US3518594A

US3518594A - Transformer having variable number of turns

Info

Publication number: US3518594A
Application number: US687082A
Authority: US
Inventors: William Kelvin Bottomley
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1966-11-16
Filing date: 1967-11-13
Publication date: 1970-06-30
Anticipated expiration: 1987-06-30
Also published as: GB1207955A

Description

June 30, 1970 w. K. BOTTOMLEY TRANSFORMER HAVING VARIABLE NUMBER OF Filed Nov. 13. 1967 TURNS LS ShooL=3-Shout 1 q R w m N tmdo kzmxmfi Q 200 2mm km ko m mmk N at a y Q m 1 f 536 R202 QMEIQEQ ZOQQQQZOQ MQE 9; moktm kbk INVENTOR WILLIAM KELVIN BOTTOMLEY June 30, 1970 i; w. K. BOTTOMLEY 3,518,594

TRANSFORMER HAVING VARIABLE NUMBER OF TURNS Filed Nov. 13, 1967 3 Sheets-Sheet 2 INVENTOR WILLIAM KELVIN BOTTOMLEY ATTORNEY United States Patent 3,518,594 TRANSFORMER HAVING VARIABLE NUMBER OF TURNS William Kelvin Bottomley, Hamilton, Scotland, asslgnor to Honeywell Inc., Minneapolis, Minn., a corporation of Delaware Filed Nov. 13, 1967, Ser. No. 687,082 Claims priority, application Great Britain, Nov. 16, 1966, 51,324/ 66 Int. Cl. H01f 21/04 U.S. Cl. 336-15 9 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a variable transformer wherein the primary winding means is relatively fixed while means are provided for varying the relative number of turns in the associated secondary winding. That means includes a rotatable member for supporting the secondary winding in close proximity to the associated primary winding. The turns ratio of the transformer is changed in accordance with the magnitude of a control signal.

This invention relates generally to transformer units, and more particularly to feedback elements for use in electrical recording and/or indicating apparatus.

In a form of electrical recording and/or indicating apparatus commonly used hitherto, an electrical input signal is derived from a variable which is to be recorded and/ or indicated. The input signal is fed to a comparison circuit Where it is compared with a second signal provided by the apparatus in a manner shortly to be described. The output of the comparison circuit is an error voltage dependent on the difference between the two compared signals. This error voltage is amplified and the amplified voltage drives a control motor, for example a two-phase induction motor, which controls the position of a visual index on a scale, where an indication of the variable is required, or a recording pen resting on a recording medium such as a moving paper band, where a record is required, or both where a recording and an indication of the variable are required. The motor may also actuate a controller to make a corrective adjustment to the variable from which the input signal was derived.

The second signal previously referred to is derived from the motor by a feedback element driven by the motor, which feedback element generates the second electrical signal which is dependent on, and normally proportional to, the pen position. Thus the feedback element provides a signal with which the input signal may be compared in order to produce an error voltage which will cause the motor to move the index and/or the pen in accordance with any change in the value of the input signal.

The feedback element has hitherto usually included a potentiometer supplied with a stabilized voltage, the movable contact of the potentiometer being moved along the resistance element of the potentiometer by the motor in order to pick off a voltage for feeding as a second signal to the comparison circuit.

This type of feed-back element is subject to many disadvantages. Normally the resistance element is in the form of a resistive coil and the movable contact makes and breaks contact with the turns of the coil one by one as it is moved, so that the output voltage changes in discrete steps and not in a continuous manner. Dust or corrosion may prevent a good electrical contact being made between the movable contact and the resistance element and any metal particles produced by wear of the movable contact or the resistance element may build up between ice the turns of the resistance element and thus cause short circuits between adjacent turns.

A solid bar of plastics material impregnated with conductive material has also been used as a resistance element in a moving contact potentiometer but it has'been found extremely difficult to produce such resistance elements having linear resistance characteristics, which are most commonly required.

