TECHNICAL FIELD

This invention relates to supplying of electrical energy and particularly to supplying an unlimited range of direct currents with superimposed sinusoidal and non- sinusoidal alternating currents.

BACKGROUND ART There are a number of methods for providing direct current (DC) with a superimposed alternating current (AC). In each of these methods it is necessary to provide isola¬ tion between the DC and AC sources. A typical method of such decoupling is illustrated by a diagram in Fig. 1. An inductance 3 blocks the flow of alternating current into a DC source 1 while a capacitor 4 prevents an AC source 2 from short circuiting the DC power supply. A power supply of this type which provides an AC current superimposed on DC current to a load 5, however, becomes unwieldy in applications where large currents (of the order of thousands of amperes) are required as in the case of some electrochemical installa- tions. In such _. cases the values of inductance 3 and capaci¬ tance 4 become quite large and proh biti ely expensive,

A method that provides a partial solution to these problems is illustrated by a diagram in Fig. 2. Alternating

current is provided by a transformer 11 with a center tap 12. A DC power source 13 is connected between the center tap 12 on the transformer and the common point 14 of two loads 15 and 16. If the loads are identical, then AC voltage across the DC supply 13 is zero and no inductance is required to prevent the alternating current from flowing through the DC source. Also no capacitor is required in this circuit. The main disadvantage of this approach is the requirement that the two loads be identical. The need for balanced loads creates a number of difficulties in practica applications and has the effect of increasing the cost of industrial.processes which require the use of such supplies.

DISCLOSURE OF INVENTION

In brief, the present invention overcomes most of the problems encountered in the existing methods for supply ing direct current with a superimposed alternating current. The new power supply does not use blocking inductive ele¬ ments, requires no blocking capacitors and can work with single loads. For these reasons the supply provides a con- venient and inexpensive method and system of generating an almost unlimited range of currents from very small to ex¬ tremely large values.

In all conventional rectifier circuits one end of a load (which will be referred to henceforth as the first load terminal) is essentially connected to a rectifier cir¬ cuit element or to a common point of several such elements. The other end of the load (which will be referred to hence¬ forth as the second load terminal) is connected to a trans¬ former winding or to another rectifier circuit element or to another common point of several such elements. In the case where a filter capacitor is connected across the load, the filter capacitor terminal which is connected to the first load terminal is designated "the first capacitor

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3 terminal", and the capacitor terminal connected to the second load terminal is designated "the second capacitor terminal." A conventional rectifier circuit is designed to have AC potential difference between the first and second load terminals equal or nearly equal to zero. We will consider this case as one when AC potentials of the load terminals are balanced. In a full-wave center tapped rectifier, for example, balancing is achieved by connecting the second load terminal and the second capacitor terminal to the center tap of the transformer.

According to the invention, superposition of AC voltage on DC voltage is obtained through purposely unbal- ancing AC potentials of the load terminals by changing AC potential of the first load terminal and/or of the second load terminal so that an additional difference between these potentials will appear across the load. This change in AC potentials may or may not be accompanied by a change in DC potentials of the load terminals depending on the method used for varying the AC potentials.

If the second load terminal is connected to the center tap of the transformer winding, the AC potential of only this terminal may be changed, and subsequently the un- balancing will follow, by disconnecting the terminal from the center tap and connecting said terminal to other points along the winding. A transformer winding which is used for "unbalancing" will hereinafter be referred to as the "un¬ balancing winding." The position of capacitors initially shunting the load should remain unaltered; i.e., the capaci¬ tor should remain connected between the first load terminal and the center tap of the transformer in this case. No change of DC voltage across the load will be observed here since the AC potential of the first load terminal remains unchanged.

A change in AC potential of only the first load terminal may be accomplished by connecting one or more coupling capacitors between the first load terminal and

different points along the unbalancing winding. These capacitors may initially shunt the load or they may be specially added to change the potential of the first load terminal. Since the AC potential of the first load ter¬ minal changes, the AC voltage across the rectifier elements also changes. This process in turn alters the magnitude of the rectified DC voltage.

The same method may be used to change the AC potential of the second load terminal when this terminal is connected to a common point of several (at least one) rectifier circuit elements. It is understood that AC and DC potentials of both load terminals may be changed simultan¬ eously to obtain the desired value of AC and DC voltages across the load.

