US3657678A

US3657678A - Multi-purpose, multi-voltage transformer

Info

Publication number: US3657678A
Application number: US44214A
Authority: US
Inventors: Carl A Schwenden
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-06-08
Filing date: 1970-06-08
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

Multi-purpose multi-coil single-unit transformer for accommodating a number of phases and voltages by making different appropriate external connections between the transformer coils. The core has at least two parallel legs connecting two appropriate yokes of transformer iron. At least one coil encircles each of the legs and at least one coil encircles all other coils. C-shaped magnetic shunt members abutting opposed faces of the yoke extend essentially parallel to the legs, and embrace the periphery of the coils, to create a magnetic-flux bypass beyond the coils. Parallel and series connections of the coils encircling said legs with interconnections between said coils so that the flux generated by a primary a.c. current passing through said coils is forced to flow partially through the magnetic shunt members, and generates a secondary voltage in the coil encircling all other coils. Two sets of three identical coils wound on a two-leg core with magnetic shunt members, each set consisting of two coils wound around one leg each and a third coil encircling the two coils, operate to generate a secondary 3phase a.c. current from one set of coils when the other set is energized by a primary 3-phase a.c. current. The multi-coil transformer operates as a saturable core reactor if a d.c. current is passed through one coil electrically insulated from the other coils with an a.c. current flowing through the other coils to a load, and with interconnections between the latter coils to render the desired reactance of the coil/core configuration.

Description

United StatesPaten t S chwenden [54] MULTI-PURPOSE, MULTI-VOLTAGE TRANSFORMER [72] Inventor: Carl A. Schwenden, 1218 C Edith Street,

Alhambra, Calif.

[22] Filed: June 8, 1970 [21] Appl. No.: 44,214

[52] U.S. Cl ..336/160,336/l84,336/2-12 [51] Int. Cl. ..H01f2l/08, l-I0 1f 27/28 [58] Field ofSearch ..336/5,12,155,160,165,170,

[56] References Cited UNITED STATES PATENTS 2,267,382 12/1941 Vance ..336/155 3,308,413 3/1967 Schroeder et a1. 336/170 X 3,290,634 12/1966 Stevens ..336/160X 3,241,048 3/1966 Lee .336/165 X 2,060,477 11/1936 Unger ..336/160 2,735,989 2/1956 Williams ...336/155 2,253,962 8/1941 Vance ..336/155 FOREIGN PATENTS OR APPLICATIONS 1,129,403 9/1956 France ..336/212 Primary Examiner-Thomas .l. Kozma Attorney-Sellers and Brace [151 "3,657,678 [451 Apr. 18,1972

[s7 ABSTRACT Multi-purpose multi-coil single-unit transformer for accommodating'a numberof phases and voltages by making different appropriate external connections between the transformer coils. The core has at least twoparallel legs connecting two appropriate yokesof transformer iron. At least one coil encircles each of the legs and at least one coil encircles all other coils. C-shaped magnetic shunt members abutting opposed faces of the yoke extend essentially parallel to the legs, and embrace the periphery of the coils, to create a magnetic-flux by-pass beyond the coils. Parallel and series connections of the coils encircling said legs with interconnections between said coils so that the flux generated by a primary a.c. current passing through said coils is forced to flow partially through the magnetic shunt members, and generates a secondary voltage in the coil encircling all other coils. Two sets of three identical coils wound on a two-leg core with magnetic shunt members, each set consisting of two coils wound around one leg each and a third coil encircling the two coils, operate to generate a secondary 3-phase a.c. current from one set of coils when the other set is energized by a primary 3-phase a.c. current. The multi-coil transformer operates as a saturable core reactor if a dc. current is passed through one coil electrically insulated from the other coils with an a.c. current flowing through the other coils to a load, and with interconnections between the latter coils to render the desired reactance of the coil/core configuration.

18 Claims, 21 Drawing Figures Patented April 18, 1972 '3 Sheets-Sheet 1 V 5466. Z I

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This invention relates to electric induction devices and more particularly to a multi-purpose multi-coil transformer usable to generate a variety of voltages depending upon external connections between the various transformer coils.

In conventional transformers, the secondary voltage is determined by the ratio of turns of the secondary coil to the number of turns of the primary coil, assuming a fixed value for the primary voltage. In principle the magnitude of the secondary voltage is affected by the flux through the transformer core section around which the secondary coil is wound. Disregarding the case of a biasing field generated by a dc. current through an auxiliary coil, modifications of the flux in the core of conventional transformers is customarily effected by changing the number of windings of the primary coil through which a current generating a secondary voltage is passed. In conventional transformers, it is not customary to compensate for a portion of the primary flux by an opposite flux generated by an in-phase a. c. current flowing through an auxiliarycoil. Op-

posite fluxes generated in the same core section by two coils through which oppositely phased currents are passed for the purpose of compensating for part or for all of the flux generated by one of the coils lead to strong eddy currents in that core section. These eddy currents cause overheating of the transformer core and serious damage or destruction of the transformer.

In the transformer of the invention, the flux in the transformer core section encircled by the secondary coil can be influenced by the flux generated by a second primary coil to which primary a.c. current is fed. The detrimental effect of flux superpositions is eliminated, and the above-mentioned generation of eddy currents and the ensuing overheating of the transformer core is prevented by the provision of magnetic shunt members arranged beyond the periphery of the transformer coils between appropriate core sections. Depending upon the number of turns of the second primary coil and the connection of the same with respect to the first primary coil, the flux through the transformer core is modified by bypassing magnetic flux through the shunt members'and, consequently, the magnitude of the secondary voltage can be increased above, or decreased below the voltage which would result without operation of the second primarycoil. Magnetic flux generated by both primary coils is passed through the magnetic shunt members thereby changing the flux through the secondary coil and, hence, the magnitude of the secondary voltage by proper connections between the primary coils, without changing the number of turns in the primary coils.

As will be described below, the coils of the transformer of the invention can be connected for operation with single-, two-, or three-phase a.c. current. In another mode of operation of the transformer of the invention, a biasing d. c. current is passed through one of the coils and the transformer then assumes the function of a saturable core reactor.

It is an object of this invention to provide a versatile, new, simply constructed single-unit transformer having at least two primary coils and at least one secondary coil and magnetic shunt members cooperating to vary the magnetic flux in the transformer core by passing appropriately directed a.c. currents through the primary transformer coils.