It is one general object of the present invention to provide a variable transformer unit, and a further object is to provide such a unit which may be used as a feedback element in electrical recording and/or indicating apparatus and which does not involve the use of a moving contact.

From its broadest aspect the present invention provides a transformer unit comprising a transformer having two inductively coupled windings, one of which is constituted by part of a flexible electrically conductive element, and means for winding and unwinding turns of said one windmg.

In principle, either of the windings may be used as a primary winding, the other then being the secondary winding. The flexible conductive element may be a fine wire or a flexible, and possibly elastic, tube filled with mercury.

In a transformer unit according to the invention, the turns ratio and hence the ratio of output voltage to input voltage can be varied without the use of any sliding contact, by physically altering the number of turns which constitute one winding of the transformer. It will be appreciated that where there is a reference in this specification to increasing or decreasing the number of turns of a winding, this expression is intended to comprehend increasing or decreasing the number of turns by any fraction of a turn as well as any whole number of turns.

It has been found preferable in practice, especially when the transformer unit is to be used as a feedback element, to have a fixed number of turns in the primary winding, and therefore preferably in a transformer unit according to the invention the secondary winding is constituted by said part of a flexible conductive element. Such a transformer unit may be used as a feedback element in recording and/or indicating apparatus of the kind referred to.

Preferably, the turns of the secondary winding are carried on a rotatable member so that rotation of the member in the appropriate direction will either increase or decrease the number of turns in the secondary winding.

In one particularly convenient construction according to the invention one portion of the flexible conducting element, intermediate the ends of the element, is wound on a part of the rotatable member in proximity to the primary winding, so as to form the secondary winding, and another portion of said element, also intermediate the ends of the element, is wound on another part of the same rotatable member but out of proximity to the primary winding, to form an inactive portion of said element, the winding of the two portions on the rotatable member being such that rotation of the member will wind more turns into one portion while unwinding turns from the other portion.

Although the transformer in a unit in accordance with the present invention may be an air transformer, in which case the primary winding is preferably Wound on a former extending within that part of the rotatable member on which the secondary winding is wound, this arrangement requires the two portions of the flexible element to be spaced relatively widely apart in order to reduce inductive coupling between the primary winding and the inactive portion of the flexible element to a low enough level to be acceptable in practice. Such coupling 3 may give rise to non-linearity of the transformer output which in many applications is undesirable.

It is advantageous for the primary to be wound on the central post of a ferrite core which has an outer annular portion defining an annular space between the annular portion and the central post. The part of the rotatable member which carries the variable secondary winding can then be contained in this annular space and the shape of the ferrite core will keep the field of the primary winding away from the inactive turns. Even though the inactive portion of the flexible element may be wound on the rotatable member just beyond the end of the core, the inductive coupling between the primary and the inactive windings, and hence the non-linearity, is considerably reduced. This construction enables the overall length of the rotatable member to be reduced as compared with that required for an equivalent air transformer.

The linearity of the transformer can be still further improved in accordance with a further aspect of the invention.

From this latter aspect, the invention provides a transformer unit in which the transformer has two primary windings, and two secondary windings which are in series and are associated with respective primary windings, one secondary winding being constituted by one portion of a flexible conducting element which portion is wound on a part of a rotatable member in proximity to one of the primary windings, and the other secondary winding being constituted by another portion of the flexible conducting element, which other portion is wound, 'in the same sense as said one portion, on another part of the rotatable member in proximity to the other primary winding, whereby rotation of the rotatable member will wind more turns into one of the secondary windings while unwinding turns from the other one, the primary windings respectively being capable of inducing opposed voltages into the two secondary windings when suitably supplied with A.C. voltages.

In such a construction, any unintentional inductive coupling between one primary winding and that secondary winding associated with the other primary winding will be to some extent counterbalanced by a coupling in the opposite sense between the other primary winding and the other secondary winding, so that the non-linearity which tends to occur where inactive windings are present is substantially reduced.

