EP0095398B1 - Electric transformer with modular, selectively fed primary circuits - Google Patents

Abstract

An electric transformer for supplying an adjustable electric magnitude, especially for regulating purposes, includes several modules having therebetween a binary progression relationship with respect to their power handling capabilities. Each module has a switching network for selectively rendering it operative or inoperative and, each module has at least one primary circuit input coil, the primary coils of each of the modules cooperating with one single secondary output circuit which is common to all of the modules. The switching network of each module being so connected as to allow the neutralization of the effect of the electronic induction of its respective primary coil or coils on the common secondary circuit, while maintaining the magnetic activity of the primary circuits.

Description

The present invention relates to a variable voltage transformer.

We know that this kind of transformer tends to solve the problem of varying an output voltage from a constant nominal input voltage.

• en galvanoplastie ils permettent d'ajuster le courant en fonction du produit traité et de la surface intéressée ;
• en transmission mécanique ils entrent dans la constitution de variateurs de vitesse ;
• en métallurgie ils assurent une alimentation des fours à induction réglables.
• Jusqu'à maintenant, les transformateurs connus présentent de très nombreux inconvénients.

Such transformers have multiple applications:

• in electroplating they make it possible to adjust the current according to the product treated and the surface concerned;
• in mechanical transmission they are part of the constitution of variable speed drives;
• in metallurgy they supply adjustable induction ovens.
• Up to now, known transformers have had numerous drawbacks.

Thus, for example, in plug-in transformers, the secondary has several output terminals and a switch is applied selectively to one or other of these terminals to give a variable operating voltage depending on the height of the secondary to which the socket is located.

With such a device, very serious jumps and breaks in load are encountered, which require extreme limitation of the powers to be regulated and, despite this, the installation undergoes rapid wear.

There are also known "slider auto-transformers which provide a single outlet on the secondary but this is mobile and constituted by a roller made of carbon.

It is easy to understand that when the roller passes from one turn to another it puts them in short circuit and that causes a heating of the coal which wears out quickly and which obliges to limit the power of variation.

However, there is an advantage here which comes from the regulation which is almost continuous, since the pitch is of a single turn, so that if we imagine that the secondary is formed by a winding of 220 turns, the pitch is equal to 1/220.

Magnetic amplifiers or "transducers include a magnetic circuit and a choke coil, which makes it possible to obtain an adjustment without moving mechanical part, because one acts by saturating and desaturating the magnetic circuit by causing a phase shift.

This results in a non-sinusoidal regime and, consequently, a shift between the current and the voltage which leads to a very bad cosine.

Finally, developments in electronics have made it possible to use thyristors which give a non-sinusoidal output signal and analogous, therefore, to that of a transducer, but without phase shift.

On the other hand, each interruption causes unacceptable current surges in practice.

• commutation avec rupture de charge,
• puissance d'utilisation limitée,
• régime sinusoïdal déformé,
• cosinus y variable,
• puissance massique identique à celle d'un transformateur normal,
• incidence économique défavorable par le coût élevé.

In summary, the known devices have the following drawbacks:

• switching with load break,
• limited operating power,
• distorted sinusoidal regime,
• cosine y variable,
• mass power identical to that of a normal transformer,
• adverse economic impact by the high cost.

Furthermore, document FR-A-2406 908 is known which describes a regulator composed of several transformers each comprising a primary circuit, a magnetic circuit and a secondary circuit, the latter being electrically connected to the secondary of neighboring transformers. In addition, each primary is associated with switching means which are intended to short-circuit it when it is not in use.

Such an assembly cannot be applied to installations of significant power, since the short-circuiting of the primaries would generate a very significant overvoltage due to the self formed by all the secondary ones which remain in circuit.

In addition, the presence of several secondary in series requires the presence of connectors.

The present invention overcomes all these drawbacks as will be seen below, since a transformer according to the invention knows no power limit, operates with a pure sinusoidal regime, has a constant cosine, does not cause breakage of load and allows an adjustment of a finesse as great as is desired because the pitch of this adjustment can be practically insensitive.

The invention will be better understood from the detailed description below made with reference to the accompanying drawing. Of course, the description and the drawing are given only by way of an indicative and nonlimiting example.

