Claw pole type synchronous motor

Abstract: A shield plate (24) for preventing the leakage of a magnetic flux and for reducing acoustic/electromagnetic noise leaking out from the interior of the motor is arranged so as to cover the corresponding end surface of an armature (13). The inner diameter of the shield plate (24) is made smaller than the outer diameter of a rotor (14) in order to prevent the rotor (14) from being detached from a stator (27).

Description: The present invention relates to a claw pole type synchronous motor used as a spindle motor of a memory device such as an FDD (floppy disk drive) or an HDD (hard disc drive) or a CD-ROM. The spindle motor of a memory device such as an FDD, an HDD or a CD-ROM is required to have high operating efficiencies such as a high rotational accuracy, very small magnetic leakage and low acoustic/electromagnetic noise. Conventionally, therefore, a three-phase brushless DC motor which has high operating efficiencies has been used. Since, however, it has been strongly demanded recently that the manufacturing cost of memory devices should be reduced, an expensive three-phase brushless motor has not come to meet this demand. In order to fulfill the requirements of high operating efficiencies and low cost, it is considered to use a claw pole type synchronous motor which can be manufactured at a low cost as the spindle motor. However, since the exciting coil of the motor is a solenoid coil, there occurs leakage of a large alternating magnetic flux which reverses its direction along the axis of rotation at each excitation time. The alternating magnetic flux intersects the reading/writing head of the memory device, resulting in the head having a degraded output characteristic (S/N). This makes it impossible to use the conventional synchronous motor. EP-A-0 685 843 discloses a low profile motor powered disk assembly for a recording/reproducing device, wherein a shield is stationarily disposed between the motor and the disk of a memory disk assembly. JP-A-05284710 discloses a motor, wherein a magnetic shielding plate is fixed above the coil with screws to shield a leakage flux to a magnetic recording medium. US-A-4 695 419 discloses the manufacture of a miniature electric motor having double stator claw pole features. US-A-5 410 201, which is considered as being the closest prior art document, discloses an electric motor used in a magnetic disc drive. For preventing magnetic leakage the motor is provided with a magnetic shielding plate. The magnetic shielding plate is so configured that protrusions are formed at several locations of the inner periphery of an aperture of an annular part of the magnetic shielding plate. The inner extremities of the protrusions are positioned closer to the rotary shaft than the outermost periphery of the permanent magnet of the rotor. With this configuration, the magnetic shielding plate confronts the permanent magnet of the rotor only at the protrusions rather over the entire inner periphery. As a result, the area of the magnetic shielding plate confronting the driving permanent magnet is reduced, so is the magnetic attracting force acting on the magnetic shielding plate. Sticking of the magnetic shielding plate to the rotor due to the magnetic attraction is thereby avoided. It is the object of the present invention to provide an inexpensive claw pole type synchronous motor which can be used as the spindle motor of a memory device and which can be operated in a stable manner. This object is achieved by a motor as defined in claim 1. The dependent claims 2 and 3 show advantageous further developments of the motor of claim 1. Especially, a claw pole type synchronous motor includes an armature, a rotor having a rotating axis and a shield plate, (I) the armature having two stator assemblies superposed in a direction of the rotating axis and each of the stator assemblies comprising: (A) stator yokes made of soft steel material and each provided with (a) a circular doughnut shaped base having an inner peripheral edge, an outer peripheral edge and an axis, (b) pole teeth bent at the inner peripheral edge so as to extend in a direction of the axis, (c) an outer wall bent at the outer peripheral edge so as to extend in the direction of the axis, and (d) an annular armature coil receiving portion defined by the base and the pole teeth; and (B) an armature coil formed by winding insulated wires and mounted in the armature coil receiving portion, (II) the rotor having an outer diameter, facing the pole teeth with a small gap provided between the rotor and the pole teeth and comprising: (C) the stator assemblies provided with a flange and a bearing provided on said flange, and (D) a field magnet of a permanent magnet type having an end face corresponding to the rotor inserting side face of said armature, and (III) the shield plate having an inner diameter smaller than the outer diameter of the rotor and fixed to the rotor inserting side face of the armature in such a manner that the shield plate covers the rotor inserting side face of the armature and the end face of the field magnet, for