US2957151A

US2957151A - Current transformer

Info

Publication number: US2957151A
Application number: US639537A
Authority: US
Inventors: Jennings Jo Emmett; Hugh C Ross
Original assignee: Jennings Radio Manufacturing Corp
Current assignee: Jennings Radio Manufacturing Corp
Priority date: 1957-02-11
Filing date: 1957-02-11
Publication date: 1960-10-18
Anticipated expiration: 1977-10-18

Description

Oct. 18, 1960 J, 5, JENNINGS ETAL 2,957,151

cumm TRANSFORMER Filed Feb. 111 1957 INVENTORS JO EMMETT JENNINGS BY HUGH 0. R088 their ATTORNEY United States Patent CURRENT TRANSFORMER Jo Emmett Jennings and Hugh C. Ross, San Jose, Calif.,

assignors to Jennings Radio Manufacturing Corporation, San Jose, Calif., a corporation of California Filed Feb. 11, 1957 Ser. No. 639,537

2 Claims. (Cl. 336-174) Our invention relates to electrical transformers; and particularly to a vacuum insulated current transformer. One of the objects of our invention is the provision of a vacuum insulated transformer characterized by close spacing of the primary inductor and secondary element.

Another object of the invention is the provision of a high potential vacuum insulated current transformer of small size and light weight.

Still another object is the provision of a substantially fireproof vacuum insulated transformer.

A still further object is the provision of a vacuum insulated transformer in which the secondary element may be easily replaced by another possessing a different current rating.

Another object is the provision of a vacuum insulated transformer capable of measuring with equal case either open or shielded type circuits.

A further object is the provision of a vacuum insulated transformer useful over a wide range of Voltages without increasing the size of the primary inductor or secondary element.

.A still further object is the provision of a vacuum insulated transformer capable of handling test voltages approximately five times as high as the rated working voltage of the primary inductor.

Another object is the provision of a vacuum insulated to the showing made by the said description and the drawings, since we may adopt variant forms of the inven tion within the scope of the appended claims.

Referring to the drawings:

, Fig- 1 is a vertical half-sectional view of one form 'of our vacuum insulated transformer.

Fig. 2 is a vertical half-sectional view showing a modi- -fied form of our vacuum insulated transformer.

. Both figures are drawn approximately one-half actual size.

In the measurement of large, low potential alternating currents it is common practice to employ a device known as a current transformer. The primary conductor of this device is inserted in series with a circuit conductor, and by measuring the proportional voltage and current induced in the secondary of the transformer, it is possible to calculate the current flowing through the circuit conductor. Measurement of currents in high potential conductors however, presents difliculties due to the inherent limitations in conventional transformers, which require wide spacing between the primary and secondary conductors, and a large amount of insulation therebetween to render them safe under high potential operation. As potentials are increased, the spacing and amount of insulation must be correspondingly increased. As the spacing is increased, however, the ability of the transformer to perform its intended function is reduced, thus destroying element of the transformer.

the efficiency of the conventional current transformer and rendering it unsuitable for the measurement of high potential currents.

To obviate the disadvantages inherent in conventional current transformers, we have provided a current transformer or vacuum pothead characterized by close spacing of the primary inductor and secondary element to provide high efficiency; and effective vacuum insulation interposed between the primary and secondary, rendering our transformer safe and useful for both low and high potential current measurement.

Broadly considered, our vacuum insulated current transformer comprises a conductor constituting the primary inductor of the transformer, supported within a vacuumized envelope. Terminals outside the envelope are conductively connected to the primary inductor and serve to serially connect the device into an alternating circuit. Inductively encircling the primary inductor and positioned outside the vacuumized envelope is an annular coil constituting the secondary element of the transformer. Alternating current flowing through the primary inductor thus induces a proportional alternating voltage and current in the secondary element. Connecting the secondary element with an appropriately calibrated voltmeter or ammeter will thus given a direct reading of the potential or current flowing in the conductor under test. The ability to safely and efiiciently handle large potentials is augmented by enlarging one end of the envelope and providing it with a series of spaced circular corrugations or convolutions.

More specifically, the vacuum insulated current transformer of our invention comprises an evacuated envelope formed by a hollow, generally cylindrical dielectric shell 2, hermetically closed at opposite ends by metallic end caps 3, hermetically united with the dielectric shell. The shell illustrated is conveniently formed from high dielectric strength glass hermetically united with the mutually reaching end cap flanges 4 by conventional metal-toglass seals. Where the use of a ceramic shell is desirable, the end caps may conveniently be brazed on the metallized ends of the ceramic shell.

Mounted within the envelope and conductively extending between the metal end caps is a conductor 6, constituting the primary inductor or conductor of the transformer. The primary inductor is conveniently cylindrical and brazed at each opposite end within a hollow externally projecting boss 7 on each end cap. Axial alignment of the primary inductor with the enclosing shell is preferred, with the primary inductor extending the full length of the envelope. The end cap bosses 7 serve as terminals for connecting the transformer into an alternating circuit.

Encircling the primary inductor in spaced inductive relation is an annular coil 8 constituting the secondary The coil is wound on a flanged cylindrical hub 9 proportioned to slidably engage the outer peripheral surface of the shell. A bead '12 integrally formed on the shell to one side of its midpoint functions as a stop to limit downward movement of the coil when the device of Fig. 1 is mounted in a vertical position as shown. It will be understood that the device may be positioned horizontally if desired, in which case the bead 12 serves to position the coil midway between the ends of the envelope.

