GB2200255A - Ferro-resonant constant voltage transformer - Google Patents

Abstract

A ferro-resonant constant voltage transformer which includes first, second and third juxtaposed subcores (C1, C2, C3), a primary winding (L1) around one limb of the first subcore (C1) and one limb of the second subcore (C2), an output winding (18) around one limb of the third subcore (C3), and at least a first control winding (20) around one limb of the second subcore (C2) and one limb of the third subcore (C3) <IMAGE>

Description

FERRO-RESONANT CONSTANT VOLTAGE TRANSFORMER This invention relates to a constant voltage transformer and line conditioner of the ferro-resonant type.

The continuous supply of a clean and stable mains voltage is of critical importance to the correct functioning of computers and similar sophisticated electronic equipment. Transient voltages between line and neutral, or between neutral and earth1 of an electrical supply system can render the aforementioned type of equipment totally inoperative or unreliable. Similarly excessive variation, upwardly or downwardly, of a mains supply voltage is unacceptable.

Ferro-resonant constant voltage transformers known to the applicant are stable in the sense that their output voltages are stabilised against certain input voltage fluctuations.

However this statement is true only for unity power factor loads and assumes that the frequency of the voltage supply is itself stable. For example, it is known that if the mains frequency changes by 1 percent the output voltage changes by up to 1,6 percent. Similarly if the load has a power factor of 0,8 leading the output voltage can increase by up to 4 percent, while if the load power factor is 0,8 lagging, the output voltage can reduce by up to 5 percent.

An international norm for a constant voltage transformer is that the output voltage should be kept stable within approximately 2 percent for a mains supply voltage lying between - 20 percent and + 10 percent of the rated supply voltage. The output voltage stability of approximately 2 percent must be maintained from no-load to full-load.

It is known to provide constant voltages through the use of devices such as thyristors, triacs or other switching mechanisms which use feed-back controllers. However, this approach is expensive and in certain instances unreliable.

For a description of the theory of operation of ferro-resonant constant voltage transformers reference may be had to the dissertation submitted to the Faculty of Electrical Engineering of the University of the Witwatersrand by Dr Herbert Silbermann entitled "An investigation of the ferro-resonant constant voltage transformer", and to the specification of South African patent number 77/2240.

To achieve a constant voltage output with a ferro-resonant constant voltage transformer a part of the transformer core must operate at high magnetic flux density i.e. in the saturated part of the magnetising curve. It is not unusual that a flux density of up to 2,2 tesla is required.

The output voltage of a constant voltage transformer contains harmonics which distort the wave form. Distorted AC voltage wave forms are only suitable for DC rectification and are unsuitable for loads such as transformers, motors and solenoids. Generally the harmonic distortions arise from the odd harmonics of the fundamental frequency i.e. the third, fifth, and seventh harmonics, and so on. To achieve near-sinusoidal wave shapes, filter circuits have to be used to eliminate the harmonics. These circuits are expensive and time consuming to adjust correctly.

The use of external filters has been avoided through the use of special designs of transformer cores using cruciform steel laminations. A special winding in combination with an air gap is tuned to the odd harmonic frequencies and then connected in anti phase to the main winding on the transformer core. This approach does result in the harmonic voltages being cancelled and in the output wave form becoming more sinusoidal but has inherent disadvantages such as the wastage of expensive electrical steel in making up the transformer core, and in high temperatures which arise in the windings which are enbedded tri the core. In addition it is not possible to adjust the output voltage except by the provision of taps on the output winding.

Further the assembly of this type of constant voltage transformer requires the use of special jigs and tools.

The specification of the aforementioned South African patent number 77/2240 describes a constant voltage transformer which makes use of an external filter choke to suppress harmonic voltages0 The output voltage can be adjusted to a limited extent by varying an air gap in the transformer core. However, once the transformer is vacuum impregnated, no adjustments are possible unless externally accessible taps are provided on an output winding. This is a distinct disadvantage for the output voltage varies positively and negatively as the frequency of the main supply changes and, as has been indicated, a 1: percent change in the mains frequency causes an output voltage change of the order of 1,6-percent. Also the output voltage changes as the load to power factor changes from unity, whether leading or lagging.

It wouldybe desirable to provide an improved ferro-resonant constant voltage transformer The invention provides a feFro-resonant constant voltage transformer which includes first, second and third juxtaposed subcores, a primary winding around one limb of the first subcore and one limb of the second subcore, an output winding around one limb of the third subcore, and at least a first control winding around one limb of the second subcore and one limb of the third subcore.

