US 3223923 A
Description (OCR text may contain errors)
Dec. 14, 1965 E. K. HOWELL PULSE TRANSFORMER Filed July 2, 1962 Inven'tor: Edward Hei th H weLL b9 tow-neg United States Patent f 3,223,923 PULSE TRANSFORMER Edward Keith Howell, Hendersonville, N.C., assignor to General Electric Company, a corporation of New York Filed July 2, 1962, Ser. No. 206,612 4 Claims. (Cl. 32376) This invention relates generally to transformers and, more particularly, to a transformer which will deliver a high voltage pulse to effect the starting of discharge lamps.
In starting of discharge lamps, particularly compact xenon arc lamp types, a high voltage pulse is required to ionize the gas. It is an object of this invention to provide a pulse transformer suitable for the above-mentioned lamp. Another object of this invention is to provide such a transformer which is economical to manufacture and of limited power dissipation. It is a further object of this invention to provide an improved transformer which is of compact design and of lighter weight than conventional constructions.
To maintain the transformer of a reasonable size in the lamp starting circuit, the duration of the high energy pulse must be short. Still another object of this invention is to provide an improved transformer having a capacitor incorporated into the winding which is utilized to provide the high energy pulse initially, the design being such that the capacitor does not create serious losses when the transformer is operated continually in the lamp circuit .at lower voltage.
Briefly, in one form of the invention, the transformer is constructed with windings of a plurality of conducting sheets separated by insulation with the outer ends of the sheets being separated to permit insertion of a capacitive layer insulated from and diffused between the separated turns. The result is a transformer having an integral capacitor. Electrical connections from opposite layers or plates of the capacitor may be placed in close proximity to each other to form -a spark gap.
Further objects and advantages of the invention will appear from the following specification.
FIG. 1 shows the basic circuit for a transformer const-ructed in accordance with this invention;
FIG. 2 shows an equivalent magnetic circuit;
FIG. 3 is a cross sectional view of the transformer taken along the line 33 of FIG. 4; and
FIG. 4 is a top plan view of the transformer.
In FIG. 1, a general electrical circuit is shown schematically including a pulse transformer shown in dotted outline and which is made in accordance with the invention. It is connected-to :a high voltage source 11 by conductors 12 and 13 and to a capacitive load 14 and bypass capacitor 40 by conductors 15 and 16. The pulse transformer includes an autotransformer winding 17, a storage capacitor 18, and a spark gap 19. The capacitor 13 is connected to source 11 across conductors 12 and 13. The spark gap 19 is connected to conductor 12 and to tap 20 of the coil winding 17 of transformer 10.
The capacitive load 14, as shown by way of example, is a xenon arc lamp comprising an envelope 21 filled with Xenon gas having a thoriated cathode 22 and anode 23. One form of commercial lamp of this type will operate on 100 amps at 20 volts DC. but requires a high voltage pulse to ionize the gas and form a conductive path between the electrodes for starting. The pulse required is of the order of 3050 kilovolts when applied to capacitive load of the order of 30 picofarads. After the lamp has started, the pulse transformer must be able to carry continuous D.C. current on the order of 100 amperes, supplied by low voltage source 41.
The equivalent circuit is shown in FIG. 2, wherein 24 is the capacitance of the transformer, 25 the primary magnetizing inductance, and 26 the total effective capacitance 3,223,923 Patented Dec. 14, 1965 of the output referred to the primary. The total leakage reactance referred to the primary is shown at 27 and the primary circuit inductance external to the transformer at 28. Switch 29 represents the spark gap 19.
To obtain maximum transfer of energy from the transformer capacitance to the output capacitance 26, it is necessary that transformer capacitance 24 be at least equal to output capacitance 26. It is further necessary that the primary magnetizing inductance be large by a factor of about 10 with respect to the leakage reactance 2'7. Similarly, it is necessary that the primary circuit inductance external to the transformer 28 be smaller by a factor of about 10 than the leakage reactance 27.
In order to minimize the inductance external to the transformer the capacitor 18 is made an integral part of the transformer winding. While separate capacitors may be obtained which would satisfy the inductance requirements of the circuit, such capacitors would be large and expensive. In accordance with the invention the inductance external to the transformer can be reduced substantially to the spark gap by using the construction shown in FIG. 3.
As shown in FIG. 3, the pulse transformer includes a core 30 of magnetic material; the main conductor 31 in the form of fiat sheets is wound spiral fashion about the core 30. Insulation 32 in sheet form is provided between the turns of the conductor and is wound with the conductor. The outer sheets of conductor 31 are separated into two sections 31a and 31b. Between the separated sections of conductor, conducting capacitor foil 33 is Wound integrally into the transformer. The capacitor foil 33 is separated from the split main conductor by sheet insulation 34. The outer ends of the separated conductor sections 31a and 31b are pressed together to form an electrical connection, as shown at 316. This way the outer end of capacitor foil 33 is protected and sealed Within the transformer construction.
