U.S. patent application number 09/870148 was filed with the patent office on 2002-01-31 for grain-oriented electrical steel sheet for low-noise transformer.
Invention is credited to Fujikura, Masahiro, Mizokami, Masato, Mogi, Hisashi, Sakaida, Akira.
Application Number | 20020012805 09/870148 |
Document ID | / |
Family ID | 26592923 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012805 |
Kind Code |
A1 |
Mogi, Hisashi ; et
al. |
January 31, 2002 |
Grain-oriented electrical steel sheet for low-noise transformer
Abstract
The present invention provides a grain-oriented electrical steel
sheet, for a low noise transformer, capable of reducing higher
harmonics which are highly audible and lowering noise effectively,
and relates to a grain-oriented electrical steel sheet for a low
noise transformer, characterized by imposing a film tension of 0.5
to 6.0 MPa on the surface by coating or a method corresponding
thereto, without forming a glass film on the grain-oriented
electrical steel sheet or, if a glass film is formed, after
removing the glass film by an arbitrary method.
Inventors: |
Mogi, Hisashi; (Futtsu-shi,
JP) ; Sakaida, Akira; (Tokyo, JP) ; Fujikura,
Masahiro; (Futtsu-shi, JP) ; Mizokami, Masato;
(Futtsu-shi, JP) |
Correspondence
Address: |
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
26592923 |
Appl. No.: |
09/870148 |
Filed: |
May 30, 2001 |
Current U.S.
Class: |
428/469 |
Current CPC
Class: |
Y10T 29/4902 20150115;
H01F 1/18 20130101; Y10S 428/926 20130101; Y10S 428/928 20130101;
Y10S 428/90 20130101; C21D 8/1288 20130101; Y10T 428/31678
20150401 |
Class at
Publication: |
428/469 |
International
Class: |
B32B 015/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2000 |
JP |
2000-160511 |
Apr 2, 2001 |
JP |
2001-103588 |
Claims
1. A grain-oriented electrical steel sheet, for a low-noise
transformer, having a film that imposes a tension of 0.5 MPa to 4.0
MPa on the steel sheet.
2. A method for producing a grain-oriented electrical steel sheet,
for a low noise transformer, characterized by imposing a film
tension of 0.5 MPa to 4.0 MPa on the surface by coating or a method
corresponding thereto, without forming a glass film on the
grain-oriented electrical steel sheet or, if a glass film is
formed, after removing the glass film by an arbitrary method.
3. A grain-oriented electrical steel sheet for a low noise
transformer according to claim 1, characterized by having an
.lambda..sub.19 of 1.5.times.10.sup.-6 or less.
4. A grain-oriented electrical steel sheet for a low noise
transformer according to claim 1 or 3, characterized by having a
sheet thickness of 0.27 mm or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electrical steel sheet,
for a low noise transformer excellent in magnetostriction property,
used for an iron core of a transformer and the like.
[0003] 2. Description of the Related Art
[0004] In magnetic materials widely used for electrical and
electronic apparatuses the lengths change when a magnetic field is
applied (this is referred to as "magnetostriction") to cause of
transformer noise and the extent thereof has become an important
evaluation item in quality control. In recent years, regulation of
noise generated by electrical apparatuses is becoming stricter with
an increased demand for a better living environment. Therefore,
studies for lowering noise, by decreasing magnetostriction, are
being carried out.
