US2462106A - Electric transformer - Google Patents

Description

Feb. 22, 1949- w. KRAM ELECTRIC TRANSFORMER 3 Sheets-Sheet 1 Filed April 26, 1946 W f. m 7 e Z 4 m V 0 ,4 0 W 4 M M as m \9 8 5 g k m A F4 3/3? Feb, 22, 1949. w, KR M 2,462,106

ELECTRIC TRANSFORMER Filed April 26, 1946 3 Sheets-Sheet 2 Patented Feb. 22, 1949 ELECTRIC TRANSFORMER Walter Kram,

London, England, assignor, by

memo assignments, to International Standard Electric Corporation, New York, N. Y., a corpcration of Delaware Application April 26, 1946, Serial No. 665,047 In Great Britain April 27, 1945 7 Claims. 1

The present invention relates to improvements in double screened electrical transformers. Such transformers have been employed for a long time for use in alternating current bridges and for coupling high frequency circuits together, and for similar uses. It has been found, however, that the designs hitherto adopted for these transformers prove inadequate when the frequencies employed extend into the radio ire.- quency range. It is the principal object of this specification, therefore to disclose the main cause for the defects of the existing designs and to show how they may be remedied.

The invention will be described with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic circuit diagram of a known type of double screened transformer;

Figs. 2 and 3 show two test circuits employed in the explanation of the distinction between the static and dynamic capacitances associated with such a transformer;

Fig. 4 shows a perspective view of the winding and screening arrangements of a double screened transformer;

Figs. 5, '7 and 8 show diagrams illustrating one method of designing a double screened transformer according to the invention;

Fig. 6 shows a schematic circuit diagram to illustrate the effect of the screen electromotive forces in a double screened transformer;

Fig. 9 shows equivalent circuits employed in the explanation of the action of the screen electromotive forces;

Fig. 10 shows a double screened transformer with neutralising arrangements according to the invention;

Figs. 11 and 12 show another neutralising arrangement according to the invention;

Fig. 13 shows still another arrangement according to the invention;

Fig. 14 shows a schematic circuit diagram of a screened impedance bridge of known type;

Fig. 15 shows a simplified circuit diagram of Fig. 14 to illustrate the effect of the screen electromotive forces;

Fig. 16 shows a transforming arrangement ac cording to the invention applicable to the bridge shown in Fig. 14; and

Fig. 1'7 shows a simplified circuit to illustrate the action of Fig. 16.

Referring to Fig. 1, which shows a schematic circuit diagram of a double-screened transformer of well known type, it will be seen that the transformer consists of a primary winding 5 coupled inductively to a secondary winding 2.

The secondary winding is substantially completely enclosed in an inner metal screen 3, and this screen is in turn substantially completely enclosed in an outer metal screen 4 over which the primary winding i is wound. The whole is then enclosed in an outer metal case 5 which is connected at one point to the screen l. Terminals 6, i for the primary winding l are located inside the outer screen, and terminals 8, 9 for the secondary winding are located inside the inner screen. A terminal it) for the inner screen 3 is located inside the outer screen and H is the terminal for the outer screen. The construction of the transformer will be explained in fuller detail below with reference to Fig. 4.

When the transformer is connected in a circuit, suitable singleor double-screened connecting leads are employed to maintain the continuity of the screening from the external circuit components right up to the transformer terminals.

The principal screening requirements for such a transformer have hitherto been that the direct capacitance between the primary winding l and the inner winding 2 or the inner screen 3 should be substantially zero, and also that the direct capacitance between the inner winding 2 and the outer screen 4 should be substantially zero. This is principally for the purpose of ensuring that no alternating electromotive forces or poten tial differences are produced on the secondary side of the transformer by currents flowing in the primary winding except the electromotive force generated in the secondary winding 2 by direct magnetic coupling. It is also for the purpose of definitely fixing and localising the capacitances associated with the transformer so that their effects can be properly controlled. All this is, of course, well known practice.