In one particularly convenient arrangement, each of the two primary windings is wound on a former within, and directed along the axis of, the rotatable member, one primary winding being encircled by one secondary winding and the other primary winding being encircled by the other secondary winding, and the two primary windings are in series with each other but are wound in opposite senses so as to induce opposed voltages in the two secondary windings when an A.C. voltage is applied across the primary windings.

For compactness, each of the two primary windings may be wound on the central post of a respective ferrite core, the two central posts being co-axial, and each core having an outer annular portion defining space between the annular portion and the central post, the parts of the rotatable member carrying the two secondary windings being located within the respective annular spaces of the two cores.

In accordance with the invention, whether the transformer has one or two secondary windings, the rotatable member may be driven by means of the flexible conductive element itself. Such a transformer construction may comprise a wheel, one of the end portions of the flexible element being engaged round the outside of the wheel in one angular direction and the other end portion of the flexible element being engaged round the outside of the wheel in the opposite angular direction, the flexible element being held taut between the wheel and the rotatable member, whereby rotation of the wheel will effect said winding and unwinding of the element on the rotatable member. Alternatively, using a similar construction, the rotatable member may be driven directly andthe movement transmitted to the wheel by the flexible conductive element.

Preferably the wheel has a considerably larger diameter than has the rotatable member, so that a single rotation of the wheel, or less, will wind and unwind several turns on the rotatable member.

Conveniently the two portions of the flexible element which engage the outside of the wheel are guided in respective grooves around the periphery of the Wheel. The wheel arrangement just referred to in combination with the rotatable member carrying the variable secondary winding or windings, allows electrical connections to be made to both ends of the secondary winding or windings without the unwound portions of the flexible element forming a large amount of slack, as will be apparent from the more detailed description hereinafter.

In some applications, for example when recording temperatures measured by a thermocouple, which generally will have a non-linear temperature response, it may be required to introduce a non-linear characteristic into the transformer unit. In accordance with the present invention, this may be done by giving the outside of the wheel a predetermined non-circular shape in order to impose a non-linear relationship between the angular position of the wheel and the number of turns in the secondary winding or windings.

A further construction in accordance with the present invention which avoids the occurrence of slack in the flexible element when winding or unwinding it from the rotatable member comprises two pulleys lying in the same plane as each other with their axes perpendicular to the plane, the pulleys being mounted for movement in unison along the direction of an imaginary line joining the pulleys, each end portion of the flexible element being led tautly from the rotatable member round the extreme part of the periphery of a respective one of the pulleys to an anchor point, whereby said movement of the pulleys in unison will effect said winding or unwinding of the flexible element on the rotatable member.

Preferably the flexible element is held tautly between the rotatable member and the said pulleys by respective guide pulleys which lead the flexible element on to and off the rotatable member substantially at right angles to the axis of rotation of the rotatable member.

Where linearity is of prime importance, the magnetic field produced by the primary winding or windings should be highly linear over the range occupied by the secondary winding or windings.

In accordance with the present invention this is achieved by providing a screen of electrically conductive materila, for example copper, silver or aluminium, between the or each primary winding and its associated secondary winding, the screen extending over the whole distance which may be occupied by the secondary winding and the screen having a gap therein so as not to form a short-circuited secondary turn.

Such a screen may be formed by spraying, or otherwise applying the conductive material on to the outside of the primary winding, over a suitable interlayer of insulating material such as a plastics material.

A preferred embodiment of the present invention which is to be described more fully hereinafter is an apparatus for recording and/ or indicating the value of an electrical input signal, the apparatus comprising a comparison circuit having input terminals for connection to means for providing an electrical input signal, and an output connected to an amplifier, a motor driven in dependence on an output signal from the amplifier, and a feedback element actuated. by the motor to feed a second electrical signal dependent on the motor position into the comparison circuit for comparison with said electrical input signal to produce an error signal at the output of the comparison circuit, the motor being driven in such a direction as to tend to reduce the error signal to zero, wherein the feedback element comprises a variable transformer unit constructed in accordance with the invention as already referred to, the motor being operable to wind and unwind the flexible electrically conductive element, and the flexi ble electrically conductive element being connected to the comparison circuit for feeding to it the second electrical signal.