[0005] Among magnetic materials, for grain-oriented electrical
steel sheets used for iron cores of transformers, a means for
decreasing magnetostriction by reducing closure domains has been
applied. The term "closure domains" as cited here means domains
having magnetization oriented in the direction perpendicular to the
direction of an applied magnetic field. Magnetostriction is caused
when this magnetization is turned to a direction parallel to the
magnetic field by an applied magnetic field. Accordingly, the fewer
the closure domains, the smaller the magnetostriction. The
following methods are known as means for decreasing
magnetostriction:
[0006] {circle over (1)} a method for avoiding the formation of
closure domains, that cause shape changes by magnetization
rotation, by means of aligning the <001>direction of crystal
grains with the rolling direction. (T. Nozawa et al, "Relationship
between Total Losses under Tensile Stress in 3 Percent Si-Fe Single
Crystals and Their Orientations near (110) [001], " IEEE Trans. on
Mag., Vol. MAG-14, No.4, 1978),
[0007] {circle over (2)} a method for eliminating closure domains
by releasing plastic strains (Japanese Unexamined Patent
Publication No. H7-305115, [Development of Revolutionary Oriented
Silicon Steel Sheet, ORIENTCORE HI-B]: OHM 1972. 2), and
[0008] {circle over (3)} a method for eliminating closure domains
by applying a tension film to a steel sheet (T. Nozawa et al,
"Relationship between Total Losses under Tensile Stress in 3
Percent Si-Fe Single Crystals and Their Orientations near (110)
[001]," IEEE Trans. on Mag., Vol. MAG-14, No.4, 1978).
[0009] These three methods have decreased magnetostriction and have
contributed to lowering the noise of electrical apparatuses.
SUMMARY OF THE INVENTION
[0010] When a transformer is fabricated from a grain-oriented
electrical steel sheet and magnetized, various modes of vibrations
are generated in the structure and the oscillation frequencies of
harmonics are also generated. In particular, a fundamental
frequency of magnetization (for example, 100 Hz in the case of an
exciting current of 50 Hz in frequency) and frequencies obtained by
multiplying the fundamental frequency by integral numbers (for
example, 200, 300, 400 Hz - - - in the case of an exciting current
of 50 Hz in frequency) have especially high intensity among
transformer noises. Among these frequencies, relatively low
frequency components vibrate an iron core proper directly, while
high frequency components resonate the auxiliary units of a
transformer, such as a tank, a chiller, a conservator and the like.
However, since the vibrations decrease their intensity
exponentially and become less influential at higher frequencies,
technologies for reducing the low-frequency components have mainly
been tackled so far.
[0011] However, demands for further lowering the noise are strong
and more sophisticated technologies are required.
[0012] The object of the present invention is to provide an
electrical steel sheet, for a low noise transformer, excellent in
magnetostriction properties, capable of reducing high frequency
components in a magnetostriction waveform and lowering noise
effectively.
[0013] The embodiments of the present invention are as follows:
[0014] (1) A grain-oriented electrical steel sheet for a low noise
transformer, having a film that imposes a tension of 0.5 MPa to 4.0
MPa on the steel sheet.
[0015] (2) A method for producing a grain-oriented electrical steel
sheet for a low noise transformer, characterized by imposing a film
tension of 0.5 MPa to 4.0 MPa on the surface by coating or a method
corresponding thereto, without forming a glass film on the
grain-oriented electrical steel sheet or, if a glass film is
formed, after removing the glass film by an arbitrary method.
[0016] (3) A grain-oriented electrical steel sheet for a low noise
transformer characterized by having a .lambda..sub.19 of
1.5.times.10.sub.-6 or less.
[0017] (4) A grain-oriented electrical steel sheet for a low noise
transformer according to the item (1) or (3), characterized by
having a sheet thickness of 0.27 mm or more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows waveforms of magnetostriction.
[0019] FIG. 2 shows profiles of magnetostriction.
[0020] FIG. 3 shows examples of applying Fourier analysis and
audibility correction to magnetostriction.
[0021] FIG. 4 is a graph showing the relationship between film
tensions and magnetostriction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] The gist of the present invention is a grain-oriented
electrical steel sheet, for a low noise transformer, which reduces
the higher harmonic components of vibrations and greatly lowers
noise perceivable by human ears by means of applying an adequate
amount of coating on the surfaces without forming a glass film or
after removing a formed glass film by an arbitrary method and
controlling the magnetostriction waveform.
[0023] As described above, all studies have, up to now, been
focused on decreasing magnetostriction by reducing closure domains.
However, the present inventors found that noise in a relatively
high-pitched sound zone, where human ears become more sensitive, is
scarcely lowered even if a material having smaller magnetostriction
was used, and carried out an extensive study. The explanation
thereof will be given hereunder based on the experiment.
