US3611026A

US3611026A - Ballast circuit for low wattage gaseous discharge device

Info

Publication number: US3611026A
Application number: US874899A
Authority: US
Inventors: Joseph A Crawford
Original assignee: Advance Transformer Co
Current assignee: Advance Transformer Co
Priority date: 1969-11-07
Filing date: 1969-11-07
Publication date: 1971-10-05
Anticipated expiration: 1988-10-05

Abstract

A ballast circuit for a low-wattage gaseous discharge device, for example, a rapid start fluorescent lamp, comprises an iron core having a primary winding and a secondary winding, the principal part of which is loosely coupled relative to the primary winding, the primary winding being isolated from the secondary winding. A lesser portion of the secondary winding is closely coupled relative to the primary winding, and arranged in bucking relationship to the principal part of the secondary winding. The circuit provides leads for connecting the gaseous discharge device in series with the secondary winding and a series condenser of such value to provide a leading current in the gaseous discharge device. There are also filament windings closely coupled with the primary winding and arranged to be connected for continuous energization of the heater filaments of the lamp.

Description

United States Patent [72] Inventor Joseph A. Crawford Chicago, Ill. [21 App]. No. 874,899 [22] Filed Nov. 7, 1969 [45] Patented Oct. 5, 1971 [73] Assignee Advance Transformer Company Chicago, Ill.

[54] BALLAST CIRCUIT FOR LOW WA'ITAGE GASEOUS DISCHARGE DEVICE 14 Claims, 2 Drawing Figs.

52 use] 315/239, 315/276, 336/155 [51] lnt.Cl ..'....H05b4l/l4 [50] FieldofSearch 315/105, 235, 239, 244, 276, 278, 281; 336/155 [5 6] References Cited UNITED STATES PATENTS 2,958,806 11/1960 Lord X 2,988,670 6/1961 Genuit 3l5/lO5X Primary Examiner-Raymond F. Hossfeld Attorney-Silverman & Cass ABSTRACT: A ballast circuit for a low-wattage gaseous discharge device, for example, a rapid start fluorescent lamp,

comprises an iron core having a primary winding and a secondary winding, the principal part of which is loosely coupled relative to the primary winding, the primary winding being isolated from the secondary winding. A lesser portion of the secondary winding is closely coupled relative to the primary winding, and arranged in bucking relationship to the principal part of the secondary winding. The circuit provides leads for connecting the gaseous discharge device in series with the secondary winding and a series condenser of such value to provide a leading current in the gaseous discharge device. There are also filament windings closely coupled with the primary winding and arranged to be connected for continuous H energization of the heater filaments of the lamp.

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BALLAST CIRCUIT FOR LOW WA'I'IAGE GASEOUS DISCHARGE DEVICE BACKGROUND OF THE INVENTION The field of the invention is the art of ballasts and ballast circuits for gaseous discharge devices, and more particularly, such ballasts and circuits for igniting and operating fluorescent lamps. Fluorescent lamps are normally elongate glass envelopes filled with some inert gas such as argon or a mixture of inert gases, and having several droplets of mercury in the envelope. The inside surface of the glass envelope is coated with phosphors which are capable of fluorescing when activated by ultraviolet radiation. This radiation is produced when the gas within the envelope is ionized, such ionization including the breaking down of the mercury gas as a principal source of the ultraviolet radiation. Variations exist in these details, but for the most part, the fluorescent lamp is used for illumination, and the normal construction is as described.

'I'He fluorescent lamp is one example of a negative resistance characteristic device in that it requires a higher voltage to start the are or discharge than is required to maintain the discharge. Thus, when a voltage high enough is a applied to a gaseous discharge device, be it a fluorescent lamp, a welding arc, a neon tube or the like the discharge occurs and the impedance of the gap is substantially reduced. As the impedance drops, the current drawn by the device rises at such a rapid rate that the discharge device may literally destroy itself, unless the current is contained. This can be done by using some form of impedance which limits the current flow to some value that will provide the most efficient utilization of the function of the device. In the case of a fluorescent lamp, the current limiting impedance can be resistance, capacitance, inductance or a combination of these. in the case of DC operated fluorescent lamps, the current limiting impedance is only resistance, but the DC use of fluorescent lamps is quite minimal in the world today. Furthermore, the more desirable fluorescent lamps in use today cannot withstand the effects of direct current, as will be pointed out shortly.

