U.S. patent application number 11/131249 was filed with the patent office on 2005-09-22 for method of manufacturing a stator core.
Invention is credited to Eguchi, Shigeki, Hasegawa, Hideo, Koshiishi, Hiromichi, Ogata, Tomiya.
Application Number | 20050206267 11/131249 |
Document ID | / |
Family ID | 32718793 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050206267 |
Kind Code |
A1 |
Koshiishi, Hiromichi ; et
al. |
September 22, 2005 |
Method of manufacturing a stator core
Abstract
A motor stator core for achieving improved magnetizing feature
in lower magnetic fields and reduced iron loss, and improving motor
power. The stator core fabriated out of non-oriented electrical
steel sheets is annealed by applying a magnetic field to the heated
stator core at least in the temperature range from a temperature
immediately above a Curie point thereof to 300.degree. C. in the
process of cooling the stator core. The magnetic field has the same
direction as the direction of exitation of a stator in the motor
when used to drive a motor. This increases the magnetic induction
in lower magnetic fields in particular and reduces the hysteresis
loss, with a reduction in the total iron loss of the stator. A
motor using this stator core increases in saturation induction
under exciting currents of higher frequncies, allowing enhanced
motor power.
Inventors: |
Koshiishi, Hiromichi;
(Kitakyushu-shi, JP) ; Ogata, Tomiya;
(Kitakyushu-shi, JP) ; Eguchi, Shigeki;
(Munakata-shi, JP) ; Hasegawa, Hideo; (Sakura-shi,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
32718793 |
Appl. No.: |
11/131249 |
Filed: |
May 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11131249 |
May 18, 2005 |
|
|
|
10688929 |
Oct 21, 2003 |
|
|
|
Current U.S.
Class: |
310/216.006 ;
29/596 |
Current CPC
Class: |
Y10T 29/49078 20150115;
Y10T 29/4902 20150115; H02K 1/02 20130101; Y10T 29/49009 20150115;
H01F 41/0233 20130101; H02K 15/02 20130101 |
Class at
Publication: |
310/216 ;
029/596 |
International
Class: |
H02K 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2003 |
JP |
P2003-005840 |
Apr 10, 2003 |
JP |
P2003-106674 |
Sep 1, 2003 |
JP |
P2003-309297 |
Claims
What is claimed is:
1. A stator core for use in a motor to be driven in a lower
magnetic field of 1.0 T or less in magnetic induction, the stator
core being made of non-oriented electrical steel sheets and
annealed with a magnetic field applied thereto at least in a
process of cooling from a temperature immediately above a Curie
point thereof, said magnetic field having the same direction as
that of excitation of a stator in the motor when used to drive the
motor.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 10/688,929, filed Oct. 21, 2003.
BACKGROUND OF THE INVENTION
[0002] This application claims the benefit of Japanese Patent
Application No. 2003-005840, filed on Jan. 14, 2003; Japanese
Patent Application No. 2003-106674, filed on Apr. 10, 2003; and
Japanese Patent Application No. 2003-309297, filed on Sep. 1,
2003.
[0003] 1. Field of the Invention
[0004] The present invention relates to a motor stator core made of
non-oriented electrical steel sheets. In particular, the invention
relates to a motor stator core of improved magnetizing feature in
lower magnetic fields and a method of manufacturing the same.
[0005] 2. Description of the Related Art
[0006] Recently, in view of environmental and resources problems,
energy savings and improved efficiency have been increasingly
demanded of electric equipment. The biggest request for motors is
also an efficiency improvement, which in turn requires reductions
of iron loss, copper loss, and mechanical loss. Such motors use a
lamination of non-oriented electrical steel sheets for their core.
It is well known that the core has a large impact on iron loss.
[0007] Motor cores are made of non-oriented electrical steel
sheets, which are manufactured through the application of cold
rolling, including one or more processes of annealing, to
hot-rolled steel sheets that are adjusted to predetermined chemical
compositions. Some of the manufacturing steps vary between
semi-processed non-oriented electrical steel sheets and
full-processed non-oriented electrical steel sheets. Semi-processed
non-oriented electrical steel sheets are predicated on the
application of stress-relief annealing after punching. The
cold-rolled steel sheets are thus annealed at relatively low
temperatures for a short time. The annealing also includes skin
passed rolling in its final step. In contrast, full-processed
non-oriented electrical steel sheets are not necessarily predicated
on the stress-relief annealing of the core. The cold-rolled steel
sheets are thus annealed at higher temperatures for a longer time
than the semi-processed non-oriented electrical steel sheets are.
To ease this annealing condition, the hot-rolled steel sheets are
sometimes annealed.
[0008] By the way, the design concept of high-speed high-efficiency
motors typified by electric vehicle motors, which are under
intensive development and commercialization recently, is toward
higher frequencies (400 to 600 Hz) and lower magnetic induction
(1.0 to 0.5 T). A further improvement for lower iron loss is thus
required of non-oriented electrical steel sheets, or motor core
material, accordingly.