[0024] The present inventors considered that the reason for
generating magnetostriction with higher harmonic waves when a
transformer was magnetized at a certain frequency (usually 50 Hz or
60 Hz) might be explained by a non-smooth magnetostriction
waveform, and studied the relationship between its smoothness and
film tensions.
[0025] FIG. 1 shows the change with the passage of time in the
magnetostriction of two grain-oriented electrical steel sheets each
having a different film tension when they are magnetized at 1.9 T
and 60 Hz. It is known that the magnetostriction waveform of a
grain-oriented electrical steel sheet varies greatly when laser
irradiation conditions, film tensions and the like are changed. In
this example, grain-oriented electrical steel sheets produced by a
conventional method were employed, and, in test sample 1, the
amount of film coating was decreased by 80% from the usual amount
to reduce the film tension to some extent. A film tension was
calculated from the amount of bending of a steel sheet developed by
removing the film on one surface with an acid (formula 1). Here,
the amount of bending of a sheet, H (mm), was determined by setting
a test sample upright on a flat plate, and measuring the distance
between the tangential line at one corner of the bent sheet and the
other corner.
.sigma..sub.B=E/(1-.nu.).times.T.sup.2/3t.times.2H/L.sup.2 (formula
1)
[0026] where .sigma..sub.B: Film tension (g/mm.sup.2)
[0027] E: Young's modulus (kg/mm.sup.2)
[0028] .nu.: Poisson's ratio (=0.3)
[0029] t: Thickness of steel sheet (mm)
[0030] T: Thickness of test sample (mm)
[0031] H: Bending amount of sheet (mm)
[0032] L: Length of test sample (mm).
[0033] From FIG. 1, .lambda..sub.p-p, the difference between the
maximum value and the minimum value of magnetostriction, is
0.62.times.10.sup.-6 in the case of test sample 1 and
0.64.times.10.sup.-6 in the case of test sample 2. It is estimated
that the noise of both test samples 1 and 2 are at almost the same
level in view of magnetostriction amplitude which is one of the
conventional indicators.
[0034] In FIG. 2, is shown the relationship between magnetization
magnetic flux density B and magnetostriction .lambda..sub.B
(.DELTA.l/L of a steel sheet when magnetization magnetic flux
density is BT; a so-called O-p value. .lambda..sub.19 represents
the O-p value when B=1.9 T) when these test samples 1 and 2 are
magnetized in a range from 0 to 1.9 T at 60 Hz.
[0035] With regard to .lambda..sub.B in FIG. 2, .lambda..sub.B
values are positive in the full range of magnetic flux density in
case of the test sample 1 having a low film tension. However, as
for the material having a usual tension, .lambda..sub.B grows
bigger in the negative direction until B reaches 1.7 T, and
afterwards becomes positive at a higher magnetic flux density.
[0036] Three-phase transformers of 630 kVA were fabricated using
these two materials, and their noise levels were measured when they
were magnetized at 60 Hz and 1.9 T. There occurred a big difference
in noise, 66 dB in case of using the test sample 1 and 73 dB in
case of using the test sample 2, despite .lambda..sub.p-p being
almost equal in both cases. Then, the waveforms of these materials
were studied in detail.
[0037] FIG. 3 shows an A-weighted vibration velocity levels (LvA)
at each frequency component for each of the test samples 1 and 2
when they are magnetized at 60 Hz and 1.9 T. These are the values
obtained by decomposing the values of speed, which is converted
from the change with the passage of time in magnetostriction, into
the intensity at each frequency by Fourier transform, and then
correcting the decomposed values at each frequency according to the
audible level (A-weighted). Here, the correction according to the
audibility level means to be multiplied by a coefficient
corresponding to audibility at each frequency.
[0038] As seen in FIG. 3, when the LvA at each frequency component
are compared, it is observed that, although the LvA of the test
sample 2 having a larger noise when it is fabricated into a
transformer is equal to that of the test sample 1 at the
fundamental frequency component (120 Hz), the LvA of the test
sample 2 is rather higher at the second harmonic component (240 Hz)
or above. Since the audible level becomes higher as the frequency
becomes higher in a range up to 4 kHz, the higher the frequency is,
the more the intensity is corrected in the range up to 4 kHz when
the audibility correction is applied. For this reason, the noise
level of the test sample 2 is high when it is fabricated into a
transformer.
