JPH0536504A - High energy zinc oxide varistor - Google Patents

Abstract

PURPOSE: To provide a system based on infrared-measurement for predicting capability for handling energy of specified varistor disc. CONSTITUTION: Electrodes 150 and 151, extended to a terminal part at least, are provided on the counter faces of zinc oxide varistor disc 100 and between these electrodes, heat stress is reduced in a varistor comprising an insulating member on a cylindrical face 130 of this disc 100.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a disk-shaped zinc oxide-based varistor for high voltage or high current, and more particularly to improvement of its physical stability.

[0002]

2. Description of the Related Art A varistor based on zinc oxide is usually manufactured by a sintered disc made of zinc oxide and an additive, and the disc is a pair of electrodes facing each other. have. To avoid arching reductions called fringing currents or flashovers, what is typically done for electrodes on this surface is to extend the electrodes only part way up the periphery of the disc. For example, Tapan K.
U.S. Pat. No. 4,460,497 to Gupta et al., Eugene Sa
See US Pat. Nos. 4,451,815 and 4,450,426 to Kshaug et al. In addition, the perimeter is usually protected from electrical insulation by components or aspects of the manufacturing process. For example, US Pat. No. 4,3, John E. May et al.
71,860 and Steven P. Mitoff et al., No. 3,13.
See No. 8,686.

Zinc oxide varistors exhibit a non-linear current-voltage relationship which is considered to be I = C × V ^a . Here, “a” is greater than 1. In other words, the varistor acts as an insulator for low voltages and acts as a conductor for high voltages. Therefore, it can provide an overvoltage protection function, function as a voltage stabilizer, a surge absorber or an arrester, and overcome a current surge.

Due to the Joule heating, the inside of the disk reaches a high temperature, the peripheral edge of which remains at ambient temperature. This condition is exacerbated by the anti-arching design where the perimeter of the surface (margin) is left bare. The electric field, like temperature, drops rapidly near the periphery, resulting in a heat shock condition. This can result in physical damage to the edges and significant damage to the device.

The following US patents, referenced as typical prior art, disclose a means of preventing arching and minimizing this problem, namely extending the electrodes at least to the periphery. U.S. Patent No. Inventor 4,692,735 Moritaka Shogi et al. 4,423,404 Gary L. Goedde et al. 4,272,411 Theodrore O. Sokoly et al. In the '006 patent, the process of manufacturing and installing the insulation is complicated and has temperatures above 500 degrees which can destroy the varistor.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to have a more uniform Joule heating without the occurrence of arching between the surface electrodes, and thus to be more peripherally damaging, to a conventional zinc oxide varistor. It is to provide an improved zinc oxide varistor that is more stable. It is an object of the present invention to provide an infrared measurement based system for predicting the energy handling capacity of a particular varistor disc.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 and 2 illustrate a conventional zinc oxide varistor, generally indicated at 10. The varistor consists of a disc made of sintered zinc oxide and additives, which has a bulky interior 2
5 and the cylindrical surface 3 extending between the opposing surfaces 40 and 41
Has 0. A pair of electrodes 50 and 51 are attached to the opposing surfaces 40 and 41, but extend only partway through the cylindrical surface 30 to eliminate arching. Surrounding 60
Is the part of the surface 40 not covered by the electrodes.
Cylindrical surface 30 is typically covered with an insulator (not shown), usually a thin glass coating, to protect this surface from parts or aspects of the manufacturing process.

When the conventional varistor 10 is exposed to a high current for a single hour, the interior 25 of the disc has a substantially uniform electric field. If the surrounding area 60 is narrow, a strong electric field gradient is generated. Due to Joule heat, the interior 25 easily reaches temperatures up to 160 degrees above ambient temperature. Cylindrical surface 3
The area of the disk near 0 stays near ambient temperature as heat is rapidly dissipated to the surroundings. Moreover, this region has a much lower current. Due to the current slope and heat dissipation characteristics of the disc, a tight thermal slope may occur across the disc. These thermal gradients create heat shock conditions that can result in physical damage to the disc.

FIGS. 3 and 4 illustrate a known composition zinc oxide varistor, generally designated 100, according to the present invention. The varistor comprises a disk-shaped body 155 of sintered zinc oxide and additives, the disk having an outer cylindrical surface 130 and opposing surfaces 140 and 141. The pair of electrodes 150 and 151 has the opposite surface 1
It is attached to 40 and 141 and extends to the end of the outer cylindrical surface 130. A collar 160 having a high dielectric constant and high temperature insulation is then placed around the outer cylindrical surface 130. In the preferred embodiment, the collar 160 is a high temperature polymer.

The collar 160 can be formed from polyetherimide (ULTEM) which has a high dielectric strength of 33 KV / mm at 1.6 mm in air. Polyetherimide is well within the expected range of use 21
It can be used for applications at temperatures above 0 degrees. Metallizations 170 and 171 extend across the top and bottom electrodes 150 and 151, and the bonding of the polyetherimide collar 160 and the disk 110 can be accomplished by various techniques such as vacuum metallization, flame / arc spray. It can be formed by one. This metallization covers the electrodes (15
0, 151) are effectively stretched to generate a more uniform electric field gradient across the disc-shaped body 155. In other embodiments, the collar 160 is made of enamel, thermoplastic or a suitable polymer.

