Definitions

• the present invention
  relates to a mica tape for an insulated coil used in electrical equipment such as rotating electric machinery, and also relates to an insulated coil. More particularly, the present invention is concerned with a mica insulated coil formed by a vacuum-pressure impregnation.
  
• the insulated coil for use in high voltage rotating machinery
  is generally manufactured by steps of winding mica around a coil, impregnating the mica with a solventless thermosetting resin by a vacuum-pressure impregnation, and curing the resin with heat.
  
• a mica paper or an aramid fibrid mica paper
  has been used as the mica tape employed in manufacturing such an insulated coil.
  
• the mica paper
  is formed, by a paper-making process, from fine mica particles and water alone.
  
• the aramid fibrid mica paper
  (hereinafter, referred to as “aramid mica”) is formed, by mixing 5 ⁇ 3 wt % of aromatic polyamide pulpy particles (hereinafter, referred to as “fibrid”) in fine mica particles and subjecting the resultant mixture to a paper-making process, as described in Jpn. Pat. Appln. KOKOKU Publication Nos. 43-20421 and 1-47002.
  
• the aramid mica
  contains fibrid, which is present among fine mica particles. Since the fibrid takes hold of the fine mica particles, the resultant aramid mica insulated tape is excellent in mechanical strength, heat resistance, and impregnation properties, as compared to a mica paper tape containing no fibrid.
  
• the excellent insulation properties
  can be attained together with excellent heat resistance and impregnation properties by mixing the fibrid to the fine mica particles and subjecting the mixture to a paper-making process.
  
• the aramid mica
  has a problem in that it contains an aromatic polyamide.
  
• the aromatic polyamide
  is known to generate toxic hydrogen cyanide when it burns, since it contains nitrogen in its structural formula (see, for example, Proceedings of Electric & Electron Insulation Conf. 1987. VOL. 18th Page 181-187).
  
• the insulated coil
  when an insulated coil is manufactured by winding the aramid mica around a coil and impregnating the aramid mica with a solventless thermosetting resin in pressurized vacuum conditions, the insulated coil also generates the toxic hydrogen cyanide when it burns.
  
• the toxic hydrogen cyanide
  will be a significant problem to workmen’s health. This problem makes it difficult to design a pollution-free recovering process of the conductive material from the insulated coil in order to obtain a recycle resource.
  
• An object of the present invention
  is to provide a mica tape generating no toxic hydrogen cyanide when it burns, and having high heat resistance, high impregnation properties, and excellent insulation properties.
  
• Another object of the present invention
  is to provide an insulated coil formed by using the mica tape generating no toxic hydrogen cyanide when it burns, and having high heat resistance, high impregnation properties, and excellent insulation properties.
  
• a mica tape
  formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material having an average diameter of 0.1 to 20 ⁇ m.
  
• a mica tape
  formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material having an average length of 0.5 to 10 mm.
  
• a mica tape
  formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material, the surface of the fiber being treated with a surface active agent.
  
• an insulated coil
  comprising a conductive coil and a mica tape wound around the conductive coil, wherein the mica tape is formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material having an average diameter of 0.1 to 20 ⁇ m, and the mica tape is impregnated with a solventless thermosetting resin.
  
• an insulated coil
  comprising a conductive coil and a mica tape wound around the conductive coil, wherein the mica tape is formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material having an average length of 0.5 to 10 mm, and the mica tape is impregnated with a solventless thermosetting resin.
  
• an insulated coil
  comprising a conductive coil and a mica tape wound around the conductive coil, wherein the mica tape is formed by a paper-making process from a mixture of fine mica particles and at least one selected from the group consisting of a short fiber made of a nitrogen-free organic material and a short fiber made of an inorganic material, the surface of the short fiber is treated with a surface active agent, and the mica tape is impregnated with a solventless thermosetting resin.
  
• FIG. 1
  is a graph showing the impregnation properties of the insulated coil with a resin, according to examples of the present invention as compared to those of comparative examples;
  
• FIG. 2
  is a graph showing tan ⁇ – voltage properties of the insulated coil according to examples of the present invention as compared to those of comparative examples.
  
• the mica tape according to the present invention
  is characterized in that it is produced by mixing fine mica particles and a short fiber made of an inorganic material or a short fiber made of a nitrogen-free organic material, and subjecting the resultant mixture to a paper-making process.
  
• Examples of the short fiber made of an inorganic material, suitable for use in the mica tape of the present invention
  include short fibers made of E glass (electrical glass), S glass (high strength glass), silica glass, alumina, and the like.
  
• Suitable examples of the short fiber made of a nitrogen-free organic material
  include short fibers having an excellent heat resistance, made of polyethylene terephthalate, polyethylene naphtharate, polyether sulfon, polyphenylene ether, and the like.
  
