US3160843A - Ventilated primary bar for current transformer - Google Patents

Description

Dec. 8, 1964 H. A. THORPE 3,160,843

VENTILATED PRIMARY BAR FOR CURRENT TRANSFORMER Filed Aug. 3, 1961 fizlm dli Hwy (19. 2720;

United States Patent ()fiice 3,160,843 Patented Dec. 8, 1964 3,160,843 VENTILATEE) PRIMARY BAR FGR CURRENT TRANSFGRMER Harry A. Thorpe, Somersworth, N.H., assignor to General Electric Company, a corporation of New York lied Aug. 3, 1961, Ser. No. 129,092

2 Claims. (Cl. 336-174) This invention relates to current transformers, and more particularly to a ventilated primary for use with a current transformer.

As is well known to those familiar with the general field of current transformers, it is normal to use a singleturn primary with such current transformers. The single turn primary is usually of a solid piece of copper material, either a cylindrical rod or a fiat strip or bar, extending through an opening in the secondary winding. The secondary winding is generally of a toroidal configuration and is embedded in a casing of electrical insulating material. An example of these types of current transformers are found in Patent No. 2,436,636, which shows a cylindrical rod primary, and Patent No. 2,618,689, which shows a fiat bar primary. Where a cylindrical rod is used, it is necessary either to secure flat terminal pads to the end of such bar for connecting such primary to the bus bars or conductors of the electrical system on which it is to be used. Such fiat terminal pads connected to the end of a cylindrical rod is shown in the aforementioned Patent 2,436,636. It has also been known to fiatten the ends of the cylindrical rod to form the desired primary pads. An example of this latter type is shown in Patent 2,815,493. Of course, where a flat bar is used for the primary, the ends of such bar form the desired terminal pads without the necessity of doing anything more than drilling the necessary holes for the desired connection.

As will be understood, when the primary of a current transformer is subjected to high currents, the primary begins to heat up. The portion of the primary which is within the current transformer tends to heat .at a much faster rate, and to a much higher degree than the portions of the primary which are exterior of the current transformer. This heating of the primary limits the amount of current which can be carried by any given primary, and thus limits the rating of the current transformer using such primary. Furthenwhere it is necessary to weld terminal padsto the primary, extreme care must be taken to prevent damaging the insulation of the current transformer, which surrounds such primary.

From the above, it will be apparent that in the current transformer field there is a present need for a primary which will enable larger currents to be utilized with a given size current transformer. Further, it is clearly desirable to provide a primary to which terminal pads may be attached, without any danger of damaging the insulation of the current transformer, which surrounds such primary. Of course, it is always desirable to produce a current transformer which will be more efiicient and less expensive to manufacture than those presently available.

Therefore, it is an object of this invention to provide a current transformer having a primary with ventilation to lessen the heat built up in such primary.

It is a further object of this invention to provide a current transformer with a primary which uses less material than equivalent rated primaries of present day current transformers.

It is a futher object of this invention to provide a cur-.

rent transformer with a primary whereby the terminal pads may be welded thereto, without damaging the insulation about such primary.

A still further object of this invention is to provide an improved current transformer, having a novel primary, which is less expensive to manufacture than present day current transformers.

Briefly, this invention in one form comprises a current transformer having a toroidal-shaped secondary enclosed in an electrically insulated casing. The casing is provided with a window opening therein, and a primary in the form of a hollow cylindrical tube fits within such window opening. Terminal pads are provided at each end of the primary tube, being welded thereto to opposite sides of said tube across the diameter thereof.

The invention which it is desired to protect will be specifically set forth and distinctly claimed in the claims appended hereto. However, it is believed that this invention, and the manner in which its objects and advantages are obtained, as well as other objects and advantages thereof, will be better understood from the following detailed description of a preferred embodiment thereof when taken in connection with the accompanying drawing, wherein:

FIGURE 1 is a perspective view of a current transformer showing one form of improved primary of this invention used therewith; and

FIGURE 2 is a side view, partially in section, of the form of improved primary of this invention shown in FIGURE 1.

