US2615067A - Luminescent tube system and apparatus - Google Patents

Description

Oct. 21, 1952 J. H. BRlDGES LUMINESCEN'I TUBE SYSTEM mo APPARATUS 2 SHEETSSHEET 1 Original Filed March 5, 1945 1&1

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dome HEROLD EmoeES %,%7 7- HS ATTORNEY Oct. 21, 19 2 J. H. BRIDGES 2,615,067

LUMINESCENT TUBE SYSTEM AND APPARATUS Original Filed March s, 1945 2 snsms smzw z INVENTOR.

Jon-m HEROLD BRmGES H l5 ATTORNEY Patented Oct. 21, 1952 LUMINESCENT TUBE SYSTEM AND APPARATUS John Herold Bridges, Fairburn, Ga., assignor to National Inventions Corporation, a corporation of New Jersey Original application March 5, 1945, Serial No. 581,055, now Patent No. 2,510,209, dated June 6, 1950. Divided and this application October 1, 1946, Serial No. 700,428. In Canada August 1 Claim.

My present application for patent is a division of my copending application, Serial No. 581,055, filed March 5, 1945, now Patent No. 2,510,209, issued June 6, 1950, and entitled Luminescent Tube System and Apparatus, which in turn is a continuation-in-part of my copending application, Serial No. 448,471, filed June 25, 1942, and entitled Luminescent Tube System, now Patent 2,370,635 of March 6, 1945, and the invention relates to transformers and power units for powering fluorescent systems and other tube loads of negative resistance characteristics. As well, it relates to the lamp system itself, including the tube load.

An important object of my invention is to provide a transformer unit characterized by its small iron and copper requirement, its consequent low weight, its compact, self-contained and unitary design, with small space requirements, its physical sturdiness and low first cost, both of materials and of assembly, and, as well by its low core loss and high efliciency.

Another object is to provide a new transformer of such construction that detrimental over-loading is effectively prevented at all times, in which all necessity of protective fuses or other safety mechanisms is avoided, the entire construction being in full compliance with all requirements of the Fire Underwriters.

Another object is to provide a power unit comprising a transformer, together with a single power-factor correcting auxiliary, which power unit is small, compact, extremely rugged, and comparatively light in weight, and which is capable of operating its associated load in reliable,

efficient and entirely satisfactory manner.

Yet another object is to provide a tube-lighting system embodying my new power unit, which, capable of energizing a plurality of fluorescent lighting tubes or similar load in the substantial absence of detrimental stroboscopic effect, gives rise to appreciably improved wave-form, a system power-factor of substantially unity value, with quick-starting characteristics even under coldweather operation, with subsequent steady operation, and which will permit the satisfactory use of hot-cathode tubes, where desired, with attendant appreciable increase in tube life and freedom from the requirement of constant supervision.

Another object is to provide a lighting assembly of the type described in which current flow is at all times limited within safe values, even in the instance of failure or short-circuit of one or more elements of the assembly, and which at the same time is substantially immune to damage from unauthorized or negligent handling, system failure, or the like.

Other objects in part will be obvious and in part pointed out hereinafter during the course of the following description.

My invention accordingly resides in the various elements and features of construction, and in the combination of parts, the scope of the application of all of which is more fully set forth in the claim at the end of this specification.

In the drawings wherein I have disclosed several embodiments of my invention which I prefer at present,

Figures 1 and 2 schematically illustrate two circuits embodying my inventive thought, while Figure 3 illustrates, in perspective, my new power unit.

Throughout the drawings like reference characters denote like parts.

In my copending application, it has been fully revealed how fluorescent tube lighting, over say the past decade, has met with widespread acceptance throughout the arts, commerce, industry, and as well, in almost all types of household utilization. The marked advance in efficiency, and in reduced operational costs, the absence of heat and many other characteristic advantages as contrasted with the Edison type of filamentary lamp readily explains the warm reception with which this type of illumination has been received.

Basically, illumination occasioned by space discharge through a suitable gas medium, with resultant emission of visible radiation, is old. The mercury arc lamp, introduced in the late 1800s, is the prototype of arc discharge illumination.

Its cold, unfriendly color, however, displeasing' and unsatisfactory to the normal eye, limited its use to those few instances where bold relief and absence of shadows transcended in importance a pleasing and quieting effect. Printing industries in particular, certain factory operations on precision parts, and industrial uses in general, defined the sphere of application of this mode of lighting.

Space discharge tubes containing a filling of various ones of the rare or monatomic gases next were evolved, producing illumination of characteristic colors which were effectively embodied in display and advertising arrangements, wherein an unusual, eye-catching appeal was desired rather than brilliance or intensity, in both of which qualities they were almost entirely lacking.