• the diameter of these short fibers
  is preferably from 0.1 to 20 ⁇ m, and more preferably, from 1 to 10 ⁇ m.
  
• the diameter of the short fiber
  is less than 0.1 ⁇ m, the impregnation properties may be lowered.
  
• the diameter
  exceeds 20 ⁇ m, it may be difficult to produce a tape form since water dispersion of the fiber is low.
  
• the length of the short fiber
  is preferably from 0.5 to 10 mm, and more preferably, from 3 to 6 mm.
  
• the length of the short fiber
  is less than 0.5 mm, it may be difficult to form clearances between the fine mica particles in the mica tape.
  
• the length exceeds 10 mm
  water-dispersion of the fiber may be low, with the result that the fiber may be easily agglomerated.
  
• the amount of the short fiber
  is preferably from 0.5 to 20 wt %, and more preferably, from 3 to 15 wt % based on the mica paper.
  
• the amount of the short fiber
  is less than 0.5 wt %, it may be difficult to obtain excellent mechanical strength, heat resistance, and impregnation properties.
  
• the amount
  exceeds 20 wt %, the dispersion property of the short fiber in the fine mica articles may deteriorate, with the result that the short fiber may be easily agglomerated.
  
• the surface of the short fiber
  is preferably treated with a surface active agent. If the surface of the short fiber is treated with a surface active agent, susceptibility of the fiber to a resin is improved. As a result, the strength of adhesion interface between the short fiber and the impregnating resin is enhanced, thus improving electrical characteristics such as tan ⁇ (dielectric loss)-voltage properties of the resultant insulated coil.
  
• Examples of usable surface active agent
  include fluorine-base surface active agent having a perfluoroalkyl group, amine salt type cationic surface active agent, and the like.
  
• the mica tape of the present invention
  is manufactured as follows:
  
• a mica ore
  is calcinated at, for example, 700 to 1000° C., to remove foreign materials, and crushed into pieces of a predetermined size. Then, jet water is applied to the mica pieces, thereby obtaining fine mica particles. To the fine mica particles thus obtained, the aforementioned short fiber is added. The mixture is blended in water, leading to a mica-fiber dispersion solution.
  
• the dispersion solution
  is subjected to a paper-making process to make a paper on a cloth and dried to obtain a mica paper.
  
• a glass-fiber cloth
  serving as a backing material
  
• an adhesive agent
  is bonded with an adhesive agent.
  
• the mica tape
  is formed.
  
• the insulated coil
  is obtained as follows.
  
• the mica tape
  is wound around a coil formed of a conductive material in a predetermined number of times in a partly overlapped fashion.
  
• the coil wound with the mica tape
  is impregnated with an impregnating resin in pressurized vacuum conditions. Then, the impregnating resin is cured by heating it to 130 to 180° C. In this manner, the insulated coil is obtained.
  
• Examples of the impregnating resin for use in obtaining the insulated coil
  include an epibis-series epoxy resin, cycloaliphatic epoxy resin, and the like.
  
• the insulated coil of the present invention
  uses a mica tape produced from the mixture of fine mica particles and a short fiber made of an inorganic material, harmful hydrogen cyanide will not be generated when it burns. Furthermore, since the mica tape used in the present invention is obtained by a paper-making process from a mixture of the fine mica particles and a short fiber made of an inorganic material of 0.1-20 ⁇ m diameter and 0.5-10 mm length or a short fiber made of a nitrogen-free organic material, the short fiber pieces of the inorganic or the organic material are dispersed in the mica fiber, solution. As a result, clearances are formed between the fine mica particles, facilitating the permeation of the resin into the fine mica particles.
  
• Impregnation of the mica tape with the resin
  is thereby improved. Further improvements: reduction of impregnation time, impregnation with a high-viscosity resin, and permeation of a resin into a thick insulation layer for obtaining an insulated coil for high-voltage use, can be attained.
  
• the surface of the short fiber
  is treated with a surface active agent having good compatibility and dispersion properties, susceptibility of the short fiber to the impregnating resin is improved. As a result, the adhesion between the short fiber and the impregnating resin is enhanced, improving electrical characteristics of the insulated coil, such as tan ⁇ (dielectric loss)-voltage properties.
  
• fine mica particles
  are held by the short fiber made of an inorganic material having high heat resistance and high rigidity or the short fiber made of an organic material excellent in heat resistance and mechanical strength. Hence, even if heat deterioration takes place during operation, no blisters are produced in the insulation layer. The dielectric strength will not be lowered.
  
• mica tapes
  corresponding to Examples 1 to 4 were formed.
  
