CA1059848A - Method of insulating electromagnetic coils - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A method of insulating electromagnetic coils comprising coating an electromagnetic coil with epoxy resin containing a cross-linking agent consisting essentially of acid anhydride, said coil having a bonding layer consisting of epoxy resin having a melting point of higher than 60°C, and then subjecting to bridging said epoxy resin containing a cross linking agent by heating.

Description

BACKGROUND OF THE INVENTION:
Electromagnetic coils of electrical machinery and appa-ratus such as transformer are made of covered wires. The covered wires are prepared by coating copper or aluminum wires with insulating resins such as polyimide, polyester, polyesterimide, polyurethane, epoxy resin, nylon resin and the like. In order to ; improve the dielectric strength and the heat resistance of the electromagnetic coils, the coils are coated with epoxy resin, polyester resin or acrylic resin. Such coating is effected by stic]sing powders of the resin to the heated coils and then fixing the resin to the coils. Alternatively, said coating is effected by allowing to adhere electrostatically charged powders to the coils and then fixing the resin to the coil by fusing the resins.
In the accompanying drawings, Figure 1 shows a cross section of a coil coated with powdered resin; and Figures 2 and 3 show cross sections of coils coated by methods of the present invention.
Referring first to Figure l, the numbers 1, 2, 3 . . . 20 represent covered wires and the order of windings of the wires. ~ ~

The coil is coated with an insulating layer 21 of resin, which is ~ -:' fixed to a surface of the coil. In this coil, voids 22 are formed.
Owing to the voids, the insulating layer of coating is deteri-; orated by using repeatedly the electrical apparatus, and the dielectric strength thereof deteriorates.
As high voltages are generated in the windings 1, 10, 11, ; 20 and 5, 6, 15, 16, high dielectric strength of the coil is required. In the coil coated with the conventional method as stated above, discharge is apt to occur on the windings on account of the voids. ~-It is an object of the present invention to provide a method of insulating electromagnetic coils by which the coils

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having high dielectric s-tren~th and heat resistance can be ob-tained.
~UMM~RY OF THE IN~ENTION:

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The present invention relates to a me-thod of insulating electromagnetic coils of electrical machinery and apparatus, and more particularly to a method of insulating electromagnetic coils by which hi~h dielectric strength and heat resistance o~ the coils can be obtained, said coils having been made of a covered wire coated with conventional insulating resins.
In one aspect of this invention there ls provided a method for forming an insulated electromagnetic coil. According to the method, an electromagnetic coil is first formed (i) by winding a self-bonding wire comprising a covered wire and a ;~
bonding layer on the covered wire, said layer being made of epoxy resin having a melting point higher than 60C, or (ii) by forming an electromagnetic coil from the covered wire and coating said electromagnetic coil with the bonding layer. The electromagnetic coil is then coated with a composition consisting of a powdered epoxy resin and a cross-linking agent consisting of an acid ~0 anhydride. Finally, the coated electromagnetic coil is heated to ; cross-link the powdered epoxy resin. The heating step is pre-ferably conducted at a temperature of 150 - 250C.
As stated above, the method of the present invention is characterized in that epoxy resin having a melting point of higher - than 60C and powdered epoxy resin containing a cross linking agent consisting essentially of acid anhydride are used and the powdered epoxy resin is subjected to bridging by heating.
More particularly, there is provided a method for forming an insulated electromagnetic coil which comprises~
(a) forming an electromagnetic coil by (i) winding a self-bonding wire comprising a covered wire and a bonding layer on the ,J ~ ~.
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covered wire, said layer being made of epoxy resin having a melting point higher than 60C or (ii) by forming an electromag-netic coil from the covered wire and coating said electromagnetic -coil with the bonding layer;
(b) coating said electromagnetic coil with a composition con-~-~ sisting of a powdered epoxy resin and a cross-linking agent con-sisting of an acid anhydride; and (c) heating said coated electromagnetic coil to cross-link the powdered epoxy resin.

There is also provided a method for forming an insulated electromagnetic coil which comprises: -(a) forming an electromagnetic coil by winding a self-bonding wire comprising a covered wire and a bonding layer on the covered wire, said layer being made of epoxy resin having a melting point ' higher than 60;
` (b) coating said electromagnetic coil with a composition con-sisting of a powdered epoxy resin and a cross-linking agent con-sisting of an acid anhydride; and ;`~
(c) heating said coated electromagnetic coil to cross-link the ,~
20 powdered epoxy resin. ~ ;
There is further provided a method for ~orminy an insu-lated electromagnetic coil which comprises~
(a) forming an elec:tromagnetic coil from a covered wire;
(b) coating said electromagnetic coil with a bonding layer of `
epoxy resin having a melting point higher than 60C;
~` (c) coating said coil coated in step (b) with a composition con-sisting of a powdered epoxy resin and a cross-linking agent con-sisting of an acid anhydride; and (d) heating said coated coil of step (c) to cross-link the powdered epoxy resin.

