US2958806A - Lamp starting and ballast circuit - Google Patents

Description

Nov. 1, 1960 w, LORD 2,958,806

LAMP STARTING AND BALLAST CIRCUIT Filed NOV. 20, 1957 Fig.1 9

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eUnimdsw-res P tf t 10 LAMP STARTING AND BALLAST CIRCUIT Harold W. Lord, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 20,1957, Ser. No. 697,595

7 Claims. (Cl. 315-400) The present invention relates to improved lamp starting and ballast circuits and particularly to such a circuit having improved regulating and load current wave shape characteristics.

Some types of electric loads such as fluorescentlamps and mercury vapor lamps, for example, impose many requirements on the circuit from which they are energized and, as a result, there have been developed a large number of circuitsfor starting and running these lamps from an alternating current supply circuit. Since lamps of this type have a negative resistance characteristic it is necessary to provide some ballasting impedance in circuit with them. It is also desirable to provide automatic regulation to take care of supply line voltage fluctuations and it is necessary to provide sufiicient voltage, which is higher than the running voltage, for initiating the lamp discharge. It is also desirable, from the standpoint of operation and life of the lamp, that the current wave shape I through the lamp be relatively flat topped. In circuits of this type, it is common practice to use a capacitor in circuit with the lamp for operating the circuit as a slightly leading power factor load on the'supply circuit and the electrical characteristics of sucha capacitor also contribute problems that should be taken care of by the remainder of the circuit components.

' It is accordingly an important object of the present invention to provide a lamp starting and ballasting circuit having improved current wave shape in the lamp circuit while maintaining good regulating characteristics. It is a further object of the present invention to provide improved regulating and load current wave shape characteristics in a ballast circuit which is readily producible by modern manufacturing methods and which may be manufactured at a competitive cost.

In accordance with a preferred embodiment of my invention a high reactance transformer is provided having a core providing a closed magnetic circuit. The core includes a section with which a primary winding is inductively related and a section with which a secondary winding is inductively related; The core is shaped to provide substantial flux leakage between these core .sections and is provided with a section of reduced crosssectional area of a substantial'length in the section with which the secondary winding is associated, this section having a cross-sectional area selected so that it is saturated during a part of the alternating current supply voltage even during open circuit conditions in the circuit of the secondary winding. Preferably, this section of reduced crosssection lies directly under the secondary winding. The restriction in the secondary core section in accordance with the present invention is of greater length relative to the mean length of the secondary magnetic circuit and of lesser extent than that provided, by known bridge-gap constructions. A lamp load is energized from the device through a series capacitor. The secondary winding of the device is preferably connected in series with a portion of the primary winding so connected that the voltage thereof opposes the voltage of the 2,958,806 Patented Nov. 1, 1960 secondary winding. As will be brought out in detail this voltage provided by a part of the primary winding in the lamp circuit contributes both to improved regulation and lamp current wave form characteristics of the circuit while the saturated core section associated with the secondary winding permits the attainment of an adequate open circuit voltage to start the lamp. The secondary saturation under open secondary circuit conditions permits a much smaller average value of secondary voltage than would be required to start the lamp if a sinusoidal secondary voltage were generated during open secondary circuit conditions. This lower value of average open circuit secondary volts reduces the operating secondary voltage and thereby reduces the volt-ampere rating required for the series capacitor. Since the peak capacitor stored energy is a source of current peaks in the lamp circuit this reduction in capacitor volt-ampere rating contributes in large measure to the reduction of the lamp current wave form problem. The substantial inductance presented by the restricted core section when operating in the saturated region limits the discharge of the capacitor which also limits the peak lamp current. This saturated inductance is not sufiicient to seriously impair the regulating characteristic of the circuit with respect to line voltage variations.