It is a further object of this invention to provide a new and simple transformer of the specified type in which, by proper connection of at least two of the primary coils, the flux through the transformer core can be modified and, hence, the magnitude of the effective secondary voltage can be changed in definite ratios, without changing the number of turns in any primary or secondary coil.

It is another object of this invention to provide a transformer of the specified type with N primary coils and at least one secondary coil and witha 1:1 turn ratio of all coils, wherein, by making proper connectionsbetween the primary coils, the secondary voltage can be varied in steps of UN times the primary voltage applied to each coil, or in multiples of the primary voltage up to a maximum of N times the primary voltage.

It is another object of this invention to provide a transformer which, by proper connection of its coils, can be operated with single-, two-, or three-phase a.c. current and which delivers secondary voltages of the same number of phases or a smaller number of phases.

It is still another object of this invention to provide a new and simple transformer of the specified type in which a biasing dc. current can be sent through one of the coils for the purpose of modifying the flux through the core so that the transformer functions as a saturable core reactor.

This invention will be better understood from the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

FIG. 1 of the drawings is a perspective view of an illustrative embodiment of the invention;

FIG. 2 is a cross section taken substantially along the center plane of FIG. 1;

FIG. 3 is a schematic of the electric circuit equivalent to the magnetic circuit of the embodiment of the invention shown in FIG. 1 with the magnetic field lines produced in the two primary coils opposed to one another;

FIG. 4 is a schematic of the electric circuit equivalent to the magnetic circuit of the embodiment shown in FIG. 1 with the magnetic field lines produced in the two primary coils flowing in the same direction;

FIGS. 5 and 6 show two modes of connecting the transformer of FIG. 1 with the primary coils in series;

FIGS. 7 and 8 show two modes of connecting the transformer of FIG. .1 with the primary coils in parallel connection;

FIG. '9 is a perspective view of the FIG. 1 embodiment showing the flux distribution in the core and the magnetic shunt members of the invention transformer;

FIG. 10 is a perspective view of another preferred embodimerit of the invention transformer;

FIG. 11 is a perspective view of part of the core and of a sec tion of a shunt member used in the embodiment shown in FIG. 10;

FIGS. 12-17 are schematics of the connection of the transformer of FIG. 10 in various exploitations;

FIGS. 18 and 19 are schematics of the connections for operating the transformer of FIG. 1 as saturable core reactor;

FIG. 20 is a schematic of the connections for operating the transformer of FIG. 1 with three-phase a.c. current; and

FIG. 21 is a schematic of the connections for deriving two secondary a.c. phases from the FIG. ltransformer operated while energized with 3-phase a.c.

Referring first to FIGS. 1 and 2, there is'shown a preferred embodiment of the transformer of the invention having a core 1 with two legs la and lb, two magnetic shunt members 2a, 2b, a coil 3a encircling leg la, a coil 3b encircling leg lb, and a coil 4 wound on top of coils 3a, 3b and encircling both legs la, lb. Core 1 consists of two stacks of conventional, essentially U-shaped laminated sections of a type usually employed in power transformers. The two stacks have essentially the same size and smooth snugly-abutting end faces at the end of each leg. The two stacks abut at these end faces, in a plane concealed inside between the opposite ends of coils 3a, 3b. As far as the magnetic properties of the material and the shape of the magnetic shunt members are concerned, magnetic shunt members 2a,2b closely resemble legs 10 and lb of core 1, though it will become apparent from the following description that the shape of the magnetic shunt members may differ from the form shown in FIG. 1 without departing from the spirit of the invention.

Coils 3a, 3b, 4 are conventional wire wound inductance coils. The winding ends to which electrical connections are made are denoted by '5a, 5b for coils 3a, by 6a, 6b for coil 3b, and by 7a, 7b for coil 4. The ends of each coil are brought out separately so that a variety of connections can be established between the coils.

The various ways of connecting the coils and operatingthe invention transformer are most easily understood from first considering the analogy between electric and magnetic circuits. For example, coils 3a, 3b, legs 1a, lb, and magnetic shunt members 2a, 2b can be considered as two magnetic circuits, wherein coils 3a, 3b perform the functions of sources of magnetic field lines when currents flow through these coils. It is well known from elementary electromagnetic theory that a complete analogy exists between electric and magnetic circuits and that magnetic circuits of any complexity can be analyzed like electric circuits when the magnetic flux (b, the magnetomotive force nl, and the reluctance R,, of the magnetic circuit are respectively compared with the current i, the

electromotive force E, and the resistance R of electric circuits.

Thus, coil 3a acts as a magnetomotive force nI, wherein n denotes the number of turns of coil 3a and I the current which is passed through coil 3a. For the analogy considerations, current I can be assumed as direct current. The magnetic flux in the entire core/shunt-member configuration must be considered, as well as the reluctance of all sections, in order to ob tain the full magnetic circuit in which the magnetomotive force generated by coil 3a acts. Similar considerations are applicable to coil 3b. When it is assumed in the following description of the magnetic circuit that all the core and shunt sections are identical (a restriction which need not be made, but which facilitates the description), the simplified equivalent electric circuit representation of the magnetic circuit is that shown in FIG. 3, it being understood that R relates to the magnetic reluctance between the center of each U- shaped core or magnetic shunt-member section and the end of the same section. The internal resistance of each source of electromotive force, or corresponding y, magnetomotive force can be ignored without loss in generality. To include this internal resistance amounts to taking into account magnetization losses which in the case of conventional core materials are immaterial for an explanation of transformer operation at low frequencies.

Kirchhoffs laws can be applied to the electric circuit equivalent to the magnetic circuit, expressions in terms of electric quantities can be derived, and the results can be reconverted into magnetic quantities to obtain a full description of the functioning and operation of the transformer of the invention. This application is facilitated by denoting by i vthe electric circuit current corresponding to the flux in leg la, by i,, the corresponding flux in leg lb, by i the flux in both identically shaped magnetic shunt members 2a, 2b, by E, the electromotive force corresponding to the magnetomotive force generated by coil 30, and by E,, the corresponding magnetomotive force generated by coil 3b.

With reference to FIG. 3 wherein the directions of the flux lines in legs la, lb are opposite so that closed magnetic field lines pass through core 1 when the magnetic flux through shunt members 2a, 2b vanishes, the following three equations are found to be valid:

1. at point A (or B) F u' 2. E, circuit E,,-2i,,R+i R=0 3. E, circuit E -2i,,R'i R==0 number of turns and the ac. currents flowing through coils 3a,

3b, then the currents i,,, 1' and i can be expressed in terms of R,-E and E thereby to express the corresponding fluxes in the magnetic circuit by known magnetic circuit parameters.

