US2920297A - Current transformer - Google Patents

Description

Jan. 5, 1960 G. w. sPlcER 2,920,297

l CURRENT TRANsFoRMER Filed April 28, 1955 Trae/V575 United States Patent O A 2,920,291 CURRENT TRANsFoRMER George-W. Spicer, Philadelphia, Pa., assignor to I-T-E ACircuit Breaker Company, Philadelphia, Pa., a corpo- Y ration of Pennsylvania Application April 2s, 1955, serial No. 504,454 z claims. (ci. 336-174) secondary. In order that the secondary remain properly and securely positioned with respect to the primary, it has been necessary to provide metallic feet or other external supports for the secondary windings. This arrangement, although satisfactory for many installations, has been found to be a rather cumbersome one, particularly when the space available for installation of the `transformer is rather limited. Moreover, under crowded orcrarnped conditions, the relaying accuracy of such transformers, when constructed with separate coils instead of a toroidal winding, was severely limited.

.Y ,I propose to overcome these and other diiiiculties inherent in the devices of the prior art by providing a current transformer in which the'secondary is rigidly secured to the bushing type insulating tube of the primary conductor by means of a cast epoxy resin.

Accordingly, i-t is an object of my invention to provide va novel current transformer particularly suitable for use in installations where the available spaceis limited. j

. It is another object of my invention to provide a current transformer of high relaying accuracy and having excellent mechanical and dielectric properties.

A further object of my invention is to provide a current transformer in which the primary insulating tube is rigidly secured to the secondary by an epoxy resin thereby resulting in an integral unit suitable for installations of limited space.

These and other objects of my invention will become more apparent when taken in connection with `the following description and drawings in which:

Figure 1 is a view showing, in perspective, my novel current transformer with a portion of the secondary cut away to show the construction thereof.

Figure 2 is a cross-section taken along the lines 2 2 of Figure 1.

Essentially, my invention involves a current transformer in which the toroidally wound secondary turns are rigidly secured to the insulating tube provided for the primary conductor by a body of cast epoxy resin.

By epoxy resin, I refer to the reaction product of epichlorhydrin and Bis-phenol A, which forms a resin having superior heat stability, chemical resistance and electrical properties.

These materials are sold under various trade names such as Epon," Araldite or HysoL Although essentially they are the reaction products of epichlorhydrin and Bis-phenol A, they may also be the reaction product of any polyphenol and polyhalohydrin. Compounds of the type to which I refer are described, for example in United States Patent Nos. 2,324,483, 2,458,796 and 2,637,- 716.

2,920,297 Patented Jan. 5, 1960 These epoxy resins are readily cast into desired shapes Vwith a minimum of shrinkage during curing, thereby making a cast product having a minimum of internal stresses. Moreover, the cast resin readily adheres to other materials, has excellent moisture and chemical resistance, and high mechanical strength. For purposes of economy, and to reduce therrnal'expansion, inorganic llers such as slate powder, kaolin, quartz, talcum, silica, mica may be added to the resin mix. Generally fillers are added in amounts such that they constitute between about 60 and 70 weight percent of the nal mixture.

Referring now to the drawings, the primary of the transformer to which I refer comprises a hollow bushing- Atype sleeve or insulating tube 11 which can readily be placed laround a single-turn conductor such as a bus bar. The secondary is inthe shape of a doughnut and contains a desired number of turns of a toroidally-wound wire which is wound around iron core 14.

The core is constructed of laminated steel as shown in Figure 2. Between the iron core and the wire coil is a layer of tape insulation 15. After the coil 20 is wound about insulation 15 another layer of tape insulation 17 is wound around the entire assembly. The tape which is used in insulating the secondary may be conventional vinsulation tape having a cotton or polyvinyl chloride base.

An insulation tube 11 assumes a shape so that it Awill t securely to the primary conductor. The insulating material used here is an electrical grade phenolic kraft paper of the type commonly used for electrical insulation. The insulating paper is free from impurities which may lead to heat distortion and is of rather high dielectric strength. Moreover, the paper should not contain iillers which would make it hygroscopic.

. Y The secondary windings 20 are wound in a toroidal fashion so that the secondary is of a doughnut shape.

In bonding the primary and secondary the transformer windngsand insulating bushing are placed in position and then dried in a circulating oven at 250 F. maximum temperature. Y, Theiepoxy resin which is cast supplied the bond between the insulating tube 11 and secondary 20 and may be cured with either acid or amine hardeners.