• a calcinated muscorite mica paper
  was used.
  
• the short fibers
  inorganic fibers and organic fibers were employed which had good water-dispersion properties and blending properties in the fine mica particles and had a 0.1-20 ⁇ m diameter and a 0.5-10 mm length.
  
• the mica tape of Example 1
  was obtained by mixing short E glass fiber pieces (3 parts by weight) having an average diameter of 6 ⁇ m and an average length of 3 mm-length and the hard calcinated muscorite mica paper (100 parts by weight), and subjecting the mixture to a paper-making process.
  
• the mica tape of Example 2
  was obtained in the same manner as in Example 1 except that the surface of the short E-glass fiber pieces was treated with a fluorine-base surface active agent having a perfluoroalkyl group (for example, F-177(trade name) manufactured by Dainippon Ink & Chemicals, Inc).
  
• a fluorine-base surface active agent having a perfluoroalkyl group
  for example, F-177(trade name) manufactured by Dainippon Ink & Chemicals, Inc.
  
• the mica tape of Example 3
  was obtained by mixing short fiber pieces made of polyethylene terephthalate (15 parts by weight) having an average diameter of 20 ⁇ m and an average length of 10 mm and a hard calcinated mica paper (100 parts by weight) and subjecting the mixture to a paper-making process.
  
• the mica tape of Example 4
  was obtained in the same manner as in Example 3 except that the surface of the short fiber pieces made of polyethylene terephthalate was treated with a fluorine-base surface active agent having a perfluoroalkyl group (for example, F-177(trade name) manufactured by Dainippon Ink & Chemicals, Inc).
  
• a fluorine-base surface active agent having a perfluoroalkyl group
  for example, F-177(trade name) manufactured by Dainippon Ink & Chemicals, Inc.
  
• an aramid mica tape
  was used which was obtained by a fibrid (5 parts by weight) in the mica paper (100 parts by weight).
  
• test method
  JIS C 2116-1992, 13
  
• dielectric strength
  test method: JIS C 2116-1992, 22
  
• impregnation properties
  test method: JIS c 2116-1992, 24.
  
• the test results
  are shown in Table 1.
  
• each of mica tapes
  (Examples 1-4 and Comparative Example 1-2) is wound around an aluminum rectangular bar (10 mm ⁇ 50 mm ⁇ 800 mm) in a half overlapped fashion so that the mica insulation layers of the individual bars had the same thickness. After the both ends of the wound mica tape were sealed, an electrode was provided at the center of the mica-tape wound bar.
  
• the mica tape
  was impregnated with an epibis-series epoxy resin having a viscosity of 0.6Pa.s in pressurized vacuum conditions. Thereafter, state of impregnation was checked by measuring an electrostatic capacity of the mica insulation layer.
  
• each of the mica tapes of Examples 1-4 and Comparative Examples 1-2
  was wound around an aluminum rectangular bar (10 mm ⁇ 50 mm ⁇ 800 mm) in a half overlapped fashion in a predetermined number of times.
  
• the mica tape wound bar
  was set on imitation slots.
  
• the mica-tape of the bar
  was impregnated with an epibis-series epoxy resin in pressurized vacuum conditions, the introduced resin was cured with heat. In this manner, each of insulated coils were obtained.
  
• the insulated coils thus obtained
  were checked for electrical insulation properties immediately after the formation and after being subjected to thermal degradation at 200° C. for 40 days.
  
• a break down voltage (BVD)
  was measured and appearance of the insulated coil were visually observed.
  
• BDV
  was measured in oil at a constant elevation rate of voltage (1 kV/sec). The results of the test are shown in Table 2 below.
  
• BVD values of the insulated coils wound with the mica tapes of Examples 1-4
  are superior to that obtained in Comparative Example 1 both at the initial time and after the thermal degradation. No blisters were observed on the insulated coil after the heat deterioration. Furthermore, tan ⁇ -voltage properties of the coils immediately after the formation were measured. The results are shown in FIG. 2.
  
• the insulated coils using the mica tapes of Examples 1-4
  exhibit lower values of tan ⁇ -voltage properties than those of Comparative Examples 1 and 2. This tendency was remarkable in the surface active agent treated insulated coils of Examples 2 and 4.
  
• mica insulation layers
  were cleaved out from the insulated coils using the mica tapes of Examples 1-4 and baked in an electrical furnace at intervals of 100° C. within the temperature range of 300° C. to 800° C.
  
• the gases generated at the time of baking
  were collected in a collection bottle and quantitatively analyzed with respect to hydrogen cyanide. As a result, no hydrogen cyanide was detected in any one of Examples.
  