• Figures 1 and 2 are schematic views, respectively in plan and front, of an elementary transformer according to the invention.
• Figures 3 to 6 are diagrams showing different solutions for the selective individual supply of the different primary modules that a transformer according to the invention comprises.
• Figure 7 is a diagram showing the possible design of a transformer according to the invention.
• Figure 8 is a schematic view of a particular embodiment of the invention.
• Figure 9 is a partial diagram of an alternative embodiment.
• FIG. 10 schematically shows a possibility of winding “in eight for the secondary common to two primary circuits.

The subject of the invention is an electrical transformer intended to deliver an adjustable electrical quantity, in particular for regulation purposes, comprising several modules which have a binary progression relationship between them and which are associated with switching means making them selectively operative or inoperative for electrical transformation, each module comprising at least one primary circuit winding, characterized in that each of the primaries of each of the modules cooperates with a secondary magnetic circuit which is common to all the modules, in that the switching means are connected to neutralize the effect of the electrical induction of the respective primaries on said common secondary while retaining the magnetic activity of said primaries.

• Les moyens de commutation sont constitués par un connecteur individuel à chaque module et ayant deux positions dans l'une desquelles le bobinage de circuit primaire est alimenté normalement par une tension d'origine nominalement constante et dans l'autre desquelles le circuit magnétique reste activé à partir de la même tension d'origine mais en induisant dans le secondaire une tension nulle par soustraction.

According to other characteristics of the invention:

• The switching means consist of an individual connector to each module and having two positions in one of which the primary circuit winding is normally supplied by a nominally constant original voltage and in the other of which the magnetic circuit remains activated at starting from the same original voltage but inducing in the secondary a zero voltage by subtraction.

The supply voltage and each of two terminals of the primary circuit winding of each module are interposed two electronic switches such as thyristors or transistors mounted head to tail and selectively controlled to ensure the reversal of the supply of said circuit winding primary.

Each module comprises two primary circuit windings selectively supplied, either to ensure induction in the secondary circuit winding, or to neutralize the effect of this induction on said secondary.

Two primary circuit windings are in opposite directions, a connector being provided to establish contact between the original voltage and one or the other of these two primary circuit windings.

Two windings are in the same direction and in that an inverter is interposed between the terminals of only one of these windings on the one hand and the original voltage which simultaneously supplies the other winding on the other hand.

The primary circuit winding of the first module is supplied by the two main lines of a voltage source and that the corresponding winding of the following modules is supplied at one of its ends by one of the two main lines and at its other end by a derivation from an intermediate point, notably in the middle, of the primary circuit winding of the previous module.

The modules have a nominal voltage providing with a given current a power which is different for each module, the total of these individual powers being substantially equal to the maximum admissible power for the single secondary circuit.

Each module comprising two magnetic circuits, one for each primary of this module, these magnetic circuits have constant lengths and widths while their thicknesses follow, from one module to another, a binary variation.

Referring to FIGS. 1 and 2, it can be seen that a transformer according to the invention can comprise, as here, two modules which each comprise a single primary winding respectively 1 and 2 each associated with a magnetic circuit respectively 3 and 4, these two primary circuits being associated with a single secondary circuit winding 5 common to the two modules 1 and 2.

Each primary winding 1 and 2 is independently connected to a nominally constant voltage source 6, for example 220 volts.

For each winding, there is a switch respectively 7 and 8 which can occupy two positions in one of which it establishes the normal circuit (connector 8 in Figures 1 and 2) while in the other (connector 7 in Figure 1), it puts the corresponding winding in short circuit.

In this way, depending on whether the switch of each primary circuit winding is in one or other of its two positions, the single secondary circuit 5 has a voltage which corresponds either to that which results from the winding 1 only, or to that which results from the winding 2 only is that which results from the action of the two windings 1 and 2 at the same time.

A complete installation comprising a transformer according to the invention is equipped with a control device, possibly programmed, and acting on all the connectors to act selectively on the individual power supply of each primary module.

As a result, by properly calibrating the primary modules, a very high degree of precision and fine tuning can be achieved.

Naturally, a simple version consists in providing a plurality of modules all equal to each other.

However, according to a more elaborate version, it is advantageous to provide that the modules have a nominal voltage which is established to provide, with a given current, a power which is different for each module.

The total of these individual powers is substantially equal to the maximum admissible power for the single secondary circuit.

According to a variant of the invention which proves to be particularly advantageous, the modules are regular decreasing fractions of the total power and are established according to a binary code.

