US2079843A - Transformer - Google Patents

Description

1937- w. DALLENBACH ET AL 2,079,843

TRANSFORMER Filed May 17, 1935 I5 Sheets-Sheet 1 Horne f5 y 1937. w. DALLENBACH ET AL 2,079,343

TRANSFORMER Filed May 17, 1935 3 Sheets-Sheet 2 May 11, 1937. w, DALLENBACH ET AL 2,079,843

TRANSFORMER Filed May 17, 1955 3 Sheets-Sheet 5 n uw? )0 r5 WaHer D522! )Oac)? & Gm; L ze'schne'r- Patented May'll, 1937 UNITED STATES PATENT OFFICE TRAN SFORMEB lands Application May 17, 1935, Serial No. 22,068 In Germany May 18,. 1934 19 Claims.

The invention relates to a transformer, particularly to a transformer with variable ratio of transformation adaptedto regulate alternating E. M. F.

One use of such a transformer, for example, is to maintain at a constant value the potential of an A. C. net, despite varying loads or fluctuations of the feed potential.

Another use is the generation of variable potential, for example, for supplying current to the motors of electric locomotives.

In such cases it must be possible to vary the transformation ratio of the transformer without thereby interrupting the electric currents. In passing from one regulating stage to the next, short-circuiting, for a short period of time, of the part of the regulator coil that is connected in or cut out is unavoidable. The result is that in transformers heretofore used there arise powerful short-circuit currents, which cause dangerous heating of the transformer and the formation of arcs which may destroy the contacts.

The invention relates to a transformer in which the above mentioned defects are avoided by subdividing the core of the regulator coil into a plurality of pole or core sections which together make up the whole core; these sections being magnetically parallel, that is, forming parallel sections of a magnetic flux circuit. The flux generated through a coil which embraces the entire cross section of the core is distributed over these partial pole pieces according to the magnetic resistance of the latter. If a portion of the regulator coil is short-circuited, this short-circuit, as will be explained hereinafter in detail, forces the partial tem of an alternating flux conductor.

Fig. 7 is a perspective view of a modified form of iron core for a transformer of the type shown in Fig. 1.

Fig. 8 is a side elevation of a modified form of iron core for a transformer of the type shown in Figs. 4 and 5.

Fig. 1 shows, for example, ten different alternating magnetic flux conductors, such as iron cores, which are so conductively connected with each other as to transmit magnetic fiux at the branch points I, 2, 3, 4, 5, B, 1. Each of these alternating flux conductors is encompassed by a. coil. In order to simplify the following explanation, it is assumed that all coils have the same number of windings. For such branched system of alternating magnetic flux conductors, excited by alternating current, it is possible to establish laws which correspond to the Kirchhoff laws of a branched system of current conductors. These laws are:

1. The sum of the alternating E. M. F. in the coils which encompass the alternating flux conductors which extend from any one branch point is zero. This follows from the condition that the sum of the alternating fluxes meeting in a branch point must be zero, inasmuch as leakage fields may be disregarded outside the flux conductors.

2. The sum of the currents in the coils which encompass the alternating flux conductors of any simple closed circuit is zero. This is true under the assumption that the alternating magnetic fiux conductors conduct perfectly, or that the drop in magnetic potential within the alternating flux conductors, and therefore also the magnetizing currents, may be disregarded.

For the case in which the coils have varying numbers of windings, there are corresponding laws wherein instead of potentials (voltages) and/or currents, the quotients and/or products of potential and/or current and number of windings must be considered.

These laws provide a series of linear equations between alternating potentials (E. M. F.) and alternating currents in the windings encompassing the alternating fiux or conductors. For example, if there is an alternating E. M. F. in a certain coil and all the other coils are closed across known impedances, it is possible to calculate the currents and potentials in all the coils.

