US2577707A - Pulse transformer - Google Patents

Description

Dec. 4, 1951 Q. A. KERNS ET AL PULSE TRANSFORMER Filed Aug. 8, 1950 INVENTORS. QUE/vr/N A KMA/5 @M6/MMM A from/Ex Patented Dec. 4, 1951 UNITED STATES PATENT OFFICE PULSE TRANSFORMER Energy Commission Application August 8, 1950, Serial No. 178,267

8 Claims. (Cl. 323-44) transformers are quite different from those of conventional transformers; however, the exact differences and the means of overcoming the attendant disadvantages and utilizing the advantages present have not to date been extensively explored. With the advent of modern means of communication and the advance of pulse echo systems and the like, as well as the recent emphasis upon particle acceleration in connection with research in the field of physics, the need for improved pulse equipment has arisen.

'It is, of course, apparent to those skilled inV 4`the a'rt that the low power requirements nor- Amally associated with pulsed circuits affect the types and amount of insulation required in elements employed therein and further that the high voltages commonly employed and the limitations upon size of equipment imposed in many applications present new difliculties requiring not only improved techniques but also basic revisions in conventional transformer design.

Accordingly, it is an object of the present invention to provide an improved pulse transformer.

Another object of the present invention is to provide a new and improved method and means for developing very large pulsed voltages.

A further object of the present invention is to provide an improved pulse transformer of compact construction, small size, and large voltage rating.

Still another object of the present invention is to provide an improved pulse transformer having a low leakage inductance.

A still further object of the present invention is to provide an improved pulse transformer capable of efficiently operating upon voltage pulses having a very short rise time and duration and a very rapid repetition rate.

Many other objects and advantages of the invention will be apparent from the following disclosure and claims taken together with the accompanying drawing wherein:

Figure l is a longitudinal view of one embodiment of the invention with portions cut away as indicated to show the internal relationship of the elements;

Fig. 2 is a fragmentary view in cross section taken on line 2-2 of Fig. 1;

Fig. 3 is a block diagram of a suitable electrical circuit wherein the invention may be employed.

Considering the invention as shown in Fig. l, it will be noted that the illustrated pulse transformer I is provided with a hollow open-ended cylinder 2 preferably formed of a non-magnetic metal, such as brass, and having flared ends. Wound about cylinder 2 is the primary winding 3 of the transformer I consisting of a relative few turns. As shown in Fig. 2, primary winding 3 may consist of a thin conducting foil 4 such as .00025 inch aluminum separated by a. plurality of insulating tissue paper strips 5 which may, for example, have a thickness of .001 inch. Primary winding 3 may be advantageously formed by winding an elongated strip of foil l together with strips of insulation 5 about cylinder 2 with the inner end of conducting foil 4 electrically contacting cylinder 2 and extending in part at least past insulation 5 on the outside of the winding to provide a terminal for electrical energization of the winding.

Internal to cylinder 2 and concentric therewith there is provided a second or inner cylinder 6 which is also preferably formed oi' a nonmagnetic electrically conducting materialsuch as brass. Inner cylinder 6 may also be open-ended and end bells 'I having a generally spherical configuration may be disposed over the ends of cylinder 6 in closing relation thereto and may be secured in position by the mating of corresponding annular indentations and protrusions about cylinder 6 and end bells l, respectively. It will, of course, be appreciated that the construction and connection of cylinder 6 and end bells I is not critical and that no limitation is intended by the description and illustration of one specic embodiment.

The secondary winding 8 of transformer I is wound about inner cylinder 6 and extends therefrom to outer cylinder 2 thereby providing convenient spacing means for providing the abovenoted fixed relation between inner and outer cylinders Ii and 2. Secondary winding 8, which may be formed in the same manner as primary winding 3, as illustrated in Fig. 2 and described above, also comprises a continuous or connected strip of conducting material extending from the outer portion of the winding to the inner portion. Secondary winding 8 is preferably electrically connected at its outer edge or circumference to outer cylinder 2 and at its other end or inner circumference to inner cylinder 6.

With reference to the electrical connections of transformer I, a terminal 9 is provided on outer cylinder 2 which contacts one end of both primary and secondary windings 3 and 8 and which may therefore be grounded if desired. A connection II is provided to the outside of primary winding 3 for energization thereof and a connection I2 is made to end bell 1 which is electrically connected to the inner end of secondary winding 8 and therefore constitutes the high voltage connection of the transformer.

Inner cylinder 6 is hollow, as noted above, and may enclose only air or other suitable gas which thereby constitutes the core I3 of the transformer. It will be appreciated that an air core is desirable in that total weight of the transformer is thereby reduced, an external magnetic return path need not be provided, and the problem of residual magnetism isobviated. Certain applications of pulse transformers which require extreme loading of the transformer may best be accomplished by employing another type of core, as for example oriented steel or Permalloy in appropriate laminated form. In order to employ a high permeability core it is necessary to modify the design of end bells 1 as an open path should be provided for the return of the magnetic flux from the core.