In other words, the denominator of each fraction is an integer power of 2 so that the most powerful module is equal to half of the total power P, or P / 2, the other halves having a power equal to respectively P / 4, P / 8, P / 16, P / 32, P / 64, etc ...

It can thus be seen that the adjustment step, or minimum jump, is equal to the smallest fraction of P provided in the transformer.

It is therefore possible in practice, without any difficulty, to make a transformer composed of a plurality of modules of decreasing dimensions which, eight in number provide a fineness of adjustment equal to 1/255 for the last module which is excellent performance (1/255 = P / N-1).

When the requested power is equal to P / 2, only module N ° 1 is activated. If one wishes to increase it by P / 255, module N ° 8 is also activated. To increase it further from P / 255 we activate the module? 7 by simultaneously neutralizing module No. 8 etc ...

Each module being selectively controlled, all combinations are possible with a fine adjustment equal to P / N-1, N-1 being the fraction of the power of the last module of a given set.

It should be emphasized, because this is an interesting characteristic of the invention, that each primary module is not very bulky and that the passage from a setting at 1/127 to a setting at 1/255 requires only the addition of a extra transformer of very small dimensions.

Referring now to FIG. 3, we see an electrical diagram of a transformer according to the invention comprising six identical modules 10 to 15 and, of course, always a single secondary circuit winding 16.

Each module comprises a coil 17 to 22 associated with an individual magnetic circuit 23 to 28.

The AC voltage source 30 is connected to two main lines 31 and 32 to which the individual power supplies of each primary module are connected respectively 33 to 38 and 39 to 44.

The type of connection chosen is that which has already been described with reference to FIGS. 1 and 2, that is to say that each module 10 to 15 is associated with a connector 45 to 50 respectively movable between two positions corresponding to the power supply normal or when each winding 17 to 22 is short-circuited.

To simplify the drawing, all the modules 10 to 15 have been shown as if they all had the same dimensions.

However, in reality, if we adopt the preferred embodiment according to which the modules provide different powers, it is clear that they have dimensions the smaller the power developed is itself low.

If we take the binary variation of this power with respect to the total power P admissible by the single secondary circuit winding 16, we have for module 10 a power equal to P / 2, for module 11 an equal power to P / 4, etc ...

Assuming that the installation is supplied with 220 Volts with an effective current of 48 Amperes, we obtain the results grouped in the table below:

It emerges from the description above, that the voltage of the secondary is a function of the number of modules under voltage and that it can, therefore, vary from 0 to 100 per 100 by modifying the ratio of the number of turns of the primary windings the number of turns of the secondary winding, taking the precaution of shunting the windings of the non-activated modules to give them zero impedance, this which is indeed the case since the secondary then becomes conductive.

The single transformer according to the invention functions, in a way, like a series of transformers, the secondaries of which are all in series.

This solution results in a very significant mass saving since the power of such a transformer-variator is the same as that which a conventional transformer would have without adjustment.

Referring now to FIG. 4, we will describe a transformer of the same type as that which has been described previously, that is to say of which each module comprises only one primary winding, but with the use of head-to-tail thyristors to act selectively on the induction produced by the primary winding.

In this figure, only two modules 51 and 52 have been shown, respectively, but the outline of the module 53 shows that, as can be seen from the preceding explanations, any number of modules can be provided above two.

Each module includes a primary circuit winding 54 and 55 respectively and an individual magnetic circuit 56 and 57.

Each module is supplied from a voltage source 58 connected to two main lines 59 and 60 to each of which are connected the terminals of the two windings 54 and 55. Each of these terminals 54a and 54b on the one hand, 55a and 55b on the other hand leads to a branch with two branches on each of which is a thyristor, which are mounted head to tail for the same branches: 61 and 62-63 and 64-65 and 66-67 and 68.

These thyristors are controlled by any known electronic means such that in the same given primary winding is sent a current, either in one direction or in the other.

It follows that in one case the single secondary winding 70 is the seat of a voltage which, if necessary, is added to the other voltages arising from the other modules, while in the other case no voltage is induced in the secondary winding.

It can thus be seen that, depending on the direction in which the winding is traversed, the induction effect is neutralized.

But, here again, the magnetization of the magnetic circuits is permanent, whatever the effective direction of the current. In other words, the magnetic circuits are always supplied, but the voltage in the secondary winding is either effectively induced or zero.