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By a method of the present invention, high dielectric strength and high heat resistance of an elec-tromagnetic coil can be obtained. In the coi] having an insulating layer of epoxy resin of about 0.3mm in thickness, the dielectric strength of 8kV
and the heat resistance tempera-ture of 170C can be obtained.
The coils shown in Figure 2 have been formed in accord-ance wi-th the method descrihed in the above aspect of this inven-.~
tion in which in the electromagnetic coil forming step thealternative (i) was employed, whereas Figure 3 shows coils which were formed by the same method but the alternative (ii) was employed. -- Voids as presented in Figure 1 cannot be seen in Figures :~ 2 and 3. A bonding layer 23 of epoxy resin having a melting point of higher than 60C is formed between covered wires and a layer 21 ; of epoxy resin containiny a cross linking agent.
Epoxy resin used in the present invention is bisphenol : ~
~;~ A diglycidyl ether (i.e. 2,2-bis(4'-glycidyloxyphenyl)propane) having the following structural formula~

C\2/C~I-CH2 ~ O - ~ bC33 O-C~2-CI~l-C~

` CH
- O ~ 1 ~ O-CH2-cH-/ H-wherein n is zero to 12.
At least one of epoxy resins (1)-(4) listed below may be added to bisphenol A diglycidyl ether.

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(1) Novolak-type epoxy resin f 2 o If~12 ~ o fH >

: CH2 fH2 1 2 O O O

` ~ CH2 _ ~ CH2 R R R
. wherein R is hydrogen or an alkyl group of Cl to C4 and n iS 1 ~: 10 or 2.
(2) hydantoin-type epoxy resin O=C fH2 CH2-~CH~CH2-N~ /N-CH2 C\/cH2 , O .. `,;

(3) alicyclic epoxy resin `::
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CH2-C\ o~CHb~CH2 .` 20 CH3 H3C

(4) epoxidized oil O /0\
CH2-o-c-(cH2)n-cH Cff (CH2)n 3 ~; CH -O-C-(CH2)n-CH-CH-(CH2)n-CH
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_o-c-(CH2)n~CH~CH (CH2)n C 3 wherein n is 1 to 5.
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In the method of the presen-t invention, bisphenol ~
diglycidyl ether is used as the chieE ingredient. Epoxy resin having a melting point of higher than 60C is bisphenol A digly~
cidyl ether having a value of n of higher than 1.5. Such epoxy resin (bisphenol A diglycidyl ether) having n of 1.5 may be obtained, for example, by mixing epoxy resin having n of 1 and epoxy resin having n of 2 in a ratio of 1 to 1. Epoxy resin having desired value of n may be obtained by mixing epoxy resins having different values of n. Accordingly, bisphenol A diglycidyl ether having desired melting point may be obtained by mixing epoxy resins having different value of n. Other epoxy resins listed in (1) to (4) above optionally may be added to bisphenol A diglycidyl ether to obtain epoxy resin having a melting point of higher than 60C.
The reasons by epoxy resin having a melting point of higher than 60C must be used, are as follows:
Epoxy resins having a melting point of l~wer than 60C
are soft and sticky at room temperatures, and wires coated with such epoxy resins are stuck to each other in the winding opera~
tion of the wires, and further a coil having a layer of such epoxy resin is inferior in the heat resistance.
Epoxy resins which are used by mixing with cross linking agents are bisphenol A diglycidyl ether having the value of n of zero to 12. Epoxy resins listed in (1) to (4) above may be added in an amount of 1 to 50 parts by weight based on 100 parts by weight of bisphenol A diglycidyl ether.
Typical acid anhydrides which may be used as a cross ;~
linking agent are as follows:
maleic anhydride, phthalic anhydride, succinic anhydride, citraconic anhydride, itaconic anhydride, tricarballylic anhy-dride, linoleic acid adduct of maleic anhydride, maleic anhydride :-adduct of methylcyclopentadiene, pyromellitic dianhydride, ;~

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C ,~ o -o ~ cycropentanetetracarboxylic dianhydride, ~ ~ I
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benzophenonetetracarboxylic dianhydride, O o . ~

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ethylene glycol bistrimellitate ~ .
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O-c~2-C~2-o-c ~ ~