The output winding and the first control winding may be connected in anti phase.

A second control winding may be located around the same limbs of the second and third subcores as the first control winding.

In one form of the invention a capacitor is connected to the output winding and to the second control winding. A terminal for a voltage which is output by the constant voltage transformer may be provided on one side of the capacitor.

A further output winding may be provided around a limb of the first subcore. A first terminal of this further output winding may be connected to a junction point between the first and second control windings and a second terminal of this output winding may form a terminal for the said output voltage.

In a variation of the invention a further control winding is located around the same limbs of the first and second subcores as the primary winding. A capacitor may be connected to this further control winding and to the output winding.

The transformer of the invention may include capacitor means connected between the output winding and the first control winding. The capacitor means may be variable to compensate the output voltage for load power factor variations.

Alternatively or additionally inductive means may be connected between the output winding and the first control winding. Again it is possible to make the inductive means variable to compensate the output voltage for load power factor variations.

According to a further variation of the invention a further control winding is located on one limb of the third subcore and a resistor is connected in series with the further control winding. The resistor may be variable. This arrangement enables unwanted harmonics to be reduced.

The resistor may be replaced by any suitable circuit element, such as a semi-conductor, which preferably is controllable so as to vary the amount of current flowing through it.

The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 illustrates the construction of a prior art ferro-resonant constant voltage transformer of the kind described in the specification of South African patent number 77/2240i Figures 2 to 5 illustrate different types of constant voltage transformers in accordance with the present invention, Figure 6 illustrates an equivalent circuit for a ferro-resonant constant voltage transformer of the kind shown in Figures 2 to 5, and Figure 7 illustrates a further variation of a constant voltage transformer according to the invention.

For a better understanding of the present invention reference is made initially to Figure 1 which illustrates a prior art device 10 of the kind described in the specification of South African patent number 7?/2240.

The device 10 shown in Figure 1 includes first, second and third juxtaposed subcores designated C1, C2 and C3 respectively, a primary winding L1 located around adjacent limbs of the first and second subcores, an output winding t2 and a first control winding L3 located around adjacent limbs of the second and third subcores, a second control winding L4 located around the same limbs of the first and second subcores as the primary winding, a third control winding L5 located around the outer limb of the first subcore, and an external filter choke 12. Each subcore has two airgaps, in opposed limbs, respectively, shown by heavy tines.

The transformer 10 supplies its output voltage across the windings L2, L3 and L4. The output voltage depends on the amount of turns of these windings and the turns ratio between the windings L1 and L3. Changes to the output voltage can be made by adjusting the size of an air gap 14 in the outer limb of the third subcore. However once the transformer is vacuum impregnated no adjustments can be made unless taps are provided on the winding L2. As has been pointed out this is a distinct disadvantage inasmuch as the output voltage of the transformer is frequency dependent and further is dependent on the load power factor.

The external filter choke 12 is used to suppress harmonic voltages The applicant has discovered that the winding L4, in combination with the external filter choke, can adversely affect the operation of the constant voltage transformer. The windings L1 and L4 are adjacent each other and it has been found that a capacitance exists between these two windings whereby high frequency transient voltages are transferred from the primary winding L1 to the secondary windings. This is detrimental to tr-ansient voltage rejection. The winding L4 contains harmonics especially the third and fifth harmonics which are detrimental to the output voltage wave shape.

In the present device the external filter choke is dispensed with and this makes it possible either to do away with the winding L4 or to reduce the problems which are associated with its use.

Figure 2 illustrates the construction of a constant voltage transformer 16 according to a first variation of the invention.

The transformer, as are the remaining examples of the invention, is based on the use of first, second and third juxtaposed subcores designated Cl, C2 and C3 respectively. A primary winding L1 is wound around adjacent limbs of the first and second subcores. An output winding 18 is wound around the outer limb of the third subcore and first and second control windings 20 and 22 respectively are wound around adjacent limbs of the second and third subcores. A third control winding 24 Is located around the same limbs of the first and second subcores as the primary winding L1 and a fourth control winding 26 is positioned around the-outer limb of the first subcore C1.

The subcores, in this and in the other examples of the invention, have airgaps similar to those described with reference to Figure 1.

