US2391873A - Transformer - Google Patents

Description

Jan. 1, 1945. c. P. BOUCHER TRANSFORMER Original Filed Sept. 15, 1959 5 Sheets-Sheet l i m w f EM'ZesPBamer Bu d d f /1 /5 flrrO Jam. 1, 1946. c. P. BOUCHER 2,391,373

' TRANSFORMER Original Filed Sept. 15, 1939 3 Sheets-Sheet 2 ('arZayPB mi Jan. 1, 1946. I c. P. BOUCHER TRANSFORMER Original Filed Sept. 15, 1939 5 Sheets-Sheet 5 panel 1... 1. 1946 2,391,373

UNITED s'mrss PATENT OFFICE TRANSFORMER Charles Philippe Boueher, Paterson, N. 1., assignor, by mesne assignments, to National Inventions Corporation, a corporation of New Jersey Original application September 25,- 1939, Serial ruary 26, 1942, Serial No. 432,505

No. 296,522. Divided and this application Feb- 1 Claim.

My application for U. 8. Letters- Patent s a division of my copending application, Serial No. 296,522, filed September 25, 1939, Patent No.

2,321,357, and entitled Transformers, and the incost with which it can be either replaced in its entirety or serviced at the site where it is installed.

- Still another object is to'produce a new method of operating a plurality of luminescent gaseous discharge tubes by transformers, characterized by its simplicity and its flexibility in the disposition of both luminous and transformer units.

Other objects will be obvious in part, and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, features of construction and arrangement of parts, and the. relation of, each of splits the voltage, so that the effective voltage between the outermost turn of the transformer and the ground is but one-half the potential difference existing between the terminals of the vention relates to electrical transformers, and 5 transformer secondary. Thus, while the full more particularly concerns transformers capable rated potential is impressed across the output 0! perating luminescent gaseous discharge tubes, electrodes of the transformer, no more than half and to systems including the same. of that potential will pass to any object shorted An object of my invention accordingly is toproacross the secondary at some point along its duceanew high potential transformer unit which extent. It will be seen that the possibility of is characterized by its extreme simplicity, eflimischance resulting from accidental contact or ciency and flexibility, its increased load capacity, abnormal operation is minimized. the low cost with which it can be connected to 9. Additionally, such multi-coil construction with sign to be energized thereby, and by its ru gedness grounded midpoint permits decreasing the amount and dependability under all operating conditions. of insulation required, because of the lower volt- Another object is to produce suchatransformer ages to be insulated against. Because the inunit which can be readily and simply produced sulation represents an important item in the at appreciably reduced first cost, requiring aminicost of the transformer, an important saving in mum of dies and tool equipment, and which is the quantity of such material employed is folcharacterized by the comparative ease and low lowed by a sharp decrease in the production costs.

The significant point of the foregoing discussion of the present-day transformer designed for luminous tube duty is that such transformers,

even in their simplest embodiment and in their the same to one or more of the others, the scope I of the application of the invention being indithe better the insulation is, the longer is the life cated in the claim at the end of the specifics.- of the transformer. tion- I In connection with what follows, it should be In the drawings: kept prominently in mind that transformers of Figs. 1 and 2 show in front and side elevation the type under discussion represent a. highly derespectively, a shell-type transformer construcveloped and highly competitive field of commerce. tlon embodying my invention. Thus the realization of items of economy in the Fig. 3 illustrates in front elevation my invenproduction of such transformers, be those econtion as applied to a core-type transformer unit. omies immediate or far-reaching in their effect, Figs. 4 through 9, inclusive, illustrate the comis of the utmost practical importance. Parbination of transformer units according to my ticularly is this true when no loss in efllciency or, invention in various circuit connections. serviceability attends the practice of such econ- As conducive to a more thorough understand omies. ing of my invention, it may be pointed out that My investigations demonstrate thatlnthe great in the operation of high potential negative loads, majority of instances, transformers of the type particularly in the operation of luminescent gaseso under discussion having two secondary coil secous discharge tubes, it is customary to use high tions are discarded, due to the defective opervoltage constant currenttransformers. It isthe ation of but one of the secondary coil sections. usual practice to construct the secondary wind- It is seldom indeed that the two secondary coil ings of such transformers in two coil sections and sections are' damaged simultaneously, and I have to ground the midpoint of the secondary. This smallest units, comprise at least a core, a primary coll section, two secondary coll sections, and a casing.