[0039] The present inventors considered that the reason why LvA of
test sample 2 was high at higher harmonic wave components as
mentioned above might be explained by the difference in the
magnetostriction waveform. It is observed from FIG. 2 that
.lambda..sub.O-B of test sample 2 having a higher tension has the
inflection point in the vicinity of the magnetization magnetic flux
density of 1.7 T, and magnetostriction increases sharply at a
magnetic flux density exceeding 1.7 T.
[0040] Since test sample 1 with a lower film tension has a smaller
induced magnetic anisotropy compared with test sample 2, closure
domains that suppress the leakage of magnetic flux from surfaces
increase in the vicinity of 1.7 T. These closure domains begin to
disappear at 1.7 T and above, and magnetostriction increases
accordingly. The present inventors consider that, since the
variation of closure domains is large in test sample 1, the
magnetostriction increases sharply, and that creates higher
harmonic components and affects LvA. Such a sharp change causes the
generation of noise having higher frequency components and
undesirably raises the whole noise level.
[0041] On the other hand, there is no sharp change of
magnetostriction in test sample 1, and the magnetostriction
waveform increases smoothly in the whole range of magnetic flux
density. It is considered that the reason for this smooth increase
is because the magnetostriction increases gradually from a low
magnetic flux density since the generation of closure domains is
relatively low.
[0042] FIG. 4 shows the relationship between film tensions and LvA.
LvA lowers as a film tension increases, reaches the minimum point
at 2.0 MPa and then increases again. Here, the reason for measuring
at 1.7 T is that closure domains are easily formed at this magnetic
flux density and .lambda..sub.B tends to be negative. Another
reason is that the magnetic flux density used for the design of
transformers is in this vicinity.
[0043] As a product having a film tension slightly lower than
conventional products, a grain-oriented electrical steel sheet
having an insulating film coated on a steel sheet was produced
using a process where a glass film was not formed. (Y. Yoshitomi,
O. Tanaka, et al: Ultrathick glassless high-permeability,
grain-oriented silicon steel sheets with high workability. JMMM,
160, (1996), 123). This product has a coated film equal to
conventional products (8 MPa) and its tension is higher than the
tension claimed in the present invention. Further, in U.S. Pat. No.
5,961,744, disclosed is a steel sheet coated with an insulating
film, which is the same as that coated on a conventional
grain-oriented electrical steel sheet, on a material having no
glass film thereon. However, these materials are for iron cores of
turbine generators and the like, claim a film thickness of 2.5.mu.
m or more (=8 MPa or more), and are intended to improve punching
property and suppress shearing burr but not to lower
magnetostriction.
[0044] From the above viewpoints, the present inventors considered
that the noise of electrical apparatuses such as transformers, etc.
could be lowered effectively by providing a grain-oriented
electrical steel sheet having a smooth magnetostriction waveform
with moderate steepness realized by providing an appropriate film
tension to reduce the higher harmonics, which were, in
magnetostriction, highly influential on noise. Thus the present
inventors realized the present invention.
[0045] Next, reasons for specifying the present invention will be
explained hereunder.
[0046] In the present invention, the tension imposed on a steel
sheet by a film is specified to be in the range from 0.5 to 4.0 MPa
based on the result in FIG. 4, to realize of moderate steepness in
an actual magnetostriction waveform in order for the material to
have excellent effects in lowering noise.
[0047] The reason for setting the lower limit at 0.5 MPa is that,
since steel sheets are bound together so as not to be loosened when
iron cores of transformers are fabricated and the compression force
thereof is 0.5 MPa or more, a tension capable of withstanding the
stress of this degree is required, otherwise strains will be
developed in the steel sheet by external stresses and
magnetostriction grows. Also, the reason for setting the upper
limit at 4.0 MPa is that, if the film tension exceeds 4.0 MPa, the
waveform smoothness will be lost and LvA will increase since steel
sheets contract and then stretch sharply toward saturation at 1.7
T. Further, the preferable range of a film tension is from 1.0 MPa
to 3.0 MPa.