[0009] Motor power P is expressed by the following general
equation:
P=k.times.f.times.N.times.i.times.B.times.S,
[0010] where k is a proportionality constant, f is a frequency, N
is the number of windings, i is a current, B is a magnetic
induction, and S is the sectional area of the core. From the
foregoing equation, it is first shown that with consideration given
to the miniaturization and weight reduction of the motor, the power
P may be increased by raising the frequency f of the exciting
current. Frequencies as high as 10 times or so the frequencies of
commercial power sources are available recently owing to the
invention of inverters. For this reason, the frequency f and the
magnetic induction B have been evaluated for optimum solutions in
the range of frequencies higher than those of the conventional
commercial power sources. The feasible range of magnetic induction
B at a given frequency f depends on the occurrence of
anti-magnetizing force from the motor core material, or the
non-oriented electrical steel sheets. Consequently, the frequency f
is typically set within the range from 400 to 600 Hz, and the
magnetic induction B the range from 1.0 to 0.5 T. In such numerical
ranges, the magnetic induction B remains at a decreasing rate of
1/2 or so for 8.times. to 10.times. frequencies f. This promises a
power increase of the order of f.times.B=4 to 5 times. The
numerical ranges of the values k, N, i and S in the foregoing
equation are automatically determined once the frequency f and the
magnetic induction B are determined.
[0011] The iron loss W of a non-oriented electrical steel sheet, or
motor core material, is expressed as:
W=Wh+We,
[0012] where Wh is hysteresis loss which is given by
Wh=k1.times.f.times.B.sup.16,
[0013] We is eddy current loss which is given by
We=k2.times.(t.sup.2.times.f.sup.2.times.B.sup.2)/.rho., and
[0014] k1 and k2 are constants, t is a thickness, and .rho. is a
resistivity.
[0015] For the sake of reduction in this iron loss, steel sheets
have been reduced in thickness and increased in resistivity. Some
of the most excellent non-oriented electrical steel sheets at
present have thicknesses t as small as 0.20 mm or so and
resistivities as high as or above 55 .mu..OMEGA.-cm.
[0016] As a result of these measures for reducing iron loss by
means of reduced thicknesses and increased resistivities,
non-oriented electrical steel sheets have been considerably reduced
in iron loss as compared to heretofore. Nevertheless, the reduction
in iron loss resulting from the reduced thickness and increased
resistivity of the electrical steel sheets is ascribable to a
reduction of the eddy current loss out of the iron loss expressed
by the foregoing equation. The hysteresis loss is not reduced by a
reduction in thickness or an increase in resistivity. As the eddy
current loss decreases, the ratio of the hysteresis loss to the
entire iron loss increases relatively from conventional 70% or so
to 90% or so. For future measures for a reduction in iron loss, it
is thus becoming increasingly important to reduce the hysteresis
loss.
[0017] The hysteresis loss of an electrical steel sheet has a close
relationship with magnetic induction, and magnetizing feature can
be improved to reduce the hysteresis loss.
[0018] Consequently, improvement in the magnetizing feature matters
for the sake of a reduction in hysteresis loss. The magnetizing
feature can be improved by making the crystal direction of an
electrical steel sheet to be a random cubed direction. For concrete
means, one of the inventors has developed a non-oriented electrical
steel sheet which is switched from a conventional Si-rich
composition to an Al--Mn rich composition to achieve both reduced
iron loss and improved magnetizing feature (see Unexamined Japanese
Patent Publication No. 2002-146490).
[0019] To fabricate a typical motor core, hoops of non-oriented
electrical steel sheets oiled with punching oil are press-punched
into a predetermined shape, and then a lamination of the resultant
is firmly bonded by clumping or welding. Subsequently, heat
treatment is performed to remove the adhering punching oil,
followed by annealing.
[0020] The primary purpose of this annealing is to remove strains
occurring at the time of punching as well as to promote the growth
of crystal grains for improved magnetizing feature. It is
semi-processed non-oriented electrical steel sheets that are
predicated on the application of this stress-relief annealing to
the core after punching. Conventionally, motor cores have been
annealed under the condition that they are kept in a non-oxidizing
or reducing atmosphere at soaking temperatures of approximately
750.degree. C. for approximately 2 hours. Recently, it has been
proposed to perform this kind of core annealing in a magnetic
field. For example, Unexamined Japanese Patent Publication No. Hei
11-340030 describes a method in which the electrical steel pieces
of a core to be excited in two or more directions inside are
annealed in magnetic fields having the same directions as the
directions of their excitation. Moreover, Unexamined Japanese
Patent Publication No. Hei 11-341749 describes a method of
annealing a core under the application of a magnetic field, wherein
the magnetic field to be applied to the core is caused by a coil,
and the core is heated for annealing by the heat generated by the
coil or through the application of a high-frequency magnetic field
from the coil.
[0021] The technology for applying heat treatment to metal in a
magnetic field has been studied long. Even with Fe--Si alloys,
however, it has been concluded in the 1960s that "heat treatment in
a magnetic field has been developed for the purpose of improving
Fe--Si alloys' permeability and produced excellent outcomes in
laboratories, but of little practical value since it requires a lot
of work and cost." No practical use has thus been attempted so
far.
