US 3665356 A
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United States Patent Douglas et al.
115 1 3,665,356 1451 May 23, 1972  DIFFERENTIAL TRANSFORMER WI BALANCING MEANS I  inventors: Ellwood S. Douglas, Orinda; Wallace W.
Wahlgren, Oakland, both of Calif.  Assignee: The Rucker Company, Oakland, Calif.
 Filed: Apr. 23, 1969 [2 l] Appl. No.: 818,558
52 us. c1 ..336/73, 33 /84, 336/175,
7 336/212 511 1111.0. ..H01fl7/06  FieldofSearch ..336/173,l74,l75,84,2l2, 336/73  References Cited I UNITED STATES PATENTS 3,525,964 8/1970 Stevenson ..336/84 Brennan et a1 ..336/73 433,702 8/1890 Tesla ..336l212 X 1,832,662 11/1931 Schmutz.. ..336/84 3,296,364 1/1967 Mason ..336/174 X 3,343,074 9/ l 967 Brock 336/175 X FOREIGN PATENTS OR APPLICATIONS 288,606 3/1929 Great Britain .i ..336/ 1.75
1,086,341 8/1960 Germany ..336/ l 74 Primary Examiner-Thomas J. Kozma Attorney-Elem, l-lohbach, Test, Albritton 8L Herbert 57 7 ABSTRACT A differential transformer having a balancing sleeve interposed between its primary and secondary windings to provide a substantially identical degree of coupling between each of theprimary windings and the secondary winding. The balancing sleeve is made of magnetic and/or electrically conductive materials.
2 Claims, Drawing Figures Patented May 23, 1972 3,665,356
2 Sheets-Sheet l INVENTORS ELLWOOD S. DOUGLAS BY WALLACE W WAH LGREN ATTORNEYS Patented May 23, 1972 2 Sheets-Sheet 2 INVENTORS ELLWOOD S. DOUGLAS BY WALLACE W WAHLGREN ATTORNEYS 1: DIFFERENTIAL TRANSFORMER wrrn BALANCING MEANS v BACKGROUND OF THE INVENTION l-leretofore, differential transformers have been provided wherein a currentis induced in a secondary winding in proportion to the difference between the currents flowing in two or more primary windings. A problem exists in such transformers in providing balanced coupling between each of the primary windings and the secondary winding' This problem is particularly severe'with differential trans-- formers whichare used in ground fault detectors of the type described in US. Pat. No. 3,213,321, issued Oct. 19, 1965. Such transformers must be capable of detecting leakage currents on the order of 5 milliamperes. Thus, for typical primary currents on the order of 100 amperes, a sensitivity on the order of 50 parts in a million is required. In order to have a sensitivity ratio of this magnitude, it is necessary to'have a very high degree of balance among the primary windings of the transformer. The magnitude of the requisite degree of balance becomes apparent when one considers that the current due to unbalanced coupling should be at least an order of magnitude smaller than the leakage current which is to be detected. In
the example given, the ratio of current due to unbalanced SUMMARY AND OBJECTS OF THE INVENTION The present invention provides a differential transformer having a plurality'of single turn primary windings, at least one secondary winding, and balancing means interposed between the primary and secondary windings to provide a substantially identical degree of coupling between each of the primary windings and the secondary winding.
It is in general an object of the'present invention to provide a differential transformer wherein a substantially identical degree of coupling is provided between each of the primary windings and the secondary winding.
Another object of the invention is to provide a differential transformerof the above character which can be readily and economically produced.
Additional objects and features of the invention will be apparent from the following specification in which the preferred embodiments of the invention are described in detail and illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING ferential transformer incorporating the present invention.
FIG. 5 is a sectional view of a fourth embodiment of a differential transformer incorporating the present invention.
FIG. 6 is a sectional view of a fifth embodiment of a differential transformer incorporating the present invention.
Like reference numerals are used to designate corresponding elements in each figure of the drawing.
DESCRXPT ION OF THE PREFERRED EMBODIMENTS The embodiment of the difi'erential transformer shown in FIGS. 1 and 2 comprises generally a toroidal core 10, primary windings 11, a secondary winding 12, and a balancing sleeve 13.