Fig. 2 shows a single phase transformer for which use is made of these laws. The iron core 9 is a system of five alternating flux conductors. A coil l l with a fixed number of windings is disposed around the arm or pole piece In. Hitherto, with single phase transformers of the usual type, the second coil, which has a variable number of windings, has been disposed on the same or on a second pole piece. Instead of this second pole piece, in the transformer of Fig. 2 there are four partial pole pieces .or sections i2, i3, II, and 15. These partial pole pieces are encompassed by coils IS, l1, l8, l9 respectively. The last named coils Hi to 19, each have the same number of windings, are inter-connected in series, and are provided with taps or leads which are connected to the groups or lines of contacts 22, 23, 24, 25 which are mounted on insulating supports, such as 22'. These taps subdivide the coils into divisions preferably having substantially equal numbers of turns or windings. The movable contacts 26, 21, 28, 29 are slidable on these contact groups, and are wider than the spaces between the individual contacts thereof, so that when one of these movable contacts is moved from a stationary contact to the following one the connection with the first contact is not broken until after the next contact is reached. Fig. 3 shows the position of one of the movable contacts, such as 26, during passage from one contact to the next of the group 22, and from this figure it may be seen that the movable contact, in order to fulfill the conditions stated, must be somewhat broader than the space between two stationary contacts.

The operation of the transformer described above is as follows:

If the coil ii is connected to a source of alternating potential, for example an A. C. net, an alternating flux will be generated in pole piece Hi. If the magnetic resistances of the partial pole pieces l2, l3, l4, ii are equal, each of them will be traversed by one fourth of the alternating flux generated in pole piece ID. If a part of the regulator coil on one of the four partial pole pieces is short-circuited by one of the movable contacts during the switching operation, the short-circuit current cannot increase freely because, according to the second of the previously given laws, the sum of the currents from a closed path of alternating flux conductors must be zero. The intensity of the short-circuit current can thus not exceed the order of magnitude of the operating current. Dangerous heating of the short-circuited part of the coil, therefore, cannot take place, nor can injurious arcs form when short-circuiting occurs. A counter-flux is generated by the short-circuit current in the partial pole piece encompassed by the short-circuited coil, which counter-flux counter-balances the flux generated by the primary coil, which latter flux is then, so to speak, forced off to the three other partial pole pieces. In order, however, that such transfer of the flux from one pole piece to the others may be possible without increasing the magnetic induction too greatly in the other pole pieces, the movable contacts are moved alternately, so that no more than one pole piece at a time is ever encompassed by a coil section short-circuited by a movable contact; in other words, the coil sections are never short-circuited by more than one contact simultaneously.

The movable contacts should not be moved simultaneously, and caution is also to be observed since for perfect operation of the transformer it is important that the flux generated through the primary coil must (when no section is short-circuited) be distributed as uniformly as possible over all the partial pole pieces. Accurate, uniform distribution is possible only when the parts of the coils around the individual partial pole pieces traversed by theoperative current have the same number of windings. This, however, is impossible, as the contacts must be moved alternately. In order to hold the irregularity in the distribution of the flux over the different partial pole pieces to a minimum, the contacts are moved so that the differences in the number of windings of the individual coils traversed by the operative current are as small as possible, thus never being greater than the number of windings of a coil section lying between two taps.

This requirement is fulfilled when the contacts are moved in cyclic change. On Fig. 2, for example, contacts 26 and 21 are on the third, contacts 28 and 29 on the fourth contact of their contact groups. If the voltage is to be increased contacts 28 and 29 must not first be moved on to the fifth contacts, since then the number of windings of the parts of coils l8 and I9 traversed by the operative current, on the one hand, and Ii and II on the other, would differ by the number of windings in two coil sections. n the contrary, first the movable contact 21, and then the movable contact 26, are moved on to the fourth stationary contacts. If the voltage is to be further increased we begin again with the movable contact 29, which reaches the fifth stationary contact, contacts 20, 21, 26 following in order. If the potential is to be lowered the operation is performed in the opposite direction and order.