Further objects and advantages of my invention will become apparent as the following description proceeds, referenec being had to the accompanying drawing in which:

Figure 1 illustrates schematically one embodiment of my invention;

Figure 2 illustrates a modification of my invention employing a different physical magnetic circuit configuration;

Figures 3 and 4 illustrate current and voltage curves which are helpful in understanding the invention; and

Figure 5 illustrates conditions of saturation in the magnetic core of the devices shown in Figures 1 and 2.

In the particular embodiment illustrated in Figure 1 a closed magnetic circuit is provided by a three-legged core 1 made up of a stack of laminations of suitable magnetic steel preferably one of the steels exhibiting a relatively sharp-kneed saturation characteristic. The primary winding 2 and secondary'winding 3 are wound respectively on'portions 4 and 5 of the central leg. A generally rectangular opening through the stack of laminations illustrated at 6 reduces the core section with respect to the remainder of the central leg and preferably the remaining core under the secondary winding to 40% to of the primary core section. In a particular embodiment the secondary section was made with an area equal to 60% of the area under the primary. The outside legs 7 and 8 of the core are connected together and to the ends of the central leg by end sections 9 and 10. As illustrated in the drawing, core sections 11 and 12 extend inwardly from these outside legs toward the central leg intermediate the primary and secondary windings and terminate short of the intermediate leg. It is apparent that this configuration of the core section provides for substantial flux leakage between the primary and secondary winding sections and renders the transformer a high reactance transformer.

The primary winding is provided with lead wires 13 and 14 for connection with an alternating current supply circuit and with an intermediate terminal 15 which is connected to one terminal of the load circuit specifically one terminal of a lamp 16 by means of conductor 17. The other terminal of the lamp is connected to one end of the secondary winding 3 through a series capacitor 18. The other terminal of the secondary winding is connected to conductor 14 and, in this way, to the lower terminal of the primary winding. This connection completes the circuit of the lamp through the portion of primary winding between the terminal 15 and conductor 14 in such a direction that the voltage of this portion of the winding opposes the voltage of the secondary winding 3. As will be described in more detail at a later point in the specification, the amount of biasmay be adjusted for difierent applications. Substantial advantages may be obtained from the controlled saturation feature of the present invention independently of the use of an opposed bias voltage.

A brief consideration of the operation of the embodiment of my invention just described will provide a better understanding of its features and advantages. When the conductors 13 and 14 are energized from a suitable source of alternating voltage for which the device is designed the magnetic circuit in the region of the air gap 6 is saturated for a portion of each half cycle and the turns ratio of the primary and secondary windings is so selected that with rated line voltage the secondary voltage during the unsaturated portion of the cycle is sufficient to establish an arc discharge through the lamp 16. In Figure 3 there are illustrated current and voltage curves which are helpful in understanding the present invention. In Figure 3, curve A is a trace of the voltage provided in the lamp circuit by the lower portion of the primary winding 2; curve B represents the secondary voltage (the voltage impressed on the lamp and capacitor in series) and curve C represents the lamp current which is also the current through the series capacitor. The current curve may be considered to be made up of two components. During the first portion of a half cycle the lamp current rises rather rapidly to a peak. It will be noted thatthis occurs at a time when the voltage of the secondary winding is negative or a relatively low positive value and this current is essentially current resulting from the discharge of energy stored in the capacitor through the saturated inductance presented by the secondary winding. The height of this peak and its duration as far as it is the capacitor discharge current is a function of the magnitude of the capacitance, the voltage to which it is charged, the magnitude of the saturated inductance of the transformer secondary and the lamp arc voltage. The secondary winding, with its associated core, is designed to present substantial inductance even during the portion of the. cycle that the restricted core portion is saturated. Thus, a substantial inductance is presented to the flow of, the capacitor current and the peak is, in this way, controlled. This saturated inductance is controlled by the length of the restricted section and the coil configuration. This component of current becomes of relatively smaller significance during the second portion of the half cycle of lamp current during which the lamp current ismore directly attributable to the direct transformer action of the primary and secondary windings. The curve A illustrates the portion of the primary voltage coupled into the secondary circuit in series opposed relation to the secondary voltage so that this voltage is, at the time of the current peak, rapidly changing in a direction to oppose the current peak during a substantial portion of the peaked current wave. The opposed voltage of the primary section connected in circuit with the secondary winding tends to reduce the voltage available for starting and the magnitude of this opposed component may be adjusted to suit the requirements of any particular circuits recognizing that it tends to limit the magnitude of the current peak due to the capacitor discharge but also tends to increase the required turns ratio voltage of the transformer to provide a satisfactory starting voltage. This voltage of curve A also increases with increases in line voltage, and therefore tends to minimize the effect of supply line voltage fluctuations on the lamp current, thereby improving the regulation of lamp current with respect to supply line fluctuations.