In order that the invention may be more clearly understood, some embodiments thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a schematic drawing of apparatus for recording and indicating an electrical input signal to which apparatus the :present invention may be applied.

FIG. 2 shows one form of variable transformer unit constructed in accordance with the invention, I

FIG. 3 illustrates the use of a ferrite core carrying the primary winding within the rotatable member of a variable transformer unit which in other respects is the same as that shown in FIG. 2,

FIG. 4 shows an alternative construction suitable for winding and unwinding the secondary windings,

FIG. 5 shows a further form of variable transformer unit in accordance with the invention, and

FIG. 6 shows the control circuit of a recording and/or indicating apparatus embodying the present invention.

FIG. 1 shows schematically the basic components of a recorder/indicator to which the present invention may be applied. The input to the recorder/indicator is in the form of an electrical signal which may be supplied directly by a measuring instrument giving an electrical output, such as a thermocouple, or which may be derived from a measuring instrument having a non-electrical output by means of a suitable transducer.

In FIG. 1 the source of the electrical input signal is indicated at 1. The input signal is fed to a comparison circuit 2 where it is compared with a second signal which is fed into the comparison circuit 2 over a line 3. By comparing the input signal and the second signal the compari 'son circuit 2 produces an error voltage which is fed to an amplifier 4. The amplified error voltage produced by the amplifier 4 is used to drive a control motor 5 which may be a two-phase induction motor. The control motor 5 controls the position of a visual index 6 which is movable along a scale which in practice is displayed at the front of the apparatus, and also controls the position of a recording pen 7 which is movable by the motor 5 across the surface of a recording medium which may be a paper band which can be driven at right angles to the direction of movement of the pen in known manner.

The control motor 5 also drives a feedback element 8 which provides the said second electrical signal which is fed into the comparison circuit 2 over the line 3. The second signal generated by the feedback element 8 is dependent upon the position of the pen 7 and is normally required to be proportional to the displacement of the pen from the position which it occupies when the input signal is zero. I

Thus the apparatus continually compares the position of the pen, as reflected in the value of the second signal produced by the feedback element, with the value of the input signal, and thereby produces an error voltage which drives the motor to move the index or the pen in accordance with any change in the value of the input signal.

A controller 9 may also be driven by the motor 5 to effect a corrective adjustment to whatever variable the input signal is being derived from.

FIG. 2 shows a variable transformer unit in accordance with the present invention which may be included in the feedback element 8 of a recorder/indicator as shown in FIG. 1.

The variable transformer unit shown in FIG. 2 includes a transformer which has a primary winding 10 supported on a former 11 and a secondary winding generally indicated at 12 which is constituted by a part of a flexible conductive element 13, such as a fine metal wire, which part is wound on a rotatable member in the form of a drum 14. By rotating the drum 14 in one direction or the other, the number of turns in the secondary winding 12 can be increased or decreased because turns of the flexible element 13 will be wound on to or unwound from the drum 14 according to the direction in which the rotation takes place.

Another portion 15 of the flexible element 13 is wound on to the drum 14 at the opposite end from the secondary winding 12, that is to say out of proximity to the primary winding 10. The portion 15 of the flexible element 13 forms an inactive coil which is wound onto the drum 14 in the same sense as the secondary winding 12 so that when the drum 14 is rotated to wind more turns into the secondary winding 12, an equal number of turns will be unwound from the inactive windings 15 and vice versa. The drum 14 is driven by means of the flexible conducting element 13 itself from a wheel 16 which is mounted on an axle 17 which is driven by the motor 5 through a suitable transmission. One end portion 18 of the flexible element 13 is engaged round the outside of the wheel in a clockwise direction, as illustrated in FIG. 2, and the other end portion 19 of the flexible element 13 is engaged round the outside of the wheel 16 in an anticlockwise direction, as illustrated in FIG. 2. The flexible element 13 is held taut between the drum 14 and the wheel 16 by means of a spring biased tension pulley 20.

