GB2029118A - Transformers - Google Patents

Abstract

A constant voltage transformer comprising a primary coil 24, a secondary coil 30 coupled to a tuning capacitor 44, and a magnetically continuous first magnetic core 20 linking both the primary coil and the secondary coil. A magnetically discontinuous second magnetic core 22 is also provided which, as shown, links the secondary coil but not the primary coil although the reverse is envisaged. A winding 42 in series with the secondary winding 30 is disposed on the core 22, and the output is obtained across the series connection and terminals 36, 38. A further discontinuous magnetic core 18 links the primary winding and a further winding 40 which is in series with the capacitor 44 across the output terminals. Numerous variations are disclosed. <IMAGE>

Description

SPECIFICATION Transformers The present invention reiates to constant voltage transformers.

Constant voltage transformers provide good stabiiisation of output voltage with wide input voltage variation; load regulation is normally better than for conventional transformers but they do introduce frequency sensitivity (causing small output voltage variations which are proportional to the change in freqency) and waveform distortion. Distortion, however, is unimportant where the transformer output is rectified to obtain a fairly stable D.C.

Alternative electronic methods which have been used to give stable D.C. outputs include feedback controlled series or parallel stabilisers or switched mode systems. The former are very wasteful of power whereas the latter are complex and expensive.

The present invention seeks to provide an improved form of constant voltage transformer for providing A.C. or, where rectified, D.C. output voltages.

Accordingly the present invention provides a constant voltage transformer comprising a primary coil, a secondary coil coupled to a tuning capacitor, a magnetically continuous first magnetic core linking both the primary coil and the secondary coil, a magnetically discontinuous second magnetic core linking one of the primary coil and the secondary coil but not the other and at least one auxiliary coil linked by said discontinuous magnetic core and coupled to said tuning capacitor.

In a first embodiment said magnetically discontinuous second magnetic core links the primary coil but not the secondary coil, and said auxiliary coil is a compensating coil.

In a further embodiment said magnetically discontinuous second magnetic core links the secondary coil but not the primary coil.

In a preferred embodiment according to the present invention a magnetically discontinuous third magnetic core is provided linking the primary coil and a compensating coil which is series coupled with the secondary coil. The cores are spaced from one another and the first and third cores each has at least one air gap.

Normally, to achieve low distortion for the output of a constant voltage transformer the transformer core or cores are specially designed. The present invention enables standard, commercially avaiiable cores such as C-cores and toroids to be used while still providing low distortion, thus avoiding the need for special tooling. The basic form of the transformer may be adapted to more than one pattern of mechanical assembly, and the detailed mechanical construction can be varied to permit flexibility of design for specific requirements.

A transformer according to the present invention is normally of the ferroresonant type.

The present invention is further described hereinafter, by way of example with reference to the accompanying drawing which illustrates a preferred form of constant voltage transformer.

In the drawing the constant voltage transformer 10 comprises three electrical circuits 12, 14 and 1 6 primarily formed by windings in three independent magnetic circuits 18, 20 and 22.

The latter are in the form of three transformer cores, in the present instance C-cores (such cores may be strip wound, of laminated stampings or of other suitable construction) in which substantially the same magnetic flux traverses the complete magnetic length. By C-core is meant a core comprising two C- of U-shaped parts butt-jointed together. In the preferred embodiment two of the cores 1 8 and 22 have non-magnetic portions formed in this instance by air gaps 1 9 and 21. The number of cores having air gaps may be varied depending upon the particular use to which the transformer is to be put and the desired results. As an alternative to air gaps 1 9 and 21 each core 18, 22 may be in the form of a composite core in which an effective air gap is distributed throughout the magnetic path of the core.

The core 1 8 serves effectively as a magnetic shunt, core 20 is a saturable core and core 22 is a linear choke core. The cores are spaced apart to maintain their flux paths substantially independent of one another.

