Capacitor having a fuse and a weak member

A capacitor has a capacitor element including a metallized film and a case for hermetically containing the capacitor element. The capacitor element has a protective mechanism for protecting it against insulation breakdown at overvoltage or thermorunaway. The capacitor also has a safety device for keeping the capacitor safe at high internal pressure due to heating. Thus, the protective mechanism and the safety device operate surely, the capacitor is protected under various severe conditions, and the life of the capacitor becomes longer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a capacitor, and in particular to a 
capacitor for use in electric equipment. 
2. Description of the Prior Art 
A capacitor including a metallized film for use in electric equipment such 
as a washing machine comprises a capacitor element positioned in a casing 
filled with an insulator such as air, polybutene oil or a resin. The 
capacitor is required to be reliably used for a long time in electric 
equipment. The capacitor is also needed to be used safely. 
In a prior art capacitor including a metallized film, a capacitor element 
comprises small metallic electrodes formed on a dielectric film, and a 
fuse portion is provided in each small electrode as a protective mechanism 
from insulation breakdown. Thus, the capacitor element can be regarded as 
a group of small capacitors. When insulation breakdown happens between a 
pair of opposing small electrodes in the capacitor, the pair of the small 
electrodes related to the insulation breakdown is electrically separated 
at the fuse portion thereof from the remainder of the capacitor element, 
and the protective mechanism prevents insulation breakdown of the other 
small capacitors. Thus, the life of the capacitor becomes long. 
However, when the temperature increases to a high ambient temperature, the 
space between film layers in the capacitor element in the capacitor 
becomes narrower, and a start voltage of corona discharge becomes higher. 
Large insulation breakdown does not occur before high temperature 
breakdown, and only a breakdown current smaller than a minimum current 
needed to break the fuse portion flows at the high temperature breakdown. 
That is, the protective mechanism does not operate normally in this severe 
condition. Then, at thermal breakdown, the capacitor is heated without 
stop and thermorunaway happens, so that the capacitor is broken, which is 
finally accompanied by smoke or fire. 
As to breakdown at a portion of the dielectric material in a small 
capacitor, the protection mechanism operates normally because each small 
metallic electrode has a fuse. However, the protective mechanism does not 
protect the capacitor against anomalous contact between an end plane of 
spiraled metallized film with a sprayed metal formed thereon, and an 
anomaly of a lead wire connected between the capacitor element and an 
electrode. Therefore, heating is liable to occur at such anomalous 
portions, and once heated, thermorunaway happens and the capacitor is 
broken with accompanying smoke and fire. 
In another type of a prior art capacitor including a metallized film, a 
safety device is provided in the capacitor to prevent destruction of the 
capacitor when internal pressure increases due to anomalous heating caused 
by insulation breakdown, overcurrent or the like. For example, a bellows 
portion is provided in a case to reduce internal pressure. When the 
internal pressure increases, the bellows portion extends to reduce the 
internal pressure. Alternatively, a mechanically weak portion is provided 
to 10 break a lead wire supplying electrical energy to the capacitor 
element. When the internal pressure in the capacitor case increases, the 
mechanically weak portion disconnects the capacitor element from the 
external terminals of the capacitor. Then, power supply to the capacitor 
is shut off. 
In a capacitor having the safety device, insulation breakdown only at a 
portion in a capacitor element causes insulation breakdown of the entire 
capacitor chip, and gas generated at the insulation breakdown makes the 
safety device activated and the function as a capacitor is lost. Thus, the 
life of the capacitor becomes short. That is, it is not likely that the 
insulation breakdown remains partial so as to keep the remaining capacitor 
operating normally. The speed of gas generation depends largely on the 
dielectric material, applied voltage and temperature. The speed is very 
high for a capacitor for a high voltage, and the top plate of the case may 
be blown off from the case due to the high gas pressure. That is the 
capacitor has low safety. Especially, if the capacitor includes a 
capacitor element with a resin coating, the gas which has broken the resin 
expands in the case in a moment, and the top plate tends to be removed. 