The

end portions

18 and 19 are guided in respective parallel grooves around the periphery of the wheel 16, but for the sake of clarity the grooves are not shown in FIG. 2.

The

end portions

18 and 19 of the flexible element 13 are led through respective holes 21 and 22 in the periphery of the wheel 16 and their ends are joined to respective terminal pins 23 and 24 within the periphery of the wheel 16. Flexible output leads 25 and 26 are each secured at one of their ends to a respective one of the terminal pins 23 and 24, and pass between a pair of closely spaced

cylindrical posts

27 and 28 to

output terminals

29 and 30, the output leads 25 and 26 being held taut by respective tensioning springs 31 and 32.

As illustrated in FIG. 2, the number of inactive turns 15 on the drum 14 is equal to the number of turns in the secondary winding 12 on the drum 14, that is to say, the drum is midway between the limits of its rotational movement and so is the wheel 16. In this position the output lead 25 is held tautly against the cylindrical post 27 and the output lead 26 is held tautly against the cylindrical post 28. As the wheel 16 is rotated from the position shown in FIG. 2 in order to increase or decrease the number of turns in the secondary winding 12, both the output leads 25 and 26 will become wrapped partially around one or other of the

cylindrical posts

27 and 28 and at the same time the

posts

27 and 28 will be moved, by the rotation, closer to the

output terminals

29 and 30, but the small amount of slack which would otherwise occur is taken up by the tension springs 31 and 32.

Preferably, as shown in FIG. 2, the wheel 16 has a diameter many times greater than that of the drum 14 so that a single rotation of the wheel 14 will wind and unwind several turns of the flexible element 13 onto and off the drum 14.

The variable transformer unit illustrated in FIG. 2 of the drawings embodies an air transformer, which requires the inactive turns 15 to be separated by relatively large distance from the secondary turns 12 on the drum 14 in order to reduce inductive coupling between the primary winding 10 and the inactive turns 15 to an acceptable level.

FIG. 3 of the drawings shows a cross-section through a variable transformer for inclusion in a unit constructed in accordance with the invention, for example a unit as shown in FIG. 2, in which the aforementioned inductive coupling is reduced by using a ferrite core 33 to keep the field of the primary winding away from the inactive turns 15. The primary winding 10 is wound on a central post 34 of the core 33. The core 33 has an outer annular portion 35 which is joined to the central post 34 by a radially extending portion 36 and defines an annular space between the annular portion 35 and the central post 34. The end part 37 of the drum 14, which end part carries the variable secondary winding 12, the turns of which are guided in a spiral groove around the outside of the drum as indicated at 38 in FIG. 3, is con tained in the annular space within the core 33 while the inactive turns 15 which are similarly wound around the other end part of the drum 14 lie outside said annular space.

The drum 14 is rotatably mounted on the ferrite core 33. A spindle 37a is secured coaxially with the drum 14 by means of nuts 38a and 39 engaging with external threads on the spindle 37a, and a washer 40 interposed between the nut 38a and an inwardly projecting flange 41 which serves as a mounting point for the drum 14 on the spindle 37a. The spindle 37a is supported in bushes 42 which fit within the respective ends of a sleeve 43 which is fixed within a central bore 44 through the central post 34 of the core 33 by means of an annular projection 45 which is drawn firmly against one end of the central post 34 by a nut 46 which is screwed on to the other end of the sleeve 43, which end is externally threaded.

Conveniently, the ferrite core 33 is assembled from preshaped ferrite parts and the parts are clamped together by means of

end plates

47 and 48 which are drawn together by bolts 49 which pass between the two end plates and have nuts 50 screwed on to their threaded other ends.