In Figures 2 1 have shown my invention incorporated 4 in a lamp starting and ballasting system in which the magnetic core structure is physically quite different from that illustrated in Figure l, but which is magnetically, essentially the same. i

In the modification of Figure 2 a transformer connection without an opposed bias winding section is illustrated. This system utilizes the feature of providing saturation of a relatively long length, substantially one third of the mean length of the secondary magnetic circuit over a part of the cycle at normal operating voltages even under open secondary circuit conditions. In some applications the improvement in current wave shape and the regilation obtained with thesaturation feature alone provides acceptable performance. .As illustrated in Figure 2 the central or winding leg of the core structure includes a portion 19 with which the primary winding is associated and which is of substantially larger cross sectional area than a second portion 20 with which the secondary winding 3 is associated. The outer legs 21 and 22 are shaped to abut against the end portions of the central leg and to be spaced from the central leg throughout their lenngth. It will be noted that in the vicinity of the junction of portions 19, and 20 of the central leg, the leakage path between the central leg and the two outer legsis short and provides substantial leakage reactance in the transformer.

In the circuit of Figure 2 the load circuit including capacitor 18 and lamp 16 are connected in series across the secondary Winding 3. The lamp starting, ballasting and voltage regulating characteristics of the system illustrated in Figure 2 are essentially as described in connection with Figure 1, as far as the effect of saturation is concerned.

A further discussion of the manner in which the provision of saturation in accordance with the present invention produces a very efiective combination of starting voltage for the lamp, regulationof the lamp current and relatively flattop lamp current wave form will provide a better understanding of this aspect of my invention. In FigureS the curve S shows the magnetization curve for the material of the core. With the reduced core section of the secondary magnetic circuit, the bend or knee of the curve will occur at a value of B designated a. This ordinate will have the same ratio to the ordinate b corresponding to the knee of the saturation curves as the area of the restricted-secondary core section bears to the area of the primary core section. The slope of the curve 0 is representative of the saturated magnetizing inductance .of the secondarycircuit and is a measure of the discharge period of the capacitor 18. In other words, there is substantial inductance in the secondary circuit under thev conditions of saturation that exist with peak values of operating current. The slope of the portion c will depend upon the length of the saturated section, the slope being less for greater lengths of the restricted section. Under open circuit secondary conditions the restricted core in accordance with the present invention provides saturation over a relatively small portion of the alternating current cy'cle, something in the order of 60, for example. This provides an open circuitor lamp starting voltage having a wave shape such as illustrated in Figure 4 (b) as compared with the sharper peak voltage of Figure 4 (a) produced by a more conventional bridge gap type of core structure, The voltage of the wave shape of Figure 4 (b) is pre ferable from the standpoint of starting the lamp and providing enough follow-through to keep the lamp lighted once the discharge isestablished.