The three electrical circuits comprise a primary electrical circuit 12, a secondary electrical circuit 14 and a tertiary or resonant circuit 16. Although only one secondary circuit 14 is illustrated two or more such circuits may be used. All of the above-described electrical circuits may be distinct circuits or alternatively part or all of any secondary circuit may form a part of any other secondary circuit, or (as in the illustrated transformer) a part or the whole of the tertiary circuit.

The primary electrical circuit comprises a primary coil 24 which is connected to an A.C. power supply source via terminals 23, 25 and is linked to the cores 18 and 20. This linking may be achieved, as shown, by winding the coil 24 about adjacent parallel limbs 26, 28 of the two cores 1 8 and 20 or by forming the coil 24 in two series connected portions one of which is wound around one of the limbs 26, 28 and the other of which is wound around the other of the limbs 26, 28. The total number of turns in the coil 24 are selected appropriate to the nominal voltage of the power source and the coil may conveniently be provided with tapping points to cater for different nominal power source voltages.

More than one coil 24 may, of course, be used but again the coils will be interconnected in such a manner as to provide an effective total number of turns appropriate to the nominal power source voltage.

The secondary electrical circuit comprises a coil 30 which links adjacent parallel limbs 32, 34 of the two cores 20 and 22 in a similar manner to coil 24. The coil 30 may be formed in similar manner to coil 24, as described above, and equally two or more such coils 30 may be used in the secondary electrical circuit. The core 20 which magnetically links both coils 24 and 30 may be of continuous construction (i.e. toroidally wound) or assembled in parts involving one or more small effective air gaps.

Winding the primary and secondary coils 24 and 30 on separate legs bf the same core 20 reduces the capacitive and mutual inductive coupling between the two coils thus giving rise to suppression of transients in the input voltage.

The coil 30 serves as the transformer main output winding. One end 41 of the coil 30 serves as an output terminal 36 of the transformer and the other end is coupled to a second output terminal 38 of the transformer.

The tertiary or resonant circuit is formed by the coil 30 together with two auxiliary coils 40 and 42.

Although the preferred transformer is described as having three coils 30, 40 and 42 forming the tertiary circuit 1 6 the coil 42 may be coupled with the coil 30 to form any secondary and/or tertiary electrical circuit. Likewise the coil 40 may also be coupled with the coil 30 to form any secondary and/or tertiary electrical circuit. Alternatively coil 40 may be omitted.

The coils 40 and 42 are wound on the respective cores 1 8 and 22 (not on the core 20 linking the primary and secondary coils 24 and 30) in the approprite sense in series with the secondary coil 30, coil 42 coupling coil 30 ta the output terminal 38. A capacitor 44 couples the coil 40 to the output terminal 38 and has a value chosen such that, when power is applied to the primary circuit 12 2 via the input terminals 23, 25 ferromagnetic resonance occurs with, in this case, the cores 20 and 22 being in magnetic saturation i.e. during part of the alternating flux cycle the flux density is substantially above the "knee" of the B-H curve for the cores and the cores are magnetically saturated. The magnetising energy is derived from the applied power source which in this instance is mains A.C. voltage.

The flux generated by the application of power to the primary coil 24 divides between the cores 1 8 and 20. In resonance the core 20 is magnetically saturated and the flux density remains substantially constant over a reasonably wide range of input voltage. The flux density in core 18, however, exhibits a greater variation with change in input voltage and approaches the value of the flux density iin the core 20 only when the input voltage approaches its maximum value.

The core 22 in the preferred embodiment has an air gap 21 whose dimensions are such as to maintain with core 22 a magnetic circuit whose effective permeability does not vary greatly with flux density up to the saturation level obtaining in core 20. When linked, in addition to core 20, with the main secondary winding 30 this magnetic circuit increases the inductance of the tertiary or resonant circuit to provide a greater resonating energy with the same tertiary current in coil 30. While the core 22 is shown with a single airgap 21 in the drawings it may have a plurality of air gaps whose combined effect is that of the single air gap 21. It is preferable for two gaps to be provided in opposing arms of the core 22.

