Differential Transformer Mounting Arrangement Particulary For Ground Fault Interrupter Apparatus

Abstract

A coaxial conductor passing through an apertured core of a differential transformer is provided with an outer conductor in the form of a rigid tubular conductive member with one or more insulated inner conductors therein. Outside the core, the rigid tubular member is bent at two substantially right angles forming two parallel legs. At least an end of one leg is attached to a fixed support so that the coaxial conductor provides a way of fixing the electrical characteristics of the transformer as well as substantially mechanically securing the core of the transformer by a simple and effective means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to differential transformers particularly for ground fault interrupter apparatus.

2. Description of the Prior Art

Generally available ground fault interrupters show characteristics that vary in accordance with the load current on the primary conductors. That is, a device calibrated at zero load current would indicate at full load current a fault leadage current that might cause undesired tripping of the apparatus.

It has been previously recognized that there are certain advantages in providing the primary conductors of a differential transformer in ground fault interrupter apparatus in a coaxial arrangement. Such an arrangement is disclosed, for example, in copending application, Ser. No. 158,338, filed June 30, 1971 by J. R. Reeves et al. and assigned to the present assignee, for the purposes of minimizing stray currents.

Among the practical problems encountered in applying a coaxial conductor in ground fault interrupters is that the size of the load current associated with the conductors with which the apparatus is intended for use is relatively large. It would be technically feasible to use a heavy braided conductor as the outer conductor and pass an insulated wire through the braided wire to create the coaxial cable. However, the braided conductor must have its individual wires properly terminated to prevent electrical shorts and malfunctions. Also, the flexibility of the braided conductor offers a problem in mounting the cable and the magnetic core thus requiring additional mounting means and hence requiring more space and increasing the cost.

There is therefore definite interest in providing ground fault interrupter apparatus in as simple and compact form as possible while providing the required electrical characteristics, particularly in terms of avoiding changes in calibration or stray current effects.

SUMMARY OF THE INVENTION

It has been found possible to provide a coaxial conductor arrangement in a form that satisfies the electrical requirement of ground fault interrupter apparatus and also economically and effectively provides a compact mounting arrangement for the transformer core.

In accordance with this invention the outer conductor in the coaxial arrangement is a rigid tubular member of a conductor, such as copper, with means at its ends for input and output connections. One or more insulated wires is simply threaded through the tubing to create an inexpensive and readily formed coaxial member that eliminates the susceptability of the device to having the calibration level vary according to the amount of load current on the conductors. This way the calibration level is preserved from zero load current through the full range to full load current.

Where at least two inner conductors are required, it is preferred that they be provided in a twisted combination so that effectively they are coaxial although individually they are simple insulated wires.

The rigid outer conductor serves as a mounting means for the transformer core. This may be achieved by having two legs of the tubular conductor, each at substantially right angles to that portion of the outer conductor that extends through the core, so that the core is effectively pinned therebetween. Alternatively, the core with a sensing coil wound on it may be fit snugly on the rigid outer conductor. The outer conductor ends, in either case, may be secured to a fixed support member thus accomplishing the securing of the core as well. The space between the legs of the bent tubular conductor that is not occupied by the core may be utilized for associated apparatus such as a trip circuit fabricated from solid state components.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an elevation view of one embodiment of the present invention;

FIG. 2 is a partial elevation view of a modified element of the apparatus of FIG. 1; and

FIG. 3 is a partial perspective view showing an embodiment of the invention .

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawing, there is illustrated in FIG. 1 a differential transformer 10 including a toroidal core 12 of magnetic material which has as single turn primary windings the conductors 13 and 14 of a coaxial cable 15 extending through the center aperture of the core 12. A secondary winding 16 on the core 12 comprises a plurality of turns to sense differences in flux produced by the current in the primary conductors. The signal on the sensing coil 16 is supplied to a trip circuit 17 which responds to a certain level of sensed leakage current to actuate the trip coil 18 of apparatus, not shown, such as a circuit breaker, for opening the circuit of the primary conductors. Reference may be made to copending application, Ser. No. 158,338, filed June 30, 1971, by J. R. Reeves et al. and assigned to the assignee of the present invention and also to an article appearing in IEEE Spectrum, Jan. 1970, pages 55-62, for description of the general nature of ground fault interrupter apparatus and various types of trip circuits that may be provided therein.

The coaxial cable 15 comprises an outer conductor 13 that is a rigid tubular member within which is disposed at least one inner conductor 14 of insulated wire. It can be seen that no special assembly techniques are required to form the coaxial cable but merely that the inner insulated wire 14 is threaded through the tubular conductor 13. The tubular conductor 13 is bent at substantially right angles on each side of the transformer core 12. This means the core 12 is held in a relatively fixed position substantially only by the rigid tubular conductor 13. The conductor 13 may be snugly fit within the toroid (FIG. 3) or, as shown in FIG. 1, the core may be substantially held by the bent legs of the conductor 13.

As shown, the coaxial conductor 15 is U-shaped with the bottom of the U being that portion 25 passing through the core 12 and the legs 26 and 27 extending substantially in parallel therefrom at substantially a right angle from portion 25.

The exact location of the coaxial cable 15 within the core is not critical. In general it is desirable to minimize the weight and volume of the core and the coaxial cable. Suitable insulation between the elements of the differential transformer must, of course, be provided.

It is preferred that the tubular outer conductor be provided with a flange-like element 19 at at least one of its ends for making the necessary connections to a supporting surface 20. As shown in the drawing, the surface or wall 20 has output terminals 21 and 22 thereon for ultimate connection with one or more loads. The flanged end 19 is welded or otherwise secured to one of the terminals 22 for electrical connection and also to provide substantial mechanical support for the conductor 15 and, in turn, the core 12.

FIG. 2 shows only a small part of coaxial conductor 15, without associated elements, as modified for systems requiring two or more inner conductors 14 and 34. The inner conductors 14 and 34 are twisted together (at least in the portion passing through the core 12) so that they effectively comprise a coaxial pair without the expense of forming concentric conductors. Thus the apparatus of FIG. 1 may be modified for use in systems of pluralities of conductors greater than two.

Claims

I claim:

1. Ground fault interrupter apparatus of the differential transformer type comprising:

an apertured magnet core;

at least two primary conductors extending through said core, said primary conductors comprising an outer tubular conductor and at least one inner wire conductor in a substantially coaxial arrangement, said outer tubular conductor being of relatively rigid conductive material and having a first portion within said core and two other portions extending at substantially right angles to said first portion, said outer tubular conductor having an extremity secured to a fixed support member, said core being supported substantially only by said tubular conductor;

a secondary winding comprising a plurality of turns on said core;

a trip circuit responsive to sensed signals on said secondary winding, said trip circuit being at least partially disposed within the space between said two other portions of said outer tubular conductor.

2. The subject matter of claim 1 wherein: said inner conductor in the form of a single insulated wire disposed in said outer tubular conductor.

3. The subject matter of claim 1 wherein: said primary conductors number at least three and include an outer tubular conductor and at least two inner wire conductors with said inner conductors twisted together over at least the portion of their length within said core.

4. The subject matter of claim 1 wherein: said core is substantially held in place by the portions of said tubular conductor extending at substantially right angles to the portion within said core.

5. The subject matter of claim 1 wherein: said core is substantially held in place by the first portion of said tubular conductor snugly fitting within the aperture of said core.