Armature core and brush-equipped DC motor using the same

An armature core includes a central circular portion and a plurality of equi-spaced extensions extending from the central circular portion outwardly in the radial direction. The extensions and the central circular portions are in the form of an integral configuration and are formed by hot pressing magnetic metal powder particles, each of which is covered by an insulating material.

BACKGROUND OF THE INVENTION 
The present invention is directed to an armature core and a brush-equipped 
DC motor using the same. 
As well known, a conventional brush-equipped DC motor includes, as shown in 
FIGS. 12 and 13, an armature core 100 having a wire coil 101 wound 
therearound, a commutator 102 electrically connected to the wire coil 101 
and rotatable together with the core 100, a pair of brushes 103 which 
enable continuous rotation of the core 100 by providing current with 
different polarities alternately to the coil through the rotating 
commutator 102. 
For reducing production cost and iron loss of the core 100, as can be 
understood from the illustration in FIG. 12, the core 100 is in the form 
of a plurality of layered thin steel sheets. The reason is that a cast 
core or a sintered core has a high electrical resistance whereby Eddy 
currents become large when a variable flux passes through the core 100, 
thereby heating the core which causes the iron loss of the core 100 to 
increase and therefore a drop of the output relative to the input is 
considerably decreased. 
In view of the foregoing circumstances, the armature core 100 had to be 
formed from layered thin steel sheets. However, employing layered thin 
steel sheets restricts the shape design of the core. Thus, for example, 
the cross-sectional shape which is perpendicular to the axis of the core 
is difficult to change in a gradual manner in the axial direction. 
In detail, as can be seen from FIG. 13, each of the layered thin steel 
sheets of the core has a central circular portion 100a and a plurality of 
equi-pitched radial extensions 100b. In light of the fact that the layered 
thin steel sheets are planar, the axial length L of any one of the 
extensions 100b is constant. In addition, the cross-section of the 
extension 100b in the axial direction of the core becomes rectangular with 
four right-angle corners as shown in FIG. 15. 
When winding the wire 101 around the core 100, the wire 101 is in an 
overlapped condition at the circular portion 100a and the resultant 
expansion, as shown in FIG. 14, has an axial dimension or thickness M, 
thereby enlarging the whole axial dimension of the core 100 by the 
thickness M. 
In addition, as can be understood from the illustration in FIG. 15, the 
right-angled corners of the core cause the winding coil 101 to be spaced 
from the core 100. Such a spacing causes an enlargement of the axial 
length of the core 100. Moreover, the axial length L of any one of the 
extensions 100b is constant as explained previously and thus it is 
difficult to place an element such as a bearing or commutator closed to 
the core 100 in the design thereof. This also causes an enlargement of the 
entire axial length of the core 100. 
Furthermore, as can be appreciated from FIG. 15, the closed loop of the 
wire 101 forms a substantial oval, whereby the shortening of the wire 101 
makes it difficult to achieve an adequate performance. Thus it is not 
possible to increase the motor output by increasing the current flowing 
through the wire when a voltage is applied across the motor when the 
closed loop is shortened in length. 
The foregoing problems relate to the fact that the shape of the core cannot 
be formed in an arbitrary manner. 
SUMMARY OF THE INVENTION 
It is, therefore, a principal object of the present invention to provide an 
armature core whose shape can be formed in an arbitrary manner and a 
brush-equipped DC motor using the same. 
In order to attain the foregoing objects, the present invention provides an 
armature core which comprises a central circular portion and a plurality 
of equi-spaced extensions extending from the central circular portion 
outwardly in the radial direction, the extensions and the central circular 
portions being in the form of an integral configuration and being formed 
by shaping magnetic metal powder particles, each of which is covered by an 
insulating material.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Preferred embodiments of the present invention will be described 
hereinafter in detail with reference to the accompanying drawings. 
With reference to the first embodiment shown in FIG. 1, there is 
illustrated an armature core 1. The armature core 1 includes a central 
circular portion 2 and a plurality of equi-spaced extensions 3 each of 
which extends outwardly in the radial direction. Each of the extensions 3 
has a base portion 3a and a distal end portion 3b. 
As best shown in FIG. 2, the thickness or axial dimension J of the circular 
portion J is set to be smaller than the thickness or axial dimension H of 
the distal end portion 3b of the extension 3. In addition, the base 
portion 3a of the extension 3 becomes gradually smaller in the radial 
direction and is smaller than the thickness H of the distal end portion 
3b. 
Referring to FIG. 3, each corner of the base portion 3a is formed into a 
rounded configuration and therefore when a wire 4 is wound around the base 
portion 3a, the wire 4 runs along the surface of the base portion 3a 
without defining a gap therebetween unlike the illustrated conventional 
condition in FIG. 15. 
Since the core 1 has the foregoing configuration, after completion of 
winding the wire 4 the resultant wound or overlapped condition of the wire 
4 as illustrated in FIG. 2 has a thickness J1 which is substantially equal 
to the thickness H of the distal end portion 3b. Thus, little protrusion 
of the wound wire 4 occurs. 
Thus, by employing such a core 1 wherein the conventional protrusion M 
(FIG. 14) is deleted enables the construction of a motor having a shorter 
axial length, thereby providing a compact motor. 
