US2412345A - Voltage transformer - Google Patents

Description

Dec. 10, 1946. N. E. I INDENBLAD VOLTAGE TRANSFORMER Filed Feb. 3, 1943 2 Sheets-Sheet l www ArrogP/vsy Patented Dec. 10, 1946 VOLTAGE TRANSFORMER Nils E. Lindenblad, Port J eierson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 3, 1943, Serial No. 474,539

Claims. 1

This invention relates to improvements in high voltage iron core transformers, and particularly to a novel type of high voltage step-up or stepdown transformer suitable for use in high frequency lsystems operating at frequencies above one megacycle.

Among the objects of the present invention are: To provide a high voltage iron core step-up or.stepdown transformer of increased efficiency, whereby losses due to eddy currents and hysteresis effects are greatly reduced; to provide a well insulated high voltage iron cere step-up or step-down transformer which is inexpensive to manufacture and simple to assemble; to provide a highly eilicient iron core step-up transformer especially applicable at frequencies above those normally considered in connection with iron core designs; and to provide a highly efficient iron core step-up transformer especially suitable for passing microsecond duration pulses of power to such electronic devices as magnetrons, X-ray tubes, etc., and to spark circuits. The primary consideration in the design of the transformer of the present invention is to reduce the length of the magnetic path formed by the core, in order that the rapidly increasing hysteresis and eddy current losses may be considerably reduced by the consequent employment 0f lesser volumes of iron. Of equal importance in the transformer of the invention is that the possibilities of high voltage insulation be maintained. The primary winding has as low resistance as possible and its conductors have the heaviest possible cross-section. The secondary winding is made up of a plurality of secondary winding coils connected in series and wound on separate iron cores having hollow central portions. The core materials for the secondary coils are of good magnetic qualities and havedimensions as small as possible for a certain required cross-section. The primary winding is made to extend through the hollow central portions of all of the iron cores upon which the secondary coils are wound, and its conductors are designed to ll up as completely as possible the hollow centers of the iron cores, due consideration being given to insulation problems to be discussed hereinafter.

The transformer of the invention as described above contains the essential structural features of the invention. A more refined and preferred transformer embodying the principles of the invention includes individual secondary coils each of which is made up of a plurality of windings which are arranged in parallel relationship and cooperate to magnetize the core for the currents plcying a magnetron oscillator. it is preferred.

passed by the transformer. In such an arrangement, the terminals for each individual secondary coil are made to appear at points located on diametrically opposite sides of the Core, thus minimizing insulation and breakdown problems. Where the transformer of the invention is designed especially ior use in connection with a pulsing system of the radio locating type emthat the windings of each individual secondary coil carry heating power for the cathode of the oscillator, in which case the windings are so designed that the heating current does not affect the magnetic circuit of the transformer. This is done by arranging the windings of each secondary coil in parallel relation to cooperate magnetically for the pulsing current and to have these same windings in magnetic counter relation when carrying cathode heating power.

A more detailed description of the invention follows in conjunction with the drawings, wherein:

Fig. 1 illustrates a cross-section of a high voltage iron core voltage step-up transformer in accordance with the principles of the invention;

Fig, 2 is a detail explanatory of one form of construction which the individual cores may take;

Fig. 3 is a schematic showing of a preferred form of a secondary coil which is only one of a plurality of such coils comprising a secondary winding; j

Fig. 4 shows the essential details of a pulse transmission radio locating system for which the transformer of the invention was primarily del signed;

Fig. 5 shows another embodiment of a transformer in accordance with my invention; and

Fig. 6 shows, schematically, how the transformer of Fig. 5 can be used in cascade.

A better understanding of the principles of the invention may be had first by referring to the pulse transmission system of Fig. 4, in which the transformer of the invention is especially suitable. The details of Fig. 4 form part of an obstacle detection radio system which employs a transmitter for transmitting periodically re' peated radio wave pulses of extremely short duration, and a receiver for receiving the pulses which are reected by the obstacle to be detected.