The really important advance was made, however, when it was found that by providing a space 3 discharge tube with a filling of a suitable medium, such as mercury, a rare or monatomic gas, or a mixture thereof, and at the same time, by providing a lining in the tube of a suitable fluorescent salt or salts, it will emit a visible secondary radiation, when energized at approximately a fixed wave length in the ultraviolet spectrum, which is pleasing to the eye and of adequate intensity. By suitably selecting the salt or combination of salts-these salts are termed phosphorsa wide variety of characteristic colors of radiation can be obtained, without appreciable variance in intensity of emission. As has been stated, high operating eificiencies are obtainable as compared with the old type filamentary lamp. High lumen and candle-power output are achieved for minimum watt input. These efficiencies are perhaps 2 to 2 times those of filamentary lamps. Additionally, such lamps display a useful life of from 2 to 2 /2 times that of the best present day filamentary lamp. In this manner such equipment adequately compensates for the somewhat higher first cost of the fluorescent tubes. It possesses an elasticity of application totally unapproached by the relatively inflexible, one color filamentary lamp. Batteries of two, three, or more tubes may be arranged, either in single or multiple battery combinations, for floodlightin effects in which, to illustrate, various colors may be combined in brilliantly unusual display effects.

As has been stated, the more conventional type of fluorescent lighting has invaded the home, and already is in widespread use in baths, for reading and desk lamps, in kitchens, play rooms and the like.

Perhaps the principal limiting factor on the present-day utilization of fluorescent lighting is basicly economical in nature. The first cost of such equipment, although not'prohibitive, particularly when computed on a candle-power and life basis, is nevertheless somewhat higher than is true of filamentary lighting. The first cost of the tube is higher, while costly auxiliaries accompanying and comprising part of the power unit materially contribute to appreciable first cost. Despite this, however, the rich rewards showered upon the pioneers in this field enticed many manufacturers who quickly began to vie among themselves with strenuous programs, for the fruits of exploiting this mode of lighting.

Such competition within the industry itself, together with the vigorous competitive eiiorts asserted by the producers of filamentary equipment, contributed to rapid development work within the art. Changes, refinements and improvements, seemingly small in themselves, resulted in practical positional advantages in the market of important magnitude, giving to their owners a desirable position in the industry. Needless to say, such development work was directed towards generalized improvement.

Another important defect in the earlier fluorescent equipment was the slow starting characteristics of such tubes. Particularly was this noticeable in comparison with filamentary lighting. Whereas these latter illumine immediately upon manipulation of the control switch, appreciable time lag is observed in fluorescent lighting between throw of the switch and striking of the arc. Cold weather conditions emphasize this time lag. On many occasions an operator, unused to this phenomenon and observing no immediate response upon throw of the switch, will again operate the latter, thereby de-energizing the line.

Any appreciable decrease in this time lag obviously represents an important advance in the art.

Additionally, fluorescent tubes of the conventional hot cathode type, that is, employing a hot cathode at each end of the tube, While having long life as contrasted with available filamentary equipment, nevertheless do not display the long period of usefulness predicted for them. In practice, the arc was found to localize on a particular point of the filament, burning, through comparatively rapidly. Rectifier action developed, with accompanying overload on the auxiliaries, frequently attended by failure of these latter, should the defective tube be not quickly replaced. Too, the electron-emissive qualities of the filamentary electrodes would deteriorate comparatively rapidly. This phenomenon, accompanied by occlusion of gases within the tube walls, the electrodes and other parts of the tube, resulted in hardening" and increased tendency towards slow starting, particularly in cold weather operation.

Tubes of this general type required starting potentials substantially in excess of operating potentials. When operated from lines of ordinary rated voltages, the complicated, expensive and fragile auxiliaries already referred to had to be employed. Not only did these constitute an important item of first cost, but as well, they required constant supervision and frequent replacement. In such assemblies poor system performance is observed, and energy costs are increased, due to poor Wave-form resulting from the auxiliaries, and poor system power factor. Stroboscopic effect is observed, even where the tubes are employed in batteries.

In large measure a number of these difficulties, deficiencies and disadvantages of the prior art were overcome by the construction according to my said copending application. Therein an assembly is illustrated in which a high leakage reactance transformer serves to energize the tube load. This load comprises two or more tubes, each connected across a corresponding one of two secondary windings of the transformer. Magnetic shunts are disposed in the core between the primary and one secondary, to serve as ourrent-limiting means. A condenser is interposed in the line of the other secondary, this serving to limit the curent in that winding as well as to improve operating power factor of the system as a whole and reduce stroboscopic effect.