The solutions of Equations 1 to 3 are:

n f' o) and accordingly, in the magnetic circuit, the following equations give the values 'of the flux 4):

b a a b a) R: (a e b b) wherein F denotes the cross section of a leg or a magnetic shunt-member; l is half the length of a U-shaped core or magnetic shunt-member section; p. is the magnetic permeability of the core or shunt-member material; n,,, n is the number of and have the following solutions:

From these thefollowing relations are derived for the magnetic circuit represented by the core/shunt-member' configuration:

a (3 41 11 b b) The advantages and objectives of the present invention c be illustrated to advantage if coils 3a, 3b have identical numbers n of turns of wire of the same diameter. In this case, n, m, in the above expressions ford) (by, and (b In the case of oppositely directed field lines in legs la, lb, coil 4 can be considered as a secondary winding of a conventional single-leg transformer and electrically insulated from the primary winding. According to the principles of transformer theory, the a .c. voltage generated in the secondary winding of a transformer by induction is proportional to the rate of change in magnetic flux in the secondary winding and the number of turns in the secondary winding.

The flux which leads to an induced voltage in coil 4 is determined by the fluxes through legs la, lb, as well as through

magnetic shunt members

20, 2b. When in-phase a.c. currents of identical magnitude-I are passed through coils 1a, lb connected in parallel so that the magnetic field lines created by coil 3a in leg la are at any instant of time oppositely directed to the magnetic field lines created by coil 3b in leg 1b (FIG. 7), then the magnetic flux in leg 1a equals the magnetic flux in leg lb, whereas the flux through the magnetic shunt member vanishes, a fact readily apparent from the expressions for and da From these equations it will be noted that th da =nI/2C, where C l/ptF and Generally, l=l sinwTin power transformer applications. It follows that the fluxes change essentially'at the same rate as the currents in the primary coils. With the total flux generated in both primary coils 3a, 3b passing through the core, the absolute value of the resulting secondary voltage is (i i v k Lfiu sut' "sec 7; id 4 b) Q and since for the primary voltage V prlm. the relations and 'l 2% i4- apply, the secondary voltage is :ec "sec -vprlm (n denotes the number of turns of the secondary coil), i.e. with a 1:1 ratio of secondary winding to primary windings, twice the primary voltage is generated in the secondary coil of the invention transformer.

On the other hand, when the fluxes created in legs la, lb by respective coils 3a, 3b connected in parallel are opposing, i.e. when the magnetic field lines in leg la have a direction parallel to that of the magnetic field lines in leg lb at any instant of time (FIG. 8), the flux in legs 1a, lb is halved relative to the above-described case of oppositely directed magnetic field lines in the legs, and a portion of the flux which is generated by the magnetomotive forces represented by coils 3a, 3b is forced. to pass through magnetic shunt members 2a, 2b. Then, necessarily i.e. with a 1:1 turn ratio of secondary to primary windings, the secondary voltage equals the primary voltage, and half of the total flux generated in the magnetic circuit flows through each shunt member 2a, 21) (FIG. 9).

When the number of turns of the secondary coil differs from that of the primary coil, the resulting secondary voltage is determined by the total flux in the secondary coil and the ratio of the number of turns between secondary and primary coils.

In another mode of exploiting the invention transformer, coils 3a, 3b are connected in series. When this series connection is such that the field lines in the two legs are oppositely directed (FIG. 5), half the flux obtained in the case of parallelconnected primary coils 3a, 3b is obtained and, consequently, for a 1:2 turn ratio of secondary winding to primary winding, a voltage equal to the primary voltage is produced which voltage is electrically separated from the primary voltage. Conversely, when the two primary coils are connected in series in such a fashion that a flux bucking in legs 10, 1b and a flux in magnetic shunt members 2a, 2b results, the voltage generated in secondary coil 4 is half of that of the previous case, since the flux in legs 1a, lb, which determines the secondary voltage generated in coil 4, is half that in the last discussed mode of operation (FIG. 6).

In the schematic representations of the various possible connections of primary coils 3a, 3b (FIGS. 5 to 8), the flux directions are denoted by arrows. It is understood that arrows pointing in the same direction indicate oppositely directed magnetic force lines in adjacent legs which are encircled by primary transformer coils. Conversely, arrows pointing in opposite directions indicate that the magnetic force lines have the same direction in adjacent legs encircled by primary coils.

To be more specific, when each of the coils 3a, 3b, 4 has the same number of turns, and an input ac. voltage of l volts is applied to primary coils 3a, 3b connected in parallel in the oppositely directed field-line mode, there is produced a secondary voltage of 220 volts (FIG. 5). However, a secondary voltage equal to the primary voltage is generated if the primary coils are operated in the bucking mode in which the magnetic field lines in legs la, lb are parallel (FIG. 6). When the two primary coils are connected in series, the secondary voltage is equal to the primary voltage in the nonbucking mode (FIG. 7) and equal to half the primary voltage in the bucking mode (FIG. 8).

These particular modes of connecting the coils of the invention transformer provide secondary voltages whose absolute values are given by the ratio of the number of turns in the secondary coil to the number of turns in the primary coil, whereas the relative magnitude of the two possible secondary voltages is in the ratio 2: 1.

The primary and secondary coils can change their roles, and changes in the secondary voltage can be effected by changing the connections of coils 3a, 3b used assecondary coils. This equivalence between secondary and primary coils 3a, 3b, 4 makes it possible to obtain 1:1, 1:2, and 2:1 voltage ratios with 1:1 turn ratios of the coils.

It must be noted that identical primary coils 3a, 3b are fully equivalent in their importance for the operation of the invention transformer.

The core configuration of the invention transformer can be extended to three-legged cores with two pairs of magnetic shunt members (FIG. 10). The three-legged core 9 can comprise two blocks of conventional E-shaped tape-wound transformer lamina, one block of which is schematically shown in FIG. 11 along with a portion of a shunt member. The two blocks abut between the opposite ends of coils 10a, 10b, 10c. Coils 10a, 10b, 10c encircle legs 11a, 11b, 11c, respectively, while secondary coil 12 encircles all

coils

10a, 10b, 100. Two pairs l3, 14 of magnetic shunt members 13a, 13b, and 14a, 14b consisting of stacks of generally 0 or U-shaped transformer steel tape are provided at both sides of the threelegged core. The magnetic shunt members and the core have identical cross sectional areas and are held together by appropriate means, eg steel-tape straps surrounding the magnetic shunt members and the core.