A capacitor C is connected to the winding 18 and to the winding 24.

The winding 18 is connected in anti phase to the output winding 20. To achieve a rated output voltage of 220 volts, in this example, the windings 20 and 22 are adjusted accordingly. The third harmonic voltage in the winding 20 is in antiphase with the fundamental voltage but the fundamental voltage in the winding 18 is in antiphase to the voltage appearing across the winding 20. Since the third harmonic voltage of the winding 18 is in antiphase to the third harmonic voltage of the winding 20 the total harmonic distortion of the output voltage wave form is considerably reduced without the use of an external filter choke. A capacitor C Is connected to the winding 18 and to the winding 24.

It has already been mentioned that once a ferro-resonant constant voltage transormer is vacuum impregnated and tested, no means exists to change the output voltage easily other than through the use of externally accessible taps on the windings.

Designs do exist which make use of electronic feedback devices and thyristor or triac based circuits to achieve voltage changes but these approaches are complicated and often expensive.

Figure 3 illustrates a simple and inexpensive design which enables the output voltage to be changed. The transformer 28 in Figure 3 is constructed identically to the transformer 16 of Figure 2. However a connection 30 is provided to a junction point between the wlndings 18 and 20. A second connection 32 leads from one side of the capacitor C. The conections 30 and 32 terminate in a switch 34 which enables one end of a variable capacitor unit 36 to be connected either to the connection 30 or to the connection 32. The opposing end of the unit 36 is connected to the junction of the windings 20 and 22. The unit 36 includes capacitors Ca, Cb Cc and Cd.

The unit 36 operates as follows. The output voltage of the transformer depends inter alia on the amount of turns of the windings 20, 18 and 26 respectively. The total output voltage, across the indicated output terminals, in the absence of the capacitors of the unit 36, is adjusted lower than the required rated output voltage at full load and at the rated mains supply input voltage. When the capacitors Ca to Cd are connected either directly or via any switch arrangement to the winding 18 the output voltage increases, the percentage increase depending on the value or combination of values of the capacitors It is apparent that the invention is not limited to the arrangement of capacitors shown in the unit 36 and any suitable number of capacitors connected in any suitable configuration, whether in series or in parallel, may be employed. In addition it is possible to replace the capacitors in the unit 36 by inductances, in the case of a substantial leading load power factor.

Figure 6 illustrates an equivalent circuit for the transformer of the invention, The primary winding L1 is in series with a primary leakage reactace designated Xlp. In simplified terms it can be said that when the rated input voltage Vin is connected to the primary side a large magnetising current flows through X1p and through the primary winding L1 of the transformer. The current Ic' in the capacitor C, referred to the primary side1 partially cancels the magnetising current in the winding, the difference being a current I1 having a lagging power factor.

Since the current I1 flows through the inductive reactance X1p the result is a voltage drop Vp across the primary leakage reactance. By way of example only when correctly designed the voltage drop Vp is of the order of 33 volts, at the rated mains input voltage, and a primary voltage E1 across the primary winding L1 is 187 volts i.e. the difference between the rated supply voltage of 220 less the volt drop Yp.

The voltage across the winding 20 depends on various factors such as the turns ratio of the windings, the primary and secondary leakage reactances, the flux densities of the cores, the value of the capacitor C and the voltage drop across the primary leakage reactance as a result of a current having a lagging power factor.

Once the capacitors Ca to Cd are connected across the winding 20, as described in connection with Figure 3, a capacitive current having a leading power factor will flow. This capacitive current flows in the primary circuit as well as through the primary leakage reactance Xlp. Since the lagging current I1 is in anti phase to the capacitive current the volt drop Vp is reduced and the voltage E1 increases. An increase of the output voltage across the winding 20 is observed.

This voltage increase depends on the value of the capacitors in the unit 36 connected to the winding 20.

The winding 24, referred to as L4 in Figure 1 is adjacent the primary winding L1 and it has been established that a capacitance exists between these two windings which has the effect of transferring high frequency transient voltages from the primary winding to the secondary winding, This can be detrimental to transient voltage rejection. With the present approach the winding 18 ls used to reduce the harmonic distortion of the output voltage wave shape, without the use of an external filter choke, and as is illustrated in Figures 4 and 5, the winding 24 can be dispensd with.