it is not possible to completely reduce the acovementioned capacitances to Zero, on account of design and manufacturing limitations, and it has therefore been the practice to set maximum limits for these capacitances which it is judged will be sufficient to prevent the undesired effects resulting therefrom from becoming excessive. However, it has been found that when an attempt is made to employ these conventional transformers at radio frequencies, they behave as if the above-mentioned direct capacitances were much larger than their measured values. Fuller details of this effect will be given presently, but it may be stated here that an investigation of the trouble has shown that the effect is principally due to the electromotive forces which are generated in the screens by the currents in the transformer windings. These electromotive forces operate in conjunction with the capacitance between the inner and outer screens to produce unwanted currents in the circuits connected to the secondary side of the transformer.

This effect may produce serious errors in the measurements made at radio frequencies on impedance bridges employing such double screened transformers; and particularly in cases where the impedance to be measured has one component very small compared with the other, very large percentage errors may be produced in the measurement of the smaller component. Where the transformers are used at very high frequencies for coupling transmission circuits, for example, the effect may result in the introduction of excessive crosstalk or noise.

The recognition of the manor cause of this trouble has enabled several possible remedies to be applied which form features of the present invention.

When determining the values of the direct capacitances associated with a double-screened transformer, it is usual to short-circuit the primary and secondary windings so that they can be treated substantially as single unipotential conductors, and to make the measurements on a direct capacitance bridge, of which there are several well known types. Such measurements are found to produce correct values for these capacitances. When the transformer is con nected in a normal circuit, however, currents flow through the windings, and an alternating flux is developed in the core which is a condition not taken into account in the above-mentioned measurements of the direct capacitances, which are therefore not the only factors which affect the screening of the transformer. Figs. 2 and 3 show schematic circuit diagrams of two measuring circuits by which the effect of the screen electromotive forces may be made evident.

In these figures, i2 is a source of alternating current of impedance R connected through an adjustable attenuator [3 of constant characteristic impedance R. to the double screened transformer which is similar to that described with reference to Fig. 1. The output side of the transformer is connected to a detector M of conventional type, of input impedance also R. In Fig. 2 both windings of the transformer are short circuited, and terminal I is connected to the outer screen 5 through a resistance l5 equal to the characteristic impedance R of the attenuator 3. The output side of the attenuator is connected to terminals 5 and H, the latter terminal being connected to ground. The input terminals of the detector it are connected to terminals ID and H. The leads from terminals 6 and I ii should have screens connected to the outer screen 5, as indicated. The attenuator I3 is adjusted to obtain a convenient reading on the meter of the detector Hi. Suitable switching means (not shown) of known character is provided to connect the output of I 3 direct to the input of I 4, thus cutting out the transformer. The attenuator I3 is then readjusted to produce the same reading on the detector meter. The difference between the two readings of attenuator l3 then gives the attenuation of the connection formed through the direct capacitance between the primary winding and the inner screen 3. It is easily shown that this reading should be equal to A decibels where A=10 logio (1+1/4w C R (1) in which 0 is the value of the direct capacitance between the winding l and the screen 3, R is the characteristic impedance of the attenuator l3 and w is 21r times the frequency.

The value of 0 obtained from this measurement is found to agree satisfactorily with the value measured on a direct capacitance bridge.

Fig. 3 differs from Fig. 2 in that neither winding is short circuited, and terminal l is connected direct to the outer screen 5. The resistance i5 is connected between the output terminals 8 and 9. It is assumed that the transformer has a 1:1 ratio and is designed for coupling together two impedances each equal to R.

When the test is made with the arrangement of Fig. 3 in the same way as has been described with reference to Fig. 2, it is found that the attenuation measurement obtained is very much lower than before, which would indicate that the transformer is behaving as though the direct capacitance 0 were considerably larger than the measured value.

It will be convenient to call the direct capacitance measured according to Fig. 2, or by means of a direct-capacitance bridge the static capacitance, and the apparent value derived according to the equation (1) from the measurement according to Fig. 3 the dynamic capacitance.