The electronic solution which has just been described has many advantages among which we can cite the fact that the engagement and the break are made at the zero of the sinusoid so that whatever the combination used for the modules, there is conservation of the pure sinusoidal regime.

In addition, the use of thyristors or transistors to effect the selective switching of the modules makes it possible to obtain extremely high switching speeds, necessary for regulating electrical quantities with very high performance.

Referring now to Figures 5 and 6, there is shown another embodiment of the invention according to which each module comprises two primary circuit windings.

FIG. 5 shows a complete module 71 and the start of a second 72 with, as always, a single secondary circuit winding 73.

Each module includes a magnetic circuit 74 and two coils of the same pitch, respectively 75 and 76.

A voltage source 77 is connected to two lines 78 and 79, the latter directly leading to one of the terminals 76a of the winding 76 and to the terminal 80a of a connector 80.

Line 78 leads to a branch with two branches, one of which leads to terminal 76b of the winding 76 and the other is connected on the one hand to terminal 75a of the winding 75 and on the other hand, to terminal 81a d a coil 81 whose pitch is opposite to that of the coil 75 and whose second terminal 81b can be connected to line 79, like terminal 75b, depending on the position of the connector 80.

With this arrangement, it can be seen that the current which divides according to the arrows F1 and F2 always flows through the coil 76 and, depending on the position of the connector 80, flows through either the coil 75 or the coil 81.

In the first case a voltage is induced in the secondary winding 73 and this tension is added to the induced voltage of the winding 76 while in the other case the winding 81 having a reverse pitch winding of the winding 75, the induced voltage in the secondary winding 73 is no longer added to but subtracted from the voltage induced from the primary winding 76 so that, ultimately, the secondary winding 73 is traversed either by the nominal voltage of the module 71 or by a zero voltage for the same module 71.

In this case, the secondary winding 73 is traversed, if necessary, by the only voltages induced by the other modules.

Referring now to FIG. 6, we can see an assembly which also includes two primary windings for each module but, here, the neutralization of each module is obtained by different means.

Two complete modules 90 and 91 have been shown and the outline of a third 92.

Each of these two modules comprises a primary winding 93 and 94 associated with a magnetic circuit 95 and 96 and the terminals of which are permanently connected to two lines 97 and 98 supplied by a source 99.

In addition, each of these modules comprises a second winding 100 and 101 respectively. associated with a magnetic circuit 102 and 103, all the modules corresponding, as always, to a single secondary winding 104.

Between the lines 97 and 98 on the one hand and the terminals of the windings 100 and 101 on the other hand, there are interposed inverters 105 and 106 respectively.

We understand that with this arrangement, depending on the position that is given to each inverter located in each module, the corresponding winding 100, 101, etc ... is traversed in the same way as the corresponding winding 93, 94, etc. ... or is in opposition so that, as in the case of FIG. 5, the secondary winding 104 is traversed by an induced voltage which is added or subtracted from that, permanent, of the windings 93, 94, etc ...

The voltage across the secondary winding 104 is therefore the result of an addition of the voltages of each activated module.

Referring now to FIG. 7, we see that we have graphically represented the way of dividing the total power P admissible by the secondary winding into modules each having, for a given current, a power which is a regularly declining fraction of the power P.

This graph tends to show visually that one can easily reach an extreme fineness of adjustment since one could, whatever the power P, provide a last power module equal to P / 128 or even P / 256 or P / 512 etc ...

This graph also shows that obtaining this finesse is done by means of an extremely compact and therefore inexpensive module whereas generally the increase in a given performance is proportionally much more complicated and costly than the performance itself.

In Figure 8, there is shown schematically a transformer according to the invention comprising four modules of the type according to which each of these comprises two primary windings and two magnetic circuits.

We see that for each module 110,111,112 and 113 the two windings respectively 114 and 115, 116 and 117, 118 and 119,120 and 121 are equal as well as the corresponding magnetic circuits 1.22 and 123, 124 and 125, 126 and 127, 128 and 129 .

It is indeed fundamental that the two primary circuits and the two magnetic circuits of each module are strictly equal so that one can neutralize the other.

It is understood, of course, that the resulting voltage is either zero or equal to the addition of the two primary circuits of each module.

Each module has a binary function, 0 and 1, obtained by inverting the field of one primary with respect to the other.

FIG. 9 represents a diagram of a variant according to which a branch is connected in the middle of one of the two windings of each module and constitutes the supply of the corresponding winding of the following module.