preventing magnetic leakage, reducing acoustic/electromagnetic noise leaking from the interior of the motor and for preventing the rotor from being detached from the stator assemblies. Especially, according to the present invention, that part of the shield plate which corresponds to the field magnet of the rotor is provided with a plurality of projections which project toward the field magnet and which are used for preventing attraction to the field magnet. The claw pole type synchronous motor of the present invention can be manufactured at low cost. By providing the motor with the shield plate, the leakage of a magnetic flux, acoustic/electromagnetic noise and/or the like to the outside of the motor is considerably reduced, and the rotor is prevented from being detached from the stator assembly. Further, the projections are formed on the shield plate so that the shield plate does not contact the field magnet when,the shield plate is attached to the armature. The projections prevents the shield plate from being attracted to the field magnet even if the rotor oscillates in axial directions. Thus, the rotor smoothly rotates without being attracted to the shield plate. Fig. 1 is a plan view of an FDD spindle motor according to an embodiment of the present invention; Fig. 2 is a lateral side view of Fig. 1; Fig. 3 is a sectional view taken along line 3-3 in Fig. 1; Fig. 4 is a detailed enlarged cross-sectional view of a projection shown in a circle A in Fig. 3; and Fig. 5 is a characteristic diagram showing the relationship between the head noise and the rotor diameters of the present invention and the prior art. An embodiment of the present invention will now be described with reference to the drawings. As shown in Figs. 1 to 3, a claw pole type synchronous motor of the present embodiment has a plate-like flange 1 in which a recess la is formed into the shape of a shallow dish. Attachment holes 2 are formed in four corners of the flange 1 in order to attach the flange 1 to a desired attached portion of a mount (not shown) by means of screws (not shown). A stator yoke 3 comprises a circular doughnut shaped base 4, high trapezoidal pole teeth 5 extending axially of the base 4 from the circular inner peripheral edge of the base 4 and an outer wall 6 extending radially of the base 4 from the outer peripheral edge of the base 4. The stator yoke 3 is made from a soft steel plate by punching out at the same time (a) first portions having the shape and size of the corresponding pole teeth 5 and extending radially inward from a circular line, which will form the inner peripheral edge of the base 4, and (b) a second portion having the width of the outer wall 5 and extending radially outward from another circular line which will form the outer peripheral edge of the base 4, and finally by bending the first and second portions at substantially right angles in the same direction so as to form the pole teeth 5 and the outer wall 6. The base 4, the pole teeth 5 and the cylindrical outer wall 6 form an annular recessed ring-shaped armature coil receiving portion 7 having two ends, one end being closed by the base 4 and the other end being opened. An armature coil 8 is formed by winding an insulated wire such as a polyurethane copper wire or the like. The armature coil 8 is inserted in the armature coil receiving portion 7 from its other end (that is, the opposite side to the flange 1). After the armature coil 8 has been inserted, a stator yoke 9, which is identical with the above-described stator yoke except that it does not have the cylindrical outer wall 6, is fitted to the aforementioned other end of the armature coil receiving portion 7, thus forming a stator assembly 10. A ring-shaped armature holding member 11 has one end (the lower end in Fig. 3) fixed to the outer periphery of the recess 1a of the flange 1. Two stator assemblies 10 are sequentially inserted from the opposite side to the flange 1 in an armature mounting hole 12 defined by the inner circumferential surface of the armature holding member 11, with the outer circumferential surfaces of the outer wall portions 6 of the stator assemblies 10 being in contact with the inner circumferential surface of the armature holding member 11. The two stator assemblies 10 thus incorporated constitute an armature 13. The two stator assemblies 10, a bearing 19 (described later) and the flange 1 may be insert-molded with the armature holding member 11. Here, the stator assemblies 10, the bearing 19 and the flange 1 constitute the stator 27. A rotor 14 has a disk portion 15, a cylindrical skirt portion 16 integrally formed with the outer periphery of the disk portion 15, a cylindrical field magnet 17 of a permanent magnet type fixed to the outer circumferential surface of the skirt portion 16 and which has a plurality of magnetic poles, and a main shaft 18 having one end (the upper end in Fig. 3) fixed to the center of the disk portion 15 and having a rotating axis 18a which is also the rotating axis of the rotor 14. By rotatably inserting the main shaft 18 into a supporting shaft 20 whose one end (the lower end in Fig. 