A sturdy, reliable unit results, which while initially costing slightly more than the prior constructions, with their iron core ballasts and other auxiliaries, displays much longer life in operation. It is far more sturdy and reliable. Simple in detail, minimum servicing is minimized. Detrimental stroboscopic effect is substantially eliminated.

Because of the high primary flux linkage with the secondary windings when the primary coil is first energized, and before the arc is struck across the tube loads, open-circuit voltages of important magnitudes are developed almost instantaneously. As a result, arc-striking time is reduced, even under cold weather operation, from about 6 to 7 seconds or more, to a mere fraction of a second. For the first time, instantaneous starting of fluorescent tube lighting equipment is approached. Moreover, rapid reignition of the arc is ensured, when for any reason it is momentarily quenched.

It developed nevertheless, in the practice of my invention as disclosed in my said copending application, that certain defects existed, detracting to a certain extent from the utmost realization of the possibilities thereof. To illustrate, because of the close coupling between the primary and the one secondary in which the condenser is included, then if for any reason the condenser should become short-circuited, some protective device is required to prevent system failure by reason of the flow of an excessively high current. A comparatively expensive auxiliary such as a fuse, circuit breaker or the like, is required. Servicing of most such protective devices is required after they have once functioned to protect the line. To avoid this requirement has been indicated as a highly desirable expedient.

Another instantaneous starting fluorescent lighting system is disclosed in the copending application of Boucher and Kuhl, Serial No. 415,964, filed October 21, 1941, and entitled Luminescent Tube System and Apparatus. In that application, there is disclosed the idea of providing two separate primaries on the same high leakage re actance core, each primary being associated through the intermediary of magnetic shunts giving rise to high leakage reactance, with its corresponding secondary. Such disposition, however, requires substantial investment in both copper and iron.

An important object of my invention, therefore, is to provide a new fluorescent tube lighting assembly, embodying both a new transformer and a new power unit which includes such transformer, which while avoiding in large measure the aforementioned disadvantages and deficiencies, possesses the multiple advantages of re quiring a minimum investment in copper and iron, and consequently is neat, sturdy, compact and self-contained, lending itself to ready mounting on the reflector or other mounting of an associated fixture, and which displays high efiiciency with good system power factor, quick-starting characteristics under cold weather operation, in the substantial absence of detrimental stroboscopic flicker, and which is substantially foolproof, no circuit protective auxiliaries being required.

Referring now more particularly to the simplest embodiment of my invention, attention is dirooted to Figure 1 of the drawings, in which there is shown a transformer with secondary coils inductively associated with the primary winding in ordinary transformer connection. A magnetic core, indicated generally at I0, is here illustrated as being generally of the shell type, having a central, longitudinally extending leg II, constituting an inner core portion, flanked in spaced relation,

on either side thereof, by outer longitudinally-extendin legs l2 and I3, constituting outer core portions. Leg Ii is designed to accommodate, without saturation, the maximum magnetic flux developed by the primary winding later to be decribed.

It is not essential that legs I2 and I3 be of like dimensions or shape, or that they be equally spaced from central leg II. Accommodation for any variation in such design can be accomplished by compensating variations in other features of design. However, for symmetry as well as for many other reasons involving electrical and mechanical design, I prefer to construct legs I2 and, I3 of like configuration, and to space them equi-distant from central leg H. This gives a more readily balanced condition of magnetic flux.

Inasmuch as the flux from leg II divides substantially equally between legs I2 and I3, his

sufllcient that each of these have a cross-sectional area substantially half that of leg I I. End members or pieces I4, I4 and I5, I5 serve to interconnect opposed ends of legs II, I2, I3 so as to form a closed magnetic core, of the shell type. In the construction shown, these. in reality constitute end-leg extensions Y of substantially U-shaped core members, the yoke portions of which are constituted by legs [2, I3, respectively. It is entirely satisfactory, however, to construct them in any desired suitable manner, the controlling criterion being that the legs are stamped from suitable sheet laminations, and are subsequently assembled in the manner giving rise to the highest over-all efiiciency. However they be stamped and assembled, the legs II, I2, I3 and end pieces l4, I4 and I5, I5 are constructed'in stacks of lamina of suitable steel, displaying little residual magnetism. End pieces I4, I4 and I5, I5, for obvious reasons have cross-sectional areas approximately those of legs I2, I 3.

Intermediate the lengths of legs I I, I2, and I3, and extending either from leg II towards but short of legs I2, l3 respectively, or as shown, from legs I2 and I3 towards but short of leg II, are a plurality of magnetic shunts 16, I6 and I1, I1. These shunts, of laminar construction, may be formed in any suitable manner. In the present instance, they are struck integrally from legs I2 and I3, but certain savings in iron are had by employing laminated inserts wedged between the windings. Moreover, it is preferred, as generally indicated in Figure 2, that the width of shunts IT, I! be'substantially that of the shunts I6, I6. I find that shunts I1, I! need only be about one-half as wide as shunts 16, I6 for reasons dealt with hereinafter.