[0022] Nevertheless, it is highly probable that annealing a motor
stator core, or a subject matter of the present invention, in a
magnetic field can improve magnetizing feature. Under the present
circumstances, however, the relationship of the material properties
and annealing condition to the magnetizing feature of the annealed
core has not been clarified. Moreover, the magnetic annealing of a
core proposed in the foregoing Unexamined Japanese Patent
Publications Nos. Hei 11-340030 and Hei 11-341749 has made no
reference in this regard. The inventors have also filed a patent
application for a method and apparatus for annealing a motor stator
core in a magnetic field as Japanese Patent Application No.
2002-156136, whereas this prior application has not provided
sufficient resolution in this regard.
[0023] As described above, electric vehicle motors are currently
adopting exciting currents of higher frequencies in the frequency
range from 400 to 600 Hz to provide power higher than with the
exciting currents of commercial power frequencies. Nevertheless, an
increase in the frequency of the exciting current can lower the
stator cores to 1/2 or so, or even below, in magnetic induction.
Here, if the stator cores can be prevented from dropping in
magnetic induction even under exciting currents of higher
frequencies, it is possible to expect a further enhancement to the
power of the electric vehicle motors at present, or a
miniaturization of the same.
SUMMARY OF THE INVENTION
[0024] It is an object of the present invention to ascertain the
relationship of material properties and annealing conditions to the
magnetizing feature of motor stator cores annealed when magnetic
annealing is performed on the stator cores which are fabricated out
of conventional Si-rich non-oriented electrical steel sheets as
well as Al-rich non-oriented electrical steel sheets having more
favorable magnetizing feature or, in particular, the stator cores
for use in electric vehicle motors to be driven in lower magnetic
fields, and to provide a motor stator core for achieving improved
magnetizing feature and reduced iron loss in lower magnetic fields
and a method of manufacturing the same.
[0025] The motor stator core according to the present invention is
a stator core for use in a motor to be driven in a lower magnetic
field of 1.0 T or less in magnetic induction. The stator core is
made of non-oriented electrical steel sheets and annealed with a
magnetic field applied thereto at least in the process of cooling
from a temperature immediately above a Curie point thereof. The
magnetic field has the same direction as that of excitation of a
stator in the motor when used to drive the motor.
[0026] The method of manufacturing a motor stator core according to
the present invention comprises: a stator core fabrication step of
fabricating a stator core out of non-oriented electrical steel
sheets; and a stator core annealing step of heating the fabricated
stator core to a temperature above a Curie point thereof, and then
cooling the stator core while applying a magnetic field to the
stator core at least in the temperature range from a temperature
immediately above the Curie point to 300.degree. C. in the cooling
process after the heating. The magnetic field has the same
direction as that of excitation of a stator in the motor when used
to drive the motor. Here, the stator core desirably has a grain
size of 100 .mu.m or greater at the time of application of the
magnetic field.
[0027] In annealing the stator core fabricated out of the
non-oriented electrical steel sheets, the magnetic field is applied
to the heated stator core at least in the temperature range from
the temperature immediately above the Curie point to 300.degree. C.
in the process of cooling the stator core. A stator core of reduced
iron loss can thus be manufactured as one for use in a high-speed
high-efficiency motor typified by an electric vehicle motor to be
driven in lower magnetic fields. In particular, if the direction of
the magnetic field applied to the stator core in the cooling
process is rendered along the direction of excitation of the stator
when used to drive a motor, it follows that the directions of
magnetization of the domains in the crystals of the stator core
approach the direction of excitation of the stator for motor
driving. Consequently, the domains of the stator core are easily
oriented in the direction of excitation when the stator is excited.
This increases the magnetic induction and decreases the hysteresis
loss, allowing a reduction in the total iron loss of the stator.
Since the directions of magnetization of the domains in the
crystals of the stator core approach the direction of excitation of
the stator for motor driving, the stator core is prevented from
dropping in magnetic induction when under high-frequency exciting
currents of 400 to 600 Hz which are used in electric vehicle
motors. Besides, the saturation induction is increased. It is
therefore possible to achieve a further power improvement or
miniaturization of the motor that uses this stator.
[0028] Moreover, since the stator core is given a grain size of 100
.mu.m or greater at the time of application of the magnetic field,
the domains in the crystals annealed in the magnetic field become
easier to orient in the same direction of magnetization with a
further improvement to the effect of improving the magnetizing
feature.