Toroidal core 10 is made of a material having a high magnetic permeability, such' as supermalloy. Preferably, this core should be as nearly circular in shape as possible and should have a uniform cross section. While these core dimensions are not as critical as they would be in a conventional differential transformer, some improvement in balance may nevertheless be obtained by proper attention to the symmetry of the unit.
Primary windings 11 consist of a plurality of current-carrying conductors which pass through the opening in toroidal core 10, thereby forming a plurality of single turn primary windings. The size of these conductors is dependent upon the currents which they must carry, and the number of windings'is determined by the application for which the transformer is intended. The embodiment shown has four primary windings and, hence, is suitable for a three-phase, four-wire system.
While the placementof the wires is not as critical as it would be in a conventional differential transformer, some improvement in balance may be realized by arranging primary windings 11 in a symmetrical manner. ideally, all primary windings should lie exactly on the axis of toroidal core 10. In a three-phase, four-wire system, the neutral or return conductor can be located at or near' the axis, with the remaining conductors being distributed around it in a symmetrical pattern. Some additional improvement in balance can be provided bytwisting the conductors together in the vicinity of toroidal core 10.
The secondary winding 12 consists of a plurality of turns wound on the toroidal core 10. A pair of leads 14 is provided for connection to the secondary winding. Preferably, 'the secondary turns should be tightly wound and evenly distributed about the toroidal core. In addition, it is desirable that the secondary winding occupy an integral number of layers on the core. in transformers having more than one secondary winding, it is desirable. that each of these windings be tightly and symmetrically wound.
The balancing sleeve 13 is disposed between the primary windings 11 and the secondary winding 12. Sleeve 13 is generally cylindrical in shape. It encircles primary windings 1 l in the vicinity of toroidal core 10 and passes through the opening in the core. Preferably, the axis of sleeve 13 should coincide with the axis of toroidal core 10.
The dimensions of balancing sleeve 13 are not critical. It has been observed, however, that more balanced coupling is obtained when sleeve 13 has an external diameter approaching the internal diameter of core 10. Likewise, the degree of balance increases as the wall thickness of sleeve '13 is increased. Satisfactory results have been obtained with a wall thickness as small as 0.050 of an inch.
The improvement in balance provided by sleeve 13 increases within certain limits as sleeve 13 is increased in length. It has been observed that the improvement in balance decreases sharply as tube 13 is made shorter than the width of toroidal core 10. Thus, in the preferred embodiment, sleeve l3 should-be at least as long as core 10 is wide. lt has also been observed that little additional improvement in balance is provided by making sleeve 13 more than about three times as long as core 10 is wide. The axial placement of sleeve 13 is not critical as long as the sleeve extends completely through core 10. In the preferred embodiment, the sleeve is centered within the core.
The balancing sleeve 13 can be fabricated of any material having a high magnetic permeability. This magnetic material apparently serves to short out any uncancelled fluxes from the primary windings before they can reach the secondary winding. Although this magnetic sleeve eliminates a substantial portion of the uncalcelled fluxes from the primary windings, it has been observed to have no significant effect upon the sensitivity of the transformer to the leakage currents to be detected. While there may be a slight reduction in the sensitivity of the transformer, this reduction is almost undetectible in view of the ratio of the cross section of the balancing sleeve to the cross section of the toroidal core.
An additional improvement in the coupling balance can be provided by fabricating balancing sleeve 13 of a material which is electrically conductive, as well as having a high magneticpermeability, Suitable materials having these two properties include soft iron, nickel, and nickel-iron alloys, such as permalloy and supermalloy. It is thought that the electrical conductor provides a path for eddy currents, so that the eddy currents can distribute themselvesin such a manner as to improve the coupling balance.
The various elements of the differential transformer can be assembled and held together by conventional means. One particularly effective and convenient means is to.pot" the entir assembly in an epoxy rosin.
FIG. 3 shows another embodiment of the present invention which is similar to that previously described except for the construction of the balancing means 13. In this embodiment, balancing means 13 comprises an annular cylindrical sleeve 16 and an outer cylindrical sleeve 17. Sleeves l6 and 17 are concentric with respect to each other and with respect to the axis of toroidal core 10. Sleeve 16 is fabricated of an electrically conductive material, such as copper, and sleeve 17 is fabricated of a material having a high magnetic permeability.