Referring now to FIGS. 12 16 in which the reference symbols of FIG. 10 are used to indicate identical parts and with coils 10a, 10b, 10c fulfilling the function of primary coils, it will be understood that various ways of connecting the coils and of generating fluxes in legs 11a, 11b, 11c may be employed depending on the nature of the power supply and the result desired.

FIGS. 12-14 refer to series connection of the three primary coils. Assuming for the following description a 1:1 ratio of the number of turns of each primary coil to the secondary coil, the ratio of secondary to primary voltage can be derived from the above description of the manner in which the invention transformer functions.

Series connection of the three primary coils in the form of FIG. 12 (wherein arrows of same direction denote oppositely directed flux lines in the adjacent one of the three legs) is equivalent to a conventional transformer having a 3:1 turn ratio of primary to secondary coils, but with the secondary coil surrounding three identical primary coils, to each of which one-third of thetotal primary voltage is applied, the primary voltage is reproduced by the secondary coil.

When the primary coils of the FIG. 9 transformer are connected in series, as shown in FIG. 13, a bucking effect results, and part of the flux is forced from core 9 into magnetic shunt members l3, 14, thereby reducing the total core flux which determines the magnitude of the secondary voltage. The flux interaction in two adjacent coils producing parallel magnetic field lines is such that the flux is one-half that provided in the case of oppositely directed field lines in each pair of adjacent legs. However, while the flux in the third leg is not afi'ected, the contributions of the two other legs to the voltage generated in coil 12 cancel each other and the resulting net secondary voltage is equal to one third of the primary voltage, (from the leg in which no'bucking of the flux occurs).

While the transformer connection according to FIG. 13 involves a bucking efiect in only two legs of the core configuration, the third case shown in FIG. 14 corresponds to a bucking effect in all three legs, i.e., the magnetic field lines of each pair of adjacent legs are parallel. Thus, the'tlux effect in pairs of adjacent legs upon the voltage generated in the secondary coils cancels and the net secondary voltage is zero.

Series connection of the primary coils permits one to reduce the secondary voltage for a 1:1 turn ratio between each primary and the secondary coil, whereas an increase in the secondary voltage can be obtained if the primary coils are connected in parallel.

Referring first to the primary coil connection shown in FIG. 15 (three primary coils in parallel connection so that no bucking results in three legs 10a, 10b, 10c), the total magnetic flux through secondary coil 12 is tripled relative to that pro vided by a conventional transformer in which a single primary coil of the size of one of the three primary coils of the invention transformer is employed and, hence, the secondary voltage is three times the primary voltage despite a 1:1 tum ratio of secondary coil to each primary coil.

Partial flux bucking results when the coils are connected as shown schematically in FIG. 16. While the magnetic field lines in two adjacent legs are oppositely directed at any instant of time (this corresponds to parallel arrows in FIG. 15), a bucking effect results respecting the flux in one of the outer legs, with the flux generated in one leg being forced through the magnetic shunt members. The flux determining the secondary voltage is reduced relative to the previous case due to the cancellation of the voltages generated in the secondary coil sections surrounding legs with parallel flux directions, and a secondary voltage equal to the primary voltage is generated in secondary coil 12.

Finally, when the three-legged invention transformer is connected as shown in FIG. 17, bucking results in each of the legs and a portion of the total flux is forced through both pairs of

magnetic shunt members

13, 14. The flux in central leg 11b is practically cancelled by the parallel magnetic field lines generated in adjacent legs 11a, llc. The net result is that legs 11a, 11 each contribute a flux which generates half the primary voltage so that the combined efiect of coils 110, 11c reproduces only the primary voltage in the secondary coil and electrically isolated therefrom.

This design principle can be further extended to transformer cores consisting of more than three legs with magnetic shunt members positioned in pairs at the centers of core sections between adjacent legs. The number of such pairs of shunt members is smaller than the number of .legs. The cross sectional area of the shunt members is at least equal to the cross sectional area of the legs so that the magnetic flux density in the shunt members is at most equal to the flux density in the legs. In analogy to the previous embodiment, a bucking effeet can be obtained in each leg of the core in a transformer comprising N legs, by the provision of (N 1) pairs of magnetic shunt members. When all the primary coils are connected in series, by applying the connection modes described for the three-legged transformer, the primary voltage is reproduced by the secondary coil for the reasons outlined in relation to the three-legged embodiment. A bucking efi'ect is introduced in adjacent legs, which leads to by-passing the magnetic flux through the magnetic shunt members between adjacent legs in which the magnetic field lines are parallel.

Multiples of the primary voltage up to a maximum of N times the primary voltage can be obtained when all primary coils are connected in parallel. Opposing magnetic field lines in pairs of adjacent legs lead to a reduction of the maximum secondary voltage, with each such pair corresponding to a reduction of the maximum secondary voltage by a voltage equal to the primary voltage. Since (N I) such pairs are available, a stepwise reduction of the maximum secondary voltage to the value of the primary voltage is feasible by proper phase reversal of the currents through the coils encircling pairs of adjacent, essentially parallel legs.

The coil configuration on the core of the invention can be employed as a saturable core reactor with at least three separate coils through one of which a d.c. current is maintained to change the reactance of the other two coils through which a.c. is passed to a load. The magnitude of the d.c. current modifies the magnetization characteristics of the core material by introducing a biasing permanent magnetization. Changes in the d.c. current modify the reactance of the other coils whose magnetic field is superimposed on the biasing permanent magnetic field created by the d.c.

Several a.c. operated coils can be provided on the invention saturable core reactor which is characterized by the provision of magnetic shunt members to generate a magnetic flux bypass when the a.c. operated coils are interconnected to create opposing fluxes in various core sections.

The saturable core reactor of the invention resembles the transformer of FIG. I in the arrangement of the core, the coils, and the magnetic shunt members. The various ways of connecting the coils of the saturated core reactor of the invention are exemplified in the schematic circuits of FIGS. 18 and 19, which refer to a simple embodiment of a saturable core reac- -tor with a.c. and d.c. coils 3a, 3b, 4 mounted on a common core generally denoted by l and with magnetic shunt members 2a, 2b. At least one coil is connected between a source of a.c. current and an a.c. load for the purpose of controlling the current to said load through the reactive resistance represented by said coil.