[0029] The nature, principle, and utility of the invention will
become more apparent from the following detailed description when
read in conjunction with the accompanying drawings in which like
parts are designated by identical reference numbers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a graph for explaining the effect of the present
invention in conjunction with B-H curves;
[0031] FIGS. 2A to 2D are schematic diagrams showing the steps of
annealing a stator core according to an embodiment of the present
invention;
[0032] FIGS. 3A and 3B are schematic diagrams showing the steps of
annealing the stator core according to the embodiment of the
present invention;
[0033] FIG. 4 is a graph showing experimental results; and
[0034] FIG. 5 is a graph showing experimental results.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] Ordinary motors are typically used in commercial power
frequencies at magnetic induction of the order of 1.4 to 1.8 T. The
stator cores of the motors to be driven under such high fields do
not have much margin for improving their magnetizing feature
through the annealing of the stator cores, nor for improving
magnetic induction through magnetic annealing. That is, the
magnetic annealing promises little effect. With the stator cores of
motors to be driven under lower magnetic fields of 1.0 T or less in
magnetic induction, in contrast, the magnetic annealing provides a
significant effect if the cores are annealed under an appropriate
condition of field application. In this respect, description will
be given with reference to FIG. 1.
[0036] FIG. 1 shows a graph called a B-H curve which shows the
relationship between magnetizing force H and magnetic induction B.
The magnetizing force H on the abscissa indicates the intensity of
an electric current, which is the force for magnetization, in terms
of a total current per meter in units of A/m. The magnetic
induction B on the ordinate indicates, in units of T (tesla), up to
how many magnetic lines of force can be absorbed for a given
magnetizing force H. In the graph, the curve (1) shows measurements
on a sample that is made of electrical steel sheets, or the
material, as punched. The curve (2) shows measurements on the
sample of (1) that has been annealed without any magnetic field.
The curve (3) shows measurements on the sample of (1) that has been
magnetic-annealed. The material was commercially available Si-rich
non-oriented electrical steel sheets (equivalent of JIS C2552
35S230), having a thickness of 0.35 mm. The steel sheets were
punched into a ring shape having an outer diameter of 80 mm and an
inner diameter of 60 mm. The heating temperature at the time of
annealing was 750.degree. C. The magnetic field was applied in the
process of cooling from 750.degree. C. to 300.degree. C.
[0037] As can be seen from FIG. 1, a difference between nonmagnetic
annealing and magnetic annealing (the distance between the curves
(2) and (3)), i.e., the effect of the magnetic annealing, is small
in the area where the magnetic induction B is 1.4 to 1.8 T, which
is the driving condition for ordinary motors. Now, in the area of
lower magnetic fields where the magnetic induction B is 1.0 T or
lower, which is the driving condition for electric vehicle motors,
the magnetic annealing provides an extremely high effect. The B-H
curve rises at a sharp angle in the case of the magnetic annealing
in particular. The point of the maximum permeability (indicated
with a solid circle on the graph) shifts toward lower magnetizing
forces, with a significant improvement to the magnetizing feature
in lower magnetic fields.
[0038] The stator core itself is composed of crystals each
containing a plurality of magnetic domains. The domains disappear
at temperatures above the Curie point (for example, approximately
720.degree. C. in the case of Si-rich non-oriented electrical steel
sheets) when the stator core is annealed, and reappear when cooled
to temperatures below the Curie point. When a magnetic field is
applied in the cooling process, most of the plurality of domains
that reappeared are oriented with their directions of magnetization
along the direction of the magnetic field applied. Subsequently,
the application of the magnetic field is stopped at 300.degree. C.,
so that the directions of magnetization of the respective domains
are fixed, in principle, to the direction of a magnetic easy axis
(any one of crystal axes [100], [010], and [001]) that is
three-dimensionally closest to the direction of the magnetic field
applied so far.
[0039] As a result, higher magnetic induction is obtainable at
lower magnetic fields. That is, the point of the maximum
permeability shifts toward lower magnetizing forces.
[0040] Moreover, if the direction of the magnetic field applied to
the stator core in the cooling process is rendered along the
direction of excitation of the stator for motor driving, it follows
that the directions of magnetization of the domains fixed at
300.degree. C. as described above approach the direction of
excitation of the stator for motor driving. This facilitates
orienting the domains of the stator core to the direction of
excitation of the excited stator, allowing an increase in magnetic
induction. Consequently, the hysteresis loss decreases to reduce
the total iron loss of the stator, and the motor using this stator
core increases in saturation induction under exciting currents of
higher frequencies. As a result, the motor power improves.
[0041] The application of the magnetic field in the cooling process
is preferably performed in the temperature range from a temperature
immediately above the Curie point to 300.degree. C. At temperatures
higher than immediately above the Curie point, the application of
the magnetic field has no effect since the domains in the crystals
of the stator core disappear in this temperature range. Experiments
have found that the temperature at which the directions of the
domains reappearing in the cooling process are fixed to a certain
direction under the application of a magnetic field is around
300.degree. C. Thus, the magnetic field need not be applied at
temperatures lower than that.
[0042] The magnetic field to be applied preferably has an intensity
of 800 to 12000 A/m.
[0043] The reason is that at magnetic field intensities below 800
A/m, the effect of improving the magnetizing feature resulting from
the application of the magnetic field is small and unstable.
[0044] On the other hand, even if a magnetic field higher than
12000 A/m is applied, the effect of improving the magnetizing
feature reaches saturation.
