Method of annealing a core

A method of annealing a core comprises the following steps. First annealing coils are wound around the legs or yokes of a core or parallel-arranged cores made of an amorphous magnetic alloy. Secondly, the core is excited by flowing an alternating current through the annealing coils. Thirdly, the core interior is uniformly heated to an annealing temperature of amorphous magnetic alloy plates by heat generated due to iron loss.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of annealing a core of an 
amorphous magnetic alloy used for a transformer or the like. 
2. Discussion of Background 
The use of an amorphous magnetic alloy, which chiefly contain metals (Fe, 
Co, Ni) and semimetals (B, C, Si, P) and are rapidly quenched in a wound 
or laminated core of a transformer, has been studied. A plate made of this 
alloy has an iron loss 1/3 to 1/4 that of a silicon steel plate, which is 
the conventional core material when no strain is present in the material. 
This alloy, therefore, has excellent magnetic characteristics. 
However, since strain is produced in an amorphous magnetic alloy plate upon 
quenching, iron loss in this state is large and magnetic characteristics 
are thus considerably decreased. In order to obtain the original excellent 
characteristics, the amorphous magnetic alloy is annealed to remove strain 
after the core made of this alloy is assembled into a core, thus reducing 
iron loss. 
A core consisting of an amorphous magnetic alloy is annealed with an 
electric heating source. Referring to FIG. 1, a wound core 1 consisting of 
an amorphous magnetic alloy 2 and a DC magnetic field generating coil 3 
wound therearound is placed in a thermostat chamber 4 in which an inert 
gas is sealed so as to prevent oxidation of the core. Then, temperature of 
the interior of the chamber 4 is raised to a predetermined value upon 
flowing a DC current in the coil 3 from a DC power source (or battery) 5. 
Thus, the core 1 is heated to an annealing temperature and is maintained 
at this temperature for a predetermined period of time. Thereafter, the 
heater is turned off, and the core 1 is cooled while applying a DC 
magnetic field thereto. In this manner, annealing of the core 1 is 
completed. 
The annealing temperature and time vary in accordance with amorphous 
magnetic alloy materials. For example, in METGLAS2605S2 (chemical 
composition: Fe.sub.78 B.sub.13 Si.sub.9) available from Allied 
Corporation and considered the best transformer core material, the 
annealing temperature is selected to be 390.degree. C. to 410.degree. C. 
and time is approximately 2 hours. Thus, the annealing temperature range 
is fairly narrow, i.e., 400.degree. C..+-.10.degree. C. Other amorphous 
magnetic alloy materials also have a narrow annealing temperature range. 
However, in the above conventional method, since the core 1 is externally 
heated by radiation, heat at the outer surface of the core 1 cannot be 
sufficiently transmitted to the interior thereof with the result that the 
temperature distribution in the core is nonuniform. Therefore, it is 
difficult to maintain the temperatures both at the outer surface and in 
the core within the cited narrow range at the same time. For this reason, 
thermal stress arising from a temperature difference occurs in the plates 
2 of the core 1 and degrades the magnetic characteristics. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of annealing a 
core wherein the internal core temperature of an amorphous magnetic alloy 
can be increased to a uniform annealing temperature so that excellent 
magnetic characteristics can be imparted to the amorphous magnetic alloy. 
An annealing method of a core according to the present invention is 
performed in the following manner. An annealing coil is wound around a 
core made of an amorphous magnetic alloy. An AC current is flowed in the 
coil to excite the core, and the interior of the core is uniformly raised 
to an annealing temperature of the amorphous magnetic alloy by Joule heat 
generated by iron loss of the core. With this method, the availability of 
uniform increase of the internal temperature of the core to the annealing 
temperature improves the magnetic characteristics of the core. Since the 
core is not externally heated, the insulation of a transformer coil is not 
damaged even if the transformer coil is wound around the core. In this 
case, the transformer coil is wound around the core before annealing. 
Thus, a large external force is not applied to the core by winding it 
after annealing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described hereinafter using METGLAS2605S2 
available from Allied Corporation described above as an amorphous magnetic 
alloy. However, the amorphous magnetic alloy is not limited to the above, 
and other appropriate materials can be used. 