The auxiliary coil 42 on the core 22 being connected in series with coil 30 carries the same current. Its function is further to increase the inductance of the tertiary circuit 16, consequently again adding to the resonant energy for a given tertiary current. This increased resonating energy permits a higher VA rating for the transformer. Because the greater part of the tertiary circuit inductance is derived from the linking of coils 30 and 42 with the magnetic circuit comprising core 22 and air gap 21, the total resonating inductance is fairly constant, in consequence of which the tertiary circuit waveform has low distortion.

An increase in the tertiary circuit current which can be achieved by increasing the value of the resonating capacitance 44, will increase the resonating energy with a consequent reduction in constancy of the tertiary inductance. On any given embodiment of coils and cores, therefore, it is possible to increase the VA rating with the penalty of increased distortion or conversely, to improve the waveform with restricted VA rating by selection of the resonant capacitor. This feature is, of course, common to C.V.T.s of a more conventional construction but for a given VA rating-to-size ratio much lower distortion has been obtained on the present construction.

The following figures have been obtained for a C.V.T. according to the present invention compared with an equivalent conventional C.V.T. :- Distortion for V.A. rating Distortion Conventional C.V.T.

300 < 5% 20-30% 500 < 10% 20-30% The coil 40 on the outer limb of core 1 8 acts as a compensating winding and modifies the operation described above in the following manner:-- the varying fluxes generated in the three cores 18, 20 and 22 are out of phase with one another and so the total output voltage of the transformer is a vector sum of the voltages of each of the three series connected coils 30, 40 and 42. The relative phases of the fluxes in the three cores vary with operational conditions, i.e.

input voltage, frequency and transformer load current, and by suitably choosing the number of tursn in the coils 30, 40 and 42 and the dimensions of the air gaps 19, 21 in the cores 18 and 22 the phase variation can be used to improve the stability and/or regulation of the output voltage relative to that of the tertiary circuit 1 6. This may be achieved either by series connecting coil 40 in the secondary circuit 14 but not in the tertiary circuit 16, or by series connecting coil 40 in the tertiary circuit 16, but not in the secondary circuit 14, the sense in which coil 40 is connected being mutually reversed between the two methods. Alternatively, a combination of these two methods could be used by series connecting one winding on coil 40 to the secondary circuit 14 and another winding on coil 40 to the tertiary circuit 16.These two windings on coil 40 could be entirely separate, or the whole or part of one could form part of the other. For example, terminal 36 could be formed at the end of coil 40 with terminal 41 connected to a tapping on coil 40.

Coil 40 serves a further purpose in that it can be used to vary the waveform of the output voltage of secondary circuit 1 4. This is due to the fact that, although the tertiary circuit 16, comprising coils 30, 42 and possibly coil 40, variously linked with the magnetic cores 18, 20 and 22 in the manner previously described, forms an inductance of reasonably linear characteristic, the fluxes in the individual cores are considerably distorted. Therefore, whilst the voltage developed across the tertiary circuit 14, as a whole, conforms to a nearly sinusoidal waveform, the voltages obtained across the individual coils 30, 40, 42 have considerably greater harmonic content.When coil 40 is used for output voltage compensation by any of the methods described above, it will, therefore, introduce into the secondary circuit 1 4 a harmonic content of voltage not present in the voltage of the tertiary circuit 1 6. This may improve or deteriorate the output waveform relative to that of the tertiary circuit 16, according to the number of turns on coil 40 relative to the other coils and the sense of connection of coil 40.

The implication here is that the number of turns selected for coil 40, for any given number of turns on coils 30 and 42, could be chosen either for minimum variation of output voltage over a specified range of input voltages and/or load currents, or for the least distorted waveform of that voltage, or for any compromise between these two requirements. However, the selection of the number of turns in the coils 30, 40 42 or the air gaps 19 and 21 in the cores 18 and 22 is not a straighforward matter as variation of any of these parameters affects the flux distribution in the three cores.

Where the coil 40 is omitted the capacitor 44 connects directly with the end 41 of coil 30.

