Note: Descriptions are shown in the official language in which they were submitted.
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AMORPHOUS ALLOY WITH INCREASED OPERATING
INDUCTION
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation-in-part of U.S. Application Serial No.
08/796,011, filed February 5, 1997, entitled "Amorphous Alloy With
Increased Operating Induction".
BACKGROUND OF THE INVENTION
1. Field Of The Invention
l0 This invention relates to amorphous metallic transformer cores having
increased operating induction; and more particularly, to a magnetic field
annealing process that markedly increases the operating induction of large
transformer cores.
15 2. Description Of The Prior Art
Soft magnetic properties of amorphous metallic transformer core alloys
are developed as a result of annealing at suitable temperature and time in the
presence of a magnetic field. One of the purposes for such annealing is to
reduce the adverse effects of residual stresses which result from the rapid
20 cooling rate associated with amorphous alloy manufacturing processes.
Another purpose is to define the "magnetic easy axis" in the body being
annealed; i.e. to define a preferred orientation of magnetization which would
ensure low core loss and exciting power of the body being annealed.
Historically, such magnetic field annealing has been performed to minimize
25 the core loss of the annealed body, as disclosed U.S. Patents 4,116,728 and
4,528,481 for example. In addition to magnetic field annealing, annealing of
amorphous alloys while under tensile stress has also been shown to result in
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improved soft magnetic properties viz. U.S. Patents 4,053,331 and 4,053,332.
Sample configuration for tensile stress annealing has invariably been flat
strip.
The use of stress annealing in the production of large amorphous alloy
transformer cores is impracticable.
The two most important magnetic properties of a transformer core are
the core loss and exciting power of the core material. When magnetic cores of
annealed metallic glass are energized (i.e., magnetized by the application of
a
magnetic field) a certain amount of the input energy is consumed by the core
and is lost irrevocably as heat. This energy consumption is caused primarily
l0 by the energy required to align all the magnetic domains in the amorphous
metallic alloy in the direction of the field. This lost energy is referred to
as
core loss, and is represented quantitatively as the area circumscribed by the
B-
H loop generated during one complete magnetization cycle of the material.
The core loss is ordinarily reported iri units of W/kg, which actually
represents
the energy lost in one second by a kilogram of material under the reported
conditions of frequency, core induction level and temperature.
Core loss is affected by the annealing history of the amorphous metallic
alloy. Put simply, core loss depends upon whether the alloy is under-
annealed, optimally annealed or over-annealed. Under-annealed alloys have
2o residual, quenched-in stresses, require additional energy during
magnetization, and exhibit increased core loss and exciting power during
magnetic cycling. Over-annealed alloys are believed to exhibit maximum
atomic "packing" and/or can contain crystalline phases, the result of which is
a loss of ductility and/or inferior magnetic properties such as increased core
loss caused by increased resistance to movement of the magnetic domains.
Optimally annealed alloys exhibit a fine balance between ductility and
magnetic properties.
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It is difficult to achieve an optimally annealed condition in a large
transformer core, that is a core weighing from about 40 to 400 kg. The large
thermal mass of the core precludes uniform heating during the annealing
process. Specifically, the outer layers of a large core tend to become over-
annealed, whereas the interior sections of the core tend to become under-
annealed. Given these conditions, transformer manufacturers currently anneal
cores to minimize the core loss; but do not maximize the operating induction
of the core. With such processes, core loss values of less than 0.37 W/kg (60
Hz and 1.4 T) and operating induction ranging from about I .26 to 1.4 Tesla
are typically achieved.
Exciting power is the electrical energy required to produce a magnetic
field of sufficient strength to achieve in the metallic glass a given level of
induction (B). Exciting power is proportional to the required magnetic field
(H), and hence, to the electric current in the primary coil. An as-cast iron-
rich
amorphous metallic alloy exhibits a B-H loop which is somewhat sheared
over. During annealing, as-cast anisotropies and cast-in stresses are
relieved,
the B-H loop becomes more square and narrower relative to the as-cast loop
shape until it is optimally annealed. Upon over-annealing, the B-H loop tends
to broaden as a result of reduced tolerance to strain and, depending upon the
degree of over-annealing, existence of crystalline phases. Thus, as the
annealing process for a given alloy progresses from under-annealed to
optimally annealed to over-annealed, the value of the exciting power for a
given level of magnetization initially decreases, then reaches an optimum
(lowest) value, and thereafter increases. However, the annealing conditions
which produce an optimum (lowest) value of exciting power in an amorphous
metallic alloy do not coincide with the conditions which result in lowest core
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loss. As a result, amorphous metallic alloys, annealed to minimize core loss
do not exhibit optimal exciting power.