Further, when the capacitor is used for a long time, if the case has a 
defect due to rust or the like, the case cannot be hermetically 
maintained. Then, even when an anomaly happens and the capacitor generates 
heat, the safety device does not operate normally because the internal 
pressure does not increase. Then, the capacitor is broken with 
accompanying smoke or fire. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a capacitor which is safe 
and has high reliability under severe conditions. 
Another object of the present invention is to provide a capacitor having a 
long life. 
A capacitor of the invention comprises a capacitor element comprising a 
metallized film placed hermetically in a casing. The metallized film 
comprises metallic electrodes, and each of the metallic electrodes 
comprises a fuse portion as a protective mechanism. Lead wires connect the 
capacitor element to two terminals provided at the casing. A safety device 
of bellows type, shut-off type or the like is provided which breaks the 
lead wires when internal pressure in the case increases. The safety device 
protects the capacitor under a high internal pressure due to heating by 
stopping power supply to the capacitor. The protective mechanism separates 
a portion of insulation breakdown due to overvoltage from the other 
portion of the capacitor element. Preferably, an entire surface of the 
capacitor element is coated with a resin layer. Then, the capacitor 
element is not subjected to a dangerous state upon insulation breakdown or 
the like. 
An advantage of the present invention is that the capacitor has higher 
reliability under severe conditions of insulation breakdown and 
thermorunaway. 
Another advantage of the present invention is that the capacitor has a 
longer life. 
A third advantage of the present invention is that the capacitor is safe at 
a high temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference characters designate 
like or corresponding parts throughout the several views, FIG. 1 shows a 
capacitor of an embodiment of the invention having a bellows type safety 
device. A case 2 and a top plate 4 comprise a hermetic casing, and the 
case 2 has a bellows section 6 near the top plate. Two external terminals 
8 are provided at the top plate 4. A capacitor element 10 spiraled around 
a core 11 is put in the casing (only a half of the element is shown for 
clarity). The capacitor element 10 has metal-sprayed layers 12 at two ends 
thereof, and lead wires 14 are connected between external terminals 8 and 
the metal-sprayed layers 12. The lead wire 14 has a mechanically weak 
portion 16 which comprises, for example, a lead wire having a flat section 
different from a circular section of the lead wire 14. When an internal 
pressure in the hermetic casing is increased, the bellows portion 6 is 
elongated and the mechanically weak portions 14 are broken to separate the 
external terminals 8 from the capacitor element 10 to prevent the supply 
of electrical energy. That is, the bellows portion 6 and the mechanically 
weak portion 16 operate as a safety device. The lead wires 14 including 
the mechanically weak portion 16 are passed through an insulating tube 18 
to protect it from anomalous contact with the metal-sprayed layers 12. 
Polybutene oil 20 is filled in the casing 2, 4. The capacitor fabricated 
as mentioned above has a rating of 200 V and 60 .mu.F. The capacitor is 
referred to as capacitor A. 
The capacitor element 10 comprises a metallized film 30. As shown in FIG. 
2A, a deposition metallic film made of aluminum is formed at a side of a 
polyethylene telephthalate film 32 and is divided into a plurality of 
small electrodes 34. Each divided electrode 34 has a pattern as shown in 
FIG. 2 having narrow fuse portions 36 near an end of the film 32, and they 
play the role of a protective mechanism. The electrodes 34 are not formed 
near the other end of the film 32. As shown in FIG. 2B, a deposition 
metallic film 38 made of aluminum is also formed at the other side of the 
film 32. This film and another polypropylene film (not shown) are 
spiraled. Then, a metal such as zinc is sprayed on two sides of the 
spiraled capacitor element 10 to form metal-sprayed layers 12, and lead 
wires 14 are connected thereto after heating for aging. Thus, the 
capacitor comprises a group of small capacitors. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated similarly to the capacitor A of the 
embodiment except that the capacitor element is positioned in a resin case 
instead of the case 2 having the bellows portion, and the resin case is 
filled with an epoxy resin. The capacitor B does not comprise a bellows 
type safety device. Further, another comparison example (referred to as 
capacitor C) is fabricated similarly except that the protective mechanism 
or fuse portions is not provided, but includes the above-mentioned safety 
device. 
