US2878441A - Shielded transformer - Google Patents

Description

Filed July 17, 1956 mmvroas STANLEY ROGERS,

TOIVO M. DANNBACK,8 7 BY DAVID C.KAL8FELL United States Patent SHIELDED TRANSFORMER Stanley Rogers, David C. Kalhfell, and Toivo M. Dannback, San Diego, Calif., assignors to General Dynamics Corporation, San Diego, Calif., a corporation of Delaware Application July 17, 1956, Serial No. 598,270

8 Claims. (Cl. 32344) This invention relates to shielded transformers and more particularly to transformer shielding apparatus wherein shielding between windings of a transformer is driven by an external current source to thereby prevent interwinding and winding-to-core current transfer.

Shielding of components in a transformer heretofore has been done in a passive manner whereby the insertion of a shield between two elements tended to minimize current losses due to conduction through faulty insulation or due to capacitance coupling therebetween. While this type of passive shielding minimized the losses when both the primary and secondary were connected to ground, it did not compensate completely for these losses nor the additional losses which occurred when one or the other or both. windings were at an. A. C. potential above ground.

The transformer shielding apparatus comprising the present invention includes several methods of shielding in which in some instances one shield is connected to primary, another shield is connected to secondary, and a third shield is connected to the core.

These shields are driven by outside current amplified in a manner to compensate for the losses occurring by current passage between the windings. The gain of the amplifiers which receive a signal from the primary is set such that there is no current flow between the shield and its winding. This amplification may be manually set or may be made automatic.

It is, therefore, an object of this invention to provide for an improved shielded transformer.

It is a further object to provide for transformer shielding apparatus wherein an additional current is used in correcting for losses between windings and between windings and the core of the transformer.

A further object is the provision of a shielded transformer wherein compensation is made for the current losses due to capacity coupling between the elements and due to conductive losses through faulty insulation.

A still further object is the provision of transformer shielding apparatus wherein certain current losses due to conduction or capacitive coupling between elements is compensated or eliminated entirely.

Other objects and features of the present invention will be readily apparent to those skilled in the art from the following specification and appended drawings wherein is illustrated a preferred form of the invention, and in which:

Figure 1 is a schematic showing driven shielding between the primary winding and other elements with leakage paths shown, when the A. C. through the primary winding is at a potential above ground.

Figure 2 shows the shielding apparatus for correction or compensation of current losses when the A. C. through both windings are at potentials above ground.

Figure 3 is a cross-sectional perspective of the transformer with a schematic showing of its associated circuitry.

2,878,441 Patented Mar. 17, 1959 Figure 4 is a modification of the apparatus shown in Figure 1.

When one side of each winding of the transformer is connected to ground, a single shield connected between them produces satisfactory insulation and there is relatively little primary to secondary leakage. However, there is some primary loss and also distortion of secondary current since current is drawn from the primary to the shield due to capacitance between the upper windings and the shield. This error becomes greater as the frequency of the current increases. This capacitance coupling produces current losses in the primary even though the current does not leak into the secondary. There is also capacitance coupling and conductance from the secondary to other elements in the transformer which causes secondary losses. When the A. C. in the primary is at a potential above ground and the low side of the secondary is connected directly to ground, as is normally the case in computer work, in addition to the other leakage errors already mentioned there is also a current flow from the primary into the secondary due to this difference in potential, causing further losses in the primary and more distortion in the secondary output. In this situation, two shields are necessary, one is connected to the primary; while the other is connected to the core, the secondary, and to ground. While this isolates the primary from the secondary there is at capacitance current flow from the primary to ground through the shields which, if not corrected, will cause a current loss in the primary. Even with the second shield and with the secondary grounded there is still a smaller capacitance between the top windings of the secondary and the shield causing losses in the secondary. If these current losses can be compensated by an additional current so that the current into the primary would equal the current out of the primary, there would be no uncompensated current in the primary. The same is true for the secondary in correcting losses therein.

Referring now to Figure 1, there is shown the lower ends of the secondary 11, and shield 12 connected to ground, whereas the primary 13 is at a potential above ground as shown by resistance 14. Since the transformer core is connected to ground, there is no capacitance between it and the core shield 12. Therefore, the core is not shown in the drawings and all capacitance in connection therewith is referred to the shield 12. A shield 16 isolates the primary from the other elements 11 and 12. An amplifier 17 of approximately unity gain is connected between the primary 13 and the primary shield 16.