If the input voltage of the new power supply is sinusoidal and capacitors used in the circuit are big enough, the waveform of the AC component across the load is sinusoi¬ dal too. This waveform will vary, depending on the waveform of the input voltage or on the magnitude of capacitors used. Also, the waveform of the AC component may be changed with the help of thyristors used as rectifier circuit elements.

A non-sinusoidal AC waveform may also be acquired even when the capacitors in the circuit are big enough and normally provide sinusoidal waveform. The non-sinusoidal waveform in this case is ^" formed by introducing another non¬ linear element into the circuit (the first non-linear element being diodes or thyristors). This non-linear circuit ele¬ ment may be, for example, a saturable core reactor used for voltage control in the primary of the transformer. Likewise semiconductor controlled rectifiers may be used at the input of the system for voltage control providing non-sinusoidal waveform of AC component across the load.

The new method of superimposing alternating cur- rent on direct current provides an unlimited ratio of AC to DC voltage from zero to infinity.

The invented method and system are valid for a rectifier circuit with an arbitrary number of rectifier cir-

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5 cuit elements.

If the use of the invented method results in the appearance of an excessive magnetic flux in the transformer, various existing flux compensating methods can be used.

These methods may include sectionalizing of the transformer windings or providing an additional compensating winding, etc.

The new power supply may be considered as a source of a modulated voltage, wherein the DC voltage is a carrier and the sinusoidal or non-sinusoidal component is a modulating voltage. The voltage instead of current approach is important particularly when the load is non-linear and the voltage waveform, which may be more easily controlled, substantially differs from the current waveform.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be more readily understood from the following detailed description taken in conjunction with the drawings in which

Fig. 1 is a diagramatic representation of an example of prior art and shows separate DC and AC supplies; Fig. 2 is a diagramatic representation of another example of prior art and shows a DC supply with a center tapped secondary transformer winding;

Fig. 3 is a schematic block diagram which illus¬ trates the invented method and system for providing an AC voltage superimposed* on a DC voltage across a load; Fig. 4 is another block diagram which illustrates the invented method and system for providing an AC voltage superimposed on a DC voltage;

Fig. 5 shows a circuit diagram for a DC + AC power supply based on a full wave center tapped rectifier and illustrates the invented method of unbalancing AC poten¬ tials at the load terminals;

Fig. 6 is a graphic representation of different types of voltages across the load with a large capacitor 50 in the circuit of Fig. 5 connected to the center tap;

Fig. 7 is a view similar to that of Fig. 6 but with no capacitor used in the circuit;

Fig. 8 is a view similar to that of Fig. 6 but with the second capacitor terminal 52 connected to different points along an unbalancing winding;

Fig. 9 shows a circuit diagram for a DC + AC power supply based on a halfwave rectifier circuit and illustrates the invented method of unbalancing AC potentials at the load terminals;

Fig. 10 shows a circuit diagram for a DC + AC powe supply based on a fullwave rectifier and illustrates the invented method of unbalancing AC potentials at the load terminals; Fig. 11 shows a circuit diagram for a DC + AC powe supply based on a multi-phase rectifier and illustrates the invented method of unbalancing AC potentials of load termi¬ nals;

Fig. 12 shows a circuit diagram of a DC + AC power supply based on a three-phase rectifier and illustrates the invented method of unbalancing AC potentials of load termi- ^' 'nals.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will hereinafter be describe in detail.

Fig. 3 illustrates one of the principles of the invention. An AC power source 21 supplies at least a single phase sine wave voltage at frequencies up to kilohertz and more but preferably at a conventional 60Hz through a suit¬ able voltage-control device 22 such as a saturable core reactor, semiconductor control rectifiers, or an autotrans- former. If desired, the voltage-control unit may be elimin- ated where a constant voltage is needed at the output of the system. The primary 23 of a single phase or a multi-phase transformer is coupled with the voltage-control unit. The windings of the primary may be star-connected. The connec-