The principles of such a pulse radio locating system are described broadly in my copending U. S. application, Serial No. 441,311, filed May 1, 1942, to which reference is herein made.

The system of Fig. 4 illustrates diagrammatically the power system for supplying the mag-A netron oscillator with high voltage pulses of microsecond duration. In such a radio locating system there is employed a suitable oscillator (here shown by Way of example as a magnetron oscillator I) for producing ultra short Waves below one meter in length, preferably of the order of ten centimeters. This oscillator is shown conventionally as including an anode A and a cathode C. Theanode A is connected to ground while the legs of the cathode are shown connected by means of leads 2 and 3 to individual secondary windings 4 and 5, respectively, cf a high voltage iron core transformer TR having a single primary winding 6. The transformer TR consists in effect of a step-up transformer for converting a relatively low Voltage impressed on the primary windings to a relatively high voltage available at the terminals o-f the secondary windings. This transformer has been especially designed for this purpose and constitutes the essence of the present invention. Low voltage heating power, such as from a source of 110 volts, 60 cycles, is impressed on a transformer 1 for applying heating current to the cathode of the oscillator. In fact, transformer I is a step-down transformer which supplies only about 10 volts potential difference across its secondary winding. It should be noted that the secondary coils l and 5.of the high voltage transformer TR are in parallel relation for the pulsing currents and in series relation for the cathode heating currents. Condensers 8 and 9 connect the outside terminals of the secondary winding of the low voltage stepdown transformer 'I to ground in order to bypass the high frequencies or microsecond pulses. The mid-point of the secondary winding of the transformer 'I is shown directly connected to ground. In order to operate the magnetron oscillator I in pulses, there is provided a low voltage direct current source which supplies a voltage 0f, let us say, 1000 volts to a terminal I0. It is desired that this 1000 volts be converted to a relatively high vo-ltage of atA least 10,000 volts, which is to be periodically applied in pulses of extremely short duration to the cathode of the magnetron oscillator in order to cause the oscillator to produce oscillations solely during the interval in which the pulses of high voltage are appliedto the oscillator. Because the oscillator functions momentarily, it is possible to obtain a higher output from it than during a normal or continuous steady state. It is for this reason that there is applied to the oscillator a much higher voltage than normallyr employed by a magnetron electrode but for a very short period of time,vthus enabling high output power at short waves to be derived from the oscillator. To achieve this result, the 1000 volts direct current continuouscharging source is applied to a large capacity condenser il and to a terminal I2. Between the terminals I2 and I3 there is provided a continuously rotating small capacity condenser I4 Whose terminals make contact with the terminalsv I2 and I3 as it rotates. The direction of rotation of the condenser I4 is shown by the arrows. The motorv drivingl means, for theY condenser I 4 is not shown, but may consist of any suitable motor.` In practice, the condenser I not madeA to rotate but the equivalent result is Obtained by making its connections periodically reverse by, a commutator arrangement. Terminal I3 is connected to the high voltage end of the primary. winding 6 of the. transformer TR while the other end of winding 6 is connected to ground. The, 10.00 volts direct current thus charges the.

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condenser II and the condenser I4 when this last condenser is momentarily making contact with terminals I2 and I3. As the condenser I4 re- Verses its position, there is available for application to the primary winding 6 a momentary pulse of greater (about twice) voltage than would be possible solely by the use of a single condenser` The driving means for the condenser Id (not shown) is rotated at a suitable speed, and the values of the condensers II and I4 so chosen that pulses of 10,000 volts and higher are obtainable from the secondary windings l and 5 of one microsecond duration each, the interval between pulses4 being longer than the duration of each pulse. This interval between pulses can be as high as 500 to 4000 microseconds. For a more detailed description of the operation of this pulse generator scheme, reference made tomy copending U. S. applications Serial No. 479,220, filed March 15, 1943, and Serial No. 477,779 filed March 2, 1943, and issued June4 18, 1946, as United States Patent No. 2,402,422.

