MINIATURE ELECTRIC ROTATING MACHINE

Abstract: This invention relates to a miniature electric rotating machine having a stator equipped with a field pole on a peri­ pheral circle of a rotor and the field pole is constituted by a permanent magnet (8). Each permanent magnet of a conventional miniature electric rotating machine of this type can be divided into a portion on a magnetizing side and a portion on a demagnetizing side with respect to armature reaction. In the permanent magnet (8) of the present invention, an end portion (ll) on the demagnetizing side, which has directivity differ­ ent from the magnetizing directions on the magnetizing and demagnetizing sides, is disposed adjacent to the portion on the demagnetizing side.

Description: This invention relates generally to a miniature electric non-uniform magnetization rotating machine of the kind to in the precharacterizing portion of patent claim 1. Such a miniature electric rotating machine is known from FR-A-2 171 070. Permanent magnet filed system DC machines using a permanent magnet for poles are known in the art. The permanent magnet used for DC machines of this type has generally an arc shape and the magnet is an anisotropic magnet. The permanent magnet is magnetized by a magnetizer such as one that is disclosed in JP-A-43 167/1983 entitled "Magnetizer of Field System Aggregate" which was laid open on March 12, 1983. The magnet is magnetized in anisotropic directions substantially towards the center in a radial direction. Therefore, a problem occurs that when the magnetic flux of armature reaction due to an armature current acts, the permanent magnet disposed on a demagnetizing side is demagnetized. This means that the thickness of the permanent magnet must be incrased. FR-A-2 171 070 discloses different magnetizing vectors on the magnetizing side and on the demagnetizing side of the magnets, including right, acute and obtuse angles. Referring to figs. 1 and 2 of FR-A-2 171 070, the DC motor according to FR-A-2 171 070 forms a neutral zone in the air gap between the end surfaces of the two magnets, in which the flux is offset due to the armature reaction, in such a manner that the anisotropic directions of the magnet in the portions near the longitudinal flancs gradually change from one end surface to the other end surface. For preventing the demagnetization of the magnet as explained above, the thickness of the magnet of the DC motor according to FR-A-2 171 070 towards its radical direction is demanded to increase. According to DE-A-1 613 319, the magnetizing direction at a slot is approximately straight from the N pole to the S pole as shown in the figures thereof. The direction is one passing the magnetic flux. Further, the magnet of DE-A-1 613 319 is formed by a ring shape magnet. It is an object of the present invention to provide a miniature electric non-uniform magnetization rotating machine with a reduced thickness of a permanent magnet for its field system. This object is achieved with a miniature electric non-uniform magnetization rotation machine according to claim 1. Dependent claims are directed on features of preferred embodiments. The arrangement according to the invention can offset the reaction magnetic flux due to an armature in the proximity of the end portion on the demagnetizing side, so that the thickness of the permanent magnet for a filed system for offseting the armature reaction in the demagnetizing direction can be reduced and eventually, the thickness of the permanent magnet for the field system can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partial sectional of a permanent nun-uniform magnet field system rotating machine in an axial direction in accordance with the present invention; Fig. 2 is a sectional magnet view of the permanent magnet in a radial direction in accordance with one embodiment of the present invention; Fig. 3 is a diagram showing the armature reaction magnetomotive force along the circumference of the permanent magnet; Fig. 4 is a diagram showing the distribution of the magnetic flux of the armature reaction in a state where the permanent magnet is not fitted; Fig. 5 is a diagram showing the distribution of the reaction magnetic flux due to the armature reaction when the present invention is applied; Fig. 6 is a sectional view useful for explaining a magnetizer used for magnetizing the permanent magnet in the present invention; and Figs 7, 8, 9 and 10 show other embodiments of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 is a sectional view, in an axial direction, of a two-pole permanent magnet field system electric machine. In the drawing, a rotor consisting of a shaft 1, a commutator 2 and an armature formed by winding a widing 4 on an armature core 3 are supported by end brackets 6a, 6b on a fixing side through bearings 5a, 5b, respectively. These end brackets are fixed to a cylindrical yoke 7. Two permanent magnets 8 respectively having an arc shape are disposed equidistantly on the inner periphery of the yoke 7 and are fixed with an adhesive, or the like. Reference numeral 9 represents a brush, which is fixed to the bracket 6a through a brush holder 10. In the electric machine described above, the permanent magnet 8 which constitutes the gist of the invention is a ferrite type anisotropic permanent magnet and has an arc shape. As shown in Fig. 2 which is a sectional view in a radial direction, this permanent magnet 8 is produced in such a fashion that its anisotropic direction varies depending upon a peripheral position. In other words, the anisotropic direction of the permanent magnet extends towards the center axis of the