In the preferred embodiment the secondary circuit 1 4 comprises the coils 30 and 42 only and the output is taken from terminals 38 and 36; the tertiary circuit is formed by connecting coils 30, 40 and 42 in series in the appropriate sense, and the resonating capacitor is connected between terminal 38 and the coil 40.

In various modifications (not shown in the drawings) of the preferred transformer there may be more than one secondary electrical circuit and/or a separate tertiary circuit each of which has a main coil linking with the core 20 and core 22 (in the manner of coil 30) and optionally one or more auxiliary coils wound on one or both cores 1 8 and 22 (in the manner of coils 40, 42).

The auxiliary secondary or tertiary coils are series connected with their respective main coil in an appropriate sense. Voltage taps may be provided on the main coil 30 or auxiliary coils to enable parts of the secondary and tertiary circuits to be commoned where this is applicable, or to provide voltage adjustment taps where required.

A feature of the above described constant voltage transformer construction is that, where the tertiary (resonant) circuit 1 6 comprises the main coil 30 together with one or more auxiliary coils 40, 42 on one or both of the relevant cores 18, 22 phase differences occur between the fluxes linking these coils and consequently between the voltages induced across these coils. The relative phases are dependant on both the applied input mains voltage and the load current drawn from the transformer, and, by suitable interconnection of these coils 30, 40, 42 together with suitable interconnection of main and auxiliary secondary coils (where present) this phase variation can be utilised to improve stability of the output voltage with respect to input voltage changes and regulation with respect to load changes.

A further feature of the above-described constant voltage transformer is that where the tertiary circuit comprises both coils 30 and 42 and optionally coil 40, much of the resonating inductance derives from the linking of coils 30 and 42 with the core 22, which includes air gap 21 in its magnetic circuit. By suitable selection of this gap together with suitable interconnection of these coils 30, 42 and optionally 40, it is possible to make the resonating inductance reasonable constant even when one or both of the cores 20 and 22 is in magnetic saturation, thereby causing a nearly sinusoidal voltage to be developed by the tertiary circuit 16, that is a voltage with low harmonic content. The harmonic distortion of the output voltage from the secondary circuit 14 may be further reduced by suitable interconnection of the main and auxiliary secondary coils (where present).These interconnections, as required to give minimum distortion, may not be identical with those referred to in the preceding paragraph, which would give optimum stability or regulation. However, suitable selection will enable a good overall performance to be achieved with the option of alternative connections to enhance one of these features with some deterioration of the other where advantageous to a specific application.

The preferred transformed construction also provides very good overload and short-circuit protection, even under conditions where the overload is accompanied by overvoltage being applied to the primary.

While the illustrated transformer has three cores and is to be preferred it is possible to omit one of the cores, core 22. The construction and arrangement of the two core transformer is similar to the described three core transformer but with the core 22 omitted.

One core acts as the saturable secondary core carrying output and resonant coils on its outer limb and the other core, gapped, acts as a shunt, carrying an auxiliary coil.

The illustrated form of the preferred transformer according to the present invention is an inline form. However, this can be varied with the cores for example in a substantially U-shaped arrangement.

Two examples of constant voltage transformers constructed according to the present invention are given below with the test results achieved.

EXAMPLE I The transformer construction comprised three C-cores of identical pattern each weighing 3.84Kg. Two of the cores were gapped, the third ungapped. The secondary circuit comprised two coils and the transformer output was taken from terminals 36 and 38. The tertiary circuit was taken from these same two coils together with an auxiliary winding as per coil 40. A capacitor 44 was connected as shown in the drawing.The total weight of the conductors in the transformer was 5.05Kg giving a total "iron and copper" weight of 13.57Kg.

The transformer was tested (1) at 300 V.A. rating and (2) no loading of output, and the following results given in table 1 obtained over an input voltage variation of 240 V + 10%-20%, i.e. 264 V maximum, 192 V minimum. Load power factor was 1.0 and load was set at 240 V input.