.This embodiment has been found to provide a greater improvement in the balance of the coupling between the primary and the secondary windings than is provided by the embodiment shown in FIG. 1.
In the embodiment shown in FIG. 4, the balancing means 13 includes an inner electrically conductive sleeve 21 and an outer electrically conductive sleeve 22. These sleeves are generally cylindrical in shape and are concentric with respect to each other and with respect to the axis of toroidal core 10. Sleeves 21 and 22 are joined together at their ends by annular rings 23. Rings 23 are also made of an electrically conductive material and are joined to sleeves 21 and 22 by conventional means, such as soldering. A sleeve of magnetic material 24 is disposed in the space defined between sleeves 21 and 22 and rings 23. The effectiveness of this embodiment in improving the balance in coupling between the primary and secondary windings has been found to be comparable to that of the em bodiment shown in FIG. 3.
Operation of the embodiment shown in FIG. 4 is believed to be as follows. Current flowing in the primary conductors ll induces a flux in the magnetic sleeve 24. .The rate of change of this flux induces a voltage from end-to-end in the inner conductive sleeve 21. This voltage forces current to flow from end-to-end in the outer conductive sleeve 22, returning through the inner sleeve. The induced current in the inner sleeve 2] is in a direction opposite to that of the current in the primary windings. Where the sleeve assembly is provided with adequate core material and conductive sleeves, the induced current will be nearly as large as the primary current, thereby reducing the flux in the magnetic sleeve 24 to a value much less than it would be with no conductive sleeves andno in duced currents. Thus, the magnetomotive force seen by toroidal transformer core 10 is mostly due to the induced current flowing in the outer conductive sleeve 22. If one of primary windings 11 is in a position such that its coupling to inner conductive sleeve 21 varies around the sleeve, the induced current will have circumferential components at the ends of the sleeves which distribute the current more evenly over the outer sleeve 22. Thus, the net result of using this sleeve assembly is an improvement in transformer balance without any substantial effect on the sensitivity of the transformer to the leakage currents to be detected.
The embodiment shown in FIG. 5 is similar to those previously described except for the construction of the balancing sleeve 13. The balancing sleeve is formed to include a substantially cylindrical central portion 26 and flared end portions 27. This embodiment can be constructed 'more compactly than those previously described'The central sleeve portion 26 can have a length corresponding generally to the width of the toroidal core 10,- and the flared portions 27 should preferably extend radially outward to the outer surface of the toroidal core. The flared sleeve is most convenientlylfabricated in two sections which can be inserted into the toroidal core 10 from by the flared sleeve is comparable to that provided by the other embodiments heretofore described.
The embodiment shown in FIG. 6 is very similar to that shown in FIG. 5. In FIG. 6, however, the balancing sleeve 13 includes a central cylindrical member 31 and annular end rings 32. The end rings 32 are fitted over the end portions of the central member 31 and joined thereto by conventional means. The sleeve 13 in this embodiment can be fabricated of the same types of materials as the sleeve in the FIG. 5 embodiment, with a comparable improvement in the coupling balance. I
While the preferred embodiments have been described in terms of difierential transformers having toroidal cores, it is to be understood that the present invention is not limited to this particulartype of transformer. Thus, for example, the invention can also be utilized in differential transformers having a rectangular core, in which case the balancing sleeve would preferably be rectangularin cross section to correspond to the shape of the core.
It is apparent from the foregoing that there has been provided a new and improved differential transformer with balancing means which provides a substantially identical degree of coupling between each of the transformers, primary a plurality of conductors passing through said core to form a plurality of single turn primary windings, a secondary winding wound on said core, and current balancing means surrounding said conductors and extending through said core to provide a substantially identical degree of coupling between each of said primary windings and said secondary winding, said current balancing means including spaced apart inner and outer sleeve portions fabricated of an electrically conductive material disposed coaxially of said core, means electrically connecting said inner and outer sleeve portions together at their ends, and a sleeve of magnetic material intermediate said inner and outer sleeve portions, said sleeve extending 'coaxially of said core.
2. A differential transformer as in claim 1 wherein said inner and outer sleeve portions are cylindrical sleeves and the means connecting the ends of said sleeve portions together includes annular rings extending between said sleeves at the ta us w