It will be seen in FIG. 3 that the flexible element 13 passes on to the drum 14 through an opening 51 in the outer annular portion 35 of the ferrite core 33.

It will be appreciated that the transformer shown in FIG. 3 must be associated with means for taking up and paying out the flexible element 13 as the drum 14 is rotated, such as the wheel arrangement shown in FIG. 2. Alternatively, however, a second rotatable drum may be provided parallel with the drum 14 but outside the core 33, and the two drums may be geared together by gear wheels one of which is secured to each drum and which mesh with each other. The active turns will be wound on the drum 14 and within the core 33, but instead of the inactive turns 15 being also carried on drum 14, they will be carried on the adjacent parallel drum. The flexible element will be wound in such directions on the two drums that as they are rotated in unison it is wound from one to the other. In order to keep the flexible element taut between the two drums, the additional drum may be spring loaded in the appropriate rotational sense. A connection to one end of the active turns may be made by means of a wire which is led out along the axis of the drum and which is twisted as the drum rotates, and a similar connection may be made to one end of the inactive turns on the other drum, so as to complete the connection of the two sets of turns in series. In order to prevent the drums being rotated too far in either direction, the two gear wheels may conveniently have different numbers of teeth and each may be provided with a stop, so that after a given number of turns in either direction the stops will come up against each other and prevent further rotation.

FIG. 4 of the drawings shows an alternative construction to the wheel arrangement shown in FIG. 2, which enables the flexible element 13 to be wound on to and ofl the drum 14 Without creating slack in the

end portions

18 and 19 of the flexible element 13. This construction comprises two

pulleys

52 and 53 which lie in the same plane as each other and have their axes perpendicular to that plane. The

pulleys

52 and 53 are mounted at respective ends of a bar 54 which is slidably mounted in any convenient manner on a supporting structure so that the bar 54 can be slid from the central position illustrated in FIG. 4 to either side, as shown, for example, in the broken lines of FIG. 4, along the direction of an imaginary line joining the

pulleys

52 and 53. Four guide pulleys 55 guide the output leads 25 and 26 around the extreme parts of the peripheries of the two

pulleys

52 and 53 from the drum 14 (not shown in FIG. 4) and from the fixed

output terminals

29 and 30.

By sliding thebar 54 to one side or the other, the

pulleys

52 and 53 will be moved in unison to a position such as that shown in broken lines in FIG. 4 so as to increase the length of the part of the flexible end portion 18 which is contained in the loop passing around pulley 52, and to decrease the length of the flexible end portion 19 which is contained in the loop passing around the pulley 53. Thus the ends of the

end portions

18 and 19 which are not connected to the

terminals

29 and 30 can be made to execute complementary movements towards and away from the drum 14 by sliding the bar 54 from side to side, thereby effecting the required winding and unwinding of the element 13 on the drum 14.

Where the construction shown in FIG. 4 is to be used in a recorder/indicator as shown in FIG. 1, the bar 54 will be suitably driven by the motor 5, for example by means of a crank driven by the motor output shaft and a crankshaft linking the crank to the slidable bar 54.

FIG. 5 of the drawings shows diagrammatically a transformer for inclusion in a transformer unit in accordance with the present invention which will give a further improve-ment in linearity as compared with the construction shown in FIG. 3.

Referring to FIG. 5, the transformer shown therein has two primary windings 56 and 57 which are connected in series with each other and are wound in opposite senses on the central posts 34 of respective ferrite cores 33 each similar to the core 33 shown in FIG. 3. One portion of the flexible element 13 is wound on the drum 14 within one of the cores 33 to constitute a secondary winding 58 which is in proximity to the primary winding 57, and another portion of the flexible element 13 is wound on the drum 14 in the same sense as said one portion within the other ferrite core 33 and in proximity to the other primary winding 56 to constitute a. second secondary winding 59. Thus rotation of the drum, for example by means of a wheel as shown in FIG. 2, or by sliding pulleys as shown in FIG. 4, will wind more turns into one of the secondary windings 58 and 59 while unwinding an equal number of turns from the other of the secondary windings 58 and 59.