Assuming that the total heat generated due to iron loss of a core is 
accumulated in the core, the following equation can be established: 
EQU Q=CM(T1-T2) (1) 
where 
Q: heat required for annealing the core (kcal) 
C: specific heat of the amorphous magnetic alloy plate (kcal/kg .degree.C.) 
M: weight of the core (kg) 
T1: annealing temperature (.degree.C.) 
T2: room temperature (given as 20.degree. C.) 
According to equation (1), when 1 kg of the amorphous magnetic alloy of 
Allied Corporation having a specific heat of 0.11 kcal/kg..degree. C. is 
used, the heat Q required for raising its temperature to the annealing 
temperature of 400.degree. C. is 41.8 kcal. This value corresponds to 48.6 
W.h. For this reason, in order for the temperature of the core to be 
raised to the annealing temperature of 400.degree. C. in the same manner 
as in the conventional annealing method using a thermostat chamber, an 
iron loss of 25 W/kg is needed. 
According to experimental data shown in FIG. 2, when frequencies of AC 
currents flowing in the annealing coil are respectively set at 1 kHz, 2 
kHz, 3 kHz and 4 kHz under the condition in which time required for 
raising a room temperature to the annealing temperature takes 2 hours, 
magnetic fluxes generated in the core are 1.2 T (tesla), 1.0 T, 0.8 T and 
0.6 T. Therefore, when the magnetic flux is 1.0 T, the frequency of the AC 
current can be set at 2 kHz in order to raise the temperature of the core 
made of the amorphous magnetic alloy to 400.degree. C. within 2 hours. 
As shown in FIG. 3, time required for raising the core temperature to the 
annealing temperature of 400.degree. C. was experimentally obtained when 
the magnetic flux thereof was at 1.0 T. According to this experiment, it 
required about 100 minutes at a frequency of 2 kHz. At a frequency of 3 
kHZ, it took about 40 minutes. Furthermore, at a frequency of 4 kHz, it 
took about 15 minutes. 
From the above two experiments, even if a core made of the amorphous 
magnetic alloy having small iron loss is used, a magnetic flux density of 
1.0 T is applied to the core and the core temperature can be raised to the 
annealing temperature by an AC current of 2 kHz or higher. In addition, 
time required for raising the core temperature to the annealing 
temperature can be easily altered by selecting the frequency of the AC 
current as needed. 
However, in practice, the total generated heat cannot be accumulated in the 
core, and a portion thereof is radiated. Thus, the magnetic flux density 
and the AC current frequency must be selected in accordance with the 
radiation. Assuming a cubic core having the side length L, the heat 
generated in the core is proportional to the volume L.sup.3 of the core, 
and the radiation from the core is proportional to the surface area 
6.times.L.sup.2 of the core. It follows that, as the core size increases, 
the ratio of the radiation to the heat generated in the core reduces. 
Thus, the generated heat in the core is substantially equal to the heat Q 
accumulated in the core and the radiation can be ignored. 
In the first embodiment of the annealing method of the core shown in FIG. 
4, a pair of wound cores 11 made of the amorphous magnetic alloy 12 are 
arranged in series, and a transformer coil 13 is wound around two adjacent 
legs or yokes of the cores 11, i.e., inner legs thereof. Annealing coils 
14 are wound around the respective outer legs of the wound cores. The 
coils 14 are temporary ones which are used only for annealing the cores 11 
and are removed after the annealing. The coils 14 are selectively 
connected by a change switch 15 to a high frequency AC power source 16 and 
a DC power source 18 such that magnetic fluxes are applied to two adjacent 
inner legs of the wound cores 11 in opposite directions. A voltage of the 
AC power source 16 is adjusted by a voltage adjuster 17 such as a 
transformer. 
When a current flows in the annealing coils 14, a voltage corresponding to 
their turn number is applied thereto. For this reason, the turn number of 
the coils 14 is selected in accordance with the insulation breakdown 
voltage of the coils. For example, when the sectional area S of the wound 
core 11 is 100 cm.sup.2, the magnetic flux density Bm is 1.0 T, the 
frequency f is 2,000 Hz, and the applied maximum voltage is 1,000 V, the 
turn number N of the annealing coils 14 is about 12 which can be obtained 
from the following equation. 