The shunts I6, IE and IT, IT, terminating short of central leg II, provide included air-gaps GI, G2, G3 and G4 therebetween. In either the integral or insert construction, however, it will be noticed that air-gaps GI, G2 are shorter than air-gaps G3, G4 giving rise to greater reluctance. The purpose of this design will be developed at a later point. It is sufiicient here to note that these air-gaps are calibrated in accordancewith the particular load demands on the corresponding secondary windings later to be described.

The magnetic core shunts I6, I6 and I1, I1, together with the remaining elements of the magnetic core, define a plurality of compartments CI, C2 and C3, serving respectively to house primary winding I8 and secondary wind ing SI, S2. Since primary I 8 serves to energize bot-h secondaries SI, S2, only a single primary Winding I8 is required. While this winding must be of substantially larger size wire than would be required were separate primaries used, nevertheless a substantial saving in copper results from the use of one primary windingonly. Moreover, the reduction in physical dimensions made possible by the elimination of one primary winding employed in the Boucher-Kuhl construction makes feasible a substantial decrease in the total iron content of the transformer. "This is brought about by decreasing the lengths or the magnetic paths, and results in appreciable decrease in weight, with corresponding gain incompactness. A small, neat-appearing unit is thus made possible. v

Primary I8 of step-up transformer I0 is therefore constructed of a relatively small number of turns of comparatively heavy gauge wire. It is energized from any suitable source of alternating current energy indicated generally at I9. In'm'y preferred embodiment this comprises either a 7 volt or 220 volt line. Leads 20, 2I serve to establish this connection.

Paired fluorescent discharge tubes 22, 23 are included in circuit with secondary windings SI, S2, respectively. These fluorescent tubes may either be especially designed for cold-cathode operation, in which instance they will likely be fashioned with a single, solid electrode at each end of the tube, or else the conventional hotcathode tube, now readily available on the market, can be adapted in comparatively simple manner for entirely satisfactory operation under cold-cathode conditions. The electrodes of these tubes are conditioned for such cold-cathode operation by installing a jumper across the paired terminals of the filamentary electrode provided at each end of the tube. The shortcircuit thus established ensures that all parts of the electrode are at the same potential. Thus localizing of the are on a particular spot of widest voltage variance from the opposed electrode at the other end of the tube no longer constitutes a problem. Early electrode rupture and consequent tube failure no longer is observed from this cause. Even when rupture of the filament does occur following passage of time, the electrode, all parts of which remain at the same potential, continues to function satisfactorily. Copious emission of electrons from the electrodes is no longer required to establish and stabilize the arc, ionic bombardment being relied upon almost exclusively for such purpose. Thus vaporization of the oxide coating on the electrode during operation is substantially removed as a problem.

Moreover, it is noted that for some reason gradual hardening of the tube due to occlusion of gases in the electrodes and tube walls no longer constitutes a, serious phenomenon. I do not know exactly why these advantageous results are achieved, but without binding myself to such theory, I suggest that the momentary overvoltage which can be built up during each current halfcycle before the arc strikes is either sufficient to overcome the resistance of the space discharge path, despite tube hardening, or else energizes the gas particles to such extent that the ions, bombarding the points of entrapment, knock out and release the occluded gases, thereby restoring normal pressure. Perhaps the action is a combination of these two factors. tube hardening is no longer a serious problem. Appreciable increase in tube life is observed, attendant upon such cold-cathode operation. Tube replacement is no longer a serious economic factor, even when these tubes are employed in batteries or clusters of substantial numbers, as in flood lighting advertising displays and the like.

The fluorescent tubes employed are selected from a myriad variety of widely diverging color characteristics, dependin upon the particular fluorescent salt or combination of salts with which they are lined. These salts give rise to characteristic secondary radiation which, radiating through the tube walls, is energized as visible light of characteristic colors. Many pleasing and effective color combinations can be achieved. It is customary to employ these tubes in a battery or series of batteries, each such battery being mounted on a single reflector common to all tubes of the battery and on which reflector the power unit is also mounted as a neat, inconspicuous part thereof. Basically, this is all conventional in the art, so that no attempt is made to provide further illustration of such construction in this specification. Diffused radiation, non- At any rate,

brilliant and without glare, but of practically any desired intensity, is made available. Absence of appreciable emission of heat makes ventilation no longer an important problem. High light efficiency and low power costs make feasible the use of artificial illumination on a far more lavish scale than has hitherto been-resorted to.