When the two primary windings 56 and 57 are supplied with an AC. voltage from a source 60, they will induce magnetic fields in opposite directions in the two cores 33 because the primary windings are wound in opposite directions. Hence the voltages induced in the two secondary windings 58 and 59 will be opposed to each other, that is to say, will be out of phase with each other. The magnitudes of the opposed voltages in the secondary windings will depend upon the number of turns in each of the respective second windings and hence upon the position to which the drum 14 has been rotated. The total output voltage developed across both the secondary windings 58 and 59, being the algebraic sum of the voltages developed in the individual windings, will bear a substantially linear relationship to the angular position of the drum. However, starting from the limit of drum rotation in one direction and revolving the drum to the opposite limit, the amplitude of the output voltage will fall substantially linearly from a maximum value to zero with one phase, and then rise, in anti-phase, from Zero substantially linearly to the same maximum value. A continuously linear DC output votag e may be derived from this discontinuous output voltage by means of a phase sensitive demodulator of any known kind.

FIG. 6 of the drawings shows the

essential components

2, 4, and 8 of a recorder and/or indicator of the kind shown in FIG. 1 and also, within the broken line 64, the control circuitry which is associated with the feedback element 8. The feedback element 8 may be, in general, a variable transformer constructed according to the present invention," for example as already 'described* with reference to FIGS. 2, 3, 4 and 5 of the accompanying drawings. In FIG. 6, however, the. transformer unit is of the kind having inactive turns.

The control circuit 64 comprises a source 61 of electrical square waves having a frequency of 100 kilocycles per second. The relatively high frequency of kilocycles per second is advantageous in that it gives improved transformer efliciency and thereby enables fewer turns to be used in the variable secondary winding 12 while still giving an adequate output voltage.

The amplitude of the square waves is stabilized by an amplitude stabilizer 62 the output of which is applied across the primary windings 63 of the variable transformer 8.

The number of turns in the secondary winding 12 and also the number of turns in the inactive windings 15 is controlled by the motor 5 as indicated by the broken lines 65 and as hereinbefore referred to. It will be appreciated that in using a variable transformer of the kind shown, in FIG. 5, the inactive turns 15 will be replacedjby a second set of secondary windings as already explained.

The A.C. output voltage from the secondary winding 12 is applied to a demodulator 66 which employs" as a reference signal a part of the output voltage from the square wave source 61. The demodulator 66 produces a DC. voltage proportional to the A.C. input voltage. and the demodulator output voltage constitutes the second signal which is compared in the comparison circuit 2 with the input signal from the source 1.

The error voltage resulting from this comparison is applied to a chopper amplifier 4 which drives the motor 5 in such a way as to wind turns into or unwind turns from the secondary winding or windings and thereby alters the value of the demodulator output signal until the terror voltage is zero. It will be appreciated that any subsequent change of the input signal will produce an errorvoltage which will cause the motor 5 to drive the drum ofthe transformer in the appropriate direction until the error voltage is cancelled and thus the .index 6, or pen 7, or both (see FIG. 1), which are driven b'yffthe motor 5, will be moved to respond to all changes in the value of the input signal.

Although the flexible conductive element has .been shown as a single strand, it is also possible to use an insulating tape having several conductive strandsembedded therein. Each turn of the drum will then wind several turns into the secondary winding, though of course it is necessary to make the appropriate interconnections between the ends of the strands to ensure that the voltages generated in the individual strands are added together.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as folltiws:

1. A transformer unit comprising a transformer having two inductively coupled windings, of which the secondary winding is constituted by part of a flexible, electrically conductive element, the turnsof said secondary winding being carried on a rotatable member so that rotation of the member in the appropriate direction will either increase or decrease the number of turnsin the secondary winding, the primary winding of the transformer being wound on the central post of a ferrite case which has an outer annular portion defining an annular space between the annular portion and the central post, and the part of the rotatable member which carries the secondary winding is contained within the annular space.