Referring now to FIG. 18, two coils 3a, 3b are connected in series so that parallel flux directions are created in the legs, as is indicated by the opposing arrows and part of the flux is forced into the shunt members. The a.c. current passed through the two coils encounters the full reactance which can be modified by the controlling d.c. current supplied to coil 4 from a separate d.c. source.

As shown in FIG. 19, coils 3a, 3b, 4 can change their roles, i.e. direct current can be sent through coils 3a, 3b and the a.c. current to the load can be sent through coil 4. The considerations of the preceding paragraph are applicable to d.c.

operated coils 3a, 3b which are connected in bucking mode to by-pass part of the d.c. generated flux through the shunt members. By adjusting the d.c. current the reactance of coil 4 in the load circuit can be changed. In all these applications, the magnetic shunt members of the invention provide a magnetic by-pass for opposing magnetic fluxes in various core sections and act as a means for modifying the reactance.

When two sets of three identical coils are provided, the invention transformer can be operated with three-phase a.c. current. In a preferred embodiment, the two coil sets have the same configuration and are arranged one at the side of the other on the transformer core constructed, for example, as shown in FIGS. 10 and 11. The coil connections are shown in FIG. 20, wherein a schematic representation of the core has been omitted. One set of three

similar coils

1, 2, 3 represents the primary winding, the other set of coils 1, 2', 3' the secondary winding for example, the two sets of coils being duplicated and located in end-to-end relation on respective ones of legs lla,1lb, 11c of the core illustrated in the aforementioned FIGS. 10 and 11. The core is of the shunted twoleg type described above, with two of the three coils of each set wound around one leg each, and the third coil encircling the other two coils. This core configuration deviates from the conventional three-leg cores for three-phase transformers. Each set of coils is connected in the usual T-connection of a single -phase transformer, with the coils of each set interconnected for proper phase sequence.

The use of the shunt members has the advantage that with this coil configuration, the transformer can be used for singlephase as well as for three-phase a.c. current, whereas two conventional two-leg transfonners without shunt members would otherwise be required to obtain the same result in three-phase operation. The number of turns of the three identical secondary coils l, 2', 3' depends upon the secondary voltage to be generated.

It is another advantage of the two-leg two-coil set embodiment of the invention transformer that the transformer can be directly employed to derive two single-phase secondary currents from a three-phase primary current fed to three coils in 'l connection. Due to the effect of the shunt members, the magnitude of the secondary voltages can be varied by changing the interconnection of the coils of the secondary winding. As shown in FIG. 21, the set of three

identical coils

20, 21, 22 assumes the function of the primary winding which is operated with three-phase a.c. current. The interconnection of these coils is so that their fluxes mutually assist, as indicated by the arrows in FIG. 21.

Coils 30, 31 and coil 32 are the two phases of secondary voltages appearing at the terminal pairs 40, 41 and 42, 43, respectively. Both coils 30, 31 encircle one leg each, whereas coil 32 is wound around coils 30, 31 in the fashion described above. The magnitudes of the secondary voltage depend, in addition to the number of turns, on the way in which the interconnections between coils 30 and 31 are made.

The two-leg transformer provided with shunt members according to the invention makes it possible to convert in a simple way a primary three-phase a.c. current into two separate phases of secondary a.c. voltage, the magnitude of which can be changed by interconnection of the secondary coils so that 0 the magnetic flux'in the legs is reduced and a fraction of the flux by-passed through the magnetic shunt members abutting the core.

It will be apparent to those skilled in the art that a variety of modifications can be made in the coil interconnections used in single-, two-, and three-phase operation from a single unit without departing from the spirit of the invention.

While the particular multipurpose multi-coil transformer herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefo're stated, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention, and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.

I claim:

l. Transformer for single, two-, or three-phase alternating current, comprising a closed-loop core of laminated transformer iron with at least two legs and at least three coils, characterized in (1) that all coils are electrically separated from each other and the core and can be connected in a variety of ways in each of which all three coils convey alternating current; (2) that at least one coil encircles at least two coils each of the latter of which encircles a respective one of said legs; (3) the provision of at least one magnetic shunt member having its opposite ends abutting said core, extending essentially parallel to at least one of said legs encircled by a coil, and running over the periphery of any of said coils to create a path for magnetic flux beyond said coils, and (4) in that the cross section of said closed loop core and of said magnetic shunt member are substantially the same.

2. Transformer as defined in claim 1 characterized in that said magnetic shunt member consists of closed stacked essentially C-shaped steel-tape sections.

3. Transformer as defined in claim 2 characterized in that said magnetic shunt member protrudes at right angles from a first side of said core, proceeds essentially parallel to the longitudinal extension of at least one of said legs, and ends at right angles to, and at a second core side opposite to said first side of said core.

4. Transformer comprising a core of laminated transformer iron, said core consisting of a first yoke and a second yoke parallel to the first yoke and at least two parallel legs connecting said yokes, characterized in that a first and a second coil encircles a separate one of said legs, that at least one coil encircles all other coils, that essentially C-shaped magnetic shunt members of stacked steel tape abut said yokes in positions essentially in the center between adjacent pairs of said legs and extend parallel to said legs along the outer periphery of said coils to provide a magnetic flux path between said yokes and to by-pass magnetic flux from said legs.

5. Transformer as defined in claim 4 characterized in that similar magnetic shunt members have their ends abutting said first and second yokes from the opposite sides thereof.

6. Transformer as defined in claim 5 with at least two of said coils encircling said legs connected to a source of alternating voltage to generate a magnetic flux varying in time in said legs, characterized in that the magnetic fluxes in at least one pair of adjacent legs are at each instant of time opposite while the magnetic force lines in said legs are parallel, and that a magnetic flux through the magnetic shunt members is maintained to generate an ac. voltage in said coil encircling all said coils, the magnitude of said generated ac. voltage depending on the magnetic flux in said magnetic shunt members, the flux through said legs, and the turn ratio of said coils.

7. Transformer as defined in claim 6 characterized in that the coils connected to said alternating voltage source have equal numbers of turns.