To give specific examples, it was found that in the case of an equal ratio transformer designed for R=600 ohms in which the static capacitance was 0.05 F, the dynamic capacitance measured at 50 kilocycles per second was about 1.3 ,uuF, or about 26 times as much. In the case of an equal ratio transformer designed for R= ohms, in which the static capacitance was about 0.1 ,u/LF, the dynamic capacitance was found to be about 30 (L111? at the same frequency.

It will be appreciated from what has been explained already that the dynamic capacitance is only an apparent effect, the cause of which is the spurious electromotive forces developed in the transformer screens, but it serves as a convenient indication of the magnitude of the coupling produced by the said spurious electromotive forces.

In order to make clear the origin of the dynamic capacitance effect, Figs. 4 and 5 have been prepared. The core of a double screened transformer may be either solenoidal, toroidal or of the shell type; in all cases the portion on which the windings are mounted approximates either to a cylinder or to a toroid of circular section. In the perspective view of Fig. 4, the inner winding 2 is mounted directly on the central core l6 and is completely surrounded by the inner cylindrical or toroidal portion of the screen 3. This is in turn completely surrounded by the outer cylindrical or toroidal portion 4 of the outer screen. The outer winding I is wound over the outer screen portion 4 as indicated, and the whole is enclosed in an outer metal box 5 shown diagrammatically as a rectangle enclosing the rest of the assembly. The two screen portions 3 and 4 are split at I! and I8 in lines parallel to the axis of the core i 6 in order to prevent the screen portions acting as short-circuited turns. An overlap is formed with suitable insulation (not shown) between the two portions of the overlap in order to reduce the unavoidable gap in the screen produced by the split. A wire I9 is attached to a convenient point 2 l of the screen 3, a second wire for connection to the corresponding terminal Ill. 20 is attached to a convenient point 22 of the screen 4 and connects it to a single point on the outer box 5 and thence to the corresponding terminal II. This arrangement is well known, and no details are given of the means for insulating the screens from one another and from the windings, nor of the mounting and potting arrangements.

Fig. 5 shows a diagram of a section through the two screens taken perpendicular to the core it. The centre of the section is the point I3, and two dotted lines UP and 0Q have been drawn through the gaps in the two screens 4 and 3, respectively. It is to be noted that owing to the overlaps, the effective gaps are at the points indicated at the termination of the inner and outer laps respectively of the screens a and 3. The angle between 0? and 0Q is taken as Q and the radii from 0 to the connecting points 2| and 22 make angles ,8 and a, respectively, with (JP, which is the reference radius.

Now it will be understood that although no current flows in either screen when an alternating current flows in either or both of the transformer windings, yet an electromotive force is developed therein equal to the electromotive force E generated in a single turn of the secondary winding. same for both screens if it be assumed that the whole of the flux is concentrated inside the inner screen 3, which is substantially the case. Assuming also that the resistance of the screen material is negligible compared with the external cir- P-I' cuit impedance, and with the outer screen impedance, it can be assumed that in the case of the screen 4 the difference of potential between points of the screen adjacent to and on opposite sides of the line 0? is E. If the point on the clockwise side be taken as the reference point at zero potential, then the potential at other points increases proportionally to the clockwise angle measuredfrom 0P up to the value +E immediately on the anti-clockwise side. Similarly for the inner screen 3, the zero reference point is on the clockwise side of 0Q and the potential increases proportionally with the clockwise angle from 0Q up to the value +E immediately on the anticlockwise side of 0Q. These results are based on the simplifying assumption that the flux distribution in the core is uniform, and that the screens are both circular and concentric therewith.

Since discontinuities occur in the screens respectively at P and Q, it will be necessary to consider the current flow and potentials in the sectors q: and (21r separately.

Let I be the current in the impedance Z of the external circuit connected between the points 2] and 22. Take any radius DAB in the sector (21r-(p) making a clockwise angle 0, with DP. Then if a and b are the potentials of A and B respectively with respect to the point 2| It will be seen that this is independent of 01 so that the difference of potential between the two screens is constant over the sector 21r-q).