' Experience shows that despite all precautions employed in the manufactur of such transformers, it is the combination of the quality and quantity of insulation employed which is the controlling factor in determining the useful life of the transformer. Other factors being equal,

found that the primary coil section almost never ment.

ard Size transformer.

fails. From the foregoing, it follows that upon failure of but one secondary coil section, the entire transformer must be removed from service and sent 'to the shop for repair or replace This is true despite the fact that the remaining secondary coil sections will still operate effectively. While one-half of the transformer is still capable of proper functioning, the transformer must be replaced by an entire new unit. As typical instances of such failures may be listed shorting of one of the secondary coil sections to the core or casing, or the like.

It becomes evident at once that the unavoidable loss resulting from discarding a transformer because of failure of one secondary coil section is materially increased where the transformer in question is of the multi-circuit type, that is. having a large number of secondary coil sections. In either event, the loss is considerable, and represents an item of some gravity in transformer operation.

An important object of my invention therefore i to remove the foregoing disadvantages attendant upon known high-tension constant-current transformer installations, and to produce a transformer unit in which loss of otherwise serviceable parts thereof will be reduced to a minimum when the transformer unit is rendered service le due to failure of a secondary coil section thereof, and in which the total loss of investment attendant upon such failure will likewise be reduced to a minimum. A further object of considerable importance is to produce a transformer unit of such construction as to avoid the necessity of discarding the equivalent of an entire transformer of the known type upon failure of only part thereof, when the equivalent of onehalf of the known transformer is still capable of functioning in normal manner.

Also, in connection with the prior transformer art, it is customary practice to design transformers having multi-coil-secondary windings in certain standard ratings and load capacities. This practice may be due in part to the rapidity with which costs of tooling, and production costs mount up where, what I term off-sizes, are manufactured in any variety.

Adherence to the practice of producing only certain standard sizes, however, results iii-lack of emciency and economy in equipping many installations with transformers. Particularly is this true where the energy demand of the load is but slightly in excess of the rated output of a stand- To illustrate, assume a 15,000-volt transformer capable of operating say 60 feet of tubing, and also assume a sign to be energized, employing, say, 65 feet of tubing. In such case, although the amount of tubing to be energized over that which can be handled by the transformer at its rated output is only 5 feet, experience shows that such a 65-foot tube could former.

not be operated by the one transformer alone hitherto known, and as pointed out in the foregoing, and to produce a transformer unit which can be used in connection with other transformer units to produce a transformer installation nearly exactly suited to the load demand of the particular sign being energized, all at a minimum of cost of production, installation and maintenance.

In the typical instance pointed out hereinbefore. for example, and employing a 15,000-volt transformer capable of operating 60 feet of tubing, to energize a 65-foot length of tubing, a half-transformer according to my invention, of 3000- or 4000-volts rating, can be added to the first trans- This enables the 65 feet of tubing to be properly operated at a minimum cost of power and installation.

As a corollary to the foregoing statement of object, it may be said that a further object of my invention is to produce a flexible transformer unit of low cost and great durability, which can fill in the gaps in the ratings of standard voltages for which known transformer equipment is designed.

My invention may be applied with equal facility to either a shell-type transformer unit or a coretransformer unit. Examples of such constructions are illustrated in Figures 1 and2, respectively. As shown in Fig. 1, the transformer unit consists of a main leg Ill, and peripherally-disposed return legs ll, l2, l3 and It. Return legs I I and I2 are substantially parallel to main leg l0, while legs I3 and I4 are substantially at right angles ther eto. Legs ll through It comprise a frame, and the main leg ID lies in the frame thus formed and is joined to legs I3 and I l substantially at the midpoints thereof. The joints between the return legs I3 and I4 and the main leg It may be of any suitable type, such as butt joints or the like. However, I find it desirable to form them as pressed-in magnetic joints I5, I5. The said main and return legs I prefer to form of laminated construction, and preferably the lam- I inae are held together against vibration by means suct as bolts or the like. Rivets Ida are illustrated as serving as such Ibinding means. The main leg and return legs together form a closed magnetic path,

Although such construction is not essential, I prefer to construct the frame consisting of return legs II through It and shunt extensions 20 and 2| of laminations, each of which laminations comprises a single'piece in order to enjoy a minimum magnetic reluctance in the main core path. However, I desire it to be understood that the magnetic core can be made of as many parts as is found desirable, and that any manner of bolding such parts together can be used without materially altering the operation of my transformer.