armature core 3 from the portion ϑ A on the magnetizing side where the armature reaction acts in the magnetizing direction to the portion ϑ B on the demagnetizing side from the center of magnetism 0-0′ where the armature reaction acts in the demagnetizing direction, as shown in Fig. 3. However, the present invention is not limited to the above, in particular. Namely, the present invention embraces also the arrangement in which the magnetizing directions of the portions of the magnetizing and demagnetizing sides extend towards the center of the armature core such as the arrangement where the magnetizing directions are parallel to each other. On the other hand, at the portion ϑ C near the end portion on the demagnetizing side, the anisotropic direction is substantially at right angles to the line of the end portion 11 on the demagnetizing side. The anisotropic permanent magnet having these two directions can be produced by compression-molding a magnetic material, and causing a magnetic field to act upon the mold by use of magnetizer such as shown in Fig. 6 so as to provide the magnet which anisotropy. Turning back to Fig. 1, when a current is passed through the armature coil in the electric machine, the armature reaction takes place. The armature reaction magnetomotive force acts as the magnetizing force on the left side from the center of magnetism 0-0′ of the permanent magnet and as the demagnetizing force, that is, in a direction which reduces the flux permanent magnet, on the right side from the center of magnesium 0-0′. Particularly, the demagnetizing force acting upon the permanent magnet 8 is great at end portion 11. In Fig. 4, when a current flows through the armature coil 15 from the reserve to the front of the drawing, the flux 12 due to the armature reaction is incident substantially orthogonally to the end portion 11 on the demagnetizing side of the permanent magnet 8 if the permanent magnet 8 is disposed. If the permanent magnet 8 in accordance with the present invention is used, it can offset the reaction flux which is incident is such a direction as described above by the flux in the anisotropic direction 16 which crosses at right angles the end portion 11 on the demagnetizing side. This offsetting mode is depicted in Fig. 5, and the reaction flux 12 due to the armature reaction cannot be incident to the end portion 11 on the demagnetizing side. In Fig. 6, the yoke consists of an external yoke 12a and an internal yoke 12b having a projection 13, and both of these yokes cover the permanent magnet 8. The projection 13 extends in a radical direction in such a manner as to cover the end portion 11 on the demagnetizing side of the permanent magnet 11. When the permanent magnet 8 is magnetized by the use of such a magnetizer in accordance with the magnetization method described in JP-A-43 167/1983, the flux passes from the external yoke towards the projection 13 of the internal yoke so that the portion near the end portion 11 of the permanent magnet 8 is magnetized in a substantially orthogonal direction 16. On the other hand, the rest of portions other than the portion near the end portion 11 on the demagnetizing side are magnetized towards the center in the radial direction as represented by reference numeral 17. Incidentally, when the magnetizing direction of the permanent magnet 8 is not in the center direction of the armature core such as when all the magnetizing directions of the portions on both the magnetizing and demagnetizing sides are parallel to one another, the semi-spherical portion 18 shown in Fig. 6 of the internal yoke 12b may be made flat. In the electric machine using the permanent magnet magnetized in the manner described above, even when the reaction flux to the armature reaction is about to be incident to the end portion 11 on the demagnetizing side of the permanent magnet 8, the anisotropic direction near the end portion of the permanent magnet 8 impinges against the reaction flux 12 and hence the reaction flux is offset. Accordingly, demagnetization near the end portion of the permanent magnet 8 can be prevented and demagnetization resistance of the permanent magnet can be improved. Moreover, since demagnetization resistance is improved, the thickness of the permanent magnet can be reduced. Next, another embodiment of the invention will be described. Though the present invention deals with a two-pole machine by way of example, it can be applied to multiple-pole machines such as a four-pole or six-pole machine. Furthermore, though the embodiment described above uses the ferrite type permanent magnet by way of example, the effect of the invention remains the same so long as the permanent magnet is an anisotropic magnet such as are earth magnet, plastic magnet, rubber magnet, alnico magnet, or the like. Referring to Fig. 2, though the direction of anisotropy of the portion ϑ C on the demagnetizing side of the permanent magnet 8 is substantially at right angles to the line on the end portion 11 on the demagnetizing side, the angle need not always be right angles but may be an acute or obtuse angle. In other words, the direction of anisotropy may be determined depending upon the incident direction of the reaction flux due to the armature reaction of a rotating machine of each type in order to obtain the maximum effect. The angle ϑ C of the portion on the demagnetizing side of the permanent magnet 8 may be determined depending upon the model of the rotating machine while taking into consideration which of the generation flux quantity and demagnetization resistance is to be emphasized. In Fig. 6, magnetization is effected before