In order to make the movement positive the movable contacts are preferably inter-connected by a mechanical drive mechanism, such as shown in Fig. 2.

In this arrangement, the movable contacts 26, 21, 28, 29 are carried by blocks which are slidably mounted on rods 84, 85, I6, and 81, respectively. There is sufllcient friction between the rods and the blocks to hold the blocks against the action of gravity in any position to which they may be adjusted. Connected to the blocks are rods 80, Ii, 82, and 83 each provided with a series of laterally extending pins, the number of these pins being at least equal to one less than the number of contacts in each contact group. These pins lie in the-path of movement of arms l6, l1, l8, and 19 which are fixed on a shaft 15. A crank 14 may be provided for manual operation or rotation of shaft 15. The arms are arranged on the shaft at angles of 90 successively around the axis of the shaft.

It is obvious that in operation if the crank H is turned the rods 80, 8|, 82, and 83 will be moved successively and alternately upward or downward depending on the direction of rotation of the crank H. In this manner the movable contacts are shifted alternately and successively along their respective contact groups.

It is easily apparent that the operation of the transformer is in no manner dependent on the particular type of contact arrangement shown. Instead of a contact group with movable and fixed contacts use may be made, for example, of any suitable type of switching mechanism in which the succeeding contact is closed before the preceding one is opened.

The irregular distribution of flux which arises when the coil windings on the individual partial pole pieces vary in number may be regarded as a superposition on the uniform flux distribution generated by the primary coil of the irregular flux distribution which is caused by the increase or the decrease in the number of coil sections on each individual partial pole piece relative to the other partial cores. For example, if the contacts are in the position shown in Fig. 2, the partial core l2 is encompassed by' two, partial core i3 also by two, partial core l4, however, by three, and likewise partial core i by three coil elements.

According to the invention, a method of checking this unfavorable reaction of the operating current on the uniform flux distribution necessary for the regular variation of the potential or voltage to be regulated, consists in providing air gaps in series with the individual partial pole pieces, which air gaps increase the magnetic resistance in such a manner that the'additlonal or secondary flux distribution generated on the partial cores by the unequal number of coil windings ,remains small. This condition is fulfilled when the secondary flux generated by the operating .current in a coil element remains small as compared with the main flux generated by the primary coil through this cell element. This may be expressed mathematically by the ratio:

wherein B designates the flux density generated by the primary coil in volt. sec.

the number of windings of a coil element, i the operating current in amperes, s the magnitude of the air gap in cm., and =41r.10-, a measuring system constant, the so-called permeability of a vacuum. Inasmuch as B and'i are known for any particular arrangement, the conditions are the more favorable the less the windings Az of a coilelement and the larger the air gaps.

Fig. 7 shows a transformer core similar to that shown on Fig 2, but differing therefrom in that the partial cores are interrupted by air gaps 36. 3|, 32, 33 calculated according to the preceding equation.

A result of the large air gap is that the magnetizing current of the regulating transformer is greater, that is, the ampere windings necessary for bridging the air gap must be provided as a part of the primary coil. The cos of the apparatus is therefore less. Protection canbe provided from this defective action by arranging parallel with the terminals of the primary coil a condenser, which produces the total apparent lo'ad necessary for compensating the magnetizing current.

Figs. 4 and 5 show a three phase transformer regulable in each phase, and Fig. 6 shows the connection diagram for the regulator coils disposed around'one of the pole pieces of this three phase transformer. The three pole pieces 42, 43, and 44 are subdivided each into six partial pole pieces. The partial pole pieces of pole piece 42 are designated 45, 46, 41, 48, 43, and 56. All

the pole pieces are interconnected at their ends by the yokes 5|, 52, which correspond to the branch points of the construction shown on Fig. 1. Each of the eighteen partial pole pieces is encompassed by a regulating coil. The regulating coils encompassing the partial pole pieces of core 42 are designated 53, 54, 55, 56, 51, and 56. Fig. 6 shows the connection diagram for these coils and the corresponding contact groups. Each of these individual regulating windings is subdivided by taps into four sections, for exampie. These taps lead to contacts and the contacts of each two coils are combined into a common contact group, namely, the contacts of coils 53 and 54 into the contact group 59, the contacts of coils 55 and 56 into contact group 66 and the contacts of coils 51 and 56 into contact group 6|. Movable contacts 62, 63, and 64 slide on these contact groups. The coils on pole pieces 43 and 44 are arranged in a similar manner.