This electromotive force will be the Similarly, take anyradius 0CD in the. sector c making a clockwise angle 62 with HP. .Let .c and d be the potentials of C andD respectively with respect to the point 2 I. Then Thisis independent of 02 so that the difference of potential between the two screens is also'constant over the sector q), and differs from the value (2) for the other sector by E.

It will now be assumed that the admittance between the screens in each sector is made up of a conductance component and a susceptance component both of which are proportional to the angle subtended by the sector; in other words the admittance for the sector 21r-qo is and the admittance for the sector (p is likewise This assumption will be reasonably correct since the screens are usually maintained concentric by means of thin rings of low-loss insulating material.

The total current I is equal to the sum of two components contributed respectively by the two sectors, or

Substituting from equations (2) and (3) it follows that Z 1/27ry Equation 5 shows that the two screens act like a generator whose terminals are 10 and 11 (Fig. 1), whose electromctive force is and whose internal impedance is l/Y, where Yz21ry, the total admittance between the screens. In general for a well designed transformer, the conductance component of Y will be negligible compared with the susceptance component, so that Y y'wK approximately where K is the total capacitance between the two screens. It will be assumed in what follows that the conductance components of the transformer admittances can be neglected.

Although actual transformers will only be approximately in accordance with the assumptions which have been made, generally only the magnitude of the effect will differ somewhat from the magnitude indicated by the above formulae, but the character of the effect will be the same. Even when the cross-section of the core and screens is rectangular or square instead of circular (as with a shell type core) the electromotive force e and its effective internal impedance will not be greatly different from the values indicated by the formulae.

Fig. 6 shows a schematic circuit of the transformer in order to indicate how the electromotive force e acts between the points 2| and 22 of the screens through the interscreen capacitance K. The external terminals in and II are connected respectively to the points 21 and 22 by conductors which can be assumed not to have any appreciable electromotive forces induced therein.

Now that the principal cause of the dynamic capacitance effect has been fully explained, it is possible to state that the present invention consists in providing a double screened transformer with means for reducing or eliminating the undesired coupling between the input and output sides, which undesired coupling results from electromotive forces induced in the screens by alternating currents in the transformer windmgs.

It will be noted that a, c and (p in Equation 6 can be chosen so that e 'is zero. This affords one of the methods according to the invention for eliminating the effect of e. If the tapping points 2| and 22 where the leads to the terminals HI and II are connected are chosen so that 21r+( (u+fi) :0, then the undesired effect will be substantially removed. The interpretation of this condition is that the points 2| and 22 should be separated by an angle (p equal to the angle between the two gaps, but that 2| should be nearer to UP than 22 as measured circumferentially in a clockwise direction from P; in other words the inner screen point Z| should be nearer the outer screen gap I1 than the outer screen point 22. Special cases of these requirements are (a) when (p=1r, or the two gaps are diametrically opposite to one another, then the tapping points should be diametrically opposite to one another; and (b) when 7r=0, (that is the gaps are exactly aligned) in which case the tapping points should be exactly aligned in order to fulfill the condition cz+fl=21n It is worthy of note that the worst condition which will produce the largest dynamic capacitance is when on, c and (p are all zero. The effective electromotive force e then has the maximum value E.

It should be pointed out with reference to Fig. that if the two overlaps are exactly aligned, the angle (7) will not be zero, but will have a value equal to the angle subtended by the overlap. This can be seen in Fig. 7. If it is necessary for mechanical reasons that the two points 2| and 22 should be exactly aligned, then either one of the screens should be rotated slightly so that the radii 6F and [IQ coincide, which means that the two overlaps lie in immediately adjacent sectors, or as shown in Fig. 8, one of the overlaps could be made in the opposite sense, in which case BP and 0Q would coincide with the overlaps in the same sector.