2. A transformer unit comprising a transformer having two inductively coupled windings of which the secondary winding is constituted by part of a flexible electrically conductive element, and further characterized in that the transformer has two primary windings, and two secondary windings which are in series and are associated with respective primary windings, one secondary winding being constituted by one portion of said flexible conducting element which portion is wound on a part of a rotatable member in proximity to one of the primary windings and the other secondary winding being constituted by another portion of the flexible conducting element, 'which other portion is wound, in the same sense as said one portion, on another part of the rotatable member in proximity to the other primary winding, whereby rotation of the rotatable member will wind more turns into one of the secondary windings which unwinding turns from the other one, the primary windings respectively being capable of inducing opposed voltages into the two secondary windings when suitably supplied with A.C. voltages.

3. A transformer f-unit according to claim 2 in which each of the two priniary windings is wound on a former within, and directed along the axis of, the rotatable member, one primary winding being encircled by one secondary winding and the other primary winding being encircled by the other secondary winding, and the two primary windings are in series with each other but are wound in opposite senses so as t6 induce opposed voltages in the two secondary windings when an A.C. voltage is applied across the primary windings;

4. A transformer unit according to claim 2 in which each of the two primary windings is wound on the central post of a respective ferrite core, the two central posts being co-axial, and each core having an outer annular portion defining an annular space between the annular portion and the central post, the parts of the rotatable member carrying the two secondary windings being lo-.

cated within the respective annular spaces of the two cores.

5. A transformer unit comprising a transformer having two inductively coupled windings, of which the secondary winding is constituted by part of a flexible electrically conductive element, the turns of said secondary winding being carried on a rotatable member so that rotation of the member in the appropriate direction will either increase or decrease the number of turns in the secondary winding, the rotatable member being driven by means of the flexible conductive element itself, a wheel, one of the end portions of the flexible element being engaged around the outside of the wheel in one angular direction and the other end portion ofithe flexible element being engaged round the outside of} the wheel in the opposite angular direction, the flexibleelement being held taut between the wheel and the rotatable member, whereby rotation of the 'wheel will effect saidiwinding and unwinding of the element on the rotatable-member.

6. A transformer unit according to claim 5 in which the wheel has a considerably larger diameter than has the rotatable member, so that a single rotation of the wheel, or less, will wind and unwind several turns of the rotatable member.

7. A transformer unit according to claim 5 in which the outside of the wheel has a predetermined non-circular shape in order to impose a non-linear relationship between the angular position of the wheel and the number of turns in the secondary winding or windings.

8. A transformer unit comprisinga transformer having two inductively coupled windings, of which the secondary winding is constituted by part ofa flexible electrically conductive element, the turns of said secondary winding being carried on a rotatable member so that rotation of the member in the appropriate direction will either increase or decrease the number of turns in the secondary direction of an imaginary line joining the pulleys, each end portion of the flexible element being led tautly from the rotatable member round the extreme part of the periphery of a respective one of the pulleys to an anchor point, whereby said movement of the pulleys in unison will effect said winding or unwinding of the flexible element on the rotatable member.

9. A transformer unit according to claim 8 in which the flexible element is held tautly between the rotatable member and the said pulleys by respective guide pulleys which lead the'fie'xible element on to and olf the rotatable 15 member substantially at right angles to the axis of rotation of the rotatable member.

UNITED STATES PATENTS Storer 336-15 Hunt 336l5 Scofield 336-15 Bliss 336-15 Bliss 336-45 Ellis 33615 Gordon 336-45 XR Ross 33684 THOMAS J. KOZMA, Primary Examiner US. Cl. X.R.