TABLE 1 Total H Internal Input Load Output Harmonic Power Voltage VA Voltage Distortion Dissipation 192V 300 VA 318.5 1.8% 63W 264V 300 VA 329.5 3.4% 74W 192V 0 328 2.7% 61W 264V 0 334.5 4.5% 72W Thus over the stipulated range of input voltages the stability of the output voltage at 300 VA rating is + 5.5 v about a mean of 324 V i.e. + 1.7%. At zero loading the corresponding stability is + 1.0% and combined stability and regulation is + 8.0 V about a mean of 326.5 V i.e. + 2.45%. The transformer efficiency is seen to be better than 80% at 300 VA rating. A further test was carried out in which the input voltage range was widened to 240 V + 20% - 30% and the corresponding results were similarly analysed as: Output voltage stability at 300 VA: + 3.3% Output voltage stability at zero load: j 1.6% Overall stability and regulation: + 3.8% Total harmonic distortion: 1.6%-5.1 % This indicates that the constant voltage behaviour extends well beyond the stipulated range of input voltages without significant increase in distortion.

Overload and short circuit protection were tested as follows:- (i) At maximum input voltage 264 V, the load current was increased until a maximum power output was obtained. This was 600 VA and the output voltage had then fallen to 293 V (at 2.05 A). Beyond that, voltage and power of output both decreased. For example, at 3.0 A the output voltage fell to 1 35 V (405 V.A.). The maximum internal power dissipation was 80W.

Thus, the transformer output was limited to twice the rated power and was capable of operation for an indefinite period without overheating.

(2) The output terminals were short-circuited and 264 V applied to primary circuit. The measured current was 3.1 1A and the internal power dissipation was 36 W. Thus, the transformer was again capable of operation for an indefinite period without overheating. (Load current at 300 V.A. was 0.913 A).

At lower input voltages overload and short-circuit current and power dissipation were even lower.

EXAMPLE II In core and coil detail this transformer is identical with the example previously quoted; it differed in one respect only, as follows. Instead of end 41 of coil 30, together with one end of the compensating coil forming output terminal 36, this same end of coil 40 alone served as output terminal 36 whilst end 41 of coil 30 was connected to a tap on coil 40, positioned T of the total number of turns from terminal 36. Thus only T of the total compensating turns remained in the tertiary (resonating) winding whilst T of the total compensating turns, were introduced into the secondary circuit in the reversed sense.

This transformer was tested, as previously, (1) at 300 V.A. rating (2) at no loading of output and was further tested (3) at 400 V.A. rating.

The results, given in the table 2 below were obtained over an input voltage variation of 240 V + 10% - 20% i.e. 264 V maximum 1 92 V minimum. Load power factor was 1.0 and load was set at 240 V input.

TABLE 2 Total Internal Input Load Output Harmonic Power Voltage VA Voltage Distortion Dissipation 192 300 316 1.9% 62W 264 300 323 3.1% 74W 192 0 325 2.7% 59W 264 0 322 4.8% 68 W 192 400 304 1.7% 70 W 264 400 318 2.6% 75W Maximum input current at 400 V.A. was 2.4 A Thus, over the stipulated range of input voltages the stability of the output voltage at 300 V.A. rating is + 3.5 V about a mean of 319.5 V i.e. + 1.1%. At zero loading the corresponding stability is + 0.47% and combined stability and regulation is + 4.5 V about a mean of 320.5 V i.e. + 1.4%.At the further rating of 400 V.A. the stability is + 7 V about a mean of 311 V i.e. + 2.25% and combined stability and regulation is + 10.5 V about a mean of 314.5 V i.e. + 3.33%.

The efficiency at 300 V.A. is seen to be better than 80% and at 400 V.A. better than 84%.

A further test was carried out in which the input voltage variation was widened to 240 V + 20% - 30% and the corresponding results were similarly analysed as: Output voltage stability at 300 V.A. + 1.75% zero load + zeroload - 0.62% " " 400 V.A. + 3.4% Overall stability and regulation 0-300 V.A. + 3.7% " 0-400 V.A. + 4.33% Total harmonic distortion 1.4%-5.2% Maximum input current at 400 V.A. was 2.55 A. Maximum dissipation 80 W.