It may be pointed out that when one terminal of the primary winding I is connected to the outer screen 5 (as is often the case) then it is possible to balance out the effect of the electromotive force e acting through the capacitance K against the effect of the input voltage Eb applied to the primary winding I, which acts through the static capacitance 0 effective between the other terminal of the primary winding and the screen 3. This can be understood from Fig. 9, which shows the equivalent circuit of this arrangement. Terminal 1 of the primary winding is supposed to have been connected to the screen 5, and it will be seen by reference to Fig. 6 that E0 in. series with the static capacitance 0 comes effectively in parallel with e in series with the inter-screen capacity K, between the terminals l0 and II, as shown in Fig. 9A. The arrangement is identically equivalent to Fig. 9B in which V: Eoc-l-eK It is possible to arrange the tapping points 2| and 22 of Fig. 5 so that or Eo/6=K/C. However, as it may be found difiicult in practice to carry out this adjustment with sufficient accuracy, an alternative would be to make an approximate adjustment so that e is a little too small, and to connect a small trimming condenser between the screens 3 and 4 which can be adjusted to make up the value of K so that it satisfies condition (6). This condenser need not actually be installed in the confined space between these screens, but could be placed in and connected to the outer screen 5 as indicated in Fig. 10 at 23. The condenser 23 should, however, be in a separate compartment so as to be screened from the winding I, otherwise the static capacity 0 will be increased. The condenser should be connected to the screen 3 by a lead having a screen connected to the outer screen 4.

A slightly different method of satisfying Equation 6 is based on the fact that K/c is generally much greater than Eu/e, so that instead of adjusting the tapping points 2| and 22 so that e is reduced, a small trimming condenser (not shown) could be connected between terminals 6 and N). This trimming condenser then comes in parallel with the static capacitance c and could be adjusted effectively to increase 0 until equation (6) is satisfied. However, it is generally better to reduce e as much as possible by the methods already explained before applying any such external compensation means. It should also be pointed out that it may not always be easy to ensure that E0 and e are in exact phase opposition.

The method of arranging the tapping points and overlaps which has been described for the purpose of reducing the spurious electromotive force 6 substantially to zero is thus the preferred method of dealing with the dynamic capacitance effect according to the invention. Other means external to the transformer winding assembly will be presently described, and these means may be applied additionally to the adjustment of the tapping points and screen overlaps for dealing with any residual effect which may remain after such adjustments have been made.

It may, of course, be found impracticable to make the above described special adjustments on the transformer winding assembly, in which case the external means to be described may be employed instead of such special adjustments. Thus, according to another feature of the invention, instead of reducing e to zero, or instead of making the special adjustments described with reference to Fig. 10 (which latter arrangements are not permissible unless one terminal of the primary winding is connected to the screen 5), the effect of e may be compensated in the manner indicated in Figs. 11 and 12.

In Fig. 11, a compensating windingyZt of a few turns is inductively coupled with the windings and 2 and is connected in series with a trimming condenser 25 between the screens 3 and 4. The circuit arrangement is then as shown in Fig. 9 (C) which will be seen to be essentially similar to Fig. 9 (A) with the winding I replaced by the compensating winding and the static capaci tance 0 replaced by the trimming condenser 25. It will be evident from the discussion of formula (6) above that provided the compensating winding is poled so that the electromotive force 'eo generated thereby is in opposition to the screen electromotive force 6, then the trimmer condenser '25 can be adjusted so that the equivalent electromotive force V (Fig. 9B) is zero. It may be added that with this arrangement the necessary phase opposition between 80 and e is likely to be substantially exactly obtained owing to the by-passing of the leakage inductance of the transformer.

As it will be impracticable to place the elements 24 and 25 between. the screens, they may be arranged as shown in Fig. 12. A screened conductor is employed for the winding 2s and is wound on top of the primary winding I, care being taken to insulate the conductor screen from the winding I and also to'insulate it so that it does not produce any short-circuited turns. The condenser 25 is housed in aseparate compartment of the outer portion of the screen l so as to be screened from the winding 1, as described with reference to Fig. 1%. In Fig. 12 the winding 24 is shown with only one turn in order to avoid confusing the figure, but it is to be understood that there may be any suitable number of turns.