Even though only pulsed operation is contemplated for the invention it is desirable to guard against eddy current losses and interferences which tend to become appreciable under certain circumstances. In order to prevent or at least minimize eddy currents, one or more longitudinal slots I4 are formed in outer cylinder 2 and like slots I6 are formed in inner cylinder 6. Also end bells 1 may be provided with a plurality of slots I1 preferably directed normal to the diameter thereof to reduce the eddy currents thereon.

With regard to the design of primary and secondary windings 3 and 8 and inner and outer cylinders 2 and 6 it will be appreciated that electrical breakdown is one of the foremost problems present because of large voltage differentials existing during pulsed operation. In order to maximize the "potential holding qualities of the invention the insulation may be vacuum impregnated with a material such as varnish or other suitable liquid. Also the transformer may be sealed in a tank and immersed in an oil bath, not shown, in which case the impregnating may be accomplished with transformer oil. In the instance where oil impregnation is employed it may be further desirable to remove all air, water, and volatile fractions by subjecting the oil to a prior vacuum process. It will be appreciated that the degree of care required and the precautions necessary to prevent breakdown depend upon the rating and size of the transformer; however, by following the above procedure larger operating potentials are possible for a given size transformer than otherwise.

With regard to electrical breakdown prevention it is to be further noted that the potential induced in the secondary winding 8 of the transformer exists between outer and inner cylinders 2 and 6. In order for the gap between these cylinders to maintain a maximum voltage thereacross without discharge, the cylinders should be e constructed with a ratio of radii substantially equal to l/e, where e is the base of Napierian logarithms. This relationship may be derived from the equation e g-r log, R/r

wherein:

a=potential gradient between cylinders in volts/centimeter e=potentia1 between cylinders in volts r==radius of inner cylinder 8 in centimeters R=radius of outer cylinder 2 in centimeters The above equation relating to g, the voltage gradient, is in turn determined from a consideration of dielectric phenomenon relating to concentric cylinders and based upon the fundamental relation 1p=Ce wherein:

|p=ilux measured in coulombs C=permittance measured in farads e=applied voltage measured in volts.

It will be appreciated that the configuration of the magnetic and electrostatic fields at the ends of concentric cylinders is not the same as those existing along the remainder of the cylinders and in order to validate necessary assumptions in the calculations concerning the dielectric phenomenon and to prevent, discharge between the ends of the cylinders, it is advantageous to take steps to overcome or at least reduce the added displacement at the ends of the cylinders. In the present invention this is accomplished by flaring or helling the ends of outer cylinder 2 and by providng curved caps or end bells on the ends of inner cylinder 6. Construction of the transformer cylinders 2 and 6 in conformity with the above-noted ratio of 1'/R=1/e and end design maximizes the voltage differential that can exist between the cylinders without causing electrical discharge between the cylinders.

Considering now a suitable circuit wherein the above transformer may be employed, reference is made to Fig. 3 illustrating such a circuit in block form. Primary winding 3 of transformer I is connected in series with a pulse storage network 2| and a switch 22. A charging system 20 is connected to pulse storage network 2|, which may, for example, comprise only a pulse capacitor, and a trigger input 23, such as a spark gap, a gaseous discharge device, or other suitable switching means, is employed to actuate switch 22. The secondary winding 8 of transformer I is connected to a load 24 which may be composed of any one of a variety of loads requiring pulsed energy or voltage for operation. In operation the pulse storage network 2l is energized by a charging system to store energy, switch 22 is triggered by a trigger input having a precise time control, and switch 22 thereupon conducts to discharge pulse storage network 2l through primary winding 3 of transformer I. The instantaneous discharge through primary winding 3 induces a very high potential pulse in secondary winding 8 which thereby energizes load 2l. The present invention is capable of operating upon very short duration pulses having a rapid repetition rate and is therefore particularly advantageous for many applications.

As an example of one particular transformer that has been constructed in accordance with the principles of the present invention, the following specifications are given:

Diameter outer cylinder- 7.5 inches Diameter inner cylinder- 2.75 inches Primary turns 7 Secondary turns 450 Coil winding Aluminum foil Solid insulation Condenser paper 'Core .001" Permalloy strips Pulse duration 5 microseconds Pulse rise time 2.5 microseconds Pulse repetition rate 6 pulses per second Primary voltage 5 kilovolts Peak secondary voltage- 450 kilovolts Breakdown voltage 800 kilovolts It will of course be appreciated that peak output voltage and pulse rise time are functions of the primary circuit and secondary load and thus will depend in part upon the individual circuits associated with the transformer. It is to be further noted that the above specifications are given only as an example and are in no way limiting upon the invention. Y

Referring to the above design data and performance figures it will be noted that the voltage step-up or transformation exceeds the turn ratio of the windings. This results from a resonant interchange of energy between the leakage inductance and the secondary capacitance which produces a maximum voltage in excess of that indicated by the turn ratio. From the data set forth above it is apparent that the present invention is capable of producing large voltage pulses and has the added advantages of small size, low leakage reactance, simplified construction. and high efficiency.