It should be apparent that optimum annealing conditions are different for
amorphous alloys of different compositions, and for each property required.
Consequently, an "optimum" anneal is generally recognized as that annealing
process which produces the best balance between the combination of
characteristics necessary for a given application. In the case of large
transformer core manufacture, the manufacturer determines a specific time
and temperature for annealing which are "optimum" for the alloy employed,
l0 and does not deviate from that time/temperature schedule.
SUMMARY OF THE INVENTION
The present invention provides a method for obtaining maximum
operating induction in a large transformer composed of magnetic amorphous
alloys. Generally stated, the magnetic amorphous alloy is annealed to
maximize operating induction, rather than to minimize core loss. The method
of the present invention minimizes exciting power, significantly reducing the
likelihood of "thermal runaway" at the higher operating induction. Utilization
of such higher operating induction, in turn, markedly decreases transformer
core size and, therefore, cost.
It has been surprisingly found that the operating induction of the core is
maximized when the core is annealed using a soak time significantly longer
than that required to minimize the core loss. Generally stated, the annealing
process comprises the steps of (a) heating the core in the presence of an
applied magnetic field to a peak temperature; {b) holding the core.at the peak
temperature in the presence of the magnetic field for a soak time at least SO%
longer than that required to minimize power loss thereof; and (c) cooling the
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core to a temperature about 100°C lower than the peak temperature at a
cooling rate ranging from about 0.1 to 10°C/min.
Also provided by the invention is a large magnetic amorphous metallic
alloy core having an exciting power less than 1 VA/kg when measured at 60
5 Hz and an operating induction ranging from 1.40 to 1.45 Tesla. Further
provided is a ferromagnetic amorphous metallic alloy core having a power
loss less then about 0.25 W/Kg.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood and further advantages will
become apparent when reference is had to the following detailed description
and the accompanying drawings, in which:
FIG. 1 a is a graph depicting core loss as a function of temperature, the
graph illustrating the core loss dependence of straight strip
laboratory samples on 2 hour isochronal anneals conducted in
a magnetic field at various temperatures;
FIG. lb is a graph depicting exciting power as a function of
temperature, the graph illustrating the exciting power
dependence of straight strip laboratory samples on 2 hour
isochronal anneals conducted in a magnetic field at various
temperatures;
FIG. 2a is a graph depicting core loss as a function of temperature, the
graph illustrating the core loss dependence of actual
transformer cores on 2 hour isochronal anneals conducted in
a magnetic field at various temperatures;
FIG. 2b is a graph depicting exciting power as a function of
temperature, the graph illustrating the exciting power
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dependence of actual transformer cores on 2 hour isochronal
anneals conducted in a magnetic field at various
temperatures;
FIG. 3 is a graph depicting exciting power as a function of induction,
the graph illustrating the induction level dependence of exciting
power straight strip samples annealed at three different
conditions;
FIG. 4 is a graph depicting exciting power as a function of test
temperature, the graph illustrating exciting power dependence
on test temperature for straight strip samples which have been
annealed using three different conditions;
FIG. 5 is a graph depicting exciting power as a function of soak time,
the graph illustrating the transformer core soak time dependence
of exciting power
1 ~ FIG. 6 is a graph depicting exciting power as a function of induction,
the graph illustrating the induction level dependence of exciting
power for actual transformer cores which have been annealed in
a magnetic field using different soak times.
DETAILED DESCRIPTION OF THE INVENTION
As used herein, the term "amorphous metallic alloys" means a metallic
alloy that substantially lacks any long range order and is characterized by X-
ray diffraction intensity maxima which are qualitatively similar to those
observed for liquids or inorganic oxide glasses.
As used herein, the term "strip" means a slender body, the transverse
dimensions of which are much smaller than its length. Strip thus includes
wire, ribbon, and sheet, all of regular or irregular cross-section.
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The term "annealing", as used throughout the specification and claims,
refers to the heating of a material, in the presence of a magnetic field for
example, in order to impart thermal energy which, in turn, allows the
development of useful properties . A variety of annealing techniques are
available for developing these properties.
As used herein, the term "straight strip" refers to the configuration of a
sample which is subjected to magnetic property measurements. The sample
may be truly tested as a straight strip, in which case its length is much
greater
than that of the field/sensing coils. Alternatively, a more reasonable sample
length can be used if the material under test is used as the fourth leg in a
simple transformer core. In either case, the material under test is in the
form
of a_straight strip.