Tube 22 is connected by leads 24, 25 across the terminals of secondary SI, while tube 23 is energized through leads 26, 21 from secondary S2. Contributing to the elimination of stroboscopic effect, the relation of tube 23 to its secondary S2 is reversed with respect to the relation of tube 22 to secondary SI. That is, while the right terminal of tube 22 is connected to the right terminal of secondary SI, it is the left terminal of tube 23 which is connected to the right terminal of secondary S2.

A power-factor correctin condenser 28 is provided in lead 26 between tube 23 and secondary S2. It will be recalled that it is this secondary, in compartment C3, which is associated with the short shunts II, II. The purpose of this will be developed. The condenser 28 is chosen of sufficient size and capacity effectively to restore system power factor, made lagging by the highly inductive nature of the transformer load, to substantially unity value. Additionally, the leading characteristic imparted by this condenser to its corresponding secondary circuit effectively throws the tubes 22, 23 out of phase. One will ignite while the other is dark, and vice versa, so that detrimental stroboscopic effect, evidenced as an undesirable flicker is substantially removed.

It will now be in order to describe the operation of my new construction. Let us assume that for a given half-cycle of primary current, this current flow is such that a primary flux courses to the right in Figure 1, along central leg I I. At this time no arc has been struck across secondaries SI and S2, so that no limitative values are imposed on the coursing of flux due to saturation effects.

It is in order to digress momentarily at this point and call attention to the fact that while the magnetic core which I disclose and prefer is of the shell type, with symmetrically constructed and disposed legs, nevertheless, as has already been pointed out, these le s may be of asymmetrical construction and disposition. In point of fact, it is entirely possible to dispense entirely with one outer leg, increasing the dimensions of the other outer leg, and resulting in a core-type construction. I find somewhat better transformer performance with improved waveform, however, when the shell-type transformer is employed.

The flux stream interlinks secondary SI, which at that time is under open-circuit conditions, and generates a secondary voltage therein at the right end of leg II. This flux stream separates into two substantially equal branch streams. One of these courses up upper end member I4 to leg I2, and across this to the left in Figure 1, and then down upper end piece I5, back along the right through le II, interlinking secondary S2, then under open-circuit conditions, and thence back to winding I8.

At II, the other branch stream courses down across lower end piece I4, across leg I3 to the left in Figure 1, and up lower end piece to the left end of leg II. There it reunites with the stream first described, whereupon the combined 'brings about this phenomenon.

stream courses to the right along leg I I, back to the primary coil section. Inasmuch as no current flows through secondary windings. SI and S2 at this time, no back or secondary magnetomotive force is developed in the respective secondaries, impeding the coursing of the primary flux. Thus no reluctance is interposed in the main magnetic paths, interlinking the secondary windings. The primary flux courses unimpeded across them. Thus at these times air-gaps GI, G2, G3 and G4, particularly the last-mentioned, have reluctances which are comparatively much greater than those of the main magnetic path. Accordingly,

both during the coursing of the main flux stream across leg II, and of the branched streams across legs I2, I3, respectively, there is no tendency for any substantial part of the flux to course across the magnetic shunts I6, I6 and l1,

II. All of the flux is available for voltage build up in the secondary coil sections.

During the next succeeding half-cycle of primary current, the coursing of developed flux is just the reverse of that described. The main stream of primary flux now courses to the left along leg II from primary I8, inter-linking secondary S2, to the end of this leg. There the main stream divides into two substantially equal branch streams. One of these courses up across upper end piece I5, to the right in Figural across leg I2, and down upper end piece I4 to the right end of leg II. Similarly and simultaneously, the other branch stream courses down lower end piece I5, to the right along leg I3, and up lower end piece I4, to the right. end of leg II. At this point the two branch streams reunite. The combined stream then courses across leg II, interlinking secondary coil section S2, and inducing a voltage therein.

As has already been described in connection with the generation of flux during the preceding half-cycle, the reluctance of the air-gaps GI through G4, inclusive, are such compared to the reluctance interposed by windings SI, S2 under open-circuit conditions that practically no flux courses the branch paths provided'by shunts I6, I6 and II, I1. This reversal of flux during each ha1f-cycle, say' 120 times per second for ordinary 60 cycle current, ensures rapid buildup of voltage in the open-circuit secondary coils. Voltage rises and falls cyclically intheseucoils to a value substantially in excess of that required to strike arcs across the corresponding tube loads. Finally, after the Passage of a number of cycles of current, one of the tubes is conditioned to start. That is, the stresses produced across the tube electrodes so excite the gas content of the tube as to ionizethe latter,

and condition it for carrying an arc across from one electrode to the other. The rapid reversal of these stresses, and the tremendous value thereof due to the high open-circuit voltages, quickly In the space of but a fraction of a second, then, the are is struck across one of the tubes. This compares most favorably with the earlier tube assemblies,

in which a starting time of at least six or seven seconds is required, and even more during'cold weather operation. I find that on the contary, weather has little if any effect in the striking .and maintenance of the arc across the tubes of my new system. I am satisfied that this advantageous operation is accountable in large measure to high open-circuit voltage. v

For illustration, we will assume that it istube [23 across which the arc first strikesinitially.