7 tion of windings is ordinarily preferable since it brings about a more even distribution of currents in the phases of the power supply 21. The secondary 24 of the transformer has two types of windings: ordinary and unbalancing wind¬ ings. All these windings are star-connected. An ordinary winding is used exclusively for supplying AC voltage to a system 25 of rectifier circuit, elements, whereas an un¬ balancing winding is used mainly for supplying an unbal- ancing AC voltage to terminals 27 and 28 of a load 26, though this winding may also be used for supplying voltage to the rectifier system. The first load terminal 27 is connected to a system of rectifier circuit elements 25 and the second.load terminal 28 is directly connected to the secondary of the transformer so that a DC voltage is pro¬ vided across the load. If a minimum value of AC voltage component is desired across the load, the second load terminal is connected to the point of star connection of the windings. In this case, AC potentials of the load terminals ^are balanced. An additional AC voltage is introduced across the load when the second load terminal is connected to dif¬ ferent points of the unbalancing winding which provides unbalancing of AC potential of the second load terminal. The AC potential of the first load terminal 27 may be also altered with the help of a coupling capacitor 29 connected to the unbalancing winding. This capacitor affects also the waveform of the AC voltage component across the load even if it doesn't change the AC potential of the first load terminal which occurs when the second capacitor ter- minal is connected to the point of star-connection of transformer windings.

The schematic block diagram represented in Fig. 3 illustrates a plurality of power supplies wherein one of the load terminals (namely the second terminal) is connected directly to a transformer winding.

If both load terminals are connected to different points of a system of rectifier circuit elements, another block diagram applies. This diagram is represented in Fig.

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4, and the same elements as those of Fig. 3 are used here except that two coupling capacitors 39 and 40 may be em¬ ployed, one for changing AC potential of the first load terminal 37 and/or another for changing AC.potential of the second load terminal. It is understood that the value of these capacitors affects the waveform of the additional AC component across the load 36.

The principles of the invention disclosed in Fig. 3 and 4 will be further illustrated by a number of pre¬ ferred embodiments. These embodiments mainly differ by circuits connected to the secondary winding of the trans¬ former.

In the case of a two-phase power supply, usually known as a full-wave center tapped rectifier installation depicted in Fig. 5, a two-phase transformer secondary com¬ posed of two windings 40 and 41 with a center tap 42 is in circuit with two rectifier elements 43 and 45. These cir¬ cuit elements have a common point 46 which is a first out- put terminal of the rectifier system. Both rectifier elements have the same direction with respect to the output terminal 46 and, of course, with respect, to terminals of the transformer windings 40 and 41 to which they are con¬ nected. Under the term "rectifier circuit elements" we mean hereinafter diodes and/or thyristors. A load 47 is connected by its first terminal 48 to the first output ter¬ minal and by its second terminal 49 to the center tap 42 of the transformer secondary. A capacitor 50 with its first terminal 51 and a second terminal 52 is connected respec- tively to the first load terminal 48 and to the center tap 42 of the transformer. As long as the second load terminal and the second capacitor terminal are connected to the transformer center tap only, direct current with a ripple dependent on the value of the capacitor 50 will flow through the load. In this case an AC voltage component across the load is minimal and AC potentials of the load terminals may be considered as being balanced. If capaci¬ tance C of the capacitor 50 is very large (tending to infinity), a pure DC voltage with no AC component will be

applied to the load (see Fig. 6a). This DC voixage is equal to a half of the amplitude A of the AC voltage across the transformer secondary which includes two windings 40 and 41.

According to this invention an additional AC voltage component will be introduced across the load 47 when the second load terminal 52 is moved from the center tap and is connected to different points along either transformer winding, whereas the second capacitor terminal is still connected to the cente tap. Different posi¬ tions of the second load terminal are schematically indi¬ cated by dotted arrows. When the second load terminal moves along the winding 40, AC potential of this terminal changes and'becomes unbalanced with respect to the AC po¬ tential of the first load terminal. Thus, the transformer winding 40 is called "unbalancing winding". The unbal¬ ancing winding plays a dual role here: it changes the AC potential of the second load terminal and also supplies voltage to the rectifier circuit element 43. If the second load terminal 49 is connected to an intermediate point along the unbalancing winding 40 and capacitance C of the capacitor 50 is large, and also a sine wave voltage is applied to the input of the transformer, the waveform of additional component across the load is sinusoidal too.

