Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-07-10
2004-03-23
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S607000, C148S101000, C148S112000, C148S301000, C310S154210, C310S154260, C310S154290
Reexamination Certificate
active
06708388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to a compact DC (direct-current) motor suited for use in audio-visual appliances such, for example, as optical pickups for MDs or CD-ROMs and, in particular but not exclusively, to a permanent magnet field-type compact DC motor utilizing arcuate permanent magnets prepared from rare earth metal, having a low cogging torque, capable of providing a high output and capable of precisely rotating with minimized starting voltage and current. The present invention also relates to a method of making the permanent magnet field-type compact DC motor.
2. Description of Related Art
FIGS. 1A and 1B
depict a permanent magnet field-type compact DC motor and an arcuate permanent magnet used therein, respectively, to which the present invention is applicable. In these figures, reference numeral
1
represents a pair of arcuate permanent magnets; reference numeral
2
represents a soft magnetic frame; reference numeral
3
represents an armature including a brush-commutator, an armature shaft and a bearing; and reference numeral
4
represents generally U-shaped springs used to urge and fixedly retain the respective arcuate permanent magnets
1
against the soft magnetic frame
2
. Even this permanent magnet field-type compact DC motor is, as is the case with any other permanent magnet motor, required to have a compact size, a high output and a highly precise rotational performance.
However, it has been recognized that the permanent magnet field-type compact DC motor has a problem in that the use of a reduced diameter of the armature
3
renders it difficult to reduce the size of the motor without the output performance being sacrificed. Specifically, it is well known that ferrite magnets generally have a relatively low maximum energy product [BH]max regardless of the compression, injection or extrusion molding process using a sintering technique or a resinous material as a binder. Therefore, reduction in size of the ferrite magnets used in the permanent magnet field-type compact DC motor would result in that no sufficient static magnetic field is developed in a space between the permanent magnets
1
and the armature
3
, thus considerably reducing the motor output. In view of this, arcuate rare earth magnets having a so-called high maximum energy product [BH]max have long been desired for, because they can provide a strong static magnetic field in the space between the permanent magnets
1
and the armature
3
even though the motor is reduced in size.
When it comes to the arcuate rare earth magnets of a maximum usable thickness smaller than 1 mm, which are effective to reduce the size of the permanent magnet field-type compact DC motor, the method of making such arcuate rare earth magnets is currently limited. By way of example, Japanese Laid-open Patent Publication (unexamined) No. 6-236807 discloses a method of making an arcuate rare earth magnet by the use of an extrusion molding technique. According to this known method, a molten fluidized material including various rare earth magnetic powder having a characteristic ranging from a magnetically anisotropy to a magnetically isotropy in admixture with a thermoplastic resin is poured into a mold assembly and is, after having been cooled within the mold assembly down to a temperature lower than the melting point of the thermoplastic resin, extruded to form an arcuate rare earth magnet. The resultant arcuate rare earth magnet is described as having a maximum thickness of 0.9 mm±30 &mgr;m.
On the other hand, the permanent magnet field-type compact DC motor utilizing the arcuate rare earth magnets of the kind described above and capable of providing a strong static magnetic field in the space between the rare earth magnets and the armature
3
as compared with the ferrite magnets has an additional problem associated with increase of the cogging torque. Because of the presence of armature iron core teeth
31
and slots
32
on an outer peripheral surface of the armature
3
facing the permanent magnets
1
, the cogging torque results from generation of torque pulsation brought about by change in permeance incident to rotation of the armature
3
. The cogging torque is detrimental to the permanent magnet field-type compact DC motor that is required to have a reduced size, a high output and a highly accurate rotational performance for which the present invention is intended.
Regardless of whether or not the permanent magnets used are in the form of rare earth magnets, means for reducing the cogging torque defined by the shape of the arcuate magnets employed in the permanent magnet field-type compact DC motor is implemented by causing inner and outer radii of curvature of the arcuate magnets to be eccentric or by chamfering edges of circumferentially spaced, opposite end faces of the arcuate magnets to render them to have an inequality wall thickness so that the permeance of the magnets, in which a uniform material is uniformly magnetized from the center of a magnetic pole to any of opposite ends of each magnet, can change so as to render the distribution of void magnetic flux densities to represent a generally sinusoidal shape. (See, for example, Shogo Tanaka, “Kogata Mohta-niokeru Eikyuujishaku no Ouyou (Application of Permanent Magnets in Compact Motors)”, transaction of Kogata Mohta Gijutsu Symposium, pp. 7, 1983.)
On the other hand, U.S. Pat. No. 4,710,239 discloses a method of making an arcuate magnet. According to this disclosure, as the starting material, there is used rare earth-iron-based rapidly quenched and solidified amorphous flakes of a composition comprising, on an atomic basis, 10 to 50% of rare earth element Re (Nd and/or Pr), 1 to 10% of B, the balance being transition metal element TM, at least 60% of the transition metal element being Fe. This starting material is compacted at a temperature higher than the crystallizing temperature (about 600° C.) of a magnet phase RE
2
TM
14
B, but lower than 750° C. to form an isotropic fully dense magnet having a different thickness, followed by hot plasticizing process at similar temperature to provide the arcuate magnet. Since depending on the extent to which the hot plasticization is effected, easy axes of magnetization are oriented in a direction perpendicular to the direction of plastic flow, RE
2
TM
14
B can be transformed into an anisotropic rare earth magnet having a strong magnetism in a relatively thick portion thereof. Thus, the above mentioned USP discloses the arcuate magnet having portions exhibiting a strong anisotropy and an isotropy, respectively.
If the arcuate rare earth magnets each having the portions of different magnetic performances are used in the permanent magnet field-type DC motor, it is possible to vary a demagnetization curve in a circumferential direction from the center of the magnetic pole and, therefore, it is also possible to render the distribution of the void magnetic flux densities between the field magnets and the armature iron core to vary smoothly to reduce the cogging torque.
As discussed above, the use in the permanent magnet field-type DC motor may be made of the arcuate rare earth magnets each having the inner and outer radii of curvature that offset relative to each other so as to render them to have a varying wall thickness, or of the arcuate magnets with their circumferential edges chamfered to have a varying wall thickness, so that the permeance of the magnets can change from the center of the magnetic pole towards the circumferentially opposite ends and, also, the magnetism of the magnets can be varied in a circumferential direction from the center of the magnetic pole, to control the pattern of distribution of the void magnetic flux densities between the field magnets and the armature iron core so as to allow the cogging torque to reduce due to the different demagnetization curves.
However, the arcuate rare earth magnets of a thickness-smaller than 1 mm suited for use in the permanent magnet field-type compact DC motor to which t
Sasaki Yuichiro
Toda Shinji
Uenishi Eiji
Yamashita Fumitoshi
Nguyen Donghai
Vo Peter
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