[0048] The reason for limiting .nu..sub.19 to 1.5.times.10-6 or
less is that this limitation is essential for obtaining lower noise
than before in high magnetic fields.
[0049] The reason for setting a sheet thickness at 0.27 mm or more
is that many of transformers are fabricated in this thickness
range, and the aforementioned conditions have to be met at this
sheet thickness range in order to minimize noise.
[0050] According to the studies by the present inventors, the
reason why a magnetostriction waveform can be controlled by a film
tension as mentioned above is as follows. Closure domains dissipate
in a demagnetization state due to the inverse effect of
magnetostriction when a film tension is applied. The amount of
dissipation is approximately proportional to the intensity of the
film tension. These magnetic domains begin to appear when a steel
sheet is magnetized at up to a magnetic flux density of about 1.7
T, and dissipate at or above said magnetic flux density.
Accordingly, by appropriately controlling this tension, the
magnetostriction waveform can be controlled and a smooth waveform
can be developed depending on conditions.
Example 1
[0051] A grain-oriented electrical steel sheet produced by a
conventional method having the thickness of 0.30 mm was coated in
five different coating weight so that the imposed tensions fall
within the range from 0 to 7.0 MPa. The magnetostriction of each of
these five test samples when they were magnetized at 1.4 T, 1.7 T
and 1.9 T was measured by non-contact type magnetostriction
measuring device using a laser Doppler system. The results are
shown in Table 1.
[0052] Three-phase transformers of 500 kVA were fabricated using
the test sample D which conforms to the waveform conditions of the
present invention and test samples A and E which do not conform
thereto, all of which were chosen from among these test samples,
and the noise when they were magnetized at 50 Hz and 1.5 T was
measured. The results are shown in Table 2.
[0053] As for the transformer fabricated with the material
satisfying the conditions of the present invention, the noise could
be lowered.
1TABLE 1 Test Film sample tension .lambda..sub.14 .lambda..sub.17
.lambda..sub.19 No. (MPa) (x 10.sup.-6) (x 10.sup.-6) (x 10.sup.-6)
Remarks A 0 0.05 1.52 4.82 Comparative example B 0.7 0.26 0.43 1.46
Invented example C 1.5 0.03 0.43 0.95 Invented example D 2.1 0.07
0.13 0.42 Invented example E 7.0 -0.52 -1.13 0.54 Comparative
example
[0054]
2 TABLE 2 Test sample No. Noise Remarks A 49.5 dB(A) Conventional
technology D 41.0 dB(A) Present invention E 47.8 dB(A) Conventional
technology
Example 2
[0055] A grain-oriented electrical steel sheet produced by a
conventional method having the thickness of 0.30 mm was coated in
five different coating weights so that the imposed tensions fall
within the range from 0.02 to 7.0 MPa.
[0056] The magnetostriction of each of these five test samples when
they were magnetized at 1.4 T, 1.7 T and 1.9 T was measured by a
non-contact type magnetostriction measuring device using a laser
Doppler system. The results are shown in Table 3.
[0057] Three-phase transformers of 500 kVA were fabricated using
test sample C which conforms to the waveform conditions of the
present invention and test samples A and E which do not conform
thereto, all of which were chosen from among these test samples,
and the noise when 10 they were magnetized at 50 Hz and 1.5 T was
measured. The results are shown in Table 4.
[0058] As for the transformer fabricated with the material
satisfying the conditions of the present invention, the noise could
be lowered.
3TABLE 3 Test Film sample tension .lambda..sub.14 .lambda..sub.17
.lambda..sub.19 No. (MPa) (x 10.sup.-6) (x 10.sup.-6) (x 10.sup.-6)
Remarks A 0.02 1.03 2.08 4.71 Comparative example B 0.7 0.13 0.55
1.24 Invented example C 1.4 0.03 0.04 0.34 Invented example D 2.5
0.28 0.30 0.86 Invented example E 7.0 -0.63 -1.28 0.43 Comparative
example
[0059]
4 TABLE 4 Test piece No. Noise Remarks A 47.2 dB(A) Conventional
technology C 39.8 dB(A) Present invention E 46.5 dB(A) Conventional
technology
* * * * *