A s.ngle primary coil section It and a single secondary coil section I! are mounted on the main leg In, in spaced relation to each other. These coil-sections are snugly received in recesses I 8, l8 and l9, l9 respectively, provided for that purpose in the core frame.

Shunt extensions 20, 2| are provided on return legs II and I2, respectively, extending between the primary and secondary coil sections inwardly to avoid the diiiiculties in transformer practice as 76 towards and on opposite sides of the centrallydisposed main leg, to points Just short thereof. The shunt extensions are seen thus to provide short air-gaps gl and g2. These air-gaps preferably, but not necessarily, have thesame electrical constants. The functions of these air-gaps will be pointed out hereinafter. Similarly, the design of the coil sections and their function will likewise be described at a later point herein.

I prefer to enclose the transformer unit in a suitable casing, fitting snugly thereabout, and provided with a suitable closure. Such casing is illustrated at 22, having closure 23. Suitable means, such as bracket GI (Figure 2), serve to mount the transformer frame fixedly and rigidly in the center of the casing so that the dimensions of the casing may be reduced to a minimum, thus reducing costs of material. Bracket GI is formed of metal, so as to provide a suitable permanent metallic connection between the transformer core and the casing 22. This casing 22 in turn is grounded to earth, through its bottom member 24, at G2. While I have described and illustrated an embodiment in which a single transformer is housed in a casing, it is, of course, feasible to enclose several transformers in a single box, if such arrangement is found desirable.

A primary connecting block 25 is mounted on the wall of casing 22. insulating material such as porcelain or the like. Any suitable source 26 of alternating-current potential may be connected as by leads 21, 28, to this connecting block. The primary coll section I 8 of the transformer is energized from source 26 through conductors 29 and 30 which connect opposite terminals 'of said primary coil section to connecting block 25.

One terminal of the single high potential sec ondary coil section I! is connected at 3| to the transformer core. Through GI and G2 this terminal is thus maintained at ground potential at all times. The opposite terminal of coil section I! is connected by high potential conductor 32 to conducting bolt 33. Bolt 33 extends through a high potential bushing 34, constructed of porcelain or similar suitable material, fitted in the wall of casing 22.

The high potential conducting bolt connects with one end of a suitable negative load, illustrated in a typical instance as comprising a series of luminescent gaseous discharge tubes 35, 35, which at their other ends are grounded at G3.

The single secondary 'coil section I! is designed to deliver one-half the voltage which is ordinarily developed across both coil sections of hitherto known single-circuit transformers designed for negative load work and having -a plurality of coil sections cooperating to deliver the full rated secondary voltage. In other words, secondary coil section I! is equivalent to one-half the complete secondary winding of single-circuit trans.- formers having the multi-coil secondary windings referred to. It is for this reason that I choose sometimes to designate my new transformer units as half-transformers.

Assume now that current flows from source 26 to primary coil section l6, energizing the latter. Flux is generated in main leg ill, flowing in two separate paths through the return legs Ni, ii and H on the one hand, and l3, l2 and M on the other, reuniting and returning to main leg l0. Of course, in the opposite half-cycle of the charging current, or 1/120 of'a second later in the case of (SO-cycle current, the direction of the current flow, and hence of flux flow, reverses.

- As the flux courses through its path, it links the turns of coil section l1. and builds up'a voltage therein. At first, when the current in primary coil section l8 starts building up at the beginning of any half-cycle of current flow, its value is small, as so too, by consequence, is the value of the flux coursing through the magnetic core. The quantity of flux is as yet not enough to generate a voltage across the terminals of the This block is of suitable secondary coil section I! sumcient to overcome the combined starting resistance of tubes 35. That is, it has not yet reached the striking potential of the system of tubes. Consequently, no current courses through coil section II, and the coursing of flux through the main leg Ill is unimpeded. During such times, the air-gaps gl and 92 offer a relatively great impedance to the coursing of flux through shunt extensions and 2|, so that little, if any, of the flux follows these shunt paths.