As previously mentioned, these resins go under trade names such as Epon, Araldite or HysoL The utilization of such resins in a hermetically sealed resistor is described in United States Patent No. 2,685,016 which is owned by applicants assignee. This resin has the properties of high adhesion to the solid material of the coil form and the ability to be converted to a solid at a safe temperature for the material of these forms. A specific example of the resin which I may employ goes under the trade name of Hysol Resin No. 6800, manufactured by the Houghton Laboratories. This resin is of the heat setting type and is melted at about 300 F. The resin contains a quartz filler in an `amount such that the final composition comprises quartz 60 percent by weight and epoxy resin 34 percent by weight.

Au acid hardener such as phthalic anhydride is then added in order to obtain curing. Specifically, the hardener used goes under the trade name of Araldite Hardener No. H N. 901. The assembly containing 3.7 pounds of resin has added thereto 0.4 pound of hardener. 'Il-ie amount of hardener, of course, may vary with the rate of curing desired and will normally be between about 5 to 15 percent of weight of the resin used.

The assembly is cast by pouring the resin under a minimum vacuum of 28 inches of mercury. It is then cured in a circulating oven maintained between 250 and 260 F. for 14 to 18 hours. Upon cooling to room temperature the assembly is ready for use.

Referring again to Figure 1 it can be seen that two leads 22 and 23 emanate from the secondary coil. These may be of such a length so as to connect directly with the u'ysual electrical apparatus. They may also be` conepoxy resin rigidly secures it to the outer insulated` sur? `face 32 of the secondary and to the surface 33 of the phenolic paper and the bushing-type insulating tube -11.

The doughnut shaped secondary 20 is then arranged so that the axis or centerline of the doughnutvcoincides -with the vaxis of the insulating tube. The resin and secondary have a disc-like shape and'preferably the fresin isffcast yso that surfaces 40 and 41, which lare also normal -to the axis of the insulating tube, are smooth and parallel. This connection of the inner circumferential surface Vof the 'secondary to the primary insulating tube yeliminates the need for metallic feet or other supports forth'e secondary which have been necessary in devices Yof the prior'art.

These metal supports have had a tendency to reduce the f i `insulation level in tig/ht andcramped locations'andfthereby limit the relay accuracy of transformers of "this type.

.The strongly adhesivefepoxy resinis thereforevuse'd as a mechanical support between the insulating tube 11 and {transforrner by supporting the'transformeron the'insulating tube rather than on any 'external'elementsi In addition to the outstanding mechanical kproperties ofthe epoxy resins their electrical properties also `find use. Their dielectric properties also are useful iny that the resin is located in the dielectric e'ld'between the l'primary conductor and the Vsecondary winding. Thus, the properties of epoxy resins to set up free-of-airvoids is also important, since the epoxy forms between the surface path from the conductor to the grounded secondary.

My transformer, therefore, having anlinsulated sleeve `11 for the primary and a secondary 20 interconnected by a cast epoxy resin provides la Verycompact type transformer with excellent insulating properties.

In the foregoing, I have described my invention only in connection with preferred embodiments thereof.

'Many variations and modifications ofthe principles of my inventionwithin the scope ofthe description herein 'are obvious. Accordingly, I prefer to be bound not by the lspecific disclosure herein, ,but onlyby the appending claims.

I claim: Y

l. A current transformer comprising -ahollow insulating tube suitable for encasing a primary conductor, a toroidally Wound insulated -"secondary surrounding said tube, and a radial connection rigidly secured to the inner circumferential surface of the secondary and the outer surface of the insulatingtube; said insulating tube comprising an electrical grade phenolic kraft paper; said secondary comprising a 'doughnut-shaped iron core, a first layer of insulation adjacent tofsaid core, la conductive Winding adjacent tosaid first layerof insulation land a second layer of insulation adjacent -to5said conductive Winding; said radial connection comprisinga cast epoxy resin having ste VT()v percent of Ianinorganic filler incorporated therein.

2. The transformer as set forth in claim 1in which the epoxy surrounds the tube and is interposed between the tube and the in ner circumferential surface of the secondary; said v epoxyr resin being cast so that ltheFepoXy includes 'two parallelrelatively smooth surfaces normal -to lthe 4axis 'of 'the ins'ulating tube; fsaid parallel surfaces Aappearing as Iextensions of the 'secondaryfouter surface.

` References rCited' in Vtheiile of .this patent f UNITED STATES PATENTS France '.Nov. :20, 192

OTHER --REFERENCES Ethoxylines-A New Group of Triple-'Function Resins, Electrical Manufacturing, July' 1949, 4xp'p. '78481.

EpoXyCasn'ng Resins in Electronics? Modern `Plastics, July 1954, pp. 118, 121.

Typical Applications of 4Epoxy' Resins, The Rubber Sand Plastics Age,February 19'54, pp."8l1i87.V

EpoXiesf-No Wonder, Modern Plastics.` October 1952; pp. 89-94.

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