This component has an amplitude which is intermediate be¬ tween zero and A/2, whereas the DC voltage is the same as it was initially, i.e., before moving the second load ter¬ minal (see Fig. 6b). When the second load terminal reaches the end of the unbalancing winding, the amplitude of the AC component equalizes with the DC voltage (see Fig. 6c). A dramatic change of the AC component waveform will accrue from diminishing the capacitance of the capaci¬ tor 50 provided all other conditions remain unaltered. In the extreme when the capacitor 50 is disconnected (capaci¬ tance C = 0), the waveform of the voltage across the load will be as depicted in Fig. 7a if the second ^" load terminal is connected to the center tap. When this terminal is

connected to an intermediate point along the unbalancing winding, the waveform is as in Fig. 7b, and, at last, as in Fig. 7c when the terminal reaches the end of the winding. It should be noted that the average value of the voltage appearing across the load remains constant for all positions of the load terminal and will be equal to A/τr.

A similar effect would occur if winding 41 rather than winding 40 were used as the unbalancing winding. So far we have discussed the method and system for unbalancing the AC-potential of the second load termin¬ al which has the reference number 49 in Fig. 5. It is also possible to unbalance the AC potential of the first load terminal 48, -which may be accomplished by connecting the second capacitor terminal 52 to different points of either winding 40 or 41 , leaving the second load terminal 49 con¬ nected to the center tap 42.

The capac tor 50 in this case w ll act as a coupling capacitor transferring different AC potentials along the unbalancing winding to the common point 46 of the rectifier circuit elements. Since the AC voltage difference across the rectifier elements changes, it causes a change of the DC voltage component. Fig. 8 illustrates this phenomenon. When both second load and second capacitor terminals are connected to the center tap 42, the DC compo¬ nent is equal to A/2 and no AC component across the load exists, provided the capacitance C is large enough (see Fig. 8a). If the second capacitor terminal 52 is con¬ nected to an intermediate point of the unbalancing winding and the load terminal 49 remains connected to the center tap 42, the DC voltage increases and a sinusoidal component appears across the load. In the extreme, when the second capacitor terminal reaches the end of the unbalancing wind¬ ing, the DC component is equal to A and the amplitude of the AC component is equal to A/2 (see Fig. 8b).

It is also possible to unbalance the AC poten¬ tials of both load terminals si ultaneously by moving the second load and the second capacitor terminals along one or

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two unbalancing windings. An additional AC voltage com¬ ponent will appear across the load, provided the second load and capacitor terminals are not connected to the same point. In the extreme, when these terminals are connected to the opposite ends of the windings 40 and 41 , the DC voltage is equal to A and the amplitude of the AC voltage component is also equal to A (see Fig. 8c). The connection of the second capacitor terminal to different points of the winding is schematically shown in Fig. 5 by dotted arrows .

It should be noted that none of the described waveforms reverse the polarity of the potential across the load, a situation which may be essential for many electro- chemical and other applications of the invented AC + DC power supply.

A half-wave recti fier -circuit and a method repre¬ sented in Fig. 9 form a special case because in th s circuit the sinusoidal AC component may exceed the DC component across load 65. It will happen only if the AC potential of the first load terminal 66 is unbalanced, which may be accomplished by connecting second capacitor terminal 70 to different points of unbalancing winding 61 , leaving the second load terminal 67 connected to end 62 of the winding. The coupling capacitor 68 will transfer an AC voltage to the first load terminal 65, reducing the AC voltage drop across rectifier circuit element 64, thus resulting in diminishing the DC voltage rectified by this element. When the second load terminal reaches the end 63 of the unbalancing winding, the DC voltage across the load reduces to zero and the ratio of AC to DC components is equal to infinity.

No change of the DC component will happen if the AC potential of the second load terminal 67 is unbalanced by moving this terminal along the unbalancing winding 61 while the capacitor 68 remains connected to the rectifier circuit element 64 with the first capacitor terminal 69 and remains connected to the end 62 of the winding 61 with the second capacitor terminal 70. In this case the value of only the AC component will change. The amplitude of this

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component reaches A when the second load terminal is con¬ nected to the end 63 of the unbalancing winding. The wave¬ form of the voltage across the load 65 in this case is ade- quate to that of Fig. 8c which referred to the full-wave center-tapped system as depicted in Fig. 5. Moreover these two diagrams provide identical AC + DC voltages across the load not only in the previously discussed case. Identical voltages will also appear if in the circuit of Fig. 9 the second load terminal 67 is connected to the center point 71 of the winding 61 and the second capacitor terminal 70 moves along this winding in the direction of its end 62. The iden¬ tity of the voltages will also occur in the opposite situa¬ tion when the second capacitor terminal 70 remains connected to the point 71 and the load terminal 67 would move to the end 62.