This indicates that in this example too, the constant voltage behaviour extends well beyond the stipulated range of input voltages without significant increase in distortion, or in input current or internal power dissipation.

Overload and short circuit protection were tested in a similar manner to the previous example, i.e.:- (1) At maximum input voltage 264 V, the load current was increased until a maximum power output was obtained. This was 595 V.A. and the output voltage had then fallen to 259 V (at 2.3 A). Beyond that voltage and power of the output both decreased, for example, voltage at 2.5 A loading was 236 V and the output power 590 V.A. The power dissipation at maximum output was 65 W.

Thus the transformer was limited to a maximum increase of 50% beyond the 400 V.A. rating and was capable of operation for an indefinite period without overheating.

(2) The output terminals were short-circuited and 264 V applied to the primary circuit. The measured current was 2.82 A and the internal power dissipation was 36 W. Thus, the transformer was again capable of operation for an indefinite period without overheating. (Load current at 300 V.A. was 0.93 A, at V.A., 1.28 A).

At lower input voltages overload and short-circuited current and power dissipation were even lower.

Claims (14)

1. A constant voltage transformer comprising a primary coil, a secondary coil coupled to a tuning capacitor, a magnetically continuous first magnetic core linking both the primary coil and the secondary coil, a magnetically discontinuous second magnetic core linking one of the primary coil and the secondary coil but not the other and at least one auxiliary coil linked by said discontinuous magnetic core and coupled to said tuning capacitor.

2. A constant voltage transformer as claimed in claim 1 wherein said magnetically discontinuous second magnetic core links the primary coil but not the secondary coil, and said auxiliary coil is a compensating coil.

3. A constant voltage transformer as claimed in claim 1 wherein said magnetically discontinuous second magnetic core links the secondary coil but not the primary coil.

4. A constant voltage transformer as claimed in claim 2 further comprising a magnetically discontinuous third magnetic core linking said secondary coil.

5. A constant voltage transformer as claimed in claim 4 further comprising at least one auxiliary coil linked by said third magnetic core.

6. A constant voltage transformer as claimed in claim 3 further comprising a magnetically discontinuous third magnetic core linking said primary coil.

7. A constant voltage transformer as claimed in claim 6 further comprising at least one compensating coil linked by said third magnetic core.

8. A constant voltage transformer as claimed in claim 7 wherein said compensating coil is series coupled with said secondary coil.

9. A constant voltage transformer as claimed in any of claims 2 to 8 wherein at least one of each coil linked by two magnetic cores comprises at least two series connected winding portions, one of which is wound on a limb of one of said two magnetic cores and a second of which is wound on a limb of the other of said two magnetic cores.

10. A constant voltage transformer as claimed in any of claims 2 to 9 wherein the or each auxiliary coil is series coupled with the secondary coil.

11. A constant voltage transformer as claimed in any of claims 6 to 10 wherein said capacitance has a capacitance value such that ferromagnetic resonance occurs with said first and second cores in magnetic saturation.

1 2. A constant voltage transformer as claimed in any of claims 2 to 11 wherein each said magnetic core is a C-core.

1 3. A constant voltage transformer as claimed in any of claims 2 to 1 2 wherein the or each magnetically discontinuous magnetic core has at least one air gap.

14. A constant voltage transformer as claimed in any of claims 2 to 1 3 wherein at least said first magnetic core is toroidally wound.

1 5. A constant voltage transformer as claimed in any of claims 2 to 14 wherein one or more of said coils has tappings for enabling adjustment of operating parameters of the transformer.

1 6. A constant voltage transformer as claimed in any of claims 2 to 1 5 wherein one or more of said cores is a composite core.

1 7. A constant voltage transformer substantially as hereinbefore described with reference to the accompanying drawing.