The term "large magnetic core", as used herein, refers to a magnetic
component which is used in any number of electrical applications and devices
and which has a weight ranging from about 40 to 400 kg. A magnetic core is
usually constructed from magnetic strip or powder.
The term "peak temperature", as used herein, refers to the maximum
temperature reached by any portion of the transformer core during the
annealing cycle.
The term "soak time", as used herein, refers to the duration over which
a core is actually at the annealing temperature, and does not include core
heating and cooling times.
The terms "saturation induction" and "operating induction" refer to two
magnetic induction levels relevant to transformer core materials and the
operation thereof. Saturation induction is the maximum amount of-induction
available in a material. Operating induction is the amount of magnetic
induction used in the operation of a transformer core. For amorphous metallic
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alloys, saturation induction is determined by alloy chemistry and by
temperature. Saturation induction decreases as temperature is increased.
The operating induction of a magnetic material is determined by the
saturation induction. Transformers are designed to operate at magnetic
induction levels less than the saturation induction. The primary reason for
this
design requirement involves the permeability (fit) of the magnetic core
material. Permeability is defined as the ratio of the magnetic induction (B)
to
the magnetic field (H) required to drive the material to that induction; i.e.
p=B/H. Permeability decreases as the magnetic induction is increased to
levels approaching the saturation induction. If a transformer core is operated
at a magnetic induction too close to the saturation induction of the core
material, a disproportionally large magnetic field will be required to achieve
the additional magnetic induction. In transformers, magnetic field is applied
by passing electric current through the primary coil. Thus, a large increase
in
the required magnetic field necessitates a large increase in the current
through
the primary coil.
A large increase in the primary current of a transformer is undesirable
for a number of reasons. Large current variations through a single transformer
can degrade the quality of electric power through the neighboring electric
power grid. An increase in the primary current will also result in increased
Joule (IZR) heating within the primary coil. This electrical energy lost by
conversion to heat detracts from the efficiency of the transformer. In
addition,
excessive current will cause excessive heating of the primary coil, which can
lead to the physical deterioration and failure of the electrical insulation
used
within the coil. Failure of the electrical insulation will lead directly to
failure
of the transformer. The heat generated in the primary coil can also heat the
magnetic core of the transformer.
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The latter effect described above, heating of the magnetic core of the
transformer, can lead to a condition called "thermal runaway". As the
temperature of the magnetic core is increased, the saturation induction of the
magnetic material decreases. For a transformer performing at a fixed
operating induction, the thermally induced decrease in saturation induction
creates the same effect as an additional increase in the operating induction.
Additional electric current is drawn through the primary coil, creating
additional Joule heating. The temperature of the magnetic core of the
transformer is further increased, exacerbating the situation. This
uncontrolled
increase in transformer temperature associated with "thermal runaway" is
another common reason for failure of transformer cores in the field.
To avoid'these undesirable conditions, transformers are typically
designed such that the operating induction of the core under standard
conditions is no more than about 80 to 90% of the saturation induction of the
core material.
The present invention provides a method for annealing large magnetic
cores composed of amorphous metallic alloys that permits increased operating
induction and decreased exciting power without inducing thermal runaway. It
is desirable to operate a large magnetic core at as high an induction level as
possible so that the cross-section of the core can be minimized. That is, a
transformer core works on the basis of the number of lines of magnetic flux,
not on the flux density (induction). The ability to increase operating flux
density permits use of smaller magnetic core cross-sections, while utilizing a
given flux. Substantial benefits are thereby derived from manufacture of
magnetic core sizes that are smaller for transformers of given ratings.
As described hereinabove, the optimum annealing temperature and time
for amorphous metallic alloys presently used in transformer manufacture is a
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temperature in the range of 140°-100°C below the crystallization
temperature
of the alloy, for a time period ranging from 1.5-2.5 hours for minimized core
loss.
The dependence of magnetic core loss on annealing temperature for
5 straight strip samples of METLAS~ alloy 2605SA-1, after having been
annealed for 2 hours, is shown in Figure 1 a. At lower temperatures, core loss
is high because of insufficient annealing, which results in the magnetic easy
axis not being well-defined. In contrast, core loss is high at higher
temperatures because of the onset of crystallization in the amorphous metallic
10 alloy. The lowest core loss is seen to result at about 360°C for the
straight
strip samples. Figure 1 b shows the dependence of exciting power on
annealing temperature for straight strip samples of METLAS~ alloy. 2605SA-
l, after having been annealed for 2 hours. In this case, the optimum
(minimum) exciting power is seen to-result when annealing for 2 hours at
about 375°C. This difference in optimization temperatures is very
significant
because both technical and patent literature have taught the annealing of
amorphous metallic alloys to optimize core loss only, whereas the reason for
transformer core failure is high exciting power.