Although the present invention has been disclosed with reference to but a sole preferred embodiment, no limitations are intended thereby for it will be apparent to those skilled in the art that numerous embodiments are possible within the true spirit and scope of the invention and thus attention is directed to the following claims for a definition of the novelty of the invention.

What is claimed is:

1. A pulse transformer comprising a pair of concentric hollow split cylinders, a primarywinding about the outer circumference of the larger of said cylinders and electrically connected at the inner end thereof to said larger cylinder, a secondary winding about the smaller of said cylinders and electrically connected at one end to said smaller cylinder and at the other end to said larger cylinder, means impressing pulsed energy between the outer end of said primary winding and said larger cylinder, and means for removing transformed pulsed energy from between said larger and smaller cylinders.

2. A pulse voltage transformer comprising a flrst hollow split cylinder formed of a nonmagnetic electrically conducting material, a secondary winding about the outer circumference of said flrst cylinder and including alternate layers of sheet conducting material and sheet insulation, a second split cylinder concentric about said first cylinder and contacting said secondary winding about the outer circumference thereof, a primary winding about the outer circumference of said second cylinder and including alternate layers of sheet conducting material and sheet insulating material, and electrical terminals at the electrical extremities of said primary and secondary windings whereby said primary windings may be energized from an external s source and said secondary windings may connected across a pulsed load.

3. A pulse transformer comprising'first and second metallic hollow cylinders disposed concentrically, the first of said cylinders being larger and having open ends flaring outwardly from the axis thereof, said second and smaller cylinder having semi-spherical ends thereby substantially enclosing an air core within said cylinder, a pair of transformer win ngs each comprising a flat sheet of electrica iy\conducting material having fiat sheets of electrically insulating material on each side of said conducting material, the first of said windings being wound a few times about the outer circumference of said first cylinder and comprising a primary transformer winding, said second winding beingwound many times about the second of said cylinders and comprising a secondary transformer winding. and electrical terminals at each end of said pair of windings whereby electrical connections may be made thereto.

4. A pulse transformer as set forth in claim 3 further defined by said first and second cylinders having slots formed therein substantially parallel to the axis thereof whereby eddy currents thereon are minimized.

5. A pulse transformer comprising first and second hollow open-ended metallic cylinders of different diameters, said cylinders being disposed adjacent one another with the largest cylinder being concentric about the smaller cylinder, a first winding about said inner cylinder extending therefrom to said outer cylinder and electrically connecting the two cylinders, a second winding about the larger of said cylinders in electrical contact therewith at one end of the winding, and electrical terminals disposed one at the outer end of the second of said windings and one on each of said cylinders, said cylinders having a ratio of radii substantially equal to l/e wherein e is the base of Napierian logarithms.

6. A pulse transformer comprising a first hollow metallic open-ended cylinder having an annular depression about each end thereof, a pair of hollow metallic end bells of substantially semispherical configuration and of substantially the same diameter as said first cylinder, each of said end bells having an anular depression about the open end thereof engaging the like deformities in said first cylinder whereby said end bells are removably fixed to said first cylinder in closing relation thereto, a first transformer winding wound about said first cylinder and having a plurality of turns each including sheet electrical conducting material bounded by sheet insulating material, a second hollow metallic open-ended cylinder disposed about said first cylinder and concentric therewith, said second cylinder having the open ends thereof flared outwardly from the axis of said cylinder, and a second transformer winding formed the same as said first transformer winding and including few turns wound about said second cylinder.

7. A pulse transformer comprising a first hollow metallic open-ended cylinder encompassing an air core, a second hollow metallic open-ended cylinder disposed about said first cylinder and concentric therewith, a first transformer winding wound about the outer surface of said second cylinder and electrically connected thereto. and a second transformer winding wound about the outer surface of said first cylinder and extending to the inner surface of said second cylinder and secured to each cylinder whereby the relative 7 disposition of said cylinders is maintained, said second winding being electrically connected between said iirst and second' cylinders, said cylinders having a ratio of radii substantially equal to 1/ e where e is the base oi Napierian logarithms and whereby the air insulation between said cylinders is maximized.

8. An air core pulse transformer comprising a first hollow open-ended brass cylinder having ilared ends, a primary transformer winding wound about the outer circumference of said cylinder and electrically contacting said cylinder at one end of said winding, a second hollow openended brass cylinder of lesser diameter than said ilrst cylinder and disposed within said iirst cylinder concentric therewith. a secondary transformer winding wound about the outer circumference oi' said second cylinder, said secondary winding being electrically connected at one end to said rst cylinder and at the other end to said inner cylinder, and electrical terminals disposed one at the outer end ot said primary winding and one on each oi said cylinders, the ratio of the radius of said rst cylinder to the radius of said second cylinder being substantially equal to e" where "e is the base of Napierian logarl. whereby the insulation in air between said cylinders is maximized.

No references cited.

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