[0045] For further enhancement of the effect of the magnetic
annealing, the stator core desirably has a grain size of 100 .mu.m
or greater at the time of field application. The greater grain size
the stator core has upon field application, the smaller the total
area of the grain boundary becomes. The domains after the magnetic
annealing thus become easier to orient in the same direction of
magnetization with a corresponding improvement in the magnetizing
feature. Then, the stator core desirably has a grain size as great
as possible, at least at the point in time when the magnetic field
is applied at the temperature immediately above the Curie point. In
particular, grain sizes of 100 .mu.m or greater enhance the effect
of the field application. It is therefore desirable that the stator
core itself at the time of field application be given a greater
grain size by either of the following means: fabricating the stator
core by using non-oriented electrical steel sheets having a grain
size of 100 .mu.m or greater (such as equivalents of JIS C2552
35S230); and fabricating the stator core by using non-oriented
electrical steel sheets having a grain size below 100 .mu.m (such
as equivalents of JIS C2552 35S300) and heating the stator core to
around 850-900.degree. C. in an H.sub.2 or (H.sub.2+Ar) atmosphere
for grain growth. In the case of the stator core that is made of
non-oriented electrical steel sheets of 100 .mu.m or greater in
grain size, the heating up to the temperature immediately above the
Curie point may be immediately followed by the field application
and cooling. On the other hand, in the case of the stator core that
is made of non-oriented electrical steel sheets of below 100 .mu.m
in grain size, the crystal grains are preferably grown in the
foregoing manner before the magnetic field is applied thereto in
the cooling process. For example, when the stator core is made of
non-oriented electrical steel sheets that have been given skin
passed rolling in the final step of fabrication, the crystal grains
of the stator core can grow to a grain size of 200 .mu.m or so when
the stator core is heated for annealing.
[0046] This annealing of the stator core accompanied with field
application can be effected, for example, by using an annealer that
is described in Japanese Patent Application No. 2002-156136. This
annealer is a stator-core annealer comprising a seat, a pipe
member, a cover, a spiral winding coil, and linear coils. The seat
is intended to stack a plurality of stator cores thereon. The pipe
member is attached to an axial rod of the seat, and the stator
cores stacked with the pipe member passed through their through
holes. The cover shields the internal and external surfaces of the
stacked stator cores from ambient air. The winding coil is arranged
outside the cover. The linear coils are arranged linearly along the
bore surface and peripheral surface of the stacked stator
cores.
Embodiment 1
[0047] FIGS. 2A to 3B are schematic diagrams showing the steps for
annealing a stator core according to the embodiment of the present
invention. The shown steps are of processing prior to the start of
annealing.
[0048] FIG. 2A shows a seat 1, an axial rod 2, and a pipe member 3
for stacking stator cores (hereinafter, referred to as cores) C to
be annealed. The pipe member 3 is a heat-resistant silica tube.
FIG. 2B shows five cores (C1 to C5). In the present embodiment,
each of the cores C is a lamination of non-oriented electrical
steel sheet layers as material which have been punched in a ring
shape. Each core has an outer diameter of 300 mm, an inner diameter
of 200 mm, and a thickness of 200 mm. The height of the five cores
stacked is 1000 mm.
[0049] FIG. 2C shows the five cores C stacked. In the diagram, the
reference numeral 4 represents a core mount, and 5 a heat insulator
of alumina wool which is wound around the pipe member 3. The five
cores C are stacked on the core mount 4 which is placed on the seat
1. The inner peripheries of the cores C and the outer periphery of
the heat insulator 5 have a gap of the order of 10 mm
therebetween.
[0050] FIG. 2D shows the stacked cores C with a heat insulator 6 of
alumina wool wound around. The heat insulator 6 is wound around the
pipe member 3 and the cores C for the purpose of providing heat
insulation when the cores C are heated by a winding coil 8 for
heating the stacked cores C to be described later, and when they
are cooled off.
[0051] FIG. 3A shows the state where a cover 7 for shielding
against ambient air is installed. The cover 7 is an open-bottom
cylindrical member having a hole 7a for the pipe member 3 to pass
through in its top. With the cover 7 arranged over the stacked
cores C (including the heat insulator 6), the gap between the cover
7 and the seat 1 can be sealed so that the cores C are shielded
from the ambient air. Atmospheric gas such as inert gas,
non-oxidizing gas, and non-nitriding gas is supplied to a space 9
between the outer peripheries of the cores C and the inner
periphery of the cover 7 through a hole (not shown) which is formed
in the seat 1.
[0052] FIG. 3B shows the layout of a heating coil and field
applying coils. A winding coil 8 for heating the cores C is
arranged outside the cover 7. In the present embodiment, the
winding coil 8 is a spirally wound coil of water-cooling copper
pipe. When energized with a high-frequency current of 500 Hz, the
winding coil 8 heats the cores C to 750-850.degree. C.
[0053] Two linear coils 10 (10a, 10b) are arranged across outside
the cover 7 and inside a hole 3a of the pipe member 3 at the center
of the cores C. The linear coils 10 are intended to apply a
magnetic field to the cores C during cooling. In the present
embodiment, copper wire coils capable of applying a 12000-A/m
magnetic field are used. The winding coil 8 and the linear coils 10
described above can be moved upward, and thus are retracted upward
when the cores C, the cover 7, and the like are mounted and
dismounted onto/from the seat 1.