Of course were it the tube 22 across which the arc is first established, the operation would be just the reverse of that described, as will be clearly apparent to those skilled in the art. We will further assume that at this moment, the half-cycle of primary current is such that pri mary flux courses to the right along leg I I, from primary I8. As soon as the arc strikes across tube 23, a current flow occurs across this tube. Because of the negative resistance characteristics of this tube load, this current may attain a substantial value. The secondary current induces a secondary or back magnetic fiux which links leg II and courses in a direction opposite to the primary flux, effectively bucking the same. On the other hand, unless adequate protective devices are provided across the line, the secondary current will build up,.indefinitely, resulting in rapid failure oreven destruction of the tube ,or system, or both.

It is for this purpose, and to interpose effective and permanent control, simple in nature, that the shunt leakage paths are provided. Whereas the leakage path interposed by airra s G3 and G4, of fixed magnetic reluctance, were heretofore of substantially greater reluctance than the main magnetic path or paths on Jpen-circuit conditionsQthey are now of substantially smaller reluctance than the main magnetic paths, across which the back magnetomotive forces hold sway, bucking the primary flux.

To illustrate, let us trace the primary 'fiux paths under these conditions. Primary flux courses to the right alongleg II, fromprimary I8. It interlinks secondary SI, across which no load has as yet been established. Separating at the right end of leg II, one branch courses up, and .the other down, the end pieces I4, I4. Since the two branch paths. are in paralleland arealmost exactly alike, it will be suiiicient, for illustration, to describe but a single one of them.

Theflux courses up the upper end piece I4, to

the left along leg I2. Because of the minimum reluctance interposed by ,coil section SI,- little flux courses across air-gap Gl.

When the primary flux comes opposite the region defining compartment C3, however, hous ing secondary S2, it'is met and buckedlby a substantial flow of reverse or secondary flux. Choosing the path of least reluctance, the greater part ofthe primary-flux courses down-across shunt I! and air-gap G3 to leg II, tothe' leftfof primary I8, and thence back to the right along this leg to primary I8. Thus the greater part of the flux combines'with th back magnetic flux in coursing downthis branch leakage path and .across air-gap G3, the secondary flux returning the shortest path to secondary S2, and

the primary flux similarly returning by shortest path to primary I8.

The calibrated design of shunts II, I! is such,

relative to the intended secondary load, that only that quantity of primary flux will at'this time course to the left along leg I2, down upper end piece [-5, and to the .rightalong leg II, interlinking S2, as will be sufficient to-generate a flux is short-.circuited across the high reluctance shun-t path, directly back to primary I8. I 7

During the next primary current half-cycle,

the situation is just'the reverse of that described.

For convenience, and illustratively, we will confine ourselves to tracing the circuit conditions maintaining in the lower parallel path, including leg I3. It is to be kept in mind that circuit conditions are exactly similar in the upper parallel path. In this instance, the current flow is such that induced primary flux courses to the left along leg II from primary I8. Opposite shunt air-gap G4, it is bucked by induced secondary flux from coil section S2. Accordingly, by far the greater part of the primary flux courses down across high reluctance shunt leakage path comprising shunt I1 and included airgap G4, and thence back to the right along leg I3 to end piece I4, leg II, and back to the right side of primary I8, interlinking secondary SI. Since the latter still operates under open-circuit conditions, full primary flux interlinks this coil. There is but little tendency for the primary flux to seek shunt I6 and included air-gap G2. Only sufficient flux continues to flow along leg II to the left, interlinking secondary S2, and coursing down lower end-piece I5, and the left portion of leg I3, and is required to induce in secondary S2 a voltage and current sufficient to maintain the arc across the tube. The greater part is shorted directly across the shunt path back to primary I8. The calibration of shunts IT, IT and included air-gaps G3, G4 relative to condenser 28, and so as to fulfill the requirements herein referred to, will be described in greater detail at a later point.

The transitory phase wherein an arc has been initially established across one of the tubes, while the arc has not yet struck across the other, is just the same where it is the tube 22 which strikes first. The circuits are just the reverse of those already described, and to detail them herein would involve needless and confusing repetition.