With the present invention the ratio of the open circuit voltage required for lamp starting to lamp voltage is smaller than with circuits not providing saturation under open circuit conditions or with the conventional bridge gap constructions. This lower ratio of secondary voltage to the lamp running voltage reduces the volt ampererating v, required of the series capacitor 18 and,

arrests in this way, tends to minimize the lamp cm'rent wave form problem resulting from the presence of this capacitor and the uncontrolled saturation characteristics of the secondary core portion of a conventional gapless type of core. I

From another point of view the provision of a circuit which provides substantial saturated magnetizing inductance in the secondary circuit and a lowervolt ampere rating of the capacitor tends to .reduce the magnitude of the harmonic currents that flow. during the discharge 'of the capacitor and particularly to reduce the harmonics higher than the third. In this way, the current peak such as shown in curve C of Figure 3 is kept to a small value. At the same time, .the inductance of the secondary circuit under saturated conditions is not so great as to impair seriously the regulation of the lamp current. As stated before the presence of this inductance is illustrated by-the slope-of-the curve c in- Figure 5 and is dependent upon the lengthof the saturated portion of the secondary circuit; e V v -In the drawing-,the-restricted section is produced by cutting the rectangular gap in the secondary core section as shown in Figure 1 or by reducing its transverse dimension as shown in Figure 2. It will be readily appreciated by those skilled in the art that the reduction may also be accomplished by openings of different configuration or by reducing the number of laminations in the secondary portion of the stack. Also, in order to minimize stray flux it is preferable to provide the restricted section as shown in the illustrated embodiments of the drawing in that portion of the secondary magnetic circuit lying under the secondary coil. In the embodiments illustrated the length of the restricted or saturated section is in the order of 25 to 35 of the mean length of the secondary magnetic circuit, This length is determined somewhat by the characteristics of the magnetic material and the amount of the inductance required of the secondary circuit under conditions of saturation. Lengths in excess of 5 percent of the secondary path may, in some cases, be sufiicient. From another point of view the design of the saturable section of the core involves proportioning the core so that the magnetizing inductance of the secondary winding when the restricted section is saturated has an effective value which, at the supply frequency, is between 4 and 20% of the capacitive reactance of the series capacitor 18 at the supply frequency.

It will be apparent from the foregoing description that '1 have provided a lamp ballasting and starting circuit having improved characteristics particularly with respect to the provision in a single system of adequate lamp starting voltage, relatively good current Wave shape, that is, a wave shape having a relatively flat top or, in other words, a relatively low ratio of the peak to average current. Also, the present invention provides for the retention of good regulation characteristics by' controlling the extent of the saturated inductance. In the embodiment of the invention illustrated in Figure 1 an opposed voltage in the secondary derived from the primary circuit is utilized to improve further the current wave shape and the regulation of the magnitude of the lamp current with variations in supply circuit voltage.

While I have described and illustrated the particular embodiments of my invention it will be apparent to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects and I aim, therefore, in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A lamp starting and ballasting circuit for controlling the starting and operation of an electric discharge device from an alternating current circuit comprising a transformer having a core of magnetic material providing a 6, a closed magnetic circuit, a primary winding inductively coupled to a first section of said core, a secondary winding inductively coupled with a second section of said core, a capacitor, means connecting said capacitor, said secondary winding and a portion of said primary winding in series with the voltage of said secondary winding and the voltage of said portion of said primary winding in opposed relation to provide an output circuit for energizing an electric discharge device from an alternating current circuit connected to energize said primary winding, said second section of said core having a length thereof exceeding five percent of the mean length of said second section of said core restricted in cross section to forty percent to eighty percent of the cross section of the remainder of the length of said core which saturates for a portion of each cycle of voltage of the supply circuit when said output circuit is opencircuited.