At this time, by far the greatest majority of fluorescent lamps is operated on alternating current of different voltages, frequencies and under different conditions. Briefly, the art has advanced to a condition that the following facts are well known for fluorescent ballasts and ballast circuits operated with alternating current:

a. Fluorescent ballasts are known which serve as step-up transformers to raise line voltage to values which will ignite lamps requiring higher than line voltage to start. Such ballasts may have their secondary circuits loosely coupled with their primary circuits, so that on open circuit the greatest portion of the transformed step-up voltage is available for ignition without loading the line, but upon ignition when current flows in the secondary circuit, the high leakage reactance comes into play and provides the impedance necessary to limit the current flow through the lamps while providing the lower operating voltage therefor;

b. The use of a ballast circuit which draws a leading current from the line is not as desirable as one which operates at a high power factor. Even a somewhat lagging power factor is not as undesirable as leading. Fluorescent lamp circuits use various arrangements to correct for power factor, since the winding inductance causes lagging current. Circuits are known which provide the leading current for the lamp while not upsetting power factor too greatly; and

c. An important lamp which is in use today is known as a rapid-start lamp, as opposed to an instant-start lamp. The rapid-start lamp has filaments, or heaters as they will be referred to in thisspecification, these being used continuously. They are ehergized by filament windings and thus produce the emission of electrons which make ignition and operation voltages lower. Furthermore, these heaters are rugged and remain energized at all times while the lamps are operating normally without damage to themselves. The instant-start type of lamp is still in widespread use today, and is characterized by the absence of heaters and the need for much higher starting and operating voltages. The electrodes of this type of lamp are constructed to provide hot spots during use for the flow of electrons across the gap to maintain the are.

This invention is primarily concerned with a ballast and circuit for the operation of a relatively low-wattage rapid-start lamp which has provided considerable design problems, all of which are solved by the invention in an unobvious way. These problems will be set forth hereinafter.

The low-wattage rapid-start lamp requires a series condenser to prevent rectification phenomena. If one heater of a rapid-start lamp burns out, the other heater becomes polarized, and hence conducts only on alternate half-cycles. The resulting pulse current produces excessive losses in the ballast which increase the ballast temperature and lead to ballast failure. The existence of a capacitor in series with the lamp will prevent current flow if one heater burns out and the current becomes unidirectional, thus saving the ballast and indicating to the user that the lamp has failed. The series condenser thus serves as a safety device but also provides compensation for the reactance of the ballast windings looking toward correction of power factor.

A second problem which is related specifically to lowwattage rapid-start lamps is the effect of open circuit voltage on the lamp. Too high an open circuit voltage applied across the lamp heaters will' cause the lamp to operate as an instantstart lamp, that is, it will ignite prior to the heating of the filaments. This produces ion bombardment and lessens lamp life. Accordingly, the total open circuit voltage serving the lamp must be limited. Since the requirements of safety dictate that the primary winding should be isolated from the secondary winding when disconnect sockets are not used, the entire open circuit voltage for igniting the lamp must be provided by the turns of the secondary winding.

According to the normal methods of ballast design, the

capacitive reactance for providing the proper power factor and operating voltage for the lamp, with an inductive reactance represented by the secondary turns apparently needed, will require a large series capacitance. For example, the specific lamp in connection with which the invention is utilized is a 30-watt rapid-start lamp known as an F30- T12 which has an operating voltage of at 430 milliamperes RMS. Open circuit voltage ranges between 150 and 205 volts RMS. These are all, of course, alternating currents and voltages, as are all quantities referred to in this application. Line frequency is always taken as 60 Hertz. The normal line voltage with which the ballast for these lamps is intended to be used is 120, and the minimum and maximum values are measured at percent and percent of line voltage, respectively. This value of capacitance computed for a normal ballast design is approximately 6 microfarads, and the voltage rating in view of the circuit requirements is volts.

It will be appreciated that the capacitor which has been specified above is large physically, and is quite expensive. One of the problems which has been solved by this invention is to decrease substantially the cost of the condenser to be used in the circuit.

It would appear that the use of a lower capacitance in the circuit does not seem practical because this represents a higher capacitive reactance. Considerations of power factor and required inductive reactance suggest a secondary winding whose turns ratio would provide much too high an ignition voltage for the lamp.