At a certain point in each current half-cycle, however, secondary voltage becomes sufficient to overcome the starting resistance of tubes 35. When the value of secondary terminal voltage, known as the striking potential of the system of tubes, is reached, an arc is struck across tubes 35, and current begins to flow through coil section IT. This secondary current itself develops a magnetomotive force which opposes the fiow of the main body of flux, it tends to impede the coursing of the main body of flux through the portion of main leg ID that is linked by the secondary coil section H. In short, the reluctance of the main flux path has momentarily become greater than that of the parallel shunt paths containing the air-gaps gl and 92.

As a consequence, after the flux had divided during one half-cycle of current flow at the righthand end (Figure 1) of main leg II), it flows in two separate paths to return loss II and I2. Being blocked in large art at the left-hand end of main leg I 0, the two streams traverse shunt extensions 20 and 2|, across air-gaps of and g2, and thence back through right-hand end of main leg Hi. Just sufficient flux courses through the left-hand end of the main leg to ensure that the required current is developed in the latter. With current flow in primary coil section l6 still in the same direction, the value of current later begins to fall oil, until the instantaneous voltage induced in secondary coil section I! is no longer sufficient to maintain the discharge in tubes 35. Tubes 35 then extinguish, and the flux, no longer impeded, renews its coursing through the lefthand end of main leg l0. By consequence, coursing of the flux through the shunt paths 2' and 2| is substantially terminated.

During the next half-cycle the direction in which the flux courses through the core is re versed. When the voltage across the terminals of coil section I! reaches the striking potential of tubes 35, a glow discharge is struck and the shunt paths again come into operation. This time the flux courses through main leg ill towards the left in Figure 1, from primary coil section I6. It divides before it reaches coil section I! (because of the counter magnetomotive force generated by the latter), and passes in two par allel paths across air-gaps y I and g2, returning to the right in Fig. 1 through the return legs, to the right-hand end of main leg in. Just sufficient flux courses the left-hand end of main leg l0, returning through return legs H and I2,

to ensure that sufficient current is developed in coming short-circuited. For example, assuming that coil section ll! becomes short-circuited in any manner, the current tending to flow therethrough quickly builds up a counter magnetomotive force which results in flow or substantially all of the main flux through, the shunt air-gaps.

Inasmuch as one terminal of coil section I! is grounded, the opposite terminal thereof, which is connected to tubes 35, is first operated at the rated potential of the'transformer unit above ground potential, and during the next half-cycle at the same potential below ground.

The basic construction of the core-type transformer unit of Figure 3 is quite similar to that ofthe shell-type transformer unit just described. A main leg Illa and return leg l2 are provided, as are return legs I31: and Ma, similar to legs I3 and ll of Figure 1. Shunt'extension 2| and air-gap 92 are provided, while return leg ll,

shunt extension 20 and air-gap gi, all included in the construction of Figures 1 and 2, are omitted in their entirety. Coil sections l6 and I! are mounted on main leg I ll. As described in connection with the construction of Figures 1 and 2, when a discharge is struck across the load on the transformer unit, the shunt extension 2| and air-gap 92 offer the path of least reluctance. The fiux courses through this shunt path, to the exclusion of the left-hand end of main leg l0,

save for just suiiicient flux to maintain the current demand in coil section II.

I construct my new transformer units from substantially the same materials as those used in a single-circuit transformer having a multi-v coil section secondary winding of the same type and engineering. One of my new transformer units requires slightly more than half the iron, steel and copper employed in producing a transformer having a multi-coil secondary such as described in the foregoing. However, where two secondary coil section thereof can be operated from a ground at one end thereof to an outside terminal at the other end thereof without interfering in any way with good results of high induced secondary voltage, secondary current control, and the like. This was found to be true regardless of the polarity of the coi1 sections, provided only that the load connected to the secof my half-transformers" are employed together to energize a negative load the increase in cost of materials over a transformer with a multi-coil secondary is more than compensated by the mark edly decreased cost of servicing or replacing required by defective operation, as by aging of insulation, defective workmanship, accidents, and the like, as well as by the greater flexibility, represented by lower iirst cost of installation. Particularly where servicing is diflicult or costly, manipulation of one of my transformer units, having a weight only slightly more than half that of a transformer with multi-coil secondary, represents a great advantage over practices hitherto required. In general, where luminescent sign tubes are to be serviced where they are located, I find that a transformer having two secondary coil sections can be replaced at a decided advantage by two of my transformer units as described herein. 7

As illustrative of the flexibility of my new transformer units, and to demonstrate their ready adaptability to various loading conditions, attention is directed to the several embodiments depicted in Figs. 4 through 9,- inclusive.