The core 1 is formed in accordance with the following steps. First of all, 
iron powder is prepared with each particle of the powder having a diameter 
ranging from 25 to 150 .mu.m. Such particles are covered with a glass 
material for insulation. The resultant particles are input into a cavity 
with an amount of resin powder and are subjected to a hot press procedure 
(temperature: 275 degrees in Celsius) for 60 minutes. Upon completion of 
this procedure, the core 1 is configured to be the illustrated 
configuration shown in FIG. 1. It is to be noted that instead of 
iron-nickel, cobalt, and other magnetic materials whose characteristics 
vary upon being magnetized are available. 
The resultant core 1 is formed of glass-insulated iron powder particles, 
which means that the electrical resistance thereof becomes larger as a 
whole. Thus, an Eddy current generated in the core 1 is smaller, thereby 
restricting the iron loss of the core 1 to a minimum. 
Referring to the second embodiment shown in FIG. 4, there is illustrated an 
armature core 11 which is formed by a method similar to the method for 
forming the core 1 in accordance with the first embodiment of the present 
invention. 
As shown in FIG. 4, the armature core 11 includes a central circular 
portion 12 and a plurality of equi-spaced extensions 13 each of which 
extends outwardly in the radial direction. Each of the extensions 13, has 
a base portion 13a and a distal end portion 13b. As is apparent from FIG. 
5 and the following description, the circular portion 12 has a smaller 
thickness than the base 13a of the extension 13. 
As illustrated in FIG. 5, a ring-shaped permanent magnet 16 is fixed on an 
inner surface of a motor casing 15. Within the permanent magnet 16, there 
is provided the core 11 upon which a wire 14 is wound thereon. The core 11 
is rotatable relative to the motor casing 15 via a ball bearing 17. A 
commutator 18 is fixedly mounted on a shaft 11a and has a hooked portion 
11a in electrical contact with ends of the wire 14 about the base 13a of 
the extension 13. An electrical connection is established via a pair of 
brushes 90 between the commutator and a power supply (not shown). 
As can be appreciated from the illustration in FIG. 5, the axial length or 
thickness of the circular portion 12 is smaller than that of the extension 
13, thereby forming a recess into which the hooked portion 18a of the 
commutator 18 extends. A corresponding recess is formed at the opposite 
side of the core 1 which partially accommodates the bearing 17 which is 
located in a space defined by a circular protuberance 15a in the bottom of 
the motor casing 15. Thus, the ball bearing 17 and the commutator 18 can 
be positioned close to the core 11, thereby shortening the entire axial 
length of the motor. 
With reference to the third embodiment shown in FIG. 6, the armature core 
21 is formed by a method similar to the method of forming the core 1 in 
accordance with the first embodiment of the present invention. 
As shown in FIG. 6, the armature core 21 includes a central circular 
portion 22 and a plurality of equi-spaced extensions 23 each of which 
extends outwardly in the radial direction. Each of the extensions 23, has 
a base portion 23a and a distal end portion 23b. 
As can be seen from FIG. 7, the axial cross-section of the base portion 23a 
of the extension 23 is configured to be an ellipse. In addition, as shown 
in FIG. 6, a plurality of projections 25 is provided on an outer surface 
of the central circular portion 22 so as to extend along an axial 
direction thereof. As can be understood from the illustration in FIG. 8, 
the axial outer periphery of each projection 25 has a convex 
configuration. Such a structure enables shortening the length of the wire 
24 in the form of the closed loop without reducing the magnetic flux area 
obtained by the structure in FIG. 13. Thus, a lower electrical resistance 
of the core can be established than that of the structure in FIG. 13, 
thereby increasing the output. 
With reference to the fourth embodiment shown in FIG. 9, the armature core 
31 is formed by a method similar to the method for making the core 1 in 
accordance with the first embodiment of the present invention. 
As illustrated in FIGS. 9 through 11, the armature core 31 includes a 
central circular portion 32 and a plurality of equi-spaced extensions 33 
each of which extends outwardly in the radial direction. Each of the 
extensions 33 has a base portion 33a and a distal end portion 33b. The 
central circular portion 32 is also provided thereon with an integral 
axial cylindrical extension or annular ring 32 which acts as a partition. 
The axial length or extension degree of the annular ring 32 is set to be 
larger than that of wound wire coil 34. The distal end of the annular ring 
32 is chamfered. 
The illustration in FIG. 10 illustrates a condition wherein the shaft 31a 
of the core 31 is in the process of being fitted with a commutator 38 in 
press-fit manner. Upon completion of such a fitting, the condition in FIG. 
11 is established wherein one end of the commutator 38 is within the 
annular ring 32a. Although a distal end 34a of the wire 34 slacks 
temporally during the change from FIG. 10 to FIG. 11, after the first 
engagement of the distal end 34a of the wire 34 with the chamfered end of 
the annular ring 32a, the resultant engagement remains unchanged with the 
application of adequate tension to the distal end 34a of the wire 34, 
which enables positioning of the commutator 38 close to the core 31. Thus, 
the overall axial length of the motor can be made smaller and no means are 
required such as providing varnish on the distal end 34a of the wire 34 
for the prevention of slack thereof. In addition, chamfering the distal 
end of the annular ring 32a can distribute the stress generated at the 
distal end 34a of the wire 34 during the mounting of the commutator 38 on 
the shaft 31a, there by adjusting the tension of the wire 34 to a suitable 
value. 
The invention has thus been shown and described with reference to specific 
embodiments, however, it should be understood that the invention is in no 
way limited to the details of the illustrated structures but changes and 
modifications may be made without departing from the scope of the appended 
claims.