Fig. 13 shows another very satisfactory method of neutralising the screen electromotive force 2 by the use of two double screened transformers which are connected so that the screen electromotive force of one is in opposition. to and neutralises that of'the other. The two transformers shown are each the same as shown in 1 and their parts have been given the same designation numbers distinguished by the letters A and B. They should be of exactly similar design and manufacture and should be selected by tests so that the values of the screen electromotive force of the inter-screen capacitance K are as nearly as possible the same for both. The two pr. .ary windings are then connected in parallel by suitable leads so as to maintain the screenin but should be connected in opposition, that is terminal EA should be connected to EB, and 613 to 5A. It then 6A and EA are used as the input terminals, the screen electromotive forces e will be of opposite sign because opposite directions in the two primary windings. If the terminals NBA and 16B of the inner screens and 55B are connected together then the resultant screen. electromotive force will be substantially zero.

The two secondary windings are preferably connected in series opposing, for example by connecting terminals 8A and 8B and using 9A andBB output terminals. The secondary windings could-also be connected in parallel opposing if desired, but it can be shown that the effect of any slight residual screen electromotive force is minimised by increasing the number of turns of the secondary winding. This is generally true of any of the transformers which have been discussed: the effect of a given value of e can always be reduced by increasing the number of turns on the secondary winding.

In Fig. 13 the primary windings could have been connected in series opposing if desired. This arrangement gives excellent compensation over a wide frequency range, but has the disadvantage of doubling the inter-screen capacitance K, and also the bulk and cost. Thus the cost of the arrangement likely to be considerably greater than any of the other schemes which have been described, but the final result is likely to be on the whole better.

The methods according to the invention which have been described so far are applicable to a the current flows in 10 double screened transformer irrespective of the particular use to which it will be put. Another method which will now be described is useful where double screened transformers are employed in impedance bridge circuits.

In non-symmetrical bridge networks (that is, those with unequal ratio arms, or those of the so called product arm type) it is not generally desirable to allow the system of screens to impose a significant capacitance across more than one arm of the bridge, though inter-screen capacitances which fall across either of the diagonals can be disregarded, except when considering questions of bridge sensitivity. This restriction involves the use of either triple screened transformers (which would usually be impracticable owing to their complexity), or of pairs of double screened transformers connected in tandem.

shows one example of a screened impedance bridge of known type with two tandem- .conne'cted input transformers. The four corhers of the bridge are lettered A, B, C and D. AB and BC contain the ratio arms Z1 and Z2. CD includes the terminals for the impedance Z4 to be measured, and Z3 is the adjustable impedance .used to balance the unknown impedance. Two input transformers 2G and 21 are connected in tandem and are of the conventional double screened type. The secondary winding of 21 is connected between A and C, the primary winding of 21 is connected to the secondary winding of 26, and the primary winding of 26 is connected to the input terminals 28 and 29 to which the test oscillator (not shown) is intended to be connected. The output terminals 36 and 3f for the detector (not shown) are connected to B and D, respectively.

The elements of the bridge are provided with a system of three screens, the outermost of which is shown in double weight full lines and is connected to the corner D and earth. The outer screen of the transformer 26 is a D screen.

The innermost screen surrounds the leads connected to the corner C and is connected to A.

The inner screen of the transformer 27 is also an A. screen. This screen is shown in dashed lines.

Between the D and A screens is the intermediate screen which surrounds the impedances Z1 and Z2, the leads connected to the corner A, and the C screen. This intermediate screen is shown in. single weight full lines, and is connected to B. It includes the inner screen of transformer 25 and the outer screen of transformer 21. Both windings of both transformers have one end connected to the correspondin screen. It will be seen that with this arrangement the capacitance between the A and B screens shunts the arm AB and that between the B and D screens shunts the arm BD, and there is no screen capacitance shunting any other arm of the bridge.