I have found that the conventional type of iron core transformer is not suitable for use in transforming pulses of high power and of microsecond duration because of the extremely high losses engendered in them. The high voltage iron coreV transformer now' to be described has been designed to materially reduce these high losses.. k

Referring to Fig. 1, the transformer of the invention comprises a primary winding I5 of several turns of conductor. The conductor for this primary winding has the heaviest possible crosssection in order to create as low a resistance as possible. In practice, I have. employed in different transformers of the invention insulated metallic ribbons and also, as an alternative, large insulated wires. The individual turns of the primary winding are individually insulated by cotton tape impregnated with a thermal setting varnish or equivalent material which will serve the same purpose.

The secondary winding is shown as comprising a plurality of individual secondary coils I6, I6 which are Wound on separate iron cores I1 having hollow central portions. The primary winding I5 is further insulated fromrthe individual secondary coils I6, I6 by means of insulating tubes I8, I8 which extend through the central hollow portions of the iron cores. Insulating tubes I8, I 8 may consist of mica flakes suitably bound together, ceramic materials, glass, or other good insulating material. The secondary coils l 6, I6 are insulated from one another by insulating washers or. discs I0, I9. These secondary coils are connected in series relation by means of leads 20, 20, as shown. The` terminals of the secondary winding have been labeled T, T, while the terminals of the primary winding have been labeled T1.

The iron cores upon which the individual secondary coils are wound are made up of materials of good magnetic qualities, one form of which is shown in Fig. 2. Thecore material shown in Fig. 2 consists of a laminated core made up of a thin ribbon of soft iron of good magnetic quality. Iheri-bbon is insulated by enameling so that the Y adjacent surfaces of the ribbon are insulated from each other as they are wound or rolled up to make up the core.

By using a secondary consisting of several coil elements each having a separate core surrounded by its winding, the voltage from the secondary winding to core has been proportionally rewinding and core can be reduced. This makes it possible to use smaller diameter cores for a given hollow central area necessary for the primary. Furthermore, this method results in distributing the overall secondary voltage in such a way that no high voltage differences occur between any adjacent portions of the secondary winding. Thus, each core surrounded by a secondary coil will assume approximately the mean potential between the terminals of the coil. In order to nx this mean potential for the core, so as to obviate possible changes due to losses in the transformer, I may, if desired, electrically conneet the mid-point of each secondary coil to the core material itself.

Let us assume that the primary winding has 1000 volts across it, and that it is proposed to stepup the voltage to 10,000 volts. Since the secondary winding is made up of ten secondary coils connected in series, as illustrated, and by way of example let us assume that each coil has ten turns per primary turn, then the voltage available at the terminals of the secondary winding as a whole will be 10,000 volts, although the voltage across the terminals of each individual secondary coil will be only 1000 volts. The core surrounded by each secondary coil will then have a mean potential of 500 volts relative to the terminals of its surrounding coil. In this way I have been able to distribute the potential throughout the transformer, which is of considerable advantage because it reduces the chance of the insulators breaking down.

The assembly shown in Fig. 1 enables me to reduce the volume of iron in the transformer and the consequent eddy current and hysteresis losses, thus permitting the transformer to be used at extremely high frequencies. The reduction of the potential difference across each individual secondary coil permits the diameter of thecore to be made correspondingly small and still provide the necessary inside secondary coil dimensions which are lxed by the secondary to primary insulation requirements. The compactness and simpliiied mechanical construction of the transformer illustrated, which enables easy assembly, are additional advantages of the invention.

The high voltage iron core transformer of the invention has been described in connection with Fig, l only, with due regard to its essential elements. Each secondary coil may consist of the conventional one winding. It is preferred, however, for reasons which will now be given, that each secondary coil have two windings of the same number of turns and current carrying ability. By using two windings in a manner schel matically illustrated in Fig. 3, the potential difference between the terminals of each coil can be made to appear at diametrically opposite sides of the secondary coil, thus reducing the chances of insulation breakdown to a minimum. These two windings for each secondary coil are arranged to be used in parallel relation for the power transferred from the primary to the secondary. Thus, the currents through the two windings are in the same direction and cooperate magnetically for the pulses of power transferred from the primary to the secondary winding. The actual number of turns illustrated in Fig. 3 is by way of example only, for purposes of explanationy and not to be construed as limitative.