[0054] Next, description will be given of the method of annealing a
motor stator core by using the annealer which has the basic
structure shown in FIG. 3B.
[0055] The cores C, the cover 7, and others are loaded on the seat
1 as shown in FIG. 3B.
[0056] Atmospheric gas (for example, Ar gas) is supplied to the
space 9 between the outer peripheries of the cores C and the inner
periphery of the cover 7. After the winding coil 8 and the linear
coils 10 are arranged in predetermined positions, the winding coil
8 is energized with a high-frequency current of 500 Hz from a
high-frequency power supply (not shown) so that the cores C are
heated to 750-850.degree. C. The non-oriented electrical steel
sheets, or the material of the motor stator cores, have a Curie
point of approximately 720.degree. C., provided that they are
Si-rich. Heating the cores C to 750-850.degree. C. thus removes
strains occurring in punching the steel sheets, and provides the
effect of improving the magnetizing feature through the growth of
crystal grains in the material. In particular, the effect of
improving the magnetizing feature due to field application is
significant when the material has a grain size of 100 .mu.m or
greater. If the material has a grain size below 100 .mu.m, it is
thus preferable to heat the material to 850-900.degree. C. so that
a grain size of 100 .mu.m or greater is achieved. Incidentally,
when the material is non-oriented electrical steel sheets that have
been given skin passed rolling in the final step of fabrication,
the crystal grains of the cores C can grow to a grain size of 200
.mu.m or so when the cores C are heated for annealing.
[0057] A thermometer 11 is used to confirm, from its temperature
detection value, that the internal temperature of the cores C
reaches 750-850.degree. C. Then, the heating by the winding coil 8
is stopped to cool the cores C. After the start of cooling, at the
point that the internal temperature of the cores C falls to
approximately 750.degree. C., the linear coils 10 are energized
with DC currents of 12000 A/m from a DC power supply (not shown) so
that the magnetic field is applied to the cores C. The cooling
accompanied with the field application is continued until the
internal temperature of the cores C falls to approximately
300.degree. C. Below 300.degree. C., the field application is
stopped for natural cooling. In this cooling process, a plurality
of domains in the crystals of the cores C reappear which have
disappeared due to heating to the Curie point and over. Because of
the field application in the cooling process, most of the plurality
of domains reappearing are oriented with their directions of
magnetization along the direction of the magnetic field applied.
Subsequently, when the application of the magnetic field is stopped
at 300.degree. C., the directions of magnetization of the
respective domains are fixed to the direction of a magnetic easy
axis (any one of crystal axes [100], [010], and [001]) that is
three-dimensionally closest to the magnetic field applied so far.
Consequently, high magnetic induction is obtainable in lower
magnetic fields.
[0058] In addition, the linear coils 10 for field application are
arranged as shown in FIG. 3B so that the direction of the magnetic
field applied coincides with the direction of excitation of the
stator for motor driving. The directions of magnetization of the
domains are thus fixed close to the directions of excitation of the
stators for motor driving. Consequently, the domains of the stator
cores are easily oriented in the direction of excitation when the
stators are excited for motor driving. This allows an increase in
magnetic induction and a decrease in hysteresis loss, with a
reduction in the total iron loss of the stators. The motors using
these stator cores also improve in power under exciting currents of
high frequencies.
[0059] The following experiments were conducted to confirm the
effects of the present invention.
EXPERIMENTAL EXAMPLE 1
Experimental Condition
[0060] Used material: non-oriented electrical steel sheets (sample
symbol: HTH1500) 0.20 mm in thickness, no skin passed rolling, 60
.mu.m in grain size
[0061] Sample dimensions: 80 mm in outer diameter, 60 mm in inner
diameter
[0062] Heating temperature: 850.degree. C., 750.degree. C.
[0063] Field application: in the process of cooling from
750.degree. C. to 300.degree. C.
[0064] Field intensity: 0 (nonmagnetic annealing), 800, 6000
(A/m)
Experimental Results
[0065] Table 1 shows the experimental results.
1TABLE 1 Annealing Field intensity B0.5 W7/50 Grain size condition
(A/m) (T) (W/kg) (.mu.m) Heating 6000 0.726 0.469 70 temperature
800 0.629 0.497 70 850.degree. C. 0 0.587 0.509 70 Heating 6000
0.553 0.513 60 temperature 800 0.528 0.528 60 750.degree. C. 0
0.434 0.554 60 Non-annealed -- 0.342 0.633 60 steel sheets NOTE:
B0.5 and W7/50 were measured in accordance with ASTM A596-69
(revised in 1979).