In a second embodiment of the invention, a capacitor element 10 fabricated 
in the above-mentioned embodiment is positioned in a casing having a 
shut-off type safety device as shown in FIG. 3. As shown in FIG. 3, a case 
102 and a top plate 104 comprise a hermetic casing. Two external terminal 
108 are provided at the top plate 104. The capacitor element 10 is put in 
the case 102 (only a half thereof is shown for clarity). The capacitor 
element 10 has metal-sprayed layers 12 at two ends thereof. Lead wires 114 
are connected between external terminals 108 and the metal-sprayed layers 
12, and a connection portion of the lead wire 114 to the terminal 108 is 
provided as a mechanically weak portion 116. When an internal pressure in 
the casing is increased due to over-voltage or the like, the top plate 104 
is moved outside to open the mechanically weak portion 116 to separate the 
external terminals 108 from the capacitor element 10 to prevent supply of 
electrical energy. That is, the top plate 104 and the mechanically weak 
portion 116 operate as a shut-off type safety device. Polybutene oil 120 
is filled in the casing 102, 104. The capacitor fabricated as mentioned 
above has a rating of 200 V and 60 .mu.F. The capacitor is referred to as 
capacitor A. 
Further, a comparison example (referred to as capacitor C) is fabricated 
similarly, except that the protective mechanism is not provided, but 
includes the above-mentioned safety device. (A comparison example, having 
the same protective mechanism and fabricated similarly to the capacitor A 
of the embodiment except that the capacitor element is positioned in a 
resin case, is not fabricated in this embodiment because it corresponds to 
the capacitor B of the first embodiment. Capacitors B are not fabricated 
in some embodiments explained below for the same reason.) 
In a third embodiment of the invention, the capacitor element fabricated in 
the above-mentioned first embodiment comprising a metallized film is 
entirely coated with an epoxy resin, and it is positioned in the case 
having the bellows type safety device as shown in FIG. 1, and a polybutene 
oil is filled therein. The capacitor fabricated as mentioned above has a 
rating of 200 V and 60 .mu.F. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated similarly to the capacitor A of the 
first embodiment except that the capacitor element is positioned in a 
resin case (nit shown) without the bellows type safety device and that the 
case is filled with an epoxy resin. Further, another comparison example 
(referred to as capacitor C) is fabricated similarly except that the 
protective mechanism is not provided, but it includes the bellows type 
safety device. 
In a fourth embodiment of the invention, the capacitor element fabricated 
in the above-mentioned second embodiment having a metallized film is 
coated with an epoxy resin entirely, and it is positioned in the case 
having a shut-off type safety device as shown in FIG. 3, and polybutene 
oil is filled therein. The capacitor fabricated as mentioned above has a 
rating of 200 V and 60 .mu.F. The capacitor is referred to as capacitor A. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated Similarly to the capacitor A of the 
second embodiment except that the capacitor element is positioned in a 
resin case without the shut-off type safety device instead of the case 102 
and that the case is filled with an epoxy resin. Further, another 
comparison example (referred to as capacitor C) is fabricated similarly 
except that the protective mechanism is not provided, but it includes the 
shut-off type safety device shown in FIG. 3. 