In using the transformer of Fig. l in the particular circuit arrangement shown in Fig. 4, the two windings on each secondary coil are'arranged Lil) to be in parallel relation for the pulse current in the secondary winding, and in series relation for the heating currents for the cathode of the oscillator. The windings are, of course, so wound that the pulse currents ltherethrough are in the same direction and cooperate magnetically relative to the core, while the heating currents through the two windings of each secondary coil are in series and in opposite directions, thus cancelling insofar as their magnetizing effects on the core are concerned. Thus, the potential difference between the terminals of each winding of the secondary coil, measured between points on diametri-cally opposite sides of the core can be of the order of 1000 volts, while the potential difference between adjacent terminals of the two windings of the same secondary coil measured on the same side of the core would be of the order of 10 volts. It should be noted that one of the windings on the core of Fig. 3 has its midpoint connected to the core by a lead 25. This connec- -tion iixes the mean potential for the core relative to the terminal voltages. The full length arrows on .the windings of Fig. 3 indicate Ithe direction of the pulse currents -through the two windings, while the arrows shown dotted indicate the direction through the two windings of the heating currents for the cathode of the oscillator. It should be noted that the pulse currents are in the same direction and thus in parallel relation, whereas the heating currents are in opposite directions and in series relation.

Where it is not desired .to use the secondary windings to pass heating currents for the lament of the oscillator, then the adjacent terminals of ,the two windings of Fig. 3 can be connected together, in which case, as before, the potential difference for each secondary would still appear on diametrically opposite parts of the core.

In employing a plurality of windings for each secondary coil in the manner illustrated in Fig. 3, it should be borne in mind that if a certain turn ratio between the primary winding and secondary winding of .the transformer is desired, each winding on each secondary coil should have this same turn ratio, Thus, in the event .two parallel windings are employed on each secondary coil, and it is 4desired that each secondary coil have a ten-to-one ratio, there will be required on each core a total of twenty turns for each primary turn. Obviously, if there are three primary turns in the primary winding and it is desired to have a ten-to-one ratio, then each individual winding of each secondary coil as shown in Fig. 3 should have thirty turns, making a total of sixty turns for the individual secondary coil.

In one embodiment of the invention of Fig. l actually tried out in practice in connection wi-th the radio locating system of the type shown in Fig. 4, I was able to step-up the voltage from 1600 volts across .the primary to approximately 23,000 volts across the secondary, and to apply these stepped-up voltages to the cathode of the magnetron oscillator in pulses of one microsecond duration separated by intervals as high as 4000 microseconds, using a 1 to 15 voltage step-up transformer.

Fig. 5 shows another embodiment of the invention wherein 4the primary and secondary windings consist of con-centric tubings of minimum radial dimensions permissible when using the best possible insulating material. The arrangement of Fig. 5 also achieves a considerable reduction in the radius of the iron core.

Referring to Fig. 5 in more detail, `the primary whirling consists of three metallic tubesj |00, |0|

the ends of one of these tubular conductorsrepresent the terminals of the primary winding The secondary winding consists of .an insulated conductor |04 which is looped through the primary tubes |00, I0! `and |02 in series relation. The conductor |04 is insulated from each of the tubes through which it is looped by means of a highly enicient dielectric material |05, which separates the wire 04 from the surrounding tube. The insulation material |05 may consist of high grade rubber, ceramic material or thermal setting varnish which fills the space between the primary tubular conductor and the interiorly located secondary conductor. The terminals of the secondary winding are labeled |06, |00. Each primary tubular conductor |00, |0| and |02 is surrounded over its leng-th by a multiplicity of spaced iron cores |01. These iron cores are laminated affairs and may consist of a thin ribbon of soft iron of` good magnetic quality which is enameled so that the adjacent surfaces of the ribbon are insulated from each other as they are wound or rolled up to make a core. The core may, of course, be made up of iron washers or discs of thesame material which are insulated from each other and stacked up to constitute the core. These cores, when' made of a ribbon as described above, are spaced from one another by air gaps which enables the heat to escape from the edges of the cores. These air gaps are very small to permit as many cores as possible to be placed around each tubular conductor. When washers are used instead of a ribbon to make up the core, there may be one core for the entire length of each prima-ry conductor, the necessity of sectionalizing the core then disappearing due It0 the fact that the edges of each washer will, by contact with the air, provide suicient heat radiation and convection.