Reference will now be made to the drawing, wherein like numerals are used to indicate like parts throughout both figures thereof. As shown in the drawing, the invention comprises a current transformer 10 having a novel ventilated primary 12 connected thereto. Terminal pads 14 and 16 are shown, firmly secured to each end of the primary 12. It is preferred that the pads 14 and 16 be secured to the primary 12 by welding, as indicated at 18 of FIGURE 2.

Referring now particularly to FIGURE 1, for a more detailed description of a preferred embodiment of this invention, there is shown a current transformer 10 which may be of a window or bar type, as shown in FIGURE 1, in the preferred embodiment, a bar type transformer 10. The current transformer 10 comprises a secondary winding (not shown) which is preferably of toroidal shape, as disclosed in the aforesaid Patent No. 2,436,636. The secondary windings (not shown) are enclosed in an electrically insulated casing 20, which may be, for example, butyl rubber. The secondary winding has its terminals brought out of the casing 20 as indicated at the secondary terminals 22. A window opening 24 is provided in the casing 20, and an insulated bushing 26, preferably integrally molded therewith, extends through said window opening 24 on opposite sides of said casing 20. As is well understood, the bushing 26 provides the desired creep insulation between the primary 12 and the secondary terminals 22, to prevent a voltage breakdown between the primary and the secondary terminals.

Where desired, mounting means may be provided, for example, in the form of brackets 28, which are secured to the casing 20 by means of bolts 30. The bushing 26 will also insulate the primary 12 from the brackets 23, which are usually at ground potential, thereby preventing any voltage breakdown between such brackets and the primary 12. An opening 31 is provided in the bushing 26, and the primary 12 is inserted therein. It will be understood that the primary 12 may be securely attached to the bushing 26 so as to be non-removable or it may be slidably inserted therein so as to be removable therefrom. This of course will depend upon the type and rating of the current transformer, as well as the use to which it is to be put. As will be understood by those skilled in the art, the bus bars or conductors (not shown) of the electrical system with which the current transformer 12 is to be utilized will be attached to the terminal pads 14 and 16 by means of the opening 32, provided in each of the terminal pads. The current flowing through the primary 12 is induced into the secondary windings (not shown) and is taken from the secondary terminals 22 to the desired meters or relays, or other industrial purposes for which it is to be used.

Referring now to FIGURE 2 of the drawing, the ventilated primary bar of this invention is shown in the form of a hollow copper tube 12, being circular in cross-section, and having the terminal pads 14 and 1.6 welded across each end thereof. As is well understood, as the current flows through the primary bar 12 the temperature of such primary bar will rise above that of the surrounding air. Clearly, those portions of the bar, which are within the insulated bushing 26, will heat up faster and rise to a much higher temperature than the ends of the bar 12, which extend beyond the bushing 26. In general, it is the heat of this primary bar due to the currents therethrough which determines the rating of the current transformer. However, by means of the hollow primary tube 12 of this invention, as the air in the center of the bar is heated, the air will expand and flow out to the open ends of the primary bar, and will be replaced by the cooler outside air. Thus, the primary bar of this invention will remain at a lower temperature than the solid copper rods of the prior art. Further, it will be apparent that very little welding is required to secure the terminal pads 14 and 16 to the primary bar 12. As can be seen, especially in FIGURE 1, it is only necessary to weld the terminal pad to the thin Wall of the primary tube 12, diametrically across its open end.

As will be understood, when solid primary bars are used, the center portion of such bar does not carry an appreciable amount of current. Such center portion is generally filled with eddy currents and the like which do not aid the current capacity of such primary, but do contribute to the heat generated therein. By means of the hollow tube primary bar of this invention the center core is eliminated, without sacrificing the current carrying ability of the primary. Of course, the eddy current losses and heat are also eliminated. Thus it can be seen that the hollow tube primary bar of this invention provides substantially equivalent current carrying ability with a substantial savings in material over the solid primary bars of the prior art.