According to the invention, a high inductance is achieved to enable low capacitance, but the ignition voltage is not raised inordinately and the power factor is maintained at a high value.

It should be understood that the type of discharge device with which this invention is concerned ignites at a higher-thanline voltage, but operates at a lower-than-line voltage.

SUMMARY OF THE INVENTION Considering the above discussion, the invention is characterized by a departure from normal ballast design in using a circuit which provides the necessary starting and operating voltages through the use of a secondary winding that is formed to two parts, one part of which is the principal part and is in high leakage reactance relation to the primary winding and loosely coupled to the other part. Said other part is closer coupled to the primary winding, but bucking in an instantaneous voltage sense relative to the principal part of the secondary winding. Thus, for open circuit where the capacitor is not active in the lamp circuit, the available voltage is the inducedvoltage difference between the two secondary parts. On operating, the net higher inductive reactance of the secondary winding portion of the lamp circuit demands a higher capacitive reactance than that dictated by conventional design, but this is achieved by a smaller capacitor albeit operating at a higher voltage than normally expected of the capacitor. Thus, instead of the 6-microfarad condenser operating at 175 volts, the practical circuit of the invention for the 30-watt lamp uses a 4.6-microfarad condenser operating at a voltage of about 233 volts.

The smaller condenser with higher-rated voltage is substantially more economical than the larger condenser with lowervoltage rating, because the difference between the insulation costs for condenser ratings of approximately I75 volts and 250 volts is minimal, if anything, but the construction required to produce the difi'erence in electrical capacitance is substantial. The savings is greater than the added expense occasioned by the bucking secondary winding.

The circuit described will provide the rated operating voltage of approximately 80 volts for the 30-watt lamp.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram of a ballast circuit in accordance with the invention; and

FIG. 2 is a plan view with portions in section illustrating the transformer portion of the ballast apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is illustrated a ballast circuit 10 constructed according to the invention, for igniting and operating the gaseous discharge device L which in this case is shown as a rapidstart fluorescent lamp L having the heater filaments H1 and H2 in the ends of its glass envelope. The inductive portion of the apparatus comprises a transformer T which is illustrated in more detail in FIG. 2, having a primary winding P connected across an alternating current source represented by the terminals l2 and 14. These may comprise the power main, being a approximately 120 volts and 60 Hertz. The

terminals

12 and 14 are connected to the ends of the primary winding P by the leads I6 and 18, respectively. The primary winding P is mounted on the laminated iron core of the transformer T and as will be seen is isolated from the secondary windings S and S to eliminate shock hazard. The secondary winding S is the principal part of the overall secondary winding and is mounted on the core 20 loosely coupled to the primary winding P, either on the basis of physical spacing or by virtue of a shunt 22, the later not being used in the practical circuit to be described.

The smaller portion of the overall secondary windings is designated 8,, and it is coupled to the primary winding closer than the part 5, but is in instantaneous voltage-bucking relationship relative to the said part S. Since the two secondary parts S and S are in series one with the other and with the lamp L, the open circuit voltage which is available for ignition of the lamp L is the difference between the voltages of these two secondary windings. The nomenclature used hereinafter to describe these windings will generally be as follows: the part S will be called the main or principal secondary winding, the part 5,, will be called the bucking secondary winding, and

sometimes the both together will be called simply the secondary winding 5,5

The lamp L is a rapid-start lamp, as previously mentioned, and hence its heaters H1 and H2 require permanent energizing connections with filament windings of the transformer T. The filament winding F1 is connected to the heater H1 by the leads 24 and 26 and the filament winding F2 is connected to the heater H2 by the leads 28 and 30. The secondary winding 8,8,; is connected in series with a condenser C and, together therewith, the winding is connected between the leads 26 and 30. In this way, the secondary winding is connected across the lamp L. The lamp fixture 32 is connected to the casing or canister which houses the components of the ballast, this canister being designated 36 and represented by the broken line box, this being at ground potential. One side of the

line

12, 14 is also grounded as shown, this being the terminal 12. Conveniently, a high-ohmage resistor of 5 to 7 megohms may be connected between the primary terminal 18 and the terminal of the secondary S as an aid in starting. This resistor is indicated in broken lines. The resistor which is normally connected across the terminals of the capacitor C is not shown.

' This latter is to leak the charge off the condenser when the ballast is not in use.