Thus, for example, the module 140 is supplied with 220 Volts from the source 141 by two main lines 142 and 143. This voltage is applied to the winding 144 associated with a counterpart 145.

From the middle of the winding 144, a branch 146 leaves which ends at the end of the corresponding winding 147 associated with a counterpart 148 of the next module 149. The other end of the winding 147 is connected directly to the line 142 so that the supply voltage of winding 147 is only 110 volts.

In the same way, the winding 150 associated with the winding 151 of the module 152 is supplied with 55 Volts and so on, the secondary 153 being always unique and common to all the modules.

The first module 140 has a power of P / 2 with a current of i / 4 for a nominal current of i. Module 149 has P / 4 and i / 8. Module 152 has P / 8 and i / 16 etc ...

With this variant, the number of turns of the primary windings is constant for all modules, regardless of their power. In this way, a standardization is achieved which leads to a saving in labor.

On the other hand, savings are made in raw material (copper) since the current in each module has a decreasing value for which the diameter of the winding wire can be exactly adapted. For a set of modules, an excellent match is obtained between the current and the quantity of copper necessary and sufficient.

• plutôt que d'avoir des dimensions rationnelles, certes, mais peu pratiques comme celles qui sont schématisées sur la figure 7, il est préférable de prévoir pour les circuits magnétiques des dimensions constantes pour ce qui est de la longueur et de la largeur tandis que l'on fait varier seulement l'épaisseur selon la puissance désirée pour chaque module.

The practical details of the construction of a transformer according to the invention are within the reach of those skilled in the art, but the following provisions may however be cited:

• rather than having rational dimensions, certainly, but impractical like those shown in Figure 7, it is better to provide for magnetic circuits constant dimensions in terms of length and width while l 'we only vary the thickness according to the desired power for each module.

This construction method is shown diagrammatically in FIG. 8.

A numerical example is found in the table on page 8 where the last column indicates the thickness of the magnetic circuit in millimeters as the only variable dimension from one module to another, the length and the width being established once and for all.

Rather than building a large transformer composed exclusively of modules according to the invention, it is possible to associate a common transformer with a transformer according to the invention and comprising several low power modules, this transformer acting as regulator of the first.

An application of this principle is found in line transformers to which we can add a few modules that automatically regulate voltage variations.

• la réalisation d'un transformateur complet conforme à l'invention dans une seule structure ;
• la réalisation dans une première structure d'un transformateur à tension constante et d'une partie modulaire conforme à l'invention puis, dans une deuxième structure, un ensemble de modules ne servant qu'aux réglages fins de la puissance minimum :
• la réalisation dans deux structures différentes d'un transformateur à tension constante et d'un transformateur modulaire conforme à l'invention, ces deux structures étant électriquement connectées ;
• la réalisation d'un ensemble de régulation comprenant d'une part un transformateur à tension constante aux bornes du secondaire et d'autre part, un transformateur à tension réglable au primaire dans la plage de -15 à + 15 %.

The possibility of combining a large transformer with constant voltage and a transformer according to the invention with adjustable voltage, leads to four possible solutions which are respectively:

• the production of a complete transformer according to the invention in a single structure;
• the production in a first structure of a constant voltage transformer and a modular part according to the invention then, in a second structure, a set of modules used only for fine adjustments of the minimum power:
• the production in two different structures of a constant voltage transformer and a modular transformer according to the invention, these two structures being electrically connected;
• the production of a regulation assembly comprising on the one hand a transformer with constant voltage across the secondary and on the other hand, a transformer with adjustable voltage at the primary in the range from -15 to + 15%.

As indicated above, the control of the various modules can be obtained either by electro-mechanical means, or by electronic means.

When the primary circuit of the modules has two windings and two magnetic circuits. the winding can be obtained either independently for each of them or continuously for the two windings by giving each turn a “8” path, as shown in FIG. 10.

The sinusoidal regime remaining pure at all times, a transformer according to the invention makes it possible to supply a rectifier while retaining its natural residual ripple.

It should be noted that the arrangement of two magnetic circuits with two primary circuits for each module gives results quite different from the assembly known as the “magnetic shunt because it has no primary winding. This results in leaks creating a large current-voltage phase shift, hence a mediocre cosine <p.

According to the invention, on the contrary, a cosine close to 1 is obtained with the embodiments of the type shown in particular in FIGS. 1 and 8.