Shortly after the arc has struck across tube 23, the full primary flux, interlinking secondary SI, conditions the latter for striking. This is all but a matter of a'fraction of a second. As soon as the arc strikes across tube 22, load conditions are established across this secondary circuit, and current begins to flow of substantial value. A secondary magnetomotive force is developed, just as in the case of secondary S2, giving rise to a secondary magnetic flux of substantial importance. This is in a direction which is the reverse of one which bucks th primary flux. It effectively prevents the interlinking of all the primary flux with secondary SI. Only enough flux interlinks the turns of this winding to induce a voltage sufiicient to provide required secondary current across tube 22 to maintain the established arc. This phenomenon is occasioned by the high leakage reactance shunts I6, I6 and included air-gaps GI, G2.

For the portion of the current half-cycle that are is established and secondary current flow occurs, then the primary flux circuit can be traced through the upper parallel path-the lower parallel path being exactly the sameas follows: From primary I8 the flux will be assumed to course to the right along leg I I. Bucked by secondary flux at the left end of secondary SI, it courses in large measure across shunt I6 and included air-gap GI, and thence to the left along the leg I2. Only enough flux courses to the right along leg II, interlinking secondary SI, as is required to provide the operating current therein. When the primary flux reaches shunt I1 and included air-gap G3, it is confronted with either of two conditions: (1) the tube 23 is momentarily ignited, or (2) its arc is temporarily extinguished. Should the are be extinguished, the primary flux courses to the left, down upper end piece I5, to the right along leg II, interlinking secondary S2. It quickly builds up voltage within the half-cycle to the point where it will re-establish the arc across the ionized path within the tube. Should however, the arc be momentarily established, the developed back-magnetomotive force will buck the primary flux, causing the greater part of this latter to seek the path of least reluctance, down across shunt I! and air-gap G3, whence it flows directly back to the left side of primary I8, along leg II.

During the next current half-cycle, and now having attention to the lower parallel magnetic path, and assuming that tube 23 is momentarily energized, that is, that voltage within the current half-cycle has developed to a value sufficient to re-establish the arc, then the greater part of this branch stream of primary flux, with the exception of a calibrated part sufficient to maintain the arc until the developed voltage drops off to a point insuflicient to continue the same, will course down across shunt I1 and air-gap G4. There it courses to the right in Figure 1. When it comes opposite shunt I6, the primary fiux stream is confronted by two conditions. Either: (I) the arc is momentarily ignited across tube 22, or (2) it is momentarily extinguished. If it is ignited, a back magnetomotive force prevails, bucking the primary flux. This latter courses up across shunt I6 and air-gap G2, and

thence back along leg II to primary I8. Only sufiicient flux courses the main path and interlinks secondary SI, as is required to develop energizing current sufficient to service tube 22 under load conditions. If it is extinguished, substantially all the primary flux courses to the right to the end of leg I3, up lower end piece I4, and thence to the right along leg II, interlinking secondary SI, back to the right side of primary I8. Voltage is quickly built up in SI until the arc is re-established across tube 22. When this condition maintains, a back magnetomotive force is developed, generating flux which bucks the primary flux. This latter, always seeking the path of least reluctance, courses up across shunt I6 and included air-gap G2, and thence directly back along leg II to primary I8. By far the greater part of this primary flux bypasses secondary SI under these conditions. Only sufficient flux, under the calibration of shunts I6, I6, courses the main path, interlinking secondary SI, as is required to develop sufficient current to service tube 22 under load conditions.

In the embodiment of Figure 1, a three-coil transformer has been disclosed in which the secondaries and primary are only inductively connected, and are physically independent of each other. This, for convenience, I term ordinary transformer connection. It is entirely feasible under many circumstances, however, and from an economic standpoint, oftentimes preferable, to employ autotransformer connections between the primary and secondary windings. When these connections are employed, and the design is such that secondary maximum voltages do not substantially exceed 600 volts, so as to comply with the requirements of the Fire Underwriters, a substantial saving is accomplished in copper required, inasmuch as duplication of winding is avoided. Additionally, corresponding savings in physical dimensions of the core are attendant, with possible diminution in over-all dimensions of the core. A neater, smaller, lighter and more compact power unit is thus achieved, with substantial savings in iron content.

In Figure 2 I have disclosed the mode of carrying into effect such autotransformer connection. Therein lead 24 from secondary SI is connected to lead 2| of primary I8 at junction 29, while lead 21 from secondary S2 is connected to lead 20 of primary I8 at junction 30. Primary current flow is just as has been traced in Figure 1. Secondary circuit is established from the left side of secondary Si, lead 25, tube 22, lead 24, junction 30, lead 20, primary I8, lead 2 I, junction 29, lead 24, and back to the right side of primary SI. For secondary S2, a similar current can be traced: From the right side of S2, lead 26, including condenser 28, tube 23, lead 24, junction 29, lead 2i, winding I8, lead 29, junction 30, and lead 21, back to winding S2. For reversal of primary current, during reverse half-cycle, the current flow is just the opposite to that traced.