Fig. 15 shows a simplified equivalent circuit of the bridge showing how the screen electromotive forces of the transformers 25 and 21 act upon the bridge network. In this figure e1 and Y1 are the screen electrcmotive :force and interscreen admittance of the transformer 25, and er and Y2 are the same quantities for the transformer 2?. t will be evident that the errors produced by 61 and c2 cannot be easily allowed for; in fact any attempt to apply corrections would be impracticable.

However, the effect of ca for the transformer 21 can be dealt with by any of the methods already described, and c2 can be reduced substantially to zero. The eifect of in for the transformer 26 1 I could also be dealt with in a similar way, but in this case an alternative scheme is possible as shown in Figs. 16 and 17.

Fig. 16 shows a modification to the connections of the two transformers 26 and 27 of Fig. 14, the rest of the figure being unaltered. The inner winding of 26 and the outer Winding of 21 are each disconnected from the corresponding screen and are connected together by a pair of wires inside the B screen. A difierential condenser 32 is arranged inside the B screen with the fixed plates connected respectively to the above mentioned connecting wires, and the movable plate connected to D. Fig. 1'7 shows a schematic circuit of the connection between the two coils, including the screen electromotive force c1 and screen admittance Y1 of the transformer 26. The circuit is seen to be a bridge in which 31 and .92 indicate the effective capacitances between the terminals 6, 3 and 7, 9 respectively, and the B screen, and v1 and 222 the capacitances of the differential condenser 22. It is assumed that the admittance Y1 can be taken to be substantially a capacitance K1.

It can easily be shown by applying Kirchhofis laws to the bridge network that the difference of potential between the B and D corners of the bridge of Fig. 17 will be zero if in which ,u1=61/E1 for the transformer 26. It will be evident that provided the condenser 32 has a suitable range, it can be adjusted to satisfy the above equation and so the difference of potential between B and D resulting from e1 is balanced out.

It should be pointed out that the above assumes that e1 and E1 are in the same or in opposite phase, and that $1 and .92 are pure capacitances. Both of these conditions are only approximately fulfilled, but it is found that the arrangement generally produces a considerable reduction of the errors caused by e1. It will be understood that the arrangement does not affect the electromotive force e2 for transformer 27 which must be dealt with by one of the methods previously described.

What is claimed is:

1. A double screened electrical transformer comprising a substantially cylindrical core, a secondary winding on the said core, a substantially cylindrical inner screen surrounding the secondary winding, a susbtantially cylindrical outer screen surrounding the inner screen, each screen having a narrow gap therein running parallel to the axis of the core, a primary winding over the outer screen and a connecting lead tapped 01f at a point on each screen, the said tapping points being so chosen that the undesired difiference of the potentials of the said points resulting from the electromotive forces induced to the screens by alternating currents in the windings is brought within a specified limit, means for deriving from the alternating flux in the core of the transformer a potential difference acting between the screens to oppose the said undesired difference of potential, and means for adjusting the resultant potential diiference substantially to zero.

2. A transformer according to claim 1 having a small direct capacitance between the primary winding and the inner screen, in which one terminal of the primary winding is connected to the outer screen, comprising a condenser connected between the inner and outer screens having such a capacitance that the difference of potential between the screens is substantially zero.

3. A transformer according to claim 2, in which the condenser is an adjustable condenser.

4. A transformer according to claim 2, in which the condenser is enclosed in the outer screen and is screen-ed from the primary Winding.

5. A transformer according to claim 1 in which cne terminal of the primary winding is connected to the outer screen, comprising a condenser connected between the inner screen and the other terminal of the primary winding and having such a capacitance that the difference of potential between the screens is substantially zero.

6. A transformer according to claim 1 comprising a third winding coupled to the primary and secondary windings and connected in series with a condenser between the inner and outer screens, the capacitance of the condenser and the numbers of turns and direction of winding of the third winding being so chosen that the potential difference between the screens is reduced substantially to zero.

7. A transformer according to claim 6 comprising a secondary winding wound over a core, an inner screen surrounding the secondary winding, an outer screen surrounding the inner screen and a primary winding wound over the outer screen, the said third winding consisting of a shielded conductor is connected to the said outer screen.

WALTER KRAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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