A capacitance and conductance leakage path 21 exists between primary winding 13 and the driven shield 16. The presence of amplifier 17 between the primary and shield maintains almost exactly the same instantaneous potential across the ends of the leakage path 21 so that a negligible current flows through this admittance and hence reduces spurious primary current. The leakage path 22 is that path between the driven shield 16 and the grounded shield 12. This causes no spurious currents between the primary and secondary windings but merely acts as a load upon the amplifier 17. This requires that the amplifier have sufficient output power to maintain a gain close to unity in spite of the losses through path 22. Leakage path 23 is between the secondary 11 and the grounded shield 12. Since the secondary is at ground potential, the potential at each end of this path is the same and no current flows through the path. If the secondary were above ground, the leakage path 23 would cause spurious currents to flow in the secondary unless another shield is used, as will be described in connection with Figure 2. Leakage paths 24 and 26, between primary 13 and shield 12 and between primary 13 and secondary 11, have negligible a current flow i'f'the shield 16 is properly made to be a complete electrostatic enclosure.

Figure 2 shows the preferred embodiment when the potentiometer of the secondary 11 is also above ground. Except for the addition of a driven shield 27 for the secondary winding, this embodiment is veryysimilar to that shown in Figure 1. Here the current loss in path 21 is compensated by its amplifier 17 and the current'loss in path 23 is compensated 'by its'amplifier '28 so that a negligible current flows through these paths. Leakage path-s 22 and 29 act as loads on amplifiers 17 and 28, respectively. It should be noted that with shield 12 connected to ground, there can be no direct capacitive coupling between the shields 16 and 27. The shield 12 also prevents interaction between amplifiers 17 and 28.

'The third shield 12,further improves the total shielding'between the primary and secondary windings. In Figure 2 both the primary and secondary are shown as having an A. C. in both windings at a potential above ground as shown by resistances 14 and 30, respectively. This takes care of everyconceivable usage to which the transformer might be put.

Figure 3 is a cross-sectional view of a preferred construction of the transformer in Figure 2 with a schematic showing of circuitry connected thereto. Here the windlugs 11 and 13 are formed separately and stacked over core 33, which may be one leg of a closed loop core if desired. Around the core is a cylindrical sleeve 34 with a collar 36 connected thereto extending outwardly to separate the windings. The sleeve and collar comprise shield 12 which is connected to ground. Sleeve 34.may be omitted if the core can .be grounded effectively. Shield 27 consists of a collar 37 between shield 12 and secondary 11 and a sleeve 33 extending between the secondary 11 and that portion of .the sleeve .34 of shield .12 extending through the secondary v11. Shield 16 .is similarly constructed and its collar 39 and sleeve 41 isolates primary 13 from shield 12 and other elements. The shields are of a non-magnetic electrically conducting material so as not to disturb the desired magnetic field lines. While more complex amplifiers may be used, amplifier 28 is shown as a simple cathode follower, wherein the grid 42 is connected to a secondary terminal and the shield 27 is connected to the cathode 43 of .the triode 44.

The simplified systems of Figure 1 and Figure 2 may be further improved by the system of Figure 4. The cur rent flowing into the primary of Figure 1 is not exactly equal to the current flowing out of the primary, since only one part of the primary winding is at exactly the same potential as its shield. Althoughthis error is small in practical systems, it may be eliminated automatically with the system of Figure 4. In Figure 4, block 46 designates a differential amplifier which is excited by a voltage proportional to the difference between the input and output currents of the primary 13. Amplifier 46 has a high gain in contrast to amplifier 17 whose gain was nearly unity. The voltage-drops across equal resistances 18, 19 on either side of the primary winding 13 excites the differential amplifier 46 to control automatically the drive on shield 16. Here the small equal resistors 18, 19 in series with each end of primary winding 13 will have the same voltagedrops across them when the amount of current entering point A leaves point'E. By connecting these voltagedrops, as shown by connecting like letters in Figure 4, to the input of differential amplifier 46 or equivalent, any difierence in voltage-drops will be an input signal to the amplifier. This amplifier is of conventional design'and may be of the type'shown, for example, on page 206 "of Electronic Analog Computers by Korn and Korn, published by McGraw-Hill Book Company. The polarity of the output is selected so that such differences will drive the shield 16 so as to feed an opposing current from the shield 16 to the winding 13. By'using an amplifier of high gain, the difference between the voltage-drops can be 4- held very small despite variations in operating conditions.