1 2. A lamp ballast circuit comprising a transformer including a primary winding for energization from a supply of alternating current voltage, a secondary winding, a magnetic core structure inductively coupling said windings and providing a primary magnetic circuit for said primary winding and a secondary magnetic circuit for said secondary winding, a capacitor connected in series circuit with said secondary winding, said secondary magnetic circuit having a length of restricted cross-section to produce saturation of said secondary magnetic circuit for a portion of each cycle at normal voltage of said supply and with the circuit of said secondary winding open circuited, said secondary magnetic circuit having a length of restricted cross-section exceeding five percent of the mean length of said secondary magnetic circuit to provide a magnetizing inductance of the secondary winding when the restricted section is saturated equal to 4 to 20% of the capacitive reactance of said capacitor at the supply frequency.

3. A lamp ballast circuit comprising a transformer including a primary winding for energization from a supply of alternating current voltage, a secondary winding, a magnetic core structure inductively coupling said windings and providing a first magnetic circuit for said primary winding and a second magnetic circuit for said secondary winding with substantial leakage therebetween, a capacitor connected in series circuit relation with said secondary winding, means connecting said capacitor and said secondary winding in series relation with a portion of said primary winding, the voltage of said secondary winding and the voltage of said portion of said primary winding being in opposed relation, said secondary magnetic circuit having 215% to 35% of its mean length of a crosssection restricted to 40% to of the cross-sectional area of said first magnetic circuit to produce saturation of said secondary magnetic circuit for a portion of each cycle at normal voltage of said supply and with the circuit of said secondary winding open circuited.

4. A lamp ballast circuit comprising a transformer including a primary winding for energization from a supply of alternating current, a secondary winding, a magnetic core structure inductively coupling said windings and providing a primary magnetic circuit for said primary winding and a secondary magnetic circuit for said secondary winding, a capacitor connected in series circuit relationship with said secondary winding, said secondary magnetic circuit having a length of cross-section restricted from 40 to 80 percent of the cross-sectional area of said primary magnetic circuit and providing the secondary winding with a magnetizing inductance equal to 4 to 20 percent of the capacitive reactance of said capacitor at supply frequency when the restricted section is saturated.

5. A lamp ballast circuit comprising a transformer including a primary winding for energization from a supply of alternating current, a secondary winding, a magnetic core structure inductively coupling said windings and providing a first magnetic circuit for said primary winding and a second magnetic circuit for said secondary winding with substantial leakage therebetween, a capacitor connected inseries. circuit relationship With said secondary winding, said secondary magnetic circuit having 25 to- 35. percent of its mean length of a cross-section restricted to 40 to 80 percent of the cross-sectional area of said first magnetic-circuit, .said restricted cross-section providing a magnetizing'inductancein the secondary Winding equal to 4 to 20 percent of the capacitive. reactance of said capacitor at supply frequency when the restricted section is saturated.

6. A tlampballast circuit :comprising a transformer including a primary winding for-energization froma supply of alternating current, a secondary winding, a. magnetic core structure inductively coupling said windings and providing a first magnetic circuit for said primary winding and a second magnetic circuit for saidsecondary Winding with substantialleakage therebetween, a capacitor connected in series circuitrelationship with said secondary winding, said secondary magnetic circuit having the entire length of the restricted section thereof lying under said secondary winding, said secondary magnetic circuit having to percent of its mean length of a crosssection restricted to to percent of the cross-sectional area ofsaid firstmagnetic circuit, said restricted'c'ross-section providing a magrietiiing, inductance intthe secondary winding equal to 41520 percent of the capacitivereactance ofsaid capacitor at the supply frequency when the restricted section is saturated.

7.. The lamp ballast circuit set forth in claim 5 wherein said circuit includes means connecting said capacitor andsaid secondary winding .in series relation with a portion of said primary winding, the voltage of said secondary winding and the voltage of said portion of said primary winding being in opposed relation.

References Citedin the file of this patent "UNITED STATES PATENTS 2,458,516 Klemperer' Jan. 11, 1949 2,488,742 Schwennesen Nov. 22, 1949. 2,568,553 Maurer Sept. 18, 1951 2,694,177 Sola Nov. 9, 1954 2,824,263 Strecker Feb. 18, 1958

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