Claw pole type synchronous motor

A shield plate 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. The inner diameter of the shield plate is made smaller than the outer diameter of a rotor in order to prevent the rotor from being detached from a stator assembly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
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. 
2. Description of the Related Art 
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. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an inexpensive claw pole 
type synchronous motor which can be used as the spindle motor of a memory 
device. 
In order to achieve the object, a claw pole type synchronous motor 
according to the present invention 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. 
Moreover, according to the present invention, that part of the shield plate 
which corresponds to the field magnet of the rotor can be provided with a 
plurality of dowels 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 a low cost. Also 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 dowels 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. That is, dowels prevent 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 1a 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 comprise 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 comprise 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, 0.5 mm) from that end face of the field magnet 17 which 
corresponds to the field magnet corresponding portion 25. 
A plurality of dowels 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 dowels are arranged at equal 
intervals in the circumferential direction. The number of the dowels may 
be four to ten, instead. The height or radius L of each dowel 26 is, for 
example, 0.2 mm smaller than the interval K between the field magnet 
corresponding portion 25 and the field magnet 17 in order to normally 
prevent the dowels 26 from colliding and interfering with the field magnet 
17 during the rotation of the rotor 14. Even if the dowels 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 dowels 26. Thus, the shield plate 24 does not contact 
the field magnet 17 except for the dowels 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 18 mm 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 25 mm 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 25 mm, the limit value is 0.6 V 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 0.25 V, 0.4 V 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 
0.6 V. 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 0.19 V, 0.36 V and 0.55 V 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 by 
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 projecting dowels 
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.