8. Transformer as defined in claim 7 characterized in that said coils encircling said legs and connected to said alternating voltage source are all connected in series and that at least two coils encircling adjacent legs are connected so that the magnetic force lines in said adjacent legs are parallel'and that a magnetic flux is generated in said magnetic shunt member located in an intermediate position between said adjacent legs.

9. Transformer as defined in claim 7 characterized in that said coils encircling said legs and connected to said alternating voltage source are all connected in parallel and that at least two coils encircling adjacent legs are connected so that the magnetic force lines in said adjacent legs are parallel and that a magnetic flux is generated in said magnetic shunt members located in an intermediate position between said adjacent legs.

10. Transformer for single-phase alternating current, comprising a core of laminated transformer iron, said core consisting of a first essentially U-shaped yoke and a second essentially U-shaped yoke abutting in smooth surface contact with said first yoke so that two parallel legs are formed, with a first coil encircling one of said legs and a second coil, identical to said first coil, encircling the other of said parallel legs, a third coil encircling both said first and second coil, the windings of said three coils insulated from each other and from said core, a pair of essentially U-shaped magnetic shunt members of transformer iron extending from both sides of said first yoke from the center position between said two legs, said magnetic shunt members running essentially parallel to said legs over the periphery of said third coil and terminating in smooth contact with both sides of said second yoke, a source of primary essentially sinusoidal alternating current to which said first and second coils are connected in parallel so that the magnetic force lines generated by said alternating current in said legs are parallel at any instant of time thereby to create a flux in said magnetic shunt members and to generate in said third coil a secondary essentially sinusoidal alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said legs, the magnetic flux in said pair of shunt members, and the turn ratio of said third coil to said first and second coil.

11. Transfonner for single-phase alternating current, comprising a core of laminated transformer iron, said core consisting of a first essentially U-shaped yoke and a second essentially U-shaped yoke abutting in smooth surface contact with said first yoke so that two parallel legs are formed, with a first coil encircling one of said legs and a second coil, identical to said first coil, encircling the other of said parallel legs, a third coil encircling both said first and second coil, the windings of said three coils insulated from each other and from said core, a pair of essentially U-shaped magnetic shunt members of transformer iron extending from both sides of said first yoke from the center position between said two legs, said magnetic shunt members running essentially parallel to said legs over the periphery of said third coil and terminating in smooth contact with both sides of said second yoke, a source of primary essentially sinusoidal alternating current to which said first and second coils are connected in series so that the magnetic force lines generated by said alternating current in said legs are parallel at any instant of time, thereby to create a flux in said magnetic shunt members and to generate in said third coil a secondary essentially sinusoidal alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said legs, the magnetic flux in said pair of magnetic shunt members, and the turn ratio of said third coil to said first and second coil.

12. Transformer according to claim 10 characterized in that the turn ratio of said third coil to said first and second coil is one to one so that the voltage generated in said third coil is twice the voltage of said primary alternating current source.

13. Transformer according to claim 11 characterized in that the turn ratio of said third coil to said first and second coil is one to two whereby the voltage generated in said third coil is one quarter the voltage of said primary alternating current source.

14. Transformer having a laminated core with two parallel end portions interconnected by N number of legs, a first set of identical coils encircling a respective one of said N legs, a second set of at least one coil encircling all said first coils, the windings of both sets of coils insulated from each other and from said core, (N- 1) pairs of essentially U-shaped magnetic shunt members of transformer iron having smooth coplanar and faces held in abutting contact with the opposite sides of the end portions of said core in areas thereof spaced between and parallel to adjacent ones of said core legs, said magnetic shunt members embracing said second set of coils, said first set of coils being connectable in parallel to a source of alternating current so that the magnetic force lines generated thereby are opposite at any instant of time in at least one pair of adjacent legs of said N legs, thereby to create a magnetic flux in the magnetic shunt member and to generate in said second set of coils a secondary alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said pairs of adjacent legs, the magnetic flux in said pairs of magnetic shunt members between each pair of adjacent legs, and the turn ratio of the coils of said first and second sets.

l5. Transformer having a laminated core two parallel end portions interconnected by N number 'of legs a first set of identical coils encircling a respective one of said N legs, a second set of at least one coil encircling all said first coils, the windings of both sets of coils insulated from each other and from said core, (N 1) pairs of essentially U-shaped magnetic shunt members of transformer iron having smooth coplanar end faces held in abutting contact with the opposite sides of the end portions of said core in areas thereof spaced between and parallel to adjacent ones of said core legs, said magnetic shunt members embracing said second set of coils, said first set of coils being connectable in series to a source of altemating current so that the magnetic force lines generated by said alternating'current are opposite at any instant of time in at least one pair of adjacent legs of said N legs, thereby to create a magnetic flux in the magnetic shunt member and to generate in said second set of coils a secondary alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said pairs of adjacent legs, the magnetic flux in said pairs of magnetic shunt members between each pair of adjacent legs, and the ratio of the number of turns in the coils of said first and second sets.

16. Transformer as defined in claim 14 characterized in that all N coils of said first set have the same number of turns and that said second set of coils consists of only one secondary coil having the same number of turns as the coils of said first set, thereby to generate a secondary voltage which is a selected multiple not in excess of N times the primary voltage applied to the coil of each leg and a minimum value equal to the primary voltage applied to the coil of each leg, the value of said multiple depending upon the number of pairs of adjacent legs of said N legs in which a flux of opposing magnetic force lines is created.

17. Transformer as defined in claim 4 characterized in that a second set consisting of atliii'd and a fourth coil similar to said first and second coil, respectively, and a fifth coil encircling said first and second coils, and a sixth coil encircling said third and fourth coils are provided, said first, second, and fifth coils having identical numbers of turns, and said third, fourth, and sixth coils having identical numbers of turns, one connection of each of said first, second, and fifth coils and of each of said third, fourth and sixth coils connected in common so that the force lines generated by a three-phase alternating current applied between said common point of said first, second, and fifth coils and the remaining connection of each of said coils are essentially parallel in said two legs so that the flux through the shunt members is maintained and that a secondary threephase alternating current can be drawn from said third, fourth, and sixth coils which are connected so that the fluxes generated in said two legs are mutually assisting when said secondary three-phase current is drawn.