With a transformer having the design of Fig. 5, I am able to obtain iron cores of smallest possible diameter, with a consequent saving of the volume of the iron required to make the core. An inspection of Fig. 5 will show that there is a voltage transformation ratio of one-to-one between each individual tubular conductor |00, |0i or |02 and the wire looped therein, but there will be a total over-all voltage transformation ratio of one-to-three between primary and secondary due to the fact that the voltages of the three portions of the conductor |013 within the three tubular conductors additively combine. if one end of th-e primary winding is grounded and one terminal |06 `of the secondary winding is connected to the other end of the primary winding, it is possible to obtain an additional voltage transformation stepup from the output of the secondary winding which is equal to the primary voltage. In doing this, however, care should be observed in selecting the particular terminal of the secondary which is to be connected to the end of the primary farthest removed from the grounded end, in or- In Fig. 6, |||l represents one step-up transformerV of, let us say, 2000 volts, in which case there will be obtained from the terminals of the secondary Winding a voltage of 6000 volts due tothe one-tothree step-up voltage transforma-tion ratio. The iron cores may be made up of a ribbon affaineach one inch wide, so that there are seven cores per tubular conductor. If, however, as described above, one end of the primary is grounded and the proper terminal of the secondary connectedv .to the ungrounded end of the primary, therewill be obtained .a voltage of 8000 volts between ground and the other or high potential terminal of the secondary.

Fig. 6 shows a system employing'a plurality of transformers generally of the type shown in Fig. 5, in cascade arrangement, in order to obtain additional step-up voltage transformation ratios.

and l represents another step-up transformer. It should be noted that the output of transformer l0, represented by lead I5, is applied to the pri- 1. A high voltage iron-core step-up or stepdown transformer comprising a low resistanceV primary winding of a plurality of insulated turns arranged in substantially rectangular form, insulating tubes surrounding the turns of said primary on a pair of opposite sides of said rectangular form, a plurality of secondary coils wound on separate iron cores located on each insulating tube, said cores having good magnetic qualities and hollow central portions, insulating Washers separating adjacent secondary coils, leads connecting said secondary coils in electrically series relation, a connection from the midpoint of each secondary coil to the iron core which it surrounds for establishing the mean potential of the iron core, said iron cores having relatively small `diameters and being closely spaced from the insulating tube which it Vsurrounds, whereby there is obtained a compact assembly having a short length of magnetic path which engenders a minimum of loss.

2. A high voltage transformer comprising a first winding, and a second winding inductively coupled thereto, said second Winding being made up of a plurality of coils arranged in electrically series relation and wound on separate metallic cores, said cores having hollow centralY portions and being made up of material having good magnetic qualities, said rst winding extending through the hollow central portions of all of said cores, each of said plurality of coils comprising a pair of windings of the same number of turns wound on different portions of the same core in such manner as to cooperate to magnetize the core for the currents to be passed by said transformer, the terminals of said last windings being located on diametrically opposite sides of the' same core, and separate connections between the pair of windings on each core and similar windings on an adjacent core.