Slidability of the secondary coil on the outer peripheral surface of the envelope permits its ready detachability and replacement by differently rated coils, thus widely extending the range of applicability of the device. In some installations the coil may also function as a support for the envelope, leaving the envelope ends free for connection into the circuit. In this case, the coil is independently supported on a suitable bracket or panel as in Fig. 2. Electrically connecting the secondary coil to an appropriately calibrated ammeter 13, results in a dlrect indication of the current load in the circuit being tested or metered.

In the modified form of our vacuum insulated transformer or pothead shown in Fig. 2, the bead 12 has been omitted and the transformer is supported in a panel 14 equipped with a flanged collar 16 secured on the panel by screws 17, and slidable on the cylindrical periphery of the shell 2. At one end the glass shell 2 is enlarged diametrically and provided with a plurality of circular, axially spaced corrugations or convolutions 18 integrally formed in the shell. This construction eliminates corona loss, which is the prolific source of radio and television interference, and increases the external resistance over the surface of the envelope.

Conventional transformers or potheads commonly use dielectric compounds such as transformer oil to reduce corona loss. These oil-filled devices have proven unsatisfactory due to their tendency to leak the oil, which acts as a dirt catcher, eventually resulting in a flashover of potential on the surface of the pothead. Also, conventional potheads are generally brown-glazed and opaque, and cannot be inspected internally. The vacuurn insulated transformer or pothead of our invention, when formed of clear glass, permits observation during abnormal operating conditions with complete safety to the observer, whereas oil-filled potheads are potential fire hazards, tending to explode outwardly when they are over internally.

Manufacture of our vacuum insulated transformer to meet the specifications of the National Electrical Code requires that each instrument be tested at a voltage approximately five times the working voltage of the primary inductor. For example, our vacuum insulated transformer designed to operate in a kv. circuit, must be tested at a voltage of approximately 75 kv. rms. Translated into peak values, this would mean that the peak voltage would be about 106,000 volts.

In order to protect the dielectric shell against destructive electrostatically imposed strain at these high peak test voltages, an electrostatic shield is provided. The shield may be a true Faraday shield having a plurality of slits in its periphery, or it may be formed, as illustrated, from a closed cylindrical portion of copper tubing 19. The cylindrical shield is hermetically sealed Within the envelope, and is proportioned to fit close against the inner surface of the shell between the shell and the primary inductor, and in axial alignment with both. The length of the shield is proportioned so that the shield will project on both sides of the coil. The shield thus will lie wholly within the magnetic field projected from the centrally disposed primary inductor. Since the field density is constantly varying, an intermittent electrostatic charge will be induced on the shield. The electrostatic charge is channeled to ground as alternating current through a hermetically sealed terminal post 21. The inner end of the post 21 is conductively connected with the shield, and functions as a means for fixing the shield in position within the envelope. In Fig. 1 the inner end of the post is brazed directly to the shield, and in Fig. 2, the conductive connection is conveniently accomplished by a heavy lead22. During the testing operation the outer end of the post is conveniently connected to ground by conductor 23. As a safety precaution, the shield may also be grounded during normal operation.

We claim:

1. A vacuum insulated current transformer comprising an elongated generally cylindrical unitary dielectric envelope, metallic end caps for each end of the cylindrical envelope having a cup-like shape with a bottom and generally cylindrical wall hermetically sealed to the ends of said envelope, said bottom of the end caps being formed with a recess centrally disposed therein and protruding outward from the bottom for mounting a primary inductor, said envelope being evacuated and hermetically sealed, a cylindrical conductor forming the primary inductor of the transformer and extending coaxiall'y within said envelope and supported at each end in said recess formed in the end caps for receiving and supporting the same, an annular coil constituting a secondary element of the current transformer and slidably received over said evacuated envelope in spaced coaxial inductive relation to said cylindrical conductor, said cylindrical wall of the envelope having formed therein an outwardly protruding portion intermediate the ends at a position to form a stop means for positioning said coil at a middle position along said elongated envelope, and an electrostatic shielding means of generally cylindrical shape mounted concentrically about said primary inductor and in abutting relation to the surrounding inside of said envelope and extending axially beyond each end of the secondary coil member and having ground conductor means connected thereto and extending through the wall of said envelope and externally thereof. 2. A vacuum insulated current transformer according to claim 1 wherein one end of said evacuated envelope is of uniform diameter which slidably receives in adjustable position therealong said annular coil and the balance of said evacuated envelope is formed with a plurality of spaced corrugations therein, the corrugation nearest the center of the envelope being positioned to form stop means for positioning said coil forming the secondary element at a middle position along said elongated evacuated envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,253,736 Sieber Jan. 15, 1918 1,688,748 Paul Oct. 23, 1928 1,962,275 Holst et a1. June 12, 1934 2,328,150 Kniepen 5...- Aug. 31, 1943 2,714,184 Peck July 26, 1955 FOREIGN PATENTS 7 460,783 Great Britain Feb. 4, 1937 612,070 Germany Apr. 12, 1935 618,549 Germany Sept. 10, 1935