₁ )]. In this case, more than 90% of the magnetic flux is concentrated from the magnet surface toward the inside of the magnet to a depth (πD ₂ /2P) that is half the magnetic pole spacing. Here, K ₁ (constant) is determined experimentally as described later. A cylindrical permanent magnet may include a portion with low magnetic flux density, that is, a portion that is not completely anisotropic, but a portion into which magnetic flux does not penetrate [K ₁ (πD ₂ /2P) Since the outer part (toward the center of the magnet) is unnecessary, the effective inner diameter of the magnet is D ₁ = D ₂ −K ₁ πD ₂ /P, and dividing both sides by D ₂ is D ₁ /D ₂ = 1−K. ₁ π/P. That is, the optimum inner/outer diameter ratio T is shown by the above equation. Next, an example of experimentally determining K ₁ will be explained with reference to FIG. Figure 5 shows outer diameter D ₂ = 26 mm, number of magnetized poles P =
In the case of 24 poles, it shows how the surface magnetic flux density Bo and the moment of inertia I change as the inner diameters D ₁ and K ₁ change. As shown in the figure, when K ₁ is 1.0 (the radial thickness of the magnet is half the magnetic pole spacing), Bo reaches 90% or more of its maximum value, and when K ₁ is 1.76 or more, Bo is almost saturated.
Furthermore, considering that the lower the inertia force is, the better it is for a rotor magnet, 1.76≦K ₁ ≦2.5 was set. The optimum dimensions of the cylindrical permanent magnet body of the present invention were determined as described above. In addition, under conditions other than the above, the surface magnetic flux density Bo and the moment of inertia I due to changes in the inner diameter D ₁ and constant K ₁
When the change in was determined, results substantially similar to those shown in FIG. 5 were obtained. Results for representative dimensions and numbers of magnetized poles are shown in FIGS. 6, 7, and 8. Figure 6 shows outer diameter D ₂ = 20 mm, number of magnetized poles P =
24 poles, outer diameter D ₂ = 20 mm in Figure 7, number of magnetized poles P = 12
Figure 8 shows the changes in Bo and I as the inner diameters D ₁ and K ₁ change, respectively, when the outer diameter D ₂ =14 mm and the number of magnetized poles P = 12 poles. In addition to the main composition, the cylindrical permanent magnet body of the present invention contains an appropriate amount of additives for shape retention, and contains 14 to 20% water to enable the particles to rotate when a magnetic field is applied. It is obtained by applying a magnetic field to a molded body, arranging particles in the direction of lines of magnetic force, and then magnetizing the molded body after sintering. The reason for setting the moisture content to 14 to 20% is that if it is less than 14%, the molded product becomes hard and it becomes difficult to orient the particles for anisotropy, resulting in insufficient magnetic properties.If it exceeds 20%, molding becomes difficult. This is because the shape retention of the body is significantly reduced and molding becomes impossible. Hereinafter, experiments of the present invention will be explained. [Experiment 1] Sr ferrite (SrO・
5.6Fe ₂ O ₃
A molded body with a diameter of 33 mm, an inner diameter of 23 mm, and a length of 30 mm was obtained.
Insert it into the multi-pole magnetizing yoke A as shown in the figure.
After applying a magnetic field of 3000 Oe or more, the molded body 3 was
After drying for an hour, it was fired at 1200°C. Furthermore, the obtained fired body 3 with an outer diameter of 26 mm, an inner diameter of 18 mm, and a length of 20 mm was magnetized using a multi-pole magnetizing yoke similar to the magnetizing yoke A in Fig. 2, and the surface magnetic flux density Bo was measured. .
Table 1 shows a comparison of these values with those of conventional isotropic magnets, ring anisotropic magnets, and plastic magnets.