The leads illustrated in FIG. 1 have been marked with the color code conventionally employed by ballast manufacturers and users in the United States at this time. The usual manufacturer sells the portion of the apparatus 10 which is enclosed by the box 36, the interior being filled with a potting compound formed of asphalt, silica and perhaps other ingredients. This compound insulates the components electrically, conducts the heat from the components to the canister 36 and prevents the components from moving relative to one another within the canister. The purchaser receives the box as a so-called ballast, with the leads l6, 18, 24, 26, 28, and 30 protruding as "pigtails and connects them to the lamp Lin accordance with a detailed label carried on the canister. The label (not shown) identifies the specific lamp for which the ballast is designed, gives the conditions under which the lamp and ballast are to be used, and contains a circuit diagram not too much different from that of FIG. 1 to show how the connections are to be made. The only difference might be that nothing is shown on the interior of the box 36 on the label.

Referring now to FIG. 2, the transformer T is there illustrated as an iron core 20 comprising a T-shaped central member 40 having a slot 42 formed therein for purposes which are well known in this art. Such a slot as 42 is commonly known as a bridged gap and its purpose is to limit flux in the area of the secondary winding S to prevent saturation in the core 20. This will thus prevent excessive distortion of the wave shape of the lamp current. A pair of outer legs 46 and 48 are arranged in relationship to the central winding leg 40 to provide the elongate side windows 50 within which the windings will be disposed. The core 20 is made up of thin larninations of steel stacked and held together by suitable clamps, one being symbolically illustrated at 52 along with the potting compound 54 and the canister 36.

The principal part S of the secondary winding is shown mounted on the left end of the windows, and the other windings are all on the right-hand end. These windings are made by conventional methods, and usually are wound upon rectangular paper forms such as indicated at 56, fitted over the central winding leg 40. The layers of turns may be separated by paper or sheets of other insulating material, and the entire winding may be impregnated with some compound such as varnish, but the invention does not exclude the likelihood of windings being made of the so-called precision type in which there are little or no insulating materials used other than the enamel coatings of the wires themselves.

The detailed structures of the windings P,S ,Fl, and F2 are i not shown in FIG. 2 since these can be arranged in many different waysyside-by-side, one on top of the other, etc. All are closely coupled one with the other. The winding S is shown separated from the others, but principally being separated physically from the primary winding P by the paper structure indicated at 58. When conventional windings are made, the physical article itself has end margins which may amount to some fraction of an inch. A typical separation between the actual windings themselves would be of the order of one-quarter l/4) inch for a ballast of the type contemplated by the invention. This being an elongate type of structure, the resulting electrical effect will be the production of a high leakage reactance during operation of the ballast. If this is not sufficient, small metal shunts such as suggested by the lines 58 of FIG. 1 may be used instead of depending upon the-nonmagnetic gap.

The physical details of a practical structure constructed in accordance with the invention for use with a 30-watt rapidstart lamp are given below:

The overall length of the core 20 was 4 $4 inches, and FIG. 2 being drawn fairly accurately to scale, the remaining core dimensions may be derived therefrom. The stack height was 0.7 inch, the laminations being conventional in character and thickness.

The windings were constructed as shown in the table: Primary winding P 800 turns 028 A.W.G. wire, copper Secondary part S 1980 turns 028 A.W.G. wire, copper Secondary part S, 320 turns 028 A.W.G. wire, copper The windings F1 and F2 were chosen to give proper heater voltages suggested by the lamp manufacturer.

Condenser C was 4.6 microfarads with a 250 volt rating.

Voltages Open Circuit Operation Acmcs P I20 120 Acroau S 242 290 Across S. 48 48 voltage across the secondary winding S,S is 193 volts, which is sufficient to ignite the lamp L without danger of instant-start operation. The condenser C does not participate in the igniting of the lamp L. The open circuit voltage of the secondary winding S is reduced by the open circuit voltage of the winding 8,, almost directly as will be noted.

After the lamp 1. ignites and current flows in the secondary circuit, the high leakage reactance is produced because of the loose coupling of the secondary part S with the primary winding P. The resulting voltage across the secondary winding S rises to 290 volts, but the phase relationship of this voltage with the other voltages in the lamp circuit and with the primary voltage has changed. The phase of the voltage across the secondary part S, does not change with respect to the primary winding P because it is closely coupled therewith. The flow ofcurrent through the capacitor C causes a voltage drop across it which is out of phase with the other voltages.