[0066] As can be seen from Table 1, the application of magnetic
fields of 800 A/m or above in the process of cooling from
immediately above the Curie point to 300.degree. C. resulted in
improved magnetizing feature as compared to the case of ordinary
annealing without field application. The heating temperature of
850.degree. C. grew the crystal grains of the material by 10 .mu.m
or so, which improved the magnetizing feature as compared to the
case with the heating temperature of 750.degree. C. When the
magnetic field applied was 800 A/m in intensity, the effect of
improving the magnetizing feature was somewhat less stable than in
the case with 6000 A/M.
Experimental Example 2
Experimental Condition
[0067] Used material: non-oriented electrical steel sheets (sample
symbol: 35H230) 0.35 mm in thickness, no skin passed rolling, 100
.mu.m in grain size
[0068] Sample dimensions: 80 mm in outer diameter, 60 mm in inner
diameter
[0069] Heating temperature: 750.degree. C.
[0070] Field application: in the process of cooling from
750.degree. C. to 300.degree. C.
[0071] Field intensity: 0 (nonmagnetic annealing), 6000, 12000
(A/m)
Experimental Results
[0072] Table 2 shows the experimental results.
2TABLE 2 Annealing Field intensity B0.5 W7/50 Grain size condition
(A/m) (T) (W/kg) (.mu.m) Heating 12000 0.749 0.522 100 temperature
6000 0.683 0.531 100 750.degree. C. 0 0.460 0.606 100 Nonannealed
-- 0.343 0.676 100 steel sheets NOTE: B0.5 and W7/50 were measured
in accordance with ASTM A596-69 (revised in 1979).
[0073] As can be seen from Table 2, the use of the material having
a greater grain size enhanced the effect of improving the
magnetizing feature. The application of the 6000-A/m magnetic field
and that of the 12000-A/m magnetic field did not show much
difference in the effect of improving the magnetizing feature.
Experimental Example 3
Experimental Condition
[0074] Used material 1: non-oriented electrical steel sheets
(sample symbol: 50HS01)
[0075] ingredients of C: 0.002%, Si: 0.80%, Mn: 0.20%, and B:
0.004%
[0076] skin passed rolling at a draft of 6%
[0077] 0.50 mm in thickness, 15 .mu.m in grain size
[0078] Used material 2: non-oriented electrical steel sheets
(sample symbol: 50HS11)
[0079] ingredients of C: 0.002%, Si: 0.20%, Mn: 0.20%, Al: 1.00%,
and P: 0.10%
[0080] skin passed rolling at a draft of 6%
[0081] 0.50 mm in thickness, 15 .mu.m in grain size
[0082] Sample dimensions: 80 mm in outer diameter, 60 mm in inner
diameter
[0083] Heating temperature: 810.degree. C.
[0084] Field application: in the process of cooling from
810.degree. C. to 300.degree. C.
[0085] Field intensity: 0 (nonmagnetic annealing), 12000 (A/m)
Experimental Results
[0086] Table 3 shows the experimental results. For comparison,
Table 3 also shows the experimental results on the material of the
experimental example 2.
3TABLE 3 Annealing BS B0.5 W7/50 Grain size Sample condition (T)
(T) (W/kg) (.mu.m) 50HS11 Magnetic 2.10 1.40 0.652 200 annealing
Nonmagnetic 2.10 1.20 0.746 200 annealing 50HS01 Magnetic 2.12 0.88
0.932 200 annealing Nonmagnetic 2.12 0.46 1.132 200 annealing
35H230 Magnetic 1.99 0.75 0.522 100 annealing Nonmagnetic 1.99 0.46
0.606 100 annealing NOTE: BS listed are saturation values of the
magnetic induction in the B-H curves. BS, B0.5, and W7/50 were
measured in accordance with ASTM A596-69 (revised in 1979).
[0087] FIG. 4 shows B-H curves representing the magnetizing feature
graphically. In the chart, the curves (2) and (3) are the same as
those in FIG. 1. The curves (4), (5), (6), and (7) show the results
of the nonmagnetic annealing of the sample 50HS01, the magnetic
annealing of the sample 50HS01, the nonmagnetic annealing of the
sample 50HS11, and the magnetic annealing of the sample 50HS11,
respectively. As can be seen from Table 3 and FIG. 4, even the
sample 35H230 made of steel sheets without skin passed rolling, if
given the magnetic annealing, achieved approximately 1.6 times of
improvement in the magnetic induction B0.5 in lower magnetic fields
and approximately 14% of reduction in the iron loss W7/50 with
respect to the case of the nonmagnetic annealing. The sample 50HS01
made of skin-passed rolled steel sheets was lower than the sample
35H230 in Si content and thus inferior to the sample 35H230 in
terms of magnetizing feature, whereas it had a large grain size and
thus displayed a high effect of cooling in a magnetic field. The
magnetic induction B0.5 of the sample 50HS01 in lower magnetic
fields was higher than that of the sample 35H230. The sample 50HS
11 made of Al-rich steel sheets, even with the nonmagnetic
annealing, showed a greater magnetic induction B0.5 in low magnetic
fields than the samples 35H230 and 50HS01 made of Si-rich steel
sheets with the magnetic annealing did, and the magnetic induction
B0.5 with the magnetic annealing was even greater. The foregoing
confirms that the application of magnetic annealing to a core can
improve the magnetizing feature, that the use of the electrical
steel sheets given skin passed rolling in the final step provides a
further improvement to the magnetizing feature as compared to the
case where electrical steel sheets without skin passed rolling are
used, and that the use of Al-rich electrical steel sheets achieves
still a further improvement to the magnetizing feature.