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: As a cross linking agent, pyromellitic dianhydride may preferably be used. Eowever, acid anhydrides as listed above "!`' ~ ?~:
also may be used alone or in a mixture thereof.
`~ Other cross linking agents as listed below may be added - to acid anhydrides:
: 30 dicyandiamide, diethylenetriamine, diaminodiphenylsulfone, 2-methylimidazole, 2-ethyl-4-methylimidazole and 1-benzyl-2 methylimidazole.
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Cross linking ayents are used in an amount of 0.5 to 90 parts by weiyht basecl on 100 par-ts by weight of epoxy resins.
Inorganic matters (i.e. fillers) as listed below may be added to a mixture of ~poxy resins and cross linking agents in an amount of 0.5 to 70 parts by weight based on 100 parts by weight of the mixture:
silicon dioxide, titanium oxide, calcium carbonate, magnesium oxide, zirconium silicate, aluminum oxide, aluminum hydroxide, beryllium oxide, chrome dioxide, ferric oxide, clay~
talc, mica and glass fiber.
A method of the present invention may be effected as follows.
A wire is coated with epoxy resin having a melting point of higher than 60C, said wire being a covered wire with poly-imide, polyester or polyesterimide. A coil is made of the coated wire. A mixture of lOOg of epoxy resins and 5g to 90g of cross linking agents is stuck to the coil and then the coil is heated at a temperature of 150C to 250C to bridge the epoxy resin. In this way, the bridged epoxy resin is fixed to the coil.
An alternative method is as follows:
A coil is made of a covered wire as shown above, and the coil is coated with epoxy resin having a melting point of higher than 60C in a thickness of 5~ to 500~ A mixture of epoxy resin and cross linking agent is stuck to the coil and then - the coil is heated to fix epoxy resin to the coil by repeating the same procedure as described above.
' In the above-mentioned method, fillers may be added to the mixture of epoxy resins and cross linking agents, and the epoxy resin is principally bisphenol A diglycidyl ether and the :
cross linking agent consists essentially of acid anhydride.

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A covered wire or a coil may be coated with epoxy resin having a meltin~ poin-t of higher -than 60C by applying the molten epoxy resin to the wire or coil.
A coil may be coated with a mixture of powdered epoxy resin and powdered cross linking agent or powders of a mixture of epoxy resin and cross linking agent by Eluid bed technique or electrostatic fluid bed technique as illus-trated below: ;
Fluid bed technique:
A heated coil is dipped into powders suspended in air.
Said powders may be suspended by air blown up through a porous plate. Powders adhere -to -the coil and the powders are fixed to the coil by cooling. -Electrostatic fluid bed technique:
Electrostatically charged powders at high voltage are attracted to a coil, and the attracted powders are fixed to the ~;
coil by heating.
DESCRIPTION OF T~E PRE~ERRED EMBODIMENTS:
The following examples are given by way of illustration only and are not intended as limitation of this invention.
Example 1 A covered wire (diameter lmm) coated with polyesterimide ^ ?
was coated with bisphenol A diglycidyl ether (molecular weight 3800; melting point 160C) in a thickness of about 40~. A coil : having 2000 turns was made of the coated wire. A mixture of 100g of powdered bisphenol A diglycidyl ether (molecular weight 1500; ~-, melting point 105C), 10g of pyromellitic dianhydride and 100g of -silicon dioxide (particle size 10~ - 100,~) was stuck to the coil heated at a temperature of 205C by fluid bed technique. A ,`
` thickness of layer of the powders was 0.3mm. The coil having the 30 powders thus applied was heated at a temperature of 200C for ~;
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30 minutes to form an insulating layer on a surface of the coil.
A coil having an insulating layer as shown in Figure 2 was obtained.
The dielectric breakdown voltage of the insulating layer was 8kV and the heat resistance temperature thereof was 170C.
Comparison tests were carried out as follows~
(1) An insulating layer was formed on a surface of a coil by repeating the same procedure as described above except that a covered wire coated with polyesterimide was not coated with bis-10 phenol A diglycidyl ether (m.w. 3800; m.p. 160C). A coil havingan insulating layer as shown in Figure 1 was obtained. The di-electric breakdown voltage of the insulating layer thus obtained was 6kV and the heat resistance temperature thereof was 140C.
(2) An insu]ating layer was formed on a surface of a coil by repeating the same procedure as described above except that ; dicyandiamide, diethylenetriamine or diaminodiphenylsulfone was ~ ~
used instead of pyromellitic dianhydride as a cross linking agent. ~ -The dielectric breakdown voltage of the insulating layer was 6kV
and the heat resistance temperature thereof was 130C.
~ 20 Example 2 ; A coil having 2,000 turns was made of a covered wire ;~-(diameter lmm) coated with polyesterimide. The coil was coated ~
with bisphenol A diglycidyl ether (molecular weight 3800: melting ;
` point 160C) in a thickness of about 40~ A mixture of bisphenol "
A diglycidyl ether, pyromellitic dianhydride and silicon dioxide as used in Example 1 was stuck to the coil and an insulating j layer was formed on a surface of the coil by repeating the same procedure as described in Example 1. A coil having an insulating -~
~- layer as shown in Figure 3 was obtained.
`, 30 The dielectxic breakdown voltage of the insulating layer ::~
was 8kV and the heat resistance temperature thereof was 170C. ~ ~