It means that a half-wave system of Fig. 9 may be represented in the majority of cases by a full-wave center- tapped system of Fig. 5 with one of two rectifier elements disconnected. Suppose the circuit element 45 is discon¬ nected; then the ordinary phase winding 40 is used exclu¬ sively for rectification whereas the second phase winding 41 is used exclusively for unbalancing. The idea of pro¬ viding a special winding which is used exclusively for un- balancing is very beneficial for multi-phase systems, as will be discussed below.

In systems of Fig. 5 and Fig. 9 the second load terminal is connected directly to the transformer winding in compliance with the principle disclosed in Fig. 3, where- as the first load terminal is connected to at least one rectifier circuit element. According to the principle dis¬ closed in Fig. 4, the second load terminal may also be con¬ nected to at least one rectifier circuit element which is different from the element connected to the first load terminal. This may happen, for instance, in a system which is based on a full-wave rectifier bridge represented in Fig. 10. Four rectifier circuit elements 74, 75, 76 and 77 forming a bridge rectifier circuit are connected to one-

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phase transformer secondary 71. Two of these elements, viz. 74 and 75, constitute a first group having a common -point 78 which is a first output terminal of the rectifier system. Both these elements have the same direction with respect to this output terminal. A second group includes elements 76 and 77 having a common point .79 which is a second output terminal of the rectifier system. The elements of the second group have the same direction with respect to the second output terminal but this direction is opposite from that of the elements of the first group. Therefore, DC potentials of the first and second output terminals are of opposite polarity. A load 80 is connected with its first terminal 81, to the first output terminal and with its second terminal 82. to the second output terminal. AC po¬ tentials of load termianls are in this case balanced and an AC ripple voltage across the load is minimal. The un¬ balancing of AC potential of the first load terminal is accomplished with the help of a first coupling capacitor 83 connected to the first load terminal 81 with a first capacitor terminal 84 and connected to any point of the unbalancing winding 71 with a second capacitor terminal 85. The same method may be used to unbalance AC potential of the second load terminal. A second coupling capacitor 86 is used respectively for this unbalancing with its first terminal 87 connected to the second load terminal and a second capacitor terminal 88 connected to any point of the unbalancing winding albeit different ^• f om the point of con¬ nection of the terminal 85. A multi-phase embodiment of the present invention is illustrated in Fig. 11. A secondary of the multi-phase transformer is formed by six star-connected windings with reference numbers from 91 to 96. Four of these windings, namely 93, 94, 95 and 96 , are ordinary windings which are used exclusively to supply voltage to rectifier circuit elements 99, 100, 101 and 102 which have a common point 103, this point constituting the first output terminal. The remaining two windings 91 and 92 are unbalancing windings

and each of them plays a dual role: it unbalances AC po¬ tential of one of the load terminals and also supplies voltage to a rectifier circuit element which is 98 for the winding 91 and 110 for the winding 92. A load 104 is con¬ nected to the first output terminal 103 with a first load terminal 105. A second load terminal 106 is connected directly to any point of the unbalancing winding 92, thus introducing an additional AC voltage component across the load. Evidently, no additional voltage will be introduced if the second load terminal is connected to a point 97 of star connection of the windings. The unbalancing of AC potential of the first load terminal 105 is accomplished in the manner described in previous embodiments: a couplin capacitor 107 is used, this capacitor being connected to th first load terminal with a first capacitor terminal 108 and to the unbalancing winding 91 with its second terminal 109. It should be pointed out that the use of a special unbal¬ ancing winding 91 for changing AC potential of the first load terminal is gratuitous. In this case the same unbal¬ ancing winding 92 which is employed for changing AC poten¬ tial of the second load terminal may be used. It is eviden that if the second capacitor terminal 109 is connected to the point 97 of star connection of the windings, no unbal- ancing of the first load terminal will occur. But the role of the capacitor 107 is still important here since it essen tially affects the waveform of the AC component across the load, this component being introduced by connecting the second load terminal 106 to different points along the unbalancing winding. If the windings of the transformer secondary provide sine form voltage and the capacitor 107 i large enough, ie.e,, it has a large capacitance C, the wave¬ form of the AC component across the load is sinusoidal too. Being connected to the point 97 of star-connection, the coupling capacitor 107 becomes a wave shape forming only.