[Table] As shown in Table 1, the magnet body according to the present invention has a surface magnetic flux density of about 40% compared to the conventional magnet.
This is an improvement. The cylindrical permanent magnet used in this experiment had an outer diameter of 26 mm and an inner diameter of 18 mm.
mm, the length is 20 mm, the number of magnetic poles is 24, and the fifth
As shown in the figure, _K1 is between 1.76 and 2.5, which is the optimum size for obtaining a low inertial force without decreasing the surface magnetic flux density. Table 2 shows the results of measuring the holding torque when this permanent magnet was used as the rotor of a stepping motor. During this measurement, the voltage applied to the coil was 12V DC. The table also shows the magnetic properties of permanent magnets.

[Experiment 2]

Cylindrical permanent magnets having seven types of magnetic pole numbers ranging from 4 to 24 poles were manufactured under the same conditions as in Experimental Example 1, except that the dimensions of the compact and fired body and the number of magnetic poles were changed. These permanent magnets were visually inspected for crack occurrence. For each number of magnetic poles, K ₁
Table 3 shows the occurrence of cracks when changing.

【table】

[Table] From Table 3, the number of magnetic poles is 12 to 24 and K ₁ is 1.76.
It can be seen that cracks do not occur in the above cases. Furthermore, even if the number of magnetic poles is 12 or 16, cracks may occur if K ₁ is small (P = 8,
12 and 16, cracking occurred under condition 1). When the number of magnetic poles was 4 and 6, cracks were observed in all samples under each condition. As explained above, the cylindrical permanent magnet of the present invention has surface multipolar anisotropy before sintering and particles are arranged in the magnetization direction, so it has high magnetic properties with high surface magnetic flux density. Since it is a sintered permanent magnet body, and the optimum dimensions that do not reduce the above-mentioned high magnetic properties are given by the above formula, it is lighter in weight and has a lower inertial force than conventional permanent magnet bodies. Furthermore, it is possible to prevent the occurrence of cracks, and furthermore, it has the effect of saving material costs by reducing the weight. The above explanation has focused on the case where it is used as a stepping motor, but if the permanent magnet body of the present invention is made long in the axial direction, it can also be used in a copying machine etc. with a magnetic brush phenomenon. .

[Brief explanation of drawings]

Fig. 1 is a diagram showing the arrangement direction of particles of a cylindrical permanent magnet body, Fig. 2 is a partially enlarged sectional view of the permanent magnet and magnetizing yoke, and Fig. 3 is a diagram showing the direction of the magnetic field lines of the permanent magnet. Figure 4 shows the wall thickness and surface magnetic flux density of the permanent magnet.
Explanatory diagrams showing the relationship with inertial force, Figures 5 and 6,
FIGS. 7 and 8 are diagrams showing how the surface magnetic flux density and moment of inertia change as the inner diameter changes. 1: Yoke, 2: Coil, 3: Sintered body, A: Magnetized yoke.

Claims (1)

[Claims] A sintered cylindrical permanent magnet having a composition of 1 MO·nFe ₂ O ₃ (M is one or more of Ba, Sr, and Pb, n = 5 to 6). , a surface multipolar anisotropy of 12 to 24 poles is imparted to the cylindrical portion of the magnet body,
And, the ratio of the inner diameter (D ₁ ) to the outer diameter (D ₂ ) of the cylindrical portion is represented by the following formula, where P is the number of magnetized poles and K is a constant of 1.76 to _2.5 . A cylindrical permanent magnet body. D ₁ /D ₂ = 1−K ₁ π/P 2 Substantially composed of MO·nFe ₂ O ₃ (M is one or more of Ba, Sr, and Pb, n = 5 to 6), By applying a magnetic field to the outer circumferential surface of a cylindrical molded body containing 14 to 20% water to give it surface multipolar anisotropy of 12 to 24 poles, it is sintered and then magnetized.
The ratio of the inner diameter (D ₁ ) and outer diameter (D ₂ ) of the cylindrical part is, when the number of magnetized poles is P and the constant of 1.76 to 2.5 is K ₁ ,
A method for manufacturing a cylindrical permanent magnet, characterized by obtaining a permanent magnet represented by the following formula. D ₁ /D ₂ =1−K ₁ π/P