EQU E=4.44.times.f.times.N.times.Bm.times.S.times.10.sup.-4. 
Note that wires coated with an inorganic insulator having a high breakdown 
voltage and thermal resistance (e.g., wires coated with ceramics) can be 
used for the annealing coil 14. 
From the experiment, the magnetizing force, which applied 1.0 T of the 
magnetic flux B to the unannealed wound core 11 having a sectional area of 
100 cm.sup.2, is about 350 AT/m. When the magnetic path of the wound core 
is 1 m, the exciting current is about 29 A. 
As apparent from the above description, the turn number and the current of 
the annealing coil 14 can be arbitrarily determined in accordance with the 
size, frequency, applied voltage, and length of the magnetic path of the 
wound core 11. 
The operation of the embodiment shown in FIG. 4 will be described 
hereinafter. The annealing coils 14 are connected to the high frequency AC 
power source 16 by the switch 15. The voltage of the power source 16 is 
adjusted to a predetermined value by the voltage adjuster 17, and the 
frequency of an AC current is selected to be 2 kHz or higher. The wound 
cores 11 are excited by the AC currents flowing through the annealing 
coils 14. Then, eddy currents flow in the coils 14, thus generating Joule 
heat due to iron loss in the wound cores 11. For this reason, the interior 
of each core 11 is uniformly heated and its temperature is raised. When 
the temperature of the inner portion of each core 11 reaches the annealing 
temperature of the amorphous magnetic alloy 12, i.e., 400.degree. C., the 
voltage from the power source 16 is adjusted by the adjuster 17 to 
maintain the temperature of the cores 11 at 400.degree. C. for a desired 
period of time, e.g., 30 minutes to 2 hours. Since the magnetic fluxes in 
the two adjacent legs of the two cores 11 are directed opposite to each 
other, a voltage is not induced in the transformer coil 13. 
After the cores 14 have been heated for the period of time as mentioned 
above, the annealing coils 14 are connected to the DC power source 18 by 
the change of the switch 15 and the wound cores 11 begin to be cooled with 
a DC magnetic field formed in the cores 11. 
Thus, the lower temperature of the outer atmosphere around the cores 11 
than that of their interior allows the cores 11 to be more rapidly cooled 
than with the conventional method, even if they are large. 
After the cores 11 have been cooled, the annealing coils 14 are removed 
from the cores 11, thus to complete the annealing of the wound cores 11. 
In order to prevent oxidation of the cores 11 during the process, annealing 
is preferably performed in the atmosphere containing an inert gas, i.e., 
nitrogen gas. 
FIG. 5 shows an embodiment for annealing a 3-phase 5-leg wound core. 
FIGS. 6 and 7 show an embodiment for annealing a 3-phase wound core, 
respectively. 
In both embodiments, transformer coils 13 are first wound around the 
adjacent legs of the paired wound core 11, and temporary wound coils 14 
are wound around the corresponding cores 11. The coils 14 are selectively 
connected to the AC power source 16 and the DC power source 18 such that 
the directions of the magnetic fluxes are opposite to each other in the 
adjacent legs of the paired core 11 which extends through the transformer 
coils 13. Note that a frequency, a current, the turn number of the 
annealing coil and an operation are the same as those of the embodiments 
in FIG. 3. 
In the above embodiments, the temperature distribution in the interior and 
on the surfaces of the wound cores can be kept uniform. For this reason, 
thermal stress will not substantially occur, and degradation of magnetic 
characteristics can be prevented. Since outer atmosphere of the wound 
cores are not heated, the transformer coils will not be damaged upon 
annealing. Furthermore, since the transformer coils need not be wound 
around the wound cores after annealing, the wound cores will not be 
damaged by such winding. 
In the third embodiment, the temporary wound coils are used as the 
annealing coils but transformer coils can also be used if the transformer 
has a low voltage. However, since a transformer with a high voltage has 
many turns, transformer coils cannot be used for annealing because the 
desired magnetic flux density cannot be obtained or because insulating 
breakdown may occur. 
When cooling the wound core, while applying a DC magnetic field, 
transformer coils can be used in place of temporary wound coils. 
The annealing method of the present invention, as described for a wound 
core, can also be applied to a laminated core made of an amorphous 
magnetic alloy.