In a fifth embodiment of the invention, a capacitor comprises multi layer 
type capacitor elements 210 (FIG. 4). The multi layer type capacitor 
elements 210 are fabricated in a known way. Deposition films of aluminum 
are formed at both sides of a polyethylene telephthalate film, and the 
deposition films formed at one of the sides has a pattern comprising 
segments of electrodes and having fuses portions similar to those 36 as 
shown in FIG. 2A as a protective mechanism. The film and another 
polypropylene film are layered to have a cylindrical shape. Then, it is 
cut into rings, and each of them is sprayed with a metal onto two sides 
thereof, and they are heated for aging. They are cut further to form 
capacitor elements 210, and lead wires 214 are connected thereto. Next, 
the multi layer type capacitor element 210 is positioned in a casing 
comprising a case 202 having a bellows type safety device 206 and a top 
plate 204 with terminals 208, and polybutene oil 220 is filled therein. 
The capacitor fabricated as mentioned above has a rating of 200 V and 60 
.mu.F, and it is referred to as capacitor A. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated similarly to the capacitor A of the 
fifth embodiment except that the capacitor element 210 is positioned in a 
resin case instead of the case 202 and the case is filled with an epoxy 
resin. The capacitor B does not comprise a bellows type safety device. 
Further, another comparison example (referred to as capacitor C) is 
fabricated similarly except that the protective mechanism is not provided, 
but it includes the bellows type safety device. 
In a sixth embodiment of the invention, the capacitor element 210 
fabricated in the above-mentioned fifth embodiment having a metallized 
film is positioned in a case having a shut-off type safety device as shown 
in FIG. 3, and polybutene oil is filled therein. The capacitor fabricated 
as mentioned above has a rating of 200 V and 60 .mu.F. 
Further, a comparison example (referred to as capacitor C) is fabricated 
similarly except that the protective mechanism is not provided, but it 
includes the shut-off type safety device. 
In a seventh embodiment of the invention, the capacitor element 210 
fabricated in the above-mentioned fifth embodiment having a metallized 
film is entirely coated with an epoxy resin, and it is positioned in the 
case having a bellows type safety device shown in FIG. 5, and polybutene 
oil is filled therein. The capacitor fabricated as mentioned above has a 
rating of 200 V and 60 .mu.F. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated similarly to the capacitor A of the 
fifth embodiment except that the capacitor element is positioned in a 
resin case without the bellows type safety device and the case is filled 
with an epoxy resin. Further, another comparison example (referred to as 
capacitor C) is fabricated similarly except that the protective mechanism 
is not provided, but it includes the bellows type safety device. 
In an eighth embodiment of the invention, the capacitor element 210 
fabricated in the above-mentioned fifth embodiment having a metallized 
film is coated with an epoxy resin entirely, and they are positioned in 
the case having the shut-off type safety device as shown in FIG. 3, and 
polybutene oil is filled therein. The capacitor fabricated as mentioned 
above has a rating of 200 V and 60 .mu.F. 
Next, a comparison example (referred to as capacitor B) having the same 
protective mechanism is fabricated similarly to the capacitor A of the 
fifth embodiment except that the capacitor element is positioned in a 
resin case without a shut-off type safety device and the case is filled 
with an epoxy resin. Further, another comparison example (referred to as 
capacitor C) is fabricated similarly except that the protective mechanism 
is not provided, but it includes the shut-off type safety device. 
Forced breakdown at an applied overvoltage is tested on the capacitors A 
and comparison capacitors B and C of the above-mentioned embodiments in a 
maximum rated temperature of +30.degree. C. under an elevated alternating 
voltage. As a representative example, FIG. 5 shows a result on ten samples 
of the capacitors A of the first embodiment and ten comparison 5 examples 
of each of the capacitors B and C for the first embodiment. The abscissa 
represents alternating voltage applied to the capacitor, and the ordinate 
represents a ratio of capacitors remained not broken. 
When the voltage is increased near 600 V, all the capacitors B having the 
protective mechanism with no safety device are broken accompanied with 
smoke because they are liable to be broken thermally. On the other hand, 
the capacitors A having the protective mechanism and the safety device of 
the bellows type and of the shut-off type of the invention remain without 
breakdown at a higher voltage. They are also broken, but without smoke due 
to the safety device. The comparison capacitors C having only the safety 
device are also broken, but without smoke because the safety device is 
used. The ratio of capacitors remaining that are not broken of the 
capacitors A is much larger than that of the comparison capacitors C 
having only the safety device. This is ascribed to the fact that even if 
insulation breakdown occurs at a pair of electrodes, it remains locally 
due to the protective mechanism for the capacitors A, while the entire 
capacitor of the comparison capacitors C is broken due to the safety 
device. 