18. Transformer as defined in claim 4 characterized by a third coil encircling both said first and second coils, the first, second, and third coil having the same number of turns, one connection of each of said first, second, and third coils connected in common so that the force lines generated by a threephase alternating current applied between said common point of said first, second, and third coils and the remaining connection of each of said coils are parallel in said two legs and the magnetic flux in said shunt members is maintained, a second set comprising a fourth, fifth, and sixth coil, the fourth and fifth coil having the same number of turns and interconnected so that the flux generated in said magnetic shunt members is a minimum when a first phase of alternating current is derived from said interconnected fourth and fifth coils and a second phase of alternating current from said sixth coil.

UNITED STATES PATENT oFFIcE QERTIFICATE ()F CECTION Dated April 18, 1972 Patent No. 3,651,678

Inventor(s) Carl A. Schwenden It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The attached

columns

1, 2, 3 and 4 should be inserted in the grant only.

Signed and sealed this 26th day of December 1972.

(SEAL) Attest:

EDWARD M,FLETCHER,JR. ROBERT GO'ITSCHALK v Attesting Officer Commissioner of Patents FORM P0-1050 (10-69) USCOMM-DC scan-Pen W U.S. GOVERNMENT PRINTING OFFICE: l9" 0-366-33,

Carl A Schwenden April 18, 1972 PAGE 2 MULTI-PURPOSE, MULTI-VOLTAGE TRANSFORMER This invention relates to electric induction devices and more particularly to a multi-purpose multi-coil transformer usable to generate a variety of voltages depending upon external connections between the various transformer coils.

In conventional transformers, the secondary voltage is determined by the ratio of turns of the secondary coil to the number ofturns ofthe primary coil, assuming a fixed value for the primary voltage. In principle the magnitude of the secondary voltage is affected by the flux through the transformer core section around which the secondary coil is wound. Disregarding the case of a biasing field generated by a dc. current through an auxiliary coil, modifications of the flux in the core of conventional transformers is customarily effected by changing the number of windings of the primary coil through which a currentgenerating a secondary voltage is passed. ln conventional transformers, it is not customary to compensate for a portion of the primary flux by an opposite flux generated by an in-phase a. c. current flowing through an auxiliary coil. Opposite fluxes generated in the same core section by two coils through which oppositely phased currents are passed for the purpose of compensating for part or for all of the flux generated by one of the coils lead to strong eddy currents in that core section. These eddy currents cause overheating of the transformer core and serious damage or destruction of the transformer.

In the transformer of the invention, the flux in the transformer core section encircled by the secondary coil can be influenced by the flux generated by a second primary coil to which primary ac. current is fed. The detrimental effect of flux superpositions is eliminated, and the above-mentioned generation of eddy currents and the ensuing overheating of the transformer core is prevented by the provision of magnetic shunt members arranged beyond the periphery of the transformer coils between appropriate core sections. Depending upon the number of turns of the second primary coil and the connection of the same with respect to the first primary coil, the flux through the transformer core is modified by bypassing magnetic flux through the shunt members and, consequently, the magnitude of the secondary voltage can be increased above, or decreased below the voltage which would result without operation of the second primary coil. Magnetic flux generated by both primary coils is passed through the magnetic shunt members thereby changing the flux through the secondary coil and, hence, the magnitude of the secondary voltage by proper connections between the primary coils, without changing the number of turns in the primary coils.

As will be described below, the coils of the transformer of the invention can be connected for operation with single-, two-, or three-phase ac. current. In another mode of operation of the transformer of the invention, a biasing d. c. current is passed through one of the coils and the transformer then assumes the function of a saturable core reactor.

It is an object of this invention to provide a versatile, new, simply constructed single-unit transformer having at least two primary coils and at least one secondary coil and magnetic shunt members cooperating to vary the magnetic flux in the transfonner core by passing appropriately directed a.c. currents through the primary transfonner coils.

It is another object of this invention to provide a transformer of the specified type with N primary coils and at least one secondary coil and with a 111 turn ratio of all coils, wherein, by making proper connections between the primary coils, the secondary voltage can be varied in steps of UN times the primary voltage applied to each coil, or in multiples of the primary voltage up to a maximum of N times the primary voltage.

It is another object of this invention to provide a transformer which, by proper connection of its coils, can be operated with single-, two-, or three-phase ac. current and which delivers secondary voltages of the same number of phases or a smaller number of phases.

It is still another object of this invention to provide a new and simple transformer of the specified type in which a biasing dc. current can be sent through one of the coils for the purpose of modifying the flux through the core so that the trans former functions as a saturable core reactor.

FIG. 2 is a cross section taken substantially along the center plane of FIG. 1;

FIG. 3 is a schematic of the electric circuit equivalent to the magnetic circuit of the embodiment of the invention shown in FIG. I with the magnetic field lines produced in the two primary coils opposed to one another;

FIG. 4 is a schematic of the electric circuit equivalent to the magnetic circuit of the embodiment shown in FIG. I with the magnetic field lines produced in the two primary coils flowing in the same direction;

FIGS. 7 and 8 show two modes of connecting the transformer of FIG. I with the primary coils in parallel connection;

FIG. 9 is a perspective view of the FIG. 1 embodiment showing the flux distribution in the core and the magnetic shunt members of the invention transformer;

FIG. 10 is a perspective view of another preferred embodiment of the invention transfonner;

FIG. 11 is a perspective view of part of the core and of a section of a shunt member used in the embodiment shown in FIG. 10;

FIG. 20 is a schematic of the connections for operating the transformer of FIG. I with three-phase ac. current; and

FIG. 21 is a schematic of the connections for deriving two secondary a.c. phases from the FIG. 1 transformer operated while energized with 3-phase a.c.

Referring first to FIGS. 1 and 2, there is shown a preferred embodiment of the transformer of the invention having a core 1 with two legs Ia and lb, two

magnetic shunt members

20, 2b, a coil 3a encircling leg la, a coil 3b encircling leg lb, and a coil 4 wound on top of coils 3a, 3b and encircling both legs la, 1b. Core 1 consists of two stacks of conventional, essentially U-shaped laminated sections of a type usually employed in power transformers. The two stacks have essentially the same size and smooth snugly-abutting end faces at the end of each leg. The two stacks abut at these end faces, in a plane concealed inside between the opposite ends of coils 3a, 3b. As far as the magnetic properties of the material and the shape of the magnetic shunt members are concerned. magnetic shunt members 2a,2b closely resemble legs la and lb of core I, though it will become apparent from the following description that the shape of the magnetic shunt members may differ from the form shown in FIG. I without departing from the spirit of the invention.