3. A high voltage iron-core step-up transformer comprising a primary Winding of low resistance, a secondary winding composed of a plurality of coils connected in series and Wound on separate iron cores having hollow central portions, said primary winding extending through. the hollow central portions of. -all of saidA cores, each of said coils having a plurality of windings o-f the same number of turns wound on different portions of the same core in such manner as to be in parallel relation for the currents to be passed by said transformer, the terminals of said last windings being located on diametrically opposite sides of the same core, and separate connections between the pair of windings on each core and similar rings on an adjacent core.

d. A high voltage transformer comprising a first winding, .and a second winding inductively coupled thereto, said second winding being made up of a plurality of coils arranged in electrically series relation and wound on separate metallic cores, said cores having hollow central portions and being made up of material having good magnetic qualities, said first winding extending through the hollow central portions of all of said cores, each of said plurality of coils comprising a pair of windings of the same number of turns wound on different portions of the same core in such manner as to cooperate .to magnetize the core for the currents to be passed by said transformer, the terminals of said last windings being located on diametrically opposite sides of the same core, said transformer being devoid of conductive connections between the windings on the same core.

5. In combination, a high voltage transformer comprising a primary winding and a secondary winding, said secondary winding including a coil composed of a plurality of windings wound on the same metallic core, the output terminals of said last windings being located adjacent each other on the core and diametrically opposite the input terminals of the same windings, an electric tube having a iilament whose legs are connected to said output terminals, a source of low voltage heating power for said filament coupled to said input terminals, and means for applying periodic voltage pulses of very short duration to said primary winding, said coil windings being so arranged that -they are in parallel relation for the pulse currents which are in the same direction through said windings and cooperate magnetically relative to the core, but in series relation for the heating currents which are in opposite directions through the coil windings and thus ineffectual in their magnetizing eifects on the core.

6. In combination, a high voltage transformer comprising a primary winding and a secondary winding, said secondary winding including a coil composed of a plurality of windings wound on the same metallic core, an electric tube having a filament whose legs are connected to the output terminals of said last windings, a source of low voltage heating power for said filament coupled to the input terminals of said last windings, and means for applying periodic voltage pulses of very short duration to said primary winding, said coil windings being so arranged that they are in parallel relation for the pulse currents which are in the same direction through said windings and cooperate magnetically relative to the core, but in series relation for the heating currents which are in opposite directions through the coil windings and thus ineffectual in their magnetizing eifects on the core.

7. A transmitter for sending out pulses of high frequency energy comprising an electron discharge levice oscillator having a ilament, a transformer including a primary winding and a secondary winding, said secondary winding including a coil composed of a plurality of windings wound on the saine metallic core, connections from the output terminals of said last windings to lthe legs of said nlament, a source of low voltage heating power for said filament coupled to the input terminals of said last windings, and means for applying periodic voltage pulses of very short duration to said primary winding, said coil windings being so arranged that they are in parallel relation for the pulse currents which are in the same direction through said windings and cooperate magnetically relatve to the core, but in series relation for the heating currents which are in opposite directions through the coil windings and thus ineffectual in their magnetizing effects on the core.

8. In combination with a source of short duration pulses, a load on which it is desired to impress short duration pulses of an amplitude different from the lirst-mentioned pulses, a transformer comprising a pair of windings magnetically coupled to each other, a circuit coupling said source to said windings, capacitive by-pass means connecting together one end of said pair of windings, and connections from the other end of said windings to points on one side of said load circuit, the constants of said transformer windings being such as to produce short duration pulses at said load.

9. In combination with Aa source of short duration voltage pulses, an electron discharge device oscillation generator having a filament on which it is desired to impress short duration pulses of an amplitude different from the firstmentioned pulses, a transformer comprising a pair of windings magnetically coupled to each other, a circuit coupling said source to said wings, capacitive lay-pass means connecting together one end of said pair of windings and connections from the other end of said windings to the legs of said lament, a capacitive connection across said lament, and means coupled to said pair of windings for supplying heating current to said nlament.

l0. A transmitter for sending out pulses of high frequency energy comprising an electron discharge device oscillator having a filament, a transformer including a primary winding and a pair of secondary winding coils, a source of pulses coupled to said primary winding, connections from one end of said coils to the legs of said lilament, a source of filament heating power coupled to the other end of said coils, capacitive bypass means connecting together said last end of said coils, said pair of secondary winding coils being so arranged that they are in parallel relation for the pulse currents which are in the same direction through said coils but in series relation to the iilament heating currents which are in opposite directions through said coils.

NILS E. LINDENBLAD.

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