The transformer 40 shown in Figure 4 is similar to the transformer 16 of Figure 2 but the winding 24 is omitted. In addition the capacitor C Is connected to the windlngs 18 and 22. In the transformer 42 of Figure 5 a variable capacitor unit 36, similar to that shown in Figure 3, is connected to the transformer 40 in the same way as the unit 36 is connected to the transformer 16.

The elimination of the winding 24 results in better transient rejection, reduces harmonic voltage distortions and improves the output voltage stability with respect to mains supply input voltage exchanges.

From tests carried out on the transformer 42 shown in Figure 5 the following results were obtained, at a nominal input voltage of 220 volts.

Vin = input voltage Vout = Output voltage 242 volts i.e.+ 10%) 217 volts (-0,91% change) 220 volts 219 volts 198 volts 221 volts 189 volts 221 volts 176 volts(i.e. -20%) 221 volts (+ 0,91 X change) There is zero change in the output voltage, for Vin=220 volts, from no load to full load.

The total harmonic distortion at Vin = 220 Volts, from the 3rd to the 19th harmonic of the fundamental frequency, is 2,54X.

Connecting Ca (see Figure 5) Vout = 221 volts " Cb " " " " = 222 volts " Cc " " " " = 224 volts " Cd " " " " = 225 volts The winding 18 on the outer limb on the third subcore C3 reduces the harmonic voltage distortion to less than three percent without the use of an external filter choke.

It is possible to reduce the third harmonic voltage further, to a level of -55dB. This is of the order of 0,89 volts. At -55dB the third harmonic of the output voltage wave shape is reduced by 78%.

It is to be noted that in the case of three constant voltage transformers connected in a primary star configuration, the third harmonic voltages are additive at the star point. For fnstance if the third harmonic is reduced by 42dB the third harmonic emf between the star point and neutral or earth is 11,94 volts. However if the third harmonic is reduced by 55dB then this emf is only 2,67 volts. High third harmonic- voltages between star point and neutral or earth are detrimental to communication systems.

The transformer 40 shown in Figure 4 has three subcores each of which is made of grain oriented electrical steel and has air gaps of different lengths. The air gaps are adjusted to give a high magnetising current so that the subcore C2 is driven deep into magnetic saturation. The subcores C1 and C2 with their respective windings constitute primary and secondary leakage reactances. These reactances can be individually adjusted by changing the air gap lengths. In core type transformers the leakage flux outside the cores contains high third harmonics.

The third harmonic content, in the transformer 42, across the windings 18 and 26 is approxlmately -42dB which equals 3,98 volts. In this transformer the windings 20 and 22 are series connected with the capacitor C. The winding 18 is connected in anti phase to the windings 20 and 22 resulting in a reduction of the potential across the winding 20. Consequently the arrangement results in a reduction of the third harmonic voltage across the winding 20 to the aforementioned -42dB.

In order to achieve a further reduction of the third harmonic, without the use of auxiliary filters, the arrangement shown in Figure 7 is resorted to and a control winding L7 is wound around the outer limb of the third subcore C3. A variable resistor designated R is connected in series with the control winding.

By changing the value of the resistor R the current through the winding L7 and the resistor R can be altered. Since the windings L7 and 18 are closely coupled the current through the winding L7 affects the reluctance of the subcore C3 and thereby changes the secondary leakage reactance. This results in a further reduction of the third harmonic of the output voltage wave form. It has been established that by increasing the current through the control winding L7 the output voltage of Lhe constant voltage transformer can be reduced without affecting the third harmonic which under these conditions is reduced to -55dB. The third harmonic voltage in the absence of the compensating windings 18, 26 and L7 was measured to be -36dB which is the equiv-alent of 7,943 volts.Through the use of all three of these compensating windings the third harmonic is reduced to -55dS which equals 0,89 volts and therefore a total reduction of 89 percent of the third harmonic is achieved. (All the voltages mentioned are for a mains voltage of 220 volts).

The following measurements were made on a transformer of the kind shown in Figure 7.

Harmonic distortion with windings 18, 26 and L7 connected: Harmonic (order) 3 5 7 9 11 13 15 17 19 decibels minus 55 47 47 60 50,5 85 56 70 64 Input voltage = 220V Output voltage - 220V Total percentage-equals 1,7 percent Output voltage (Vout) versus variation in input voltage (Vin) at full load.