3) is fixed via the bearing 19 to the flange 1, the rotor 14 is set coaxially with the armature 13 in a recessed columnar rotor receiving portion 21 defined by the inner circumferential surface of the armature 13. The diameter of the outer circumferential surface of the rotor 14, in other words, the diameter of the outer circumferential surface of the field magnet 17 is made slightly smaller than the diameter of the inner circumferential surface of the armature 13. In other words, the field magnet 17 is mounted in the recessed columnar rotor receiving portion 21 which is defined by the pole teeth 5 of the armature 13, so that the field magnet 17 faces the pole teeth 5 with a predetermined small gap provided between the field magnet 17 and the pole teeth 5. As shown in Figs. 1 and 2, an outer peripheral portion of the free surface (the upper surface in Figs. 1 and 2) of the disk portion 15 is provided with a drive pin 22 for transmitting rotation of the motor to the corresponding floppy disk. As shown in Fig. 1, leading-out lines 23 are drawn out from the armature coil 8. As shown in Fig. 3, that end face of the armature 13 which is opposite to the flange 1 is covered with a circular doughnut shaped shield plate 24 which is made of ferromagnetic material. The outer diameter of the shield plate 24 has such a value as to allow the outer periphery of the shield plate 24 to be fixed by caulking or the like to the corresponding portion of the armature holding member 11. Moreover, the inner diameter of the shield plate 24 is smaller than the outer diameter of the field magnet 17. By covering part or the whole of the armature 13 and the field magnet 17 with the shield plate 24, the magnetic flux leaking from the armature 13 and the field magnet 17 to the outside of the motor are reduced considerably. As shown in Fig. 2, the shield plate 24 covers that end of the armature 13 which is opposite to the flange 1. The outer periphery of the shield plate 24 is fixed by caulking or the like to the corresponding end (the upper end in Fig. 3) of the armature holding member 11. An inner peripheral part of the shield plate 24 has a circular doughnut shaped portion 25 (hereinafter referred to as "the field magnet corresponding portion") which corresponds to the field magnet 17. The field magnet corresponding portion 25 is formed so that it is spaced by a predetermined interval K (for example, 0.5mm) from that end face of the field magnet 17 which corresponds to the field magnet corresponding portion 25. A plurality of integral projections 26 having a hemispherical shape or another smooth shape and projecting toward the field magnet 17 from that surface of the field magnet corresponding portion 25, which faces the field magnet 17, are arranged so as to be spaced in a circumferential direction of the field magnet 17. In the case of Fig. 1, three projections are arranged at equal intervals in the circumferential direction. The number of the projections may be four to ten, instead. The height or radius L of each projection 26 is, for example, 0.2mm smaller than the interval K between the field magnet corresponding portion 25 and the field magnet 17 in order to normally prevent the projections 26 from colliding and interfering with the field magnet 17 during the rotation of the rotor 14. Even if the projections 26 happen to contact the filed magnet 17, the whole portion of the field magnet corresponding portion 25 does not contact the field magnet 17 because of the presence of the projections 26. Thus, the shield plate 24 does not contact the field magnet 17 except for the projections 26. Fig. 5 is a characteristic diagram showing the relationship between the rotor diameters of the present invention and the prior art and the head noise which is defined as acoustic/electromagnetic noise leaking from the interior of the motor to the outside thereof. As can be understood from this diagram, the rotor of the claw pole type synchronous motor according to the present invention provides a considerably reduced head noise (V) as compared with the claw pole type synchronous motor of the prior art in any one of the cases of 0, 40 and 75 tracks when the rotor diameters of the motors are 18mm or greater. Because the head noise is reduced, the claw pole type synchronous motor of the present invention can be used as a spindle for use in a magnetic memory device such as an FDD or an HDD. The head noise problem will be described in more detail. When the claw pole type synchronous motor of the present invention is used as the spindle motor of the above-mentioned memory device, the motors each having a rotor with a diameter of 25mm are employed in the most cases. However, in the claw pole type synchronous motor of the prior art which has no shield plate, the amount of head noise which indicates the level of the magnetic flux leaking to the outside of the motor exceeds the limit value allowed to the memory device on which the motor is mounted. When the outer diameter of the rotor is 25mm, the limit value is 0.6V in any one of the cases of 0 track, 40 tracks and 79 tracks. In the prior