In the preferred embodiment, the terminal pads 14 and 16 are provided with slots 34 and 36 which are designed to exactly fit about the thin wall 38 of primary tube 12. The terminal pads 14 and 16 may then be inserted into the primary tube 12, such that, the slots 34 and 36 will receive the portions of the thin wall 38 of the primary tube 12. Each of the pads may be Welded to the primary 12 about the slots 34 and 36. Thus, it is seen that much less weld material is required. Due to the open tube construction of the primary 12, where desired, cooling air can be provided to cool the area around the weld during the welding process, such that the temperature 4.- may be maintained below that which would damage the insulation of the bushing 26. It will be understood that the slots could be provided in the sides of tube 12 rather than in the terminal pads 14 and 16, if desired.

Of course it will be obvious to those skilled in the art, that the primary ventilated bar of this invention may be used with current transformers where the insulated bushing 26 is not utilized. In such instances, the ventilated primary bar of this invention may be molded directly with the molded butyl casing 20, if desired, or may be later inserted therein; such as, for example, in the manner set forth in patent application Serial No. 129,094, filed August 3, 1961, and now abandoned, in the names of Peter E. Kraus and Harry A. Thorpe, entitled Variable Primary Bar for Current Transformers and the Like Having Locking Means and assigned to the same assignee as this invention.

While there has been shown and described a preferred embodiment of this invention, it will, of course, be obvious to those skilled in the art that many changes may be made without departing from the spirit and scope of the invention set forth herein. It is therefore, to be understood, that such changes may be made in the preferred embodiment herein. This invention is not to be construed as limited by the specific preferred embodiment set forth herein, but only by the scope of the claims appended hereto.

What is claimed as new and which it is desired to secure by Letters Patent of the United States is:

1. A ventilated primary bar current transformer having an insulated casing and a primary extending through said insulated casing, the improvement which comprises said primary being in the form of a hollow cylindrical tube having thin walls open to the atmosphere at the opposite ends thereof, said primary being provided with terminal pads at said opposite ends, said terminal pads being secured to said thin Walls of said cylindrical tube diametrically across the openings thereof and without closing said openings, whereby cooling air may flow through said hollow cylindrical tube forming said primary.

2. A current transformer as claimed in claim 1 in which said terminal pads are provided with slotted means which fit around the thin walls of said cylindrical tube.

References Cited in the file of this patent UNITED STATES PATENTS 687,141 Everest Nov. 19, 1901 1,526,023 Steinberger Feb. 10, 1925 1,766,048 Trogner June 24, 1930 1,820,018 Gebherd Aug. 25, 1931 2,436,636 DEntremont Feb. 24, 1948 2,568,003 Hartmann Sept. 18, 1951 2,618,689 Cook Nov. 18, 1952 FOREIGN PATENTS 580,973 Canada Aug. 11, 1959 1,093,003 Germany Nov. 17, 1962

Claims (1)

1. A VENTILATED PRIMARY BAR CURRENT TRANSFORMER HAVING AN INSULATED CASING AND A PRIMARY EXTENDING THROUGH SAID INSULATED CASING, THE IMPROVEMENT WHICH COMPRISES SAID PRIMARY BEING IN THE FORM OF A HOLLOW CYLINDRICAL TUBE HAVING THIN WALLS OPEN TO THE ATMOSPHERE AT THE OPPOSITE ENDS THEREOF, SAID PRIMARY BEING PROVIDED WITH TERMINAL PADS AT SAID OPPOSITE ENDS, SAID TERMINAL PADS BEING SECURED TO SAID THIN WALLS OF SAID CYLINDRICAL TUBE DIAMETRICALLY ACROSS THE OPENINGS THEREOF AND WITHOUT CLOSING SAID OPENINGS, WHEREBY COOLING AIR MAY FLOW THROUGH SAID HOLLOW CYLINDRICAL TUBE FORMING SAID PRIMARY.

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