In addition to the said individual coils encompassing the partial pole pieces, pole pieces 42, 43, and 44 are provided also with primary coils '61, 68, and 63, respectively, which encompass all the partial pole pieces. This arrangement of the primary cell is advantageous, as compared with that of Fig. 2, in that it is very closely coupled with the coils encompassing the individual partial pole pieces, so that the leakage between the primary and secondary windings is particularly small. If necessary, intermediate spaces may be provided between the coils for cooling and/or for oil or air circulation.

As long as the movable contacts 62, 63, 64 are in the lowermost position shown in Fig. 6 and the lower ends of the coils 53 and 54 and/or the six partial pole pieces are connected in series the maximum value of the potential generated in the regulating coil is given between terminals 65 and 66.

If the movable contacts 62, 63, and 64 are moved upward, the potential generated in the regulating coil decreases and becomes zero when the movable contacts reach-the upper end position.

The operation of the movable contacts is as follows: g

If we begin with movabfe contact '62 and move it upward so that, instead of bridging the fixed contacts 10 and II, it bridges the fixed contacts H and 12, the coil section 13 of regulating coil 53 is cut out and the potential between terminals 65 and 66 is reduced by the amount of the partial potential induced in the coil portion 13. If the current passing through the regulating coil is not to be interrupted during this switching operation the movable contact 62 must reach contact I2 before it leaves contact 10. There is thus 'a time interval during which the coil section I3 is short-circuited overcontacts I0, 62, 12. According to the statement made above relative to Fig. 4, the intensity of this short circuit current cannot exceed the order of magnitude of the operating current; the interruption of the current on opening the short circuit therefore does not cause any injury to the contact surfaces.

In order that a coil section may never be shortcircuited on more than one of the partial pole pieces of a complete pole piece, the contacts here also are moved alternately in cyclic change. However, inasmuch as in two successive switching operations of the same contact two coil sections on different partial pole pieces are contacted, the requirement that the number of windings of the individual coils traversed by operative current must never differ by more than the number of windings of a single coil section will be fulfilled, even ifeach of the movable contacts makes two switching movements directly after each other, and only thereafter the next movable contact makes in its turn two switching movements.

As stated relative to Fig. 2, in case of the shortcircuiting of a coil section on one of the partial pole pieces, the increase of the magnetic induction on the other partial pole pieces is the less, the greater the number of partial pole pieces, as the flux is always forced from one partial pole piece to the (11) other partial pole pieces.

In order to utilize the iron core properly, the

four at least, is still better, six being indispensable in the example of Figs. 4 to 6. In the arrangement of contact paths described in the exemplary embodiment shown there must always be an even number of partial pole pieces.

To avoid the irregularity inthe distribution of the flux caused by the difference in the number of ampere windings on the individual partial pole pieces, the latter may also be interrupted by air gaps in the transformer shown in Figs. 4 to 6.

Fig. 8 shows an iron core corresponding in arrangement to that of the pole pieces and partial pole pieces of Fig. 5, with the partial pole pieces interrupted by air gaps 1'4, 15, 16, 11, l8, 79. In order to increase the mechanical strength these air gaps may be filled with insulating material, as shown in Fig. 8.

It is obvious that the invention described in the foregoing, as applied to a single phase and a three phase transformer can also be applied for any other number of phases.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described. for obvious modifications will occur to a person skilled in the art.