art, however, the amount of head noise is 0.25V, 0.4V and 1.3 V respectively in the cases of the tracks corresponding to those described above, as shown in Fig. 5. In the case of the tracks 79, the head noise exceeds the limit value of 0.6V. Thus, the prior art the claw pole type synchronous motor cannot be used as the spindle motor of the memory device. In contrast, according to the above-described embodiment of the present invention, the amount of head noise is 0.19V, 0.36V and 0.55V in the cases of 0 track, 40 tracks and 79 tracks, respectively, and thus the amount of head noise does not exceed the limit value in any case. Hence, the claw pole type synchronous motor of the present invention can be used as the spindle motor of the memory device. It is clear that an advantage can be attained even if the shield plate 24 is arranged only at the portion corresponding to the head, and this structure is also included in the present invention. The claw pole type synchronous motor of the present invention has an advantage in that it is simple in structure and, accordingly, can be provided at low cost as the spindle motor of a memory device in accordance with the demand of the market. By arranging the shield plate for covering the armature and the field magnet at that end surface of the motor which is fixed to an apparatus, the magnetic flux leaking from the armature and the field magnet is reduced, and the acoustic noise coming out of the interior of the motor is also lowered, making the motor advantageously suitable as the spindle motor of the memory device. Further, by making the inner diameter of the shield plate smaller than the outer diameter of the rotor, the advantage of the prevention of the detachment of the rotor from the stator assembly is attained. Moreover, by providing integral projections on that part of the shield plate which faces the field magnet, the adherence of the shield plate to the field magnet is prevented so that the rotation of the rotor is not adversely affected.

Claims: A claw pole type synchronous motor, including a plate-like flange (1), an armature (13), a rotor (14) mounted on said flange (1) coaxially with said armature (13), and a shield plate (24) made of ferromagnetic material for preventing magnetic leakage and reducing acoustic/electromagnetic noise from said motor, wherein: (I) said armature (13) has two stator assemblies (10) superposed in a direction of a rotating axis (18a) of said rotor (14), one of said stator assemblies (10) being mounted on said flange (1), each of said stator assemblies (10) comprising: (A) stator yokes (3) made of soft steel material, each of said stator yokes (3) including (a) a circular doughnut shaped base (4) having an inner peripheral edge, an outer peripheral edge and an axis, (b) pole teeth (5) bent at said inner peripheral edge so as to extend in said direction of said axis (18a), (c) an outer wall (6) bent at said outer peripheral edge so as to extend in said direction of said axis (18a), and (d) an annular armature coil receiving portion (7) defined by said base (4) and said pole teeth (5); and (B) an armature coil (8) formed by winding insulated wires and mounted in said armature receiving portion (7), (II) said rotor (14) has an outer diameter and is arranged to face said pole teeth (5) of said stator yokes (3) of said stator assemblies (10) with a small gap provided between said rotor (14) and said pole teeth (5) and said rotor (14) includes a field magnet (17) of a permanent type having an end face at an opposite side of said magnet (17) to said flange (1), and (III) said shield plate (24) has a central hole with an edge defining an inner diameter smaller than said outer diameter of said rotor (14) so that said rotor (14) is prevented from being detached from said stator assemblies (10) and said shield plate (24) is fixed to said armature (13) so that said shield plate (24) covers said armature (13) and said end face of said field magnet (17) with a space provided between said shield plate (24) and said end face of said field magnet (17), characterized in that said shield plate (24) has a plurality of projections (26) integrally formed therewith and circumferentially arranged in a portion of said shield plate (24) which is at said field magnet (17), each of said projections (26) having a contiguously projecting shape toward said field magnet (17) so that an intermediate portion of each of said projections (26) faces said field magnet (17). The claw pole type synchronous motor according to claim 1, characterized in that each of said projections (26) has a substantially hemispherical shape extending toward said field magnet (17). The claw pole type synchronous motor according to claim 1 or 2, characterized in that said projections (26) are one to ten in number.

Inventors: FUJITANI SAKAE, ISHII TAKASHI, SUZUKI YUZURU, FUJITANI, SAKAE, ISHII, TAKASHI, SUZUKI, YUZURU, Fujitani, Sakae, Menebea Co., Ltd., Ishii, Takashi, Menebea Co., Ltd., Suzuki, Yuzuru, Minebea Co., Ltd.
Applicants: MINEBEA KK, MINEBEA CO., LTD.
Application year: 1997
Main classification: H02K21
IPC codes: H02K5, H02K21