Polyetherimide has a higher coefficient of thermal expansion than zinc oxide, and when a polyetherimide collar is used, it fits nicely into the disc 100 when cooled and is in a stretched state, The disc 100 is compressed. FIG. 5 discloses another embodiment that differs from the embodiments described above in that the electrodes 140 and 141 do not extend outwardly beyond the cylindrical surface 130.

When the varistor 100 according to the present invention is exposed to a high current or a high voltage, a substantially uniform electric field extends inside the disk, and the current distribution is made uniform. This is due to the electrodes extending completely across the face of the disc. Problems due to high electric fields in the region near the outer cylindrical surface are reduced by the high dielectric constant properties of the collar. This results in more uniform heating of the varistor disc for a given current and heat dissipation, while at the same time reducing the arching problems associated with conventional electrodes extending to or beyond the cylindrical surface.

Therefore, the Joule heating is more evenly distributed through the disc, resulting in a lower thermal gradient across the disc. Reducing the thermal gradient makes it less susceptible to physical damage caused by thermal shock conditions similar to those in conventional varistors. The present invention further provides a method of determining the energy handling capacity of a disc, which is the ability to resist failure of a particular disc, prior to incorporating the varistor. This improves the performance of the finished varistor where discs with low unacceptable heat dissipation capacity are discarded.

It is known from both experiments and analysis that the varistor disc, which is most resistant to failure, exhibits axisymmetric Joule heating when a voltage is applied across its plane.
On the contrary, the varistor, which is the weakest against damage, exhibits an asymmetric temperature distribution during Joule heating. This distribution creates thermal stress that can damage the disc. The isotherms on the zinc oxide discs that undergo high energy pulses are similar to those seen after a relatively low energy input has been input for a long time.

FIG. 6 is a diagram showing a constant temperature pattern, which is thermometry across the surface of a typical varistor disc. Only three curves labeled "A", "B" and "C" are shown. However, more profiles can be used in many applications. These profiles are used to predict the stability of the finished varistor as described below.

FIG. 7 is a block diagram illustrating the process used to evaluate the thermal properties of a particular varistor disc. Each of the newly manufactured zinc oxide varistor discs 195 receives a low energy pulse. For example, when 27 kJ energy is rapidly applied, the average temperature of the varistor is about 6
Increase by 0 degrees C. The thermograph is recorded by an infrared camera recorder and converted into digital form. This data is analyzed by a digital computer to determine the thermal stress on the varistor disc.

Further, the thermal stress analysis involves calculating thermal stresses at various locations on the varistor disk, especially on and near the outer cylindrical surface, which is the area of maximum stress. All calculations in this thermal stress calculation process are accomplished directly from the thermal data described above. The calculations are accomplished in closed form, thus requiring very little computer memory and numerical results are immediately available. Once the thermal stresses near the outer cylindrical surface have been calculated, the energy handling capability process compares this data to the known physical properties of the material to determine the maximum energy handling capability of a particular disc. Discs with a heat capacity below the desired value are discarded. This allows a varistor of a given capacity to be manufactured with a smaller disc volume.

[Brief description of drawings]

FIG. 1 is a sectional view of a conventional zinc oxide varistor,

2 is a perspective view of the conventional zinc oxide varistor of FIG.

FIG. 3 is a sectional view of a zinc oxide varistor according to the present invention,

4 is a perspective view of the zinc oxide varistor shown in FIG.

FIG. 5 is a sectional view of a zinc oxide varistor according to a second embodiment of the present invention,

FIG. 6 shows isotherms of a typical varistor,

FIG. 7 is a block diagram showing a process of predicting thermal stability of a varistor disc.

[Explanation of symbols]

100 baristas 130 External cylindrical surface 140, 140 sides 150, 151 electrodes 160 colors

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Howard F Ellis             United States New York 12804               Queensbury Stonehurst             Drive 7

Claims (7)

[Claims] 1. A. A matrix composed of zinc oxide and other additives, comprising a cylindrical outer surface and first and second surfaces facing each other substantially parallel to each other; b. First and second electrodes attached to the first and second facing surfaces, respectively, and extending outward to at least end portions of the first and second facing surfaces; and c. A varistor with reduced thermal stress comprising an insulating member contacting the outer surface and extending between the electrodes. 2. A varistor with reduced thermal stress as set forth in claim 1 wherein said first and second electrodes extend across said cylindrical surface of said disc. 3. The varistor with reduced thermal stress of claim 2 further comprising a collar affixed to the outer cylindrical surface and a portion of the electrode extending beyond the outer cylindrical surface. 4. A. Sintering zinc oxide and other additives to form a varistor disc, b. Applying a predetermined pulse of electrical energy to the disc, c. Plotting the heat distribution of the energy of the disc, d. Calculating thermal stress in the disk based on the heat distribution, e. Select a varistor disc satisfying a predetermined stress state according to the result of the energy dissipation characteristic, f. An improved method for forming a varistor comprising the steps of attaching electrodes to opposite surfaces of a varistor disc satisfying predetermined dissipation characteristics. 5. The method of claim 4 including the step of selecting the diameter of said varistor such that the diameter of said electrode is at least equal to the diameter of said disc. 6. The method of forming a varistor according to claim 5, wherein the electrode is selected to have a diameter larger than the diameter of the disc. 7. The method further comprising affixing a collar to the outer diameter of the disc, the collar extending between portions of the electrode extending beyond the ends of the disc. A method for forming the described varistor.