We claim:

1. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means for moving the individual movable contacts at separate times so that no more than one of the movable contacts short-circuits a coil section at any one time.

2. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means to move the movable contacts alternately and in succession over the contact groups so that the numbers of windings of the connected parts of the last mentioned coils do not differ at any position of the movable contacts by more than the number of windings of a single coil division.

3. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions ct said movable contacts being such that contact ,use of a large number of partial pole pieces,

at any point is broken only after contact has been closed at the next succeeding point, and means to move the movable contacts alternately and in succession over the contact groups so that no more than one of the movable contacts shortcircuits a coil section at any one time and so that the numbersof windings of the connected parts of the last mentioned coils do not differ at any position of the movable contacts by more than the number of windings of a single coil division.

4. A transformer, comprising an iron core having a plurality of pole pieces at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, each partial pole piece having an air gap therein.

5. A transformer, comprising an iron core having a plurality of pole pieces at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, said last named coils being connected in series, taps subdividing said last coils, and contact groups connected to said taps, each partial pole piece having an air gap therein.

6. A transformer, comprising an iron core having a plurality of pole pieces at least one pole piece of said core consisting of a plurality of magnetically parallel pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connccted to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, each partial pole piece having an air gap therein.

7. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means for moving the individual movable contacts at separate times so that no more than one of the movable contacts short-circuits a coil section at any one time, each partial pole piece having an air gap therein.

8. A transformer, comprising an iron core having a plurality of pole pieces at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means to move the movable contacts alternately and in succession over the contact groups so that the numbers of windings of the connected parts of the last mentioned coils do not differ at any position of the movable contacts by more than the number of windings of a single coil division, each partial pole piece having an air gap therein.

9. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial .pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing saidlast coils, roups of contacts connected to said taps, cooperating contacts movable along said'groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means to move the movable contacts alternately and in succession over the contact groups so that no more than one of the movable contacts short-circuits a coil section at any one time and so that the numbers of windings of the connected parts of the last mentioned coils do not differ at any position of the. movable contacts by more than the number of windings of a single coil division, each partial pole piece having an air gap therein.

10. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of an even number of magnetically parallel pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, an even number of other coils each encompassing one of said partial pole pieces, taps subdividing said last named coils, groups of contacts, the taps of each pair of such coils being connected alternately to the contacts of a common contact group, and movable contacts one cooperating with each contact group and of such dimensions as to connect at all times at least two contacts of the group, whereby the movable contact is always in engagement with contacts connected to the taps of both coils of the pair and the coils of the pair are connected in series.

11. A transformer, comprising an iron core having a plurality of pole pieces at least one pole piece of said core consisting of at least four magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces.

12. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of six magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, an even number of other coils each encompassing one of said partial pole pieces, taps subdividing said last named coils,

groups of contacts, the taps of each pair of such coils being connected alternately to the contacts of a common contact group, and movable contacts one cooperating with each contact group and of such dimensions as to connect at all times at least two contacts of the group, whereby the movable contact is always in engagement with contacts connected to the taps of both coils of the pair and the coils of the pair are connected in series.

13. Polyphase transformer, comprising an iron core having a plurality of pole pieces, a number of such pole pieces equal to the number of phases of the current to be transformed each consisting of a plurality of magnetically parallel partial pole pieces, a number of coils corresponding to the phase number each encompassing the entire iron cross section of one of the pole pieces, and a number of coils corresponding at least to twice the phase number each encompassing one of said partial pole pieces.