Vin Vout 242 218 220 (rated) 220 (rated) ]76 218 Change = + 10X and -20% Appr. 1X Effect of current through winding L7 I in L7 Vin out 3rd harmonic at Vin=220 volts 242 218 Zero 220 221 -46dB 176 221 242 217 0,1 Amp 220 219 -50dB 176 218 242 217 0,2 220 219 -51dB 176218 242 217 0,3 Amp 220 218 -55dB 176 216 242 215 0,4 Amp 220 217 -54dB 176 213 242 215 0,5 Amp 220 215 -50dB 176 212 It follows therefore that through the use of the winding L7 on the outer limb of the subcore C3 the harmonic distortion of the output voltage wave form is considerably reduced. The third harmonic is brought down to -55dB which is 0,89 volts.

The third harmonic reduction is achieved without adversely affecting'higher harmonics. The variation of the value of the resistor causes the output voltage of the transformer to be changed in a stepless manner without unduly affecting the harmonic distortion. This enabtes an output voltage increase to be compensated for in case the load has a leading power factor.

The transformer of Figure 7 has been described with reference to the use of a variable resistor in series with the control winding L7. Other devices can of course be used in place of the resistor so that a variation of current flow through the control winding L7 is achieved.

Claims (22)

1. A ferro-resonant constant voltage transformer which includes first, second and third juxtaposed subcores, a primary winding arnund one limb of the first suhcore and one limb of the second subcore, an output winding around one limb of the third subcore, and at least a first control winding around one limb of the second subcore and one limb of the third subcore.

2. A transformer according to claim 1 wherein the output winding and the first control winding are connected in anti phase.

3. A transformer according to claim 1 or 2 which includes a second control winding around the same limbs of the second and third subcores as the first control winding.

4. A transformer according to claim 3 which includes a capacitor connected to the output winding and to the second control winding.

5. A transformer according to claim 4 wherein a terminal for an output voltage is provided on one side of the capacitor.

6. A transformer according to claim 3 which includes a third control winding around the same limbs of the first and second subcores as the primary winding.

7. A transformer according to claim 6 which includes a capacitor connected to the output winding and to the third control winding.

8. A transformer according to any one of claims 1 to 6 which includes a fourth cnntrnl winkling arnilnfl a limh of the first subcore.

9. A transformer according to claim 8 when dependent on claim 3 wherein a first terminal of the fourth control winding is connected to a junction point between the first and second control windings.

10. A transformer according to claim 9 wherein a second terminal of the fourth control winding forms a terminal for an output voltage.

11. A transformer according to any one of claims 1 to 10 which includes capacitor means connected between the output winding and the first control winding.

l?. A transfnrmer according tn claim 11 wherein the capacitor means is variable.

13. A transformer according to any one of claims 1 to 12 which includes inductive means connected between the output winding and the first control winding.

14. A transformer according to claim 13 wherein the inductive means is variable.

15. A transformer according to any one of claims 1 to 14 which includes a fifth control winding on one limb of the third subcore and a circuit element connected in series with the fifth control winding.

16. A transformer according to claim 15 wherein the circuit element is a variable resistor.

17. A ferro-resonant constant voltage transformer which includes first, second and third juxtaposed subcores, a primary winding around one limb of the first subcore and one limb of the second subcore, an output winding around one limb of the third subcore, first and second control windings around one limb of the second subcore, and one limb of the third subcore, a third control winding around one limb of the first subcore, and a capacitor connected to the output winding and to the second control winding.

18. A transformer according to claim 17 which includes variable capacitor means connected to a junction between the first and second control windings, and to a junction between the output winding and the first control winding.

19. A ferro-resonant constant voltage transformer which includes first, second and third juxtaposed subcores, a primary winding around one limb of the first subcore and one limb of the second subcore, an output winding around one limb of the third subcore, first and second control windings around one limb of the second subcore, and one limb of the third subcore, a third control winding around one limb of the first subcore, a fourth control winding around the same limbs of the first and second subcores as the primary winding, and a capacitor connected to the output winding and to the fourth control winding.

20. A transformer acording to claim 19 which includes variable capacitor means connected to a junction between the first and second control windings, and to a junction between the output winding and the first control winding.

21. A transformer according to claim 17 which includes a further control winding around the same limb of the third subcore as the output winding and a resistor connected in series with the further control winding.

22. A ferro-resonant constant voltage transformer substantially as herein described with respect to any one of the accompanying drawings Figures 2 to 7.