The whole presents a relatively complex vector relationship,

which, while capable of qualitative analysis by circle diagrams and the like, nevertheless cannot be rigorous because of the nonsinusoidal wave shapes which are involved. It was found, however, that the proper rated voltage appeared across the lamp L with the current flow also being that which is recommended by the manufacturer of the lamp.

The concept of a bucking secondary part like S, provides a highly flexible tool for the construction of a ballast of the type needed for efiicient operation of a low-wattage lamp. Adjustment of turns, physical dimensions to provide proper leakage reactance and other characteristics can be undertaken by those skilled in this art to meet the requirementsof other low wattage gaseous discharge devices similar to, that described, but not necessarily having the same physical and electrical specifications. A

What it is desired to secure by Letters Patentpf the United States is: v v

I claim: I y

1. Apparatus for providing the igniting and operating voltages for a gaseous discharge device of relatively low wattage from a source of alternating voltagewhose normal'value is higher than the operating voltage of the gaseous discharge device and lower than the igniting voltage of said gaseous discharge device, the gaseous discharge device being of the type which has continuously energized heaters therein, said apparatus comprising:

A. an elongate iron core having a central winding leg and a substantially closed magnetic circuit including said winding leg;

B. a primary winding mounted on said winding leg at one end thereof, and said primary winding having electrical leads connected with the terminal ends thereof adapted to extend a connection from said primary winding to a source of alternating voltage of the value aforesaid;

C. a secondary winding also mounted on said winding leg,

and said secondary winding comprising:

1. a first part mounted on said winding leg end-to-end relative to said primary winding, and being loosely coupled relative thereto;

2. a second part mounted on said winding leg is closer proximity to said primary winding than said first part and being looselycoupled relative to the first part of said secondary winding;

D. the parts of said secondary winding being connected in series one with the other and with a condenser, and together therewith having electrical leads for forming a series loop connection with a gaseous discharge device having characteristics of the type aforesaid;

E. the first part beingof a substantially greater number of turns than said second part, and the parts being in instantaneous voltage-bucking relationship one to the other and shock hazard isolated from said primary winding; and

F. filament windings in close coupled relation to said primary winding and having electrical leads for connecting said filament windings to the heaters of said gaseous discharge device.

2. The apparatus as claimed in claim 1 in which there is a space between the end of said first part of the secondary winding and the remainder-of said windings.

3. The apparatus as claimed in claim 1 in which there is a bridged gap in the portion of the magnetic circuit adjacent the first part only of said secondary winding.

4. The apparatus as claimed in claim 3 in which the gap is in the central winding leg enclosed between the ends of said first part.

5. The apparatus as claimed in claim 1 in which the turns ratio of the parts of the secondary winding is approximately six to one.

6. Apparatus as claimed in claim 1 in which the said iron core has a generally framing structure providing elongate parallel side legs one spaced from each edge of the central winding leg whereby to form a pair of elongate windows between said central winding leg and said side legs, and in which the windings are fonned as tubular structures whose bodies occupy thewindows and whose centralbores are engaged by said winding leg.

7. The apparatus as claimed in claim 6 in which the first part of the secondary winding is formed as a physically independent tubular structure and is mounted on said central winding leg physically spaced from all other windings.

8. The apparatus as claimed in claim 6 in which the first part of the secondary winding is formed as a physically independent tubular structure and is mounted on said central winding leg physically spaced from all other windings with a magnetic shunt disposed in the space.

9. The apparatus as claimed in claim 7 in which there is a bridged gap in the central winding leg fully enclosed by said first part of the secondary winding.

10. A fluorescent lamp lighting system comprising:

A. a source of alternating current of predetermined voltage;

B. a fluorescent lamp of the rapid-start type having an igniting voltage higher and an operating voltage lower than said predetermined voltage;

C. a transformer having an iron core and windings, and consisting of l. a primary winding connected to said source and shockhazard isolated from the remainder of said windings;

2. filament winding coupled to the primary winding and connected to heaters in said lamp ends;

3. a secondary winding and a condenser connected togetberin series between the lamp heaters,

a. the secondary winding having two parts,

b. one part having a substantially greater number of turns than the second part and having looser coupling with said primary winding than said second part, the two parts being loosely coupled relative to one another,

c. and the two parts being disposed on the iron core in instantaneous voltage-bucking relationship one to another considering the lamp series circuit in which both are connected.