14. A three-phase transformer comprislngan iron core, having a plurality of pole pieces, three of said pole pieces each consisting of a plurality of magnetically parallel partial pole pieces, three coils each encompassing the entire iron cross section of one of said pole pieces, and at least six coils each encompassing one of said partial pole pieces.

ing a plurality of pole pieces, at least one pole piece of said core consisting of an even number of magnetically parallel partial pole pieces, each of said partial pole pieces having an air gap therein, at least one coil encompassing the entire iron cross section of one of the pole-pieces, an even number of other coils each encompassing one of said partial pole pieces, taps subdividing said last named coils, groups of contacts, the taps of each pair of coils being connected alternately to the contacts of a common contact group and movable contacts one cooperating with each contact group and of such dimensions as to connect at all times at least two contacts of the group whereby the movable contact is always in engagement with contacts connected to the taps of both coils of the pair and the coils of the pair are connected in series, the movable contacts being of such dimensions as to move out of contact with the contact of any coil only after it engages the succeeding contact of the same coil, and means to move the individual movable contacts of the various groups alternately and successively over the contact groups so that no more than one movable contact short-circuits a coil section at any one time and so that the numbers of windings of the last mentioned coils do not differ at,any position of the movable contacts by more than the number of windings of a sin gle coil division.

16. Transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, each of said partial pole pieces having an air gap therein, at least one coil encompassing the entire iron cross section of one of the pole pieces, a plurality of other coils each encompassing one of said partial pole pieces, said last named coils being connected in series, taps subdividing said last 'coils, groups of contacts connected to said taps,

cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means to move the individual movable contacts of the various groups alternately and successively over the contact groups so that no more than one movable contact shortcircuits a coil section at any one time and so that the numbers of windings of the last mentioned coils do not differ at any position of the movable contacts by more than the number of windings of a single coil division.

17. Polyphase transformer, comprising an iron core having a plurality of pole pieces, a number of such pole pieces equal to the number 01' phases of the current to be transformed each consisting of a plurality of magnetically parallel partial pole pieces, each partial pole piece having an air gap therein, a number of coils corresponding to the phase number each encompassing the entire iron cross section of one of the pole pieces, a number of coils corresponding at least to twice the phase number each encompassing one of the said partial pole pieces, said last named coils being connected in series, taps subdividing said last coils, groups of contacts connected to said taps, cooperating contacts movable along said groups, the dimensions of said movable contacts being such that contact at any point is broken only after contact has been closed at the next succeeding point, and means to move the individual movable contacts of the various'groups alternately and successively over the contact groups, so that no more than one movable contact short-circuits a coil section at any one time and so that the numbers of windings of the last mentioned coils do not difier at any position of the movable contacts by more than the number of windings of a single coil division.

18. Three phase transformer, comprising an iron core having a plurality of pole pieces, three pole pieces of said core each consisting of six magnetically parallel partial pole pieces, each partial pole piece having an air gap therein, three coils each encompassing the entire iron cross section of one of said pole pieces, eighteen coils each encompassing one of the said partial pole pieces and arranged in pairs, taps subdividing said last named coils, groups of contacts, the taps of each pair of coils being connected alternately to the contacts of a common contact group, and movable contacts one cooperating with each contact group and of such dimensions as to connect at all times at least two contacts of the group, whereby the movable contact is always in engagement with contacts connected to the taps 01' both coils of the pair and the coils of the pair are connected in series, the movable contacts being of such dimensions as to move out of contact with the contact of any coil only after it engages the succeeding contact of the same coil, and means to move the individual movable contacts of the various groups alternately and successively over the contact groups, so that no more than one movable contact shortcircuits a coil section at any one time and so that the numbers of windings of the last mentioned coils do not differ at any position of the movable contacts by more than the number of windings of a single coil division.

19. A transformer, comprising an iron core having a plurality of pole pieces, at least one pole piece of said core consisting of a plurality of magnetically parallel partial pole pieces, at least one coil encompassing the entire iron cross section of one pole piece, and a plurality of coils each encompassing one of said partial pole pieces, taps subdividing said last coils, switch means adjacent each of said partial pole pieces for electrical connection of one subdivision before breaking the electrical connection of the adJacent subdivision and means for moving said switch means one at a time.

WAL'I'ER DALLENBACH. GEORG LEISCHNER.

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