Coils 3a, 3b, 4 are conventional wire wound inductance coils. The winding ends to which electrical connections are made are denoted by 50, 5b for coils 30, by 6a, 6b for coil 3b, and by 7a, 7b for coil 4. The ends of each coil are brought out separately so that a variety of connections can be established between the coils.

The various ways of connecting the coils and operating the invention transfonner are most easily understood from first considering the analogy between electric and magnetic cir-

Claims

1. Transformer for single-, two-, or three-phase alternating current, comprising a closed-loop core of laminated transformer iron with at least two legs and at least three coils, characterized in (1) that all coils are electrically separated from each other and the core and can be connected in a variety of ways in each of which all three coils convey alternating current; (2) that at least one coil encircles at least two coils each of the latter of which encircles a respective one of said legs; (3) the provision of at least one magnetic shunt member having its opposite ends abutting said core, extending essentially parallel to at least one of said legs encircled by a coil, and running over the periphery of any of said coils to create a path for magnetic flux beyond said coils, and (4) in that the cross section of said closed loop core and of said magnetic shunt member are substantially the same.

6. Transformer as defined in claim 5 with at least two of said coils encircling said legs connected to a source of alternating voltage to generate a magnetic flux varying in time in said legs, characterized in that the magnetic fluxes in at least one pair of adjacent legs are at each instant of time opposite while the magnetic force lines in said legs are parallel, and that a magnetic flux through the magnetic shunt members is maintained to generate an a.c. voltage in said coil encircling all said coils, the magnitude of said generated a.c. voltage depending on the magnetic flux in said magnetic shunt members, the flux through said legs, and the turn ratio of said coils.

8. Transformer as defined in claim 7 characterized in that said coils encircling said legs and connected to said alternating voltage source are all connected in series and that at least two coils encircling adjacent legs are connected so that the magnetic force lines in said adjacent legs are parallel and that a magnetic flux is generated in said magnetic shunt member located in an intermediate position between said adjacent legs.

11. Transformer for single-phase alternating current, comprising a core of laminated transformer iron, said core consisting of a first essentially U-shaped yoke and a second essentially U-shaped yoke abutting in smooth surface contact with said first yoke so that two parallel legs are formed, with a first coil encircling one of said legs and a second coil, identical to said first coil, encircling the other of said parallel legs, a third coil encircling both said first and seCond coil, the windings of said three coils insulated from each other and from said core, a pair of essentially U-shaped magnetic shunt members of transformer iron extending from both sides of said first yoke from the center position between said two legs, said magnetic shunt members running essentially parallel to said legs over the periphery of said third coil and terminating in smooth contact with both sides of said second yoke, a source of primary essentially sinusoidal alternating current to which said first and second coils are connected in series so that the magnetic force lines generated by said alternating current in said legs are parallel at any instant of time, thereby to create a flux in said magnetic shunt members and to generate in said third coil a secondary essentially sinusoidal alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said legs, the magnetic flux in said pair of magnetic shunt members, and the turn ratio of said third coil to said first and second coil.

14. Transformer having a laminated core with two parallel end portions interconnected by N number of legs, a first set of identical coils encircling a respective one of said N legs, a second set of at least one coil encircling all said first coils, the windings of both sets of coils insulated from each other and from said core, (N - 1) pairs of essentially U-shaped magnetic shunt members of transformer iron having smooth coplanar end faces held in abutting contact with the opposite sides of the end portions of said core in areas thereof spaced between and parallel to adjacent ones of said core legs, said magnetic shunt members embracing said second set of coils, said first set of coils being connectable in parallel to a source of alternating current so that the magnetic force lines generated thereby are opposite at any instant of time in at least one pair of adjacent legs of said N legs, thereby to create a magnetic flux in the magnetic shunt member and to generate in said second set of coils a secondary alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said pairs of adjacent legs, the magnetic flux in said pairs of magnetic shunt members between each pair of adjacent legs, and the turn ratio of the coils of said first and second sets.

15. Transformer having a laminated core with two parallel end portions interconnected by N number of legs a first set of identical coils encircling a respective one of said N legs, a second set of at least one coil encircling all said first coils, the windings of both sets of coils insulated from each other and from said core, (N - 1) pairs of essentially U-shaped magnetic shunt members of transformer iron having smooth coplanar end faces held in abutting contact with the opposite sides of the end portions of said core in areas thereof spaced between and parallel to adjacent ones of said core legs, said magnetic shunt members embracing said second set of coils, said first set of coils being connectable in series to a source of alternating current so that the magnetic force lines generated by said alternating current are opposite at any instant of time in at least one pair of adjacent legs of said N legs, thereby to create a magnetic flux in the magnetic shunt member and to generate in said second set of coils a secondary alternating voltage whose magnitude depends at any instant of time upon the magnetic flux in said pairs of adjacent legs, the magnetic flux in said pairs of magnetic shunt members between each pair of adjacent legs, and the ratio of the number of turns in the coils of said first and second sets.

17. Transformer as defined in claim 4 characterized in that a second set consisting of a third and a fourth coil similar to said first and second coil, respectively, and a fifth coil encircling said first and second coils, and a sixth coil encircling said third and fourth coils are provided, said first, second, and fifth coils having identical numbers of turns, and said third, fourth, and sixth coils having identical numbers of turns, one connection of each of said first, second, and fifth coils and of each of said third, fourth and sixth coils connected in common so that the force lines generated by a three-phase alternating current applied between said common point of said first, second, and fifth coils and the remaining connection of each of said coils are essentially parallel in said two legs so that the flux through the shunt members is maintained and that a secondary three-phase alternating current can be drawn from said third, fourth, and sixth coils which are connected so that the fluxes generated in said two legs are mutually assisting when said secondary three-phase current is drawn.

18. Transformer as defined in claim 4 characterized by a third coil encircling both said first and second coils, the first, second, and third coil having the same number of turns, one connection of each of said first, second, and third coils connected in common so that the force lines generated by a three-phase alternating current applied between said common point of said first, second, and third coils and the remaining connection of each of said coils are parallel in said two legs and the magnetic flux in said shunt members is maintained, a second set comprising a fourth, fifth, and sixth coil, the fourth and fifth coil having the same number of turns and interconnected so that the flux generated in said magnetic shunt members is a minimum when a first phase of alternating current is derived from said interconnected fourth and fifth coils and a second phase of alternating current from said sixth coil.