Referring more particularly to Fig. 4, it will be seen that in a typical instance, Iground one end of secondary I! to the core Illa, in turn grounded at G2. The other end of the secondary I connect through lead 32a to one or more tubes 35, which at their opposite ends are grounded at G3. The secondary circuit may be traced as follows:

" Ground G2, core 10a, secondary II, lead 32a,

tube 35 and ground G3. Since one end of secondary I1 is grounded. the other end thereof alternately rises and falls above and below ground potential, as indicated by the plus and minus signs on Fig. 4.

It has previously been demonstrated with reference to multi-circuit transformers that any iii) ' one end thereof.,

It may quite possibly happen that the load, represented by a luminescent gaseous discharge tube sign, for example, may be too large to be operated by a single transformer unit according to my invention. Or it may be impossible to ground the midpoint of such a tube assembly, or the ends of individual lengths of tubes. when the total resistance of the series of tubes constituting the load is too great for one transformer unit, then according to my invention I operate the tubes with two half-transformer units simply by connecting one end of the series of tubes to one transformer unit, and the other end of the series of tubes to another transformer unit. The two transformer units are preferably so selected that together they provide a voltage output substantially that required by the series of tubes. It is desirable that the transformers have substantially the same current ratings in order to enjoy maximum efliciency of both units. It is also desirable that the transformer units have like voltage ratings. However. this last requirement is not essential, and for example, the transformer at one end of the series of tubes may have a 7500- volt rating, while that at the other end may have say a 6000-volt rating.

One example of the foregoing is depicted in Fig. 5, wherein the secondary windings of the transformer units are shown in series-opposition.

' In this case, the free end of the secondary winding of one transformer unit is connected to one end of a series of tubes, while the free end of the secondary winding of the other unit is connected to the other end of the tubes. The polarities of the secondaries, which may be changed as desired merely by reversing their respective primaries, are such that the voltages at their free ends are always positive or negative simultaneously. It is essential in such installations that theseries of tubes have its electrical mid-point grounded. Parallel circuits may be traced from the secondary windings ll, through conductors 82a, tubes 35 and grounds G3 back to the two secondary windings, grounds G2 and cores ill'a.

Such series-opposition assembly may be likened to a parallel connection. It is also feasible to operate my transformer units in full parallel connection, wherein the secondary windings of two or more uniits are connected in parallel at one or both ends of a series of tubes, or one long tube, to increase the availablesupply of current.

As a further possibility, I may connect two or more transformer units in series-aiding relationship. Such an assembly is illustrated in Fig. 6. In that figure, the connections are much similar to those of Fig. 5, except that the polarity of either one of the two transformers is now reversed with respect to what it was in Fig. 5. This ensures that the voltage across theoutside terminals of the string of tubes 3! will be the sum 2,891,878 of the voltages developed by the two secondary windings. Thus the available voltage at tube terminals with the connections shown in Fig. 6 will be twice that available with connections according to Fig. 5. The further difference exists between th series-aiding connection of Fig. 6 and the series-opposition connection of Fig. that while in the last-mentioned case a grounded midpoint of the chain of tubes is mandatory, in the first-mentioned case it is optional. Asa practical matter, it may be dispensed withsince the voltage to ground, assuming transformer units of equal voltage ratings, is never in excess of the voltage developed in one secondary. To illustrate that the transformer units of equal voltage ratings, is never in excess of the voltage developed in one secondary. To illustrate that the transformer units need not be of the same secondary voltages, the right-hand secondary winding II in Fig. 6 is shown as comprising more turns than the left-hand secondary, corresponding to say ratings of 7500 and 6000 volts, respectively.

An example of the manner in which series-opposition connections can be used to advantage is illustrated in Fig. '7. Therein the polarities of all the secondary windings of the several transformer units are the same. The tubes are all disposed extending in the same direction, with their grounded terminals all at the same end, and their transformer-connected terminals at the other end. In this manner, the tubes may be disposed closely adjacent each other, in order to produce a desired pleasing and effective display, without puncture or rupture of the tubes under electrostatic stress. regardless of the materials from which the tubes are formed. In point of fact, the tubes may actually touch each other, or two or more of them may use a common wall, without detrimental effect on their operation, because the potential difference existing across the wall between the tubes is substantially negligible at any point along the tube length.