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Example 3 An insulating layer was formed on a surface of a coil by repeating the same procedure as described in Example 1 except that 1 to 20% of bisphenol A diglycidyl ether (m.w. 1500, m.p. 105C) `;~
was substituted by novolak-type epoxy resin, hydantoin-type epoxy resin, alicyclic epoxy resin or epoxidized oil. The dielectric breakdown voltage of the insulating layer was 8kV and the heat resistance temperature thereof was 170C. The same results as those in Example 1 was obtained.
Example 4 An insulating layer was formed on a surface of a coil by repeating the same procedure as described in Example 1 except that ~;
cyclopentanetetracarboxylic dianhydride, benzophenonetetracar~
boxylic dianhydride, ethylene glycol bistrimellitate or other acid anhydride such as maleic anhydride was used instead of pyromel-litic dianhydride as a cross linking agent. The insulating layer thus formed showed the results similar to those in Example 1.
.
The formation of voids amonq covered wires by which a coil is made, can be prevented and therefore the dielectric strength and the heat resistance of the coil can be improved by coating the covered wires with epoxy resin having a melting point of higher than 60C and forming an insulating layer on a surface ;
of the coil. The insulating layer may be formed by sticking powders on a surface of the coil and heating it, said powders having been prepared by pulverizing a solid obtained by melting a ~ ;
mixture of epoxy resins and acid anhydrides as a cross linking ~-;~: :
agent. "~
~ method of the present invention as illustrated above is preferably used for insulating magnetic coils of transformers or leakage transformers. The leakage transformer has a leakage core between a high-tension coil and a low-tension coil, and -~
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:, ~' :' -' "' ' , ' '' , . .', . ,' ' ' :-' ' ':' " ' ': '- ~ . ' " ', '' ~': , partial discharcJe is apt to occur between the leakage core and ~ ., the high--tension coil. Such partial discharge can effec-tively be prevented by insulating the high-tension coil according to a method of the present invention. Further, the dielectric strength and the heat resistance of the low-tension coil can be improved by insulating the low-tension coil according -to a method of the present invention.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for forming an insulated electromagnetic coil which comprises:
(a) forming an electromagnetic coil by (i) winding a self-bonding wire comprising a covered wire and a bonding layer on the covered wire, said layer being made of epoxy resin having a melting point higher than 60°C or (ii) by forming an electromagnetic coil from the covered wire and coating said electromagnetic coil with the bonding layer;
(b) coating said electromagnetic coil with a composition consisting of a powdered epoxy resin and a cross linking agent consisting of an acid anhydride; and (c) heating said coated electromagnetic coil to cross-link the powdered epoxy resin.

2. A method for forming an insulated electromagnetic coil which comprises:
(a) forming an electromagnetic coil by winding a self-bonding wire comprising a covered wire and a bonding layer on the covered wire, said layer being made of epoxy resin having a melting point higher than 60°;
(b) coating said electromagnetic coil with a composition consisting of a powdered epoxy resin and a cross-linking agent consisting of an acid anhydride; and (c) heating said coated electromagnetic coil to cross-link the powdered epoxy resin.

3. A method for forming an insulated electromagnetic coil which comprises:
(a) forming an electromagnetic coil from a covered wire;

(b) coating said electromagnetic coil with a bonding layer of epoxy resin having a melting point higher than 60°C;
(c) coating said coil coated in step (b) with a composition consisting of a powdered epoxy resin and a cross-linking agent consisting of an acid anhydride; and (d) heating said coated coil of step (c) to cross-link the powdered epoxy resin.

4. The method according to Claim 2, wherein the heating step is carried out at a temperature of 150°C
to 250°C.

5. The method according to Claim 3 wherein the heating step is carried out at a temperature of 150°C
to 250°C.

6. The method according to Claim 2, 4 or 5 wherein the electromagnetic coil is an electromagnetic coil of a transformer.

7. The method according to Claim 2, 4 or 5 wherein the electromagnetic coil is an electromagnetic coil of a leakage transformer.

8. The method according to Claim 2, 4 or 5 wherein the cross linking agent is contained in the composition in an amount of 0.5 to 90 parts by weight based on 100 parts by weight of the epoxy resin.

9. The method according to Claim 2, 4 or 5 wherein said epoxy resin and said powdered epoxy resin are the same and consist essentially of bisphenol A diglycidyl ether.

10. The method according to Claim 2, 4 or 5 wherein said acid anhydride is at least one selected from the group consisting of pyromellitic dianhydride, cyclopentanetetracarboxylic dianhydride, benzophenone-tetracarboxylic dianhydride and ethylene glycol bistrimallitate.

11. The method according to Claim 2, 4 or 5 wherein said covered wire comprises a copper or aluminum wire coated with electrically insulating resin.