• the mica tape
  is formed by mixing fine mica particles with a short fiber made of an inorganic material or a short fiber made of a nitrogen-free organic material, and subjecting the mixture to a paper-making process.
  
• the short inorganic fiber pieces or the short organic fiber pieces having a predetermined shape
  are dispersed among the fine mica particles, with the result that clearances among them are increased.
  
• the resin
  is thereby facilitated to permeate in the fine mica particles. Impregnation properties can be improved.
  
• the fine mica particles
  are held by the short fiber made of an inorganic material having high heat resistance and high rigidity or by the short organic fiber excellent in heat resistance and mechanical strength, no blisters are formed in the insulation layer even if thermal degradation takes place during operation. Therefore, the dielectric strength does not decrease.
  
• the insulated coil having excellent insulation properties
  can be provided.
  

Images (1)

Classifications

• • H—ELECTRICITY
    
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    
  • H02K—DYNAMO-ELECTRIC MACHINES
    
  • H02K3/00—Details of windings
    
  • H02K3/30—Windings characterised by the insulating material
    
    
    
• • H—ELECTRICITY
    
  • H01—ELECTRIC ELEMENTS
    
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    
  • H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
    
  • H01B3/02—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of inorganic substances
    
  • H01B3/04—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of inorganic substances mica
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
    
  • Y10T428/252—Glass or ceramic [i.e., fired or glazed clay, cement, etc.] [porcelain, quartz, etc.]
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/25—Web or sheet containing structurally defined element or component and including a second component containing structurally defined particles
    
  • Y10T428/254—Polymeric or resinous material
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    
  • Y10T428/2911—Mica flake
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    
  • Y10T428/2913—Rod, strand, filament or fiber
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    
  • Y10T428/2913—Rod, strand, filament or fiber
    
  • Y10T428/2933—Coated or with bond, impregnation or core
    
  • Y10T428/2938—Coating on discrete and individual rods, strands or filaments
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    
  • Y10T428/2982—Particulate matter [e.g., sphere, flake, etc.]
    
  • Y10T428/2991—Coated
    
  • Y10T428/2993—Silicic or refractory material containing [e.g., tungsten oxide, glass, cement, etc.]
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    
  • Y10T428/2982—Particulate matter [e.g., sphere, flake, etc.]
    
  • Y10T428/2991—Coated
    
  • Y10T428/2993—Silicic or refractory material containing [e.g., tungsten oxide, glass, cement, etc.]
    
  • Y10T428/2996—Glass particles or spheres
    
    
    
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    
  • Y10T428/00—Stock material or miscellaneous articles
    
  • Y10T428/31504—Composite [nonstructural laminate]
    
  • Y10T428/31942—Of aldehyde or ketone condensation product
    
    
    

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• Chemical & Material Sciences
  (AREA)
• Inorganic Chemistry
  (AREA)
• Engineering & Computer Science
  (AREA)
• Power Engineering
  (AREA)
• Insulating Bodies
  (AREA)
• Inorganic Insulating Materials
  (AREA)
• Insulating Of Coils
  (AREA)

Abstract

Description

                                  TABLE 1                                 
__________________________________________________________________________
                              Comparative                                 
                                    Comparative                           
  Property Example 1 Example 2 Example 3 Example 4 Example 1 Example      
__________________________________________________________________________
                                    2                                     
amount of fibrid                                                          
          0    0    0    0    0     5                                     
  (% by weight)                                                           
  amount of short fiber 3 3 15 15 0 0                                     
  (% by weight)                                                           
  tensile strength 113 115 109 112 110 118                                
  (N/10 mm width)                                                         
  dielectric strength 3.4 3.5 3.3 3.5 3.4 3.6                             
  (kV)                                                                    
  impregnation property 610 620 580 560 1850 680                          
  (second)                                                                
__________________________________________________________________________

                                  TABLE 2                                 
__________________________________________________________________________
                                   Comparative                            
                                         Comparative                      
  Property Example 1 Example 2 Example 3 Example 4 Example 1 Example      
__________________________________________________________________________
                                         2                                
initial time                                                              
        BDV (kV)                                                          
               56   58   55   57   49    55                               
  after deterioration BDV (kV) 51 53 50 52 40 49                          
   insulating layer no no no no blister no                                
   of coil blister blister blister blister  blister                       
__________________________________________________________________________

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US6153301A

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• 1997
  • 1997-10-21
    US
    US08/955,241
    patent/US6153301A/en
    not_active
    Expired – Lifetime
    
  • 1997-10-23
    EP
    EP97118448A
    patent/EP0915484B1/en
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    BR9705178A
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    IP Right Cessation

  • 1997-10-30
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    CN97122816A
    patent/CN1086500C/en
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• 1999
  • 1999-10-06
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    HK99104347A
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