Experimental Example 4
[0088] In the experimental examples 1 to 3, the samples before and
after the annealing were measured for the magnetic induction under
a direct-current magnetic field, and for the iron loss at a
frequency of 50 Hz. In the present experiment, the samples before
and after annealing were subjected to alternating-current magnetic
fields of different frequencies and measured for the saturation
induction at that time, assuming the cases where the motor stator
core of the present invention is applied to an electric vehicle
motor.
Experimental Condition
[0089] Used material: non-oriented electrical steel sheets (sample
symbol: 35H250)
[0090] ingredients of C: 0.002%, Si: 0.30%, Mn: 0.20%, and Al:
0.80%
[0091] 0.35 mm in thickness, no skin passed rolling, 150 .mu.m in
grain size
[0092] Sample dimensions: 80 mm in outer diameter, 60 mm in inner
diameter
[0093] Heating temperature: 810.degree. C.
[0094] Field application: in the process of cooling from
810.degree. C. to 300.degree. C.
[0095] Field intensity: 0 (nonmagnetic annealing), 8000 (A/m)
[0096] Frequency of the exciting current: 50, 200, 400, 600, 1000
(Hz)
Experimental Results
[0097] Table 4 shows the experimental results.
4 TABLE 4 Frequency of exciting current (Hz) 50 200 400 600 1000
Rotational speed (min.sup.-1) 500 2000 4000 6000 10000 Magnetic
Saturation 1.54 1.39 1.17 1.08 0.39 annealing induction (T) Output
77.0 278 468 648 390 factor Nonmagnetic Saturation 1.55 1.24 0.62
0.48 0.25 annealing induction (T) Output 77.5 248 248 288 250
factor No annealing Saturation 1.55 1.09 0.44 0.37 0.21 induction
(T) Output 77.5 218 176 222 210 factor NOTE: The rotating speeds
are for a 12-pole motor at the respective frequencies.
[0098] The saturation induction is a measurement under
alternating-current magnetization at the respective
frequencies.
[0099] The output factors are listed as the indexes of the motor
outputs, each shown in terms of the product of the frequency and
the saturation induction.
[0100] FIG. 5 is a graphical representation of Table 4. The broken
lines show the saturation induction, and the full lines the output
factors. The marks hollow circle, solid triangle, and solid circle
represent magnetic annealing, nonmagnetic annealing, and no
annealing, respectively.
[0101] As also shown in FIG. 1, when a direct-current magnetic
field is applied to the non-annealed sample punched out of the
non-oriented electrical steel sheets, the magnetic induction
increases with increasing magnetizing force. Nevertheless, the
magnetic induction reaches saturation at magnetizing forces of
around 100000 A/m, with a saturation induction of approximately 2.0
(T). On the other hand, when an alternating-current magnetic field
is applied, the saturation induction decreases with an increasing
frequency of the exciting current because of the occurrence of
anti-magnetizing force. Here, the rate of decrease of the
saturation induction depends on the amount of strains remaining in
the sample and the orientations of the domains. Stress-relief
annealing reduces the residual strains to lower the rate of
decrease of the saturation induction. In addition, magnetic
annealing brings the orientations of the domains close to the
direction of the magnetic field applied, and allows a further
reduction in the rate of decrease of the saturation.
[0102] As shown in Table 4 and FIG. 5, the non-annealed sample and
the nonmagnetic-annealed sample sharply drop in saturation
induction when the frequency of the exciting current exceeds 200
Hz. The output factors, expressed in terms of the product of the
frequency and the saturation induction, show little increase at
frequencies of 200 Hz and above. In contrast, the saturation
induction of the magnetic-annealed sample gently falls as the
frequency of the exciting current increases. In particular, in the
range from 400 to 600 Hz, which is the frequency range of the high
frequency exiting current to be employed in electric vehicle
motors, the sample provides output factors at least twice those of
the non-annealed or nonmagnetic-annealed sample. This confirms that
when the motor stator core according to the present invention is
used for an electric vehicle motor, it is possible to provide motor
power at least twice that of the electric vehicle motor using a
conventional motor stator core, or to miniaturize the motor for the
same power.
[0103] Up to this point, the configuration and experimental
examples of the present invention have been described on the
assumption that the motor stator core according to the present
invention would be applied to an electric vehicle motor.
Nevertheless, the motor stator core according to the present
invention is not limited to electric vehicle motors in application,
but is generally applicable to motors for use in lower magnetic
fields with the effects of reduced iron loss and improved
power.
[0104] While there has been described what is at present considered
to be a preferred embodiment of the invention, it will be
understood that various modifications may be made thereto, and it
is intended that the appended claims cover all such modifications
as fall within the true spirit and scope of the invention.
* * * * *