For simplicity, it is highly desirable at times to connect the primary coil sections of a number of the transformer units in series across a source of supply. For example, four 110-volt primaries may be connected in series across a 440-volt supply, two 55-volt primaries across a 110-volt supply, etc. In general, itis suflicient, in order to employ this connection safely and advantageously, that the number of transformer units will be so large that upon failure of one secondary winding through open-circuiting,'for example, an excessively high voltage will not be induced in remaining secondary windings. Practical operation and economical construction can be obtained, for example, by connecting flve or six of my transformer units with their primary windings connected in series. In a typical case, by way of illustration, five 22-volt primary windings or six 18.3 volt primary windings may be successfully operated on a. 110-vo1t circuit. Such a connection is shown in Fig. 8, wherein the five primary windings l6 are connected in series, by leads 21, 28, across a common source of supply 26, of conventional voltage output.

.A series-connection of the primary windings possesses the advantage of cheapness; but the dis- Finally, as illustrative of the great flexibility of my new transformer units, and their adaptability to all sorts of practical problems encountered in commercial practice, reference should be had to the construction of Fig. 9, wherein is depicted a transformer 36 of multi-coil secondary design, having grounded midpoint 62 and high potential bushings 31, 38 for leading high potential current from the secondary circuits to tubes 39, 40. Energizing current from source 26 enters transformer 36 over leads 21, 28 at connectors 40, 4|.

In the instance illustrated, the secondary coil sections of transformer 36 are considered as in series-aiding connection, although a series-oppo- .,sition relation is perfectly feasible. One secondary transformer terminal 42 is connected through lead 45 to tube 39.

Now by way of illustration, let it be assumed involved in attempting to operate the 66 feet of tubing by such a transformer. It was also suggested that great loss in investment, economy and efficiency resulted in using, for handling such a load, a second transformer of like rating, or a transformer of the next higher standard rating.

However, the problem is readily solved by the use, in conjunction with the main transformer 36, of one or more comparatively small transformer units according to my invention. Accordingly, a 3000 or 4000 volt or like transformer unit following my new invention is shown as connected at the one'end of tube 40 opposite to transformer 36 and in series-aiding relationship with respect thereto. Similarly, a. second transformer unit of 3000 or 4000 volts output is connected in seriesaiding relationship at the free end of tube 39. In this manner, the lacking voltage is provided with a fair degree of precision, and loss resulting from installation, operation and possible repair of equipment of unnecessarily large capacity, is avoided.

g It becomes apparent from the foregoing description that the transformer units according to my invention represent a decided advance in the art, having a lower first cost due to simplicity of tooling and low cost of dies and being subject to comparatively ready servicing or replacement when this is required. The lower production costs make it possible, without increasing the initial cost over known multi-coil secondary transformer installations, to improve the insulation of the secondary coil sections, thereby producing a transformer of longer life and increased durability and ruggedness.

My transformer units, all as pointed out in earlier parts of this specification, possess a flexibility not hereto known, in various combinations, either among themselves or along with known transformers having multi-coil secondary windings, they can be adapted closely to the requirements of widely varying load conditions.

advantage is present that an increase in secondary voltage occurs when one or more secondary coil sections becomes inoperative. It is to be understood, of course, that the type of connection employed for the primary coil sections of my transformer units is independent of the type used with the corresponding secondary windings.

Since the size and construction of my transformer units make it possible to dispose them closely adjacent the tube assembly or other load, the quantity of high potential cable, and hence the cost thereof can be reduced to a minimum. At the same time, a tube assembly is produced which is more compact than is hitherto known. Servicing in situ, hitherto accomplished only with ditllculty, can, by the practice of my invention, be

accomplished with comparative facility. Such servicing has been facilitated by reducing the weight of the transformer units to a minimum.

As many possible embodiments may be'made of my invention, and as many changes may be made in the embodiments hereinbefore set forth, it will magnetic material snugly receiving said core with the coils mounted thereon, the core being secured to the casing by a metallic ground connection; and external terminal means therefor comprising a single high potential bushing assembly protruding through one side of said casing and two relatively low potential terminals protruding through another side, one end of said secondary coil sec tion being grounded to the core and other end thereof being connected with said high potential bushing assembly, and the ends of said primary coil section being connected with said low potential terminals, whereby there is achieved a compact unit with good heat eliminating properties in combination with minimum disturbance of operating characteristics by the casing.

CHARLES PHILIPPE BOUCHER.

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