The data in Figures 2a and 2b are similar to those of Figures 1 a and 1 b,
except that they now pertain to magnetic cores for electric power utility
transformers. It is significant that the benefit of annealing straight strip
samples at higher temperatures are also realized for the these magnetic cores.
This demonstrates the commercial utility of the present invention.
Another way in which the results of the present invention can be
illustrated is given in Figure 3. The curves in Figure 3 show the induction
level dependence of exciting power for straight strip samples which were
annealed according to the times and temperatures indicated. The benefits of a
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higher temperature anneal are clear. For example, if a given exciting power
level is chosen, a higher operating induction can be used for samples which
have been annealed at higher temperature. The data in Figure 3 indicates that
as much as a 5% increase in operating induction could be realized.
A further advantage of the present invention is illustrated in Figure 4, in
which the dependence of straight strip sample exciting power on sample test
temperature is shown. It is readily apparent from Figure 4 that the benefits
derived from the invention are greater at higher sample temperature. This is
important because transformers operate at temperatures greater than ambient
and can achieve even higher temperatures when going into an overload
condition. Thus, the teachings of the invention have a particularly useful
benefit.
Annealing is a time/temperature process. As such, Figure 5 shows the
dependence of exciting power on "soak time" during annealing of a magnetic
core. It is significant that, again, exciting power decreases with increased
soak time. This illustrates the option of using either annealing cycle soak
time
or temperature to develop the method of the present invention on a
commercial scale. As Figure 3, Figure 6 shows the dependence of magnetic
core exciting power on induction for cores which have been annealed using
different soak times.
EXAMPLE 1
Sixteen single phase wound magnetic cores for use in commercial
distribution transformers were made using 6.7" wide METGLAS~ alloys SA
1, having a nominal chemistry Feg°B"Si9. Each core weighed about 75 kg.
These sixteen cores were broken into groups of four, each group being
annealed at about 355°C with a different soak time. The baseline anneal
soak
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time, to achieve minimum core loss, is about 20 minutes. The three other
groups were annealed using soak times of 30, 40, and 60 minutes, which soak
times represented an increase of SO%, 100% and 1 SO%, respectively. Results
of for all of these cores have already been shown in Figures S and 6. A
S significant decrease in exciting power was evident for each of the increased
soak times. Further, it was found that longer soak times resulted in lower
exciting power.
EXAMPLE 2
Three single phase wound magnetic cores for use in commercial
distribution transformers were made using 6.7" wide METGLAS~ alloy SA-1,
having a nominal chemistry FegoB"Si9. Each core weighed about 118 kg, and
care was taken to minimize thermal gradient effects in the cores during heat-
up and cool-down. These three cores were annealed using a soak time of 20
IS minutes and a peak temperature of about 370°C rather than the
normally used
peak temperature of about 3SS°C. The results of exciting power and core
loss
measurements on these cores, which were annealed at higher temperature, are
shown in comparison to those of cores which have been annealed
conventionally in Figure 2a and 2b, respectively. It is clear that a
substantial
decrease in exciting power is realized when the peak temperature used during
anneal of the core is increased, while only incurring a small increase in core
loss. The results of Example 2, produced by annealing at increased peak
temperature, are comparable to those produced in Example 1 by annealing for
extended soak times.
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EXAMPLE 3
Straight strip laboratory samples were made using 6.7" wide
METGLAS"~' alloy SA-l, having a nominal chemistry Fe8aB"Si9. These
straight strip samples were subjected to two hour isochronal anneals
conducted in a magnetic field at various temperatures. The results of exciting
power and core loss measurements on these straight strip laboratory samples
are depicted as a function of temperature in Figures 1 a and 1 b. It is clear
that
a substantial decrease in exciting power is realized when the peak temperature
of the anneal is increased by at least 5°C.
EXAMPLE 4
Straight strip laboratory samples were made using 6.7" wide
METGLAS~ alloy SA-1, having a nominal chemistry Fe8oB,iSi9, These
straight strip samples were subjected to two hour isochronal anneals
conducted in a magnetic field at various temperatures. Figure 4 shows the
exciting power measured at the temperature indicated, after having been
annealed. The results indicate an even greater exciting power reduction at
elevated temperatures, at which transformer cores operate, than at room
temperature.
Having thus described the invention in rather full detail, it will be
understood that such detail need not be strictly adhered to, but that various
changes and modifications may suggest themselves to one skilled in the art,
all falling within the scope of the invention, as defined by the subjoined
claims.