Conductive tabs 35 and 36 are connected to the outer end of the conductor winding 31 and to the inner end of capacitor layer 33, respectively (FIG. 3). Another tab 37 is connected to the inner end of conductor 31 and serves as an output connector. All of these tabs are brought from the interior of the transformer outwardly between the layers of sheet insulation 32 and 34. Mounted on the outer ends of tabs 35 and 36 in close proximity to the edge of insulation 34 is the spark gap 19. The spark gap 19 includes cars 38 and 39 formed integrally with tabs 35 and 36 and spaced from each other a distance such as to provide the proper breakdown voltage.
In one form of the invention, the core 30 may be a one-inch diameter iron powder rod which is wound with ten turns of conductor 31. The conductor 31 is composed of fourteen layers of .003 inch copper foil which has, therefore, an area sufiicient to carry at least amperes D.C. continuously. The outer two turns of the main conductor are separated into seven layers each and inserted therebetween is the capacitive foil of one layer of 3 mil copper foil. The insulation between turns of conductor 31, and between the separated conductor layer 31a, 31b and the capacitive layer 33 is provided by several layers (approximately four) of 5 mil Teflon film. As shown in FIG. 4, the Teflon film overlaps the edges of the conductive foil to prevent any short circuiting.
Thus, as discussed with reference to FIG. 2, the pri mary circuit inductance external to the transformer is small in comparison with the total leakage reactance referrred to the primary. A small iron core is used, providing a primary magnetizing inductance restricted to the range from .5 to 5 microhenries. The leakage inductance is approximately .05 microhenry and by making the capacitor integral with the transformer the primary circuit inductance is less than .01 microhenry.
A suitable electric current source 11 being provided and conections being made as in FIG. 3, capacitor 18 is slowly charged towards 15 kv. until the spark gap arcs .over. The spark gap is set to discharge at approximately 10 kv.,at whichtimev the energy from capacitor 18 is discharged into the primary of the autotransformer 17. The outer two separated turns 31a and 31b of the copper foil constitute the primary turns of the transformer out of the ten turns wound on the core 30. The turns ratio being thus properly regulated to give a -to-1 step-up ratio, the output of the transformer 17 is approximately 50 kv. peak amplitude to strike the arc in the lamp. Should the lamp fail to light, a similar voltage will again be built up on the capacitor 18 and the spark gap arcing over will pulse the lamp repeatedly until the lamp starts. The lamp is suitably ballasted during normal operation from the low voltage source 41.
By constructing the transformer of layers of sheet metal material, such as copper, and insulating the layers with sheet insulating material, it is possible to construct a wound transformer of low cost and having a compact design. In such a construction it is possible to incorporate the capacitor directly intov the transformer by separating layers of the sheet metal conductors over any chosen length and use them with an auxiliary sheet as the plates of a capacitor which thus forms part of the transformer winding.
While the invention'has been disclosed by way of a specific embodiment, it will be understood that it may be widely modified within the spirit and scope of the appended claims.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A transformer comprising a core, a conductive winding of sheet material having inner and outer ends and having spaced turns separated by insulation, said sheet material branching into separated portions at its outer end, a conducting layer having inner and outer ends and lying between and insulated from said separated portions of said sheet material, a first conductor connected to the inner end of said sheet material and leading to the exterior of the transformer, a second conductor connected to the outer end of said sheet material and leading to the exterior of the transformer, and a third conductor connected to the inner end of said conducting layer and leading to the exterior of the transformer, said second and third conductors being arranged with their exterior ends in close proximity to each other to form a spark 2. A transformer comprising a core, a spiral winding of conductive foil having an outer end with separated portions, insulating means between the spaced turns of said winding, a conducting layer insulated from and lying between said separated outer end portions of said winding, a conductor leading from the outer end of the winding and a conductor from the inner end of said conducting layer and conductive ears providedbetween said conductors to form a spark gap.
3. A transformer comprising a core and an electrical winding placed in inductive relation thereto, a conductive layer placed in capacitive relation to the electrical winding and a spark gap electrically connecting the capacitive layer and the transformer winding.
4. In a combined transformer and capacitor construction, a core, a conductor in the form of thin layers of sheet material and having inner and outer ends, insulation in the form of thin layers of sheet material, said conductors and said insulation being assembled together and wound around said core to provide a spiral winding extending outwardly from said core with the conductor windings being insulated from each other, said conductor layers being separated from each other adjacent the outer end of said conductor, a capacitive layer of conducting material located between said separated conductor layers, insulating material on each side of and between said capacitive layers and the adjacent conductor layers, and means forming electrical connections to the opposite ends of the wound conductor and with-the capacitive layer including a spark gap between the electrical connections to the capacitive layer and the outer end of the wound conductor.
References Cited by the Examiner UNITED STATES PATENTS LLOYD MCCOLLUM, Primary Examiner.