Tests are performed at various temperatures on the three types of 
capacitors A, B and C for the eight embodiments. Table 1 shows a result on 
ten samples the capacitors A of the first embodiment and ten comparison 
examples of each of the capacitors B and C for the first embodiment. The 
data in Table 1 means a number of capacitors remain normal at test vs. the 
number of capacitors tested. At room temperature, all the capacitors 
operate normally, as expected. On the other hand, at high temperature 
(+30.degree. C.), the seven capacitors B having only the protective 
mechanism among ten are broken accompanied with smoke, while the 
capacitors A and C remain normal without breakdown. However, the safety 
device of the capacitors C does not operate if the case has a defect so as 
not to keep the casing hermetic, or all the capacitors C are broken. On 
the contrary, the capacitors A having the protective mechanism and the 
safety device operate normally at high temperatures and in a case having a 
defect. 
TABLE 1 
______________________________________ 
Test results 
Room High Case with 
temperature 
Temperature 
defect 
______________________________________ 
Capacitor B 
10/10 3/10 -- 
Capacitor C 
10/10 10/10 0/10 
Capacitor A 
10/10 10/10 10/10 
______________________________________ 
NB. The data in Table 1 means a number of normal capacitors/a number of 
capacitors tested. 
As explained above, the capacitors of the invention are more safe for 
overvoltage and thermorunaway because they have the protective mechanism 
and the safety device simultaneously, and synergistic advantages are 
observed. Even when a protective mechanism does not operate at a high 
ambient temperature or when breakdown occurs due to heating at the 
sprayed-metal layer, a power supply to the capacitor can be shut off due 
to an increase in internal pressure because the capacitor element is 
positioned in a case having a safety device. Thus, the capacitor is not 
broken with smoke or fire. 
Further, because the capacitor element comprises the protective mechanism, 
the capacitance decreases only due to insulation breakdown of one small 
capacitor, and the life of the capacitor can be lengthened without thermal 
breakdown of the entire capacitor. 
Even when the case has a defect after it has been used for a long time and 
it cannot be kept sealed, the capacitor has a long life because of the 
protective mechanism including small divided electrodes. 
The speed of gas generation is slow because insulation breakdown occurs at 
a small portion between a pair of small electrodes, while disconnecting 
the remaining part of the capacitor element electrically. Thus, a sharp 
increase in internal pressure in the casing can be prevented. Therefore, 
even if the capacitor is broken, it does not accompany fire or smoke. 
If the entire capacitor element is covered with a thermosetting resin, the 
capacitor does not become dangerous upon insulation breakdown. 
In the above-mentioned embodiments, the capacitor element comprises a 
metallized film of polyethylene telephthalate film and a polypropylene 
film. However, one of polycarbonate film, a polystyrene film, a 
polyethylene, paper and the like or a combination thereof may be used 
instead of polyethylene telephthalate film and polypropylene film. In the 
embodiment, both sides of the polyethylene telephthalate film are 
deposited with metal. However, a film may comprises a deposited metallic 
layer only on a side, and a pair of such films may be layer for 
capacitance. The metal to be deposited on a film may be zinc instead of 
aluminum. The capacitor element comprising a metallized film may be 
hardened with an urethane resin instead of epoxy resin. The capacitor 
element may have any form such as tubular, rectangular or flatness type. 
Though polybutene oil or epoxy resin is filled in the casing in the 
embodiments, the capacitor elements may be enclosed in a casing with air 
therein. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as included within the scope of the present invention as 
defined by the appended claims unless they depart therefrom.