As such, it has a minimal AC voltage drop and gives the ad¬ vantage of employing the least expensive electrolytic type capacitors which cannot be otherwise employed as coupling capacitors when a substantial AC voltage is applied across

their terminals.

It is not crucial for the unbalancing winding to supply a voltage for a rectifying system along with supply- ing the unbalancing voltage. The last function of the wind¬ ing may be the only one. In this case the rectifier circuit element 110, which is shown by dotted lines in Fig. 11, is disconnected.

Another preferred embodiment of the present inven- tion, which is depicted in Fig. 12, is a particular case of the just now described multi-phase system. It is a three- phase system where a sine form voltage of industrial fre¬ quency, predominantly of 60 or 50 cycles per second, is applied from a source 110 to a transformer primary through a voltage-control system which here is a saturable core reactor with three windings 111, 112, and 113.

The three-phase transformer primary consists of threeΔ -connected windings 114, 115 and 116. The Δ-connec- tion is preferable since the source appears to be more evenly current loaded therein. Still, a Y-connection may also be employed here. A saturable core reactor is purpose¬ ly chosen in this system for voltage control to provide the following two additional functions: it assists in equaliz¬ ing line currents and also changes the sine form voltage at the input into non-sinusoidal waveform at the output of the reactor to secure a non-sinusoidal waveform of voltage .component across a load. This waveform is very important in some applications, for instance when the load is an aluminum anodizing installation. Of course, other types of voltage control, such as an autotransformer or semi¬ conductor control rectifiers, may be used too. An auto¬ transformer would not change the waveform of the controlled voltage, whereas semiconductor control rectifiers do change this waveform but would not provide the same equalizing effect for the line currents as the saturable core reactor does. Three phase windings of the transformer secondary, namely 117, 118 and 119, are star connected in a point 120. Windings 118 and 119 are ordinary windings and are used

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exclusively for supplying voltage to rectifier circuit elements 121 and 122, both elements being connected to a common point 123 and having the same direction with respect to this point. The winding 117 is an unbalancing winding and is used here exclusively for supplying voltage to change AC potential of a second terminal 125 of a load 124 which is connected to the first output terminal 123 with its first terminal 126. The unbalancing of the second load terminal is accomplished by connecting this terminal to different points of the unbalancing winding 117 which is indicated schematically in Fig. 12 by several dotted arrows. A capacitor 127 is connected to the first load terminal with a first capacitor terminal 129 and to the point of star connection 120 with the second capacitor terminal 128. An electrolytic capacitor or a plurality of capacitors con¬ nected in parallel may be used as a capacitor 127. The higher the capacitance of this' capacitor, the closer to a sinusoid will be the waveform of AC component across the load, provided the sine voltage is applied to the secondary windings of the transformer. Disconnecting the capacitor or diminishing its value would greatly affect the waveform of the AC component unless the load itself has capacitive,- reaction and the capacitance of the load is high enough. The last two embodiments of Fig. 11 and Fig. 12 represent a multi-phase system implemented in compliance with principles of the block diagram in Fig. 3, where one of the load terminals is directly connected to a winding o-f the transformer secondary and the other load terminal is connected to at least one rectifier circuit element. A multi-phase bridge rectifier system known to the skilled in the art may also be employed for a DC + AC power supply. In this system both load terminals are connected to dif- ferent groups of rectifier circuit elements according to principles of the block diagram in Fig. 4, and AC poten¬ tials of load terminals are altered in this case with the help of coupling capacitors. A Δ*-connection instead of a

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Y-connection of windings of the transformer secondary may be used in this system.

Although certain embodiments of the invention have been shown in the drawings and described in the specification, it is to be understood that the invention is not limited thereto, is capable of modification, and can be arranged without departing from the spirit and scope of the invention.