1 l. The system as claimed in claim 10in which the iron core is elongate and has a central winding leg and a pair of parallel side legs spaced from the winding leg to form windows on opposite sides of the central winding leg, and in which the windings are formed as tubular structures whose bodies occupy the windows and whose central bores are engaged by said winding leg.

12. The system as claimed in claim 11 in which the first part of the secondary winding is physically independent from all of the other windings and spaced therefrom.

13. The system as claimed in claim II in which the first part of the secondary winding is physically independent from all of the other windings and spaced therefrom with a magnetic shunt disposed in said space.

14. The system as claimed in claim 12 in which there is a bridged gap in the central winding leg fully enclosed by said first part.

Claims

1. Apparatus for providing the igniting and operating voltages for a gaseous discharge device of relatively low wattage from a source of alternating voltage whose normal value is higher than the operating voltage of the gaseous discharge device and lower than the igniting voltage of said gaseous discharge device, the gaseous discharge device being of the type which has continuously energized heaters therein, said apparatus comprising: A. an elongate iron core having a central winding leg and a substantially closed magnetic circuit including said winding leg; B. a primary winding mounted on said winding leg at one end thereof, and said primary winding having electrical leads connected with the terminal ends thereof adapted to extend a connection from said primary winding to a source of alternating voltage of the value aforesaid; C. a secondary winding also mounted on said winding leg, and said secondary winding comprising: 1. a first part mounted on said winding leg end-to-end relative to said primary winding, and being loosely coupled relative thereto; 2. a second part mounted on said winding leg is closer proximity to said primary winding than said first part and being loosely coupled relative to the first part of said secondary winding; D. the parts of said secondary winding being connected in series one with the other and with a condenser, and together therewith having electrical leads for forming a series loop connection with a gaseous discharge device having characteristics of the type aforesaid; E. the first part being of a substantially greater number of turns than said second part, and the parts being in instantaneous voltage-bucking relationship one to the other and shock hazard isolated from said primary winding; and F. filament windings in close coupled relation to said primary winding and having electrical leads for connecting said filament windings to the heaters of said gaseous discharge device.

2. a second part mounted on said winding leg is closer proximity to said primary winding than said first part and being loosely coupled relative to the first part of said secondary winding; D. the parts of said secondary winding being connected in series one with the other and with a condenser, and together therewith having electrical leads for forming a series loop connection with a gaseous discharge device having characteristics of the type aforesaid; E. the first part being of a substantially greater number of turns than said second part, and the parts being in instantaneous voltage-bucking relationship one to the other and shock hazard isolated from said primary winding; and F. filament windings in close coupled relation to said primary winding and having electrical leads for connecting said filament windings to the heaters of said gaseous discharge device.

2. The apparatus as claimed in claim 1 in which there is a space between the end of said first part of the secondary winding and the remainder of said windings.

2. filament windings coupled to the primary winding and connected to heaters in said lamp ends;

3. a secondary winding and a condenser connected together in series between the lamp heaters, a. the secondary winding having two parts, b. one part having a substantially greater number of turns than the second part and having looser coupling with said primary winding than said second part, the two parts being loosely coupled relative to one another, c. and the two parts being disposed on the iron core in instantaneous voltage-bucking relationship one to another considering the lamp series circuit in which both are connected.

6. Apparatus as claimed in claim 1 in which the said iron core has a generally framing structure providing elongate parallel side legs one spaced from each edge of the central winding leg whereby to form a pair of elongate windows between said central winding leg and said side legs, and in which the windings are formed as tubular structures whose bodies occupy the windows and whose central bores are engaged by said winding leg.

10. A fluorescent lamp lighting system comprising: A. a source of alternating current of predetermined voltage; B. a fluorescent lamp of the rapid-start type having an igniting voltage higher and an operating voltage lower than said predetermined voltage; C. a transformer having an iron core and windings, and consisting of

11. The system as claimed in claim 10 in which the iron core is elongate and has a central winding leg and a pair of parallel side legs spaced from the winding leg to form windows on opposite sides of the central winding leg, and in which the windings are formed as tubular structures whose bodies occupy the windows and whose central bores are engaged by said winding leg.

13. The system as claimed in claim 11 in which the first part oF the secondary winding is physically independent from all of the other windings and spaced therefrom with a magnetic shunt disposed in said space.