the permanent magnet 8 is assembled. However, if magnetization is made by the use of an internal yoke having one projection for one pole while the permanent magnet is assembled in the yoke, an anisotropic permanent magnet can be obtained simultaneously in a multiple-pole machine as shown in Fig. 2. The end portion 11 on the demagnetizing side of the permanent magnet 8 having the shapes such as shown in Figs. 7 and 8 may be provided with anisotropy that offsets the reaction flux of the armature reaction of an electric rotating machine such as the rotating machine of the present invention. In Fig. 7, both end surfaces 11a are cut. In Fig. 8 the gap length at both end portions of the permanent magnet 8 is expanded and the center of an outer diameter is different from that of an inner diameter. In a field permanent magnet equipped with a magnetic sheet, such as a soft iron sheet 14 having high permeability, on the demagnetizing side inside the permanent magnet 8 facing the armature core 3 as shown in Fig. 9, demagnetization resistance of the permanent magnet can be further improved by providing the magnet with anisotropy according to the present invention. Fig. 10 shows a permanent magnet field system D.C. rotating machine equipped with auxiliary poles, which has a construction wherein a field pole is constituted on the inner periphery of the yoke 7 by the permanent magnet 8 and auxiliary poles made of a magnetic material and acting upon the magnetization of the armature reaction. The effect of the present invention will remain the same if the anisotropy of the end portion 11 on the demagnetizing side of the permanent magnet 8 of this field pole is the same as that of the present invention. In a composite permanent magnet consisting of a high coersive force portion and a high residual flux density portion, the end portion on the demagnetizing side of the high coercive force portion may be provided with anisotropy so as to offset the armature reaction as in the present invention. Through the description given above illustrates an example wherein an anisotropic magnet having at least two magnetizing directions are disposed in the same pole, the present invention is not particularly limited thereto. For example, it is also possible to employ the arrangement wherein two anisotropic magnets each having at least two magnetizing directions are produced separately and are arranged adjacent to each other. In accordance with the present invention described above, the direction of anisotropy in the magnetizing direction of the end portion of the permanent magnet on the demagnetization side is set in a direction which offsets the reaction flux, and the demagnetization resistance of the permanent magnet can be improved. Therefore, the tickness of the permanent magnet can be reduced as described already, and a compact, light-weight rotating machine can be produced economically. The magnetic flux leaks from the end portion on the demagnetizing side to the yoke at the time of non-load where the armature reaction is small, the effective flux incident to the armature core can be reduced relatively when compared when the prior art apparatuses. Therefore, the non-load number of revolution can be improved by about 15% if ϑ A + ϑ B + ϑ C = 63° and the angle of ϑ C at this time is 10°.

Claims: A miniature electric rotating machine including a rotor (3) and at least one pair of non-uniformly magnetized anisotropic permanent magnet segments (8,8) disposed around said rotor (3) with a predetermined circumferential space therebetween, each of said anisotropic permanent magnet segments (8,8) extending in the axial direction of said rotor (3), and being adapted to rotate into one predetermined direction so that one half (ϑ B , ϑ C ) of each of said anisotropic permanent magnet segment (8,8) is subjected to demagnetization due to armature reaction caused by the rotation of the rotor (3) and the other half (ϑ A ) thereof is subjected to magnetization due to the armature reaction caused thereby, Characterized in that ; one end portion (ϑ C ) of each of said anisotropic permanent magnet segments (8,8) being magnetized into a first magnetizing direction which cancels out the demagnetization due to the armature reaction along substantially the entire axial length thereof, and the remaining portion ( ϑ A , ϑ C ) of each of said anisotropic permanent magnet segments (8,8) being magnetized into a second magnetizing direction different from the first magnetizing direction directing toward said rotor (3). A miniature electric rotating machine according to claim 1 characterized in that, each of said non-uniformly magnetized anisotropic permanent magnet segments ( 8, 8 ) is shaped in such a manner that the circle center of the outer circumference of each of said non-uniformly magnetized anisotropic permanent magnet segments (8,8) is offset from that of the inner circumference thereof so that air gaps between each of said non-uniformly magnetized anisotropic permanent magnet segments (8,8) and said rotor (3) are formed larger at the both end portions of each of said non-uniformly magnetized anisotropic permanent magnet segments (8,8) than at the center thereof. A miniature electric rotating machine according to claim 1 further characterized in that, an anxiliary pole (15) of a magnetic material is disposed adjacent to each of said non-uniformly magnetized anisotropic permanent magnet segments (8,8) at the other end portion thereof.

Inventors: ABUKAWA, TOSHIMI, TAHARA, KAZUO, TAKAHASHI, NORIYOSHI, TOMITE, TOSHIO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H02K23
IPC codes: H02K23, H02K15, H02K1