-An alternate method of automatically providing the correct amount of current'for driving shield 16 is to replace resistors 18 and 19 with coil windings on an auxiliary transformer and connecting its output to the grid of a triode whose plate is connected to the shield. It is thought that from the foregoing description, other ways of automatically driving shield 16 to compensate for current loss in winding 13 will readily occur to those skilled in the art. If desired, the secondary shield 28 of Figure 2 also might be driven by the differential secondary current analogously to the primary shield drive herein described.

While certain preferred embodiments of the invention have been specifically disclosed, it is understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

What we claim is:

1. A shielded transformer comprising a core, primary and secondary windings around said core, a shield separating said primary winding from said core and from said secondary winding, a current source connected to said shield, means for adjusting said current source to produce zero capacitive coupling for alternating currents between said shield and said primary winding.

2. A shielded'transformer wherein capacitance coupling and conductance losses therein are compensated, said transformer comprising a primary winding, a secondary winding, a core, a primary shield between said primary winding and core and between said primary winding and said secondary winding, a second shield separating said secondary winding from said primary winding, said second shield being connected to ground, current means'connected to said primary'shield and to said primary windingfor supplying current to said shield of phase and amplitude controlled by current in said primary winding, said current means comprising a pair of equal value resistances serially connected to said primary winding at each end thereof, respectively, a differential amplifier whose input is connected to each of said resistances for obtaining a differential current, said amplifier supplying current to said shield of phase and magnitude indicative of said differential current.

3. A shielded transformer wherein capacitance coupling and conductance losses therein are compensated, said transformer comprising a primary winding, a secondary winding, a core, a primary shield betweensaid primary winding and core 'and between said primary winding and said secondary winding, a secondary shield between said secondary winding and said core and between said secondary winding and said primary winding, a third shield separating said core from said primary winding and said secondary winding, said core and said third shield being connected to ground, first current means connected to said primary shield for supplying current thereto, said first current means being connected to said primary winding for phase and amplitude control thereby, and a second current means connected to said secondary shield for supplying current thereto, said second current means being connected to said secondary winding for phase and amplitude control thereby.

4. A shielded transformer as in claim 3, said first and second current means comprising first and second cathode follower amplifiers of adjustable gain, said amplifiers includingvacuum tubesthaving grids connected to said primary and secondary windings, respectively, first and second variable cathode resistors connected to the cathodes of said vacuum tubes for adjusting the gain of said amplifiers, said shields being connected, respectively, to the cathodes of said tubes.

5. A shielded transformer as in claim 3, resistances of equal value connected serially to said primary winding at each'en'd thereof for measuring current into said winding and current out of said winding, said first current means comprising a differential amplifier connected to said resistors to obtain a differential current therefrom, said amplifier connected to said primary shield for supplying current thereto indicative of said differential current, secondary resistors of equal value serially connected at each end of said secondary winding, said second current means comprising a second differential amplifier connected to said secondary resistors for obtaining a differential current therefrom, said second amplifier connected to said secondary shield for supplying current thereto indicative of said last-mentioned differential current.

6. A shielded transformer comprising a core, a primary and secondary having windings on said core, a pair of shields separating said primary from said secondary and said core, a current source of adjustable magnitude connected to one of said shields, the other of said shields being connected to ground, means measuring current flow into said primary, means measuring current flow out of said primary, and means responsive to the difference in current flow in and out of said primary for automatically controlling said current source.

7. A shielded transformer comprising a core, a primary and secondary having windings on said core, a pair of shields separating said primary from said secondary and said core, a current source of adjustable magnitude connected to one of said shields, the other of said shields being connected to ground, said current source being a differential amplifier operable by the difference in current flow into the primary winding of said transformer and the current flow from said primary winding.

8. A shielded transformer comprising a core, a primary and secondary having windings on said core, shields separating said primary from said secondary and said core, a current source of adjustable magnitude connected to one of said shields, said current source being a differential amplifier operable by the difference in current flow into one of said windings and current flow from one of said windings.

References Cited in the file of this patent UNITED STATES PATENTS 1,837,245 Wheeler Dec. 22, 1931 1,907,400 Davis May 2, 1933 2,724,108 Hayes et a1 Nov. 15, 1955

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