A modular transformer can be provided in accordance with the invention using mixed electro-mechanical and electronic solutions, in particular by providing thyristors for the last module only, these thyristors acting only on a tiny fraction of the sinusoid and causing a variation in power only in absolutely insignificant proportions with a very reduced distortion rate.

• A tout instant la charge appliquée au secondaire reste sous tension et, celà, même pendant le temps des interruptions dues aux commutations.

It appears that the above description, that the invention makes it possible to produce a transformer with adjustable voltage which includes extremely marked advantages compared with known devices:

• At all times the load applied to the secondary remains energized and this, even during the time of interruptions due to switching.

The module actuation circuits are cut and closed at 0 on the sinusoid since the modification of the number of modules used has an effect on the amplitude of the sinusoid.

The leakage surface is reduced to a minimum and is identical to that of a normal high performance transformer and in particular with the embodiments shown in Figures 1 and 8.

Switching takes place without a load break.

The adjustment is made in an extremely fine step-by-step progression.

Stabilization devices of all known types can be used.

The power is unlimited.

A transformer according to the invention has an efficiency equal to that of a normal transformer.

The adjustment is made without any interruption of the power supply.

The voltage variation is carried out using a step-by-step servo system which avoids setting the secondary voltage to zero when it has to be modulated.

Claims (9)

1. Electrical transformer intended to deliver a variable electrical quantity, in particular for control purposes, comprising several modules which have a binary progression relationship between them and which are associated with switching means which selectively render them operative or inoperative for the electrical conversion, each module comprising at least one primary circuit winding (1, 2), characterised in that each of the primaries (1 and 2) of each of the modules cooperates with an individual magnetic circuit (3 and 4) and all the primaries (1 and 2) cooperate with a single secondary circuit (5) which is common to all the modules, in that switching means (7 and 8) are connected to enable neutralising the electrical induction effect of the respective primaries (1 and 2) on the said shared secondary (5) whilst maintaining the magnetic activity of the said primaries (1 and 2).

2. Transformer according to claim 1, characterised in that the switching means comprise an individual connector (7-8) for each module and having two positions, in one of which the primary circuit winding (1-2) is supplied as normal with a nominally constant initial voltage and in the other of which the magnetic circuit (3-4) remains activated on the basis of the same initial voltage but whilst inducing a nil voltage in the secondary (5) by subtraction.

3. Transformer according to claim 1, characterised in that between the supply voltage and each of two terminals (54a-54b and 55a-55b) of the- primary circuit winding (54-55) of each module (51-52) are interposed two electronic switches such as thyristors or transistors (61 to 68) arranged head-to-tail and actuated selectively to ensure the inversion of the supply of the said primary circuit winding (54-55).

4. Transformer according to claim 1, characterised in that each module (71) comprises two selectively supplied primary circuit windings (75 and 81), either to ensure induction in the secondary circuit winding (73) or to neutralise the effect of this induction on the said secondary (73).

5. Transformer according to claim 4, characterised in that the two primary circuit windings (75 and 81) have opposed directions, a connector (80) being provided to establish contact between the initial voltage and one or other of these two primary circuit windings (75 or 81).

6. Transformer according to claim 4, characterised in that the two windings (93-100, 94-101) have the same direction and in that an inverter (105-106) is interposed between the terminals of one only (100-101) of these windings on the one hand and the initial voltage which simultaneously feeds the other winding (93-94), on the other hand.

7. Transformer according to claim 1, characterised in that the primary circuit winding (144) of the first module (140) is supplied by the two principal lines (142 and 143) of a voltage source (141) and that the corresponding winding (147-150) of the following modules (149-152) is supplied at one of its extremities by one (142) of the two principal lines (142 and 143) and at its other extremity by means of a branch circuit (146) originating from an intermediate, and in particular central, point of the primary circuit winding (144) of the preceding module (140).

8. Transformer according to claim 1, characterised in that the modules have a nominal voltage securing with a given current a power which is different for each module, the total of these individual powers being substantially equal to the maximum power acceptable for the single secondary circuit.

9. Transformer according to any one of claims 4 to 8, characterised in that, each module (110 to 113) comprising two magnetic circuits (122 and 123, 124 and 125, 126 and 127, 128 and 129), one for each primary (114 to 121) of this module, these magnetic circuits (122 to 129) have constant lengths and widths, whereas their thicknesses follow a binary variation from one module to the other.

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