In Figure 3 I have disclosed the relation of the condenser 28 to the transformer unit in the assembled power unit, the combination of transformer and condenser conveniently being referred to as the power unit. It is entirely possible, in final assembly, to contour condenser 28 differently from what is shown in the drawing, and to associate it closely and snugly with the transformer unit in the interest of small compass, compactness and self-contained construction, of pleasing appearance to the purchaser. The general relation of the several parts of the transformer of Figure 3 is exactly in conformity with Figure 1, so that no amplification is necessary.

I have stated hereinbefore that the design of both sets of transformer core shunts is in calibrated conformity with the nature of the particular tube loads serviced by the corresponding secondaries. The load of secondary SI, for example, gives rise to a comparatively low reluctance when the tube 22 is in operation. Accordingly, were the leakage reactance of the associated shunt path to be of appreciable value, the primary flux, always seeking the path of minimum reluctance, would still in large measure traverse the main flux path, even when the arc had been struck across tube 22. In nicely calibrated manner, therefore, the reluctance of the air-gaps GI, G2 is closely computed, and the air-gaps designed accordingly, to provide just the proper ratio under load conditions between the quantities of primary flux shunted across the leakage paths, and that still traversing the main path.

In the case of the secondary S2, servicing the power factor correcting condenser and associated tube load 23, the condenser 28 interposes a high reactive flux, so that if air-gaps G3, G4 have no more reluctance than gaps GI, G2, they will bleed substantially all the primary flux when the tube load is energized. Insufficient primary flux would course the main circuit to maintain the required are voltage. The are would extinguish, and the tube would remain de-energized. Accordingly, therefore, the air-gaps G3 and G4 are made substantially longer than the air-gaps GI and G2. Moreover, I have found that the width of shunts I1, I! need only be about half as wide as shunts I6, I6, thus permitting a very compact unit which nevertheless is protected in the event of a short-circuit of condenser 28 or grounding of tube 23. In ad- 14 dition, I find a better distribution of flux is had with shunts I'I, ll being narrower than shunts I6, I6, there apparently being a functional relationship between the length of air-gaps and narrowness of the associated shunts.

It is readily apparent that with the excess of voltage in open-circuit conditions made available by my new construction, it is possible to diminish the applied line voltage, and still provide for an induced secondary voltage of sufficient value to strike the arc. The only difference would be that the arc would strike later in each half-cycle, and extinguish earlier. Thus a dimmer operation is available, of practical utility.

By the practice of my invention, iron and copper requirements have been minimized, so that a more compact and self-contained unit has been made possible, at a substantial saving in initial investment. The new assembly is sturdy and subsequently fool-proof, it being virtually impossible to damage the installation permanently, even upon short-circuit in both secondaries. Not only the transformer unit considered alone, but as well, both the power unit and tube assembly, rigidly comply with all requirements of the Fire Underwriters. All necessity of protective auxiliaries, expensive in themselves, has been effectively eliminated. No fuse or other circuit protecting device is required in the condenser circuit because full protection against excessive short-circuit current is had with the foreshortened core shunts. Stroboscopic effect has been avoided, and good system power-factor achieved. Long and satisfactory tube life has been made possible, even under cold weather conditions, and quick-striking characteristics have been imparted to the tubes.

All these and many other thoroughly practical and important advantages have been imparted by the practice of my invention. Since many modifications may be made of the embodiments which I have disclosed, and since many embodiments of my basic principles may be evolved, the foregoing description is to be considered as merely illustrative and not by Way of limitation.

I claim:

A power unit comprising in combination a three coil transformer which in turn comprises a single shell-type magnetic core, two paired sets of shunts, the shunts of one set being of higher reluctance than those of the other, extending between the'longitudinal members of the core but providing included air-gaps, said shunts dividing the core into three compartments only, one primary coil only in the intermediate compartment, and two secondary coils one being in each end compartment; and a power factor-correcting condenser disposed physically contiguous one end of the transformer and electrically connected in circuit with the secondary coil associated with the set of shunts of high reluctance.

JOHN IIEROLD BRIDGES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 931,114 Conrad Aug. 17, 1909 1,895,231 Pearson Jan. 24, 1933 2,346,621 Sola Apr. 11, 1944 2,370,633 Boucher Mar. 6, 1945 2,382,638 Keiser et al Aug. 14, 1945

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